Hours per gallon as a function of throttle throttle setting

[jimrockireland]

I have a 1983 Mercruiser 305/5L MCM228 V8 engine (Serial#6285627) on a 19 foot Wellcraft Sunhatch 198 boat. I have not yet had this boat in the water since the engine has a cracked block. I am weighing the decision to rebuild the engine and one of the factors I'm weighing is how many hours/gallon of gas should I get if the throttle is half open and wide open. Does anyone have an approximate estimate for hours/gallon under these 2 conditions?

 

[alldodge]

Re: Hours per gallon as a function of throttle throttle setting

No such thing as hours per gallon, it's more like gallons per hour, or miles per gallon. A boat that size with a 5.0 would get somewhere around 3 to 4 miles to the gallon. If your pulling tubes or skiers this will be reduced. A 5.7 will usually get beter milage then a smaller engine due to it works less, that is so long as you stay out of the throttle. Everytime you go out just think of at least 20 gallons for the day, if your pulling and tubing add another 5 or 10 gallons. WhaI figure a C note and have a good day out

 

[Bob_VT]

Re: Hours per gallon as a function of throttle throttle setting

Fuel economy depends on finding the sweet spot for the specific boat/prop/trim set up. The 305/350 blocks are VERY available and worth the rebuild. If economy is the primary factor I would look for a boat with a newer 3.0L which has newer technology.

 

[alldodge]

Re: Hours per gallon as a function of throttle throttle setting

Have a look here
http://forums.iboats.com/boat-topic...on-motor-size-vs-fuel-consumption-397741.html

 

[QC]

Re: Hours per gallon as a function of throttle throttle setting

Exactly what is it that you are trying to understand? A typical 305 like you describe would probably burn 20 gallons per hour at wide open throttle. 50% is literally half that, but poorer efficiency.

And FWIW, the thought that a larger engine will deliver better fuel efficiency in the same boat has been proven false. Old rule of thumb that just isn't true. I only post this for accuracy. Generally speaking, if all else is equal, for the same given load, the smaller the engine displacement, the better the efficiency. That is theoretical, and you can mess it up if you go too small to plane for example.

 

[alldodge]

Re: Hours per gallon as a function of throttle throttle setting

wow, relax, do not agree but OK

 

[Bondo]

Re: Hours per gallon as a function of throttle throttle setting

Exactly what is it that you are trying to understand? A typical 305 like you describe would probably burn 20 gallons per hour at wide open throttle. 50% is literally half that, but poorer efficiency.

And FWIW, the thought that a larger engine will deliver better fuel efficiency in the same boat has been proven false. Old rule of thumb that just isn't true. I only post this for accuracy. Generally speaking, if all else is equal, for the same given load, the smaller the engine displacement, the better the efficiency. That is theoretical, and you can mess it up if you go too small to plane for example.

Ayuh,... Ya mean My favorite ole wives tale has been Debunked,..??

But What If,..... ya bump the compression, 'n utilize the quench factor,..??

Or What If,......

 

[oldjeep]

Re: Hours per gallon as a function of throttle throttle setting

Beyond all that, you have to take into account which way the wind is blowing

I will definitely agree with the smaller displacement getting the better fuel economy, as I have owned both a 5.0L 195 and a 3.0L 195 that were basically the same boat. The 5.0 was a lot nicer to ski behind since it could get up and out faster, but for cruising at 35 MPH, the 3.0L kicks the 5.0L rear for fuel consumption.

 

[QC]

Re: Hours per gallon as a function of throttle throttle setting

It's a physics thing . . . And remember, I said "all else is equal"

Out of respect for Don, I must report, he thought I was full of carp

 

[alldodge]

Re: Hours per gallon as a function of throttle throttle setting

Would use a nickles worth but a nickle comes out to 2 bits today (no accronyms). My Rinker 232 (7.4L) left Louisville side by side with my buddies Rinker 212 (5.7L) I had 3 folks buddy had 4. We ran from Louisville to Madison OH, did some other running and the end he burned about 3 gallons more then I. Maybe that was the extra person but I weigh more then he did.

But Iboats is a place of opinions just like other stuff

Cheers, and engine room where the heck is my drink

 

[aerobat]

Re: Hours per gallon as a function of throttle throttle setting

A 5.7 will usually get beter milage then a smaller engine due to it works less

sorry to say this, but thats a wrong myth. actually, an engine has the the best specific fuel consuption at WOT - so working as hard as it can do.

things are a little more complicated than this one statement and depend on rpm,s and load ( gear / propratio ) to reach a given cruise, but as a basic rule of thumb : its a wrong myth to say a bigger engine will burn less fuel at a given boat / speed configuration because it has to work "less hard"

cheers

 

[trendsetter240]

Re: Hours per gallon as a function of throttle throttle setting

sorry to say this, but thats a wrong myth. actually, an engine has the the best specific fuel consuption at WOT - so working as hard as it can do.

things are a little more complicated than this one statement and depend on rpm,s and load ( gear / propratio ) to reach a given cruise, but as a basic rule of thumb : its a wrong myth to say a bigger engine will burn less fuel at a given boat / speed configuration because it has to work "less hard"

cheers

Actually this is not a myth if you are talking about fuel economy and not fuel efficiency. While you are correct that a smaller displacement engine will be more efficient at WOT that doesn't translate into fuel economy (MPG).

