What you may not be aware of is that "overpropping" ie. too much diameter or too much pitch will overload the motor at almost any speed or throttle position. I observed the following with my Helsen 23 motorsailer about 3500 pounds loaded. 9 x 6 two blade got 4 mph top speed at 60 amps full throttle.
12 x 8 three blade got 5 mph top speed at 75 amps full throttle.
12 x 15 three blade got 6.4 mph top speed at 150 amps and hot, hot.
12 x 13 three blade got 6.4 mph top speed at 100 amps , not hot with 2 v-belts.
Same prop with cogged belt gets 6.7 mph in calm water.
You should always prop to motor's continuous rating, in my case 100 amps at full throttle to not overload the motor. I can run with closed engine hatch for 3-4 hours straight and easily hold my hand on the motor. Cool also means reliable. Bigger may be hydronamically more efficient, but can cause overloading and death to electronics.
Dave Brooks
From: aweekdaysailor <aweekdaysailor@yahoo.com>
To: electricboats@yahoogroups.com
Sent: Mon, May 31, 2010 10:53:22 AM
Subject: Re: [Electric Boats] Safety weekend
12 x 8 three blade got 5 mph top speed at 75 amps full throttle.
12 x 15 three blade got 6.4 mph top speed at 150 amps and hot, hot.
12 x 13 three blade got 6.4 mph top speed at 100 amps , not hot with 2 v-belts.
Same prop with cogged belt gets 6.7 mph in calm water.
You should always prop to motor's continuous rating, in my case 100 amps at full throttle to not overload the motor. I can run with closed engine hatch for 3-4 hours straight and easily hold my hand on the motor. Cool also means reliable. Bigger may be hydronamically more efficient, but can cause overloading and death to electronics.
Dave Brooks
From: aweekdaysailor <aweekdaysailor@
To: electricboats@
Sent: Mon, May 31, 2010 10:53:22 AM
Subject: Re: [Electric Boats] Safety weekend
Thanks James, I had one before - not marine rated - which self destructed. But I had hopes to be able to add enough heatsink for passive cooling to work. (what I really want is to bolt it to a water heater! :)
I run normally at 40amps - but 40kts on the nose required 100 - luckily for only about a minute to clear the seawall. I am wondering if lowering the pitch on my prop would make sense? It's a 13x13 3-blade and I can definitely exceed max amps at full throttle. If I am understanding all the prop/pitch posts correctly that should give me more oomph (bollard pull?) at the cost of less top speed (which I never use anyway)
-Keith
(sorry to the southers - heading out again today in some epic spring weather - you can brag come December)
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