On Mon, 23 Nov 2009 09:54:12 -0800 (PST), Frogwatch
wrote:



Unfortunately, when it comes to hydrodynamics, anybody here is more
qualified than me.
From what little I understand about planing boat performance, weight
is the big issue for fuel economy. You are supporting the entire
weight of the boat on that tiny portion of the hull in contact with
the water meaning you gotta push it at a minimum speed to get it to
plane. Maybe it is like friction where the force due to friction is
proportional to weight.

Think it's more due to prop inefficiency. Slippage increasing at
higher speeds. With a car once you get enough torque to lock up
the converter, efficiency increases. Of course your wheels aren't
slipping. Water and concrete are different, but analogies can be
useful.
So how do you make the prop more efficiently push water?
Beats me, but maybe how you feed water to the prop at the front end
could help, which is one reason I find the tunnel hulls interesting,
even aside from what else they do in floating the boat.
Sometimes I wish I was an engineer. Not often though.

At any rate, Renn Tolman says weight is the primary issue for fuel
economy so his boats are very lightweight. My 20' Tolman weighs about
800 lbs so is very economical on fuel (if you can call 5 mpg
economical) optimum speed is roughly 20 mph with 5 people aboard.
Even using it to take the kids kneeboarding in an afternoon it uses
little fuel.
Tolman also has a design for a "SeaBright" skiff that uses a diesel
that supposedly gets extreme fuel economy at speeds of about 18 mph.

Think the "Rescue Minor" in this link is the first time I became aware
of the tunnel hulls.
http://www.fishyfish.com/renn_tolman/index.html
Neat. How much of the economy comes from the light weight and use of
a diesel I don't know.

If you do not mind going slow, check out the "Diesel Duck", a 38'
"troller cruiser" that supposedly uses 3/4 gal/hour of diesel at 8
kts. It also has sail assist if you want to use even less.
For me, I thought fuel economy would be a worry but so far with the
Tolman it never has been an issue. I have never used more than 7
gallons of fuel in a whole day. The larger Tolman's (such as the
Jumbo at 23') using 4 cycle motors are supposed to get over 6 mpg with
a 125 hp 4 cycle.

From what I've seen you can get pretty good economy from displacement
hulls if you stay away from hull speed and have
the propulsion gear optimized.
Even planing hulls don't have to be planed.
There's a guy with a 24' Carolina Skiff DLX who fishes mostly creeks
and such, but sometimes goes into the Chesapeake Bay and the
Virginia Eastern Shore.
His engine is a tiller 9.9 4-stroke. Says his typical mileage is 16.5
mpg. Probably goes about 5-6 knots. He didn't say.
All of this will matter more when/if fuel gets to 10 bucks a gallon.

--Vic