Marinette Boat Forum

At 10 mph, I think you'll be pushing a lot of water instead of efficient planing-- but of course, will see a lot of scenery and that alone will make the extra fuel consumption worthwhile. this is a SWAG (scientific wild-assed guess) but would guess you'll "enjoy" about .75 mi/gal or a little less depending on what weight you are carrying such as full water / fuel tanks, number of passengers, cases of adult beverages.

I just completed a 140 mile trip to move from one marina to another. At about half way I had to fuel up so after starting with a full tank I logged 67 miles from marina 1 to the refuel marina. I took 70 Gal of gas to fill her back up.

I was running around 13-14 MPH slower coming out of the marina and had a couple of sprints to 20 mph just to see how the boat was running.

Over all right at about 1 mile per gal.

I did not refuel at the destination marina so can not tell you how we did on the second half of the trip.

I have a 1973 32' express with flying bridge.

I was happy with the mileage.

Will have fuel sensor in place for spring but did not get it set up before the trip.

L

Nice record keeping and reporting. One note to add. Boats ain't cars. Tide, current, wind will have a much greater effect on fuel use than with a car. Oh, and the amount of weight you're carrying.

Ok, so here's a thought.. Why has no one put a "automotive" type tranny on a boat, to turn the prop faster at lower RPM's.? Instead of this direct drive system we use now for a 100 years... I would think a mechanical clutch would not be hard to configure..

Someone has, but not in normal boating. The drag racing boats shift to a higher gear after they get on plane.

Jeff

on my 28 express single i will get on plane and throttle up to 2800 rpm .I then will start tabbing down until i see my rpms jump and speed increase on gps. Then I will throttle back down to 2800 rpms. I figure that is where i am most efficient.

To answer Coastie's question of why boats don't use muti-gear transmissions, like say a 3 speed automatic in an old Buick, you have to get into the study of water hydraulics. When any boat is being designed, multiple factors are figured in: weight (boat and crew), hull displacement/type, engine size and power output, prop pitch, etc... This is then factored against the RPM of the prop shaft. On Big M's, ours for the most parts use Velvet drives with a 1:1 ratio, 1:1.5, or 1:19. For the sake of simplicity, we'll look at the simple 1:1. It's basically a direct drive, meaning if you engine is spinning 2500 RPM, so is your prop shaft. With an certain diameter prop, with a certain pitch, and an engine with X horse power, the boat will perform as designed at an acceptable speed-to-fuel consumption ratio. One can change and experiment with different size/pitch props to achieve at one end, good fuel economy or at the other end, more top speed. With all those factors staying constant, adding a second or even third gear would not produce the same results like in a car, the prop will either bog the engine down (like a big truck trying to pull a mountain pass in high gear) or the prop would spin so fast that it would cavitate and the boat just won't go any faster, just make an awesome rooster tail. And that would be on a glass smooth inland lake. In choppy seas the trans would shift from gear to gear, wildly hunting for what gear it "thinks" it should be in. Also, in a car, you are instinctively and constantly adjusting your gas pedal to compensate for climbing hills or coasting down grades and keeping a consistent speed. Imagine trying to keep a boat with an auto trans happy, while constantly adjusting twin hand throttles ! Maybe I'm overthinking it but I don't believe it would work, at least on a large cabin cruiser.

Jim,
IMO....
Well stated!!

Bill

One more thing...The Germans (who else?) developed a surface drive propulsion system with a variable pitch propeller.

Of all the difficult prop pitch selection deals, the surface running drives are the worst. Half the prop is out of the water at full plane, yet it's all under the water at rest. That generates a massive over-pitched conditions at plane off, then an over reving condition at WOT. The German design allows a shallow pitch it get on plane, then adds pitch at the speed builds up--like an auto tranny would do.

Slick, eh?

Jeff

PS: One mire thing: A guy I met who runs an off shore catamaran race boat once told me he could not get her on plane with the prop pitch the motors needed for top speed (around 160 mph as I recall)--and his engines EACH put out 1,400 hp! The solution was to pipe exhaust to just before the props which made them cavitate enough to get the revs up--like slipping a clutch.

Thanks for the reply, and sorry it's been so long getting back to the post. That really explains a lot, as far as I'm concerned. When I first fell in love with boating, my grandfather said B.O.A.T. stood for "break out another twenty", but now you can't get a good coffee maker for $20.! And gasoline was about 25 cents a gallon.! But we still run on an old, but proven to work, system.. So I guess short of switching to a 4 stroke outboard system, there will never be a more fuel saving configuration for bigger boats..