Join Date: Dec 2003
Thanked 0 Times in 0 Posts
Rep Power: 14
The thought process makes sense.
For item one, the static measurement of reverse thrust from the engine should be straight forward. Both are static forces, i.e. the thrust from the propeller in reverse at 80% is a static anchor setting force, the wind load in assumed ideal conditions of a 30 knot wind for comparision purposes would be treated as a static load as well. And a direct comparison can be made. But, one thing that does not to me is the windage load in foot lbs. That is not what the anchor line will see in a loading situation. I would not think that the resultant force from the windage would not be a torque on the rope but a force.
On item two, I sort of disagree. While thrust appears to be the output to compare, I would almost say that the output to compare would be speed. This is the thought process that I went through as well when I reproped. The relationship of the hull to the water and the force it takes to move the boat varies with the speed of the boat. As well as the efficiency of the propeller varies at different speeds. Some of the things that I read term this slip, efficiency, what ever, you get the picture I lumped both variables together and assumed drag on the boat / propeller slip was one variable.
What I found when I was mapping my existing propeller I had 32% slip at 1000 rpm, up to 45% slip at 3600 rpm. By using this, then could I compare the various propeller options assuming that the slip model remained the same for the new propeller. I am limited to a 13/15HP, Yanmar 2gmf.
When I think of achieving max RPM at wide open throttle and your last example couple thoughs come to mind at least with limited HP, lets I have two pitch setting that achieve and slightly exceed Max RPM at WOT. Then the higher pitch would be the best for conditions with no wind or current (could run the motor at a lower rpm to achieve the speed required for maxium speed to fuel usage), but against a strong wind and heavy seas the lower pitch would be more be most effective, since at the higher pitch maximum rpm would not be able to be achieved. Overall I tend to agree with the maximum pitch setting that achieves slightly greater than WOT. A rough sea will increase the slip or the thrust required.
I hope this made sense, I have five interuptions while I was typing.
Feel free to email me.
C&C 32, Hull #237
Mt. Hope Bay