turn fins on tub

[Marty Davis]

Martys fishing again!

A turn fin on both sponsons with a small fin on the left and a large fin on the right sponson WILL snap roll, not just theory, tried many combinations years ago. Will not work.

The correct turn fin on the right sponson does not have to pull the right sponson down to make it work. The correct fin holds the sponson in the same position as in the straight -a- way. The boat just does a slow pivot around it. When it is right, the boat "Sweeps" the turns, loosing very little speed. The correct shape doesn't have to be a "Hooked" or a curved turn fin either.

While we are playing with different ideas, what about WHEELS on the sponsons. Don't laugh, they work great in the straights and so-so in the turns. They are just a bear to get up on plane.

Charles

Charles:

I remember the fin that Steve and Jack O'Donnell had on your Boss Boat when they came out to Indy for the Masters. It looked like it wouldn't work, but boy did it work. Was a 1 bend fin like the Unlimited's but with a flat on the bottom that was important. The boat was amazing.

I tried that and got it to work. Not nearly like your setup though.

So, I went in a different direction. Ed Lackey and I made hundreds of fins in an effort to work towards something that was as good as your setup. Getting the snap in the turns was difficult, but we finally got there. You are correct, a fin does not have to pull the sponson down. My fin does not, it runs neutral. Takes some tuning, but that is important because a fin that pulls down takes a lot of speed from you.

I tried fins on both sponsons, but I spent almost all my time on just the right side fin setup. I didn't really work at 2 fins much, so I can't say that that system won't work. Just haven't developed it.

Your early efforts were SUPER IMPRESSIVE. I would like to see how far you have come from that point......

 

[Charles Perdue]

One major requirement of a consistent handling, good turning rigger is hull rigidity and sponson bracket rigidity. I am referring to torsional rigidity, the ability of the hull to resist ANY twisting under load and sponson bracket flexing, in particular, the right rear sponson bracket.

When a rigger is operating at race speeds in a turn, the load on the turn fin and rudder is trying to twist the hull. ANY twisting movement between the rudder and turn fin will have the tendency to unload the turn fin, showing up in poor turning and erratic handling. Remember that the loads go up to the SQUARE of the speeds, the decreasing radius of the turn and the weight of the boat. There are a lot of other factors that come into play also but I believe that these are the most important.

This is why the curved turn fins work on some boats better than the straight fins.The hull/right sponson bracket are flexing. The straight fins are critical as to the angle that the bottom of the fin is leaned in where the curved fins, even though they are flexing,due to their shape, they still have a lot of bite in the water.

I am not trying to convince anyone that one style is better than the other, but I run only flat/ straight turn fins.

Your turn Marty.

Charles

 

[Marty Davis]

One major requirement of a consistent handling, good turning rigger is hull rigidity and sponson bracket rigidity. I am referring to torsional rigidity, the ability of the hull to resist ANY twisting under load and sponson bracket flexing, in particular, the right rear sponson bracket.

When a rigger is operating at race speeds in a turn, the load on the turn fin and rudder is trying to twist the hull. ANY twisting movement between the rudder and turn fin will have the tendency to unload the turn fin, showing up in poor turning and erratic handling. Remember that the loads go up to the SQUARE of the speeds, the decreasing radius of the turn and the weight of the boat. There are a lot of other factors that come into play also but I believe that these are the most important.

This is why the curved turn fins work on some boats better than the straight fins.The hull/right sponson bracket are flexing. The straight fins are critical as to the angle that the bottom of the fin is leaned in where the curved fins, even though they are flexing,due to their shape, they still have a lot of bite in the water.

I am not trying to convince anyone that one style is better than the other, but I run only flat/ straight turn fins.

Your turn Marty.

Charles

Charles:

I agree with you about the fin rigidity.

I double up the fin with 1/8" carbon plate to keep the fin totally rigid. As for the center section flexing, that goes to design. My boats do not flex much if any.

I run only curved fins because I didn't like the negative of a slight left turn rolling the boat on it's top. That probably can be offset to a large degree with sponson spacing. More spacing will lenthen the lever arm making it difficult to roll.

