thruster rotating fins

General Discussion
1

been playing with a thruster RFS. The way it is set up now…

The rear fin is a standard fixed fin.

The two rail fins each rotate. At their maximum toe-in, they roughly match the toe-in angle of a normal thruster fin. But at rest their toe-in is parallel to the rear fin. So when you paddle in you have three fins aligned. When you turn both rail fins align with the flow - but subject to resistance to turning and end-limits.

Rotational resistance is also assymetric. They are forced to a normal toe-in with much resistance, but fold away in reverse toe-in quite easily. The purpose of this was to get rid of forces on the fin when it is the outside rail fin, just let it go with the flow. When it is the inside rail fin, however, make it stiff. If you’re confused - pushing the rear of the fin towards the rail is very very hard. Pushing it towards the center of the board is very very easy. But the side forces are independent - they do not fight each other.

And, honestly, its nice, so far. There is AFAICT one really noticeable benefit, and a few smaller ones. Start small…

The board paddles and planes more easily than it used to (this is a retro-fit thruster shortboard, a 6’4"). This I expected, and it didn’t blow me away.

The outside rail fin is a little freer on turns. But this hasn’t blown me away.

What has been a big effect is the board coming off the bottom. And I think I understand why. I used a fin with a longer chord length than standard thruster fins - a bigger fin fore-aft, but the same depth as normal fins. You couldn’t use this as easily in a fixed fin system because chord length is related to range in angle of attack. A fin too long fore-aft stalls at lower angles. This fin is definitely on the longer chord length end of the spectrum for thruster fins.

However, as the fin can rotate, it will not stall, and when all is said and done, you go into a bottom turn with 20-30% more power. You hit the bottom turn. The fin is fully rotated, with a normal toe in, and a much stronger than normal fin - a fin that would have substantial negatives in a fixed fin system. But in the rotating system it is just plain juice…

Still got a lot more testing to go, thought I’d share.

Also, it’s pretty small and light.

2

very cool…i think anything that would minimize stall in progressive surfing is a step in the right direction

some thoughts…

toein helps to lift the tail out of the water when paddling for waves so straight toein there would be undersirable - so maybe adjust the board dimensions a little…wider thicker tail.

at higher AOAs the fin adjusts accordingly and then…???..lose drive? Or is reduced stalling/drag make up for the reduced drive?

isnt Speeed fins doing the same thing?

you use one plug to secure the fin to the board? seems dicey…

where’s my set?

3

what about using a rsf in a bonzer set up as the trailer or having these rotating side fins on say a twin or a quad, what about having every fin on the board rotate?m this could be a big jump for progressive surfing.

4
Quote:

toein helps to lift the tail out of the water when paddling for waves so straight toein there would be undersirable - so maybe adjust the board dimensions a little…wider thicker tail.

The board I am working on now is a retro-fit.

It paddles and planes easier with the fins set straight.

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at higher AOAs the fin adjusts accordingly and then…???..lose drive? Or is reduced stalling/drag make up for the reduced drive?

isnt Speeed fins doing the same thing?

Speeed fins has flex fins. I have a flex box. The box allows me to set an upper limit on toe-in - so that the fin never exceeds a normal thruster toe in. You can stall in a turn just as easily. But the drag when you are going negative AOA (turning the other way) or when you are going straight is much less. Again, these were, I felt, the smaller effects.

Another point of consideration is that I can have the following advantages over flex fins.

  1. flex fins become convex on the high pressure side of the foil, and concave on the low pressure side

  2. flex fins, if they were to be set up as loose as my box, would be unstable. My flex box much more stable because the rotational forces only work in one hemi-plae of rotation each. So, there is preload in the middle. Even a little preload breaks up the tendency to oscillate.

  3. Flex fins need to design the flex into the planshape. I optimize the planshape and design the flex into the box.

A lot of little things that add up.

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you use one plug to secure the fin to the board? seems dicey…

Still working on that aspect. The plug may ultimately need to be enlarged a little. The 1.5 inch axle on the singles works really well with the fin to box bond. I downsized to a 1 inch axle for the thruster (and redesigned everything else to minimize size and weight). Its possible I’ll end up a little bigger, but only if the task demands it.

