i was thinking

[John Finch]

I prefer to make the fin from spring steel. Put the hook in it in the annealed state, and then harden/temper it. A 90 fin is ~0.065" thick and the 21 can be ~0.035" thick. Protect them with either black oxide, or graphite paint. The down side to the spring steel is that it is virtually impossible to adjust the hook once the material is hardened.

Sounds like something to make after you experiment with the aluminum first. I tried carbon fibre. Fabrizio from Italy made it for me. He does great carbon/kevlar/glass work. The fin didn't last long before the leading edge started to come apart. Looked like a good idea. The leading edge was the only problem.

 

[dr gary turner]

I recently tlked with and looked at the turn fin that Charles Purdue is running on his 21 hydro. It is completely flat and "bent" , the entire turn fin, at an angle to the inside plate of the sponsons, there is no cup or bend line in the water. It is vary thin as well. It would be interesting to have him give his thoughts on his turn fin. Gary

 

[John Finch]

I recently tlked with and looked at the turn fin that Charles Purdue is running on his 21 hydro. It is completely flat and "bent" , the entire turn fin, at an angle to the inside plate of the sponsons, there is no cup or bend line in the water. It is vary thin as well. It would be interesting to have him give his thoughts on his turn fin. Gary

Gary,

Yes, the straight fin angled inward without the curve is the only other option I have tried that works. Because it is straight for it's full depth it does not scoop the water off the pond. I have used those type fins on many 1/8 scale boats. I feel the scoop fins we use on the riggers today are better for the high speed turns than the straight angled fin. But, that's just an opinion. If someone has a straight fin that is doing the job, that's great. I have been using the scoop fins on my sport gas boat and recently tried several straight fins, but went back to the scoop fin because the straight ones just would not let the boat hang on all 3 buoys in a high speed turn. Like Marty said...........I go into the corners wide open. Marty likes to chop the throttle, set the boat, and then blow thru the corners. I have to drive my scale boats that way when I use the straight fins. It's all relative to speed too. On my stock sport boat the straight fin works great. 10 mph difference.

I'm sure you have noticed with your twins that the faster you go the more critical everything becomes. Remember when we used to run straight turn fins ony two inches deep back in the 70's? The servos back then were not strong enough to hold a turn with fins like we have today. We used to mount the fins on the sponson rather than behind the sponson so the rear or the boat would swing out to make the turn.

 

[John Finch]

Another thing to keep in mind......Small 20 sise boats carve the same turn as the bigger boats, but in relation to their size on the course, and the weight of the boat, they take less of a sharp turn per their size in relation to the size of the course.

Did I say that right? They take less of a scale turn and with less weight to hold in the corner than a big boat like the twin. To put it in perspective.....imagine a full size unlimited trying to make a 75 foot turn on an IMPBA course. Size does matter!

That would be a U turn for a boat that big. Impossible! So as the boats get smaller the turn is less sharp. For some of the guys reading this post it is imperative that you know there are many ways to skin a cat and what works on one boat may not work on another. Try to keep it all in perspective.

 

[ek kustomz]

I prefer to make the fin from spring steel. Put the hook in it in the annealed state, and then harden/temper it. A 90 fin is ~0.065" thick and the 21 can be ~0.035" thick. Protect them with either black oxide, or graphite paint. The down side to the spring steel is that it is virtually impossible to adjust the hook once the material is hardened.

Sounds like something to make after you experiment with the aluminum first. I tried carbon fibre. Fabrizio from Italy made it for me. He does great carbon/kevlar/glass work. The fin didn't last long before the leading edge started to come apart. Looked like a good idea. The leading edge was the only problem.

Were you able to figure out why the fin started to fray??

BTW cluttered or not I would read through this stuff anyways. TONS of great info here, especially for someone wanting to design thier own boat!!

