Big Boat vs Small Boat Fuel Economy........

[mmetzger1028]

Im in the initial stages of researching different boats in order to decided which will be best for the wife and I. My question is, how much of a difference is there in the fuel economy of say a 24ft flybridge model boat (single engine) vs say a 32ft dual engine flybridge model? Obviously the larger the boat and engines, the worse it gets, but is it "that" much worse? Say 1.5mpg to .5mpg.........

http://seattle.craigslist.org/est/boa/4648660892.html

vs

http://seattle.craigslist.org/see/bod/4641621192.html

Any input would be appreciated!

 

[ssobol]

All other things being equal a twin engine boat will get half the mileage of a single just for starters. In the case of a small single versus a larger twin the difference might be more the half the mileage.

The rule of thumb is max hp/10 equals gallons per hour per engine at WOT. If you have some idea of the WOT speed you can figure the full speed mpg.

 

[Robbabob]

Mileage depends on many factors. To actually answer your question, it is likely worse than you might imagine with the larger vessel.

​Choose which vessel based on what your needs or desires are. I assume you are not new to boating and have experience in the waters you'll be boating.

Good luck!

 

[25thmustang]

My 34'er gets around .75 mpg at around 2900-3100 RPM cruise. A heavier boat will probably get worse and a lighter boat will get better.

 

[Scott Danforth]

here is some reference. a 260 SeaRay sundancer weighing in at 8400 with a single 300hp 350 Mag with a bravo drive will burn 22 gph at WOT (that is about 2.27 miles per gallon). a 31' Fountain weighing in at 10,500 with twin Merc 250's on back will burn 22gph at WOT (that is about 3.0 miles per gallon). A 17' Avanti weighing in at 2000# with a single 3.0 liter OMC burned about 22gph at WOT (that is about 1.82 miles per gallon)

fuel consumption has more to do with hull design, wetted surface, wind resistance and operators right hand than it does with the size of the motors (once you have enough motor to get the boat up to plane at a reasonable clip)

 

[smokeonthewater]

yes, yes it really is THAT bad, or worse

 

[redneck joe]

yes, yes it really is THAT bad, or worse

yep.

 

[dingbat]

No idea why people have to chime in with irrevlent, WOT burn data, when a boat is run at cruise rpm 95% of the time. .....

 

[dingbat]

No idea why people chime in with irrevlent, WOT burn data, when the boat is run at idle or cruise rpm 95% of the time. .....

 

[mmetzger1028]

Thanks so much for the input everyone. I live in the Seattle area and plan on keeping the boat in Everett. Mainly I just like being on the water, zero interest in going "fast" etc. Initially I will totally be content with just cruising out into the sound and floating around with the wife/friends/family. However Im sure after a while I start wanting to take trips to Friday Harbor in the San Juans. I don't make much $$ But Im very handy, but no amount of "handiness" will help filling a tank for $450. So I just don't want to feel like I can't go out enough only due to the high fuel cost. Or else I might even go for something like this

http://bellingham.craigslist.org/boa/4641875846.html

 

[airdvr1227]

It gets really interesting if you start to look at HP v. weight.

 

[H20Rat]

here is some reference. a 260 SeaRay sundancer weighing in at 8400 with a single 300hp 350 Mag with a bravo drive will burn 22 gph at WOT (that is about 2.27 miles per gallon). a 31' Fountain weighing in at 10,500 with twin Merc 250's on back will burn 22gph at WOT (that is about 3.0 miles per gallon). A 17' Avanti weighing in at 2000# with a single 3.0 liter OMC burned about 22gph at WOT (that is about 1.82 miles per gallon)

fuel consumption has more to do with hull design, wetted surface, wind resistance and operators right hand than it does with the size of the motors (once you have enough motor to get the boat up to plane at a reasonable clip)

Some errors in your numbers...


Looking at boattest, your first boat comes in at 23.5 mpg and 2.6 mpg, so close there...

Merc optimax 250's are rated at 23 gph, but you left out the fact that is EACH. That boat burns roughly 50 gallons per hour give or take at WOT. Looking at boattest, there is a 31 fountain with twin 300 merc's, the absolute best on-plane mpg's that it achieved were 1.7 mpg. The 250's are going to be very similar... No where close to 3 mpg. In fact, the WOT mpg was 1.0.

