T-Pain Is On A Boat: Re: [Electric Boats] Re: Sailboat Trolling Motor Options

Monday, August 23, 2010

Re: [Electric Boats] Re: Sailboat Trolling Motor Options

Dennis , just a little thing.

If the small petrol engines use a gear reduction , wouldn't that increase the torque values , and allow them to run a larger prop ?.

How do the "bigfoot" outboards work ?.

My understanding is that they have a big gear reduction , and a large prop.

And if you are going to use the bottom end of an outboard to convert to electric power , why not a "bigfoot" type outboard ?.

Regards Rob J.

From: dennis wolfe <dwolfe@dropsheet.com>
To: electricboats@yahoogroups.com
Sent: Tue, 24 August, 2010 3:51:28 AM
Subject: Re: [Electric Boats] Re: Sailboat Trolling Motor Options

This is the old 1 electric hp = 2 or 3 gas hp argument. Of course that's baloney; a hp is a hp whether it is generated by gas, diesel electric or steam.

The point is that a 2 hp gas engine can be replaced by a 1 hp electric motor and boat will probably perform satisfactorily.

Eric is right, the prop is a big part of it. Most small gas engines need to turn 5000 rpm to make their rated hp, they might have a 2:1 gear reduction so spin the prop 2500 rpm, which implies a small prop. The electric motors we talk about might only turn 2000 rpm max and its easy to gear down to 800 - 1200 prop rpm, meaning you can swing a larger, more efficient prop.

A large slow turning prop can be much more efficient (i.e. more hp applied to moving the boat) than a smaller, faster turning one; both props absorbing the same input hp.

A Torqeedo 1003 is 1000 watts input, that's 1.5 hp. It has an 11.5" prop turning 1200 rpm max. A Honda 2 hp gas OB has a 7.25" prop turning 2600 rpm. This makes a huge difference.

On my 25' launch I could measure about 5% loss of speed for a given amp draw when changing from a 12" dia to 11" dia prop, same pitch, same mfr, style.

My trolling motor experiments showed a 30% increase in efficiency (speed per amp) switching from the 9" weedless stock prop to a 10" RCA airplane prop.

Another notion behind the 1 elec = 2 gas idea is that few people will want to run a gas engine WOT for very long. No such reservation with a quiet electric motor.

----- Original Message -----

From: Eric

To: electricboats@yahoogroups.com

Sent: Monday, August 23, 2010 1:02 PM

Subject: [Electric Boats] Re: Sailboat Trolling Motor Options

Sid,

I'm with you about the power produced by motors and engines, 1hp output = 1hp output regardless of the type.

But you did make a big assumption in the last paragraph. Since trolling motors and Torqeedos have the motors right down at the prop, they do not incur some of the losses that come along with typical outboards. I don't believe that "normal" outboard propellors are considered to be particularly efficient designs either. Torqeedo did put some time making their props more efficient and a few people on this list have noticed that replacing trolling motor props with 2 blade model airplane props provide more range (efficiency) from the same motor/battery setup.

So we may be dealing with an apples to oranges kind of thing. The deciding factor would be comparing the same boat with different drives, noting the top speed and acceleration. I'll try to get my 2hp Tohatsu running so that I can collect specific data concerning their potential power. The batteries are more of a "size of fuel tank" issue and should be left out of the comparison.

Fair winds,
Eric
Marina del Rey, CA

--- In electricboats@yahoogroups.com, "sirdarnell" <sirdarnell@...> wrote:
>
> According to Ray Electric (one of the oldest makers of electric outboards) 1 hp = approx 49-54 lbs of thrust.
>
> This would make the Cruise 4.0 a 4hp motor and the Cruise 2.0 a 2hp motor.
>
> Of course these are max HP numbers and an electric motor of the same max HP will out accelerate a gas motor, due to more low end torque. It will feel like you have more power because it has more thrust out of the gate, but an electric motor with a max of 4hp (assuming same gearing and prop) will not ultimately push a boat any faster than a gas 4hp engine.
>
>