i was thinking

[John Finch]

I wish I could say "see ya at the nats", but I don't know if I can get that much time off. I wish the nats were shorter. Dick Tyndal makes the turn fins that I have been experimenting with. He just lives down the street so to speak. He does such a pretty job with them. I ask him to make a certain size and bend and he makes a mold of sorts to be able to reproduce them. I have build a lot of trial and error fins and then asked Dick to make some pretty ones with the same shape. The ones I build are always ugly! One time I asked for three identical fins and he made them just a bit different. ONLY a bit different. He said try them and let me know what you find. Only the original worked well on the boat. The design that works best on my boats is a take off of the roadrunner. I guess you had something to do with that original design. Is my fin location of 3/4 close to what you are using? Oh, I still have my roundnose Bardahl that I ran in 97 at the nats. It is set up like your Vernors was. Same prop too. If you want to send me a fin I will try it on the boat and let you know if it works better than what I have.

John:

MOST of the fins that I made in the effort to optimize were UGLY. No reason to make them all look good since they won't be on the boat long anyway. The next one is already in my head.....

Some people can't stand to make ANYTHING UGLY, but since time is so limited, I have ZERO problem not finishing them completely. I would suggest that people use that approach so that they can cover more ground quickly.

Your question can not be answered directly, since all boats are different in their proportions. Also, the cg varies with a full tank, an empty tank or one that is 1/2 full. Now that I understand the dynamics, I can use any of these cg points.

We are getting into an area that has taken me a LONG time to figure out and I am not comfortable talking in very specific terms. I would like to give information that will save people time in their own general search.

Marty Davis

I understand. I will put this out there.................Marty brought up a very good point. Boat setup varies with how the boat is weighted at the time of the measurement. For example......If you put your boat on a flat table (setup board) and measure sponson angles for example. You will get a different reading when the boat has the engine in it as opposed to not having the engine in the hull. TWO COMPLETELY DIFFERENT MEASUREMENTS. If you don't believe it......try it!

John:

The amount of specific information that we have talked about should provide a GREAT starting point for people to do their own analysis, boat building and testing. It would be great to have this type discussion on a variety of things. We used to have Chats and discussions like this on my RcBoat.com Listbot.

Maybe we can do more of this. I have REALLY enjoyed it.

Of course, we have but scratched the surface on this topic, so I would bet that it won't die at this point.

Marty Davis

Ok, here goes my 2 cents worth on using angled flat turn fins.

The first time that the ANGLED flat turn fin was used in serious competition, to the best of my knowledge, was on one of Marty's 40 Crapshooters that we ran at the NAMBA Nationals in Monterey California in 1979. This and other modifications ( the beginning of the BOSS setup) was very successful. We won the 40 hydro class with a perfect score. Marty you and some of your friends from Indy were there, you should remember the maroon and gold boat.

The boat worked ok but there was room for improvements. On the trip back from that race, in the motor home,the BOSS design was expanded to include some improvements that were needed to help

the boat turn better and more consistent in different water conditions. The BOSS boat was born.

It took about a year to get the bugs worked out of the design. The first BOSS was a 20 size. In 1981 we won the district 19 championship (not finishing in first place only twice all year) and also won the 20 hydro class at the NAMBA Nationals at Amarillo, TX with 4 first place heats and fast time in the class. All this using an angled flat turn fin.

Jack O'Donnell, Steve O'Donnell and myself took a weeks vacation and built the 60 BOSS from scratch using what was learned from the 20 and 40 BOSS(You may be asking what about the 40 BOSS. It had 2 first place heat wins in Amarillo when a rudder servo decided to go out of off bouy 5 and I think it still holds the record for going the deepest into the bank at that lake, in past the sponsons). Only about a month after the 60 BOSS was built, Jack and Steve took it back to Indy for the record trials. The first lap of the 2 lap timed oval was run at about 10 seconds. The disc rotor in the engine went out on the second lap and the boat slowed considerably and the still broke the old record by about 2 seconds. The boat was over-built as for strength and it weighed in at 9 1/2 pounds. All this and it was done using an angled flat turn fin.

There is more to using this type of turn fin than just bolting it on. The entire boat and engine combinations has to be designed around it to optimize its benefits.

Whether it is better than the curved turn fin, I don't know since I sent all of my time on the angled flat design. Each design has its good points and bad points. You just can't count either of them out.

