In a message dated 5/16/2005 12:06:14 A.M. Eastern Daylight Time,
malcolm@tennantdesign.co.nz writes:

... The idea that  you will be using less fuel by running on one engine is a
myth and is not  sustained by testing.
Malcolm,

This sounds a little like the old argument about whether it's  better to let
the prop spin or to lock it when you're under  sail, it's my guess is that
there's not a single answer for  every case.  Are you saying this is a myth just
for powercats, or  for ALL twin engined boats?  There must be much accumulated
data on  this topic over the years -- certainly for monohulls at least?  And,
what about Dennis Raedeke of Wild Wind IV?  I believe he recently made  the
statement, "I cruised leisurely at 8 - 10 kts on one engine to save  fuel".
Maybe he only believed he was saving  fuel???

During our brief charter of a PDQ 34 powercat, I tried  some single-engine
operation.  I found that for an  economical speed of 6.5 kts, which occurred at
1400 RPM on both  engines, it took about 2100 RPM on a single engine to get
the  same speed .  Unfortunately, I have no way of making an  intelligent fuel
consumption estimate as the Yanmar-published charts show  almost nothing at
these low speeds.  Intuitively, though I would  have guessed that the
single-engine mode was the more economical of  the two.

Regards,
Henry Clews

Answer to Henry Clews:-

Yes Henry there is data and it supports the use both engines concept.
Unfortunately the intuitive answer is not always supported by the data.

Dennis Raedecke certainly did not "believe" he was using less fuel. He was
using less fuel! but that was because he was using controllable pitch
propellers which was why using one engine worked. Being able to adjust the
pitch of the propellers using an exhaust pyrometer is also a major
advantage.He has full fuel flow instrumentation so he had good data on his
actual performance/fuel consumption. Of course the drawback of controllable
pitch propellers is that they are very expensive and can only be justified
if you are doing long distances. I believe Dennis is already approaching
30,000 nautical miles so in his case the added cost is probably justified.
It would certainly be interesting to hear Dennis' thoughts on controllable
pitch propellers after his experiences.

You are right, this is very much like the fix it or let it rotate argument
for propellers except that I know the data on the single.v.two engine
approach but of course there are a lot of variables. There have been
experiments done of the fixed .v. rotating propeller and I believe a folding
propeller on sail boats [and including a self pitching propeller] but
unfortunately I can neither remember where it was done, nor what the answers
were.

We have looked at the self pitching propellers, which appear on paper to
have some of the advantages of the controllable pitch propeller, for some of
our power cat designs but as yet none of our clients has been brave enough
to commit to the idea.

Shame you could not use the Yanmar fuel consumption charts. Our experience
is that those manufacturers fuel charts are usually within about 2% unless
there is something very strange going on.

I hope this sheds a little light on the issue.

Regards,

Malcolm Tennant.