Fuel Usage????

T37Chef · **Posted:** 03 Sep 2007 18:35

Hello,

I have been monitoring my fuel consumption for a couple of years now to 1.7 gallons per hour. That seems high?

I have an Auto Prop, W50, and usually motor at about 2400 rpm at 7 +- knots.

What kind of fuel usage do you get?

I am thinking I may need to calibrate the prop?

Thoughts?

Sam · **Joined:** 31 Oct 2006 19:43 **Posts:** 37

I posted a quote from the internet about fuel consumption that claimed 20 HP per gallon per hour so 1.7 gallons per hour would be 34 HP worth of fuel for the 7+ kts you mentioned. I have read that 0.4 pounds of diesel per HP/Hour was a credible consumption rate, diesel is listed as 7.1 pounds/gallon so that works out to 17.75 HP per gallon per hour. At 1.7 gallons/per, the HP is 30 so I guess the question is does a T37 need 30 to 35 HP to motor at 7+ kts? I have claimed that at 5.9 to 6.2 I observed a rate of 0.75 gal/hr so it seeems that one extra kt after 6kts is requiring a gallon of fuel each hour. I have crewed on a boat fitted with an Auto Prop made by Brunton's in England, I think it is an excellent device and if your prop is what I am referring to, there are no adjustments, the prop feathers itself into the apparent water flow as it is providing thrust, as the boat moves, the blades twist to match the flow caused by boat speed and the built in blade camber adds a slight additional angle of attack to drive the boat forward. While sailing, the blades swing around the hub and feather to reduce drag. The prop I am familiar with has no gears in the hub and each of the three blades is independent of the others, the flow of water past the blades causes them to swivel into the flow, motor sailing with this prop is its strong point because the blade will swing into the flow at sailing speed while the blade camber just adds thrust so the effect is like motor sailing with a proppellor with a large amount of pitch, say 15" but while straight motoring without sail the effective pitch may look to the engine as a 13" prop and in really rough going with the boat laboring, the prop may look to the engine as if it has only 10" of pitch. My explaination may be flawed but the propellor works, the faster the boat goes, the more pitch the propellor provides. If you have this prop, it is set at the factory and the only way to change is to get different blades. I think that if you experimneted traveling at 6 kts you may get the fuel use down to 1 GPH or less.
A few months ago there was a discussion about holding the throttle lever from creeping, I use a string loop just as some suggested. For my boat it is set at about 5.5 kts I have never let my wife see that it can go faster so as far as she knows 5.5 kts is full speed.

Sam

Tortuga's Lie · **Joined:** 27 Oct 2006 10:32 **Posts:** 827

Our boat has a W-40 and if we have the RPM's at around 2200, with little/no current/windage, we'll travel about 6.5 knots with a consumption of around .75 gal/hour, maybe a little higher if the current is against us.
We have a 16" Martek folding prop, two blade, that came with the boat.

T37Chef · **Posted:** 05 Sep 2007 19:30

Tortuga's Lie wrote:

Our boat has a W-40 and if we have the RPM's at around 2200, with little/no current/windage, we'll travel about 6.5 knots with a consumption of around .75 gal/hour, maybe a little higher if the current is against us.
We have a 16" Martek folding prop, two blade, that came with the boat.

WOW...something is definitely up then. .75 to 1 is what I expected and always have thought my fuel consumption was way out of whack... Shocked

I am thinking to pull the engine over the winter and do a minor overhaul. Should be a good winter boat project Crying or Very sad

Sam · **Joined:** 31 Oct 2006 19:43 **Posts:** 37

WOW...something is definitely up then. .75 to 1 is what I expected and always have thought my fuel consumption was way out of whack... I am thinking to pull the engine over the winter and do a minor overhaul. Should be a good winter boat project