Marty, I enjoy reading your comments, and John, I enjoyed reading your articles and your books, you guys keep up the good work passing along your great knowledge about RC boating.

Charles:

Yes, I sure remember coming the NAMBA Nats in Monterey. It was a really fun time for us. We also enjoyed a small earthquake just for us. That was the Nats that Jack Garcia left his boat on the dryer at home and didn't realize it till he got to Monterey. Good times....

I remember Steve and Jack showing up for the Hydro Masters. That is in fact, what got me started on turn fins and how much they could contribute to the ability of a boat to be AWESOME. As I remember, the fin that they were using was a flat fin with the bottom of the fin perfectly flat. They indicated that the flat area on the bottom of the fin was important, but I never figured that out.

We spend a lot of time with the flat fin and couldn't get past the left turn problem. That is when we moved to a fin that was 90 degrees to the water with the bend just as far toward the bottom as we could get. That way we eliminated the left turn problem. In fact, I was tempted this year to enter my 20 hydro in Open Offshore class where you have to turn left. I was not well received in my attempt to enter it in that class. The 20 boat will turn left just fine and I could race where I had to turn left.

I will tell you a little story about the Hydro Masters that year. Steve brought a 40 and a 60 boat. We sanctioned the qualifications so that any times that were records would be official. Gregg Huey set the 40 oval time and Steve came back and took it away from him with an amazing time. The 60 boat was having some trouble with consistency. Steve made the A Main in both classes. He had the 60 boat set up so that he could turn right, but had little ability to turn left. In fact he had a flex cable arrangement hooked to the rudder and when pushing it was supposed to turn left. The flex cable did not allow him to turn left much at all. In the A Main there were 7 boats so he found himself in bad situations where he needed to be able to turn left and could not. I don't think that he finished a heat. The boat was amazing, but the problem with the left turn, kept him from doing well.

As I remember, the boat also had air traps on the bottom of the sponsons. Gregg Huey bought Crapshooter from me shortly after that time and produced some Crapshooter with the air traps. I never did really study that feature and to this day do not know if it was good or bad. I was so interested in the turn fins that I kind of left that alone and concentrated on the turning.

We would be interested in seeing any pictures of your new efforts, as I can guarantee everyone that they will be "cutting edge".

Marty Davis

The information that I gave above about the BOSS 60's development and the trip back to Indy came directly from Jack O'Donnell, Steve O'Donnell AND RC Modeler Magazine.

The primary problem with the boats performance was the fuel and the disc valve on the engine.

There was minor problems with the combination engine, boat and setup in the beginning as there is with most things that are that new. These were rectified shortly after and the BOSS 60 won the next SEVENTEEN heats that it ran in. Quiet an accomplishment for a boat that flies easily and will not turn left.

As for the performance of the BOSS 40 boat at Indy, that would have been hard to do since the BOSS 40 never went to Indy. Must be a case of mistaken identity.

ENOUGH OF THAT!!!

One of the things that is needed for the angled flat turn fin setup is a very minimum of flex, not only in the turn fin but also the sponson brackets and most importantly the hull in the torsional direction. (The twisting of the hull) When this happens it changes the entire setup as the load / speed increases in the turns making the boat act in a very erratic manner.

This is why some boats react differently to the different setups.

One other requirement is more engine torque is needed to carry the boat thru the turns without falling off as the load increases. This was an area that Jack O'Donnell helped me in to optimize the setup.

Flex.....oh ya. I can only guess how much, but I suspect from man handling my boat that the angle of attack of the right sponson changes dramatically when the turn fin grabs. I use the black glass booms and see quite a bit of flex.

 

[John Finch]

Don, I was fishing. No one bit, so I'll use different bait. I am concerned with the relationship of the rear sponson ride points vs the prop location. Is there a sweet spot there. I think there is for heat racing. Rears are out of the question for straightaway.

 

[Ray Sametz]

I'll bite, back of rear sponsons even with front of prop or drive dog. The way the old A/S Wagner boats were set up. Ray :unsure: :unsure:

 

[Don Ferrette]

Don, I was fishing. No one bit, so I'll use different bait. I am concerned with the relationship of the rear sponson ride points vs the prop location. Is there a sweet spot there. I think there is for heat racing. Rears are out of the question for straightaway.

