thruster fin physics

[surfersteve]

I remain intrigued about the physics behind thruster fins. Over the past few years I have told that thrusters do all of the following: 1) make a board looser, 2) provide extra grip, 3) increase lift because of more total fin area, and 4) affect the pressure on the center fin and its drive. It seems hard to believe all of these things are true. Any physicists out there who wish to pontificate?

[isobars]

Physics, schmysics. Go try a multifinned board (thrusters if that's your specific objective) in frightening conditions, pushing it way beyond your normal control limits in very rough terrain at high lateral loads (extreme slashing, knee-buckling high-g jibes, landing jumps sideways sheeted in, ripping high upwind in heavy chop, etc.. IF it's well-designed and everything is well-integrated, your limits will expand. If not, or after years of experience pushing your limits in this manner, you will see little difference ... primarily at the very edges of your control envelope ... and won't need the thrusters unless you start sailing on that edge all the time.

I can think of no reason we'd need to run greater overall fin area with thrusters, because thrusters let us run smaller center fins. On a single-fin board I'd need at least a 9.5" fin with my 6.2; On my 95L thrustered board 8", maybe even less, is fine. On my 75L thrustered board with sails of 4.2 or less, I do fine with the stock 16 cm/6.25" Fin. That fin would be a joke without the extra area provided from the thrusters.

Sure, it ultimately boils down to physics, but even with a simple waterstart, the variables are so numerous and inconsistent that experimentation and feel are much more useful and practical than physics.

[hilton08]

Here is what I have learned from sailing tri-fins in the Gorge.

1. The ability to use a smaller main fin does loosen up the board and get rid of many of the control problems (i.e. tailwalking) when overpowered.
I like an 18-20cm main fin for 3.7-5.2 sails. Any smaller and you start to loose the ability to push on the fins as on a single fin board.

2. Having the extra fins near the rail does increase the grip in turns, especially when carving back up the swell (bottom turn?) against the wave. I like smaller 9-10cm side fins to minimize the drag. Any bigger and I do notice the board getting slower.

3. The tri-fin setup does seem to plane and go upwind at least as well as a single wave fin would, but is still not as efficient as a larger single freewave or freeride fin.

4. This one is harder to explain, but the tri fin does seem to maintain it's speed better when turning and surfing swell. The single fin would often get bogged down and loose its speed in the trough of the swell, but the tri fin seems to generate a bit of extra speed from each turn. It's almost like you are pumping the fins (like a sail) for extra power when you turn the board.

[isobars]

Well-put, and without any Reynolds' numbers or Euler equations.

[WMP]

Hi Steve,

As a former tri-fin OO enthusiast for 10 years, I am now a devoted single fin OO enthusiast for more than 5 years. Main reason for the change is the tendency for the board to pearl with thrusters installed when getting to the bottom of a swell. OO thrusters have a way of bringing the nose down, at least the ones that are angled like on the OO. Nose gets burried too easily was my experience. Don't know if that's the case with other boards.

I honestly feel that my board is much looser without the thrusters, greater sense of freedom. Speed is about the same, no desire to go any faster. Grip has not changed enough to complain about. OP conditions are no problem, in fact I find more control with a single fin since it's easier to keep the nose out of the water.

Of course, I'm old schoolin' it..... so your experience could be much different?

[kmf]

Old school Open Oceans have really narrow pointy noses, with the mast track way forward of where industry standard is now focused. These design features result in no volume up front where the mast is located. Thus if one puts on a lot of mast foot pressure along with stepping forward during transitions, it is easy to bury the nose. When I had my new OO made this spring, I made it 7'10" long 75 litre and put a 10" mast track in it, with the center of the track about 2" back from Brian orthodoxy. I find that if I run the mast base way toward the rear of the track, the board works fine and is even looser than the 8' 70ltr old school board that I copied the shape from. I also don't bury the nose as I could with the old school 8' board that I used as a template, as I have a lot more board volume up there to counter the submarine nose tendencies of the older board.
I do use a much smaller fin than my single fin boards, 8 and 7.75" fins for 4.7 down instead of a 9" fin that i would use in single fin mode. The board is much looser with the shorter fins, still points high and is tolerant of my heavy back foot

Brian uses fins that are foiled on the rail side of the fins, unlike say the flat fins on the Real wind boards. They are also canted by 20 (?) degrees from straight, and they still work fine.

KMF.

[isobars]

The pearling issue is board shape and layout, not fin count. OO's sensitivity to too much sail is the primary reason Brian has always posted a narrow sail size range for them and scoffed at his main competitor's claim of twice the sail range. When OO was claiming a sail range of about 1.0 to 1.5 sq meters (e.g., 3.5 to 4.7), Bonzers (5 fins) claimed more like 3.0 to 5.0. Brian laughed at that, but my head to head testing of the two marques not only verified the OO's narrow range, but extended the Bonzer's range beyond its claims. I had zero problem sailing Bonzers from hammered on a 2.8 to marginally powered (or hammered) on a 5.2. Les and Brian went round and round about it, while I just blasted off into the sunset on 2.8s to 5.2s all summer long.

I have never had a Bonzer (or my Naish tri-fins) pearl, not even diving dead downwind fully or over powered on any Gorge swell. OTOH, putting a slightly too-big sail on an OO (at modern fin box location) made it feel nose-heavy, inviting a pearl.

Again ... we can overanalyze, or we can go sailing. The first four letters in "analyze" tell one story, while TOW tells quite a different one.

[dllee]

Multi fin boards....
Short fins, quicker and less resistance rail to rail.
Some can pivot off the sidefins, like twin fin boards.
Overall sq inche's of total fins is usually more than single fins.
More fins, one or two might spin out, cavitate, or whatever, but one always holding in.
Speed is not an issue when picking multi fin boards.
Sideways resistance gives multi fins decent early planing.

[kmf]

My 75 litre OO easily does 3.4 to 4.7...I haven't tried anything bigger, or smaller. I have other boards for big sails. My friend with a 85 litre OO uses it from 3.0 to 5.2. It is his only gorge board for the most part. These are what we have been using this year. I don't know when Mike was testing boards.

Sorry for high jacking this thread Steve....I will shut up now.

KMF

[isobars]