Bow steering, spin out and transom mount spint fin

[Kez]

Hi All,

Transom mount anti-spin fins are very common on today's monos and vees. When I got into this hobby in the early 70's, most monos ran fully submerged drives and I did not remember seeing any spin fins on the transom. I guess as speed increases, a boat needs more help to keep it on the water. The look of a huge fin hanging on the transom has always bothered me. Furthermore, I am sure a fin like this will add to the drag. I only install them if have to. In fact I can get away without any spin fins most of the time but I have to be careful while driving or I will spin out.

Since full size boats do not have these fins, I started reading up on some articles on full size crafts. Bow steering in full size crafts is caused by the CG being too far forward and/or the boat trimmed to run too flat (correct me if I am wrong for those of you who run a full sized boat). In extreme cases, bow steering will end up with a spin out. This all make sense to me. But the problem is that my experience with RC monos and vees is the complete opposite. If I trimmed a boat to run loose; either by moving the CG aft or by moving the trim plates up, I am more likely to have a spin out while turning even with a spin fin.

While I am rebuilding a couple of old vees, I started experimenting with set ups and removed the spin fins. I have found that the standard rule of thumb for CG at 33% works quite well. In fact, a well trimmed out boat does not need any spin fins to turn properly at high speeds.

My question is: Do we really need a spin fin? Please share your experience.

Thanks,

kez

 

[Steve Seebold]

You need some kind of a turn fin. Whether your fin be mounted on the transom or forward under the engine. At high speeds, without a turn fin of some kind, these boats will slide sideways in a high speed turn, and that can cause major problems

I speak from experience. A few years ago, a mono I was running had one of the mounting screws that held the turn fin fall out and the fin was just flopping around on the back of the boat. As I entered the last turn heading for the start, my boat slid sideways from lane 3 to about lane 15. I was penalized a lap for that infraction. That's when I started using nyloc nuts.

 

[Joe Warren]

John Finch wrote many great articles in ADVANCED RC/BOAT MODELING years ago that apply to basic set ups for Mono & why they work. Still great articles today. I do not recall if he touch on he fact that wnen the monos went from sub surface to surface drive the breaking point on the bottom of the boat moved 3-4 inches also. This is what makes it impossible to get a Older Subsurface drive mono to work well by making it Surfae drive set up. The Boat builders themselves especially would know about this...... Then Seaducer came along with the boat with Hook in the Bottom and many Basic set ups do not aply to that type of bottom.. its best to use their set up sheet for their boats.. But Johns articles touched on High CG monos Low CG monos and the proper placement of transom hardware & why & when to use a turn fin & to set it up with progression.

 

[Tim_Duggan]

Attached is a document that surfaced on here a few years ago. I can't recall the gentleman's name to give credit, but its an interesting read.

Rudder+turnfin.pdf

 

[David Bryant]

Seaducer came along with the boat with Hook in the Bottom and many Basic set ups do not aply to that type of bottom.. its best to use their set up set for their boats

sounds like the "hook" incorporated along the bottom of the hull surface creates a similiar effect as trim plates? Bringing the bow down?

 

[John Finch]

Nice article on harware setup. I might add......on a mono the keel is very importand on turning. A sharp keel forward of and about at the CG point will cause the boat to dart quicker in a turn. Rounding that area of the keel will let the front of the boat dig in less and make turning more linear if the boat likes to turn too agressively in a high speed turn. The article said the boat would corner with less drag if it did not lean as hard to the right. We found out by mistake one time why Dick Tyndal's 20 mono was the only twin craft 20 boat that did not bank in the turns. The mistake was a hook on the right side strake that the other boats did not have. The strake hook lifted the right side of the boat while cornering. It won many big races running flat in the corners. Amazing how so many things change the dynamics of how a boat rides.

 

[Andy Greene]

Nice article on harware setup. I might add......on a mono the keel is very importand on turning. A sharp keel forward of and about at the CG point will cause the boat to dart quicker in a turn. Rounding that area of the keel will let the front of the boat dig in less and make turning more linear if the boat likes to turn too agressively in a high speed turn. The article said the boat would corner with less drag if it did not lean as hard to the right. We found out by mistake one time why Dick Tyndal's 20 mono was the only twin craft 20 boat that did not bank in the turns. The mistake was a hook on the right side strake that the other boats did not have. The strake hook lifted the right side of the boat while cornering. It won many big races running flat in the corners. Amazing how so many things change the dynamics of how a boat rides.