Yes there is & I just experienced it on my single F hydro. We talked about it the other week..... B)

 

[John Finch]

I'll bite, back of rear sponsons even with front of prop or drive dog. The way the old A/S Wagner boats were set up. Ray :unsure: :unsure:

ok Ray, I just glued two sponsons on the rear of my experimental boat exactly where you stated. Great minds think alike. Now, If we could just find two great minds! Just kidding. Seriously though, I just glued on two sponsons like you said because I was too impatient to wait for a reply on the site. I will let you know how our idea works.

 

[Ray Sametz]

John, Only problem I found with that setup is positioning the rudder so it's in clean water. IE in front of prop. Ray :unsure: :unsure:

 

[Mike Larson]

Learning.... learning.... my answer to the rear sponson question is multiple, and 'probly not even close like, having the rears farther back gives you a longer balance fulcrum in heat race conditions..... possibly keep the prop from submerging quicker in heat race water...... along that same line ( I thought Ray was right on) allowing cleaner water at a constant to the prop....... novice words.... teach me! mike

 

[John Finch]

Ray, I did something that worked several years ago on a boat of mine. I cut out the inner sides of the rear of the sponsons and I am using the outer bottom of the sponsons for the ride surface. They are wide body sponsons so cutting out the inside to clear the rudder is not a problem. It gives a wider stance on the rears too. What I have been experimenting with is depth,width,length, and ride surface width of rear sponsons for the last 4 years. The tell tale sign that you need wider or further reaward rear sponsons is a flutter in the straightaway when hitting rough water where the front of the boat leaves the water a few inches, followed by the front sponsons slapping the water left to right like a see saw, then followed by the boat blowing off the water. I started out with no rear sponsons. Then went to sponsons under the hull, then sponsons on the sides of the hull, then extended rear sponsons further to the outside of the boat with a connector between the hull and the sponsons. etc. As I see it, and correct me if I am wrong, If the prop is far behind the rear sponsons and the boat gets loose in the front, the boat teaters on the prop with little help from the rear sponsons because when the boat is at a positive angle of attack the rears are out of the water. If you move the sponsons further back, they tend to be in line with the prop and closer to the water which gives stability to the boat thus allowing the boat to settle back on to the water without the flutter that causes the boat to flip. I run no rears on my oval record boat because they are drag, but the oval boat will only run in perfectly glass water.

Our heat racing boats need stability more than speed. GOT to first finish before you can finish first.

 

[Don Ferrette]

If the prop is far behind the rear sponsons and the boat gets loose in the front, the boat teaters on the prop with little help from the rear sponsons because when the boat is at a positive angle of attack the rears are out of the water.

Like I said- wheeliebars!

This is like what we saw that day on my single F hydro at Albert's except it happened coming off the corners, the right sponson was completely off the water, the left was hardly touching at all & the boat was barely hanging on the hook of the turn fin. All I did was bring the strut closer to the transom & that issue disappeared. As it turns out it seems there are a couple different Speedmaster hydro strut brackets out there & the last batch of new ones I got the strut blade mounting holes are just about 3/4" further back than what they typically were on previous ones. :blink:

Also something to keep in mind is where the rears are positioned in relation to the ride. If they run at the same depth as the strut they don't have to extend as much past the transom, if at all, as opposed to them being say 1/8" up. Then you definitely want them extending past the transom so they make contact when the boat gets loose. B)

 

[Marty Davis]

Since we are on turn fins, think about this..............When the boat goes straight there is no water coming off the turn fin. When you throw the boat into the corner the fin throws a wall of water up in the air. To do that the fin and boat has to be sliding to the outside of the course. Agreed? Now, if water is being thrown up in the air off the trailing edge of the fin, shouldn't the angle of the trailing edge help to control where that water is thrown. Relate it to a propeller. If you change the rake of the trailing edge doesn't that change the lifting characteristic of the prop. Remember grade school and the law that states for every action there is an equal and OPPOSITE reaction. The water is shooting up, so does that actions/reaction push the sponson down to keep the sponson on the water? Thoughts????

John:

You would think that would be correct. BUT, if you think of the fin as a pivot point and NOT as something that is sliding sideways, you will be talking about the correct fin. One that throws a lot of water into the air is NOT working very well. One that hardly has ANY WATER being thrown in the air is what you are looking for. In fact, you have discovered one of the MOST IMPORTANT testing items in turn fin optimization. The amount of water is the KEY.

