Big diesels at displacement speeds

We have rather large old diesels (1972 10.4 litre 270 hp) which are turbo's and we tend to bimble along at 1600rpm which uses about 20 litres an hour and with the tide gives us around 10 knots, as Hayden says in a beam sea it does roll a bit. If its a bit too rolly we use 2000rpm as the boat stiffens up at those revs. Had no problems with the ancient technology so far and we do give it a bit of a thrash every now and then just to clear it out.
 
That would be unfair on the F34 as this would be as ineficient as the F34 at (say) 20knots.
To get an efficient displacement figure for the F34 its speed should be about 7 or 8 knots.

Our Sealine is bigger hence a bigger displacement speed - we do quite a bit of cruising at about 9 knots when we arent in a rush or meeting a tidal gate (relates to 850 rpm on the D12s).

Normal cruising for us is about 2 galls per nautical mile (24 knots)
Displacement cruising is about .7 to .8 galls per nautical mile (as I say about 8/9 knots).
 
Apples and oranges. Not sure what the comparison would tell? Maybe that a cat at it's most efficient uses x amount of fuel and a planning boat at it's LEAST efficient uses x amount of fuel. You wouldn't compare the figures to give a comparison would you? Or maybe YOU would /forums/images/graemlins/grin.gif
 
Modern FP cat showed 3.9 miles per gal at 9 knots. A displacement cat like FP will be much more efficient than the planing mono like sealine.

For higher speeds above 20 knots the planing sealine would be better (FP cannot reach these speeds anyway!!!) . However, it would be a different story if we put a planing cat against the sealine. The planing cat will be more fuel efficient at these sorts of speeds than sealine, particularly if fitted with a hydrofoil.

It was rather strange to put a displacement boat and a planing boat head to head in the article.
 
My boat, wich is SD hull does about 2.4 mpg at 8.5 knots, about 1 mpg at 10 knots and as little as ).33 mpg at 23 knots.

IMHO
A planing hull of the same length would give a poorer consumption at 8.5 knots but a lot better - say 0.5 mpg at 23 knots. At low speeds such a hull could roll badly and would not be that comfortable in any sort of decent sea.

The displacment cat boats of the same size would knock spots of both of them for fuel consumption and come out weel ahead throughout the range. Its twin narrow hulls have a much higher displacement speed.
 
Few people in UK leisure boating understand the displacement cat and planing cat concepts. The only benefit I can maybe see for derogation is a big change in peoples perception. I hope to see a lot more exciting designs built to match this demand. It could be very exciting times compared to the stale, revamped, conservative, boring offerings from manufacturers over the years. Lets see a few of the fuel efficient exciting drawings in UK boat design competions actually built.
 
Some of my most enjoyable trips last year were made on one [either] engine on auto pilot coast hopping around the i.o.w up on the flybridge sunbathing. Dont have to go everywhere on the plane if you have plenty of time. At 1200rpm the one engine pushes along at about 6kts without the turbo blowing. You cant hear the engine running. Dave.
 
yes davedpc agree completely. We travel like this all the time. we have twin 225turbos in a 38 footer s/d and the economy at 6/8 knots on one engine is out of all proprtion to our upper cruising speed of 15/16 knots.
Brighton to Fecamp in about 7/8 hours - who cares how long it takes if its a nice day!
 
Just because I've nothing else to say I'm going to say that last year we did 140 something hours and averaged 11.6kts over the whole year. We cruise at 22-23kts but we do have a fairly lengthy 6kt trundle up Chichiester Harbour to get to the marina. If we cruise at 18kts which is what we've worked out to be most efficient cruising speed (about 1.6mpg) then annually we'll average near displacement speed anyway.

The log is religiously switched off when we stop so the average isn't being distorted by sitting around at anchor doing precisely 0kts!
 
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