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On Nov 8, 2016, at 1:54 PM, Kev captainyoung@gmail.com [electricboats] <electricboats@yahoogroups.com> wrote:
Yes, larger slower turning props are more efficient for displacement hulls.Less blades are more efficient. The reason for more blades, is due to diameter constraints of the prop. You need 2 inches minimum from the edge of prop to the hull. So if say the most efficient propeller would be 23 inch diameter, but you can only fit a 20 inch propeller, then a 3 blade might be more efficient.
This is a nice tool to guide you in choosing a propeller:
https://www.vicprop.com/displacement_size.phpIf you are not trying to get every little bit of efficiency out of your propeller, this will give you a pretty close estimate.If you want the most efficient propeller for your boat at 6 knots, it will probably take some trial and error, by testing different propellers and reducing gears/belts combinations.You can get a more accurate selection if you know how much power is required to move your boat at your desired speed of 6 knots. Say if you have done tests with other propellers. This calculator tries to calculate the power required based on 4 values: waterline length, beam at the waterline, draft and displacement. This will give you a good idea, but there are many other variables which cause your boat to require more power to go your desired speed, such as keel, rudders, wind resistance, smoothness of hull...The book that will help you do the calculations is call "The Propeller Handbook" by Dave Gerr. But I would just use the calculator above, and see if you could work out a deal with the company you buy the propeller from, to see if you can try a few different diameter/pitch configurations, and do some tests at your desired speed.On Sun, Nov 6, 2016 at 7:06 PM, king_of_neworleans <no_reply@yahoogroups.com> wrote:Also common on trawlers. Actual trawlers not trawler yachts. About 8 to 10kts is the break even point, typically, and above that speed the increased drag results in a reduced efficiency. At normal trawling speeds they can save substantial fuel. Call it 3kts. if your E-boat will be cruising that slow then it might be worth considering. For a sailboat, forget it. Astern propulsion is noticeably weaker. Lateral thrust is of course reduced a great deal. Lots of tradeoffs including the actual cost of nozzle, and prop modification. When you get a net in the wheel it ain't nothing pretty. I have considered homebrewing one but I always ended up discarding the idea as having slightly more cons than pros. For a new build to be powered with electric drive, I might again give it some thought. It very likely wouldn't get past the thoughting stage, tho.
Posted by: L Schmitz <terminalift@yahoo.com>
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