Turn fin location?

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Terry Keeley

Terry Keeley

Well-Known Member
Supporting Member
Joined
Jul 24, 2002
Messages
7,124
Needed to upgrade the turn fin on my 90 hydro, it was letting go in the marbles.

I went from 18 ga (0.048") to 14 ga (0.075") Ti and made it about 25% bigger.

Did a bunch of testing with it yesterday, at first I mounted it right on the sponson, the LE was 6" fwd of the CG. Boy did it turn hard! Problem tho was it was so "twitchy" it was undrivable. When the boat hit the pipe it did a hard right, with a little left correction it flew off. I moved it back 1.5" and it was much better, still turned well but way better drivablity.

Just curious, whereabouts is the LE of the fin WRT the CG on some of the other popular hulls? Crapshooter? Roadrunner? JAE?




 
Terry
Just keep moving it back for the way you like to drive the boat Yes it will turn quick with the fin moved forward The trailing edge of the turn fin should be around the flywheel or the CG.
 
Terry,

As Dave mentioned, I've found that the TE is the proper gaging point, and it usually finds its way to the CG, or thereabouts. If the TF is exceptionally "long" (along the waterline, front to back), you may find the TE needs to be shifted back a bit.

What is really going on, in my observation, is that the TF center area, being a pivot point, is actually ahead of the CG, the aft-mass is slung into a spin in a turn about that pivot point, and the rudder keeps that sling in check. Make sense?

Thanks. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine and Precision LLC
 
Terry Keeley said:
Needed to upgrade the turn fin on my 90 hydro, it was letting go in the marbles.

I went from 18 ga (0.048") to 14 ga (0.075") Ti and made it about 25% bigger.

Did a bunch of testing with it yesterday, at first I mounted it right on the sponson, the LE was 6" fwd of the CG. Boy did it turn hard! Problem tho was it was so "twitchy" it was undrivable. When the boat hit the pipe it did a hard right, with a little left correction it flew off. I moved it back 1.5" and it was much better, still turned well but way better drivablity.

Just curious, whereabouts is the LE of the fin WRT the CG on some of the other popular hulls? Crapshooter? Roadrunner? JAE?




Click to expand...

TK, I really think testing is the best location finder for a fin. We always test from right to left and make the turn in front of you and your helper,you will see alot more of whats going on. In know the Indy guys at one time were actually timing the turn from bouy 1 to bouy 3 and bouy 4 to bouy 6 that also showed what was really working. Pay attention to the very last portion of the bottom curve, there's alot to to gained in that area.
 
Thanks guys.

As it turns out with the LE 4.5" ahead of the CG the TE is right at the CG. Much smoother, less hooking and left corrections no problem. I guess you could move the fin fwd from there and test until your driver was "uncomfortable", then back it off a bit, lol.

Hey Zee Man, you mean turn in front of you from L - R?
 
Terry Keeley said:
Thanks guys.

As it turns out with the LE 4.5" ahead of the CG the TE is right at the CG. Much smoother, less hooking and left corrections no problem. I guess you could move the fin fwd from there and test until your driver was "uncomfortable", then back it off a bit, lol.

Hey Zee Man, you mean turn in front of you from L - R?
Click to expand...
we did r to left ,Iguess whatever way you can see better.
 
That's a big fin. Pay attention to the shape of the LE. The angle and amount of radius on it will effect things as well.

Sometimes making the smaller size fin just go a little deeper and a little more hook will help with that letting go in the marbles.
 
Mike Betke said:
That's a big fin. Pay attention to the shape of the LE. The angle and amount of radius on it will effect things as well.

Sometimes making the smaller size fin just go a little deeper and a little more hook will help with that letting go in the marbles.
Click to expand...

Yup, I hear you on both accounts. Maybe a little more testing this weekend.
 
Terry,

Little story.....

A number of years ago, a well respected boater had acquired one of my BlackJack boats, and had it running wicked fast. He called me with a suggestion on the turn fin, as he was having trouble holding lane one at the speeds he was getting. After the conversation, I made up a new turn fin that was 3/8" wider and 1/4" deeper than the original design. I took it to the pond to try it out. It was considerably better in the turns, but was far more likely to lift the boat into a cartwheel with hardly ANY left adjustment. Over the course of several trips to the pond, I started whittling the leading edge back, from the top of the curl down, as it transitioned from vertical to horizontal, sweeping it back smoothly as I went. I eventually I worked it to the point it would still hold the turn, but was much more friendly to left adjustments. It still had the vertical section width I needed, but pretty much ended up with the horizontal section of the original design, with a smooth sweep from one to the other, which was still plenty enough to hook up in the turns.

Food for thought......

Thanks. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine and Precision LLC
 
Brad Christy said:
Terry,

Little story.....

A number of years ago, a well respected boater had acquired one of my BlackJack boats, and had it running wicked fast. He called me with a suggestion on the turn fin, as he was having trouble holding lane one at the speeds he was getting. After the conversation, I made up a new turn fin that was 3/8" wider and 1/4" deeper than the original design. I took it to the pond to try it out. It was considerably better in the turns, but was far more likely to lift the boat into a cartwheel with hardly ANY left adjustment. Over the course of several trips to the pond, I started whittling the leading edge back, from the top of the curl down, as it transitioned from vertical to horizontal, sweeping it back smoothly as I went. I eventually I worked it to the point it would still hold the turn, but was much more friendly to left adjustments. It still had the vertical section width I needed, but pretty much ended up with the horizontal section of the original design, with a smooth sweep from one to the other, which was still plenty enough to hook up in the turns.

Food for thought......

Thanks. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine and Precision LLC
Click to expand...

I hear that too! I've also found over the years that radius from the LE to the bottom has to be quite big or it will want to lift big time with any left correction.

Julio Almazan said:
Hi:
This is a link to the KEP´S download page.
I have seen that the location of the turn-end in KEP´S 21
it is a little backwards compared to the C.O.G.
I hope this helps you
View attachment 282239View attachment 282238
http://www.ne-stuff.net/2008/12/keps-21-freebie-pdf-drawing.html
Click to expand...

Thanks.

What about some of the other hulls out there? Where is the TE of the fin (at the water line) relative to the CG?

Crapshooter? Roadrunner? Eagle? JAE? Betke?

Inquiring minds want to know...
 
Terry Keeley said:
I hear that too! I've also found over the years that radius from the LE to the bottom has to be quite big or it will want to lift big time with any left correction.



Thanks.

What about some of the other hulls out there? Where is the TE of the fin (at the water line) relative to the CG?

Crapshooter? Roadrunner? Eagle? JAE? Betke?

Inquiring minds want to know...
Click to expand...
Terry,

I think you will find that most, if not nearly all, of the well performing designs will have the turn fin in about the same position, relative to CG. It's kind of tied into the physics at play.

Consider loading a trailer. If you load it too heavy behind the axel(s), what happens? That sway you feel is the equivalent of that over-reactive, darty behavior we see when the turn fin is too far forward of the CG. The axel(s) of the trailer are the equivalent of our turn fin, for the purpose of this discussion, only we are moving the axel(s) fore and aft on our boats, instead of shifting mass.

Thanks. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine and Precision LLC
 

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