What would you say if I asked you how important the location of the the center of pressure of the fin is in relation to the dynamic CG? Would the optimimum position be within 1/2", 1/4", 1/8", 1/16" or ????

Marty Davis

Are we talking sailboats and wings type pressure differences when you say center of pressure or a specific location in the fin?

John:

Do you have Autocad capability? If so, you can convert the shape of the fin area below the water line into a polyline, then to a region and use the Mass Properties to tell you where the center of pressure is. I would think that an easy way of describing the center of pressure would be to describe the place on the fin that if you put the head of a pencil against the fin at 90 degrees it would support the fin against pressure from the other side, still staying supported by the head of the pencil. All different shapes will have a center of pressure that will move as the shape changes. That point on the fin has a DIRECT relationship to the cg. The question that I asked you about the precision of the location of the fin is LESS THEN 1/6". When you hit the perfect position it will totally amaze you in the way the boat acts and lack of drag in the turns. The boat will actually appear to accelerate in the turns. You ONLY need to look at the fin in relation to the area of the fin that is in the water. Sorry, this is getting VERY technical, but in order to understand the dynamics of turn fins, it is required to be this detailed. Lets say that the shape of the fin is similar to one of these random shapes http://www.rcboat.com/cp.jpg The center of pressure is shown on each of them. That point is relative to the cg.

Marty Davis

Thanks for the great thread .... to ALL!

Marty,

For the sake of efficiency to find that optimal "sweet spot", can I add or take away Ballast in small increments if I mount some sort of an enclosure on top of the strut??

I'm just talking turning now. I was wondering if the adding/subtracting weight scenario would give me a more flexible way to acheive the same thing without using the calipers every time I adjusted the fin.... and makes sure the fin angle etc does not move...

Just thinking out loud.

Thanks again for your efforts in sharing this critical concept.

Sincerely,

David Jensen

David:

I suppose that you could do that, why not just move the fin.

I HATE adding weight to a boat to correct something. Reason, as my friend Tom Grannis used to tell me: It "takes more power to the square" to accelerate for each ounce a boat weighs. I try to make my boat as light as possible.

One time years ago I made a boat that had a flat lead weight that I bolted to the bottom of the center section which had multimple holes to move forward and back to adjust it. It was to move the cg in large amounts to test some things: props loading, turn fins, attack angles. It didn't really work very well or I didn't apply the test very well.

Marty Davis

 

[Marty Davis]

OK, Jokes aside. Cover #8 turn fin parallel to tub. Some riggers set front sponsons offset to left to compensate for prop walk. Should fin still be parallel to tub???????? Ray :unsure: :unsure:

Turn fin should be parallel to the tub. This is one of the most often overlooked things I've seen on riggers, especially on those that run a little toe in. You'd be surprised how much as little as a 1/16" toe throws the turn fin off.

I beg to differ on this one. The toe in puts the fin at a slight left angle to compensate for prop walk. I have boats, and my oval boat is one of them, where the fin bends and finds a set it really likes.

HUH?? If you're toed-in on the right sponson the turn fin is off to the right making it track that way. I've solved alot of tracking right issues by setting the turn fin parallel. :blink:

Actually you were the one who got me straight on that way back when I first started messing with riggers. The first Roadrunner I had was tracking right a little & when I was looking at the rudder you gave me that little hint of "you're looking at the wrong end".

If there is toe in the fin makes the front of the boat track left. This can make the boat go straight if there is a prop walk problem. Not the best way to solve the problem, but it works. Andy's sgx fins are sharpened on the right side only. that does the same thing as the toe in on a roadrunner. It forces the front of the boat to the left. These methods can make the boat crab as it goes down the straightaway. I would rather sharpen the prop, put more drag on the left sponson or throw the left sponson further from the tub to compensate for prop walk. Funny thing on my gas sport hydro. The fin bends, finds a set, and still the boat handles really well. Can't turn left though.

I agree totally and apply this myself on all my boats.