That was the only thing that I didn't like when Jack and Steve came out, the problem with left turns. I do remember that Steve had trouble with left turns in the Masters or I believe he would have won easily. To compound the left turn problem, he had a flex cable as his stearing linkage and it was causing him a lot of problems.

The left turn negative was the reason that I went in the direction of keeping as much of the fin 90 degrees to the water and using the cup in the fin to do the work. Currently, I have reduced the depth of the fin while maintaining the same area (below the water line) of the fin in order to further reduce the left turn effect. I had a 67 boat that was a handful until I widened the fin and shallowed it at the same time. Now I can turn left and it still turns perfectly.

We both got to the same place using 2 different methods. Both work well.

Lets talk about the leading edge of the fin a little too.....

 

[Jeff Lutz]

One thing to point out is when I say a pressing down turn fin,I do not mean to the point of dragging the ride pad in the water.But if you see water break off the top of your fin,the reaction is trying to push it down. On the leading edge of the fin, the more angle towards the back of the boat is less drag down the straights,but is not as snappy through the turns.

 

[Charles Perdue]

When the boat goes thru the turn, the optimum is no "Roostertail" off of the turn fin. This tells you that the boat is not sliding sideways. If it is sliding sideways the turning ability of the boat will never be consistent. I prefer to DRIVE the boat thru the turns. You also have to be careful and not make the turn fin too large in area. This is unneccessary drag.

I prefer the turn fin to be the in the shape of a parallelagram (Spelling) with sharp corners. The amount of "sweep" on the turn fin varies to the type and weight of the boat. For rigger use, it usually works out to about 22 to 25 degrees. The more sweep the, the larger "sweet spot" in the front to rear balance on the position of the turn fin but it also has the greater tendency to drive itself out of the water in a hard turn causing the fin to unhook.

Next.

Charles

 

[Marty Davis]

One thing to point out is when I say a pressing down turn fin,I do not mean to the point of dragging the ride pad in the water.But if you see water break off the top of your fin,the reaction is trying to push it down. On the leading edge of the fin, the more angle towards the back of the boat is less drag down the straights,but is not as snappy through the turns.

Jeff:

I must have a neutral fin because I have almost ZERO water coming off the fin. It is acting like a pivot point and is not shearing water upward.

Charles:

If you lag a rudder, it lifts. If you lag the front edge of the fin, it lifts. I try to make the leading edge of the fin with the minimum amount of sweep back.

 

[Charles Perdue]

I agree about the lag or sweep lifting, BUT when the boat goes thru the turn you have the greatest amount of down pressure on the right sponson than at any other time. The engine is at or near its peak horsepower twisting the prop to the left, the resulting torque is pushing the same amount down on the right sponson. For every action there is an equal and opposite reaction.

Sometimes a little lift is a good thing to even things out.

Charles

 

[John Knight]

Please define "lag" that you both note for the rudder and fin. Not everyone understands exactly what you are meaning.

 

[Marty Davis]

Please define "lag" that you both note for the rudder and fin. Not everyone understands exactly what you are meaning.

John:

Lag is when the bottom of the rudder is not 90 degrees to the water line. The bottom of the rudder front edge is further to the rear. When you turn the blade it tries to climb out of the water a ton. When it is going straight it tries to climb out of the water a little becuase it is a slight ramp. Same thing on the turn fin if the front edge of the fin is not 90 degrees to the water.

 

[Charles Perdue]

Hello John, Marty's term "lag" and my term "sweep" is referring to the angle from perpendicular of the leading edge of the turn fin or rudder. I guess.

???

Charles

 

[Marty Davis]

Hello John, Marty's term "lag" and my term "sweep" is referring to the angle from perpendicular of the leading edge of the turn fin or rudder. I guess.

???

Charles

I tried to say that ....

 

[John Knight]

Many of the turn fins from the rigger builders have nowhere close to 90 degrees to the leading edge of the turn fin. Essentially all have a sloped angle from the sponson down to the primary fin area and it's bend. Roadrunners, Crapshooters, Eagles, etc., all have that general characteristic. I would love to see some pics of the optimum shape. Or have I missed the point of this discussion or misunderstand it?

Marty you can count of me looking closely at your riggers when you come to Charleston in October. School will be session for me then.

 

[TomMoorehouse]

Well this is a great thread on fin design.

