Ice is the solution

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sailquik said..
Yes. Who did that and where are the findings?
Oh hang on. This is not the British surfboard study is it?

Delft towing tank, 1983. Not done by me, but I was present as observer/assistant. 1/2 scale boards and fins due to size/speed limitations of the towing tank.

One interesting result was showing ventilation (or possibly cavitation) of the tip vortex, visible in the photo on the nearest fin in the photo I posted. Tip vortex cavitation can occur at high speeds, but you would need to use a cavitation tunnel to determine which it was.

There were a couple of other interesting results, but I'll try to contact the researcher and see if i can get a copy of the original report to back up my recollections.

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Pacey said..

sailquik said..
Yes. Who did that and where are the findings?
Oh hang on. This is not the British surfboard study is it?

Delft towing tank, 1983. Not done by me, but I was present as observer/assistant. 1/2 scale boards and fins due to size/speed limitations of the towing tank.

One interesting result was showing ventilation (or possibly cavitation) of the tip vortex, visible in the photo on the nearest fin in the photo I posted. Tip vortex cavitation can occur at high speeds, but you would need to use a cavitation tunnel to determine which it was.

There were a couple of other interesting results, but I'll try to contact the researcher and see if i can get a copy of the original report to back up my recollections.

Don't know how many times we've puzzled over the drag of fin and the drag of the board on this forum. No one has yet given us a real number. Maybe we'd guess the board drag is somewhere between 1 and 10, and the fin drag between 0.5 and 3kg? Despite all the implied expertise we've basically got no idea.

Bring it on. Having a definite figure for drag would solve a whole lot of unknowns for those of us who doodle on beer coasters.

32.5 lbs is the answer for drag.....according to Jim Drake and not the Hitchiker's Guide!

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Roo said..
32.5 lbs is the answer for drag.....according to Jim Drake and not the Hitchiker's Guide!

Sounds like the total drag? i.e.. Hull + fin + rig + rider. Did he partition them up?

Drag is relative to speed and AoA... so any single number is completely meaningless. I think 42 is a better number.

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mathew said..
Drag is relative to speed and AoA... so any single number is completely meaningless. I think 42 is a better number.

Well I suppose we could assume the speed is a typical windsurfing speed of 27 knots and the the AoA is that of the optimally-sized fin to produce Roo's 68 lbs of side force with minimal drag at 27 knots. But yes doesn't give us much to play with. Might as well go with 42 lbs, easy to remember and close enough to Drake's figure of 32.5.

Guess we were never meant to know too much about this black art.

That's it Ian, you found it. Jim's analysis of windsurfing, best one I've found to date. Sadly he never broke down the drag figures into individual components before he passed away. It's a good reference to establish a base point for calculations and testing. Have built the AFTTR to withstand 200 pounds of force when testing flex/twist of fins and foils.

Almost finished 3d printing the test components for the lift/drag experiments, just waiting for a weather window to start testing, still a bit chilly here!

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sailquik said..
How could we test board drag?
Tow a board and sailor behind a speedboat at 40 knots with a rope that includes a strain scale?
How do we take into account air drag of the sailor? Stand a person in a wind tunnel?
Just to be clear. I am volunteering - to drive the boat!

No need but it may be more fun with a force gauge and a wakeboarder on a speed board.
All you need to do is measure the resulting force forward generated by the sail and the drag will equal it at constant speed.

Here's the little buggers, the first batch of 3D Roodents for testing. Had to develop a complex algorithm to shape and scale them for differing chord-lengths. Modeled in Solidworks before printing on a Prusa. If testing is encouraging will build some molds to make them from more flexible materials.

Cool looking things where do yhey go and what do they do and how do they work?

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Pacey said..

sailquik said..
Yes. Who did that and where are the findings?
Oh hang on. This is not the British surfboard study is it?

Delft towing tank, 1983. Not done by me, but I was present as observer/assistant. 1/2 scale boards and fins due to size/speed limitations of the towing tank.

One interesting result was showing ventilation (or possibly cavitation) of the tip vortex, visible in the photo on the nearest fin in the photo I posted. Tip vortex cavitation can occur at high speeds, but you would need to use a cavitation tunnel to determine which it was.

There were a couple of other interesting results, but I'll try to contact the researcher and see if i can get a copy of the original report to back up my recollections.

I've got a copy of the original report now, but I see I can't post PDFs to this forum. Anyone who wants a copy, email me at optimar at me dot com and I'll send you the PDF files.

A bit of background, this research was done by Pjotr van Schothurst, a student at Delft University in Holland in 1983, I assume as his final year project for his Naval Architecture degree. So the good news is that the report is quite comprehensive, covering three different board designs and 4 different fin configurations, bad news is that it is in Dutch.

It was done in the Delft towing tank, and more than 200 runs were performed, so a lot of good data.


I was responsible for delivering the 3 test boards, 1/2 scale models shaped by Tris Cokes of Limited Edition in Cornwall, to Pjotr in Holland, as I was doing some speed sailing contests in the area at the time. I got to help out for a couple of days and observe all the testing so I know it was good quality work.

My Dutch is up2speed
I'll send you a mail