[MHml] HP to Watts

[Alec Mackenzie]

Dan,

A Watt is a Watt is a Watt

Thrust is simply the product of the power delivered to the 
propeller and the propeller efficiency at the specific boat 
speed.

Too bad there is nothing 'simple' about propeller 
efficiency!

The vessel's resistance varies with speed as does the 
efficiency of the propeller, so no single 'thrust number' 
will work.

Torque has nothing to do with power, but everything to do 
with the delivery of that power to the water. If the power 
source has the wrong torque curve, we simply use gear ratios 
to better match delivered torque to the load requirements.

Read the Propeller Handbook by Dave Gerr to get the whole 
story.

Alec



----- Original Message ----- 
From: "Dan Frenette" <Dan.Frenette at Sun.COM>
To: "Informed discussion of multihull issues" 
<multihulls at steamradio.com>
Sent: Friday, December 14, 2007 4:00 PM
Subject: [MHml] HP to Watts


>A simple formula of 1 hp required = 746 Watts looks simple 
>but do we
> really need that many watts to get the same effective 
> power from an
> electric motor vs an outboard?
>
> I know Johnson/Evenrude claim's they are measuring HP at 
> the prop but
> what does that mean in terms of what we would need from an 
> eletric motor
> to do the same thing?
>
> Should I be looking at units of thrust from the prop or 
> ???  I don't
> know if the thrust is commonly quoted??? An eletric motor 
> has better
> torque characteristics if I understand what's going on 
> under water.
>
> Dan Frenette
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