Outrigger Turn Fins

[Marty Davis]

Thanks Ian,

I was thinking similarly....

I was thinking that if I widened the outboard sponson "into the next lane" that the rolling moment would be taken up by the bottom of the sponson as a lift component on the outboard sponson. My thinking is that this would be better than exaggerating a bad attitude by putting the load on a turn fin that has a horizontal component.

just exploring some ideas......I'm definitely open to any opinions on the subject as I don't think it's been thoroughly explored yet.

maybe it should be tested

Making the outside sponson further out should reduce the downward force required from the fin but wont eliminate the need for it. Moving the turn fin sponson out should also do the same. Reducing the CG height will help as well. But the roll moment will still try to lift the turn fin sponson, and this is something that gets worse as it starts to lift, the CG gets higher increasing the roll moment and the turnfin starts to run effectively angled 'out' so due to the slip angle water starts to get pushed down and creates more lift on the turn fin side!

The downward force provided by the turn fin to counteract the roll moment does have some adverse effects and causes the sponsons to twist:

http://gallery.intlw...bum=1092&pos=30

Ian.

You pretty much nailed what I consider as the problem with a curved fin.....

I'm going to try the vertical fin soon...I have identical .21 sponsons except one is rigged with a vertical fin and the other is set up like a Hummingbird. I hope to find some speed, and I'll definitely let you know how it turns out.

Pics to follow,

what do you think the disadvantage is to having more attack angle on the turn fin sponson while in a turn? i would think it would be a good thing, keeping the fin from pulling the sponson down into the water and causing drag in the turn only..

Martin:

I run .2 of degree more attack angle on the turn fin sponson to offset torqe. I have also found that if you have too much attack angle on the turn fin sponson the fin will become unhooked in race water. As long as you are running on smooth water the fin works fine, but as soon as the water becomes chopped, the extra attack angle causes the fin to become unhooked or at least varying in its ability to stay hooked up. Varying is WORSE because you can't predit it at all.

 

[carlnoki]

Marty;

Here is a picture of my 20 boat in turn at full throttle.

I use one of Dick Tyndalls turn fins and love it.

It is of the curved design as most everyone knows.

Carl

 

[Charles Perdue]

The curved fin came from Ed Lackey. He was running as a team member of Crapshooter in the late 70's early 80's. He built several hundred fins and that was his area to test. When he decided to curve the fin, we found turning ability that we had never seen.

As for the straight fin not turning as well. Not true....

Steve and Jack O'Donnell came to Indy with a Boss Boat that had a straight fin. It turned better than ANYTHING that we had ever seen. Was stiff as a board and didn't flex at all. Had a lot of tricks on it with a flat area on the bottom that was paralell with the bottom of the boat.

The real advantace of the curved fin was that you have most of the area of the fin as a straight section and only the curved part not 90 degrees to the running attitude.

That makes it much easier to turn left. With the Boss fin, any left turn and the boat was upside down. The boat climbed up on the straight fin very badly.

It has taken 20 years to understand the workings of the curved fin and be able to replicate the good turning of a well designed curved fin. I have seen maybe 10 boats in all these years that work super well in the turning area.

Close .......

In 1979 EVERYONE was running a flat turn fin that was set up vertical. It had to be a small fin in area so that the boat would slide in the turn or if the turn fin had enough area so that it would not slide, the boat would roll over in the turn.

I designed a flat fin with enough area that it would not slide and bent the turn fin just below the mounting holes at an 18 degree angle with the bend parallel to the bottom of the fin. I worked so well that it cracked the boat enough that the front bulkhead of the radio box popped aloose. Reinstalled the bulkhead and reinforced the area (this was a 45 crapshooter) took it to the NAMBA Nationals in Monterey and won the class with 4 first place finishes and fast time of the class with bone stock engines.

In 1980, I designed the BOSS boat and in 1981, Jack, Steve Odonnell and myself built the 60 boat that Marty was talking about using the flat/angled fin. In the beginning we had problems turning left but later we widened the sponsons out and that fixed this problem. To run the flat/angled fin the boat hull, fin and the sponson brackets had to be very stiff.

The curve fin came into being because the hull, thin turn fin material and the small sponson brackets would flex and the straight/angled fin would straighten out and the fin would "Unload". A well designed curved fin compensates for this with the curl at the bottom of the fin because as the fin straightens out, there is still enough curl left so that the fin does not turn aloose.

I am still using a similar design to the flat /angled fin today, only I use a completely flat turn fin (made of tool steel) and mount it at an about 18 degree angle to the sponson (this angle will vary as to the size and weight of the boat). When the turn fin is done this way, there is no setup for the turn fin, just mount it and go.

Just another way of accomplishing the same results.

 

[TomMoorehouse]

One thing I have learned is that the more forward a turn fin is the faster the boat turns. What would be the advantage/disadvantage of a smaller fin mounter further forward?

