Datsun Diesel drivetrain

[sal's Dad]

I am planning to build an Atkin tunnel-stern boat. Scaled up a bit, I figure the design will require 40 hp or so. Aluminum hull.

I like the looks and functionality of a boat in a museum near me - a 1930's lobsterboat, with Ford engine and transmission, hand-operated clutch lever. Not sure what was used for a thrust bearing. As I recall the differential povided power to a niggerhead (am I allowed to use a politically incorrect, but historically accurate term here?) on deck.

A family member has a rusty Datsun Diesel pick-up truck, engine still in good shape, and would like to see it used in a boat project.

I know I am nuts, but does it make any sense to pursue the idea of dropping engine and transmission into a boat?

With the right prop, transmission could allow some gearing" - low gears for towing and maneuvering, 3rd for cruising, 4th for light load and flat water, high speed.

Fresh-water cooled, using an integral keel-cooler, minimal electrics, maybe no fuel pump (a gravity feed "day tank") etc. CV joint from drive-shaft to prop-shaft, with some kind of thrust bearing. Presumably water-cooled exhaust would be quieter, but fishermen around here just use straight-pipe exhausts, and I hate holes in the hull...

I know for a gasoline conversion, pretty much all the electrics would have to be replaced, but not sure about diesel.

Thanks for any advice or suggestions, however absurd. And yes, I know it would be easier, safer, and "better" to spend $10K on a marine engine... But what would be the fun in that?

[Corpus Skipper]

(a gravity feed "day tank")

These are ILLEGAL!!! If you break a hose, all the fuel will drain to the bilge, and we know what can happen next.

pretty much all the electrics would have to be replaced, but not sure about diesel.

The starter and altenator would have to be ignition protected marine types.

I know I am nuts, but does it make any sense to pursue the idea of dropping engine and transmission into a boat?

The transmission won't do you any good, shifting gears under load would surely destroy the drivetrain, and the prop has to be chosen for a given speed and load, so is only efficient at one reduction ratio anyway. Spin it too fast, it will cavitate, too slow and you're wasting fuel. Most automotive trannies have a final drive of 1:1, or overdrive of .75:1 (or somewhere in that neighborhood), which would overload the engine and burn it up in short order. Most diesels have 2:1 reduction or higher.

[sal's Dad]

Thanks for your comments -

Originally Posted by Corpus Skipper

These are ILLEGAL!!! If you break a hose, all the fuel will drain to the bilge, and we know what can happen next.

That makes perfect sense - My experience with this system is in British Seagulls - very elegant and effective! Any leakage (and there was always some) went straight overboard.

Originally Posted by Corpus Skipper

The starter and altenator would have to be ignition protected marine types.

Again, makes sense. I wasn't sure if Diesels had different rules!

Originally Posted by Corpus Skipper

The transmission won't do you any good, shifting gears under load would surely destroy the drivetrain,

I don't follow - isn't that what a clutch is for?

Originally Posted by Corpus Skipper

and the prop has to be chosen for a given speed and load, so is only efficient at one reduction ratio anyway. Spin it too fast, it will cavitate, too slow and you're wasting fuel.

Yes, I understand. But if a prop were selected for a gear ratio of 2:1 and "typical" loaded, choppy conditions -- and you sometimes run at say 60% displacement, in flat water, with no spouse to tell you to slow down -- isn't it possible that a gear ratio of say 1.5:1 would be an improvement?

Likewise if towing (or in very rough condidtions), with the same prop, but at a speed of 5 knots, instead of 15, wouldn't a gear ratio of 3.5:1 be an improvement, instead of lugging the engine?

[FAST FRED]

"Likewise if towing (or in very rough condidtions), with the same prop, but at a speed of 5 knots, instead of 15, wouldn't a gear ratio of 3.5:1 be an improvement, instead of lugging the engine?"

You can only absorb so much power from any sized diameter of propeller.

If your setup for top or cruising speed a 25% change in tranny ratios will surly help to optomize that cruise or top speed.

Going to 3.5 to 1 would only help reduce speeds for close manuvering.

If the fuel tank is above the engine , the required top outlet (rather than bottom discharge will still alow the fuel to syphon out.

Personally we prefer the gravity feed as any fuel leaking is EZ to see, an air leak is LOTS harder to find.

FAST FRED