# Need help calculating headwind drag in watts from force in newtons

Discussion in 'Hydrodynamics and Aerodynamics' started by Jacques_clue_no, Jul 20, 2012.

1. Joined: Oct 2008
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Selecting a prop is driven by only 3 factors

Speed of water in way of the prop, power delivered to the prop and the shaft rpm. That's it, no magic.

Thus you only select for one position, not several. Unless a CPP. You wouldn't expect the same efficiencies of a prop at 3000 rpm as you would at 4000 rpm nor other characteristics. You would not change the prop diameter going from 3000rpm to 4000rpm as suggested; despite protestations to the contrary. The prop has been designed and fixed at a given point on the power curve; all other variables have been taken into account and a fixed point has been selected for the design; of the 3 above. It may not even be at MCR, for a variation of reason, but whatever is selected, it is what it is and cannot be changed just because the rpm/power is lower. All that exercise of what if's is done during the design phase (changing P/D or Diameter etc), not when on-board with the prop buzzing about!

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### FMSSenior Member

I thought the original point was regarding the what if's during the design/prop selection, not prop changes underway. When a prop is selected based on efficiency at one RPM only and ignoring the rest of the torque curve, using an online prop calculator for example, sometimes people find they have to choose another because of the torque available at a different RPM. A prop can be optimal for best cruise, fastest top end, or best hole shot, or the best compromise.

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3. Joined: Oct 2008
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What is the point of selecting a prop to be used when not under way...is the boat it is attached to not moving??

You have to decide, are you selecting a prop for max speed, or whatever...only one "point" can be selected. This is achieved, taking other variables of the design into account, during the design phase and the deisgn fixed around that "point". It is then what it is...good bad or otherwise. But so long as the prop selected archives the goal or speed, or other, then so what if it is not "optimal" at other parameters?

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### dustmanJunior Member

As far as I can gather, one newton meter per second(newton) basically equals one watt.

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