When I pulled my Catalina 30 through the harbor I first took the propeller off the boat to truly simulate how much thrust was being created.
The Catalina 30 weighed 10,200 from the factory with an Atomic 4 engine which weighs about 400 lbs. (anyone know for sure?) and all the diesel tank and diesel gear - maybe another 100 lbs.
My boat has 1,252 lbs of batteries, 100 lbs for the Propulsion Marine Electric Drive, another 200 lbs of tankage and 200 lbs of anchor chain, 145 lbs generator weight and probably another 400 lbs of wire, chargers, and gear aboard. My guess is she displaces 12,000 lbs.
The thrust is equivalent to the tension in the line when the boat is pulled without a propeller and is measured in lbs. Ft x Lbs is a torque measure.
I should also note that the boat had a good bottom job on her, we took out all the through hulls and went down to only two - one for the head, and another with a sea chest that does all the intakes plus the sink drain. I faired the bottom as well as I could, fairing out the Catalina smile work from the previous haulout.
The numbers were an average of the two runs both directions in the harbor in still water with no wind.
To extrapolate these numbers and guess what they might be for your boat a few things to consider might be:
1) a narrower boat (Catalina 30 is really wide at 10, 10") requires less thrust
2) a lighter boat requires less thrust.
But how do we deal with length?
My guess is that any boat length, of the same weight and beam requires approximately the same amount of thrust to go the same speed in calm conditions. ( This is a point of interest to me so if anyone else has similar data I am most interested in knowing about it.)
In order to size and electric drive, the best way is to tow your boat and find out the thrust required. If you don't do this, then consider all factors, especially the windage of the boat. We need to size our boat's electric drives for the worst possible conditions they will see within their own cruising areas.
Also, I believe that electric drives should be rated at their continuous output after one hour when temperature in the motor stabilizes. Because that is how these electric drives are used.
The website
http://www.propulsionmarine.com has recently been rebuilt and there is a great F.A.Q. section there under Electric. Check it out and tell me what you think,
Best,
James
Propulsion Marine Products
by
Above the Waterline, Ltd.
Santa Barbara, CA
This indicates 50ft/lbs/sec Prop Thrust for 4knots, and 100ft/lbs/sec Prop Thrust for 6knots
for Catalina 30 at 10,200lbs, through Sea Water, is that about right ?
In a message dated 3/9/2011 12:55:25 P.M. Eastern Standard Time,
ewdysar@yahoo.com writes:
GNHBus,
I know that you've seen the link posted here before. James Lambden at Propulsion marine has collected extensive data concerning his Catalina 30 Kapowai and posted that data online at http://electricboatdesign.com/ . If you look in the "data" tab, James has posted thrust requirements based on measuring the pull while towing his boat. In that table, 100 lbs of pull (thrust) occurs at 5kts. Keep in mind that this is empirical data, not theoretical like the results from the various planning worksheets. Of course, this data is most relevent for Catalina 30s, but other similar boats can probably be extrapolated from James' data. Once a boat has been converted, then specific energy data can be used to derive whether that boat design requires more or less thrust for the same speeds.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, GNHBus@... wrote:
>
> How did you calculate the 100lbs/Thrust ?
>
>
> In a message dated 3/9/2011 2:32:25 A.M. Eastern Standard Time,
> josh@... writes:
>
> I'm in the middle of my conversion. Just a few numbers for you to keep in
> mind when considering whether or not electric will work for you. Numbers
> are for a Catalina 30, which is 10k lbs and change, so about 2/3 your
> displacement.
>
> It takes about 100 lbs of thrust to move my boat through calm water at 5
> knots, or about 3.2KW with the geared system and optimized prop.
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