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where's my set?

Still making them using prototype machining techniques and getting the kinks out…no wine sold before its time.

5
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what about using a rsf in a bonzer set up as the trailer or having these rotating side fins on say a twin or a quad, what about having every fin on the board rotate?m this could be a big jump for progressive surfing.

I already make a single that rotates. I’ve offered several times to Duncan to send him a free sample for testing for this purpose, but they aren’t interested.

Other than that I’m going one step at a time…

6

I am interested. wanna hook a grom up?

7
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I am interested. wanna hook a grom up?

I sell the singles now, if interested email or call me (info at link below). The thrusters are still in R&D until I get the kinks out. I’m convinced it’s gonna be great, but not ready to send any out in their current state.

WRT free samples, I used to do those, there’s about a 90% chance it’ll never get used. And I find that unacceptable. I don’t want to send samples out to sit on people’s shelves.

8
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toein helps to lift the tail out of the water when paddling for waves so straight toein there would be undersirable - so maybe adjust the board dimensions a little…wider thicker tail.

…

hmmm just looking head on into the nose of an upright board sitting on the water with say exaggerated cant (the new FCS springs to my mind), the fins would be a bit like a wing with downwards pointing dihydral. Toe in would angle the wings down and give a downward force not lift.

9
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hmmm just looking head on into the nose of an upright board sitting on the water with say exaggerated cant (the new FCS springs to my mind), the fins would be a bit like a wing with downwards pointing dihydral. Toe in would angle the wings down and give a downward force not lift.

The standard reply (lift from rail fins when going straight) argues that the water flow under the board naturally flows from stringer to rail, so the inside of each fin is the high pressure side, and the water is forced down, generating lift.

However, this doesn’t hold with reality, because if you test the same board with straight and toe-d in fins, the board with straight fins paddles and planes easier.

I think the more relevant argument is that the water flow is different at the surface of the board and 3-4 inches down. Anything that keeps the water from flowing towards the rail will force it to follow the hull instead (and go down), and create lift. Fins with toe-in encourage the water flow towards the rail, and remove lift. Fins without toe-in encourage the water to follow the stringer line, and this also causes the water to flow down, and creates lift.

The best feeling rail fins, to me, have about 2 degrees of toe-in at the base and zero toe-in from 2.5 inches depth to the tip. I’ve now seen two different people who are both fin experts utilize this design (I think they came to the same conclusions independently), and I’ve ridden it. It works. I’m definitely going to test it out for my rotating fins, once I get the buggers I’m working on now done. Interesting that a million dollars of research cannot buy you what two innovators each working at home can find.

Anyone who is innovating in board design and not making similar efforts on fin design is certainly missing a lot of fun.

10
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Rotational resistance is also assymetric. They are forced to a normal toe-in with much resistance, but fold away in reverse toe-in quite easily. The purpose of this was to get rid of forces on the fin when it is the outside rail fin, just let it go with the flow. …

that makes sense to me, it was J Troy who pointed out to me that some of the stiffening effect of the thruster comes from the outside fin in a turn and increasing toe in on the outside fin loosens up the turn. I’ve found this to be true by playing with wedges which pack hacked future fins into my lokboxes. Trouble is a lot of toe in on the outside fin reduces drive when turning in the opposite direction ( ie when the heavily toed in fin becomes the inside fin). However your system overcomes that problem.

11

well we can theorize till were blue in the face…of all the fin adjustments one can make with standard fins today, toein has the most profound effect, by far. Now, I’ve tested the lift effect…during the same session I surfed my board with no front fins, small front fins (side bites) and regular sized fronts…the difference in tail lift is astounding…bigger fronts more tail lift. The lift is caused by good’ole fashion drag…lots of toein with zero cant and the drag will still lift the tail up…no front fins the board lags during the paddlein/drop but once up and riding it planes like butter…just try it.

