Turning Fin Info

[jtower3941]

Have some questions: Runabout turn fin

1. Boat corkscrews on straight away. What causes this??

2. How far, from stern, should the fin be placed.

3. How deep should the fin be below the bottom of boat.

4. If you put the fin closer to stern does this effect the ability for the driver to turn right and maybe the one of the answers to #1

5. What is the effect of a small fin on left side of bottom on the stern.

6. Can you have to much turning surface to the turn fin??

7. Is stainless the only way to go??

[steve jones]

BUMP-BUMP_BUMP
I want to know the answers to jtower3941's questions too!!
Any help out there in "RUNABOUT LAND" ??

[trident]

Turn Fin

Oooh!, point #7, I'd be very leery of using stainless. Yes, its pretty, but its hard to work.

But, the real issue is safety. On faster stuff, it holds up to a point, then yields suddenly. As in, folds right over! Almost put me on my head in a first turn in an EMH many years ago. Thank goodness no one was to my outside!

My preference is 4130 steel plate. I use 1/8" and support it well. Yes, it needs to be painted lightly (flat black looks nice). When it gets dinged up, I bead blast it and get out the can of Krylon. Otherwise it looks really nasty.

Available in smallish pieces from Aircraft Spruce. Nice folks to do business with. They also carry all those lovely stainless AN washers and such.

Jerry

[Tim Weber]

Jim,

The BIGGEST thing is to make sure that the fin is parrallel with the keel. If its off center even an 1/8" it will make the boat snake. If its off more than that, I could see where it could barrel roll you. If the fin is parrallel, the boat will track straight. If you want to find out without turning the boat upside down, make a run without the fin. You'll need a country mile to turn around though.

The fin location will work 36" to about 42" from the transom. If you go too far foward it will make the boat want to spin. Move that pivot point too far and look out.

Depth, I run about 4" deep. If you think that is too deep it's not. Don't worry about it. If you don't have enough fin, the boat will not lock in the turn and it will wander.

The chicken fin on the rear, I think helps hold the boat. In nice water I can't tell a difference but in race water I can feel it help hold.

Too much turning/ fin surface? I would say you wouldn't want any more than about 4"- 4 1/2" in width.

This will sound crazy but it happened to me. One of my boats was doing the left right, grabbing etc. It never did this before. Ok, I check all the above things and all are spot on. Finally figure out that the fin actually twisted. I bent the metal as if you were ringing out a sponge. It had a 1/4" twist in it.
I was using 6061 T6, 1/4" material.

I spoke with Trident about material many moons ago. He metioned the above about chrome moly. I went a step further and use 3/16" material. Problem solved. It is a pain to work with. The 4160 is very tough. The fin is attached with grade five bolts in five places.

Before switching to grade 5 bolts, I sheared the bolts off a fin in the turn at Ocoee. It's not fun. The water is cold in March. Again, thank you Trident I wish I would have had that advice sooner.

Finally, another Trident suggestion, he's a smart guy. When you grind the edge of the fin, be careful. You can grind an edge that will pull the boat.
Mine are very equally beveled. Some guys like a fin that will pull slightly to the left toward the turn. I don't. I figure that's why you have a steering wheel.

A fin can be a real pain. I have made 8-10 in the last few years and I have tried different shapes, materials etc. It may take a while to find what you like.

Tim

[jsilvestri]

Exactly what Tim said.

[trident]

Turn Fin

What Tim said, with some added info. I think he meant 4130. I grind my fins to pull very slightly to the left, rather than grind them perfectly symetrical. This is not to 'help' it turn, its to put a small load on it on the straight, so on really fast stuff (FEH comes to mind) it doesn't hunt. If the fin has a slight pull, it takes out that hunting in the water tendancy which can affect the boat stability. So long as its going to pull one way or the other, just makes sense to make it go left. Just a tiny bit, though...

My fin fine tuning has been on hydros, but the same principles apply... as Tim said, experiment, find what works for you, for your boat.

Grade 5 bolts: I highly recommend them for fins, steering bars, etc. They're really strong, but without the brittleness of Grade 8. Especially in a shear load situation, like mounting a steering bar, the Grade 5 works well... If a Grade 5 bolt isn't strong enough, I'd suggest going up in size, rather than going up to Grade 8.

BTW, if you mount your fin with a single big bolt, so it can pivot to adjust, an ordinary washer doesn't work well to lock it down, as it will 'dish' some... what you want is for the washer to clamp at its perimeter. I recommend you use a drilled out Mercury clamp button, like from a Merc 500 or whatever, as your clamping washer. Hard to drill as they are heat treated harder than a politician's heart, but drill slow, lubricate, the chamfer hole edges. Then I use a fine thread Gr. 5 bolt, a stainless AN washer on top of the clamp button and an elastic stop nut. Fin stays put.

Hope this helps.

