starcraft semi plane or planning hull/ fuel mileage

[elkhunter338]

So I have a 21' cheiftain with a 140hp I/O. I am starting to investigate how to find the best fuel mileage speeds.

Is the starcraft a semi plane hull or a planning hull?
Has anyone got any good info and the sweet spot of the hull?

My top end speed is 28 mph at 4,600 rps with a standard factory 3 blade prop. 19 pitch
The plugs look good, no black soot so it is not burning rich. I had a 48/50 main jets (sooty, foul plugs) and now have a 46/48 jets in the carb. it burns clean.

I realize reducing weight/balance weight/ shift weight for easy planning/ tune the motor all help. I have done all that. I know no wot operation because fuel mileage decreases at wot. Most boats have a sweet spot.

At time the weather dictates my speed. I would like to maximize my fuel milage without buying an expensive fuel monitor gauge.
Thanks

 

[cj8mule]

Re: starcraft semi plane or planning hull/ fuel mileage

I'll be ready to discuss this with you soon elkhunter. I've got the 165hp version though, but should be similiar. I've read before about you getting 28 mph at wot, but what speed and rpm do you plane out at. I would think that would be the key, right?

I've not ran the 140 hp, but the 120 hp in my old 18' SS had a real nice sweet spot right around 3K rpm.

 

[elkhunter338]

Re: starcraft semi plane or planning hull/ fuel mileage

Honda BF 2.3 - 5 - 8 - 9.9 - 15 - 20 - 40 - 50 - 75 - 90 - 115 - 135 - 150 - 225 - 250 hp Outboard Fuel Consumption GPH MPG Specs
I found this web site that give fuel burn rates. I plan to write down my speed vs. rpms and calculate the mpg and see what that says is the sweet spot for best mpg. This should give me an idea. I just now have to write down my gps speed at each 500 rpm increment. Next fishing trip. I would not be too concerned about this but I plan to run 200 to 600 miles this summer, alot depends on where the fish are. If they are in close maybe only 150miles if they are 8-12 miles out then I might approach 600 miles, that alot of fuel at almost $5 per gallon at the dock.

 

[airshot]

Re: starcraft semi plane or planning hull/ fuel mileage

Not sure how much this will help but here goes...I have a 1983 islander, basicly same hull but your chieftain weighs a bit more, I also have the 140 hp 3.0 engine and also swing the 19" prop. My wot is 34mph (gps) at 4600 rpm, not sure why your wot is so much slower..
Now my cruising speed varies depending on wind and waves, typically 20 - 24 mph 3000-3200 rpm seems to get me the best fuel economy, typically around 5 mpg. Now If I am bucking a headwind or crashing thru 2 ft waves then that changes everything. We like to cruise in the evening up and down the Maumee river in Toledo Ohio, after the wind dies down and the river settles down from the boat traffic and our typical ride is from 30 to 50 miles in length. Under these conditions I normally run around 3000-3200 rpm and get between 22-24 mph depending on current. I have checked this a number of times and typically end up around 5 mpg, little more/little less depending on how many go along and wind. Hope this helps.........

 

[elkhunter338]

Re: starcraft semi plane or planning hull/ fuel mileage

hmm that interesting you run 34 mph @ 4600, I too have a 19 pitch 3 blade prop and have never seen over 28 mph.

I will have to check speed vs. rpms this summer more accurately and write down the information, make sure I am at slack tide. I might have checked wot against a tide running the wrong way. I was running an old tach years ago that read wrong. It took awhile to figure that out. I had the engine under prop and was running around 4k all the time and tach read 3200. That is now fixed and I gained mpg by going back to my 19 pitch 3 blade prop.
I do have lots of fishing gear and we always have 3-4 pobs, kicker motor, 35 gals. fuel. Boat on the trailer weight is 4100 lbs on the axles, 3/4 plywood floor.

 

[airshot]

Re: starcraft semi plane or planning hull/ fuel mileage

You are carrying just a little more weight than I am as the cheiftan is a bit heavier, however mine weighs 3900 on the single axle trailer and I also have a 9.9 kicker, but have a 25 gallon alum tank, I normally have 2-3 on board when checking speed and use my gps which by the way is within 2mph of the speedometer on board. I keep my fishing gear onboard always except for the minnow bucket and cooler.
Now a good headwind or running into some rolling seas will drop that wot down to 30-32 easily. Yours does sound a bit slow I would check out your tach and be sure to check speed with a gps.