Diesel engine and propane

But it could accumulate in the ceiling. Am I wrong?

 

That would really depend on the design and layout of the ceiling, walls, doorways, vents, etc, etc, etc.
Oh, and the air currents in the enclosed space, no?

 

You are more right than a saint (Hispanic expression that means that you are very right). In any case, you will not deny that the hydrogen will tend to go towards the upper part of the enclosure that contains it.

 

Unless there is some bizarre design, boat are open on the top. Also, bilges are below the water and don't drain overboard except through a pump or gasses through a blower. A barge with no open compartments and all machinery on deck would be an exception.

 

The thing to consider is economy. Combustion engines burn carbon, be it propane, gasoline, diesel.
The power produced is a function of the mass of the fuel, not necessarily the quantity in gallons.
Unless you are getting the propane for cheap, it is a poor choice in terms of BTU per gallon or dollar. Propane is only
4.24 Lb per gallon US.
Gas is 6.1
Diesel #2 is 7.1
Propane is 91,500 BTU
Gasoline E10 is ~120,240 BTU
Diesel #2 is 138,700 BTU

Diesel engines have a couple of advantages over a gasoline engine.
They are more efficient in terms of HP/LB/HR, and the gallon of diesel weighs more too. It adds up to a formidable advantage.
Screwing around with propane to seek out some imaginary advantage is just that, screwing around. There is no magic ratio that changes the number of carbon molecules you are consuming.
In terms of transient power like a sled pull, then all the bets are off. Ultimate power extraction is the goal and nothing we should concern ourselves with for a steady state reliable power plant like a boat or genset engine.
The downsides of carrying propane for fueling a boat have been mentioned by @gonzo and gasoline also has some pretty horrific problems in the bilges too.
Folks with little propane grills clipped to the rail leave the bottles out there for a reason
Diesel wins in every regard except for initial outlay for the engine itself.
I played with methanol injection with Cummins BT in pickup truck.
No matter how I sliced it, the money was not there. I could not get the methanol cheaply enough it was always more expensive that the diesel it was trying to help.
TANSTAAFL
There Ain't No Such Thing As A Free Lunch

 

Amen.

No point in converting an engine that runs on a convenient and stable energy dense wildly available fuel, to one that has very few of those features. Unless the price is very cheap.