What I had in my head is the newer forward front sponson designs that everybody is talking about (Japanese style)and weight. We have the trailing edge of the front sponsons much further forward and then put a big extension on the back to hold the fin. It just makes me think extra weight. Mounting the fin to the tub would eliminate that weight.

I know from experience with my design that a fin mounted too far forward (standard full grimracer/vegas fin) the boat becomes extremely over sensitive to steering input. Would a smaller fin or a different design fin fix this?

 

[Marty Davis]

Many of the turn fins from the rigger builders have nowhere close to 90 degrees to the leading edge of the turn fin. Essentially all have a sloped angle from the sponson down to the primary fin area and it's bend. Roadrunners, Crapshooters, Eagles, etc., all have that general characteristic. I would love to see some pics of the optimum shape. Or have I missed the point of this discussion or misunderstand it?

Marty you can count of me looking closely at your riggers when you come to Charleston in October. School will be session for me then.

I agree that all those boats have too much lag in the fin leading edge.

Who knows what the optimum shape is?????

 

[John Knight]

Many of the turn fins from the rigger builders have nowhere close to 90 degrees to the leading edge of the turn fin. Essentially all have a sloped angle from the sponson down to the primary fin area and it's bend. Roadrunners, Crapshooters, Eagles, etc., all have that general characteristic. I would love to see some pics of the optimum shape. Or have I missed the point of this discussion or misunderstand it?

Marty you can count of me looking closely at your riggers when you come to Charleston in October. School will be session for me then.

I agree that all those boats have too much lag in the fin leading edge.

Who knows what the optimum shape is?????

You must have something that is much closer that the original manufacturer are using and that is what I am most interested in seeing. You are still the master my friend.

 

[Bill Gibson]

Many of the turn fins from the rigger builders have nowhere close to 90 degrees to the leading edge of the turn fin. Essentially all have a sloped angle from the sponson down to the primary fin area and it's bend. Roadrunners, Crapshooters, Eagles, etc., all have that general characteristic. I would love to see some pics of the optimum shape. Or have I missed the point of this discussion or misunderstand it?

Marty you can count of me looking closely at your riggers when you come to Charleston in October. School will be session for me then.

I agree that all those boats have too much lag in the fin leading edge.

Who knows what the optimum shape is?????

You must have something that is much closer that the original manufacturer are using and that is what I am most interested in seeing. You are still the master my friend.

Yeh, he really is..and ive just gotten to know him over the last year or so..wish ida met em say 20 years ago...oh the stuff id know now!!

 

[dwilfong]

Now this is the meat and potato's we need.

What a great discussion and info. This is what makes this sight in my opinion the best on the web.

David

 

[Charles Perdue]

For those of you that were following this thread on turn fins, the lifting of the left sponson that Marty was referring to when turning left with the flat bladed turn fin, were rectified with some small changes. This type of turn fin is still MY choice.

Charles

 

[Andy Greene]

Many of the turn fins from the rigger builders have nowhere close to 90 degrees to the leading edge of the turn fin. Essentially all have a sloped angle from the sponson down to the primary fin area and it's bend. Roadrunners, Crapshooters, Eagles, etc., all have that general characteristic. I would love to see some pics of the optimum shape. Or have I missed the point of this discussion or misunderstand it?

Marty you can count of me looking closely at your riggers when you come to Charleston in October. School will be session for me then.

I agree that all those boats have too much lag in the fin leading edge.

Who knows what the optimum shape is?????

You must have something that is much closer that the original manufacturer are using and that is what I am most interested in seeing. You are still the master my friend.

Yeh, he really is..and ive just gotten to know him over the last year or so..wish ida met em say 20 years ago...oh the stuff id know now!!

He still would have forgotten more than you or I combined would have learned Bill

The teaching style is tough- NOTHING is handed to you and you understand when he is done if you pay attention for sure.....

Great stuff btw guys- please continue.

Andy

 

[Mike Rappold]

For those of you that were following this thread on turn fins, the lifting of the left sponson that Marty was referring to when turning left with the flat bladed turn fin, were rectified with some small changes. This type of turn fin is still MY choice.

Charles

Charles,

Can you further explain some of the small changes that you used to keep the left sponoson from lifting?

Thanks in advance.

Mike