Tom, the trouble with using gallons per hour is that it doesn't measure
fuel economy. If I burn twice the gallons per hour, but go three times
as fast, then burning at the higher rate yields greater economy.

I will grant that if you are comparing identical boats at identical
speeds then gallons per hour would give the desired result.

BS


tomdownard wrote:

The reason we measure boats in gallons per hour and not miles per
hour? Wind and current make miles per hour impractical. With the
current against you, the engine can be running at 6000 RPM for an hour
and you have only covered two miles. I have seen that in the Inside
Passage around the Fraiser River. Too big of a veriable there. RPM's
and fuel used can be realistically monitored.

The only measure that makes sense is miles per gallon, drift, tide,
wind effects excluded, your choice of units..

Calculated against dollers per hour time on vacations aboard (what
price freedom?)

I calculate time at 10 bucks an hour for me, shopping for bargains,
whatever. The guests can do as they please.

Terry K

Terry K wrote:
The only measure that makes sense is miles per gallon, drift, tide,
wind effects excluded, your choice of units..

Calculated against dollers per hour time on vacations aboard (what
price freedom?)

I calculate time at 10 bucks an hour for me, shopping for bargains,
whatever. The guests can do as they please.

Terry K


Someone receintly said that as long as gas cost less than designer
water nobody was ging to worry about it.

Now there's a place in NYC selling water for over $50 a quart.
And people are buying it!!!

Richard

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

On Fri, 20 Jul 2007 17:12:45 +1000, wrote:

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 one thing, the off center thrust will push the hull through the
water sideways, even with rudder keeping the thing headed in a
straight line. Also on plane is more effecient than going too fast at
displacement speeds.

Casady

Stepped hydroplanes? Aeronautical work on flying boats and multi-
stepped hulls suggest power economy concerns at takeoff speed and some
intermediate speed transitions.

The hull speed equation is only one indicator.

Hull shape is important. A displacement hull will likely go faster,
further and cheaper than a planing hull at certain speed / power
combinations. it's finesse, I think, as opposed to fine-nese, though
hobie owners report incredible speeds with knife shaped hulls. It's
all about pushing an equal mass of water aside while climbing on top
of it before it can move. It's delta-vee, rocket science versus
frictional area, versus disturbances in the water (wakes) left behind
by hurried boaters.

Think about a nice sailing boat bumkin sliding down a pushing wave as
opposed to a water sucking vacuum behind a square transom. It costs
gas to keep a hole in the water charmed for a long time.

Include ball bearings made of air in there, and you are coming to
grips with most of the problem. The transition to wing in ram air
ground effect is particularly interesting, hovercraft like.

I'd like to see exhaust gas used as friction reduction near the
planing surface of a waterfoil wing almost airborne, almost
cavitating, in ground effect.

A water jet intake at the front could effect certain things, like
elimination of wake in a steady speed submarine. Would leaving a
bubbly, cavitated wake be more stealthy to a satellite looking for
large area reflective patterns on clear days?

Take a look at power / weight / speed curves with reference across all
hull forms at the surface.

Terry K