Hi Julian,
I'm gald to hear that you're more confident about your project. I think that you've getting distracted by torque values. A typical electric motor has a torque "curve" that is a flat line until you get to operating speed. Unless you're planning on doing hole shots with your boat, you've got plenty of torque. And I don't think that speeding up your prop to 600 or 700rpm should not cause any cavitation, I've only seen cavitation be a problem with speed boats that are turning their props at 1000's of rpm.
The bad news is that given your proposed setup, you may not hit 6 knots at full motor speed (1000 rpm). Given a pitch of 22", prop speed of 500 rpm and only 40% slip, the calculated speed is 5.4kts. With your given gear reduction, you'll need 1100 motor rpm to hit 6kts with 40% slip. The part that concerns me is whether you'll get cruising prop slip as low as 40%, given the size and shape of your boat. And I wouldn't recommend that full throttle is your normal cruising setting, you don't do that with your car or ICE boat, do you? Your motor and controller should have short term ratings that are higher than your continuous target. Most motors can handle twice their continuous rating for a few minutes and controllers can often run at 50% more than continuous for a little while. So you need to figure out how much power you want to pull for your 6kts cruise, while reserving some extra punch to get out of a tight spot. Personally, building a system that can handle full throttle as long as your batteries last is safest, so you might want to consider 11kW as maximum power with cruise power somewhere around half that.
With a 32' LWL, your theoretical hull speed is around 7.5kts, and 6kts is about 80% of that. That is right on target. For many of our boats, between 50 and 95% of hull speed, doubling your power in increases your speed about 1kt. If all this holds true for your boat, then you want 6kts from 5.5kW per motor (or less) and the full 11kW into each should deliver 7kts (or more) in flat water. Going back to your prop, you will need 650 prop rpm with 40% slip to hit 7kts. I'll go out on a limb and say that only 40% slip is optimistic. BTW, prop slip and prop efficiency are two different things, slip is easily measured and calculated, efficiency is not.
Surprisingly, a 17" pitch prop at 1000 rpm with 50% slip, calcs to 7kts. Maybe you should ditch the v-drive (more efficient anyway) and go direct.
When you've got your systems installed, your initial tests will let you know how well you've matched the propellor and reduction to the motors. If at full throttle, each drive pulls more than 11kW, but the motors do not reach max rpm, then the props are too agressive or the gearing is too tall. Reducing pitch or increasing your reduction will help. Alternatively, if at full throttle, the motors hit max rpm but your drives are only pulling 8kW, then you can run more prop or reduced reduction. Obviously, with direct drive, changing the props is the answer. Or if your performance at 8kW per motor meets your expectations (fast enough and strong enough), then your project is successful and complete.
Anyway, I hope that this helps rather than confuses the situation.
Fair winds and smooth seas,
Eric,
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Julian Webb <julian.proto@...> wrote:
>
> Hi Eric
>
> - Wonderful info, and between yourself and the other great folk I think I
> can sleep tonight!
>
> - My only concern was the torque being 110Nm but that's higher than a lot
> of electric drives advertised seem to be, so in theory I am OK.
> - I've seen supposed "permanent magnet, especially designed and blah, blah,
> blah" 10KW motors down to 45Nm, and with the controllers I have I can get
> a 20% increase in torque for a 10% temp rise, so with H class insulation
> and being in Europe, that's always an option.
>
> - The 1KW/Ton also doesn't take torque into the equation but my continuous
> rating of 11KW at 110Nm I still seem to have enough I think?
>
> - Here's the prop makers email to me regarding choices:
>
> I have made lots of calculations for you to look at, the rpm is the MAX you
> will see with the size:****
>
> ** **
>
> 23x22 – 500rpm = 55% eff, 2150N thrust****
>
> 23x17 – 600rpm = 53% eff, 2160N****
>
> 21x15 – 700rpm = 51% eff, 2024N****
>
> 20x13.5 – 800rpm = 48% eff, 2001N****
>
> 19x12 – 900rpm = 46% eff, 1980N****
>
> 18x11 – 1000rpm = 45% eff, 1921N****
>
> ** **
>
> It really is your call where you want to prop this, but seeing as load is
> not really an issue, I would get the most out of the system and prop the
> most propeller thrust (in N) and most efficient.
>
>
> - It seems that I/we have got it right with the prop, or at least on the
> way.
>
>
> - As I am running 6 pole motors that have a natural speed of 1000RPM, what
> will happen if I take the 23 x 22 prop above 500RPM, will I get more speed
> or will I get cavitation?
>
> - I guess it will depend on whether I'm underpowered or not?
>
>
> - All fueled up with a full larder and a couple of tubby eager crew she
> will probably come in at 10 ton, but with 22KW that should still see me
> well under the 1KW/ton rule of thumb.
>
>
> - Even the great Fisher Panda say that tubby craft for their length (which
> mine is) "
>
>
> For larger displacements 2.5kW per tonne is a very safe rule of thumb to
> achieve hull design speed in worst case conditions ie taking into account
> tide, current, high winds and manouvering power. If hull design speed is
> not a particular requirement, as is sometimes the case with sailboats &
> catamarans then you can use a factor as low as 1.0kW per tonne of
> displacement.
>
> and as my motors develop 10% more torque than theirs I reckon I'm OK.
>
> - I promised everyone many months ago that when she's in the water and
> tested I would give full specs, diagrams, suppliers, BOM and test figures
> and I again promise I will.
>
> - She is due for relaunch first week in May with immediate
> testing/trialling straight after that.
>
> Thank you Eric and thanks everyone, I'd better go before I fall asleep,
> haha, Cheers to all, Julian
>
>
>
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