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[gromarlie]

I have tested the motor set-up in my narrow boat found that It's drawing 1.9 amps in the water.
Query is - is this about correct ? Motor is an MFA 457-RE385 running a 35mm 2 blade prop.

[FullLeatherJacket]

Sounds about bang on the money to me, given the load you describe. I might be tempted to use a three or even four bladed prop, though. That would look more the part; move as much water as the 2-blade prop, and very probably draw less current.
Suit yourself.
FLJ

[gromarlie]

Thanks  - do I need to keep to the same size 3 blader ?

[FullLeatherJacket]

How long is a piece of string etc etc. In VERY general terms you'll find that the pitch of scale-type 3 and 4 blade props is less than that of the typical 2 blade props sold by most snake-oil merchants non-specialist model shops, so you can afford a larger diameter pro-rata.
 
In practical terms it's a matter of trial and error,  based largely around matching the prop to the motor to the battery to the hull. A 35mm 3-blade would be a reasonable place to start. I wouldn't go any larger in diameter; maybe try a 30mm 4 blader as well, if you can get one easily.

If prop selection was just a matter of a simple calculation then life would be even less of a mystery. (Discuss in 1000 words).

FLJ

[Ghost in the shell]

a 35mm 2blade prop sounds like a race propeller, and so quite coarsem thus making your 385 an amnp hog.  get a 3blade prop and you shold drop.  I get less 1a off a 385+30mm 3blade set up

[gromarlie]

Thanks folks - off to get a 3blade 30mm 

[Ghost in the shell]

well put it this way, I have a graupner neptun, 30mm kit prop, (plastic), 2000mah ni-cad battery and a vision-400 385 spec motor and sailed for 9 hours solid at an event ON ONE BATTERY