Outrigger Turn Fins

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,

Archie Stanton said:

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,

Click to expand...

history, i have no idea...reason, to hold the rf sponson down on the water in a turn, keeping the whole fin in the water, and keeping the boat right side up :lol:

I appreciate the reply......

but I'm thinking if you exaggerate the plane( X-Y-Z) of the boat beyond normal....the turn fin in a curved configuration makes the attitude worse.

if you always did what you were taught, then you always get what you got...... right?

Archie Stanton said:

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,

Click to expand...

Kevin,

Draw up the Free Body Diagram looking at the turning outrigger from behind. The turn fin is providing the radial acceleration below the water surface while the centre of gravity of the boat is above the water line, this difference in height creates a roll moment that wants to roll the boat 'outwards' in a turn. The outside sponson resists the extra load on it so the inside sponson is lifted by the roll moment.

The slip angle that a hooked turn fin (or even a flat plane but angled over fin) runs in the turn causes water to climb up the inside face of the fin. Causing the water to climb creates a downward force on the fin to counteract the roll moment of the boat.

Ian.

Archie Stanton said:

I appreciate the reply......

but I'm thinking if you exaggerate the plane( X-Y-Z) of the boat beyond normal....the turn fin in a curved configuration makes the attitude worse.

if you always did what you were taught, then you always get what you got...... right?

Click to expand...

yes, but try a straight fin and see if you can corner as fast as a curved one, its not even close...not really sure what the other options are here..

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

mtruex said:

Archie Stanton said:

I appreciate the reply......

but I'm thinking if you exaggerate the plane( X-Y-Z) of the boat beyond normal....the turn fin in a curved configuration makes the attitude worse.

if you always did what you were taught, then you always get what you got...... right?

Click to expand...

yes, but try a straight fin and see if you can corner as fast as a curved one, its not even close...not really sure what the other options are here..

Click to expand...

Good point........

I'm thinking about trying the vertical fin only

vertical fin of G-10/ FR4 integrated into the structure of the sponson.....should be pretty stiff . I'm guessing that the horizontal component of the curved fin is a hindrance rather than a asset.

worth a try dont you think?

Archie Stanton said:

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

Click to expand...

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:

https://www.intlwaters.com/gallery/displayimage.php?album=1092&pos=30

Ian.

nitrocrazed said:

Archie Stanton said:

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

Click to expand...

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.

Click to expand...

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,

Archie Stanton said:

nitrocrazed said:

Archie Stanton said:

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

Click to expand...

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.

Click to expand...

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,

Click to expand...

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..

Kevin,

Another pic of the same boat:

https://www.intlwaters.com/gallery/displayimage.php?album=1092&pos=29

Clearly in this shot the outside sponson is doing very little to counteract the roll moment! (Hey Wild Bill, I am not having a shot at your boat, it is just one I have good pics of! B) )

The roll moment tries to push the outer sponson down, this is resisted by that sponson creating more lift as more running surface is pushed into the water. So effectively the outside sponson becomes a pivot with the moment resisted by the mass of the boat acting at the CG, which is of course a fair way from the outside sponson. A turn fin with hook also counteracts the roll moment, without this it is only the mass of the boat! So you could also try making the boat heavier! The cornering forces are quite high however, I have previously calculated over 5G for a little over 60mph. This means that roll moment can be significant as well.

I will be interested in you test results.

Personally I look into the stiffness of the sponson boom, the boom mountings into the tub and sponsons, the turn fin stiffness and turn fin mounting stiffness.

Martin,

The sponson boom twisting means that the turn fin sponson runs a higher angle of attack in the turn, counteracting the downward force from the turn fin, so fighting it, which creates un-needed drag. Also the twist suddenly stops when the load comes off the turn fin, I have seen this cause boats to blow off the water exiting the turns as the outside sponson angle of attack suddenly increases.

Archie Stanton said:

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,

Click to expand...

i think marty and the indy group may have brought us the curved turn fin. try everything else,then you will come back to a curved one

Archie Stanton said:

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,

Click to expand...

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:

https://www.intlwaters.com/gallery/displayimage.php?pos=-16577

https://www.intlwaters.com/gallery/displayimage.php?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.

Archie Stanton said:

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

Click to expand...

It started being tested 30 + years ago .....straight fins don't work well ..curve ones work better . Get a fast hydro and prove it to yourself .

nitrocrazed said:

Archie Stanton said:

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,

Click to expand...

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:

https://www.intlwaters.com/gallery/displayimage.php?pos=-16577

https://www.intlwaters.com/gallery/displayimage.php?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.

Click to expand...

VERY interesting Ian B)

typing into this a little, as... there way bigger minds at work....... building the FF 45 boats a few back, with angles of 0, and 3 degrees respectively to aid the fin/turn relationship was inherent to the build......

With a fast 21 boat doing 7 G's in a turn, i'd state, that the stated roll over force would/may be counteracted with a way wider off side sponson, and angle degree built in.... how wide?.... the factors as the faster the boat goes the more g's/lift out, in the turn....... surely this would take a serious amount of research, as finding the balance of the static weight of the boat, and computing the forces on the spoon in the turn would be so variable..... somebody, has to have tried this, but i imagine the off side sponson would have to be so much more bouyant, you'd lose what you gain per weight and overall water friction effects running it?...... somebody, and i'd bet Stu knows, has a bunch of left sides around that this was a point of testing.........

Kev, get with Chris, surely, as, he has done extensive testing/engineering on his fins, to the point of a can full of attempts......... whatever he and his dad had done works to perfection, and surely they built it that way, purposefully..... in a nutshell, the blackbird carves the turns like glass, and releases on request without undue/unseen effects anywhere....... something is into this..... and while chris' sweat has tested it to absolution he knows what it takes.......

I had, an old school scaled down Dave Frank sport 20, that ran the straight fin, but had inherent angle 17 degrees into the mount..... granted, and for lack of building 10 of them, they'd drift in the turn the faster they went..... upon installation, of a late model fin with bends, and mount @ 90, no more drift....... theory with the bends holding the water, but i didnt have enough speed, nor G's to get close to pulling/lifting the off side out on that one........

There's guys out there that have this information..... as much as i'd like to build prototypes, and cant, there's always a new pinnacle, it never hurts to think....... mike

If the turn fin is not swept rearward it will begin to "flutter" or vibrate side to side...

True, twisting and flexing especially down the straight is bad.

Creases and curves strengthen the Material being used.

I do know a properly set up turn fin will act like a pivot and whip your boat around a corner like its on a tether.

I do it the easy way " I do what John Finch and Kentley Porter tell me to do"

I do love it when people try new approaches to age old designs.

anthony_marquart said:

If the turn fin is not swept rearward it will begin to "flutter" or vibrate side to side...

Click to expand...

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.

MartyDavis said:

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.

Click to expand...

Marty ...same boat ...Straight fin VS Curved fin ....the curved fin wins . As far as not being able to turn left at all in a race ....that would really suck ! Being able to turn left some with a little blip in the throttle can't be avoided unfortunately .