Re: Outboard prop pitch for sailboat
Hello, I just went through this procedure.<br />I found this useful info for HpWLSpeed.<br />
http://homepages.apci.net/~michalak/page3.html <br /><br /><br />To get the best hp from your motor, small pitch and larger diameter would be used. You could calculate the desired pitch given the gearcase reduction, wot rpm. Use some 10 to 40% slip of the prop in the water( from some site I can't find again) but the larger diameter prop will have less slip. You know your max speed 7 kts so design for that.<br />I'll do the calc for(all made up numbers) 9 mph and 0% slip(theoretical best. increase pitch from here), propRPM/engineRPM=.5, WOT RPM=5000.<br />At max hp, your prop will turn at .5*5000=2500 rpm. This is equal to 2500/60=42 revs each second. At 9mph, you are going 9*5280ft/mile*12inch/foot=570240 inches each hour. 570240/3600sec/hr=158.4 is your design speed in inches each second. The pitch is the theoretical inches of travel per prop revolution, so 158.4/42=3.8 inch is the theoretical design pitch for a prop to push a boat at 9mph w no slip(of course there is slip) given 5000 rpm at wot and 2:1 gearratio.<br /><br />Since you probably have hp to spare to get your boat to speed, and probably dont want to run your motor at wot all day, you should design for the 3/4 rpm setting you desire. I would make the above calc to find the theor. pitch, multiply by 1.1 and get the closest prop to that in the largest diameter size available that will fit your motor. If only small diameter sizes were available I would get more pitch. I can't help you 100 % cause I dont know how much torque a prop of a given size can develop before it loses effectiveness, but obviously, larger diameter props of a given pitch can apply more engine torque. Be careful not to pick a prop that allows your engine to overrev. <br /><br />I'm no expert so take it with a grain of salt. Also, rechecking the math would be advisable.