Marty Davis

 

[Marty Davis]

I wish I could say "see ya at the nats", but I don't know if I can get that much time off. I wish the nats were shorter. Dick Tyndal makes the turn fins that I have been experimenting with. He just lives down the street so to speak. He does such a pretty job with them. I ask him to make a certain size and bend and he makes a mold of sorts to be able to reproduce them. I have build a lot of trial and error fins and then asked Dick to make some pretty ones with the same shape. The ones I build are always ugly! One time I asked for three identical fins and he made them just a bit different. ONLY a bit different. He said try them and let me know what you find. Only the original worked well on the boat. The design that works best on my boats is a take off of the roadrunner. I guess you had something to do with that original design. Is my fin location of 3/4 close to what you are using? Oh, I still have my roundnose Bardahl that I ran in 97 at the nats. It is set up like your Vernors was. Same prop too. If you want to send me a fin I will try it on the boat and let you know if it works better than what I have.

John:

MOST of the fins that I made in the effort to optimize were UGLY. No reason to make them all look good since they won't be on the boat long anyway. The next one is already in my head.....

Some people can't stand to make ANYTHING UGLY, but since time is so limited, I have ZERO problem not finishing them completely. I would suggest that people use that approach so that they can cover more ground quickly.

Your question can not be answered directly, since all boats are different in their proportions. Also, the cg varies with a full tank, an empty tank or one that is 1/2 full. Now that I understand the dynamics, I can use any of these cg points.

We are getting into an area that has taken me a LONG time to figure out and I am not comfortable talking in very specific terms. I would like to give information that will save people time in their own general search.

Marty Davis

I understand. I will put this out there.................Marty brought up a very good point. Boat setup varies with how the boat is weighted at the time of the measurement. For example......If you put your boat on a flat table (setup board) and measure sponson angles for example. You will get a different reading when the boat has the engine in it as opposed to not having the engine in the hull. TWO COMPLETELY DIFFERENT MEASUREMENTS. If you don't believe it......try it!

John:

The amount of specific information that we have talked about should provide a GREAT starting point for people to do their own analysis, boat building and testing. It would be great to have this type discussion on a variety of things. We used to have Chats and discussions like this on my RcBoat.com Listbot.

Maybe we can do more of this. I have REALLY enjoyed it.

Of course, we have but scratched the surface on this topic, so I would bet that it won't die at this point.

Marty Davis

Ok, here goes my 2 cents worth on using angled flat turn fins.

The first time that the ANGLED flat turn fin was used in serious competition, to the best of my knowledge, was on one of Marty's 40 Crapshooters that we ran at the NAMBA Nationals in Monterey California in 1979. This and other modifications ( the beginning of the BOSS setup) was very successful. We won the 40 hydro class with a perfect score. Marty you and some of your friends from Indy were there, you should remember the maroon and gold boat.

The boat worked ok but there was room for improvements. On the trip back from that race, in the motor home,the BOSS design was expanded to include some improvements that were needed to help

the boat turn better and more consistent in different water conditions. The BOSS boat was born.

It took about a year to get the bugs worked out of the design. The first BOSS was a 20 size. In 1981 we won the district 19 championship (not finishing in first place only twice all year) and also won the 20 hydro class at the NAMBA Nationals at Amarillo, TX with 4 first place heats and fast time in the class. All this using an angled flat turn fin.

Jack O'Donnell, Steve O'Donnell and myself took a weeks vacation and built the 60 BOSS from scratch using what was learned from the 20 and 40 BOSS(You may be asking what about the 40 BOSS. It had 2 first place heat wins in Amarillo when a rudder servo decided to go out of off bouy 5 and I think it still holds the record for going the deepest into the bank at that lake, in past the sponsons). Only about a month after the 60 BOSS was built, Jack and Steve took it back to Indy for the record trials. The first lap of the 2 lap timed oval was run at about 10 seconds. The disc rotor in the engine went out on the second lap and the boat slowed considerably and the still broke the old record by about 2 seconds. The boat was over-built as for strength and it weighed in at 9 1/2 pounds. All this and it was done using an angled flat turn fin.

There is more to using this type of turn fin than just bolting it on. The entire boat and engine combinations has to be designed around it to optimize its benefits.

Whether it is better than the curved turn fin, I don't know since I sent all of my time on the angled flat design. Each design has its good points and bad points. You just can't count either of them out.

Marty, I enjoy reading your comments, and John, I enjoyed reading your articles and your books, you guys keep up the good work passing along your great knowledge about RC boating.

Charles:

Yes, I sure remember coming the NAMBA Nats in Monterey. It was a really fun time for us. We also enjoyed a small earthquake just for us. That was the Nats that Jack Garcia left his boat on the dryer at home and didn't realize it till he got to Monterey. Good times....

