Excessive gas usage and boat would not plane

[iggyw1]

I think I know the answer to this question, but: I took my boat out for the first time the other day and I found that I had a lot of water (18 gallons) trapped beneath the floor. Got that problem resolved, however, when I had it out on the lake, the boat would not plane. I suspect that it was due to all of this water under the floor. (sealed bilge area, no outlet for the water). The bow was wayyyyyyy up in the air and two big guys on board as well. only got 10 MPH out of a 75 h.p. Evinrude speedi - four on a 16 ft. Geneva fiberglass boat. Went only about 6 miles wide open and used 5.5 gallons of gas. Motor seemed to run real strong but it was struggling to get the boat on plane which it could not do. Is this gas usage due to the boat not planing??? Is that why I did not get any type of speed out of the motor, or would that amount of water not effect it that much??

ALSO, I might add that the motor was tilted on the last notch out (away from the transom). I moved it in two notches and there is one more notch to go farther in towards the transom. Cannot wait to get it back on the lake again to see the difference in the boat now that the water is gone (Cut hole into solid floor and installed a bilge pump) Boat is not leaking but had a bunch of rain water in it!! Looking for opinions on what effect the water had on the boat not planning or no speed and what effect the motor tilted out all the way would have on it too???
Thanks!

 

[H20Rat]

ALSO, I might add that the motor was tilted on the last notch out (away from the transom). I moved it in two notches and there is one more notch to go farther in towards the transom. !

Not saying you don't have water problems, but that by itself will be more than enough to keep a 16' from planing!

 

[SCduckhunter]

Do the math at 8.5# per gallon of water...20 gallons...that's 170# in addition to all the other persons and gear on board...my 19.6 ft cc with a 150,tops out at 54 mph with me and 20,gallons of fuel with fishing gear...I took on water in heavy seas last Sunday and couldn't get it to.plane out...with the throttle on the corner...water adds a lot of weight fast...the fact that it was holding water with no.outlet is a different issue..but I'd say that water was your problem

 

[jigngrub]

18 gallons of water is only 144 lbs.... a medium sized person.

Tilt your motor all the way in to the transom to start with, then move out a hole at a time until you find the sweet spot. Tilted all the way in will plane quicker, but you'll get less on top end. The second hole from the transom should be the best, but I'd try it all the way in first.

 

[Woodonglass]

Check and make sure you don't have a spun prop. Check that it's hitting on all cylinders

 

[iggyw1]

Thanks to all that replied. I never thought of the spun prop, but I will definitely check that out. Also, I will start with the motor all the way in towards the transom. By getting the water out and also having the boat go on plane, it should be way better on gas too right? I sure hope so 'cuz at one mile per gallon, plus the oil, I cannot afford to go fishing!

 

[Sea Stomper]

If you are running on 2 cylinders instead of 3, the exact same thing will happen and it will guzzle fuel badly.

 

[Chris1956]

Certainly the weight of the water didn't help, but unless there was way more than 18 gal of water, I would suspect something may be wrong with the engine. I would take a compression test and inspect the spark plugs for color. It is possible you are running on 2 cylinders only. Those V4 motors run pretty good on two. The fuel consumption you saw was about right for not planning and running on 2 cylinders. Check the points for proper gap.

 

[Watermann]

By the OB formula of old... It will consume 1 gallon an hour for every 10 HP at WOT. 7.5 gal an hour... 6 miles at 10 MPH at WOT and used 5.5 gal sounds about right.

 

[iggyw1]

By the OB formula of old... It will consume 1 gallon an hour for every 10 HP at WOT. 7.5 gal an hour... 6 miles at 10 MPH at WOT and used 5.5 gal sounds about right.

I hope I get it to plane now that I got all of the water out that was trapped beneath the floor and I moved the motor in towards the transom. Once it planes out for me, I can pull back on the throttle a bit at a good cruising speed and save some fuel as I get to my fishing hole then.

 

[jigngrub]

Let us know how it goes this weekend, I'm curious to hear about how much better it is.

If this is a "new to you" boat there are some things you should consider doing to that motor if you haven't already.

1. Carb rebuild
2. Fuel pump rebuild
3.New spark plugs
4. New impeller (water pump)
5. Change the lower unit lube

These things will help increase the performance of your motor, make it more reliable, and help ensure it's longevity.

 

[iggyw1]

Hi Jigngrub,

I had the water impellor replaced and the lower unit was resealed completely when I had it in the repair shop. As for the carb, I plan on rebuilding it during the off season here in Mich. Has new spark plugs in it. I will be running a heavy dose of sea foam in the gas soon. I have not been out since I had the water trapped under the floor. I had my wife in for surgery, and I took my truck in for warranty work on June 9. I called them yesterday (June 17) and they just started on it at 4:00 p.m. yesterday. I MIGHT get it back on Friday but unlikely!!!! Peed me off big time! No way to tow the boat to the lake. My son and best friend are both out of town for another week, so no help there.

