Thats good info, thanks. I'll post pics of my boat soon as its almost completed. Its a rebuilt 18ft day sailor, 1200lbs fully loaded, with two 36v/50a Minn Kota's. The motors are spaced almost six fit apart at the stern and I have continuous forward/reverse speed control on eadh motor via wireless remote handheld. So, the concept is steering via differential motor speed control. We shall see.
I estimate these motors are about 3hp net and should more than drive the boat to hull speed of, maybe, 6 mph. However, Im concerned the low prop pitch wont get past the 3.5 mph you calculate.
Can you suggest sources or methods for alternate props on the Minn Kota ? Im thinking 9"d x 8" pitch ? Each motor has an analog (RMS) amp meter, so I can keep an I on the motor loading.
Is that shaft type with the "D" cross section used on other types of props ? Any ideas are greatly appreciated.
--- In electricboats@yahoogroups.com, David Brooks <avantiman@...> wrote:
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> Have tried different pitches on my inboard electric with good success.
> ie 12" diameter 2 blade 10" pitch drew 75 amps wot and got about 5.mph
> 12" diameter 3 blade 8" pitch drew 80 amps wot and got about 4.6 mph
> 12" diameter 3 blade 13" pitch drew 115 amps wot and gets about 6.2 mph or hull speed
> ***** The 115 amps at 36 volts wot is within the spec for the motor. After several miles at wot the motor is very warm but can be held by your hand without burning and water on motor does NOT boil. Temp control is ok for a maginally ventilated motor space.
> *** A non-standard prop voids most trolling motor warranties bc of possible motor overloading.  The motors (almost all with 4" pitch) regardless of power rating are sold to power at fishing speeds for different weights of boats. The 36 volt motors can push a 20ft boat without overloading the motor at wot. If your motor is listed by the manufacturer at 60amps wot on a 3000-4000 pound boat and you have a 1200 pound boat, then wot will maybe only draw 35 amps at wot. IF  this is the case then a steeper prop will give more speed but draw more amps at wot. IF the amps don't exceed the 60 in the spec at wot, then you are ok as the motor won't overload except possibly when fighting high winds and or current
> **** also you are subject to the rules of hull speed calculations as this affects ultimate top speed in a NON-PLANING HULL.
> ie Hull speed is approximately 1.3 times the square root of the waterline length of the hull.
> 9 foot waterline would be 1.3 x 3 = 3.9 kts
> 16 foot waterline would be 1.3 x 4=5.2 kts
> 20 foot is 1.3 x 4.47=5.8 kts
> My 23 foot Helson motorsailor has about a 20-21 foot waterline so I need to be happy with 6.2 mph at wot
> 4 mph draws 25 amps
> 5 mph draws 50 amps
> 6.2 mph draws 115 amps
> ****** To get 7 mph by over propping I might draw 250 amps and burn the motor, NOT worth the risk.
> ***** Using a 3 blade Motorguide trolling motor prop on a Minn Kota gave me the same amp draw but 0.3 mph more forward speed and DOUBLE the thrust in reverse. Reverse gear is the brakes on a boat.  Well worth the effort, but the Minn Kota 2 blade (weedless) was better in forward in heavy weeds than the Motorguide.
> Hope this was helpful. Each boat is an experiment waiting to happen. Good luck
> Dave ( Day Dream Believer)
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> --- On Sat, 10/2/10, alex <alex04032@...> wrote:
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> From: alex <alex04032@...>
> Subject: [Electric Boats] Re: 2 trolling motors or 1 ?
> To: electricboats@yahoogroups.com
> Received: Saturday, October 2, 2010, 11:59 AM
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> John, regarding, "expect about 3.5 mph max", if the Minn Kota were larger and so moving the boat easily at that speed, then is the full available power not being used because the prop is not matched ? That is, for big Minn Kota's on small boats, would a higher pitch prop deliver more speed ? I realize this could over load the motor, but I have amp meters and can keep an eye on on current.
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> --- In electricboats@yahoogroups.com, "John" <john.zimmerlee@> wrote:
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> > Though it is more complicated than this . . . the 30 amp motors have 9" props and therefore 63.585 sq inches . . . times two or 127.17 sq inches total. The 55 amp has 11 inch prop or 94.985 sq inches of grip.
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> > The 30 lb thrust motors have been recently redesigned to a shorter length and I question their rating and their duarbility. Their motor diameter is just 3.25".
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> > The 55 lb thrust has a larger 3.625" diameter and is built stronger.
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> > Redundancy is nice to have when offshore, and differential steering might be nice too . . . but I question the single 30's ability to push your boat . . . and would enjoy a good laugh watching you steer the boat while leaning over the stern and twisting handles while coming into port. You may find it more responsive to use the motor(s) in fixed position and steer with your usual rudder.
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> > All MinnKota props have a 4" pitch and most are expected to turn about the same RPM. If you put 1 large MinnKota or 20 small ones on your boat . . . expect about 3.5 mph max!
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> > The 24 volt motors are more efficient than the 12 volt series. They cost more and will needs 2 batteries in series.
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> > John Zimmerlee
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> > Stream Dancer & Versaboat
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