We don't have to just make this stuff up off the top of our heads, there is a considerable amount of relevent information on the internet that can be used in these discussions. I've got no practical need for the info that follows (my boat is already converted and performs better that all of the "models" worked out ahead of time), but I continue to research these topics to better understand boat design and to help fellow electric boat converters. And many of these topics have been discussed here before, the search function on this Yahoo group can provide a wealth of information.
Bob, surprisingly, there seems to be some performance differences between container ships and the recreational boats that we discuss here. I think that is has something to do with scale... :)
If you look at the propeller calculators from most of the propeller manufacturers, the typical figures for displacement sailboats are around 50-55% slip. I would consider them to be a credible source.
Here's a random table from the interweb that covers prop slip in recreational boats:
Type of Boat - Speed(kn) - Slip
Auxiliary sailboats, barges - under 9 - 45%
Heavy powerboats, workboats - 9 to 15 - 26%
Lightweight powerboats, cruisers - 15 to 30 - 24%
High-speed planing boats - 30 to 45 - 20%
Planing race boats, vee-bottom - 45 to 90 - 10%
Hydroplanes, catamarans - over 90 - 7%
Given Keith's hull design, I would put it in the "barge" range or 45-50% prop slip.
But enough with secondary research sources, let's discuss a primary data source. Here's the objective prop slip data that I've posted numerous times in this group, Feb 2012 (twice), Dec 2011, Oct 2011 and originally on Aug 22, 2011 (Post #19469). Suprisingly, nobody else here has measured and posted their actual slip numbers. It seems like other reports of primary data sources would be helpful during these conversations since the topic comes up a couple of time a year.
"I purchased my 13 x 15.5 four bladed prop from a local prop shop to replace the 13 x 10 two blade that was already on the boat. You can see both props side by side in "Eric's Serenity" photo folder in this group http://groups.yahoo.com/group/electricboats/photos/album/1967401930/pic/list
With the old prop and the diesel engine with 2:1 reduction, engine rpm was about 1200 (shaft speed of 600) at 3kts and 3200 engine rpm at 5.5 kts. With the new prop and electric motor with 2.55:1 reduction, the motor speed is 830rpm (shaft speed of 325rpm) at 3kts and 1900 motor rpm at 6kts. Doing the calcs, the old prop showed 39% slip at 3kts and 58% slip at 5.5kts. The new prop calcs to 27% slip at 3kts and 37% slip at 6kts."
Perhaps not very surprisingly, my measured ICE results fell directly in line with the typical slip numbers provided by the propeller vendors for my type of boat. We can also see that going to a larger propeller with greater pitch and increased Projected Area Ratio (PAR), my slip was reduced by more than 30% at 3kts and more than 36% at 6kts. Of course, an ICE would not be able to push this prop effectively, the electric drive makes that possible.
Keith, the bottom line is 12% prop slip is not reasonable for your design.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Robert Lemke <robert-lemke@...> wrote:
>
> I think even for a small boat 40~50% is high. On the last container ship I served on was around 3% at 112 rpm. The slower the prop speed, the less slip.
> Bob
>
> --- On Sat, 5/5/12, luv2bsailin <luv2bsailin@...> wrote:
>
> From: luv2bsailin <luv2bsailin@...>
> Subject: Re: [Electric Boats] concept for an electric houseboat
> To: electricboats@yahoogroups.com
> Date: Saturday, May 5, 2012, 11:00 AM
>
> Something like 40 or 50% slip is more typical for a displacement hull. Dave Gerr's "Propeller Handbook" is a good resource.
> Jim
>
> >
> > > Slip calculations have to take so many variables that yes, I just threw
> > > 12% out there.
> >
> > Sure, I understand. Is 12% fairly reasonable?
> >
> >
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
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