It Gave me some time to work on the boat. I got my temp sensor in on the outboard and the gauge is in the dash, all hooked up. Got my new fish finder in and all hooked up. I installed a bilge pump once I cut an access hole into the floor. Put a switch on that and its all set to go. I am really anxious to see what happens as far as planning out and speed when I take it out on the lake. I will replace the plugs again when I finish the heavy dose of seafoam. They may not need replacing, but plugs are pretty cheap and there are only 4 of them. Cheap insurance! I also dropped the motor three notches closer towards the transom. I did not notice that was way out all the way before I went out last time when it would not plane until I came back to the dock that day Will post when I take her out again.

 

[iggyw1]

Jignrub, I told you I would let you know how my motor/boat re-acted after I removed the water from beneath the floor. I finally got her out on the lake to try things out. Before, I could not get the boat to plane at all and I was only able to go 10 MPH with WOT. Well things sure changed since I got the trapped water out of the area between the bot hull and the floor. The boat planed very well and I got 25 MPH out of her. Was looking for 26-28, but I will settle for 25! I also tilted the motor closer in towards the transom too. Before, it was tilted all the way out, and now I took her in only one notch out from the transom.

The boat no longer has a "sealed" bilge area. Before, there was no access to let any water out if it got under the floor. It is suppose to be a sealed floor so no water would get under it. Well, thru the years, the previous owner put brackets in the floor to hold batteries in place and to hold gas tanks down and such, and the brackets have been moved thru the years, so water got in thru the old screw holes here and there. I cut a hole in the floor at the rear where the normal bilge area is, and installed a bilge pump. Was on the lake 6 hrs yesterday, and did not get one drop of water in the boat, so I know for certain that the trapped water that was there was rain water as I suspected. I now have a bilge pump on a switch with a thru hall discharge if I ever need it. Was a cheap fix.

I am now trying to figure out why the motor is "drinking the gas". I am using a lot of gas for short trips. I think I read somewhere on here that someone replaced the diaphragm on their fuel pump and it cut way down on the gas usage in their motor. Is this possible???

 

[jigngrub]

Rebuilding the fuel pump and carbs and replacing all the old fuel lines would be a real good place to start looking for better fuel economy... but boats are gas hogs and not like cars, 5 miles per gallon is pretty good mileage in a boat.

 

[Woodonglass]

With One person on Board in a 16ft fiberglass boat and a 75 hp Evinrude that's properly set up with a properly pitched prop etc I'd fully expect to get 35 mph or more. I have a 15 footer with only a 40 hp and I get 25 mph with 2 300lb adults in her. Does your hull have foam in it. It could be waterlogged and adding a lot of additional weight. You should check to ensure your prop is the proper pitch and is not slipping on the hub. You should also hook up a Tachometer and determine the RPM @ WOT!!! There is definitely something wrong! It, for sure be making better speed.

As for your fuel usage..

Fuel useage cannot be tracked on a linear scale (although that may seem logical).

For example, a 50 hp Evinrude at idle, in gear (700 rpms) burns about .3 gallons per hours giving it the eqivalent of 8 mpg.

At 1500 rpms it burns .4 gph but because the boat speed has effectively doubled (from 2.4 to 4.9 mph on the test hull), now it's getting about 13.5 mpg.

However, when you hit 2500 rpms, the gph jumps to 1.7 and mpg drops to 4.2.

But then when it's running at 3500 rpms, the fuel usage just barely tic's up to 2.0 gph giving it 9.9 mpg (this motors "sweet spot").

From there on the fuel usage goes up and the mpg goes down until at near wide open throttle it's burn about 4 1/2 gallons per hour (not bad for a 50 horse) and getting the eqivalent of 7.2 mpg.

So as you can see the ratio of fuel useage to miles per gallon is all over the map - and each model of motor will have it's own "sweet spots".

All you can do with a given motor is -

make sure it's tuned properly (new spark plugs, fresh gas, properly adjusted carbs)

make sure it's mounted properly (correct height, trimmed properly)

make sure it's "propped" properly (running too much pitch will overwork the engine and fuel economy will suffer/too little pitch will not get you the speed for a given rpm and could tend to make the mpg suffer)

load the boat properly (too much weight in the front will cause it to plough water and waste gas etc)

and once you have done all that, you need to play around and see "where" your motor likes to run - normally somewhere in the 3500-4200 rpm range (this is why I said Hook up a Tachometer) will give you the best combination of speed and fuel economy.

I am NOT a motor guy. This is all just stuff I've gleaned here on the forum and research on the "Net". Hope it helps!!

 

[iggyw1]

Rebuilding the fuel pump and carbs and replacing all the old fuel lines would be a real good place to start looking for better fuel economy... but boats are gas hogs and not like cars, 5 miles per gallon is pretty good mileage in a boat.

The fuel lines have been replaced. I found the fuel pump repair kit at a local marina for $30, so I am going to do that this week. It doesn't look like too much to that repair in the shop manual that I have. Not many parts at all inside that fuel pump. That's an easy repair the way it looks. The carb, I plan on removing that myself then taking to a carb shop for them to rebuild it during the off season here in Michigan. I can remove and replace it onto the motor, but I don't know about the rebuild. I ran seafoam thru it yesterday and that decarboned things real nice. If I did get 5 miles per gallon on the boat, I would be real happy. I am nowhere near that now. I went about 4 - 5 miles yesterday at a cruising speed and used 3 & 1/2 gallons of gas. The last time I was out when the boat would not plane I got one mile per gallon if that, so it improved somewhat. Just wondered if a bad diaphragm in a fuel pump would cause the gas to be sucked up like it is doing???

