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.

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

On 23 Feb 2005 12:08:23 -0800, wrote:

I truly do not know how much fuel it uses but am saying 1-1.5 gal/hr as
a maximum. I have major problems filling it as the fuel backs up and
tries to backflow even though I have unclogged the breather line.

As far as finding time to sail, I am blessed with loving my work and
really cannot decide if I would ever really want to go sailing for long
periods.

It seems you are well passed the speed you want for fuel economy.
My boat is about 36,000 lbs (46') and my big 85 hp burns between 1 and
1 1/2 gph at 6 knts.
You might try a self-pitching prop like the Auto-Prop. You can
still use sail power and the prop will self-pitch to still give speed
at slower rpm. Take a look at their web site. Over time, the fuel
economy will pay for the prop. You'll also get a real prop in reverse.

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.

"Doug Dotson" dougdotson@NOSPAMcablespeedNOSPAMcom wrote in
:

I don;t actually know. The calibration of the tach is off because the
alternator is
not stock. When we first bought the boat I measured the actual RPM and
made a graph to convert from indicated to actual. Converting the
proper cruising RPM
to the indicated gives 2500 so that is where we run unless we need
some extra. I've forgotten what the actuals are. 1800 sems to stick in
my mind but
I'd have to go look it up.

Doug




Thanks. The 4-108 just seems to lazily cruise along in the 1800-2000 RPM
range. Lionheart has 90 gallons of diesel in the tank that's the starboard
seat in the center cockpit. It's amazing how long that lasts under power.

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.....

"Larry W4CSC" wrote in message
...
"Doug Dotson" dougdotson@NOSPAMcablespeedNOSPAMcom wrote in
:

I don;t actually know. The calibration of the tach is off because the
alternator is
not stock. When we first bought the boat I measured the actual RPM and
made a graph to convert from indicated to actual. Converting the
proper cruising RPM
to the indicated gives 2500 so that is where we run unless we need
some extra. I've forgotten what the actuals are. 1800 sems to stick in
my mind but
I'd have to go look it up.

Doug




Thanks. The 4-108 just seems to lazily cruise along in the 1800-2000 RPM
range. Lionheart has 90 gallons of diesel in the tank that's the
starboard
seat in the center cockpit. It's amazing how long that lasts under power.

We have 2 95 gallon tanks. Great cruising range but having that much fuel
when
one only consumes 1 GPH has it drawbacks.

Doug

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

Here's the scoop on fuel efficiency. It's an engine parameter called
specific fuel consumption. The most efficient aero recips use
0.45 lb per HP hour.

A diesel could be 50% more fuel efficient, let's say.
So 0.3 lb diesel fuel per HP.hr would need 26 lb fuel per hour
to produce 85HP
That's 4 gallons or more....for 84HP

I guess this means Doug is using full revs at 1/4 throttle,
or full throttle at 1/4 max revs....

Regards

Brian Whatcott Altus OK

On Wed, 23 Feb 2005 20:41:51 -0500, "Doug Dotson"
dougdotson@NOSPAMcablespeedNOSPAMcom wrote:

Something must be wrong with your figures or your engine/prop
situation. I run a 1985 Perkins 85 HP 4 cylinder pushing a 43'
boat. I consistantly burn 1 GPH.

Doug
s/v Callista

wrote in message
oups.com...
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.

The number I've used for marine engines is 0.055 Gallons per HP per
hour. For gas engines, use 0.1.


Brian Whatcott wrote:
Here's the scoop on fuel efficiency. It's an engine parameter called
specific fuel consumption. The most efficient aero recips use
0.45 lb per HP hour.

A diesel could be 50% more fuel efficient, let's say.
So 0.3 lb diesel fuel per HP.hr would need 26 lb fuel per hour
to produce 85HP
That's 4 gallons or more....for 84HP

I guess this means Doug is using full revs at 1/4 throttle,
or full throttle at 1/4 max revs....

Regards

Brian Whatcott Altus OK

On Wed, 23 Feb 2005 20:41:51 -0500, "Doug Dotson"
dougdotson@NOSPAMcablespeedNOSPAMcom wrote:


Something must be wrong with your figures or your engine/prop
situation. I run a 1985 Perkins 85 HP 4 cylinder pushing a 43'
boat. I consistantly burn 1 GPH.

Doug
s/v Callista

wrote in message
roups.com...

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.