how do you work out sail area to bhp?

[woody001]

how do you work out sail area to bhp?

say with a constant wind of 30 knots on 800meters sq of sail.

i now you can do it but have forgotten. and yes, of course it's going to be rough!

woody.

 

[woody001]

or how about torque?

 

[woody001]

or "thrust"?

 

[LadyInBed]

or hot air /forums/images/graemlins/smile.gif

one hell of a lot of sail for that amount of wind!!!

 

[Lakesailor]

Why?

 

[woody001]

because iam doing a spreadsheet, that includes yacht stability, and thought it would be nice to "have".
Not really needed, but i did know how to do it.

Ps. shall i upload this spreadsheet to show u what iam doing?

 

[Twister_Ken]

Once tried to do this off watch on a long boring race in a 57' that was doing about 11kts. Worked it out with lots of guesstimates as about 150 hp, but happy to accept that I could have been out by a factor of quite a lot. As I recall it involved taking the displacement of the boat, working out how many seconds it took to travel a boat length, then working out a number for the weight of water shifted per second, which was converted into foot/lbs and then converted into hp, with an allowance for smacking waves out of the way, and an allowance for air friction.

 

[alec]

I believe there is a rule of thumb on sail areas :-

1hp=50sq ft of sail area in a force 4.


Have no idea how it's arrived at.

 

[DJE]

It's going to need all sorts of assumptions and be pretty meaningless I fear.

Power is tractive force x speed. You could get a tractive force from wind speed, sail area, and a guess at the lift and drag coefficients of the sails; but what boat speed do you multiply it by? This will depend on the resistance offered by the water to the hull which will obviously vary from boat to boat.

If you fix your 800 sq. m. to the dock it doesn't develop any power at all because it's not moving. Thrust yes. Power no. The sleeker the boat you attach it to faster it will move and the more theoretical power it will develop. I think this is why jet engines are rated in thrust and not horsepower.

 

[Sgeir]

[ QUOTE ]
shall i upload this spreadsheet to show u what iam doing?

[/ QUOTE ]Yes, please!

 

[capt_courageous]

As DJE says it is not that easy. However to get a rough idea use your engine to go at max. hull speed assuming you are not a lot overpowered. Then try to get to max hull speed when sailing without being overpressed. Sail power = engine power near enough.