Before you pull your engine apart I wonder if you could run at full power to try for 8kts and observe the "squat" at the transom, even at seven Kts, I would think you are close to running with the exhuast outlet at or in the water. 7+ kts at 2400 is very good and I would be interested if you have more speed and rpm available. I have my boat out of the water so I can't test the speed but it seems like you have a very strong combination of engine and propellor and if you don't have any noises or serious problems, you may try to get a consumption curve vs. speed before the rebuild otherwise how would you know if it improved? Can I ask what is a minor overhaul? Which internal engine part(s) would cause excessive fuel consumption? Maybe you can ask another T37 to run a test at 7+ kts to see what they burn at that speed. One more thing, I have sailed with three different captans this summer Sabre 38, C&C 35, and a Camper Nicholson 35 with a Perkins 4-108 (very similar to W40). They all "calculate" the fuel remaining in the tank by logging the engine hours and multiplying by a rate that is based on past history of fill ups and hours. The C&C has run out completely unexpectedly twice while I was aboard, several years apart, but while we were on long passages, one from bermuda and the second time from Halifax,Canada. The Sabre never goes below 1/2 tank because he doesn't want to run out, and the Camper Nick is often off the predicted gallons to fill by 10% to 15%. My point is the consumption rate each skipper offers may not be accurate enough to provide incentive to overhaul an engine that may be ok.

Sam
Windfall #52

alpinista · **Posted:** 07 Sep 2007 22:52

I just completed an offshore passage with 66 hours of motoring on our 4-107 W40. We used 43 gals. of fuel for a rate of .65 gal/hr. We run the boat at 2000 rpm and average 6.5 knots with our original equipment two-bladed fixed prop.

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          /|\          Jay and Muki Kerr
         / ||\         Tartan 37 Betty Lou
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       / 37||  \       St. Helens, Oregon
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efgrina · **Joined:** 23 Jan 2007 19:04 **Posts:** 10

My 1979 W-40 (in hull 145) drives a two bladed fixed prop, to a boat speed of 6.2 knots with minimum chop, at 2150 RPM while consuming an average of .8 gal/hr. (As bottom grows hair during the season the speed falls off a half knot or so.)

You can also compare that to the performance curves for the W40 which I got off the net - I think it was a Pearson Owner's site that posted it. From the curve at 2250 RPM fuel consumption is supposed to be 1.3 gph. Horspower there is about 32. on the curve. At rated speed of 3000 RPM you get a continuous rating of 40 HP with 2.2 gph. Can't explain the difference in fuel consumption between curve and real life.

Eric Grina
T-37 Cricket

Down2TheC · **Posted:** 30 Sep 2007 08:41

We stick with 2000 RPM on our W50 w/ stock 2 blade fixed screw. We usually get .65 - .7 but this year was a hard year and we saw .75 for the first time.

jammin · **Joined:** 30 Oct 2006 15:40 **Posts:** 63

I have to agree with the other posts. We have a W50. We motor at 2000-2200 rpm most of the time with a Martec folding prop and get .65 to .7 gal per hour. We also tend to motor at 5-6 knots. No offence intended, but the first thing I would do is re-calculate the fuel consumption. Although for cruising at 7+kts it may be possible to use that much fuel. Also, are you in flat water or choppy seas? More fuel will be used in heavy seas to maintain a given speed.

I fill the tank everytime, so the amount added is always the fuel used since last fill. One tank fill up is not enough to calculate average usage.

Are you getting any black exhaust? Have you had an injector tune up lately? A stuck injector could cause excessive fuel use, but should show up as black smoke and/or fuel in the crankcase.

I wouldn't jump to do anything expensive. Try all the cheap fixed first.

JAMMIN #226

Velera · **Joined:** 29 Dec 2006 09:38 **Posts:** 656

You might check to see if you Tach is working correctly. They do not last forever. Mine started working slower (so fuel consumption appeared increased increased as gal/hr)) as it got older and gave the impression that I was using a lot more fuel. On 7000 mile trip from California to Maine, I used .9 gals/hr like clock work at 2300 rpm.

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