Electric sailboat motor: new conversion project (update: project shelved)

[MetroMPG]

The boat goes in the water tomorrow (crane day!). Woohoo!

But this presented a minor problem with the gas engine removed from the lower unit (transmission).

Namely: sinking quickly to the bottom of the harbour.

The problem is that with the ICE removed, there is an open passage up through the gear case (below the hull) into the engine compartment to feed cooling water to the ICE. And since the top of the lower unit (inside the boat) is below the waterline, you can see where this would ultimately lead. Swimming with the fishies.

(Incidentally, this is an inherently bad design. If you ever have to change the water pump impeller - not an uncommon thing to do on marine engines - you first have to remove the engine from the lower unit to get to it. Which suddenly permits water to flood into the boat. Which means of course this boat has to be lifted out of the water to replace the water pump impeller. Clearly not designed by Germans.)

So my first task was sealing up that water passage.

I removed a plastic screen from the water intake, shaped a plastic plug to fit the opening and then hammered it in. Then I epoxied over the plug (several layers):



After the epoxy cured, I slathered a liberal coating of silicone over top...



To be extra safe, I will also be sealing up the top of the cooling water passage, inside the engine compartment. That's my task for this evening.

And here's a view of the top of the lower unit, showing the input shaft that I'll be mating to the electric motor...



Edit: I'll also be removing the impeller (which is mounted directly on the input shaft), since we don't want that spinning merrily away in a dry water pump housing.

FYI, the cooling water passage is visible at about the 8 o'clock position from the input shaft.

[MetroMPG]

FYI, I'm a noob when it comes to propeller theory. I haven't read up on it yet.

But the prop on this Sail Drive is a 3 blade, 11 inch diameter, 9 inch pitch.

The gearcase has a 14:28 ratio. (Why didn't they say "1:2"?)

The maximum operating speed of the ICE is "3300-3700" RPM according to the manual.

ICE idle speed (in gear) is 600-700 RPM, and that's enough to move the boat along at about 2 knots (2.3 mph / 3.6 km/h).

[RobertSmalls]

It's proud of how many teeth it has.

[Daox]

Awesome, I love it. Its very cool to see a different project like this. Even though its fairly simple, its still just fun.

[MetroMPG]

Ran into a potential snag this evening.

I removed the impeller, and while doing so discovered the cooling water passage isn't going to be as easy to seal/contain from above as I thought. It really needs the bottom of the ICE to be sitting on it (or some facsimile!).

On top of that, looking over the manual, there's also a small chance that the seal I made in the water inlet of the gearcase (under the hull) doesn't make the gearcase 100% watertight! Water may also be able to find its way in through the cap on the end that holds the prop shaft/guts in place.

We'll know for sure tomorrow morning when the boat goes in.

Worst case scenario is I will have a slow leak, and will have to lift the stern of the boat up with the stationary crane to sort it out (to get the relevant bits above the water line. I had to do that once to change the impeller.)

Some good news though: with the impeller housing removed, the input shaft pops right out. That means I can make the coupler on the workbench, instead of having to monkey around in the engine compartment to figure it out.

[dcb]

I'm 100% certain you have thought of this, but have you considered something a little more "outboard" like? i.e. shave that wart off the bottom of the hull and smooth patch it, and come up with a small electric drive that you can tilt or lift up and out of the water when under sail? Could be bow mount or stern or whatever. Oversized trolling motor basically.

maybe a rack of trolling motors, linked together?

[dcb]

If Bryn is right, you can probably horsetrade for one or a couple reasonably large purpose built trolling motors. But getting the props out of the water alltogether under sail might actually make for an improved sailing experience.

[lightflite]

MetroMPG

Ref your cooling/sealing challenges - I have a natural law which states that "secondary effects are always Primary"

Your mind is soaring with the optimum motor, and a left-over system from history is giving you grief.

If its any help with the power required, boats like this are (used to be) powered by British Seagull longshaft outboards. Altho plated as 4 HP, I understand they measured as about 1/2HP - but they had huge reduction and swing a large coarse prop. What they lost in power and honesty they gained in propulsion efficiency.
They would give a boat your size 3 to 4 kts and be adequate for most locations (except the Maelstrom or Minches)

For your relatively short running time you could probably also add more volts to your small motor to get the required performance - and watercool it if necessary for survival

andrew

[moorecomp]

Under the water pump housing, the metal plate with the semicircular cutout could be remade without the cutout and installed with a full gasket under it. Liberal use of sealant and removal of the impeller and you should be good to go.

[MetroMPG]

Quick update: first boat in today (I don't like being the "test" boat). The good news: no crane mishaps! (Until later.) Oh, and no leaks ... it was still floating last I checked.

Some good ideas/info in the past few posts. Will reply later on.