The OMC 3.0 is rated at 10.5 gph. There is no way no how it is going through 22 gph, unless it is so horribly tuned that raw fuel is dumping out. If it were running so rich it was burning 22 gph, it wouldn't actually even stay running. Although it isn't identical, found a 3.0l carbed 18' bayliner on boattest. WOT was the same as the OMC, 10.5 gph, and best cruise was 4.8 mpg.

So in summary, the 3 boats are more or less multiples of each other. The fountain is the worse by far. The searay is a little less than double the fountain's MPG's, and the 3.0 is a less less than double the searay's mpg's.


The hull makes ZERO difference in gallons per hour at WOT. Doesn't matter if it it is a tug boat at 10 knots or a high performance racing hull, if you are running the engine at WOT and it is at the same rpm in each case, you will burn exactly the same amount of fuel. And to go further, it really comes down to horsepower. Horsepower/10 = gallons per hour. No matter the configuration or hull, the formula doesn't change. (yeah most modern engines are better than /10, but its a nice round number)

(all numbers on boattest if you want to check...)

 

[smokeonthewater]

OP here are some real world examples for you....ALL of these are I/O's

I have a 28' Welcraft cruiser w twin merc v8 260's... It is @6 miles down river to harrods creek and maybe a mile at no wake speed up the creek to my favorite hamburger joint.... It costs me right at $50 in fuel to go to the hamburger joint and back.

I trailer my boat so that is landlubber fuel prices.

My boat cruises at 26-30 mph and tops out at 48 mph.

My old 26' carver (2667 flybridge) with a single inline 6 merc 165 would make the same trip on $30 or less... it cruised at IIRC 16-18 mph and topped out at 26 mph.

My friends 17' bowrider w a 2.3 omc makes the same trip twice without moving the gas gauge .... Sometimes I think it actually MAKES fuel .......UGH

 

[Cap'nHandy]

Yup ^^^ I hear ya. Big boat = big $$$ on operations cost. WOT is not how most people operate. But you can kinda ballpark engineer it.

Wayback in the shadows of forgottenville, is something bothering me about the hp / torque / rpm relationships, and the linearity of said curve. Or lack thereof in certain parts of the curve..

For a reasonable best guesstimate, Cruise rpm / WOT rpm gives you a factor to use against the WOT fuel consumption. Short of a flow meter or empirical testing, its a good place to start. So if 200 HP at 5500 RPM = ~20gph. 3500 cruise rpm / 5500 Max = .64 X 20 gph = ~12.7 gph at cruise. Once you get up on plane, your efficiency goes up. So work your speed out for that rpm that puts you on plane and figure your best mpg. The curve is not perfectly linear, and other factors as drag are at play. Your fuel consumption in gph is a function of power produced.

Your speed is just a variable that depends on your hulls design, displacement, etc. Small light boat will get better mpg, as it goes faster at the same gph.
By design, 2 similar designs, of differing sizes, the big boat needs more power than the little boat, or, goes slower with the same power and fuel consumption.

Parasitic drag quadruples for every doubling of speed. So a speed just above getting on plane, will typically get the best fuel economy. If that also corresponds to the best power / fuel consumption point on the power curve, your in a sweet spot where the boat will give you the best range and fuel economy. I would imagine that one can prop for that spot - as well as best top speed or best hole shot or pulling.

Cheers, and hope for lower gas prices.

Blaine

 

[Scott Danforth]

smokingcrater. Im fully aware of boat test. I am also fully aware how much fuel my old boat used, and how much the searay uses (former boss). I can only reitterate how much my neighbors fountain uses (per conversation with him) and yes, when running long distances, I am usually flat out WOT for at least 30 minutes of my 60 to 100 mile excursions. both of my last two boats took about the same 25-40 gallons (depending on the trip)

the point is boats are horibly inefficient and as everyone pointed out, not everyone runs at WOT all the time. if your just tooling along, you are not using 300hp or what ever is the WOT rating so you cant boil it down to a formula. hull shape, current, wind, trim angle, etc. all play into it. the amount of hp required to push the boat at what your current speed is the factor at that moment in time, along with BMEP and engine efficiency.