Where are the fuel efficient boats?

[fasternu]

Might be a dumb question, but I need to ask it- How is the best way to figure out MPGs with a boat? can you do it with a GPS or do you have gauges that will tell you? I have just the basic Crestliner with a 90 2stroke and now I am curious what I get...

[hydro]

Figuring out mileage is a guestimate at best. If you know your speed and fill/re-fill the tank you can measure how much fuel used over time at speed and get the MPG number. The one flaw in that is that it has to be a repeatable throttle setting, with WOT being about the only way to do it.

[solbes]

Two drool worthy rigs.... Deitz, even your trailer wheels look fast!

[MuskieJunkie]

Heres what you can do to save gas. Slow down!

If your running at max RPMs of 6000 and at a speed of 45 MPH your burning gas at the fastest rate.

Slow the boat down to say 4000 RPMs and cruise at 30 MPH.

Your going to burn a lot less gas....

Frank,

You're right slowing down does save gas but not for that reason.

In that scenario you will burn less fuel per hour at the lower RPM but it will take you longer to get to your destination so it doesn't save you anything.

The savings is because of the reduced resistance, resistance is calculated with speed squared (or maybe cubed I can't remember). So your resitance becomes exponetially greater the faster you go.

[DTro]

In that scenario you will burn less fuel per hour at the lower RPM but it will take you longer to get to your destination so it doesn't save you anything.

I cannot think of a equation with these factors in which going slower will not save fuel

Of course you will get there slower, but in essence you’ve gone the same distance and saved gas/money.

Now if you could put a value on your time you could come up with an equation in which to calculate the “real” savings.

Independent of your suface tension, if your engine is turning less, you are burning less gas, hence a multigear transmission in automobiles.

[MuskieJunkie]

if your engine is turning less, you are burning less gas

Yes but the engine is turning for a longer period of time. Decreasing your speed will equally increase your time.

Look at it this way, if you cut your speed in half you double your drive time. One way your burning 5 gallons per hour for an hour the other way your burning 10 gallons per hour for a half an hour.

You do save gas going slower but it's because the resistance (wind and water) on your vehicle is exponetially less.

It makes for interesting discussion anyway.

[Surface Tension]

I could get into the resistance of water, prop, air, and even an outboards drive train.

You have to overcome resistance to move. The faster you go the more resistance that has to be overcome.

Drive down I35 at 70 MPH or drive going 65 MPH.

Which speed will you get better gas millage?

That little difference in a few hundred RPMs will save you gas.

This savings is exaggerated when you increase the resistance of a boat and prop through water. Simply put it takes more HP and high RPMs. So a few hundred lowered RPMs saved you fuel in the car.

Back off the throttle on the outboard 1k and you'll probably double your gas mileage from Point A to Point B. Sure it'll take you longer but your burning a lot less fuel.

[DTro]

Ok so say you get 5mpg at 5500RPM and 10mpg at 4500RPM.

Now let’s say you drove 30 miles.

@5500RPM you burned 6 gallons but got there faster

@4500RPM you burned 3 gallons but arrived later

Engine run time doesn’t matter because the engine is running more efficiently

Am I missing something?

[MuskieJunkie]

No, I was just trying to point out that it’s not the higher RPMs that use extra gas, it’s the higher resistance you have to overcome with that higher speed. I shouldn’t have got so techy. I probably just got excited thinking that all the time and effort I put into those College Physics courses has some real life use.

[Surface Tension]

MuskieJunkie , I know what your saying, I just said it different.

[MuskieJunkie]

MuskieJunkie , I know what your saying, I just said it different.

Yea I saw that on one of your replies. Shoot, I thought I had finally found something I could stump you on, or at least disagree with you on.

[Surface Tension]

TeHe

[turtleboy]

I find this an interesting discussion and will add to it my non professional understanding of the subject. My experience is in building several canoes, a sailboat, a 22 ft "cruiser skiff", and presently a lapstrake 14 ft wooden boat for rowing/ trolling.