 

[Marc Peternell]

As suggested on here earlier, you may want to check the pitch of your prop. Also, you could weigh the boat. A weigh station or local garbage dump can weigh it for you. Weigh your truck, trailer, and boat. Drop the boat off at your ramp, then weigh your truck and trailer.. You can see how much your boat weighs. It may be heavy and waterlogged from the water that was in it before.

Have you done a compression check on the motor?

 

[iggyw1]

With One person on Board in a 16ft fiberglass boat and a 75 hp Evinrude that's properly set up with a properly pitched prop etc I'd fully expect to get 35 mph or more. I have a 15 footer with only a 40 hp and I get 25 mph with 2 300lb adults in her. Does your hull have foam in it. It could be waterlogged and adding a lot of additional weight. You should check to ensure your prop is the proper pitch and is not slipping on the hub. You should also hook up a Tachometer and determine the RPM @ WOT!!! There is definitely something wrong! It, for sure be making better speed.

As for your fuel usage..

Fuel useage cannot be tracked on a linear scale (although that may seem logical).

For example, a 50 hp Evinrude at idle, in gear (700 rpms) burns about .3 gallons per hours giving it the eqivalent of 8 mpg.

At 1500 rpms it burns .4 gph but because the boat speed has effectively doubled (from 2.4 to 4.9 mph on the test hull), now it's getting about 13.5 mpg.

However, when you hit 2500 rpms, the gph jumps to 1.7 and mpg drops to 4.2.

But then when it's running at 3500 rpms, the fuel usage just barely tic's up to 2.0 gph giving it 9.9 mpg (this motors "sweet spot").

From there on the fuel usage goes up and the mpg goes down until at near wide open throttle it's burn about 4 1/2 gallons per hour (not bad for a 50 horse) and getting the eqivalent of 7.2 mpg.

So as you can see the ratio of fuel useage to miles per gallon is all over the map - and each model of motor will have it's own "sweet spots".

All you can do with a given motor is -

make sure it's tuned properly (new spark plugs, fresh gas, properly adjusted carbs)

make sure it's mounted properly (correct height, trimmed properly)

make sure it's "propped" properly (running too much pitch will overwork the engine and fuel economy will suffer/too little pitch will not get you the speed for a given rpm and could tend to make the mpg suffer)

load the boat properly (too much weight in the front will cause it to plough water and waste gas etc)

and once you have done all that, you need to play around and see "where" your motor likes to run - normally somewhere in the 3500-4200 rpm range (this is why I said Hook up a Tachometer) will give you the best combination of speed and fuel economy.

I am NOT a motor guy. This is all just stuff I've gleaned here on the forum and research on the "Net". Hope it helps!!

Thanks for all of the info Woodonglass! It is very informative. No, fortunately, I do not have foam under the floor. It would probably be water logged if I did with all of the water I had beneath my floor. I am quite certain I got all of the water out and everything under the deck is dry. The sections between the stringers all "bleed" into one another, so I am sure there is no more water trapped underneath. I ran a heavy dose of seafoam in the motor a few days ago, trying to decarbon things. Now I'm going to put in new plugs. Old ones look ok to me, but they are only a few bucks each and I only need four of them. I have to test the fuel pump for a bad diaphragm. I got some great tips and advice on this forum on how to do that real easy. Actually, I am HOPING I do have a bad diaphragm so I can rebuild the fuel pump and be done with things.
Installing a tach is in my plans as well. Thanks again!

 

[greenbush future]

I think the outboard you are running is a known gas hog, I am not 100% certain on this but others would pipe in if you ask these questions in the outboard section of this website. I just remember others saying the same thing about your outboard and it's thirst for fuel. Glad you have her running well, Like Wood suggested there are only so many things that can cause the boat to not run well. Getting her on the right pin on your transom is key. The prop you are using is also very key. Maybe pay attention to the rpm's as you get her on plane next time, this will tell you a lot about the prop and if you have the correct (best) one for your intended use. Running a compression check on your outboard will also tell you if it's really tired and used, or if it still has a bunch of life and just needs some tuning. Have fun dialing her in, sounds like you have made great progress so far. Get some picture up here to share with us.

 

[greenbush future]

I think the outboard you are running is a known gas hog, I am not 100% certain on this but others would pipe in if you ask these questions in the outboard section of this website. I just remember others saying the same thing about your outboard and it's thirst for fuel. Glad you have her running well, Like Wood suggested there are only so many things that can cause the boat to not run well. Getting her on the right pin on your transom is key. The prop you are using is also very key. Maybe pay attention to the rpm's as you get her on plane next time, this will tell you a lot about the prop and if you have the correct (best) one for your intended use. Running a compression check on your outboard will also tell you if it's really tired and used, or if it still has a bunch of life and just needs some tuning. Have fun dialing her in, sounds like you have made great progress so far. Get some picture up here to share with us.