Zach

 

[Jim Allen]

I recently tlked with and looked at the turn fin that Charles Purdue is running on his 21 hydro. It is completely flat and "bent" , the entire turn fin, at an angle to the inside plate of the sponsons, there is no cup or bend line in the water. It is vary thin as well. It would be interesting to have him give his thoughts on his turn fin. Gary

Gary,

Yes, the straight fin angled inward without the curve is the only other option I have tried that works. Because it is straight for it's full depth it does not scoop the water off the pond. I have used those type fins on many 1/8 scale boats. I feel the scoop fins we use on the riggers today are better for the high speed turns than the straight angled fin. But, that's just an opinion. If someone has a straight fin that is doing the job, that's great. I have been using the scoop fins on my sport gas boat and recently tried several straight fins, but went back to the scoop fin because the straight ones just would not let the boat hang on all 3 buoys in a high speed turn. Like Marty said...........I go into the corners wide open. Marty likes to chop the throttle, set the boat, and then blow thru the corners. I have to drive my scale boats that way when I use the straight fins. It's all relative to speed too. On my stock sport boat the straight fin works great. 10 mph difference.

I'm sure you have noticed with your twins that the faster you go the more critical everything becomes. Remember when we used to run straight turn fins ony two inches deep back in the 70's? The servos back then were not strong enough to hold a turn with fins like we have today. We used to mount the fins on the sponson rather than behind the sponson so the rear or the boat would swing out to make the turn.

John,

I presently use the straight, spring steel, full wedge type, skid fin on both my .90 & twin .90 boats. After experminting with both types, I feel a straight angled mounted fin (15 deg angle), that is progressively wider at the bottom, is best for both types of driving. As you & Marty have stated, different boat setups may require slightly different techniques to achieve the desired results.

Jim

 

[Jim Allen]

I recently tlked with and looked at the turn fin that Charles Purdue is running on his 21 hydro. It is completely flat and "bent" , the entire turn fin, at an angle to the inside plate of the sponsons, there is no cup or bend line in the water. It is vary thin as well. It would be interesting to have him give his thoughts on his turn fin. Gary

Gary,

Yes, the straight fin angled inward without the curve is the only other option I have tried that works. Because it is straight for it's full depth it does not scoop the water off the pond. I have used those type fins on many 1/8 scale boats. I feel the scoop fins we use on the riggers today are better for the high speed turns than the straight angled fin. But, that's just an opinion. If someone has a straight fin that is doing the job, that's great. I have been using the scoop fins on my sport gas boat and recently tried several straight fins, but went back to the scoop fin because the straight ones just would not let the boat hang on all 3 buoys in a high speed turn. Like Marty said...........I go into the corners wide open. Marty likes to chop the throttle, set the boat, and then blow thru the corners. I have to drive my scale boats that way when I use the straight fins. It's all relative to speed too. On my stock sport boat the straight fin works great. 10 mph difference.

I'm sure you have noticed with your twins that the faster you go the more critical everything becomes. Remember when we used to run straight turn fins ony two inches deep back in the 70's? The servos back then were not strong enough to hold a turn with fins like we have today. We used to mount the fins on the sponson rather than behind the sponson so the rear or the boat would swing out to make the turn.

John,

I presently use the straight, spring steel, full wedge type, skid fin on both my .90 & twin .90 boats. After experminting with both types, I feel a straight angled mounted fin (15 deg angle), that is progressively wider at the bottom, is best for both types of driving. As you & Marty have stated, different boat setups may require slightly different techniques to achieve the desired results.