Fuel efficiency in boats is a complex subject. Hull form (planeing vs displacement), weight, length, beam, and bottom shape all factor into the equation. In general, speeds at just below "hull speed", or the speed at which you start pushing forward a bow wave are the most efficient. This speed is largely a function of length and in most fishing boats in MN I believe this is in the approximately 6 to 7 mph range. Boats that are designed to run at these displacement speeds are more efficient by quite a bit than boats designed for planeing, the suction of the transom under water creating quite a bit of drag. For planeing hulls at above displacement speeds I have been told that a length to width ratio of about 3 to 1 is the most efficient, and the lighter the hull and flatter the bottom the higher the efficiency. It really comes down to what you want to pay for efficiency in terms of speed, function, safety, and comfort. A heavier beamier hull with a deep v bottom is probably more comfortable, but at quite a bit of cost. I have a home built 22 ft boat with approx 3 to 1 length to beam and shallow V which weighed in at 1400 lb without motor powered by a 90 hp ETEC. I recently traveled from Kabetogama to Crane Lake and back about 90 miles total and calculated my mileage at just under 7 mpg. I traveled at about 28 to 30 mph except in the narrows (my WOT speed is 36). This boat is quite a bit lit lighter than comparably sized aluminum or glass boats.

The boat I am presently working on I plan to use for mainly for fishing while rowing, and is designed as a displacement hull. It is light with rounded bottom, relatively narrow, and the narrow transom is above the water line eliminating the drag you feel trying to row (or power) planeing hulls. Again there are compromises to be made. Narrower, longer, lighter are more efficient (think rowing shell) but I wouldn"t want to be 5 miles down Kab and have the wind come up in a rowing shell. With he boat I am building I hope I have made the compromises I need to to make it functional, safe, comfortable, and efficient.

It will be interesting to see what the boating industry does, what boaters will accept in compromises, and what fishermen do in changes in habits and tactics as fuel prices and environmental concerns increase. (I'm not preaching here except maybe to myself, I burn plenty of gas out on the water. That 90 mile trip to Crane Lake and back was just an evening cruise to Nelsons Resort for dinner. Ever try their smorgasboard?)

[Deitz Dittrich]

Very pretty boat TB.

[hydro]

Turtleboy,

Great discussion! The aspect ratio bit is very interesting and makes a lot of sense.

Those are some very nice boats you have built. As a fellow woodworder I appreciate the level of craftsmanship it takes to put something like that together. On that last pic, it looks like there is carpet under that boat, and furniture around it. Once it's built can you get it out of the basement?????

[slipperybob]

LOL's

But the molecular density of water and air is at a constant ratio and the coefficient of drag resistance between two materials is a constant ratio. Things can change when an external force is enacted upon it such as a rise/dropp in the barometric pressure or an added material is sandwich between the two to create lubrication.

Compute in the plane lift of body in fluid vs constanct force of gravity and the distibution of object weight to the angle of body plane and the surface displacement of liquid.

Add in the value of accelaration force and the time before reaching a constant velocity. Then factor in the time of distance of travel against the fuel consumption. Plus the whole thing backwards as the boat decellerats to a full stop. There you have your MPG.

[delcecchi]

If you look at the tests in Bass and Walleye boats you will see that for each boat and motor there is an optimum speed/RPM that gets the best milage. Usually it is in the 3500 to 4000 RPM range but not always. WOT or barely planing is worse.

They have a HSOforum if you want to see for yourself.

[turtleboy]

Thanks for the compliments on the boat. Dietz, I would be willing to trade boats for a while, looks like fun. And Hydro, yes, I believe it will fit out the door with a very slight amount of room to spare, as long as I don't put too many coats of paint on it first. I'm slowly taking over the rec room for a workshop. If the boat doesn't fit out my excuse will be that that patio door is inefficient and needs replacing anyway. Actually that is an older picture and the project is nearing completion. I'm looking forward to launching in the spring and plan to be measuring fuel efficiency in miles per can of beans instead of miles per gallon of gas.

[turtleboy]

Slippery, sounds like you are on the the verge of discovering a way of hybridizing watercraft propulsion, deriving some energy back during deceleration. Go for it!

[slipperybob]

LOL's

but unfortunately you can't turn though to benefit from surface gliding. The only way to keep optimal energy propulsion is to be able to bend the shape of the boat to curve in the direction you want to turn. Assuming wind is not a factor.