Resistance

[Bertil]

I have a 12m sailboat with a Volvodiesel and saildrive with a 2-blades folding prop. I calculate the resistance using the frontal area of the saildrive and get 341Newton when the boatspeed is 3,9 m/s. It seems very much. Is my calculation OK?
I have seen on ABM Amro 1 and other boats that they fold in the prop and shaft and then close a hatch. Do someone know how this is done?

[GuestR01312011]

retractable is way better but way more expensive complicated. What flow speed did you use for the resistance calculation? the boat may be travelling at 3.9m/s but the flow of advance into the prop will almost definatley be different, and there is different speeds at different parts of the blades when they are at different positions due to the flow comming off the hull differently at various positions.

[Bertil]

My calculation is when sailing and the prop is folded in. I calculate only with the frontal area of the drive. My calculation: 0,5* 1020(dens of water) *speed *speed *0,04 (frontal area) = 310 N. Is that right? It seems much! The rest up to 341 N is wet surface resistance.

[Olav]

Bertil,

it seems you forgot to include a drag coefficient (which is around CD = 0.06 for folding propellers).

R = rho * 0.5 * v² * A * CD

Applied to your case this would lead to R = 18.6N + friction resistance.