Jim

 

[John Finch]

I recently tlked with and looked at the turn fin that Charles Purdue is running on his 21 hydro. It is completely flat and "bent" , the entire turn fin, at an angle to the inside plate of the sponsons, there is no cup or bend line in the water. It is vary thin as well. It would be interesting to have him give his thoughts on his turn fin. Gary

Gary,

Yes, the straight fin angled inward without the curve is the only other option I have tried that works. Because it is straight for it's full depth it does not scoop the water off the pond. I have used those type fins on many 1/8 scale boats. I feel the scoop fins we use on the riggers today are better for the high speed turns than the straight angled fin. But, that's just an opinion. If someone has a straight fin that is doing the job, that's great. I have been using the scoop fins on my sport gas boat and recently tried several straight fins, but went back to the scoop fin because the straight ones just would not let the boat hang on all 3 buoys in a high speed turn. Like Marty said...........I go into the corners wide open. Marty likes to chop the throttle, set the boat, and then blow thru the corners. I have to drive my scale boats that way when I use the straight fins. It's all relative to speed too. On my stock sport boat the straight fin works great. 10 mph difference.

I'm sure you have noticed with your twins that the faster you go the more critical everything becomes. Remember when we used to run straight turn fins ony two inches deep back in the 70's? The servos back then were not strong enough to hold a turn with fins like we have today. We used to mount the fins on the sponson rather than behind the sponson so the rear or the boat would swing out to make the turn.

John,

I presently use the straight, spring steel, full wedge type, skid fin on both my .90 & twin .90 boats. After experminting with both types, I feel a straight angled mounted fin (15 deg angle), that is progressively wider at the bottom, is best for both types of driving. As you & Marty have stated, different boat setups may require slightly different techniques to achieve the desired results.

Jim

Wow! 15 degrees! I have never gone past 7 degrees. That's great input. I know that you know what you are doing, so that's a new thing for me to try. Big boat, big angle. HMMMMMMMM

 

[John Finch]

I prefer to make the fin from spring steel. Put the hook in it in the annealed state, and then harden/temper it. A 90 fin is ~0.065" thick and the 21 can be ~0.035" thick. Protect them with either black oxide, or graphite paint. The down side to the spring steel is that it is virtually impossible to adjust the hook once the material is hardened.

Sounds like something to make after you experiment with the aluminum first. I tried carbon fibre. Fabrizio from Italy made it for me. He does great carbon/kevlar/glass work. The fin didn't last long before the leading edge started to come apart. Looked like a good idea. The leading edge was the only problem.

Were you able to figure out why the fin started to fray??

BTW cluttered or not I would read through this stuff anyways. TONS of great info here, especially for someone wanting to design thier own boat!!

Zach

Zach,

I think it is because of trying to get a sharp edge on the leading edge of the fin it made the carbon fibre too thin to withstand the water forces hitting it. Someone with carbon fibre experience can probably shed some light on the problem. I don't know much about the material.

 

[Marty Davis]

I recently tlked with and looked at the turn fin that Charles Purdue is running on his 21 hydro. It is completely flat and "bent" , the entire turn fin, at an angle to the inside plate of the sponsons, there is no cup or bend line in the water. It is vary thin as well. It would be interesting to have him give his thoughts on his turn fin. Gary

Gary,

Yes, the straight fin angled inward without the curve is the only other option I have tried that works. Because it is straight for it's full depth it does not scoop the water off the pond. I have used those type fins on many 1/8 scale boats. I feel the scoop fins we use on the riggers today are better for the high speed turns than the straight angled fin. But, that's just an opinion. If someone has a straight fin that is doing the job, that's great. I have been using the scoop fins on my sport gas boat and recently tried several straight fins, but went back to the scoop fin because the straight ones just would not let the boat hang on all 3 buoys in a high speed turn. Like Marty said...........I go into the corners wide open. Marty likes to chop the throttle, set the boat, and then blow thru the corners. I have to drive my scale boats that way when I use the straight fins. It's all relative to speed too. On my stock sport boat the straight fin works great. 10 mph difference.

I'm sure you have noticed with your twins that the faster you go the more critical everything becomes. Remember when we used to run straight turn fins ony two inches deep back in the 70's? The servos back then were not strong enough to hold a turn with fins like we have today. We used to mount the fins on the sponson rather than behind the sponson so the rear or the boat would swing out to make the turn.

