Hi Guys,
These numbers for my Albin 25 fit fairly closely with what Eric is saying. This is a 5000lb boat with a waterline of 22.2 ft.
Speed Drag Watts HP
3 32 490 0.7
4 60 980 1.3
5 128 1960 2.6
6 193 4300 5.8
I didn't get a lot of data points on the drag numbers so had to do some rough interpolating, but you can see the trend. You'll notice the drag builds up more at the higher end compared to the 30-footer, due to shorter waterline length. It's interesting to see how the power just about doubles for each knot. I've seen similar results on other boats and I think it's pretty typical.
Those ft-lb/sec numbers have me scratching my head a bit. Where did they come from? I'm not all that physics savvy, but according to Google, 550 ft-lb/sec equals 1 horsepower. If that's the case you've got a REALLY efficient hull there! Me thinks something is amiss.
Jim
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> GNHBus,
>
> I don't know if you tried to attach something to your post, but I don't get attachments for Yahoo group posts.
>
> James' measured data shows 20 pounds of pull for 2kts, 35 pounds of pull for 3kts, 60 pounds of pull for 4kts, 100 pounds of pull for 5kts and 180 pounds of pull for 6kts with his Catalina 30 in Santa Barbara harbor. The basic presumption is that if you're not being towed by another boat, the prop (or sails) has to provide an equal amount of thrust to achieve the same speed.
>
> This data shows the exponential type of power requirement that I am familiar with in boats. In James' data, it takes about 3 times the force to drive the boat 2kts faster. This will simple conversion rule will come apart as the boat approaches hull speed, but it seems to work from 2kts through 6kts. You should see curves with a similar shape with observed data from other boats.
>
> But when you look at the electricity required to produce that thrust, the curve gets steeper. Again looking at James' data, the rule is closer to doubling the power to gain 1kt. It is my belief that this is at least partially related to increased prop slip as the loads get higher. I plan on collecting detailed data (watts to knots to RPM) for my boat, but the weather hasn't been cooperating each time I've been at the boat for more than the last month.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
> --- In electricboats@yahoogroups.com, GNHBus@ wrote:
> >
> > 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@ 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/_
> > (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....
> > Fair winds,
> > Eric
> > Marina del Rey, CA
>
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