maybe put the post in the GPS & speed section? Plenty of smart buggers there!

So you are seeking a rule of thumb that will enable you to estimate the planing threshold of a hull given the all up weight and sail and fin sizes?

That could be a universal index for all hulls couldn't it sort of like the mast stiffness index. Manufacturers could publish a planing index alongside the volume, width and length of each model.

The variables at play in real world threshold planing speed are all up weight, hull area and aspect ratio, fin size, sail size and wind strength. It seems all too complicated to get a quantitative calculation that returns the actual threshold planing speed of a particular setup, but an index would certainly offer a valid and simple means of comparing that quality in different boards. I suspect it wouldn't be much different from a straight forward comparison of widths though, but I'm just guessing.

DL said...

http://www.hydrocompinc.com/knowledge/whitepapers/HC113-MinPlaningSpeed.pdf

This paper suggests that a rule of thumb for calculating planing speed (for boats?) is:

velocity in knts = k * LCG / SQRT (B)

k = Constant 4 for feet, 7.2 for metres
LCG = Distance from logitudinal centre of gravity to transom
B = Beam

Not sure how applicable it is to windsurfing, as you can radically change your CofG. The formula indicates that putting all your weight close to your tail would decrease your min planing speed, but practically, we know that mast foot pressure is essential to getting onto the plane early.

It also seems to disregard mass.

Little maths help?

250cm board, 65cm wide so approx:
720 * 125 / 8 = 11,250 knots???

I don't think mass would affect it. More mass will just make it harder to get to that speed. It doesn't calculate how much energy you need, just speed.
(no, i'm not going into mass and speed, nerd)

edit: d'oh k is a constant 7.2, i've converted to cm.

so... 112 knots.

But in reference to the original question you will need less energy to get those pounds (kg) moving at the required speed to plane, so less wind, smaller sail, less cake etc., you still need to go, um, 112 knots to plane

nebbian said...
Beam is 65 cm, so 0.65m.

V = k * LCG / SQRT (B)
V = 7.2 * 1 / SQRT (0.65)
V = 7.2 / 0.8
V = 8 knots.

V = 7.2 * 1 / SQRT (0.65)
V = 7.2 / 0.08
V = 90 knots.

What the formula fails to address is hull speed ( en.wikipedia.org/wiki/Hull_speed ), which will make a longer narrower board get up to planing speed easier than a short wide board. Once planing, however, the wider board should stay planing longer in lulls.

I think!

Yeah, this is power required to get onto the plane, which is where you use the most fuel, be it wind or petrol. That is it is much harder to get on the plane than stay on it.

Anyway, the less pounds/foot2 the easier it is to get to planing speed, which is relative to shape not weight.

Now we just need a formula for technique.

nebbian said...

evlPanda said...

V = 7.2 * 1 / SQRT (0.65)
V = 7.2 / 0.08
V = 90 knots.

I think our calculators disagree

No, no, I be a complete idiot.
$duh = 0.8 * 0.8;

$evlPanda =