Replumbing an Outboard for Heat

I think the reason manufacturers don't include this possibility is not for any technical reasons, because there aren't any, but because it would allow dangerously hot water to shower out of the top of the leg. That sounds dangerous.

 

It is possible to modify the water passages. However, the total flow can't be restricted or the motor will overheat and also the water pump will be damaged by the exhaust gasses. A heater core can be fed with a diverter valve. That allows the total flow to be unrestricted but directs the amount of flow to the heater core. The main engineering challenge is to modify the water passages at the point that mixes the water with the exhaust.

 

Exactly @gonzo ! It's an engineering challenge. And we like a challenge.

 

And think of the benefits. An outboard is no more than 40% thermodynamically efficient. So 60% of total fuel burn just makes hot water.

 

That's not quite correct. The exhaust gasses carry most of the waste heat. If you could collect the water at the propeller, that would recover a lot more heat.

 

You already have enough challenges, DC, at this rate you won't be on the water till 2025.

 

Well, yes. The exhaust gases leaving the cylinders absolutely do. By the time hey are on their way down the leg, a lot of the heat has been absorbed by the cooling water. Certainly not all, but if we could get all the waste heat into the boat, it would be too hot. The specific heat of water is ~3500 times higher than air, per unit volume. Your point is well taken though. If we could run the exhaust gases right out of the block through a heat exchanger, we could do some work.

 

Your water won't be hot enough to make it to the radiator inside the cabin.

Gonna be like 125 degrees after flowing to the cabin via outside passages. The typical real heater fans don't even operate below 120F in a standard system because they will feel like blowing cold air against a 98 degree human.

Anyhow, I think the temperatures are not going to be high enough on a gut hunch.

 

Run an outboard up to operating temperature on a flusher unit, and the water exiting the engine is not that hot, not the same perhaps as running under load, but the temp at the cylinder head is similar to on-water operation, and more water will be going through the engine then. That is an experiment you can conduct pretty easily, measure the temperature of the water exiting the prop

 

Those motors have a 140 degree thermostat.

 

Yeah, so by the time the lines get to the cabin; they are gonna be too cold to do much against heat losses..

Really do some heat calcs before tearing into the engine. There is some break even point or you can calc the btu provided, but gotta think about 1-2k btu is close.

Then calc the need. 50F to 70F is a 20 degree heat rise against all the windows and low r value ceiling, etc. probably 4k btu, but I could be wrong

 

They vary quite a bit. There's a whole thread about doing it on Hondas. Thermostat opens at 210 or thereabouts. The Johnson forum talks about 160 engine temp.

 

I would think outboards are like car engines the hotter you can run them the better the efficiency .. newer ones probably have to run hotter to meet stricter standards just like a car. Even at 140* you should be able to get useable heat.

 

Except ob waterpumps work on engine rpms. The faster you go; the faster the water is ejected through the engine bottom. Yeah, the powerhead may get warm, but the telltale water is always hotter at idle. And who wants to rob waterflow from a whipping high throttled engine? Not me.

 

Holy cow. Okay, I'm going to keep talking about this, but to actual engineers and mechanics. By pm. I'm going to unwatch the thread.