Small diesel working hard or Large diesel working easy?

Hi Don,
True, mine is designed as a sailing cat, but is not as yet fitted with mast and sails... at that time I had a pair of 2 bladed folding propellers similar in appearance to the "gori" folders... at 3600 I could do 8 knots or thereabouts... now with the fixed 3 blade I still get 3600 rpm so I am not 'over-propped", but have not run far/long enough to quote a top speed... I seem to get close to 2 miles (nautical) per litre of diesel consumed with both engines running at 2500, which is close to max torque and around 8 to 10 hp applied by each engine or less?...

I have sufficient grunt to achieve higher velocity but usually delay a passage to gain best mileage in a tidal flow/weather window... With a good swell behind me the boat is a willing "surfer" and has sustained better than 18 knots on several days... I feel that the engines are enough as - is - - - for my present needs...

Hypothetically, if I was to build again it would probably be a very similar design or a 40ft equivalent by Robin Chamberlin - see the C10 in my library but to cruise at nearer to 15 knots...

 

My twin 330's run at 15lph (combined) at 8 - 8.5knts in my 43ft planing hull. L/NM basically doubles if I go from 8knts up to 18 - 20knts. We also have a couple of smaller (36ft) boats with twin 160's and twin 170's in them. I haven't done a good study, but my general observation is that if I didn't need the hp to get planing I would probably get slightly better fuel burn at 8knts with twin 160's. On the other hand I wouldn't be sitting now at 4000hrs and thinking that there is another 4000 - 6000hrs left in the engines.

 

Thankyou everyone for you comments and insight. Obviously there was more to this than i thought initially,i had assumed that a bigger engine must consume more fuel than a small one but there are so many variables to account for. Michael was right when he said millions of answers.I have focussed on fuel consuption rather than "economy" and that could lead to problems.
The price difference between a large engine and a small engine can buy a lot of fuel but if you're heading for the rocks in a strong current and high winds i'm sure i'd trade the fuel and engine cost saving for some extra grunt. Sooooo many things to consider. Thanks everyone.

Masalai i did look at the C10, i have looked at it before and its not quite what im looking for, I am after decks that are flat.The C10 reminds me of some of the Schoinning designs that may be very good looking but not functional for me. We have a Jack Russel that would fall overboard very quickly on the C10. I have recently bought some study plans for a Mike Waller powercat that looks like it may be suitable for me.

Thanks Don

 

Also remember cost wise that larger engines running lower RPM and less loaded are going to last much longer than smaller engines that are spinning fast or loaded hard.


If you ever send in oil samples from an engine, you'll see that the harder loaded engines always have higher metal content in the oil from wearing faster.

Even if the bigger engine was harder on fuel, it should last a bunch longer.

 

I seem to recall that on some website for a boat firm (I think it might have been Nordhaven) that the same boat at "cruising speed" burned less gallons per mile with the John Deere 6 cylinder than John Deere 4 cylinder. The 6 cylinder was at lower revs.