Rich Hampel wrote:
Definitely somethingw WRONG here ... A 2Gm should be using 1/2 GPH
Blocked exhaust pipe at the water injection elbow will do this....

Make chart of actual boatspeed vs. rpm. Plot various points of
operation in kts. vs. rpm ... from slow to wide open throttle
operation.


Do this in calm conditions, and do not use a gps, you want water
speed, here.
-TK

The data points (well below hull speed) will describe a straight line

The faster you go in that range, the more fuel it takes to cover a
given distance, on a more or less linear scale.


.... then as the bow begins to rise, the data points will 'knuckle' and
the straight line will begin to curve upwards.
If you operate anywhere in the range of the straight line the fuel
economy will be at its best, if you operate in the curved portions, the
fuel economy will be worse, if you operate where the tha curve starts
to go straight up ..... all you're doing is using energy to lift the
bow and develop huge bow and stern waves .... and you wont go any
faster than being down the flat section of the plot.

Well, you will actually, but the extra fuel will knot buy much. (ouch!)

Hmm, sounds like a fore and aft inclinometer might be calibrated to
read efficiency. My old Omni had an efficiency indicator on the
dash. -TK

Go to your engine manual and look at the power curve HP vs. rpm. vs.
GPH.


The height (or depth) of the wake might also serve as a rough
economy guide. A 4" high wake means you have depressed an equal
amount of water downward, and it rebounds. The amount may be taken
as the displacing weight, the length, shape and speed of the boat
and possibly the depth and width of any canal will determine the
frequency of vertical oscillation of the water. The speed will be
indicated by the height of wake behind the boat in calm water. That
could be indicated with a calibrated laser indicator illuminator.

Anyone got math enough to calculate resonance effects of a mass of
water in water, Q of the tank, and energy input efficiency for a
wave making machine? There comes a longer hull bulb bow shape and
wave mechanic where the bustle is patted along by the wake, like
pushing a child on a swing, and a hull is designed for one most fuel
efficient speed. Exhaust bubbles underwater may lubricate the hull,
or ameliorate water acceleration dynamics in a lumpy hull, gas being
compressible. Skin friction is different from displacement
resistance, which equates partly with the amount of wake left behind.

Or Would the speed of the wake oscillation be determined only by the
length of the wave, or height of the oscillation, akin to a
pendulum's length in some way?

The Q may have an effect, if you consider the shape of the tank. A
hemispherical bowl barely bigger than the boat will be one limit,
open ocean the other. A tennis ball in such a bowl of water on a
vertical motion table might give some data, but might it all just
sit still while being externally agitated? How about dropping a blob
of cream into a cup of still coffee, or coloured water into clear?

Somewhere therein lie the secrets of hull efficiency when in
displacement mode, for all hull shapes including those that get
narrower. I think it boils down to skin friction, cavitation
constants and force applied to the water, moving it around the hull
as it passes. Since water cannot be compressed, it can only move
out of the way in an essentially vertical direction, like water
pushing up and down in a convoluted pipe. How slippery is a pressure
wave in water?

The bigger the wake, the more energy input required for the
inefficient wave maker machinery.

Sounds like a fun project for a math student and a couple of laser
pointers, or an array, calibrated to measure wave height, a la dam
busters' spotlights.

All we need now is a calm day, a boat, some notepads and a case for
buddies to argue over. A sextant might measure certain angles.

Oh, and a research grant and credentials to request funding for such
a question about steamboats, like it's never been done already.

The question about bucking currents and fuel efficiency is a
different one, related slightly to the silly train collision
question, recalled below, with a hint to an outside the box solution.

(If you were making a low budget movie about a train collision, how
would you minimize fuel requirements for the locomotive if plenty of
track was available, but a certain speed was required to propel
shrapnel impressively? It too boils down to Delta-vee, time and
acceleration, all rocket science, vs accountants' hourly wages.)

The best fuel economy for a motorsailer is to sail. If you have to
ask about the price of the fuel, you can't afford to sail. I used
about 3 gallons of gas last year, how long would it take for me to
recoup the conversion to diesel? Please include the cost of a new
knot meter, since I don't have one, and testing requirements to
calibrate present fuel consumption documentation and financing cost,
including lost income.

Cruising means sailing nowhere special, at economical speed,
including the mental effort. Why do you think sailors cruise?

