T-Pain Is On A Boat: Re: [Electric Boats] Thrust

Saturday, June 1, 2013

Re: [Electric Boats] Thrust

My problem with Watts & HP, is that neither has anything to do with the amount of power actually exerted at the propeller, which is the actual power that moves a boat.

I understand all the variables, but after 5-10 years of serious recreational inquiries of using a non IC engine in a displacement hull-type boat, there are no general figures that a simpleton like me can use for a real life comparison.

I have no interest in comparing a 15 HP motor, which could drive my sailboat far beyond the theoretical hull-speed, to no avail, to a 100 pound thust motor that will give me 75% of hull-speed with no more excess power available to attempt the absolute upper limits of my theoretical hull-speed.

Seriously, for a displacement type hull, I would think that a simple equation of thrust would be very worthwhile. Moreso than with a planing type hull which can take full advantage of excess power.

John

Newport 28

Port Clinton Ohio

On Sat, Jun 1, 2013 at 5:10 PM, Joe Murray <computersatsea@yahoo.com> wrote:

Roger,
Why not use thrust as a basis for comparison? The reason is right in your reply. Thrust is dependant on too many variables. It's just simpler too stay with watts and horsepower.

Joe Murray
Different Drummer #364
Panama City, FL

From: Kirk McLoren <kirkmcloren@yahoo.com>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Saturday, June 1, 2013 10:53 AM

Subject: Re: [Electric Boats] Thrust

This one?

http://ia600301.us.archive.org/7/items/speedpowerofship00tayluoft/speedpowerofship00tayluoft.pdf

From: Roger L <rogerlov@ix.netcom.com>
To: electricboats@yahoogroups.com
Sent: Saturday, June 1, 2013 4:56 AM
Subject: [Electric Boats] Thrust

I agree that the electrical numbers are helpful, but thrust is what moves the boat through the water.

Imagine that instead of a pitched propeller, what we are spinning in the water is a propeller without any pitch, or even more simply that we have tangled up the prop in a canvas tarp and now what we are spinning is a big wad of cloth.

We can measure the volts and amps required to spin that wad of cloth at any rpm, and change that into HP and torque.....but the information isn't helping much.... And the boat still isn't moving forward.

To push the boat we need for there to be a prop with some pitch to the blades. Talking about props means we are also talking about thrust.

For me, calculating thrust in fluids has always been more difficult and complicated than the fairly straightforward calculation of volts, amps, and battery capacity.....

For a while I tried to think of pitch and prop diameter as the fluid equivalent of volts and amps - and that works to a degree....

But a propeller works in a fluid so its thrust has to take fluid viscosity into account. The various energy losses due to fluid viscosity can be huge. They vary with the diameter, pitch, and speed of the propeller..... as well as the speed of the boat.

    Roger L.

If you like this kind of calculation, I recommend reading The Speed and Power of Ships by Taylor.

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

From: Craig Carmichael

To: electricboats@yahoogroups.com

Sent: Friday, May 31, 2013 11:38 PM

Subject: Re: [Electric Boats] Trolling motors ==> electric outboards

Ya, I wish these people would cut the "#s of thrust" nonsense and
tell us the watts, or at least volts _and_ amps so we'd have some
idea and a basis for comparison. I think they want to hide it from us
so we don't realize how little power they have.

Craig

=====

>A trolling motor that produces, say 50# thrust at zero boat speed
>would produce zero thrust at 6 or 8 mph. Double the prop pitch and
>thrust at zero speed will drop while thrust at 8 mph will increase.
>Motor power is the same regardless.
>
>On Friday, May 31, 2013, danbollinger wrote:
>
>To a physicist, they are very different concepts. Thrust is a
>rotational unit of force. Power is a unit of work.
>
>Energy / Time = Power

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