Pootling - I get it now.

[rustybarge]

Assuming that both the monohull and the cat have efficiently designed hulls and the same overall weight, I'd expect the cat to burn a bit less fuel.
But that has not so much to see with the hull speed point you previously raised.
In fact, even if it's true that the (max) hull speed of a 34' cat can be higher than the one of a monohull, I don't think it's correct to compare that with the hull speed of a 68' monohull.
Length remains more relevant than beam, AOTBE.

I made the mistake of asking a question like that on the boat design forum, after being bombarded with prismatic coefficents and other esoteric formula, I decided 'rule of thumb' was the best policy: 'If it's fat and heavy it'll probably use more fuel'

 

[LittleShip]

Pootling along at 7knts.....

Cough you lot thats Little Ships cruising speed mind you that about 7lts/hr.

We went pooling yesterday around Tarbert to Arran about 6knts calm and enjoyed a cuppa and watched the mull pass by. There are a couple of very nice houses along the way.

Tom.

 

[rustybarge]

Pootling along at 7knts.....

Cough you lot thats Little Ships cruising speed mind you that about 7lts/hr.

We went pooling yesterday around Tarbert to Arran about 6knts calm and enjoyed a cuppa and watched the mull pass by. There are a couple of very nice houses along the way.

Tom.

That's about the same as the Swift trawler , even though your hull is an old design.

 

[LittleShip]

That's about the same as the Swift trawler , even though your hull is an old design.

Yep I agree that the small swift can achieve that at displacement speeds but it will weigh a lot less than Little Ship.

Tom.

 

[rustybarge]

Re engine damage at low revs/loads...
Here's the info from the Boat designers forum explaining what happens:

The >problem< of under loading can simply be solved by using a bit more throttle .

Instead of say 1200rpm ,1400rpm would load the engine higher and assure a higher engine load .

The real hassle with under loading is not the carbon in the exhaust .

The rings on a diesel seal because compression pressure gets behind the rings and forces them hard against the cylinder walls.
Light loads mean less sealing pressure so blow by is increased.

The big problem is with no sealing load the rings will burnish the cylinder walls , polishing off those tiny scratches from the hone that are needed to hold lube oil.

After a while with less cylinder lubrication the cylinder wears and the engine begins to SLOBBER. Black slime coats the exhaust and engine vents.

Using a $100 EGT gauge would allow the engine to be worked hard enough to avoid all this at low load.

The choice is a bit more fuel burn when MS or finding and rebuilding an old CPP for your sized engine.

Thousands of outlay up front or an extra $50-$100 for every few thousand miles of MS operation under powered sail.

Your choice.

Dont let some ideal keep you from contemplating the math.

In your boat at probably under 2GPH a 10% reduction in the fuel burn will take a decade underway to see the just first thousand $$$ saved.

 

[spannerman]

Maybe a two stroke as it circulates its mixture both sides of the piston, but 4 strokes build up condensation in the oil.