Fuel GPH, what to expect

[Westport]

been a while since I've been here. I've got a 1977, 23' Marquis cuddie, with a 305 Gm engine with alpha '228' drive and stock prop. Today was a great day to start working on it, and get it ready to go for a river cruise on the Ohio and Cumberland rivers. When you finally get the heavy old tub out of the water and up on plane, it planes sweetly at about 32-3400 RPM. And I was wondering what type of GPH I'll be using.

When we pulled the floor out of the boat I was surprised to see it had a huge 60 gal. aluminum hull tank. So I figured his boat was made for distance.

I know this is all subjective, but the "river run" we're planning is about a 120 mile one way trip and I was wanting to calculate the approx. of distance between fuel stops so i can see the availability of fuel locations along the way.

Any help is appreciated!

Thanks!

Tim

 

[dingbat]

Re: Fuel GPH, what to expect

When we pulled the floor out of the boat I was surprised to see it had a huge 60 gal. aluminum hull tank. So I figured his boat was made for distance.

Sixty gallons is pretty small for a "distance" boat. At 3 MPG you're only looking at 180 miles.

 

[NHGuy]

Re: Fuel GPH, what to expect

Boats vary so much. I'd say take it for a 30 minute run then check fuel used. The hardest part for me is avoiding full throttle. For me fast is fun. But when I just cruise it the fuel economy is far better. You are pushing a lot of boat, you are smart to monitor fuel use.

 

[Mischief Managed]

Re: Fuel GPH, what to expect

Assuming it's propped to get to the right WOT RPM, A 3200 to 3400 RPM cruising RPM would get you to about half the WOT GPH range. At WOT, that engine should be using about .08 GPH per HP, so multiply the HP times .08 and then halve the result to get the cruising GPH. If it's over propped, you may be using a lot more fuel.

From 2800 to 4000 RPM, I'd bet the MPG (not GPH) numbers don't change a whole lot, assuming the boat is cleanly on plane.

 

[Westport]

Re: Fuel GPH, what to expect

Thanks. I considered the tank to be large for a 23 ft'r considering my 18' Chris Craft Scorpion has a 20 gal and feeds a 3.0 GM

 

[Westport]

Re: Fuel GPH, what to expect

Good suggestion. I don't like running the engine full all the time. too costly to repair if it wrecks out and I'm not really into speed, so I WOT to get it on plane then back it and trim to where it's comfortable then leave it alone. and yes, it is a large high sided boat for a 'runabout'. the previous owners used it for fishing on Lake Erie, so I figured it was capable of taking them 40-50 mi out and back without running out. but then again.. what do I know? LOL!

 

[Westport]

Re: Fuel GPH, what to expect

Thanks for the formula. i appreciate that!

 

[Westport]

Re: Fuel GPH, what to expect

Here is the boat we're talking about. Granted this pic isn't my boat and it doesn't belong to me, but mine will be like it when done. (or close enough)

 

[sw33ttooth]

Re: Fuel GPH, what to expect

you would be shocked what the factory set up may have been i bet that tank was put in by one of the last owners. my dad used to build tiara yahts and with twins he did the math once, at wot they only had enough fuel for about and hour and half, i think they had 2, 100 gallon tanks.

what i'd do is top the tank off go out and cruise at your speed 30 minuties or 1 hour 1 way turn around and go back then top the tank off again add up how long you were out and find gph [gallons per hour] use a gps or fish finder with speedo and then average your miles say 2 hours, 20 miles,, and it cost you 8 gallons of gas, then you get 2.5 miles per gallon.

"just my .02"

 

[Westport]

Re: Fuel GPH, what to expect

great! very much appreciated. one thing about this massive tank, is that it was dated the same year as the boat and looked as to fit the compartment perfectly between the stringers and cross supports, like the boat was made for the tank or the tank was made for the boat. So I'm 'assuming' it was a factory installation. There's so little info on the older Marquis line that finding true spec's is very difficult.

 

[Mischief Managed]

Re: Fuel GPH, what to expect

My boat is 25 feet and has a 74 gallon tank. My last boat, a 21 footer, had a 45 gallon tank. 60 gallons sounds like a good size for a 23 footer.

 

[Westport]

Re: Fuel GPH, what to expect

Yes, I was pretty impressed with it myself. But it did look like a factory installation., though.