I've only ever seen one Thorneycroft diesel and it was 35 years old! The remarkably few Google entries on it say it's a 110 HP engine, which likely means a "cruise" (2,500 rpm, say) consumption of around 2 to 2.5 gallons/hr.
But a lot here would depend on sea state, hull configuration and so on. That engine is large for a sailboat. More info is needed.
We had a four cylinder Thorneycroft engine in our sailing boat it was based on a Ford block, a very good/reliable engine, it served on our boat for more than 25 years without any major overhauls, the only thing that eventually gave up were the sea water fittings, First thing was the exhaust manifold rotted through (the newer manifolds are now fresh water cooled), and filled the engine with salt water (not good) we got it running by draining the water out and flushing it with thinned oil and then running and flushing again, the following weekend one the end caps on the heat exchanger fell off (the tie rod had rotted through, so fitted a new one) and we carried on our trip.
After these two episodes and a couple of other things we decided to re-engine that winter (10 Years ago) and we ended up with the same engine second time around but finished by Mermaid it is still the same ford engine/block.
One interesting thing was we managed to sell the old engine to someone we knew and he fitted into an old lifeboat, however he had it stripped before by an engineer that supplied the new engine and I looked at it and thought why were we changing it as the bores were spotless and the engine was in very good condition.
As regard to fuel consumption and RPM it depends on what you are trying to push and how easy it is pushed we have a 48ft steel ketch built in 1939, she is bilge keel (quite a bit of wetted surface, slowing, although no keel in front of the prop) we would cruise at 7.5-8 Knots with 1700 RPM, Max RPM 2000 would give a to speed of 8.2 Knots, fuel consumption at 1700 RPM about a gallon an hour but as we always had a sail set I think we were less than that 90% of the time.
The brochure said max rated RPM 2500-2600 for max HP about 62 or 68.
(Sorry some of these details are a bit vague after this length of time……)
The new Mermaid engine had been up rated to 90HP (this was another reason for changing the engine), we did get a slight increase in speed but not much (I think we are just burying the stern more). We tend to run the engine between 1500 and 1700 RPM dependant if we are in a hurry or not however at tick over we will motor at 4Kts.
Valiente I see you have a steel boat too (sorry off topic) I think the 110HP engine was for the turbo version (no intercooler) as someone fitted one to another small motor boat that I knew and I think he said it went from 60-70 HP to 100-110Hp, someone may correct me though.
I think 2500RPM might be the top end RPM again it’s a long time ago…..
Hope this helps
Interesting stuff, Mallo. As I said, I've only ever seen one Thorneycroft diesel and it was in a steel "CN 35", a semi-custom built here in Toronto in the '70s.
I have seen a Ford Lehman diesel, however, in a 33 foot, 19,000 lb. 1964 ChrisCraft sailboat (yes, it was built like a tank!). I think that was a 75 HP model.
My motorsailor has a 52 HP Westerbeke based on the Mazda R2 block as found in several thousand Mazda B2200 light pickups. Other than going from a three blade 18 inch to a four-blade 19 inch VariProp to improve maneuvering under power and less resistance under sail, we are happy with it. I have the room to go to a bigger engine, but not the desire for reasons of fuel and the fact that making a sailboat do six knots instead of five seems a little ridiculous to me...if you're in a big hurry, don't travel by sailboat!