just a brief comment about sailboats and the propellers for them.
these comments apply only to sailboats, power boats rules are different.
About 75% of all older sailboats under 35 feet in length are limited to a 13 inch maximum propeller. this limit is the result of the atomic four engine and its 12 inch propeller having had 80% of the aulillary engine market for sailboats during the time when most of the boats were being designed. Only a few of the boat manufacturers changed the hull molds when the atomic four was discontinued. There are new boats being made today with the prop space designed for the 12 inch atomic four prop. If you see a fixed three blade propeller on a new boat it probabily is the wrong size propeller beecause the boat is prop limited and a correctly sized two blade propeller will not fit on the boat. There were several such boats at the strictly sail chicago boat show this year. there are specialized propellers that can allow up to a 16 inch diameter in special situations.
sailboats and two/three blade propellers.
Two blade propellers are more efficient than three blade propellers. That can be seen in any chart of recomended propellers that has both three and two blade propellers. for any given motor and gear ratio a three blade will be only 1 inch smaller in diameter and the same pitch and some times they are same diameter and only a pitch different, (rounding of calculations can cause them to be closer than they acutally are) yet they are rated to deliver the same torque at rpm? However when the engine stops and sails are set the 3 blade will present 30% more drag than a two blade propeller and if racing protocols are followed the drag difference could go to 60% or higher.
there are only a very few and specific reasons to use a three blade propeller as avoidance of drive train harmonics and oilcaning of a thin hull in very close clearence situations.
strategically I see gear selection as.
1) select the motor, batery,control system for your hull displacement and length and boat usage.
2) find the largest diameter prop that will fit your hull with proper clearence and try to locate a selection of 2 blade propellers in that diameter. be sure to ask for stock props or you could face a 6 to 10 week lead while they make the prop) typically you will have only a short list of diameters and pitchs probably less than four, be sure to include your present prop in that list.
3) calculate the for prop size using your motor data and various gear ratios until you find a prop that is close to one of those available. That is the gear ratio you need for your system.
alternately
If you have the bucks ($1000 and up)buy a feathering prop at the max diameter that has a field adjustable pitch (usually by set screw) and fine tune the prop to your motor and gearing during sea trials.
larry
--- In electricboats@yahoogroups.com, Daniel Michaels <nov32394@...> wrote:
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> Same as with planes, a larger 2 bladed prop is more efficient than a shorter 3 bladed prop. The larger prop you can swing the better.
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> Dan
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> --- On Thu, 8/5/10, hardy71uk <p0054107@...> wrote:
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> From: hardy71uk <p0054107@...>
> Subject: [Electric Boats] Re: Hello
> To: electricboats@yahoogroups.com
> Date: Thursday, August 5, 2010, 3:55 AM
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> boats are different to airplanes because although the formulas are the same there are different practical limits . For example in boats the larger the prop the more efficient ( that is at slow and medium speed ; the normal electric range) thats why , as I understand it, Larry recommends beginning with the biggest propellor that has adequate clearance .Find the power needed for the desired performance and then select pitch ,gear ratio and motor to match.
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> Chris S
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> --- In electricboats@yahoogroups.com, Daniel Michaels <nov32394@> wrote:
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> > Thanks for the input Larry.
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> > I am more familiar with airplanes and props but I think they work about the same.
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> > We first take the max rpm we are running plug that into the chart to
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> > find out how fast the plane will go with each pitch. Shorten or lengthen
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> > the prop to let the engine run at max rpm. The pitch tells you how fast
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> > the plane will travel.
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> > I am not sure the Triton was designed around an Atomic 4. I think an
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> > Atomic 4 fit into the general usage for the Triton. The Atomic 4 went as
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> > high as 20 hp. When only 10 to 12 could really be used. Running the rpm
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> > only to 1500 gave you 11.9 BHP. It has a 13 inch 2 blade prop. So if I
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> > had 12 hp available at 1500 rpm it would do the same thing. If I found
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> > out what hp the electric motor had at 1500 rpm I should know if it will
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> > turn the prop. Alternatively I could shorten the prop and turn a higher
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> > rpm. I could also go with a 3 bladed prop. The electric motor I am going
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> > to use is 13 hp @ 48v same as the Atomic 4. Only this will far
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> > outperform the Atomic 4.
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> > This is what I have so far. this is from limited knowledge but I am learning every day.
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> > It takes 1 Kw per ton of displacement.
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> > 36 v 140 amps 1400 rpm 3500 watts E Teck motor.
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> > comes to 3500/750=4.7 hp * 2.5 conversion factor to compare it to the
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> > Atomic 4 = 11.75 hp. @ 1500 rpm the Atomic 4 is 11.9 Very close.
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> > 48 v 180 amps 4800 rpm 9800 watts E Teck motor.
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> > comes to 9800/750=13 hp * 2.5 conversion factor = 32.5 hp but at 4800 rpm if I cut the rpm in half I am at 16.25 The Atomic 4 is 16.2 @ 2000.
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> > At 36v 3500 watts the 1 kw per ton works out very close to a Triton at 4 ton.
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> > At 48v 9800 watts /2 = 4900 it comes out in my favor with a half ton to
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> > spare and that is at half throttle. If I use a reduction drive on the
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> > 48v of 2 to 1 I am out performing the Atomic 4 at every level.
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> > 36v at 140 amps run wide open
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> > 6 * 260 Ah batteries / 50% = 780 usable Ah. 780 / 140 amps = 5.5 hrs.
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> > run at half throttle or less and you will go much farther same as if you
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> > throttled back on the Atomic 4.
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> > 48v at 180 amps 8 * 260 Ah batteries / 50% = 1040 usable Ah. 1040 / 90
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> > (180/2) amps = 11.5 hrs throttle back even more and even I need some
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> > Is any of my math correct. I gave it my best shot.
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> > Dan
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