Heat recovery

Hi all.
I am exploring options for heating the boat while underway. Currently have a Webasto forced air diesel heater which works very well but it seems such a waste of energy with the engine not being utilised for heat in some sort of way.
The main engine is a Gardner 8LXB which runs very cool at 60C or 140F so not efficient to run the cooling fluid through a radiator really. So does anybody have any experience or thoughts on using the exhaust for a heat source?
It has a water cooled exhaust manifold but still lots of energy in the stainless steel dry stack. I think the exhaust is about 5 inch so fabricate a 6 inch section that could clip over a section say 1 meter length with a fan and insulated ducting to connect to the existing Webasto ductwork. I have been warned off heating water from the exhaust as it is so hot that eventually is will boil and cause headaches.
Any thoughts gratefully accepted

140F at the engine should give you plenty of heat.

My Cummins originally had 160F thermostats and I ran a coolant to air cabin heater, it worked very well. 30F outside, 75F in cabin. At idle, the heater would actually sub-cool the engine. No problem with a little load on engine.

Later I changed t-stats to 180F (for other reasons), and heater worked even better.

I would not mess with the exhaust.

I have a school bus heater, small heat exchanger with 3-speed fan. Engine says 140F, works great, not too noisy.

The easiest way to get heating out of an engine is using the coolant and buying a marine version of a car heater.

I have a hydronic system that can be heated by the mains when they're running. And I can warm the engines with the boiler. I have valves to cut the engines in.
The problem with using the exhaust is that the high temps cause minerals, etc., in the water to evaporate out and leave a film on the pipe that eventually acts as insulation. Anything but stainless rusts out.
There is a way to use the exhaust to make fresh water like an evaporator in steam plants. The navy used it in WWII on diesel powered ships. It's a lot of maintenance to clean out the salt.

, so depending on how many areused , might need some help.The main engine is a Gardner 8LXB which runs very cool at 60C or 140F so not efficient to run the cooling fluid through a radiator really.

"Not as efficient" just means the heater has to be slightly larger.

It is difficult to install in an existing boat but house baseboard heating elements stripped of their metal outer shell can frequently be used , with no maint. once installed.

Most folks will install a "bus or box heater" , with 140F water instead of 180F , these can still be zoned , a thermostat can turn on the fan as heat is desired.

The circ. water folks will usually have good heat in the heads and heated towel racks plumbed in are a great luxury.

Remember on modest cool nights after engine shut down there is a ton of hot metal in the engine room.

A small centrifugal pump can send this heat thru out the boat , quiet and cost effective .The pump may be required as bus or box heaters are an obstruction to the engine coolant flow

On my prior boat I had a coolant to coolant heat exchanger (just a pipe in a pipe) in the Webasto loop. When the engines were running the Webasto never needed to fire. Just the circulation pumps were running. Simple and effective.

First think about how you will use the boat. Are you wanting heat while underway, in which case, coolant heat could be very helpful. But if the majority of your heat needs are at anchor, I would stick with the Webasto that you already have.

Also diesel fired forced air heat is not very expensive. 20,000 btu/hr will heat a small boat, 40,000 btu/hr will heat a big one. 20,000 btu/hr at 75% efficiency is about 3/4 of a pound or a tenth of a gallon of diesel an hour which costs about 30 cents an hour for heat. Think about how many hours underway you will need heat and see if saving 30 cents an hour is worth the investment.

David

Thank you all.

More to consider, excellent