John,

I presently use the straight, spring steel, full wedge type, skid fin on both my .90 & twin .90 boats. After experminting with both types, I feel a straight angled mounted fin (15 deg angle), that is progressively wider at the bottom, is best for both types of driving. As you & Marty have stated, different boat setups may require slightly different techniques to achieve the desired results.

Jim

Jim:

I would bet that you have a problem it you need ANY left turn. I left the angled fin for that reason.

I was telling you about when Steve O'Donnell came out to race at the Hydro Master's, set a record with his 40 boat (Purdues Boss Boat) and generally impressed everyone. One thing I did not tell in this story is that during heat racing, Steve could not turn left at all and finished only a couple heats. The angled fin bit him badly.

The reason that I use a fin that has the bend at the bottom is to reduce the characteristic of the boat riding up on the fin with left turns. I try to keep the bend just as far to the tip as possible.

Marty Davis

 

[ek kustomz]

I prefer to make the fin from spring steel. Put the hook in it in the annealed state, and then harden/temper it. A 90 fin is ~0.065" thick and the 21 can be ~0.035" thick. Protect them with either black oxide, or graphite paint. The down side to the spring steel is that it is virtually impossible to adjust the hook once the material is hardened.

Sounds like something to make after you experiment with the aluminum first. I tried carbon fibre. Fabrizio from Italy made it for me. He does great carbon/kevlar/glass work. The fin didn't last long before the leading edge started to come apart. Looked like a good idea. The leading edge was the only problem.

Were you able to figure out why the fin started to fray??

BTW cluttered or not I would read through this stuff anyways. TONS of great info here, especially for someone wanting to design thier own boat!!

Zach

Zach,

I think it is because of trying to get a sharp edge on the leading edge of the fin it made the carbon fibre too thin to withstand the water forces hitting it. Someone with carbon fibre experience can probably shed some light on the problem. I don't know much about the material.

I race cars as well so we of course use a TON of cf for chassis. I know on my carpet cars I camfer all of the bottom chassis edges so they don't catch the carpet. After they have been filed I rub CA along the newly machined edge to seal it and make it solid to prevent fraying. Perhapse this would work the same on a turn fin?? The only reason I ask is because I am in the process of designing CF running harware for my 12 boat. This happend to be one of the ?'s i had was if anyone had tried to use CF for turn fins etc..

 

[Jim Allen]

I recently tlked with and looked at the turn fin that Charles Purdue is running on his 21 hydro. It is completely flat and "bent" , the entire turn fin, at an angle to the inside plate of the sponsons, there is no cup or bend line in the water. It is vary thin as well. It would be interesting to have him give his thoughts on his turn fin. Gary

Gary,

Yes, the straight fin angled inward without the curve is the only other option I have tried that works. Because it is straight for it's full depth it does not scoop the water off the pond. I have used those type fins on many 1/8 scale boats. I feel the scoop fins we use on the riggers today are better for the high speed turns than the straight angled fin. But, that's just an opinion. If someone has a straight fin that is doing the job, that's great. I have been using the scoop fins on my sport gas boat and recently tried several straight fins, but went back to the scoop fin because the straight ones just would not let the boat hang on all 3 buoys in a high speed turn. Like Marty said...........I go into the corners wide open. Marty likes to chop the throttle, set the boat, and then blow thru the corners. I have to drive my scale boats that way when I use the straight fins. It's all relative to speed too. On my stock sport boat the straight fin works great. 10 mph difference.

I'm sure you have noticed with your twins that the faster you go the more critical everything becomes. Remember when we used to run straight turn fins ony two inches deep back in the 70's? The servos back then were not strong enough to hold a turn with fins like we have today. We used to mount the fins on the sponson rather than behind the sponson so the rear or the boat would swing out to make the turn.

John,

I presently use the straight, spring steel, full wedge type, skid fin on both my .90 & twin .90 boats. After experminting with both types, I feel a straight angled mounted fin (15 deg angle), that is progressively wider at the bottom, is best for both types of driving. As you & Marty have stated, different boat setups may require slightly different techniques to achieve the desired results.

