Is nearly 6 gallons of gas excessive for a 50 HP outboard to burn in less than 2 hours?

Question

I have a 1985 Mercury 2 stroke 50 HP outboard on my 24' pontoon boat that weighs less than 2000 lbs. Last night I was out with one other adult for under 2 hours. For two thirds of the time we were at "no wake" speeds, with the remainder of the time at full throttle. I just got the boat so I'm trying to figure out if this is the typical gas consumtion I should expect, but it just seems excessive to me. I've experimented with the trim and the prop is a little banged up, but not too bad. We were out in an inland chain of lakes with just a little chop. The motor starts, runs, sounds and shifts like a champ. The only other thing I can think to mention is that the boat seems slower than I anticipated, and when I'm at full throttle, the motor seems like it should be revving a bit higher. Can anyone clue me in?? Thanks!

Answer 1

You should also familiarize yourself with the different types of boat hulls, primarily two;

Displacement and Non-Displacement

A "Displacement" hull is designed NOT to plane, your pontoon boat is a "Displacement" hull.

Displacement (ie non-planing) hulls theoretical maximum speed is governed by the length of the wave created as the boat moves through the water. Since longer waterline lengths create longer waves they are able to go faster. Keep in mind that hulls are usually designed so that waterline length will increase as the boat heels increasing the maximum speed and also that in some circumstances (IE: surfing down large waves) the boat ceases to be a true displacement hull and can exceed the theoretical maximum. You could put a jet turbine on this type hull and succeed in creating a large standing wave in front of your boat and not succeed in going much faster.

On the other hand and analogous to this discussion would be a Catamaran. Take a 18' Hobie Cat for instance, they are a non-displacement design and engineered to "Plane" if you will.

Your best bet will be to unload the 50 HP 2 stroke to someone who has a planing type hull "Small Bassboat" or something similar and get yourself a 4-stroke Yamaha or some other "High-Thrust" engine. Here is an excerpt from the Yamaha Marine Outboard Website.

Yamaha's Dual Thrust™ propeller measures 14" in diameter and 11' in pitch for the T60 and T50 models. The large diameter, low pitch design provides up to 70% more thrust in reverse and 60% more thrust in forward than a standard two stoke, making it ideal for pontoon boats and other applications where maneuverability and thrust is important.  

The 4-stroke will be much more pleasant on the pontoon boat, much quieter and better on gas.

You aren't going to be pulling any water skiers either way so why tip the wine glasses and slosh the barbecue sauce simmering away!

You sir have one of the small pleasures in this world that is now affordable, smile broadly as you efficiently motor away at 7-8 kts  while the blown v-8 monsters sit idle at the dock cause they cant afford $200.00 in gas to rip the lake up for a couple of hours!

Enjoy !

Jerry

Answer 2

general rule of thumb, estimate full throttle fuel burn rate at 1/10 of the horsepower (gallons per hour).  so, 2/3 of an hour at 5 gal per hour is just over 3 gallons.  Wouldn't be surprised if you burned a couple gallons in 1hr 20 minutes at no-wake speed.  Add some margin for error, and some left in the tank (6-gal tank, right?).

4-cylinder?  how many carbs?  make sure all the carb throttle valves are parallel and perfectly horizontal (not beyond) at WOT.

Answer 3

We had a big 30' pontoon boat at a rental marina and you couldn't load enough fuel on it for a whole day. The rule of deminishing  returns -was alive and well with pontoons. The more throttle they get the less responce you could expect. Throw in a bad or under prop situation and the mileage was terriable. Have the motor looked at for timing and carb adjustment, get a tac and see how much smaller your prop should be, but don't expect miracles. The only real answer was 4 strokes. And believe it or not the best pontoon boat motor was an old 55hp Bearcat- with the inverted crosley automobile engine.

Answer 4

WOT sucks gas, that's the bottom line.

Answer 5

Sorry for the "gas shock" with your new boat... Fact is, boats (other than sail) were never made to be fuel efficient...