I remember Steve and Jack showing up for the Hydro Masters. That is in fact, what got me started on turn fins and how much they could contribute to the ability of a boat to be AWESOME. As I remember, the fin that they were using was a flat fin with the bottom of the fin perfectly flat. They indicated that the flat area on the bottom of the fin was important, but I never figured that out.

We spend a lot of time with the flat fin and couldn't get past the left turn problem. That is when we moved to a fin that was 90 degrees to the water with the bend just as far toward the bottom as we could get. That way we eliminated the left turn problem. In fact, I was tempted this year to enter my 20 hydro in Open Offshore class where you have to turn left. I was not well received in my attempt to enter it in that class. The 20 boat will turn left just fine and I could race where I had to turn left.

I will tell you a little story about the Hydro Masters that year. Steve brought a 40 and a 60 boat. We sanctioned the qualifications so that any times that were records would be official. Gregg Huey set the 40 oval time and Steve came back and took it away from him with an amazing time. The 60 boat was having some trouble with consistency. Steve made the A Main in both classes. He had the 60 boat set up so that he could turn right, but had little ability to turn left. In fact he had a flex cable arrangement hooked to the rudder and when pushing it was supposed to turn left. The flex cable did not allow him to turn left much at all. In the A Main there were 7 boats so he found himself in bad situations where he needed to be able to turn left and could not. I don't think that he finished a heat. The boat was amazing, but the problem with the left turn, kept him from doing well.

As I remember, the boat also had air traps on the bottom of the sponsons. Gregg Huey bought Crapshooter from me shortly after that time and produced some Crapshooter with the air traps. I never did really study that feature and to this day do not know if it was good or bad. I was so interested in the turn fins that I kind of left that alone and concentrated on the turning.

We would be interested in seeing any pictures of your new efforts, as I can guarantee everyone that they will be "cutting edge".

Marty Davis

The information that I gave above about the BOSS 60's development and the trip back to Indy came directly from Jack O'Donnell, Steve O'Donnell AND RC Modeler Magazine.

The primary problem with the boats performance was the fuel and the disc valve on the engine.

There was minor problems with the combination engine, boat and setup in the beginning as there is with most things that are that new. These were rectified shortly after and the BOSS 60 won the next SEVENTEEN heats that it ran in. Quiet an accomplishment for a boat that flies easily and will not turn left.

As for the performance of the BOSS 40 boat at Indy, that would have been hard to do since the BOSS 40 never went to Indy. Must be a case of mistaken identity.

ENOUGH OF THAT!!!

One of the things that is needed for the angled flat turn fin setup is a very minimum of flex, not only in the turn fin but also the sponson brackets and most importantly the hull in the torsional direction. (The twisting of the hull) When this happens it changes the entire setup as the load / speed increases in the turns making the boat act in a very erratic manner.

This is why some boats react differently to the different setups.

One other requirement is more engine torque is needed to carry the boat thru the turns without falling off as the load increases. This was an area that Jack O'Donnell helped me in to optimize the setup.

Charles:

Jack and Steve brought the 40 boat to Indy for a record trial and set the 40 oval record, maybe another time. Was a long time ago.

Marty Davis

 

[Tom w]

.

 

[John Finch]

I think the most impotant thing we need to agree on is that the leading sdge of the fin should move rearward as it gets deeper. If the leading edge is perpendicular to the water, the leading edge at the deepest point can search and bend the fin. Like a jet wing, it needs to be raked back to prevent the unwanted flutter. I hope I explained that right. Take a flat blade and run it through a swimming pool with the deepest part forward and you will see that if you turn it in any direction it darts. If you run the same fin or plate metal, rudder, whatever with the deepest part raked back from the direction of travel the turning motion is smooth and there is no flutter.

 

[Marty Davis]

I think the most impotant thing we need to agree on is that the leading sdge of the fin should move rearward as it gets deeper. If the leading edge is perpendicular to the water, the leading edge at the deepest point can search and bend the fin. Like a jet wing, it needs to be raked back to prevent the unwanted flutter. I hope I explained that right. Take a flat blade and run it through a swimming pool with the deepest part forward and you will see that if you turn it in any direction it darts. If you run the same fin or plate metal, rudder, whatever with the deepest part raked back from the direction of travel the turning motion is smooth and there is no flutter.

John:

Are you sure about that?