12
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The standard reply (lift from rail fins when going straight) argues that the water flow under the board naturally flows from stringer to rail, so the inside of each fin is the high pressure side, and the water is forced down, generating lift.

hmmph, i hadn’t considered that Blakestah, a canted fin would have a positive angle of attack (not with leading edge, but base of attachment to board with water moving to rail. I remember your post on perspex streamered plate which you manually moved thru the water Meecrafty and you reported the stringer to rail water flow behaviour.

from

"http://www.swaylocks.com/forum/gforum.cgi?post=153928;search_string=tinkering%20stop;#153928"tinkering with magic

when I put sidebites in front of my twins in my 5 box board (which is extra toed fin area), paddling was noticeably harder (but I suppose this is drag not necessarily reduction or increase in lift). Board was noticeably slower when surfing this overfinned twinzer than in twin form altho more drive from rider effort. However for a moment it had me fooled on speed as it had the ability to hang higher and remain in the powerful pocket part of the wave. This hanging higher is i suppose a form of lift where sideslipping is reduced and tracking a hi line made easier, altho on my previous post i was thinking more in just terms of which would plane more readily on flat water eg. making a straight drop. Unfortunately I dont remember anything about about that nor did i get objective impression on wavecatching.

not sure if i’ve written anything useful or relevant here, but yes, agree, theorising is good, but testing is the ultimate fun

13

Hey Blakestah,

The System is coming along great.

Will you work the cant into the fin tab?

What kind of foil with the rail fins have?

Will you stay with the cool fin template that looks like an Orca’s pectoral fin for the rails and center fin?

Share the Stoke, Rich

14

…Hey Blakestah, in a fast skate type performing in the curl, what type of play develop these fins?-its toe in toe out, toe in toe out, etc?

please explain that

15
Quote:

Will you work the cant into the fin tab?

What kind of foil with the rail fins have?

Will you stay with the cool fin template that looks like an Orca’s pectoral fin for the rails and center fin?

Share the Stoke, Rich

Fins have been canted 5 degrees using plus-one-shaper’s casting method.

I’ve only made one set of fins so far, a second this week. Both are the planshape I’ve shown, the Orca, sized by rake and chord length for a thruster. It’s about 20-30% longer chord lengths, on average, than a G5, but with the longest chords 2-3 inches deep. They’re foiled flat on the inside, 20% WP on the outside, and the front edge is rounded with radius 25% of max thickness.

I haven’t made twist (in toe-in) fins yet, but its in the cards.

So far the center fin is standard thruster size and shape (a Futures carbon fin that was already in the board). If I use a fin as large as the Orca I will need to let it rotate, I think. I’m still not sure that will be desireable, I plan to dial in the boxes and fins first, and then try letting the rear fin rotate.

16
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...Hey Blakestah, in a fast skate type performing in the curl, what type of play develop these fins?-its toe in toe out, toe in toe out, etc?

please explain that

The fins don’t ever get wobbly. The main innovation in my design compared to the many prior rotating fins was the method of keeping the fin from getting wobbly. If you are tight in the curl and playing off one rail fin mainly, the fin will be soft when you are high and stiff and drivy when you push on it. The force of the water pressure is insanely high, the fin turns nearly instantaneously. If you go rail to rail the board feels like it flies through the transition, with both rail fins un-engaged.

17

blakestah, im really interested in trying these boxes in a traditional keel finned fish. after feeling what the single version does, i think it would make a double foiled keeled fish insane, it would probably allow tighter arcs with alot less loss of speed right? how much do you suppose these new boxes will be? the older bigger ones were too expensive to put in one board for me.

18

I’m on design first, business later.

I’d probably redesign the single fin plug for a twin, and use a single fin sized axle, but shrink the box down a lot. It would be in between in size, but should be strong enough to hold a twin fin. These little boxes I’m using now are a little small for that large a fin. But the goal was to be as small a box as FCS…not that far off comparing two FCS plugs to my one.

Cost, who knows…wait till I’m 100% happy with the prototype, then go from there.