T-Pain Is On A Boat: [Electric Boats] Prop Notes

Friday, September 12, 2014

[Electric Boats] Prop Notes

princess Cruise line deck mounted rescue boats are all Kort nozzle. Maybe they have a reason for that ?? I took pix on the last cruise, and the props are pretty big, but they have gas motors.

When I did my 15 hp EV conversion on an Evinrude 50, we reduced the pitch about 3 inches, to fit the hull configuration. It was peaking out too soon, and using too much battery power. At lower pitch I get better control, hull max speed, with some reserve for wind, waves and still around 4.5 to 5 mph cruise, as expected. A bigger prop is the answer for some boats, but you can not beat the math on water line speed ratios, and the HP required to reach max speed. At lower speeds, bigger is more drag, by a bunch, and more liable to damage or blade cavitation.

The displacement figures for all hull lengths is explained very well in -- Royces book – Powerboating Illustrated – or His Sailing Illustrated book – probably Trailerboating Illustrated too, but I don't have that one. Published over 30 years, so they must be good. I won't go into that essential knowledge, just read that or some similar book.

Cal

From: mailto:electricboats@yahoogroups.com

Sent: Friday, September 12, 2014 5:22 AM

To: electricboats@yahoogroups.com

Subject: Re: [Electric Boats] Re:New Member and question about displacement

Consider a Kort nozzle, It will also help with vibration caused by wash
hitting the hull. The nozzle also directs that water that is forced outward
into thrust making a more efficient system than just a prop by itself.
Originally the nozzle was developed to reduce canal erosion but it was found
to increase efficiency as well.

Kevin Pemberton

On Friday, September 12, 2014 10:26:33 AM you wrote:
> Absolutely.
>
> And the common boat has a prop and drive system that is very ineffcient.
> The desirable thing to do would be to go to a prop 2-3 sizes large than
> commonly used.
>
> The problem is you would need to change the driveshaft as well, as an
> efficient prop wont fit in the standard aperture.
> As such, this is almost never done.
> Also, large props are expensive.
>
> In normal boats (inboards) you have gearboxes that requite cooling, and
> large hot oil and hydraulic coolers.
> All the heat is inefficiency (as is all noise).
> Heat does not appear from zero - work and then friction creates it. All
> heat created is a loss vs. having that energy go to the prop.
>
> The driveline wastes about 1/3 the energy.
> The prop wastes about 1/3 to 2/3 the energy. Normal props are very
> inffefficient at most speeds.
>
> So, only about 1/6 of the energy actually goes to move the boat !
>
> This is illustrated by numerous conversions done with small engines, and
> or large props, where relatively tiny engines move relatively very big
> boats.
>
> On 11/09/2014 02:33, John Acord jcacord@gmail.com [electricboats] wrote:
> > From reading various sources it seemed that a larger prop turning
> > slower would give a more efficient transfer of the power.
> >
> > Also, I have heard that running the motors at lower rpm under load
> > results in undesirable heating, thus the choice of a higher gear ratio.
> >
> > I found a listing in the Michigan Propeller catalog showing that there
> > are some Evinrude & Johnson outboards with 2.42:1 gears. Since I am
> > starting from scratch, not having an existing outboard in hand,
> > thought I might look for one of those.
> >
> > thanks,
> > John