Did it have trim tabs on both sides as well John ?

 

[El Zorro]

Nice article on harware setup. I might add......on a mono the keel is very importand on turning. A sharp keel forward of and about at the CG point will cause the boat to dart quicker in a turn. Rounding that area of the keel will let the front of the boat dig in less and make turning more linear if the boat likes to turn too agressively in a high speed turn. The article said the boat would corner with less drag if it did not lean as hard to the right. We found out by mistake one time why Dick Tyndal's 20 mono was the only twin craft 20 boat that did not bank in the turns. The mistake was a hook on the right side strake that the other boats did not have. The strake hook lifted the right side of the boat while cornering. It won many big races running flat in the corners. Amazing how so many things change the dynamics of how a boat rides.

Did it have trim tabs on both sides as well John ?

Hi Andy,

My Twin Craft B Mono had trim tabs on the transom, as did most of the monos of that era. I built the boat back in 1993, and last ran it at the 2010 Fall Nationals where it finished in third place. I sort of had a hand in helping John develop the first surface-drive Twin Craft. We started out with big, wide trim tabs and over the years they gradually got smaller. The 30" long Twin Craft was really too short.......it should have been 32" long. When the water got really rough heat racing my boat would hop in the corner and "swap ends" ( do a 360 ). To cure this I extended ALL of the hardware one inch on the back of the boat; strut, rudder, turn fin and the trim tabs. When I did this I did away with about 1/3 of each trim tab, keeping the main part of the tab down next to the strut. Problem fixed! These changes made the boat " think " it was longer, and it never spun out when it hopped again. I never really said anything about the hook in the right strake, but one time at a race John came over and asked if he could look at the bottom of the boat ( professional courtesy! ). It is the only one that we have seen that came out of the mold that way. All of the Twin Craft guys in this area had some really good running boats at that time. John has always been very helpful in sharing ideas and it helped me greatly in being as successful with my Twin Crafts as I was back in the day.

By the way, that first surface-drive Twin Craft boat was given to me by John around 1990. Had a Picco .80 in it then a Picco Hydro .90. Won my first US-1 with that boat. The boat had sat in the attic of my shop for years until last year when I gladly gave it back to John. He put a gas engine in it and won more races with it last year! Old is good............

Dick Tyndall

 

[Kez]

Thanks all for chiming in and offering your experience and thoughts. The reason I started this thread was to discuss if a "fin" was really necessary on a mono hull. Can it be eliminated by properly balancing and trimming the boat?

I think the transom mounted fin on a mono, commonly called turn fin is a misnomer. Unlike a turn fin on a hydro, which actually helps the boat turn, a transom mounted fin on a mono should be called an "anti-spin fin". Its purpose is to provide a lateral surface in the water at the transom and dampen the dency to spin out.

When a fin is mounted down in the center of the keel on a mono (near the CG), it is really a turn fin. I remember seeing a few of these in the 80's but they slowly dropped out of favor. A center mounted turn fin can keep a mono from leaning into the turn. I remember seeing them on some full sized ski boats especially with shallow vees that tends to slide.

I think spinning out, swapping ends (extreme case of spin out), darting, bow steering (full size boating term) all refers to the same symptom. Is a transom mounted anti-spin fin really a solution or just a bandaid? I think it can be eliminated through trimming. I also found out a weak or slow rudder servo can give you the same symptom. In that case replacing the rudder servo completely solved the problem.

Thanks.

kez

 

[El Zorro]

Kez,

I didn't mean to stray away from your original question with what I posted. It does give you some insite on how important the hardware on the back of the boat can be. I think you answered your question with something you said at the end of your first paragraph. If you drive real careful you don't need a turn fin; if you don't, the boat will spin out. Well, when we're racing we can't always drive real careful. You also have to realize the scale speeds that we are running are much faster than real boat speeds. This multiplies everything. When the water is flat, the boat will run slower ( more drag in the water ) than it would if the water had some chop to it. The " looser " the boat runs, the less control you will have over the boat. Like the car guys say, "loose is fast ". I guess you could run a bigger, deeper in the water rudder but that would only create more drag than the turn fin does, especially in the straightaways. Everything is a trade-off. Most of the mono hulls you can purchase today come with fairly detailed instructions on how to set-up the boat. The people that designed these boats took the time to run the boats to find out what works and what doesn't. You can walk through the pits at a race and look at all of the monos and see that none of them are exactly the same. Engine placement, tank placement, trim tabs and tab placement, Center of Gravity, prop, etc., all of these figure into how the boat will perform. Best way to get a boat to run like you think it should is to do what John Finch does; Lots of trips to the lake and make a bunch of changes to see what works and what doesn't.

