C-25 fuel mileage

journey on · Posted: Tue Sep 20, 2011 4:35 pm Post subject: C-25 fuel mileage

I noticed an interesting fact about the gas mileage on Journey On this summer, and as usual, I thought I'd share.

First, I got the fuel flow transducer, (a paddle wheel type,) working again by blowing air through it and unsticking it. For those of you who replace that transducer, I'd be interested to know if you tried that with your old transducer. It's also connected to the GPS so it gives me miles/gal.

Anyway, it now works and give me some readings which should be relative, in that it should indicate higher or lower mileage even if the figure isn't exact.

So now the meat of the discussion. I've always been told to leave the outboard slightly trimmed up, with no real explanation. Now this obviously tends to push the stern down and the bow up. I've tried different trim settings, and they didn't affect the speed.

Boris

However, when I tried trimming the motor all the way down, it increased the mileage by as much as 1/4 mi/gal, which was ~10% increase at speed, for free. Didn't affect the speed or handling. Worked at 15 knts or 7 knts. Of course trimming the motor all the way down lifts the stern and buries the bow which apparently reduces the drag.

Any comments?

BTY, we normally get 2.5 GPM at 15 knts with Judy and I plus all the cruising gerar. That gear increases every summer, as Judy gets more efficient at packing the stuff. Add one daughter and 2 grandchildern plus their gear and the mileage drops to 2 GPM

Aurelia · Posted: Tue Sep 20, 2011 5:07 pm Post subject:

We are in the same ballpark at about 2.3 at 15-17 knots and 2.6-3 with some displacement speeds mixed in. I notice our motors are much quieter when trimmed all the way down but I usually trim them up a bit out of guilt and logic. I would love to run them down in quiet mode more often if I knew there was a fuel advantage. In calm water I would take the quiet over slightly more neutral handling of some upward trim.

starcrafttom · Posted: Tue Sep 20, 2011 5:12 pm Post subject:

I cant answer your question of what the best trim is. it depend on you load and where the weight is. As I run low on fuel and heavy on water, susan is always topping of the dam water, I have a hard time to getting the bow up at all. If I am full on fuel I have to use the trim or tabs to lower the motor.

what I have found out is that no matter what speed I cruise, as long as I am no plane, I get the same gas milage at 13mph as I do at 30mph. 2.2 to 2.8 mpg depending on load and seas.

Hardtail78 · Posted: Tue Sep 20, 2011 6:17 pm Post subject: Trim 25 footer in Rough Sea

My 25 is very fuel efficient i am only burning 3.9 gal per hr at 12-13 knots.
My Trim tabs are all the way down.
If there is wind and wave action I will trim to keep her level.
The engine 150 Honda 4 stroke gives incredible power I trim it just below the first level so it's almost all the way down.
I could go 370 miles at 12 knots in most sea's on one tank.

JamesTXSD · Posted: Tue Sep 20, 2011 8:58 pm Post subject:

I have used the fuel flow meter to trim, but it seems that the best trim position is generally when the the motor is nearly perpendicular to the water. Depends on weight distribution. We generally get better than 5 mpg at 6 knots, dropping to 2.5 at 18 knots or more; around 3 mpg at 16 knots. I "flight plan" for 2.5 and almost always do better than that.

Best wishes,
Jim

thataway · Posted: Tue Sep 20, 2011 11:03 pm Post subject:

My personal way of trimming the boat is to bring to RPM and speed I want, then up and down trim, until the RPM is max at that throttle setting. Then keeping that trim, drop the RPM back down to get the speed I wanted. Generally that corresponds with the best fuel flow and the most efficient speed for that RPM, assuming that the boat is on a plane. Some folks must have very light boats, no bottom paint etc to get more than mid 3 mpg. As I recollect the best we got was close to 3 over all for 1200 mile trips. That would include some time at displacement speeds. But also the boat was well loaded for a month of cruising.

T.R. Bauer · Posted: Wed Sep 21, 2011 12:50 am Post subject:

I think the funniest thing I have ever heard is, "my boat is very fuel efficient an burns 3.9 mph at 12-13 kts." I'm not saying that is burning tons of fuel, but I have a hard time believing anything that has "very fuel efficient" and "boat" in the same sentence.......

Captains Cat · Posted: Wed Sep 21, 2011 8:08 am Post subject:

helm · Posted: Wed Sep 21, 2011 9:11 am Post subject:

We figure about 2.7 mpg cruising with the Chack Chack overall. Our longest run on the loop was from Hoppies below St. Louis on the Mississippi to Kentucky Lake which is 230 miles, no fuel stops. Part is with the current - down the Mississippi, and part against the current - up the Ohio. We had 15 gallons of fuel left when we got to Kentucky Lake plus 10 gallons in Jerry jugs that I had bought in Peoria just for that leg. We gave the jerry jugs away to the dock guys at Kentucky Lake.

Just as an aside on the AGLCA email list yesterday there were some guys discussing bringing a Viking 75 from Rochester NY to Florida - their fuel burn at cruise -23 knots is 110 gph - so I guess fuel effeciency is all relative.

Eric

stevej · Posted: Wed Sep 21, 2011 10:56 am Post subject:

At 15 - 17 MPH I get around 2.5 MPG, it's not my ideal cruising speed. The 200 e-tec likes to run 3800 to 4300 RPM and will give an honest 3MPG at 22-24 MPH fully loaded. Best cruising milage I have experienced was when way over pitched running at 26MPH and getting 3.3 MPG.

stevej

TyBoo · Posted: Wed Sep 21, 2011 12:25 pm Post subject:

I average 2 mpg everytime all the time. My boat is heavy and a little underpowered at 130 hp. I wish it was faster and I wish it was more economical but it's fine like it is.

Dora~Jean · Posted: Wed Sep 21, 2011 7:24 pm Post subject:

I get 2.6-2.8 nmpg on average (16-18 knots). Have gotten over 3 a few times, must have been going with the current, downwind and only me on board... Laughing

Then there are the stormy seas crossings (20+ nm), probably running about 2.2 nmpg or so at 8-12 knots.

It's interesting that my twin F80's are nearly identical in fuel usage than the singles. I've only had Permatrims for the past couple of years, but haven't noted any particular change in fuel usage. I put bolts in the 2nd to the lowest holes in the outboard brackets to fix their lower limit to about 10-15 deg below horizontal. I run them in this position and use the trim tabs to balance or lower the bow as needed for sea and loading conditions.

journey on · Posted: Wed Sep 21, 2011 8:40 pm Post subject:

Mike, I wish I could fish like you can.

Boris

tpbrady · Posted: Wed Sep 21, 2011 11:13 pm Post subject:

Over 1700 miles this summer, we averaged 2.9 mpg based on gas put in the tank. On my 25 with a Permatrim, efficiency improves with speed. Starting at 12 mph at 2.2 up to 26 mph 2.7mpg. As the speed increases, the trim tabs come up, nearly full down at 12 mph, and full up at 26 mph and motor trim comes up. As fuel and water get used those number go up almost .3 mpg.