removehtml]Has anyone ever used two props on a single shaft? A while ago I had read about a shaft extension that would allow a larger prop, but never using two. Without technical examination of horsepower, torque, ect. has there been any research? I've seen some power boats with a screw look rather than the typical fan blades. A second question is what determines the distance between the tip of the blade and the bottom of the hull? There is probably some hard and fast rule for optimum propulsion size, any suggestions?Error: Error: expected [/URL], but found [/removehtml] instead[/removehtml]
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Power boat prop
Some power boats have a duo prop. It's a two piece device where one section is right handed, the other left. It's supposed to reduce or eliminate prop walk giving the boat more straight line power and direction. As far as putting two props on a sailboat I wouldn't see any advantage to it. For one it would place more strain on the engine trying to turn twice the blade surface area. For another I would think the forward prop would interfere with the water flow over the second. In both cases I would suspect you would actually lose speed, not gain it. Most of all there really isn't a need for it. As Tim already said any speed problems can be handled with a properly sized/pitched prop. Remember power boats are planing hulls while sailboats are displacement. You can only push a displacment hull so fast, and that's to it's designed hull speed. If you're not achieving hull speed then there are a number of things you can and should do before considering a new prop. If you're just tinkering with the thought, well that's another thing all together.
Some power boats have a duo prop. It's a two piece device where one section is right handed, the other left. It's supposed to reduce or eliminate prop walk giving the boat more straight line power and direction. As far as putting two props on a sailboat I wouldn't see any advantage to it. For one it would place more strain on the engine trying to turn twice the blade surface area. For another I would think the forward prop would interfere with the water flow over the second. In both cases I would suspect you would actually lose speed, not gain it. Most of all there really isn't a need for it. As Tim already said any speed problems can be handled with a properly sized/pitched prop. Remember power boats are planing hulls while sailboats are displacement. You can only push a displacment hull so fast, and that's to it's designed hull speed. If you're not achieving hull speed then there are a number of things you can and should do before considering a new prop. If you're just tinkering with the thought, well that's another thing all together.
Thanks for the thoughts
I'm looking for ultimate propulsion for less fuel consumption. Got caught in a blow last year near Falmouth while finishing a cruise. I had little fuel so the reefed Jenny brought us inro the middle of a procession of 80 or so boats running on power. The final leg was into Portland Harbor where I opted for power knowing fuel was slim. I avoided the regatta, but pulling into the slip was full reverse while traveling forward at 2 knots. I considered dragging a stern anchor, but maybe there's been some advances in designing props since the 3 blade I just bought was lacking. Thanks for the thoughts on hull design and turbulence.
I'm looking for ultimate propulsion for less fuel consumption. Got caught in a blow last year near Falmouth while finishing a cruise. I had little fuel so the reefed Jenny brought us inro the middle of a procession of 80 or so boats running on power. The final leg was into Portland Harbor where I opted for power knowing fuel was slim. I avoided the regatta, but pulling into the slip was full reverse while traveling forward at 2 knots. I considered dragging a stern anchor, but maybe there's been some advances in designing props since the 3 blade I just bought was lacking. Thanks for the thoughts on hull design and turbulence.
I recommend that you purchase "The Propellor Book"
by Dave Geer NA. If you read it carefully you will find answers to your questions. The clearance between prop and hull reduces the high pressure gradient between the prop tip and the hull. On small boats like most of us have, 2 inches is considered minimum.
by Dave Geer NA. If you read it carefully you will find answers to your questions. The clearance between prop and hull reduces the high pressure gradient between the prop tip and the hull. On small boats like most of us have, 2 inches is considered minimum.
Power boats..
Power boats with two props on the same shaft have a very intricate shaft within a shaft system so the props can counter rotate! A properly sized three blade prop and a SLOWER speed than 2 knots approaching a dock is best for power but will hurt sailing performance slightly. Keep in mind that the gearing in reverse is NOT the same as in forward so PLAN AHEAD. Your C-30, if it has a diesel, will sip less than .5 gph at cruise RPM! I'd really practice certain boat handling skills before you spend the money on a new prop.. Prop sizing is CRITICAL and you CAN'T just throw on a larger prop on a whim.. As always a little more detail such as what engine you have, what prop you have (dia, pitch, # of blades) etc. always helps forum members answer your question...
Power boats with two props on the same shaft have a very intricate shaft within a shaft system so the props can counter rotate! A properly sized three blade prop and a SLOWER speed than 2 knots approaching a dock is best for power but will hurt sailing performance slightly. Keep in mind that the gearing in reverse is NOT the same as in forward so PLAN AHEAD. Your C-30, if it has a diesel, will sip less than .5 gph at cruise RPM! I'd really practice certain boat handling skills before you spend the money on a new prop.. Prop sizing is CRITICAL and you CAN'T just throw on a larger prop on a whim.. As always a little more detail such as what engine you have, what prop you have (dia, pitch, # of blades) etc. always helps forum members answer your question...
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