GPH numbers on 18' w/ 115

Tom Bare · #1 03-07-2003, 07:40 PM

Just got the NAVMAN F41 installed and took her out to make sure it worked. Here's what I found out:

4200 RPM is 6.5 GPH at an even 27 knots [HOH temp 46.5]

4500 RPM is 7.6 GPH at 30 knots

WOT is 5600 RPM at 39 knots and 11.9 GPH

Pitch on prop is 17, motor is a 98' Yamaha 115 salt HOH series with less than half a tank of gas.

I had questioned how large the tank was on an 18, and I gotta say my tank has got to be greater than 30 useable. There have been several occasions I have fished 60-70 miles offshore and had plenty of fuel left upon returning. I have also taken the boat, on numerous occasions, to Walker's Cay from Ft. Pierce. That trip is 102 miles dock to dock. Again, had plenty of fuel to spare.

At 7 gallons per hour and say 30mph, with 30 gallons of fuel gives you a range of about 130 miles or so. There's no way I can run offshore 60 miles, troll all day in the stream and run back in 130 miles. At least not and have a decent amount of fuel left.

My buddy bought the boat new and swears there's a huge tank in it but he casn't remember how big. As old as it is I hate to run it dry to find out.

Any more help would be appreciated- Trayder said he can find no options available other than the standard 30 useable.
Thanks,
Tom.

Tom Bare.

Mark · #2 03-07-2003, 08:50 PM

Tom,

Why not do a long trip with the intention of running it dry while having a five gallon jug on the boat if you really want to find out? - that way you would be certain.

Run out and do a long day of fishing with the idea that you would cruise around sight seeing close to shore until it ran dry. Pour in the five gallon jug and head for the dock.

just a thought

toadfish · #3 03-08-2003, 11:03 PM

Tom, my 18' Seacraft has a 48 gallon tank from the factory....Toadfish

Tom Bare · #4 03-10-2003, 09:26 AM

Mark-

I think Toadfish solved it-mine is 48 also. I did not want to run it dry due to the age of the boat-God only knows what is in the bottom of the tank, but I'm pretty sure it is not pleasant!

Thanks again,
Tom.

Bigshot · #5 03-10-2003, 05:13 PM

No biggie running it dry, your fuel pickup is at the bottom anyway. You may want to try a 19" prop, I believe that engine redlines at 5500 but double check. If prop is alum, swap into a 17" SS and drop 100 rpm's or so. Your cruise will pick up a few mph.

David Bienvenu · #6 01-03-2004, 06:03 PM

One pound of fuel will produce 2 horse power per hour. Figure a way to do better and you can get rich fast. Gasoline weight is 6 lb/gal.I have 150 Yamaha. Running at 75% is 112.5 Hp. Formula: (112.5 x .5)/6 = 9.38 GPH. Formula works for anything. No mater if it's a lawn mower or airplane.

ocuyler · #7 01-03-2004, 08:04 PM

Mark is right. Run it dry and find out...

Then replace the filter.

Flatline · #8 01-06-2004, 06:50 PM

I have been registerd for a while but do not post much. I have a 78 18ft seacraft and mine too has a 48 gallon tank. But I have a v6 150 rude on it. To my best calculations I get about 2.2 miles to a gallon usually running at 4000 rpm. I want to install the NAV MAN but have not found a good place for it. May I ask where you installed your NAV MAN? Thanks.

Mi Time II · #9 01-12-2004, 07:21 PM

I replaced the tank in my 1975 SeaCraft 18' with a new one same the same size and it hold right at 49gal. As far as MPG goes it get about 2mpg average. It has a 1992 even 175. If your #'s are correct your getting one hell of MPG. Mine is also carb 2stroke. Good luck.

[ January 12, 2004, 06:26 PM: Message edited by: Mi Time II ]