props and fuel economy

[BoatR]

I have a 20 ft Charger STV with 175 efi merc with hydraulic jack plate.
I normally run a 26 or 28" chopper for general go fast around around my lake.
I also have a 23 thru hub cleaver>
Question: will the 23 provide improved fuel economy for a trip we want do with boat.
Most of the cruising will be at reduced speeds due to rivers and populated areas>
It seems the other props "lug" going slower just trying to keep on plane.

[engineermike]

I haven't done any testing, but I think "lugging" the motor is actually best for fuel economy - at least in cars. The idea is to get the frictional losses in the engine down (due to lower rpm) but also to reduce the vacuum the piston has to work against in the crank case.

I'll be interested to see other responses.

I have heard that 4-blades use less fuel than 3.

Mike

[HSTKevin]

Lugging a two stroke is never a good idea, for a variety of reasons, but will not accomplish better fuel economy. Props with more blade area (four blades) wil generally get better economy esp at mid range speeds. The real trick to getting good fuel economy with an air entrapment hull is to go fast enough to fly the hull, this reduces the wetted surface and thus reduces drag. You need to define what you mean though---miles per gallon or gallons per hour---for the former, fly the boat, for the later drive the boat at the speed it just planes at, say 25 to 30 mph. I would choose the 26 chopper over the cleaver

I ought to know, been in the prop business for 16 years

Kevin

[Mr. Demeanor]

I have a fuel flow gauge on my bigger boat and have been on many others with similar equipment. Propping a boat for the rev limiter on stock power usually gets the best fuel consumption at cruise. Everyone want to think speed vs. rpm but they forget about throttle position. If your lugging the engine, you need more throttle to achive the same rpm and that eats fuel and is hard on the engine.
You probably wont see huge differences at cruise. I can see me fuel flow numbers while running and can tell you its getting on plane that kills your overall mileage when in an area where your on and off plane a lot. The prop that gets you up the easiest/quickest and has a good cruise will be your best mpg. I wouldnt be surprised if that was a through hub style like a Trophy.

[KIRCHNER]

AGREED a thru-hub big diameter / blade area most efficient at lower rpm 4 blade. i wouldnt drop pitch too much

[Motv18]

Interesting enough mine gets the best off plane and pvs plugged. 8mph 1gph.

A heavy cup 4 blade will hold most boats on plane at lower rpm. I could get 22/3 with it average

but honesty this 90 maxes at 8.2gph unless your heading somewhere fuels scarce just run. And take a Jerry can.

[David - WI]

Interesting graph from the Tunnelboat simulator for my HST... takes about 8hp less to go 61 mph than 44 mph.

[engineermike]

I have lots of data that all says my 23' eliminator gets better economy at 60 than it does at 45-50.

[eli]

most v 6 mercs 175/200 like say 1986/88 got 4 mpg at 4500 rpm with 25 pitch 4 blade prop is what i got when i had said combo 20 ss switzer 200 merc 1988

[bigboy]

Interesting graph from the Tunnelboat simulator for my HST... takes about 8hp less to go 61 mph than 44 mph.

Interesting! About 60mph thats the speed I run for cruising with the HST. Flies nice and engine spins about 4500rpm.

[David]

In my limited experience with a cleaver (VooDoo) it was crap for anything but top speed

[XstreamVking]

It seems to come down to mph's vs rpm's. I have tempest plus props on my boats and they get very good fuel economy.

[Glasstream15]

My little bass boat with an InLine 115 gets best MILES per gallon at 4500 RPM. That's 45 MPH and it's just really gotten on top. Not quie on the pad but running free. Faster takes more work and more gas. Slower there's too much boat in the water. That's just my antique but I'm sure the principle applies across the board. Just to the point she's running free and the motor is at that point of maximum performance per RPM. Maybe that's a poor explanation but that's the best I can do. Get a prop that gives you really good performance and then back off about 20 to 25%. And a lower pitch prop just means the motor is turning more revolutions for the prop to push the same distance forward. Don't go up either 'cause then you're lugging and making VERY expensive noises. Not to mention the embarassment of a tow home.

[whipper]

Your best mpg is going to be at a speed that reduces wetted surface in combination with rpm. If your (lugging) and the hull is heavy in the water the added drag will not get you as far as a speed that is skimming the surface more . mpg is relative based on gph. So if I wanted distance per tank of fuel I would pick an all round in the middle prop. Higher pitch props generally give you better milage as your rpm is lower and speed \distasnce travelled takes less revolutions to get there. To low of pitch and every time you can cruise at a higher speed your burning more fuel than the higher pitch prop. I find for slow speed rough water my 25 trophy is best. But burns way more fuel in a day. My 28 pitch for long distance is buy far the faster prop and burns way less fuel in a day. My larger dia 26 is the happy median I use the most. At a little slower cruise say 35 to 55 mph I can go a long way. But still poke a fast pass when I want to to get there. 28 can cruise at 70 and poke to 90,s and still burn less fuel and get me there sooner. But if I have to go 35 to 50 the 28 will be working pretty hard to save fuel.

[Motv18]

Good explanation of gph vs mpg.

My best cruze is about 28mph 3600rpm that gets me about 5gph or 6mpg. Mines not big enough to actually lug the engine

my uncles 22 ranger is a dead rise v with a 200. Once on plane he just burns more gas the faster he goes.

Min the end you just have to experiment I know some of you can burn 15-20 + an hour.