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post #55 of Old 01-05-2019
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Re: Electric Conversion log for Kahleesea

Originally Posted by Captain Canuck View Post
Thrust dictates speed. Once the water resistance matches the thrust from the prop, acceleration stops and the speed remains steady.
I will take a stab at trying to explain why the above statement makes me cringe.

Thrust is not static. A given prop/rpm does not create say "500lbs of thrust" and when you reach "500 lbs of water resistance" you achieve equilibrium. This is not at all how this works, and is an especially misleading train of thought if you are attempting to armchair/roughly calculate efficiencies or select the correct prop.

A propeller thrusts a boat foward due to the lift generated by its blades' angle of attack relative to the water column it is spinning in (the AoA often times indirectly referred to as 'slip,' or the speed of advance of the propeller VS. the speed of the water column it is advancing in). Thrust, the force pushing forward on the prop shaft, is a dynamic force and depends on RPM, pitch of the propeller, speed of the boat. As the the boat speeds up, the angle of attack at a given RPM is reduced, and so is the lift, and so is the thrust. When the boat is not moving, at the same RPM the blades can begin to stall, create turbulence and possibly cavitate, generating very little thrust. When optimizing for efficiency, you can only optimize for a single point in the speed/thrust curve. Everything slower or faster than this particular point will not be at max efficiency of power in vs. power out of the system, and for thrust can be wildly different from what you expect.

A perfect example of this is the lowly trolling motor, which may have 100lbs of thrust at zero speed, but at roughly 4-5 kts, is generating zero thrust. You can put 5 100lb thrust trolling motors on the same boat and it will not go any faster. It can't. The propeller blades are effectively at zero AoA to the water column they are moving in.

SO let's say max efficiency is roughly 50% slip (Just throwing a number out there but this is a fairly commonly used point), you prop your boat so that at max RPM the props are going twice as fast as the water column. You're aiming for hull speed at say, 7.5kts, so you pitch it so the speed of advance of the prop is say, 15 knots. This will give you maximum thrust at 7kts, and will be great for your ideal speed run in calm water with no wind. however... when the 30knot winds on the nose are blowing, the chop is reaching 3-4 feet in the bay, your boat has slowed down, the propeller at max RPM will be at various states of stalled and will NOT generate maximum thrust where you now need it the most, to make meaningful headway against the conditions. It's like oversheeting your mainsail. You keep pulling but the boat doesn't go.

The conservative angle would be to pitch the prop for say, 10kts speed of advance. Maybe now you can't go faster than 6 kts, because you are not generating enough thrust to overcome your wavemaking resistance. But when the crap hits and forces your boat to slow.. you have a lot more thrust at 4-5 kts headway to push through the wind and chop, and your prop is not operating in a stalled speed regime.

When designing for a system with limited power -- speed is great, but it comes at the cost of low end grunt. Low end grunt comes at the cost of speed. This is the compromise you have to make when designing for efficiency, speed, or power.

Whether that thrust comes at 800 RPM or 1200 RPM is probably irrelevant. I guess I'll find out. I can't put Khaleesea on the hard until some space frees up, so I don't even know how big my prop is right now. Probably 16x10 or thereabouts. Hopefully I'll be up on the hard soon so I can take measurements.
The higher RPM means that the change in AoA across the speed regime is less drastic, so the efficiency peak, while lower, is more broad (it's over a bigger range and you are less likely to stall at the lower end of the speed range than the higher pitched, slower prop)

I can't find an inverter/charger unit that works with lithium batteries.

Are you putting your genset where your old diesel was? I can't imagine there are too many places in a 29' boat where you can put something like that. I'd also be interested to see your wiring diagram if you have one.
The boat came with an atomic four (ugh). I'm opposed to explosive things in a cave so want to avoid propane/gasoline/etc. The thunderstruck motor + belt drive take up little enough room that I can fit a 5kw genset roughly in the engine bay in front of the motor, if i build out the front of the engine bay out far enough to comprise the second step of the companionway 'ladder'. Since i had to rip the floor out to get the oil cleaned up from the ranwater flood that allowed me to pick up the boat for cheap... i'm rebuilding that, the galley, the bulkheads, etc. The new engine stringers are in now, though, and i'm rebuilding the compression post step/cabin sole now. Exciting! Take a look at the Victron Quattro/Multiplus/whatever they call them. They can adjust charging voltage in .1v increments, plenty close enough for lifepo4 @ 90-95% charge regime, and you dont really want to top lifepo4 off anyways.

Okay off to dinner...

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