 

[Charles Perdue]

Normally the closer to the CG that the turn fin is place, the quicker the boat turns as it has the tendency to pivot around the fin. The primary disadvantage of a small turn fin is it will come "Unhooked" in race water. If it does this, usually the boat will trip on the outside sponson.

Charles

 

[Archie Stanton]

The curved fin came from Ed Lackey. He was running as a team member of Crapshooter in the late 70's early 80's. He built several hundred fins and that was his area to test. When he decided to curve the fin, we found turning ability that we had never seen.

As for the straight fin not turning as well. Not true....

Steve and Jack O'Donnell came to Indy with a Boss Boat that had a straight fin. It turned better than ANYTHING that we had ever seen. Was stiff as a board and didn't flex at all. Had a lot of tricks on it with a flat area on the bottom that was paralell with the bottom of the boat.

The real advantace of the curved fin was that you have most of the area of the fin as a straight section and only the curved part not 90 degrees to the running attitude.

That makes it much easier to turn left. With the Boss fin, any left turn and the boat was upside down. The boat climbed up on the straight fin very badly.

It has taken 20 years to understand the workings of the curved fin and be able to replicate the good turning of a well designed curved fin. I have seen maybe 10 boats in all these years that work super well in the turning area.

Close .......

In 1979 EVERYONE was running a flat turn fin that was set up vertical. It had to be a small fin in area so that the boat would slide in the turn or if the turn fin had enough area so that it would not slide, the boat would roll over in the turn.

I designed a flat fin with enough area that it would not slide and bent the turn fin just below the mounting holes at an 18 degree angle with the bend parallel to the bottom of the fin. I worked so well that it cracked the boat enough that the front bulkhead of the radio box popped aloose. Reinstalled the bulkhead and reinforced the area (this was a 45 crapshooter) took it to the NAMBA Nationals in Monterey and won the class with 4 first place finishes and fast time of the class with bone stock engines.

In 1980, I designed the BOSS boat and in 1981, Jack, Steve Odonnell and myself built the 60 boat that Marty was talking about using the flat/angled fin. In the beginning we had problems turning left but later we widened the sponsons out and that fixed this problem. To run the flat/angled fin the boat hull, fin and the sponson brackets had to be very stiff.

The curve fin came into being because the hull, thin turn fin material and the small sponson brackets would flex and the straight/angled fin would straighten out and the fin would "Unload". A well designed curved fin compensates for this with the curl at the bottom of the fin because as the fin straightens out, there is still enough curl left so that the fin does not turn aloose.

I am still using a similar design to the flat /angled fin today, only I use a completely flat turn fin (made of tool steel) and mount it at an about 18 degree angle to the sponson (this angle will vary as to the size and weight of the boat). When the turn fin is done this way, there is no setup for the turn fin, just mount it and go.

Just another way of accomplishing the same results.

That's actually a pretty good explanation when coupled with Ian's diagram (https://www.intlwaters.com/gallery/displayimage.php?pos=-16577).

I cut up a plastic cup to try and model it practically. It looks like when the deflection occurs in the turn when the fin is loaded that the curved element tends to "unwrap" in a way that gives a downward component of force. This also explains why the fin is swept as this motion is exaggerated when the fin is swept more.

good stuff...thanks,

 

[Charles Perdue]

I'm curious to know the history of how the turn fin got to be curved and the thoughts/reasons behind it.

when I look a the outrigger FBD.......there only appears a need for a vertical component.

Thanks for any knowledge you can share,

In the same vein I would like to know why the turn fins are swept back, particularly the trailing edge. Being supported and bolted nearer to the front of the fin while being loaded nearer to the rear means that the fin will twist under load:

http://gallery.intlw....php?pos=-16577

http://gallery.intlw...album=241&pos=7

The twist means that the hull (and fin of course) will need to run at a greater slip angle thru the turns to generate the required cornering force.

Ian.

Hello Ian, work your computer magic on a parallelagram shaped flat turn, about 2 inches wide, mounted at a 18 degree angle and swept back at 30 degrees. No curves in front or rear edges. It would be interesting to see how the loads compare to the curled turn fin in your other photo.

Thanks, Charles

 

[Ian Inverarity]

Charles,

Here you go:

https://www.intlwaters.com/gallery/displayimage.php?pos=-16586

This is a view looking from directly behind the fin.

For deflection the result is actually worse, but this test doesnt simulate your fin. I used the same forces as for the previous tests, same support geometry, same material and same thickness. 1.6mm thick aluminium. What this test shows is that comparing shape only the curved fin resists bending a bit better. But you have said your fins are made of steel, which is much stiffer than aluminium. Also thickness is a major factor, and so is the support for the fin. In the test case the support for the fin is quite a bit above the water line, yours may not be. To properly simulate your fin I would need more detail on its geometry, mounting and material. That in itself may not tell you anything very useful that this test doesnt already show, that is the turn fin will bend outwards under load and because of the sweep back it will twist as well. And the flat shape deflects more than the curved style fins. This doesnt mean they dont work!

