Turning Fin Info

[ZUL8TR]

Fin twisting

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.

What about fin twisting under water load? Because the fin is supported by a stiff bracket and the fin is extended some depth below the bracket the fin acts as a cantilever beam loaded by water pressure on a portion of its starboard side. This pressure loading is not uniform on the fin therefore there will be a tendency to twist and flex the fin to port side side from the true centerline it was measured from. The twisting has a great deal to do with the shape of the fin that the water loads on, length of the loaded fin area from the bracket support, the material, its thickness and the design of the bracket.

At 70 mph on a 500 lb rig in a 100 ft radius turn centrifugal water loading on the boat is about 1600 lbs. If a third ( I think that is low) is assumed to be acting on the fin that is about 530 lbs. Fairly substantial to cause flexure and resultant twist.

So the real deal is to test and get the fin to perform under load with all the twisting and flexing going on and make sure the fin stays on the hull and operates the way you want. Deeper and thicker fins cause more water drag so compromise is the key here.

[Popa Sam]

Has anyone ever tried drilling holes in the fin? It works for water ski fins and worked well on 15' Allison turn fin. It allows the same grip force with less area.

[johnsonm50]

Has anyone ever tried drilling holes in the fin? It works for water ski fins and worked well on 15' Allison turn fin. It allows the same grip force with less area.

One of my boats is a classic B runabout with some extra power. The B bottom fin doesnt hold well or occasionally too well. I got an old fin for a D or more that has several holes near the base, about 3/4". Ive heard they allow more slide & that would be OK with me. Im also considering a small transome fin.

[johnsonm50]

Ive seen on a couple hydros a small fin on the aft end of the port airtrap. Anyone have the stroy on that?

[ZUL8TR]

fins

Ive seen on a couple hydros a small fin on the aft end of the port airtrap. Anyone have the stroy on that?

Here is the transom fin on my hydro. Also a shot of the sponson fin - 3/16" aluminum. Both fins have holes for up down adjustment. The top adjust hole allows the fin to be rotated out for other settings as needed. I went to a transom fin because my hydro rear was sliding to starboard (right) with the Y80 when the water was a bit choppy and the sponson fin could not prevent the slide when the rear was riding light with air compression lift. I keep the transom fin just a small amount below the bottom (about 1/4") just enough to provide the side force when the rear decides to slide starboard. Very effective and a not noticable speed drop. I recently added some depth to the air traps at the rear and this may also stop a slide and the rear fin may become obsolete? Need to test that.

[trident]

Turn Fin

2nd pix, the sponson fin... remember my discussion of how a conventional washer would dish? Excellent visual example of exactly what I was describing.

How about trying either a large clamp button here, or making a 3/8" flat 6061T6 alumininum backing plate that mimics the profile of the bracket mounting plate... put the fin in a 'sandwich'.

I also prefer a mounting that has the fin on the inside of the plate, as that supports the fin blade much better from flexing sideways. If you do that type mount, I'd suggest you radius or fair off the bottom edge of the plate where the fin 'rocks' against the plate so as to avoid a sharp shear point for the fin.

Jerry

[johnsonm50]

I also noticed that some mod hydro guys use a fin that seems larger, thicker, deeper & farther out on the sponson. Easy to see why but are these a product you can buy or are you guys making your own?

[ZUL8TR]

fin washer

2nd pix, the sponson fin... remember my discussion of how a conventional washer would dish? Excellent visual example of exactly what I was describing.

How about trying either a large clamp button here, or making a 3/8" flat 6061T6 alumininum backing plate that mimics the profile of the bracket mounting plate... put the fin in a 'sandwich'.

I also prefer a mounting that has the fin on the inside of the plate, as that supports the fin blade much better from flexing sideways. If you do that type mount, I'd suggest you radius or fair off the bottom edge of the plate where the fin 'rocks' against the plate so as to avoid a sharp shear point for the fin.

Jerry

Although it hasn't dished yet I will dump the round washer and make a sandwich clamp that matches the bracket shape that will support the fin further down its length. I agree with your preference of the fin on the inside of the bracket and have had the fin mounted there in the past and will try that again as well.

Thanks for the info

[Dave M]

Washer

I also noticed that some mod hydro guys use a fin that seems larger, thicker, deeper & farther out on the sponson. Easy to see why but are these a product you can buy or are you guys making your own?

I have ordered a few from Lee Tietz (Machine Components) and they come with a very nice thick, beveled washer to prevent what Jerry is speaking of. It is not simply a washer from a harware store, it is large, and able to withstand the amount of pressure applied. I highly recommend Lee's stuff.

[Guest]

Turn fin

We are having to rethink turn fin location for our D mod hydro. I beleive that it should go as far out on the sponson as possible.

Any comments.

Alan

[ZUL8TR]

fin placement

We are having to rethink turn fin location for our D mod hydro. I beleive that it should go as far out on the sponson as possible.

Any comments.

Alan

That is the thinking on fins but the farther out the higher up the bracket has to be on the sponson back due to the beveled sponson side. Shoot for fin centerline to be about 2" or more beyond the rear top deck outer edge so the water spray off the fin doesn't hit the rear side chine and tend to rotate the hull to the starboard side and cause the right side to dig in.

Some put several degrees of inward (to starboard) tilt to the fin and under water load to counter fin bending I suppose and if the fin is stiff enough there is a downward component of fin force on the inside of the fin to hold the sponson down.

Fin shape has alot to do with turning.

[weldkroll]

I run Dmh and everything these two guys have tought me works perfectly! They know there stuff.

[weldkroll]

Sam and Jerry that is. Everything Jerry has ever told me to do has worked to a T. Wear to mount to material. Thanks again guys for all your help and info.