6 HP Evirude fuel consumption

[Nomad57]

I bought my first boat, a 14 footer, in 1975, with a Mercury 50, four cylinder. An article in Consumer Reports suggested that at cruise it would burn about 3 U.S. gallons per hour. Since my boat was relatively light, at 400 pounds plus motor and gear, I seemed to average around 2.1 U.S. gallons per hour.

Fast-forward some twenty years to 1997, I rented a 26 foot sailboat. It had an Evinrude 6 on the transom. The motor appeared to be new, and I was told that it was. I was skeptical that this would be enough for a 5500 pound boat, but the owner said that it was 'plenty'. I checked to make sure the six gallon fuel tank was full before I left the dock, (among other things).

It was one of those dead calm days, where the water looks like a sheet of glass. Three hours after leaving the dock, the motor stopped. I checked the obvious first, (the fuel tank), and it was empty. That meant that the fuel consumption for this six horsepower motor was 3 gallons per hour, the same as my Mercury fifty. (I had been running at about two-thirds throttle).

How, in heaven's name, can a six HP motor of some 10 cubic inches, burn as much fuel as a 50 HP of 44 cubic inches?

Nomad

 

[jrs_diesel]

Re: 6 HP Evirude fuel consumption

First, that's not enough motor on a big boat. I have a 6 HP on my 22 foot (2100 lb) sailboat, and that is the right size engine for my boat.

6 gallons in 3 hours is 2 gallons an hour. Still a lot of fuel. Something is not right.

 

[raczekp1]

Re: 6 HP Evirude fuel consumption

hi.
i hade 6 hp made in 1973 and boat with weight around 150 kg.
with ful speed and 2 persons onboard motor took 3l( 1 galon) per hour

 

[SolingSailor]

Re: 6 HP Evirude fuel consumption

A couple of ideas --

First, 5500 lbs. is very very heavy for a 26 foot sailboat. Are you sure about that displacement? 6 HP is more than adequate for pushing most 26 ft. sailboats to hull speed.

Which brings me to my second thought -- Were you trying to exceed the hull speed of that boat? That would be 5 or 6 knots on a boat that size. If so, no amount of extra throttle would make the boat go faster, you would only burn more fuel, maybe as much as 2 gal./hr.

Also, if the bottom was foul with growth, you would see excessive consumption at any speed.

I'm not a powerboat expert, but I'm sure that a boat on plane uses less fuel than if trying to push a displacement boat past its hull speed.

 

[Nomad57]

Re: 6 HP Evirude fuel consumption

First, that's not enough motor on a big boat.

I knew that, straight away. I expressed my doubts to the owner.

A boat that size and weight needs a 15 (or at least a 9.9).

Nomad

 

[Nomad57]

Re: 6 HP Evirude fuel consumption

A couple of ideas --

First, 5500 lbs. is very very heavy for a 26 foot sailboat. Are you sure about that displacement? 6 HP is more than adequate for pushing most 26 ft. sailboats to hull speed.

That's what I was told was the displacement for that particular model of boat.

Which brings me to my second thought -- Were you trying to exceed the hull speed of that boat? That would be 5 or 6 knots on a boat that size. If so, no amount of extra throttle would make the boat go faster, you would only burn more fuel, maybe as much as 2 gal./hr.

I wasn't. I had been boating for twenty years at that point in time, and I knew all about hull speed. But even at full throttle, a motor that size shouldn't burn that much fuel.

Also, if the bottom was foul with growth, you would see excessive consumption at any speed.

I didn't check the bottom.

I'm not a powerboat expert, but I'm sure that a boat on plane uses less fuel than if trying to push a displacement boat past its hull speed.

Very true. I see it all the time, especially in larger boats.

Nomad

 

[Vic.S]

Re: 6 HP Evirude fuel consumption

Firstly
5500lbs is not particularly heavy for a 26ft sailing yacht. The most popular 26footer ever built in the UK, the Westerly Centaur is 6700lbs.

Secondly
I agree 6hp is too small for a 26 footer. Westerly fitted inboard engines between 10hp and 25 hp in their Centaur.

Thirdly
I agree that two gallons, even US gallons, per hour is excessive. I have a 6hp and use around 1/2 Imperial gallon per hour although that's not at WOT.

Unsuitable prop for a sail boat might be a contributing factor.

 

[jay_merrill]

Re: 6 HP Evirude fuel consumption

The question of engine size for the boat, hull speed, etc., really isn't a factor here. The issue is how a 6hp OMC ouboard could possibly burn 3 gallons of fuel per hour. I doubt it can - under any circumstance. Simply put, I don't think you could suck that much fuel through it.

I ran a pair of Johnson 6hp motors on a 15' trihull for a number of years. They were run at WOT pretty much all of the time and were trying to push the boat through its bow wave in this condition. I didn't burn 3 gallons per hour with both of them.

Something is wrong with your calculations.



???