Dick Tyndall

 

[Kez]

Hi Dick,

Your post has a lot of useful information that offer boaters alternative approach to solving same handling issue. Thanks for sharing.

I had a Prather Aggressor with standard Prather hardware which included a pretty big anti-spin fin. The boat had decent speed (for my standard) but with some minor chine walking issue. Other than that it was a pretty good handling boat. The look of that big fin had always bothered me. So I decided to remove it and see how it would affect the overall handling. Well, it did not result in any noticeable change and perhaps picked up a little speed probably due to the absence of the drag from the fin.

Thanks,

kez

 

[John Finch]

Kez,

The keel of the deep vee acts as a skid fin as does the strakes. I recon we are talking deep vee because that is what we run for heat racing with the water all chopped up. The skid fin is not needed for fun runs, perfect water conditions and reasonable speeds. For heat racing it is a must. I experimented with them a lot in the 1980s. I started out with a 1/2 inch deep turn on the right trim tab as a skid fin. As the years went by the boats got faster and the fins got deeper. Eventually I had in down to a science. A fin 2 inches deep on a Twin Craft would allow the boat to spin out once about every 3 heats in rough water with 6 to 8 boats racing. At 2 1/2 inches deep you would spin out about one heat for a weekend. At 2 and 3/4 inches deep you would not spin out ever. Its all in what you are trying to do. Personally I like to finish races and a little deeper fin will help make that happen. A 65 mph boat in rough water is common these days and the boats are literally flying. They don't turn if nothing is in the water and the turn fin can connect when the boat is aired out.

Dick is not giving himself enough credit. It was his hardware setup and his push for surface drive that made the Twin Crafts competitive back then.

You will find mounting the fin further to the side of the boat makes the boat bank harder. As you move it closer to the keel the boat banks less. Banking a mono is not a bad thing. The keel and fin keep the boat running linear and the mono becomes a wicked fast almost flat bottom boat when it is turning on it's side.

Also..... If the rudder is almost the same distance from the transom as the turn fin ....the turn fin will make rudder movement less sensitive. If you move the rudder further from the transom than the fin.....the steering becomes more sensitive. For heat racing fins don't slow you down if they are parallel with the keel because they stabilize the boat. Rocking and chine walking is what slows a mono. So...........run a fin and have fun driving a stable boat.

 

[bzubee]

I have a question to add to this topic why are a lot of the new gas monos designed to use the super long extensions for the rudder and strut? I know they do work that way but it just seems like a band aid to fix a design issue?

 

[John Finch]

I have a question to add to this topic why are a lot of the new gas monos designed to use the super long extensions for the rudder and strut? I know they do work that way but it just seems like a band aid to fix a design issue?

Bill,

Hard to tell, It may be to not push the bow down as hard when cornering or maybe that it puts the rudder back further to make the boat turn tighter as it is further back from the turn fin, or that it takes a lot of pressure off the rudder servo because of better leverage, or just because everyone else is doing it. I have been racing hydros for the last 15 years and all this came about while I was not paying attention to the mono hulls. I know back in the later 80s I used the rearward rudder to turn my twin mono to get the leverage and control with two props wanting to pull the transom to the left. Maybe the bigger props with the gas boats required the same thing.

On the struts I think it has more to do with the lift of the larger props. If close to the transom the back of the boat is lifted out of the water. When put way behind the transom the prop acts as a wheely bar and lifts in such a manner that the boat pivots on the transom rather than being lifted out of the water. I raced my old Twin Craft last year and did those experiments. That is what I came up with. May be why the rudder needs to be moved back as well because the mono hulls like the rudder beside or behind the prop in relation to distance from the transom.

John

 

[bzubee]

Yeah, I thought my Hurricane with a 5 " extension was extreme but then I saw the Rampages using a 6" extension, they all run good just seems a little strange to me.