Jim

Jim:

I would bet that you have a problem it you need ANY left turn. I left the angled fin for that reason.

I was telling you about when Steve O'Donnell came out to race at the Hydro Master's, set a record with his 40 boat (Purdues Boss Boat) and generally impressed everyone. One thing I did not tell in this story is that during heat racing, Steve could not turn left at all and finished only a couple heats. The angled fin bit him badly.

The reason that I use a fin that has the bend at the bottom is to reduce the characteristic of the boat riding up on the fin with left turns. I try to keep the bend just as far to the tip as possible.

Marty Davis

Marty,

You're 100% correct about that problem. I have seen the boat get up & run on the turn fin. I helped the problem greatly by reducing the left rudder throw to a minimum & by widening the sponson stance from a typical 11.5 inches to 16.5 inches on my crapshooter boats. Doing this requires the use of solid, high modulous carbon fiber rods for sponson mountings. I have also done the same widening on Andy Brown type hulls with the same results.

Jim

 

[Don Ferrette]

I race cars as well so we of course use a TON of cf for chassis. I know on my carpet cars I camfer all of the bottom chassis edges so they don't catch the carpet. After they have been filed I rub CA along the newly machined edge to seal it and make it solid to prevent fraying. Perhapse this would work the same on a turn fin?? The only reason I ask is because I am in the process of designing CF running harware for my 12 boat. This happend to be one of the ?'s i had was if anyone had tried to use CF for turn fins etc..

Hmmmmmm.......... I don't think CA is going to be able to hold up to the hydrodynamic force of the water against that leading edge. The problem I see with the carbon fins is the porousity (sp?) of the weave itself. A vac bagged carbon fin obviously is the best appoach to maximize sealing between the weave but once you sharpen the leading edge it would seem to me you're right back to square one with exposed CF. Water under high pressure cuts a variety of materials quite well...... :blink:

 

[Marty Davis]

I recently tlked with and looked at the turn fin that Charles Purdue is running on his 21 hydro. It is completely flat and "bent" , the entire turn fin, at an angle to the inside plate of the sponsons, there is no cup or bend line in the water. It is vary thin as well. It would be interesting to have him give his thoughts on his turn fin. Gary

Gary,

Yes, the straight fin angled inward without the curve is the only other option I have tried that works. Because it is straight for it's full depth it does not scoop the water off the pond. I have used those type fins on many 1/8 scale boats. I feel the scoop fins we use on the riggers today are better for the high speed turns than the straight angled fin. But, that's just an opinion. If someone has a straight fin that is doing the job, that's great. I have been using the scoop fins on my sport gas boat and recently tried several straight fins, but went back to the scoop fin because the straight ones just would not let the boat hang on all 3 buoys in a high speed turn. Like Marty said...........I go into the corners wide open. Marty likes to chop the throttle, set the boat, and then blow thru the corners. I have to drive my scale boats that way when I use the straight fins. It's all relative to speed too. On my stock sport boat the straight fin works great. 10 mph difference.

I'm sure you have noticed with your twins that the faster you go the more critical everything becomes. Remember when we used to run straight turn fins ony two inches deep back in the 70's? The servos back then were not strong enough to hold a turn with fins like we have today. We used to mount the fins on the sponson rather than behind the sponson so the rear or the boat would swing out to make the turn.

John,

I presently use the straight, spring steel, full wedge type, skid fin on both my .90 & twin .90 boats. After experminting with both types, I feel a straight angled mounted fin (15 deg angle), that is progressively wider at the bottom, is best for both types of driving. As you & Marty have stated, different boat setups may require slightly different techniques to achieve the desired results.

Jim

Jim:

I would bet that you have a problem it you need ANY left turn. I left the angled fin for that reason.