 

[F_R]

Re: 6 HP Evirude fuel consumption

Propped to run at a given RPM, fuel consumption would be more or less the same per hour, no matter what boat it is on. There is something wrong with that motor. Probably dumping more gas in the water than it is burning.

 

[boobie]

Re: 6 HP Evirude fuel consumption

The old "Rule of Thumb" used to be 10% of the HP at WOT was the GPH you would use.

 

[Nomad57]

Re: 6 HP Evirude fuel consumption

Something is wrong with your calculations.



???

When a full six-gallon tank is empty after three hours, there is nothing wrong with my calculations. It's not rocket science.

I've heard other people complain about these 6-9.9 Johnrudes. One fellow with a Catalina 25 got rid of his 9.9, and bought a Suzuki four-stroke.

Nomad

 

[Nomad57]

Re: 6 HP Evirude fuel consumption

The old "Rule of Thumb" used to be 10% of the HP at WOT was the GPH you would use.

Right, that's why I was expecting about .6 gph at full throttle, and .3 gph at cruise, not 2 gph.

When I looked into the tank after three hours and it was empty, you could have knocked me over with a feather.

Pretty-much ruined that trip for me, since we were dependent on the wind, (since I was unwilling to burn 2 gph with total 12 gallons on board) which was unreliable.

Nomad

 

[Nomad57]

Re: 6 HP Evirude fuel consumption

Propped to run at a given RPM, fuel consumption would be more or less the same per hour, no matter what boat it is on. There is something wrong with that motor. Probably dumping more gas in the water than it is burning.

Considering the condition of the rest of the boat, I believe you're right. Good thing I brought my tool box on that trip, I needed it. Of course, I didn't discover most of the boat's deficiencies until I was well underway. Had I known before I left the dock, I never would have.

Nomad

 

[jay_merrill]

Re: 6 HP Evirude fuel consumption

When a full six-gallon tank is empty after three hours, there is nothing wrong with my calculations. It's not rocket science.

I've heard other people complain about these 6-9.9 Johnrudes. One fellow with a Catalina 25 got rid of his 9.9, and bought a Suzuki four-stroke.

Nomad

Are you sure? You said you burned 3gph.

3 x 3 = 9.

6 / 3 = 2.

You burned 2 gph, which is still more than I would expect. I have to agree with F_R - you have to be blowing raw fuel through the engine somewhere. Either that, or you are incorrect about how long you ran the motor.

BTW, the 6hp OMC motor is a completely different engine from the 9.9hp motor. That said, both of them were hugely successful for OMC. I too have heard lots of people complain about a lot of things. Some of them have no idea what they are talking about.


???

 

[Tim Frank]

Re: 6 HP Evirude fuel consumption

It may not be "rocket science", but even in the metric system, 6 divided by 3 does not equal 3....

 

[jeremy_nash]

Re: 6 HP Evirude fuel consumption

I had a similar issue with my 25hp chrysler on my 14' fishing boat. after about 1 1/2 hours I had burned through 6 gallons of gas. didnt have spare gas, as 1 tank is more than enough for a full day fishing, and trolling battery died 1/2 way back to dock. turned out to be a torn fuel pump diaghragm. pumping fuel into the crankcase.

 

[Cornyboy]

Re: 6 HP Evirude fuel consumption

I run a 6hp Johnson 2 stroke (1970s era) on a 21' sailboat of similar vintage. In dead calm, at LOW SPEED, I can run it for about 10 hours on a 6 (US) gal. tank. In fact, I just did so last weekend. (Why I am on this thread.) I was very impressed. I sailed miles out to islands in Lake Superior one day, and motored home in dead calm the next. I swear I ran it about 8 hours total--again, at quite low speed--but only used 2/3 of a (6 gal.) tank of gas. Yay!

I bought my first boat, a 14 footer, in 1975, with a Mercury 50, four cylinder. An article in Consumer Reports suggested that at cruise it would burn about 3 U.S. gallons per hour. Since my boat was relatively light, at 400 pounds plus motor and gear, I seemed to average around 2.1 U.S. gallons per hour.

Fast-forward some twenty years to 1997, I rented a 26 foot sailboat. It had an Evinrude 6 on the transom. The motor appeared to be new, and I was told that it was. I was skeptical that this would be enough for a 5500 pound boat, but the owner said that it was 'plenty'. I checked to make sure the six gallon fuel tank was full before I left the dock, (among other things).

It was one of those dead calm days, where the water looks like a sheet of glass. Three hours after leaving the dock, the motor stopped. I checked the obvious first, (the fuel tank), and it was empty. That meant that the fuel consumption for this six horsepower motor was 3 gallons per hour, the same as my Mercury fifty. (I had been running at about two-thirds throttle).

How, in heaven's name, can a six HP motor of some 10 cubic inches, burn as much fuel as a 50 HP of 44 cubic inches?

Nomad

 

[Tim Frank]

Re: 6 HP Evirude fuel consumption

two year old thread