Jerry

[Islandmon]

Very interesting read Tim.................thanks for sharing.

Sam,

[johnsonm50]

Have some questions: Runabout turn fin

1. Boat corkscrews on straight away. What causes this??

2. How far, from stern, should the fin be placed.

3. How deep should the fin be below the bottom of boat.

4. If you put the fin closer to stern does this effect the ability for the driver to turn right and maybe the one of the answers to #1

5. What is the effect of a small fin on left side of bottom on the stern.

6. Can you have to much turning surface to the turn fin??

7. Is stainless the only way to go??

Ive asked fin questions before, this is the best set of answers yet. I use a straight edge clamped to the fin to guage straight. With nothing more than CSH I havent broken one. Yikes

[xcsp700]

Thank you for all the valuable info I have already made a prototype version out of 3/16 304 ss which has a yield strength of 30000 psi for testing purpose to find out where and what shape we are comfortable with the final version will be made out of 3/16 4130 that has a yield strength of 50000 psi for racing also I have a concern that the mounting bracket is made of 1/4 aluminum that is gusseted I hope this is strong enough or I will have to fabricate a new mounting bracket for the boat also I think I am going to add the fifth bolt if I can. Dont want to end up on the shore

[reed28n]

Hone in

The fin does need to be parrellel with the keel. I have used a square off the trailing edge of the bottom to determine the plane of the keel and then used two carpenter squares to locate the fin.

You made need to ignore other points of reference on the chine because of your design the outside of the boat may no be close to straight with the keel. Measure your depth and your vertical in relation to the planning surface. Sometimes to get the fin in the correct location forward and aft you will be at the breaks in the bottom of the boat.

You may have heard to "cant" the fin in a degree or few but make sure that measurement is from the intended zero line on the flat part of the bottom of your boat.

I mount the fin to the bracket then locate forward and aft 36" to maybe not quite 42" from the trailing point of the fin. Get it close and mount the bracket to the boat with one bolt or screw. Then get your final position before you drill for the rest of your holes.

You do not want the fin canted in too much, that will also make for an interesting ride.

[trident]

Turn Fin

The idea of putting some camber in the fin mounting is to counter the lift effect of the loaded fin. A hydro rolls ever so slightly to the outside (like a race car) as it turns. That puts the fin at a slight cant out at the bottom and the force of the water acting on the fin then lifts that corner or the boat. Spray off the fin hitting the chine does that, too. (Newton was right). On a rollup runabout, the boat rolls to the inside (more like a motorcycle), but on a side fin boat, you get the effect similar to a hydro. So, on a rollup, I'd think you want the fin square to the bottom. On the side fin boat, hydro or runabout, I'd suggest a tiny camber, then experiment to find the sweet spot.

Way back when we got this fin camber notion, I took a fin and bent a substantial angle in it, but mounted it to the existing bracket. This was on a DSH. My thought was that if we did a tiny change, we might not be sure we were getting the desired effect. A large angle change would make an unmistakable change in handling to confirm it works like we think (or not)... Whooeee!, came into a turn, and that inside sponson went Whump! right down on the water, as I turned in! Oooh, we have an adjustabe roll bar here! That took the roll out. Now lets adjust the angle to find the sweet spot. Its not necessarily the same from boat to boat. Test, test, test. Like always, too much is bad, maybe worse than not enough.

To quote Sam Hemp, what we're after is "perfectly balanced opposing forces".

Jerry

[flatsmoke]

This is really good information. What shape of fins work the best?

[T Chance]

It might not be the fin. I've run hydros, runabouts, and tunnels for a long, long time and now in my AARP days I'm only running runabouts. That said, if your boat has an ever so slight twist it could kind of corkscrew or gallop. It will roll up on the twist and then slip off and roll up again etc, etc.

Earlier it was suggested to try the boat without a fin at all. Then if the problem doesn't go away, check to see if the bottom of the boat is symmetrical. - Tim Chance

[Team B&H]

I find that a small laser is a great tool for checking that the fin is parallel to the keel or centerline of the boat. You are not limited in length with it and it clamps easily to the fin or bracket. In general that is where is runs well. However when you get really sophisticated, you may want to play with a slight amount of steering or tracking with the fin. This follows Trident's advice to test, test and then test some more in a race. Some of this comes down to personal preference and how it feels to you rather than simply following the crowd.

[Islandmon]

I find that a small laser is a great tool for checking that the fin is parallel to the keel or centerline of the boat. You are not limited in length with it and it clamps easily to the fin or bracket. In general that is where is runs well. However when you get really sophisticated, you may want to play with a slight amount of steering or tracking with the fin. This follows Trident's advice to test, test and then test some more in a race. Some of this comes down to personal preference and how it feels to you rather than simply following the crowd.

I like the laser idea, a long level or straight edge is good but but I think the laser takes you to the next level ( no pun ).

Sam,