I was telling you about when Steve O'Donnell came out to race at the Hydro Master's, set a record with his 40 boat (Purdues Boss Boat) and generally impressed everyone. One thing I did not tell in this story is that during heat racing, Steve could not turn left at all and finished only a couple heats. The angled fin bit him badly.

The reason that I use a fin that has the bend at the bottom is to reduce the characteristic of the boat riding up on the fin with left turns. I try to keep the bend just as far to the tip as possible.

Marty Davis

Marty,

You're 100% correct about that problem. I have seen the boat get up & run on the turn fin. I helped the problem greatly by reducing the left rudder throw to a minimum & by widening the sponson stance from a typical 11.5 inches to 16.5 inches on my crapshooter boats. Doing this requires the use of solid, high modulous carbon fiber rods for sponson mountings. I have also done the same widening on Andy Brown type hulls with the same results.

Jim

Jim:

Moving the lever arm of the front sponson stance is a perfect way to reduce the effect. That is also the reason that we use a rudder on the left of a small stance boat (1/8 Scale, Sport 40, etc) and a rudder on the right of a rigger. The stance offsets most of the effect of weight transfer to opposite corners. The way that you will know that you have reached beyond the limit for sponson stance is that the boat will flutter between the right and left sponson in the SAW. Reducing the stance a little will eliminate that.

Try using Fiberglass TUBES with a solid carbon rod in the middle. It does not shatter like the solid carbon rods and allows for some small amount of flex.

I would suggest that you spend some time on a fin that is 90 degrees to the water surface with the bend near the bottom. You will really find that it works better.

Marty Davis

 

[Jim Allen]

This has been one of the most imformative discussions I have seen on INTLwaters. Many thanks to John & Marty for sharing their knowledge gained over the years through many tests, & I will try the straight, curved at the bottom fin. I hope there will be more exchanges, such as this, of valid information in the future. There is more valuable information to be shared!

Jim Allen

 

[John Finch]

I prefer to make the fin from spring steel. Put the hook in it in the annealed state, and then harden/temper it. A 90 fin is ~0.065" thick and the 21 can be ~0.035" thick. Protect them with either black oxide, or graphite paint. The down side to the spring steel is that it is virtually impossible to adjust the hook once the material is hardened.

Sounds like something to make after you experiment with the aluminum first. I tried carbon fibre. Fabrizio from Italy made it for me. He does great carbon/kevlar/glass work. The fin didn't last long before the leading edge started to come apart. Looked like a good idea. The leading edge was the only problem.

Were you able to figure out why the fin started to fray??

BTW cluttered or not I would read through this stuff anyways. TONS of great info here, especially for someone wanting to design thier own boat!!

Zach

Zach,

I think it is because of trying to get a sharp edge on the leading edge of the fin it made the carbon fibre too thin to withstand the water forces hitting it. Someone with carbon fibre experience can probably shed some light on the problem. I don't know much about the material.

I race cars as well so we of course use a TON of cf for chassis. I know on my carpet cars I camfer all of the bottom chassis edges so they don't catch the carpet. After they have been filed I rub CA along the newly machined edge to seal it and make it solid to prevent fraying. Perhapse this would work the same on a turn fin?? The only reason I ask is because I am in the process of designing CF running harware for my 12 boat. This happend to be one of the ?'s i had was if anyone had tried to use CF for turn fins etc..

I tried the cyanacrolate.

 

[John Finch]

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????

 

[Marty Davis]

This has been one of the most imformative discussions I have seen on INTLwaters. Many thanks to John & Marty for sharing their knowledge gained over the years through many tests, & I will try the straight, curved at the bottom fin. I hope there will be more exchanges, such as this, of valid information in the future. There is more valuable information to be shared!
Jim Allen

Jim:

WHAT A COMPLIMENT from someone that is a Master Machinist and a great thinker.

I would love to be a fly on the wall of your shop for a year or so...

Marty Davis

 

[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

 

[John Finch]

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?