The guys telling you that your 50hp is too small - well, that's just their opinion... (we all have 'em, of course) but, your "gas usage" has nothing to do with your motor being too small.  In fact, if you want to burn more gas - simply get a bigger engine.

Your vessel has a designed hull speed... once you are at that speed - that should be it - period!  Exceeding this speed has a very dramatic effect on the energy it takes move your vessel forward and to make it go any faster...

On the other hand, the one thing that will affect your gas usage (other than your engine being tuned and in good condition) is your "prop"... The size and type of your prop may not be what you want on that boat... and believe me, it is amazing the difference in performance and fuel they can make (even dents, cracks, bends, can make a difference.

Pontoons were not designed for speed.  In fact, the very first use of these vessels were in WWI for being towed... The military used them by towing them up rivers with supplies, and also to make bridges by tying them side to side across rivers and then laying tracks across them for men and machines to move across.  Most of them were later sold as military surplus and of course, didn't become popular until the 60's after some fisherman decided to put a small motor on the back of it and a lawn chair on the deck of it.

So, have your prop checked out... I mean you can check it for dents and to see if it is cracked or bent - but you really need to take it by a knowledgeable mechanic and see if you have the right prop on it... you need a balanced prop that will give you the correct amount of "thrust" not speed at low rpms...

Now, having said all that... 6 gallons in two hours is not bad at all for a 50hp engine...  about the only way you are going to improve on that in a dramatic way is to go electric -

Happy & Safe Boating!

http://boatwrights.org

Answer 6

50hp is small for a 24' pontoon boat. Takes a lot of power from the 50hp to move that boat.  At full throttle you may be using 3 - 5 gal an hour depending on your load.

Best advise is run at about 3/4 throttle, get a tach and a speedometer.  Than check to see how much speed you gain or don't. gain with an increase in engine speed.  

This is not a speed boat, but could provide hours of fun and operated at a good steady speed be very economical.

Enjoy!

Answer 7

boats suck gas...lots of it...my friend had a 35 and we could burn 6 gal in a couple of hours easy...50 hp is small for a 24 foot boat...my neighbor has a 17 foot with a 95 hp merc...and the guy down the road has a bass boat   20 foot long with 200 hp.

Answer 8

You'll burn that in 1 hour keeping it at full throttle.

Answer 9

Full throttle will eat gas at a very high rate. Any little nick on the prop will have an impact on performance and fuel economy. So if its banged up a little, it could make a noticeable difference.

For the slower motor revs, you will need to check your specs for the motor and verify RPM to see if its in spec. Its possible you may have the wrong prop on it. Sounds like its over pitched and therefore is "geared too high"

Just cutting down throttle to 75% will make a big difference in gas consumption.

Answer 10

That is not bad at all. I run a larger motor and would not be comparinf apples to apples. My brother in law has an older 50 on a basstracker and he is getting 3.9 miles per gallon. We were boating this past weekend and he came in on fumes so we were able to get an exact on mileage. I do no think this is excessive at all but would change your driving habits. If you run anything wide open all the time it will suck down full, oil, and put major wear and tear on the motor. About once a month I will run mine wide open for a brief period of time, but normally run approx 3200 rpm (max is 5500) for my motor. My cruising speed is near 30 mph at that rpm and I find that my fuel bill is not bad at all. Coming in from the Gulf in the summers when the weather gets bad and we have to get on the throttle to get back, I almost double my fuel bill.  Back off the throttle and you will save big time.

When I get a new boat, I will quickly test its range. I keep very accurate info on average fuel use per hour, average mpg under a heavy load. When we head out fishing I never go offshore beyond 30% of it range. I know that I can troll all day, hit back and forth spots, run the coast lines, beach hop and still make it home with plenty to spare, even if I have to drop the throttle. Cars you can push the envelope if you know there is a station nearby... I have yet to see a gas station 30 miles offshore.