20hp outboard fuel consumption

[jimmwaller]

hello, I was just reading a post a few down and the poster had said that their boat was a 14' with an 18hp and 12 gallons of fuel. The consensus was that it would get like 5.5 hours of runtime, and the poster mentioned that that seemed a little low.

Made me realize that I thought it was really high, and something might be wrong with my setup.

I run a 20hp on a 17' and I have a 9 gallon tank. I feel like I get about an hour of runtime between 3/4 and full throttle. So, maybe something is wrong. But, maybe it's just my combination?

I have a '55 LoneStar riviera, and the guy I bought it from said that it was generally supposed to have like a 60-90ish HP outboard. I run a 20, because I used to run a 40 and it just felt too big. I use it mostly in a bay where the wind often kicks up and will produce 1ish foot swells, but I found that when I was above maybe half throttle on the 40 it just felt too "fast". I felt like I was hitting the swells and bouncing?it felt the swells were far enough apart that I would drop down into the trough and the bow would slam right into the face of the next one. (here I should say?I'm a relatively new boater, so I'm happy to hear advice on this. part of me thinks that if I went faster I could cut through the waves better, but I'm a little nervous to try).

SO I got a 20 and I feel more confident with it. Is it possible that the 20 just is SO inefficient that I'm getting that crappy of fuel economy? Or is something maybe wrong? It's a '73 johnson 20hp 2 stroke, so it's definitely old.

I don't know. It's clear that I'm not getting the economy I should be (thanks to the other poster for alerting me!), so figured I'd ask around and see what everyone thinks. Thanks!

 

[82rude]

Somebody can correct me if I'm wrong but in 1955 there were no 60 or 90 hp outboards.Heck a big outboard was 30 35 hp!.So id look towards properly setting up your motor so the prop is running correctly.Ibelieve it was 1958 before omc came out with the fat fifty?

 

[jbcurt00]

A small motor on a 17ftr has to work hard, and running 3/4-full throttle is going to burn lots of fuel.

Your Riviera was rated for a 50hp, so a 40hp should have been about a perfect fit.

If you've still got it, might try getting familiar w it and the Riviera on flat water 1st then try the bay again.

 

[Sea Rider]

Post pics of your set up, engine & lower leg to have a look.

Happy Boating

 

[roscoe]

a 20 hp should easily get 5 hours of 3/4 throttle run time on that 9 gallon tank


put the 40 back on and run it according to the water conditions.

sometimes faster is better than slower in chop.
After all, you said a 1' chop, so run fast enough to keep the bow up out of the troughs between the waves.

 

[Sea Rider]

If wanting to get the most out of a underpowered OB powering a large boat, need to install an induction tach to check min-max wot rpm range, dial a prop pitch that makes OB to run at full wot revs as usually loaded, once there should comsume less fuel and run at same previous speed with less throttle and consequently improving fuel comsumption and mileage. Just don't load that combo heavily, ideally lightly loaded and boating with max 2 up.

Happy Boating

 

[Silvertip]

The typical figure used for fuel consumption is 10% of the rated HP per hour at wide open throttle. Calculating what the 1/2, 3/4, or 2/3 throttle settings produce is impossible to do accurately without a fuel flow monitor. So in your case, 10% of 20 HP is 2 (gallons per hour). Therefore a 9 gallon tank/2 = 4.5 hours at WOT. Somewhat longer at lesser throttle settings. But I agree with the others, you had the best setup originally. That 20 HP is not doing you any favors and in an emergency may put you at risk. A boat should be powered with an engine that is rated at 75% of maximum or higher for best performance.

 

[jimmbo]

Somebody can correct me if I'm wrong but in 1955 there were no 60 or 90 hp outboards.Heck a big outboard was 30 35 hp!.So id look towards properly setting up your motor so the prop is running correctly.Ibelieve it was 1958 before omc came out with the fat fifty?

Mercury introduce the 60hp inline 6 in 1957. But during the late 1940s Evinrude was making 4 cylinder 50hp, the big four http://www.boatinfo.no/lib/evinrude/brochures/1949evinrudebigfour.html#/6

 

[82rude]

Thanks jimmbo.My point to the poster is 60-90 hp is a dangerous assumption by previous owner .In 1955 merc had a 40 hp did omc actually have a 50 for that year?After reading the other posts I have to go with the majority and say put the 40 back on.You can always throttle back on the 40hp .With the 20 you will have mediocre performance.Just looked at a evinrude brochure from 1955.25hp was the biggest sold that year for them.Did the big four turn out to be a dud?It looks like it was dropped?

 

[jimmbo]

I agree the 40hp is the way to go
The Big Four was only in production for a few years. It was an opposed engine design. 2 cylinders across from each other fired at the same time. So even though there were 4 cylinders there was only two combustion cycles per revolution. These engines and others like the 22.5 hp were pretty much the engines used to power a lot of the boats during WWII

Some videos of the Big Four in action

https://www.youtube.com/watch?v=XNYZLdmdxpo
https://www.youtube.com/watch?v=iE3hoBHXBFc
https://www.youtube.com/watch?v=gu75fbYV7EA
https://www.youtube.com/watch?v=m771rdEbfNs
https://www.youtube.com/watch?v=2WcC2h4z7ZM

 

[82rude]

I have a 1939 22.5 evinrude myself.

 

[jimmbo]

Then you certainly have more experience with them than I do. Any pics of it? Does it still run?

 

[82rude]

Yes its fully functional.