What happens to a rudder that is lagged? IT LIFTS. Do you really want the fin to lift because of lag in the leading edge?

Marty Davis

 

[ek kustomz]

I think the most impotant thing we need to agree on is that the leading sdge of the fin should move rearward as it gets deeper. If the leading edge is perpendicular to the water, the leading edge at the deepest point can search and bend the fin. Like a jet wing, it needs to be raked back to prevent the unwanted flutter. I hope I explained that right. Take a flat blade and run it through a swimming pool with the deepest part forward and you will see that if you turn it in any direction it darts. If you run the same fin or plate metal, rudder, whatever with the deepest part raked back from the direction of travel the turning motion is smooth and there is no flutter.

John:

Are you sure about that?

What happens to a rudder that is lagged? IT LIFTS. Do you really want the fin to lift because of lag in the leading edge?

Marty Davis

Correct me if I am wrong as I do not have a full understanding of this stuff yet, but would it not be different with a turn fin and a rudder?? The rudder pivots against the boat therefore if the rudder lags yes it would creat lift. But the turnfin pivots with the boat not against it so the pressure from the water there would not cause it to do so. Correct?/

 

[John Finch]

I think the most impotant thing we need to agree on is that the leading sdge of the fin should move rearward as it gets deeper. If the leading edge is perpendicular to the water, the leading edge at the deepest point can search and bend the fin. Like a jet wing, it needs to be raked back to prevent the unwanted flutter. I hope I explained that right. Take a flat blade and run it through a swimming pool with the deepest part forward and you will see that if you turn it in any direction it darts. If you run the same fin or plate metal, rudder, whatever with the deepest part raked back from the direction of travel the turning motion is smooth and there is no flutter.

John:

Are you sure about that?

What happens to a rudder that is lagged? IT LIFTS. Do you really want the fin to lift because of lag in the leading edge?

Marty Davis

Correct me if I am wrong as I do not have a full understanding of this stuff yet, but would it not be different with a turn fin and a rudder?? The rudder pivots against the boat therefore if the rudder lags yes it would creat lift. But the turnfin pivots with the boat not against it so the pressure from the water there would not cause it to do so. Correct?/

Marty is refering only to the leading edge. You are right about the rudder as it pivots, if it is raked back it will cause lift. I never thought about the leading edge causing lift on the turnfin, because it is stationary. I know from experience that a perpendicular leading edge will flutter. Maybe there is a certain degree of rake that works better than another, but maybe a bit of lift would be ok to compensate for the torque that is driving the right sponson downward? any thoughts ?

 

[ek kustomz]

I think the most impotant thing we need to agree on is that the leading sdge of the fin should move rearward as it gets deeper. If the leading edge is perpendicular to the water, the leading edge at the deepest point can search and bend the fin. Like a jet wing, it needs to be raked back to prevent the unwanted flutter. I hope I explained that right. Take a flat blade and run it through a swimming pool with the deepest part forward and you will see that if you turn it in any direction it darts. If you run the same fin or plate metal, rudder, whatever with the deepest part raked back from the direction of travel the turning motion is smooth and there is no flutter.

John:

Are you sure about that?

What happens to a rudder that is lagged? IT LIFTS. Do you really want the fin to lift because of lag in the leading edge?

Marty Davis

Correct me if I am wrong as I do not have a full understanding of this stuff yet, but would it not be different with a turn fin and a rudder?? The rudder pivots against the boat therefore if the rudder lags yes it would creat lift. But the turnfin pivots with the boat not against it so the pressure from the water there would not cause it to do so. Correct?/

Marty is refering only to the leading edge. You are right about the rudder as it pivots, if it is raked back it will cause lift. I never thought about the leading edge causing lift on the turnfin, because it is stationary. I know from experience that a perpendicular leading edge will flutter. Maybe there is a certain degree of rake that works better than another, but maybe a bit of lift would be ok to compensate for the torque that is driving the right sponson downward? any thoughts ?

Well I have some thoughts I guess a minimal amout of rearward rake would be necessary but if the leading edge was sharp enough wouldn't that justify the leading raked leading by minimizing the lift??

Zach

 

[Marty Davis]

I think the most impotant thing we need to agree on is that the leading sdge of the fin should move rearward as it gets deeper. If the leading edge is perpendicular to the water, the leading edge at the deepest point can search and bend the fin. Like a jet wing, it needs to be raked back to prevent the unwanted flutter. I hope I explained that right. Take a flat blade and run it through a swimming pool with the deepest part forward and you will see that if you turn it in any direction it darts. If you run the same fin or plate metal, rudder, whatever with the deepest part raked back from the direction of travel the turning motion is smooth and there is no flutter.

