Mounting 5kw electric inboard

I'm undertaking an electric conversion project and planning motor mount, originally planned direct drive but on further reflection have decided to add a reduction gear.

To handle thrust propeller shaft runs through bearing bolted to a solid metal bracket, in turn bolted to wooden block attached to hull, picture attached.



I'm intending to mount motor inline, directly above the shaft to accommodate reduction gear.

Query I have is whether or not it is sensible to also attach the main steel motor mounting plate to the face of that metal bracket?

Obviously I'd need also to brace/support it elsewhere, but advantages would seem to be the solidity of the structure and it ensures motor shaft is at exactly same angle as the prop shaft.

But it struck me that the torque/movement of motor also being transferred to metal bracket might play silly buggers with the shaft? Is this a concern.

Second pic is just to give a visualisation of what I mean - the wooden template was mocked up for the direct drive idea, the reduction plate would be taller than that with opening for prop shaft and opening and then motor mount opening would higher up above the shaft. But essentially I'm looking to bolt a steel plate where the wooden one is sitting.

Does that make sense? Any input from wiser more experienced heads much appreciated!

 

Yes you can. To handle the torque you install brackets going from the top of the plate back to the floor visible in the picture. The brackets can be as basic as a piece of pipe with flattened ends bent to the right angle and a hole for the bolt. Or you can do triangular plates with bent edges, etc.
Depending on exactly how much space you have for bolting the lower part of the plate you might also need to install brackets going forward to the former engine beds, or a right angle bracket to the keel.
For the plate I recommend 12mm or thicker Al, it needs to be stiff to keep the pulleys aligned. Thinner plate will need stiffening bars welded on.

 

thank you, that's very helpful.

 

I'm curious why you don't want a direct drive. Your boat looks like a classic heavy displacement hull. Some years ago I designed an electric boat 29 ft and displacement 4 t. Direct drive with a 10 kW 48V inboard engine and everything was working very well.
Maybe the easiest way for you is choosing an engine with proper parameters.

 

I've already got the 5kw motor, so too late to change that decision. Bare boat weight as it stands in photos is only about 400kg. Obviously that will increase by the time I've added motor, batteries, floors, seating etc but it should still be well within capabilities of 5kw.

Reason I abandoned direct drive was specs of motor indicate is at it's most efficient at 3500 rpm whereas I estimated that prop would only require about 1200 rpm to move along about 4.5 knots. Sort of back of the envelope layman's googled estimate based on 20ft waterline length and 750kg weight.

I'd be delighted to hear my estimates are wrong and I am better off with direct drive?

 

Ok. Now I understand.
3500 rpm is a lot. I checked and my 10 kW engine had a nominal 1290 rpm.
Of course a lot depends on the dimensions of the propeller, but even without full data I guess that 3500 rpm and a direct drive is not a good idea.

 

@mugball

You can pretty easily take a bunch of pitch out of the propeller to accommodate a higher RPM.
Take a little out, try it, if it's not enough, take some more out.
It's worth a try. If you don't feel comfortable doing it, have it done by a prop shop.
Or, just buy a lower pitch prop to match what you've got in direct drive.
You can also cruise below best efficiency and still be ahead in reduced boat drag from lower speed.
You need your final boat weight, loaded, to do the calculations correctly.

 

The prop that was on the boat is 16 x 18 which I figured was way too big for electric motor. I've picked up a 13 x 9 prop locally which hopefully will do the job, but I still estimated it would be better off with a reduction.