I've seen dozens and dozens of examples where a larger displacement engine gets better fuel economy than the smaller one on the same boat. How could this be? Well, the larger displacement block has a different torque profile and can spin a larger prop at a faster rate. (prop pitch / gear ratio).

If you don't believe me take a quick look at the performance reports done by Grady white on their boats. In most cases the larger motors can provide better fuel economy at cruise than the smaller ones. The cruise speed is not always the same but the amount of fuel consumed to travel a given distance is what's important in this discussion (fuel economy).

Grady-White Boats : Gulfstream 232 - 23' Walkaround Cabin

 

[QC]

Re: Hours per gallon as a function of throttle throttle setting

Tell me how fuel economy and efficiency differ if you are using the same fuel? You need to trust aerobat, he quotes facts. And I happen to have a 35 year career in the engine business. I guarantee we know of what we post.

Same statement, if all else is equal, with an Otto cycle engine (4 cycle, air throttle and spark plugs) the smaller displacement engine will deliver better fuel efficiency at the same load. This translates into better economy if the fuel is the same (same cost per BTU by volume).

I actually sell economy vs. efficiency every day as I sell natural gas conversions. I have no idea how you get a discrepancy between economy and efficiency on the tests you cite. And FWIW, and I know you think this will be backing up. There is no way to claim the Grady tests were equal. If you compile all of the available data, auto and marine, the smaller displacement, the better the efficiency for the same load (hp). This is why there is active cylinder management for efficiency, Shut down cylinders artificially decreases displacement so the throttle opens wider. This cannot be escaped. It's physics. There are practical exceptions, but it is not due to a bigger engine working less hard. All engines are not created equal. I am sure a modern 5.0 liter class V8 is more efficient than an older 4 cylinder at the same load (hp), but that's not all else being equal

BTW, Alldodge, I really think my post was respectful, not sure what upset you

 

[Outsider]

Re: Hours per gallon as a function of throttle throttle setting

Sooo, basically you guys have no answer for the OP, but you're just all frazzled to think yours might not be biggest. For OP, post your boat numbers on *** and see what you get. BTW, half throttle probably has no linear relationship to WOT ...

 

[QC]

Re: Hours per gallon as a function of throttle throttle setting

I asked the OP an honest question from the beginning, and a couple of us answered his/her question. Annnnd we are plenty helpful and friendly here. If you want to reside on another forum then I am sorry, but don't direct our forum members there. Thanks.

 

[jimrockireland]

Re: Hours per gallon as a function of throttle throttle setting

The post was prompted by my reading about someone who had an 85 gallon gas tank in his boat. A quick calculation of $300 to fill the tank got me thinking about my fuel usage for a GM Mercruiser 305 I am rebuilding. I just wanted to get an idea of how much it would run me to use this boat. I pretty much got my answer from the first couple of responses. This being the case I think I am more interested in fuel economy rather than efficiency. To me fuel efficiency is a measure of how much boat kinetic energy (a function of boat speed and mass ) I can expect burning a gallon of fuel. Some of the energy available in a gallon of fuel is lost to heat; energy to turn pumps, move valves, etc.

 

[QC]

Re: Hours per gallon as a function of throttle throttle setting

Agreed ^^^^ my point was that economy and efficiency go hand in hand IF you are using the same fuel. Gasoline make boat go this far (efficiency), cost this much (economy) in caveman terms

Always a sticky topic and no matter how politely we all post, it stirs up passions. All good.

 

[aerobat]

Re: Hours per gallon as a function of throttle throttle setting

yes, this has been discussed in a real technical and knowledged way, but going this road on every thread would be overkill and also out of interest of the avereage boater who wants to have fun and a rough number on the costs for that.

soo, with your boat keep on cruise the rpm,s midrange, trim it correctly and keep the hull clean- this will bring your the best performance and fuel economy. in an average boating day with some trolling, some planing cruise and some fun expect about 7-11 gals per hour.

cheers

 

[hivoltg]

Re: Hours per gallon as a function of throttle throttle setting

I read many many posts on this subject before buying a boat. I have a 20ft Wellcraft with a 351 v8 (2 barrel carb) in it. I went into the purchase thinking about the fuel cost being way more than I am actually seeing. My boats only using about 3 gallons of fuel an hour on average. I even took my fuel sending unit out and checked the gas tank thinking it was broken or off. It wasnt. GPH varies on alot of factors, but my results seem way lower than most of the numbers I see posted by others. Im glad they are.