T-Pain Is On A Boat: Re: [Electric Boats] Re: does anyone have current real world performance figures please

Wednesday, March 27, 2013

Re: [Electric Boats] Re: does anyone have current real world performance figures please

Hi CNC

- That was how I originally figured out the 1:3 electric:Diesel ratio myself by talking to boat builders.

- As you so rightly say, the old boats, especially working ones, had by our standards miserably small engines with slower, larger props and seemed to work just fine.

- Then the pleasure boat industry became omnipresent and when one guy starts up his own yard they have experience but no knowledge.

- The yard they left used a 300HP engine on a given design, and as they weren't paying the fuel bill but wanted to outperform, or at least sound better, then their opposition why not use a 350-400HP engine!

- Most everybody mistakes a bigger bow wave for increased speed, and so the "there's no substitute for horsepower" b------t made the jump from cars to boats.

- My only concern, as I said to Eric, was that all my hard work, reading, listening and searching can be undone with a bad prop choice, and so any input in this regard is unbelievably welcome.

Thanks again, Julian

P.S. I take it you are in the CNC game, so I wonder if you might be able to tell me where/what site might be the best place to sell a virtually unused Roland MDX-540?

On 27 March 2013 08:22, cnc sales <gcode.fi@gmail.com> wrote:

Like usual, Eric has explained it very well.
Agree 100%.

However, I would note that, imo, 99% of people overpower their boats.
You do not need as much power as is commonly believed, basically, as Eric said, by forgetting the last knot.
This reduces your power needs by 50%.

The power needed was for a long time driven by (mostly) US marketing, "you cannot have too much power" in the era of cheap gasoline and cheap ICE gas engines.
Beforehand, just like in sailboats, the needed power commonly quoted was 50% less.

Sailboats in the 3x ft/10m size were for a long time powered by something like 8-12 hp, and work very well.
11kW electric ia much more than you *really* need, so I believe you will have much more than enough.
IMO, I expect that even with 7 kW, you would have quite enough.

Commercial craft, ships, for cargo, on the open seas, in the 25 m range, used to have a 150 hp engine. In the early 1900s.
And they were NOT underpowered, rather the problems were in reliability and mechanical issues.

My friend has a 70 metric tons, ton, 25 m, displacement steel boat.
He has a 100 hp diesel engine, and is way overpowered (in that the prop will cavitate effortlessly in over 50% throttle).
The engine is similar to an electric that is has a huge amount of torque compared to an ICE.

He cannot be bothered to pay (a lot) more for a bigger prop, as he will get hull speed anyway, so why waste the money.

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
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--   -hanermo  