 

[Charles Perdue]

Charles,

Here you go:

http://gallery.intlw....php?pos=-16586

This is a view looking from directly behind the fin.

For deflection the result is actually worse, but this test doesnt simulate your fin. I used the same forces as for the previous tests, same support geometry, same material and same thickness. 1.6mm thick aluminium. What this test shows is that comparing shape only the curved fin resists bending a bit better. But you have said your fins are made of steel, which is much stiffer than aluminium. Also thickness is a major factor, and so is the support for the fin. In the test case the support for the fin is quite a bit above the water line, yours may not be. To properly simulate your fin I would need more detail on its geometry, mounting and material. That in itself may not tell you anything very useful that this test doesnt already show, that is the turn fin will bend outwards under load and because of the sweep back it will twist as well. And the flat shape deflects more than the curved style fins. This doesnt mean they dont work!

Thanks Ian, do you have a side view as well?

 

[Ian Inverarity]

Charles,

https://www.intlwaters.com/gallery/displayimage.php?pos=-16587

Ian.

 

[Charles Perdue]

Charles,

http://gallery.intlw....php?pos=-16587

Ian.

Thanks Mr. Ian, you are a Gentleman, Scholar and a Judge of Fine Cigars.

The charastics that I was looking for do not show up in the computer rendering. The flat turn fin has a much larger "Sweet Spot" as far as the setup and being sensitive to the CG goes, than the curled turn fin.

Thanks, Charles

 

[Marty Davis]

I would make this suggestion about how to make the turn fin rigid.

Get a piece of 1/8" thick carbon plate and make a doubler on the turn fin (OUTSIDE). Don't make the doubler on the inside as that will deflect the water. You want the inside smooth. Once I went to the 1/8" Carbon doubler my turning was helped a ton. I use 1/8" carbon on all boat sizes. It is sufficient even for twins and doesn't weigh hardly anything.

 

[mr_fons]

Where can a person get some of this 1/8 inch carbon plate?

 

[Chris Wood]

www.mcmaster.com

page # 3597

part #8194K56

 

[153stars]

I would make this suggestion about how to make the turn fin rigid.

Get a piece of 1/8" thick carbon plate and make a doubler on the turn fin (OUTSIDE). Don't make the doubler on the inside as that will deflect the water. You want the inside smooth. Once I went to the 1/8" Carbon doubler my turning was helped a ton. I use 1/8" carbon on all boat sizes. It is sufficient even for twins and doesn't weigh hardly anything.

How far down should doubler go ?

 

[Andy Brown]

I would make this suggestion about how to make the turn fin rigid.

Get a piece of 1/8" thick carbon plate and make a doubler on the turn fin (OUTSIDE). Don't make the doubler on the inside as that will deflect the water. You want the inside smooth. Once I went to the 1/8" Carbon doubler my turning was helped a ton. I use 1/8" carbon on all boat sizes. It is sufficient even for twins and doesn't weigh hardly anything.

How far down should doubler go ?

To the waters surface.

We sharpen the full length of the bottom edge on the outside so that it will not have much chance to create lift while running in a straight line.

 

[Norm Doerr]

Where can a person get some of this 1/8 inch carbon plate?

Aeromarine Composites has a "scrap piece area" on thier site. Just Google and you will find.

If you are using a bank saw to cut it, get yourself a couple of cheap blades to use. Carbon will wipe out a blade very fast.

Norm Doerr

 

[Marty Davis]

Where can a person get some of this 1/8 inch carbon plate?

Aeromarine Composites has a "scrap piece area" on thier site. Just Google and you will find.

If you are using a bank saw to cut it, get yourself a couple of cheap blades to use. Carbon will wipe out a blade very fast.

Norm Doerr

Aerospace Composites.

 

[Norm Doerr]

Where can a person get some of this 1/8 inch carbon plate?

Aeromarine Composites has a "scrap piece area" on thier site. Just Google and you will find.

If you are using a bank saw to cut it, get yourself a couple of cheap blades to use. Carbon will wipe out a blade very fast.

Norm Doerr

Aerospace Composites.

Yeah that's it.. http://www.acpsales.com/

No clue how I came up with Aeromarine..

 

[Marty Davis]

Where can a person get some of this 1/8 inch carbon plate?

Aeromarine Composites has a "scrap piece area" on thier site. Just Google and you will find.

If you are using a bank saw to cut it, get yourself a couple of cheap blades to use. Carbon will wipe out a blade very fast.

Norm Doerr

Actually it is http://www.acp-composites.com/acp-cat.htm Not sure if this is the same company? Might be. Looks like it IS the same company with a new website. First time I have seen their new website.

Aerospace Composites.

Yeah that's it.. http://www.acpsales.com/

No clue how I came up with Aeromarine..