Considering powering DOWN!!

[danny k]

I have a '94 150 Johnson, with about 100 hours since powerhead and carb rebuild. This is on a 28'pontoon boat. I run at about 3000rpm and even at that speed it burns fuel like crazy. 10 gals on a 45 minute run. Would I be better off powering down, maybe to a 90hp? I realize I would have to turn more rpm's to obtain the same speed but does anyone have an idea how much it would save on fuel consumption, if any, and would it be worth the hassle? I'm sure this rig was equiped with a smaller, less then a 150, when purchased as a package new in 1990.<br /><br />Thanks,<br />Danny K.

 

[red boat]

Re: Considering powering DOWN!!

1O gals in 45 minutes at 3000rpm I think you have something terribly wrong. Are you leaving a slick behind you? I would check you carbs because you are burning way too much fuel.

 

[Xcusme]

Re: Considering powering DOWN!!

Hmmm, let's see...The general rule is: 10% of motors HP in gallons of fuel per hour at WOT.<br /><br />At WOT you could expect 15 gallons of fuel used per hour. That's 3.75 gallons every 15 minutes. Since you say you ran for 45 minutes, you should have used 11.25 gallons (3x3.75). Since you not running at WOT, you fuel usage sounds about right....again, this is a rough guess.

 

[mfinucan@sympatico.ca]

Re: Considering powering DOWN!!

I have a 2000 150hp Johnson Ocean Pro on a Four Winns 180, the boat weighs in at just over 2000lbs plus engine weight. I can run at 5100rpm, 49mph and burn about 13 gals in an hour, thats at WOT. You are burning a lot of fuel. Is the boat fully loaded with people and equipment? As for dropping to a smaller engine you have to remember you will be putting the smaller engine under more strain trying to maintain the same speed. The V6 150 isn't working that hard to keep that big pontoon boat moving, a V4 90 would?!?

 

[angus63]

Re: Considering powering DOWN!!

X<br />Check out Danny's other post from yesterday. His WOT is 5500 and is burning 12.5 gph at 3000 rpm. Sounds a little thirsty to me. At 3600rpm on my 175HP, I burn about 8 gph or so. I think his hull type and size may be most of the reason.<br />Good luck Danny

 

[mfinucan@sympatico.ca]

Re: Considering powering DOWN!!

Just read yesterday's post. I think your problem is weight and drag! <br /><br />I would not even consider dropping engine size! You would kill a V4 90 with that much wieght.

 

[danny k]

Re: Considering powering DOWN!!

Thanks guys,<br /><br />I think I'll just keep the 150. It had a complete rebuild about 100hours ago so at least I know what I have. Yep,lots of drag and weigth. Next time out I'll throttle down to about 2500 and see how much a diffence it makes in GPH. After all, this is a 28'pontoon and if you're in a hurry to get somewhere this is entirely the wrong rig. Instead of it taking 45 minutes to make the normal run it will just take a little longer. Like I said, with this rig I damn sure ain't in no hurry!!<br /><br />Thanks guys,<br />Danny K.

 

[Dhadley]

Re: Considering powering DOWN!!

Why did it need to be rebuilt and was the cause fixed?

 

[danny k]

Re: Considering powering DOWN!!

It went in for routine maintenance. Low compression was found on 2 cylinders. 70 on #1 and 80 on #6. All others were in the 110-120 range. All cylinder walls looked fine. Decided to rebuild powerhead and just to be safe had carbs rebuilt also. Naturally had impeller replaced also.Engine runs great, no other problems other then GPH, which may not be an engine problem at all. Just a combination of weight, drag, etc. I took this engine off a 21' Cajun bass rig that I previously owned on a swap this for that and that for this deal. So, when I discovered how much fuel the 150 was burning on the pontoon compared to the Cajun is what raised my concern. I've owned the 150 for 6 years so I know its history and decided to spring for the $2800 for the rebuilds. At my age, 56, I figured it was time to<br />slow my butt down and spend more time cruising rather then simply fishing. The pontoon has been great for the family "quality" time thing, if you know what I mean. It's just getting hard getting used to cruising at 20mph compared to 45. So listen up guys!! When you reach my age think long and hard about making a move like mine. Hell on the gas card but great for the "family" thing.<br /><br />Hey BABY!! Grab me a Bud Lite out the fridge before you polish my shoes!!<br /><br />Thanks again guys,<br />Danny K.

 

[andys15]

Re: Considering powering DOWN!!

It takes 25% more power to go 10% faster.<br /><br />

Originally posted by danny k:<br /> Thanks guys,<br /><br />I think I'll just keep the 150. It had a complete rebuild about 100hours ago so at least I know what I have. Yep,lots of drag and weigth. Next time out I'll throttle down to about 2500 and see how much a diffence it makes in GPH. After all, this is a 28'pontoon and if you're in a hurry to get somewhere this is entirely the wrong rig. Instead of it taking 45 minutes to make the normal run it will just take a little longer. Like I said, with this rig I damn sure ain't in no hurry!!<br /><br />Thanks guys,<br />Danny K.

