Join Date: Sep 2009
Location: Kingston Washington
Thanked 4 Times in 4 Posts
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Especially for auxiliaries, diesels suffer most from not being run for sustained periods at power. Having said that different diesels have service ratings. Generally the manufacturer can advise you on this and given the specs for your boat they can make a recommendation of the power best suited for you. Where the propeller comes in is matching its' performance curve with your engine's Shaft Horse Power curve (from .94 to .96 of BHP or Brake Horse Power which is what's normally in the specs.) performance, for sailboats the goal is to get the curves to match at normally 90% power. The prop's diameter is normally limited by the hull or skeg clearance from the blade tip. You need at least 15% of the blade's diameter for clearance. The next issue is blade pitch. Too much pitch and your engine won't get up to speed, too little and it will overspeed at full throttle. After pitch comes is blade area, too little area and the prop will gravitate at full power. So if your diameter is limited you may have to go from a two bladed prop to a three bladed prop to get sufficient blade area. Propeller manufacturers will calculate the right prop given you boat and engine specifications. Michigan Wheel is a good one for this.
A note: Some manufacturers show both BHP and ISO 8665 power curves. the ISO deals with controlled testing conditions which I don't know much about. Prop manufactures seem ot with using the BHP number for thier calculations.
Again David Gerr's book has all this info