VPP with measured Drag

I've used this method to develop an empirical drag v. speed function for a car. It takes a fair number of data points to get a good result. If you can automate data collection, perhaps using onboard instrumentation, it'll make your life a lot easier. Here's the basic method.

1. Do a "coast down" from the highest speed you can reasonably achieve (or the maximum speed you want your model to work under, whichever is lower). Obviously, coasting down to zero speed is going to be time consuming, but it's also not necessary. I would guess that coasting down to 1 knot or so will give you good results. I might even cut it off at 2 knots. Once you get slow enough the effects of wind and current will begin to dominate your results even with otherwise good data.
2. Measure time and speed at as many points throughout the coast-down as you can manage.
3. Plot each run and fit a curve to it. Excel is a great tool for this because it allows you to fit an arbitrary polynomial to a set of data. In your case, I think the polynomial you'll want to fit is f(x) = a*x^2+b*x^4, where x is speed through the water, a and b are constants, and f(x) is drag force. But more knowledgeable people on this forum can provide more insight into that than I can. Note that Excel's curve fit function minimizes R^2, not sum of squares of residuals . That may or may not matter to you.
4. Repeat steps 1 through 3 for as many runs as you can practically manage. Record each run's heading along with ambient conditions, especially wind speed and direction and current speed and direction. (Your life will obviously be much easier if you can do this somewhere were current is negligible.)
5. Calculate the function coefficients for each run (step 3) and then average them.

You'll need to find a way to compensate for wind speed. I'm not sure what would be the best way to do that. It was simple with the car because we had only one fluid medium. We could just add wind component to vehicle speed to get effective air speed and, since a car's drag curve is dominated by air speed (with only small and relatively constant components from other sources) that was accurate enough. You'll face a bigger problem compensating for wind because you're trying to measure hydraulic drag only. Depending on the degree of accuracy you want this may not be a major problem if you can test in calm conditions.

 

Nonsense, I've towed lots of boats bigger than 51 feet, easy.
What "speeds" are you talking about?

 

The faster the better. Right now I need speeds from 11-15 kts.

 

I suppose driving a big stake into the bottom in the Bay of Fundy and tying your boat to it is right out. That would, at least on some days, eliminate air drag. ;-)
When coasting down, you'd need to get the props out of the water, have them fold, or carefully characterize their drag somehow.

Is there a guide to TLA's* somewhere? I haven't cracked open a PNA** this millennium. I'll admit to not knowing what VPP means. I figure it's not variable pitch propeller. Mostly, I think about aircraft, and mostly just for fun.

*Three letter acronym
**Principles of Naval Architecture

 

VPP = Velocity Prediction Program

 

Thanks!

 

Used to estimate potential sail boat speed for determining racing ratings and handicaps. Also used as a design tool.

 

At a yacht club my friend used to race at, they sometimes estimated racing handicaps by whether then boat looked funny. Or maybe it was by the amount of stainless steel hardware. Then they got mad when he sometimes beat the gold platers on handicap. Maybe a good VPP would have been better, but I think it would have required a really good naval architect and quite a bit of time.

 

No, not a no-brainer at all!
You will have to build a rig to tow it with the leeway angle (5°-10°) that the hull/keel would require to offset the leeway force from the sail.