On Thu, 21 Jun 2007 07:24:07 -0700, R Swarts
wrote:

If
one has a twin screw boat, does fuel economy increase or decrease in
running only one prop?

Bob Swarts

Hi Bob,
I sail a keeler but last year in my job I was aboard a planing hulled
launch powered by two 1250 HP turbo charged MTU diesels, each driving
its own propellor.

I don't know the physics of it, but I experienced a situation where a
single engine used far more diesel than twins.

We went at speed (about 45 to 50 knots) over a shallow patch and
somehow a stone got sucked into one of the two water intakes, smashing
the perspex (later replaced with polycarbonate) cap plate.
Unfortunately this was placed directly under the air intake for the
turbocharger which sucked the intake water directly into the starboard
engine cyclinders. Result - instant stoppage on that engine.

It was decided to slowly motor with one engine back to our home base
where repairs could more easily be done. We originally had more than
sufficient fuel to get back home uinder two engines and then some. We
ran out of fuel about two thirds of the way and had to be towed into
port. As I said, don't understand why.

For my keelboat, I normally calculate useage by the rule of thumb - a
tenth of a litre per horsepower per hour. Mine develops 37.5 HP at
full revs of about 3,000. According to the fuel usage curve in the
supplied manual, the best efficiency is at about 1800 revs which is
what I usually run it at - developing a lot less than 37 HP - probably
25 as I use about 2.5 litres per hour at those revs.

Hope this helps
Peter