Terry K

In article . com,
wrote:


Another thread discusses sailboats motoring "too much" but I think most
sailboats will use their engines to supplement sail but we still worry
about fuel economy. Even motoring alone, my yanmar 2GM seems to burn
only about 1-1.5 gals/hr at 6 kts in my 28' S2 but still I wonder about
the best way to conserve fuel in very light air.
Normally, in light air, I start with sails and no engine and eventually
get impatient with going only 3 kts and start the engine and put us up
to 5 and then eventually 6 kts or more. It is this last little bit
that I think burns the most fuel because she is most efficient at
slower speeds but as we get closer to hull speed fuel use rises
sharply.
An alternative strategy that would burn less fuel but would go a little
slower might to be ALWAYS run the engine at sufficient rpm to get to
3.5 kts and then use the sails to supplement that.

wrote:
Another thread discusses sailboats motoring "too much" but I think most
sailboats will use their engines to supplement sail but we still worry
about fuel economy. Even motoring alone, my yanmar 2GM seems to burn
only about 1-1.5 gals/hr at 6 kts in my 28' S2 but still I wonder about
the best way to conserve fuel in very light air.
Normally, in light air, I start with sails and no engine and eventually
get impatient with going only 3 kts and start the engine and put us up
to 5 and then eventually 6 kts or more. It is this last little bit
that I think burns the most fuel because she is most efficient at
slower speeds but as we get closer to hull speed fuel use rises
sharply.
An alternative strategy that would burn less fuel but would go a little
slower might to be ALWAYS run the engine at sufficient rpm to get to
3.5 kts and then use the sails to supplement that.

You have a problem. A 2GM20 shouldn't use over one gallon an hour
even running at full rated power.

At 6 knots, however, you're at 95% of hull speed, so you're well up
the curve, but since your displacement is under 8000 pounds, you
should only need about 14 HP which means about .77 gallons per hour.

Diesel use = 0.055 gallons/hp/hour

OK Jeff,
What do you estimate my usage should be with 4-108, 30' LWL, 20,000
displacement, two blade 16 x 14 prop.
krj

Jeff Morris wrote:
wrote:

Another thread discusses sailboats motoring "too much" but I think most
sailboats will use their engines to supplement sail but we still worry
about fuel economy. Even motoring alone, my yanmar 2GM seems to burn
only about 1-1.5 gals/hr at 6 kts in my 28' S2 but still I wonder about
the best way to conserve fuel in very light air.
Normally, in light air, I start with sails and no engine and eventually
get impatient with going only 3 kts and start the engine and put us up
to 5 and then eventually 6 kts or more. It is this last little bit
that I think burns the most fuel because she is most efficient at
slower speeds but as we get closer to hull speed fuel use rises
sharply.
An alternative strategy that would burn less fuel but would go a little
slower might to be ALWAYS run the engine at sufficient rpm to get to
3.5 kts and then use the sails to supplement that.

You have a problem. A 2GM20 shouldn't use over one gallon an hour even
running at full rated power.

At 6 knots, however, you're at 95% of hull speed, so you're well up the
curve, but since your displacement is under 8000 pounds, you should only
need about 14 HP which means about .77 gallons per hour.

Diesel use = 0.055 gallons/hp/hour

krj wrote:
OK Jeff,
What do you estimate my usage should be with 4-108, 30' LWL, 20,000
displacement, two blade 16 x 14 prop.

To drive a boat at a Speed-Length ratio of 1.2 requires about 1 HP for
every 700 pounds. So to push 20,000 at 6.6 knots requires 28 HP,
which should burn about 1.5 gallons per hour. It would seem that the
4-108 is overkill for this boat, but you have plenty of reserve. If
you push to hull speed, the requirement goes up to 1 HP for every 500
pounds, or 40 HP or around 2.2 gals/hours. If you back off to under 6
knots, then you only need one HP per 1000 pounds, or 1.1 gals/hour.

So how close did I come to your experience?

A prop calculation will cost you an extra 50 cents, but your goal
there should be to have a large enough prop to handle the power, and
the engine reving at a sweet part of its power curve. My rough guess
(and admit this is one area I never really understood) is that the
blade area your prop is small for the horse power you may be putting
through it - this would cause cavitation.

On Thu, 24 Feb 2005 20:26:42 -0500, Jeff Morris
wrote:

A prop calculation will cost you an extra 50 cents, but your goal
there should be to have a large enough prop to handle the power, and
the engine reving at a sweet part of its power curve. My rough guess
(and admit this is one area I never really understood) is that the
blade area your prop is small for the horse power you may be putting
through it - this would cause cavitation.