John:

Are you sure about that?

What happens to a rudder that is lagged? IT LIFTS. Do you really want the fin to lift because of lag in the leading edge?

Marty Davis

Correct me if I am wrong as I do not have a full understanding of this stuff yet, but would it not be different with a turn fin and a rudder?? The rudder pivots against the boat therefore if the rudder lags yes it would creat lift. But the turnfin pivots with the boat not against it so the pressure from the water there would not cause it to do so. Correct?/

Ed:

GREAT QUESTION....

With the rudder, you have two things in play. The lift of the rudder in the neutral position (straight ahead) if it has any rake. AND when turning, the amount of lift is increased a ton by turning it and exposing the side of the rudder as well. It will lift the rear of the boat a ton as it is turned. It also works the opposite if it is tucked forward. It will pull the boat down a ton.

What we are talking about here is the fin and assuming it is always going straight ahead.

Marty Davis

 

[Marty Davis]

I think the most impotant thing we need to agree on is that the leading sdge of the fin should move rearward as it gets deeper. If the leading edge is perpendicular to the water, the leading edge at the deepest point can search and bend the fin. Like a jet wing, it needs to be raked back to prevent the unwanted flutter. I hope I explained that right. Take a flat blade and run it through a swimming pool with the deepest part forward and you will see that if you turn it in any direction it darts. If you run the same fin or plate metal, rudder, whatever with the deepest part raked back from the direction of travel the turning motion is smooth and there is no flutter.

John:

Are you sure about that?

What happens to a rudder that is lagged? IT LIFTS. Do you really want the fin to lift because of lag in the leading edge?

Marty Davis

Correct me if I am wrong as I do not have a full understanding of this stuff yet, but would it not be different with a turn fin and a rudder?? The rudder pivots against the boat therefore if the rudder lags yes it would creat lift. But the turnfin pivots with the boat not against it so the pressure from the water there would not cause it to do so. Correct?/

Marty is refering only to the leading edge. You are right about the rudder as it pivots, if it is raked back it will cause lift. I never thought about the leading edge causing lift on the turnfin, because it is stationary. I know from experience that a perpendicular leading edge will flutter. Maybe there is a certain degree of rake that works better than another, but maybe a bit of lift would be ok to compensate for the torque that is driving the right sponson downward? any thoughts ?

John:

A couple of things.....

Do you really want the fin to lift as the speed increases? I DOUBT IT. You would have to offset this lifting with something that would be in the same proportion as the amount of lift of the fin. PROBABLY VERY HARD TO SYNCRONIZE.

The leading edge of the fin in a 100% optimum situation would be with zero rake. That way it would cut through the water with no lift. HIGHLY DESIREABLE

A neat way to imagine the effect is to assume that the fin is not sharpened. If you present this flat edge to the water, do you want it with rake? NO, IT WOULD ACT LIKE A WATER SKI. You would want it to be without rake. It would have a lot of drag, but not a lot of lift. I have been moving toward no rake with each new fin I make, and have not had the guts to make one without any rake. Maybe it is time to try that.

Just a thought.... Would you like for me to set up a chat with John and I to discuss this real time? It would move the discussion forward a lot faster than individual posts here. I haven't asked John, but I am sure he is having as much fun as I am with this discussion. Every time that we have gotton together over the years, we both enjoy bouncing things off of each other. Most times we were running on paralell courses with him running monos and me hydros. Now we are both running hydros and the discussion would be even better. Maybe we could get a semi-regular get together to discuss a set item. All kinds of options.

Both John and I love to give back to the hobby and would love to share our findings and experiences. Not sure about John, but the things that I learned the best were from mistakes made rather than from original ideas. Refinement is the way that I like to attack the hobby.

Both John and I have kind of a pet peeve. Things in model boating are much different than when we started. During that time it was necesary to understand the "WHY'S" of boating (to be competitive) rather than the current situation where you can buy a boat that is all set up or with great plans and directions. Now, you don't really have to know the "WHY'S" of model boating. I can tell you that you are missing a satisfying area.

Sorry for rambling on so much....

Marty Davis