 

[Cricket Too]

Re: Considering powering DOWN!!

Hey Jellyvision, you might want to take your prop pitch down a few notches or make some other adjustment if you are running 5100 WOT, your lugging that thing pretty bad.

 

[mfinucan@sympatico.ca]

Re: Considering powering DOWN!!

It's fine for now. If I keep the boat I plan on dropping to a stainless 17P. That should get the WOT to about 5400-5500 I should think. OMC says 4800-5500, so that's about the middle, plus I haven't ever kept a boat beyond four years before moving on. The wife hates that part, keeping up with Jones'?! LOL

 

[dajohnson53]

Re: Considering powering DOWN!!

Am I mistaken in thinking that if a motor is propped correctly, 3000 rpms will burn the same amount of fuel whether it's pushing a heavy boat (slowly) at that rpm or a light boat (faster) at the same rpm? I always thought - again, assuming equal motors and propped correctly for the boat it's on - the difference between a heavy boat and a light boat at the same RPM would be speed, not fuel consumption.<br /><br />.

 

[angus63]

Re: Considering powering DOWN!!

Hull type has a huge affect. If two boats have the same motor and wot rpm, but different hulls and loads, it will take different throttle positions to maintain 3000 rpms. The percent of throttle used, I believe, is the greatest factor governing fuel use. I'm sure there are other factors that contribute as well, but I'm hungry and tired of typing

 

[mfinucan@sympatico.ca]

Re: Considering powering DOWN!!

D. Sounds about right to me in a lot of ways. 3000 rpm is 3000 rpm no matter how much weight you have in the boat. We all may have gotten thrown off by the massive amount of weight?! Good point.

 

[dajohnson53]

Re: Considering powering DOWN!!

Originally posted by angus63:<br /> Hull type has a huge affect. If two boats have the same motor and wot rpm, but different hulls and loads, it will take different throttle positions to maintain 3000 rpms. The percent of throttle used, I believe, is the greatest factor governing fuel use. I'm sure there are other factors that contribute as well, but I'm hungry and tired of typing

What I'm writing is strictly based on thinking about it, not personal experience. But, I still don't get it. Would it follow from what you've said that the two boats would get different WOT GPH if both are correctly propped? - My assumption is they have the same engine and each has the correct prop to achieve the specified RPM for that engine. Heavy boat will have differnt prop than light boat but both will achieve same, specified RPM WOT. I would think both would burn same GPH at WOT and since 3000 (or 4000, whatever) RPM will be the same percentage of WOT for both they would burn approximately the same at any comparable RPM (I would compare RPMs that are both on plane, for instance). I still think that if my assumptions are in effect, they will both burn the same amount of fuel at any given RPM (as well as WOT) the difference would be speed at that RPM and therefore miles per gallon, not gallons per hour (and of course metric equivalents .

 

[jerrypark1]

Re: Considering powering DOWN!!

Wouldn't a good analogy be: If I keep my truck (Manual transmission) at a steady RPM while driving up Pike's Peak, I would burn X gallons of gas. However, if I maintain the same RPM, but am towing my boat up Pike's Peak, I would expect to burn greater than X gallons.

 

[angus63]

Re: Considering powering DOWN!!

Correct!!!! The amount of fuel is governed by the amount of air your motor is gulping. The amount of air you gulp is governed by the throttle plate position. Another analogy is cruise control with a manual transmission. How far down is the pedal when you go down hill, then how far is the pedal down when you go uphill? All the while your speed and RPM are the same. I'd rather pay for the downhill gas!!

 

[angus63]

Re: Considering powering DOWN!!

D<br />The factor you are omitting is the throttle position. The same motor hp on different boats will require a different percentage of full throttle to maintain the same rpm.<br />Try this.... Set your throttle at 2000 rpm and then throw a couple of buckets tied to your stern cleats over the rail. Without moving the throttle, check your rpms now. They will be somewhat lower. You'll need to advance the throttle to get back to 2000 rpm. It's not what the motors turning that governs fuel usage, it's the amount of throttle required. Hope this helps clear it up.

 

[itstippy]

Re: Considering powering DOWN!!

I'm not following your reasoning, guys. I agree, "The same motor HP on different boats will require a different percentage of full throttle to maintain the same RPM". <br />UNLESS YOU REPROP.<br />The whole idea of reproping is like shifting to a lower gear in that pickup truck going up Pike's Peak. <br />On the flats my pickup goes 60MPH at 3000RPM at half-throttle in fourth gear. I burn 5 gallons of gas per hour.<br />Going up Pike's Peak I downshift to second gear. Now I'm only going 30MPH at 3000RPM at half-throttle in second gear. I still burn 5 gallons of gas per hour. I just don't go as far in that hour.<br />D has it right.