Convinced that there is no reason that 90 degrees should be the optimum fin angle for starboard speed runs I built my first prototype. Waiting for wind.

It's canted 18 degrees. Under conditions where you may need 20kg of lateral lift, the fin will have to work a little harder and produce 21 kg of total lift. But there will be an associated 6 kg of vertical lift. That should be a win. 6 kg less work for the hull, and only 1 kg extra lift required of the fin. (Hard to believe, but check the sin and cos of 18 deg) And the fin is more efficient at producing lift - a double win.

So much for theory I'll let you know how it goes, I'm hoping it will feel pretty good on starboard and very sluggish on port. If I can't tell the difference .. I'll let you know anyway.

No one variable at a time, it is still upright and still aligned down the centre of the board. It was originally a Tekknosport 35 trim box. Used to go well in a Tiga 125.

yoyo said...

Interesting .

Fin lift is more than 2x more efficient than board lift. So should be less draggy but I can see the tail jumping out if you go too fast and get too much lift.

So, canted at 18 degrees may give 'controllable' lift at say, 30-35 knts
& then maybe 12 degrees for 35-40 knts
& the magical 7 degrees for 40+ knts

Someone may be able to do the maths - but trial & error might be more accurate.

Keep us posted

I remember years ago when Boogie was just getting started he made a really bendy fin to get much the same effect. I think he almost broke his ankles as the board tried to roll as it was getting so much vertical lift but only on the windward side of the board. Your tilted fin may have the same problem.

You're siding with the Bonominator Boogie? I'm too old to be a test pilot.

Fin lift increases with the square of velocity for a constant angle of attack but windsurfers do not sail with a constant angle of attack. The fin's connected to our feet. When sailing in a straight line we subconsciously vary angle of attack all the time to balance the lateral lift provided by the sail. The lift that the sail can generate is limited by the righting moment that the sailor can provide by hiking out. I came up with the 20kg approximation to fin lift by looking at the moments about an axis running lengthwise thru the board. If the board is kept level and the fin doesn't flex much, the vertical lift of the fin canted 18 deg will be in a fixed geometric ratio to the 20kg lateral lift. It shouldn't exceed 6 kg even as speed increases.

The lift provided by the hull is centred towards the front of the wetted surface. Taking 6 kg off the front and moving it back will alter the centre of hull lift by not much more than the shift you get by taking out a wider board. You just have to lean back a bit more, or stack on a few kilos. Look at the turning moments about a horizontal axis across the board's centre line.

I'm pretty sure i won't be launched by uncontrollable fin lift, whether we're all sailing flip flopping fins in 10 yrs time? that's a long shot.

20 knot NE forecast for Gerroa on monday, no waves forecast, hopefully give it a try then. Maybe I should duck tape a spare fin and screwdriver to the nose of the board - it's a long swim down there if something breaks.

Ian K said...

You're siding with the Bonominator Boogie? I'm too old to be a test pilot.

Fin lift increases with the square of velocity for a constant angle of attack but windsurfers do not sail with a constant angle of attack. The fin's connected to our feet. When sailing in a straight line we subconsciously vary angle of attack all the time to balance the lateral lift provided by the sail. The lift that the sail can generate is limited by the righting moment that the sailor can provide by hiking out. I came up with the 20kg approximation to fin lift by looking at the moments about an axis running lengthwise thru the board. If the board is kept level and the fin doesn't flex much, the vertical lift of the fin canted 18 deg will be in a fixed geometric ratio to the 20kg lateral lift. It shouldn't exceed 6 kg even as speed increases.

The lift provided by the hull is centred towards the front of the wetted surface. Taking 6 kg off the front and moving it back will alter the centre of hull lift by not much more than the shift you get by taking out a wider board. You just have to lean back a bit more, or stack on a few kilos. Look at the turning moments about a horizontal axis across the board's centre line.

I'm pretty sure i won't be launched by uncontrollable fin lift, whether we're all sailing flip flopping fins in 10 yrs time? that's a long shot.

20 knot NE forecast for Gerroa on monday, no waves forecast, hopefully give it a try then. Maybe I should duck tape a spare fin and screwdriver to the nose of the board - it's a long swim down there if something breaks.

Go forth guinea pig and conquer!

"20 knot NE forecast for Gerroa" got goosebumps going down my back. I'm there for Cup weekend + one week so can't wait to sail the old territory again.

Don't forget the on-board video Ian.

Icesurf (Michael H, NZ) has designed some really radical speed fins that are only 15cms deep. I 'd hate to steal his thunder (and the fact I have very little technical knowledge in respect to fluid dynamics) so I'll let him post if he wants to share.

Kudos for trying something radically different guys. A world without guinea pigs would be disappointing indeed (although somewhat not as smelly in our back yard)

Ian K said...

Convinced that there is no reason that 90 degrees should be the optimum fin angle for starboard speed runs I built my first prototype. Waiting for wind.

Someone please loan Ian a gopro because either way it's going to be good footage...

Finally got a bit of wind. Comfortably powered up on a 6.2 Hellcat, Sonic 90. Short, sharp Lake Illawarra chop. Did 30.5 knots on the canted fin and 30.05 knots on the control fin - (a 35 cm "Fins" brand reboxed to a 38 degree weedie.) That's probably about as fast as you could expect me to go in those choppy conditions no matter what I was on.

Interestingly you could probably put someone on it, without showing them the fin, and they probably wouldn't pick up on anything being particularly out of the ordinary, just like sailing someone else's board.

On the starboard tack it is a little prone to spinout in chop, but once you adjust it's fine. On the gust where i did the "top speed" I was bouncing off the chop, getting a bit of air, but putting it back in the water OK.

Going back upwind beating into the chop on the "wrong" side it was very solid, didn't look like spinning out. The chop angles were wrong to try bearing away. And I'd thought it may not even plane on this tack!

Easy to trim on both tacks, wet between the footstaps, just shows how adaptable we are.

Came back after a dozen or so runs and put in the weedie. Back to back you can notice how nice the decoupled feel of an upright fin is when picking a course thru chop. A bit like a RWD vs front wheel drive car. The weedie felt a few knots faster when I got a gust to bear away in but the GPS shows it wasn't.

It is easier to trim the upright fin on the tail... not much in it, maybe just used to upright fins.

So it is sailable. What's the theoretical gain of losing 6kg off the hull? That's probably only associated with a few percent of total drag. Halve that for potential increase in top speed. Probably lucky if there was an extra knot in it after all the appropriate tuning and getting fully comfortable with it.

If you've got plenty of wind and worried about spin out - sailing on the "dig in" side might aid control?

Conclusions. If you are reboxing a fin and don't get it exactly 90 degrees - don't worry about it, there's nothing in it.

Likewise if the fin is a little loose in the box.