===================================

Propping a sailboat is always a big tradeoff between sailing
efficiency and motoring. For motoring a big 3 or 4 bladed pop is
ideal for a displacement hull but that is deadly to sailing
performance unless you got a feathering prop, and that is another
level of compromise and expense.

Wayne.B wrote in
:

Propping a sailboat is always a big tradeoff between sailing
efficiency and motoring. For motoring a big 3 or 4 bladed pop is
ideal for a displacement hull but that is deadly to sailing
performance unless you got a feathering prop, and that is another
level of compromise and expense.


Lionheart's 3-bladed prop freewheels to operate the shaft alternator on its
4-108 diesel setup. For this reason, it has the "hydraulic transmission",
not the straight one. When the engine was swapped out, the transmission
was inspected but not changed to the other type. The mechanic said it
wouldn't be good to freewheel that shaft all the time, but I've forgotten
the "why" he told us. An extra 15-20A of 12V is easily worth the drag of
the turning screw.....

In article ,
Jeff Morris wrote:

My rough guess
(and admit this is one area I never really understood) is that the
blade area your prop is small for the horse power you may be putting
through it - this would cause cavitation.

The analogy is driving a Viper with skinny tires that light up every
time you try to apply throttle.

KeS

My 3GM30F in my previous 36' boat only burned 3/4 to 1 GPH.

"Jeff Morris" wrote in message
...
wrote:
Another thread discusses sailboats motoring "too much" but I think most
sailboats will use their engines to supplement sail but we still worry
about fuel economy. Even motoring alone, my yanmar 2GM seems to burn
only about 1-1.5 gals/hr at 6 kts in my 28' S2 but still I wonder about
the best way to conserve fuel in very light air.
Normally, in light air, I start with sails and no engine and eventually
get impatient with going only 3 kts and start the engine and put us up
to 5 and then eventually 6 kts or more. It is this last little bit
that I think burns the most fuel because she is most efficient at
slower speeds but as we get closer to hull speed fuel use rises
sharply.
An alternative strategy that would burn less fuel but would go a little
slower might to be ALWAYS run the engine at sufficient rpm to get to
3.5 kts and then use the sails to supplement that.

You have a problem. A 2GM20 shouldn't use over one gallon an hour even
running at full rated power.

At 6 knots, however, you're at 95% of hull speed, so you're well up the
curve, but since your displacement is under 8000 pounds, you should only
need about 14 HP which means about .77 gallons per hour.

Diesel use = 0.055 gallons/hp/hour

On 23 Feb 2005 10:04:20 -0800, wrote:

Another thread discusses sailboats motoring "too much" but I think most
sailboats will use their engines to supplement sail but we still worry
about fuel economy. Even motoring alone, my yanmar 2GM seems to burn
only about 1-1.5 gals/hr at 6 kts in my 28' S2 but still I wonder about
the best way to conserve fuel in very light air.

I had a Yamaha 30 with a 2GM, and I'm sure I got about 1.5 _litres_
per hour. I normally cruised around 2500 rpm, if I recall correctly.


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

In article . com,
wrote:

Another thread discusses sailboats motoring "too much" but I think most
sailboats will use their engines to supplement sail but we still worry
about fuel economy. Even motoring alone, my yanmar 2GM seems to burn
only about 1-1.5 gals/hr at 6 kts in my 28' S2 but still I wonder about
the best way to conserve fuel in very light air.
Normally, in light air, I start with sails and no engine and eventually
get impatient with going only 3 kts and start the engine and put us up
to 5 and then eventually 6 kts or more. It is this last little bit
that I think burns the most fuel because she is most efficient at
slower speeds but as we get closer to hull speed fuel use rises
sharply.
An alternative strategy that would burn less fuel but would go a little
slower might to be ALWAYS run the engine at sufficient rpm to get to
3.5 kts and then use the sails to supplement that.


You've got it, I think, but I suspect the 1.5 GPH. We don't burn 1 GPH
until we're at max throttle, which gives us a solid 7 knots.

Xan's numbers are similar to yours; I've been tracking our usage over
1200 hours and a dozen or so years. Ours is a 2GM20F; best prop we used
was a 15x11" 2 blade. They recommend 16x10, which I'd prefer.

At 6+, we burn about 0.6; 5.6 = ~.33; 5.3= ~.25; 5=~.20. Below 5, no
noticable improvement as we're only turning about 1500.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/