Approximate GPH on aft cabin cruiser (example boat enclosed)

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Sep 1, 2012
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Hi everyone, I hope it's not too rude to start a thread with my first post. I'm not new to boating, but my wife and I are new to the idea of cabin cruisers. We're trying to do some research into the possibility of purchasing a cabin cruiser in the next 5 years. Through our research, the only hold up we seem to have is some of the average GPH usage (anywhere from 8-14+ GPH) obviously dependant on the type and size of the boat and engine(s). I guess our purpose for it would be as our "cabin on the lake", some cruising up and down the Mississippi or big lakes, perhaps someday making a long journey to the gulf.

The following link, and the specs from the link below that are kind of what I'm curious about. Considering this is a lighter boat, and the type of engine and drive, what kind of GPH could one expect out of it (rough estimate is fine)?


http://www.yachtworld.com/core/list...rency=USD&access=Public&listing_id=78965&url=

Dimensions
LOA: 26 ft 8 in
Beam: 8 ft 6 in
Minimum Draft: 1 ft 4 in
Maximum Draft: 2 ft 10 in
Dry Weight: 5475 lbs

Engines
Engine Brand: Mercruiser
Engine(s) Total Power: 330 HP
Engine Model: 7.4 L (454)
Engine Type: Inboard
Drive Type: Stern drive
Cruising Speed: 18 knots
Year Built: 1989
Trim Tabs
 

QC

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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

If all you care about is GPH, it doesn't really matter about boat weight. Propeller load curves are pretty linear, and you will use about 1 GPH at idle to 30ish at full load speed (4400ish?). Straight line that up and you'll see with a cruiser (it is heavy) it will use between 15 and 20 at Cruise speeds. say 3800 - 4000 RPM.

What will matter to you is the speeds at those different GPH levels/RPM. That will then allow you to calc MPG and then your range depending on weather, conditions etc. Most cruisers of that size, single screw, get around 1.4 - 1.8 MPG at Cruise. FWIW, I used a 375 bhp 8.1 to get those estimated GPH numbers. Yes, more horsepower but also more efficient with MPI.

What I would do is scour BoatTest.com for actually test numbers with a variety of examples. Note the better efficiency of MPI (EFI) and then also note that new boats always get good test numbers as they don't have any stuff on 'em yet ;)

Good luck and welcome aboard!!! Nothing wrong with posting a new thread as your first post :)
 
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QC

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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

You might try this as an example: http://www.boattest.com/boats/boat_video.aspx?ID=555#Test-Result

However!!! Note the sellers "cruise" speed of 18 knots. My guess is the one you're looking at is significantly slower than this Bayliner. And!!! The 5.7 300 EFI MPI sips fuel compared to your carbed 454 330. Bravo 3 helps too. I would lay that 1 - 30 GPH scale over the Bayliner's speed numbers and take 10 MPH off the top. Maybe more.
 
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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

Thanks QC, I'll use that resource and get a better feel for it.
 

partskenn

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Jan 23, 2011
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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

I owned the same boat with a Merc 260. I averaged 12 - 13 Gallons per hour at 23 to 25 mph. Your engine is larger, but mine was close to underpowered, so I would think it to be fairly similar.
 
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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

So when you say "260" that's the HP or the engine size? Sorry I'm not the most engine saavy. Also trying to get a feel for the terminology when it comes to the engine specs.

Also, if the 260 was underpowered, would it theoretically use more GPH than a larger engine, or would it be kind of a wash since the larger engine would drink more even though it doesn't have to work as hard?
 

QC

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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

260 is a common name for 350's back before '90. In 1990 sterndrives began advertising and selling prop shaft horsepower. Both yours and the 260 "lost" 30 horsepower when rated at the prop vs. the flywheel. So today, the 260 would be a 230 and yours would be a 300. Not an exact science as the 260 would've had an Alpha, and yours a Bravo. But close enough.

And no, an underpowered boat, if on an efficient plane, will still use less fuel. A very common misconception, but data proves it over and over. Smaller displacement at the same horsepower (load of boat at a given speed) will almost always use less fuel. Buuuuttttt, we are talking small differences. Not worth even discussing.

Boat fuel use is much more about hull efficiency than engine efficiency. Once they break over on plane, they will use very similar amounts of fuel regardless of the engine (gasoline to gasoline, diesel to diesel). If you are stuck with not enough power to get there, then yes, the lower horsepower engine will use more fuel as it will be at full load, and the boat will not be at an efficient speed. Pretty simple if you look at fuel curves as I have suggested. You will see a rise just as they struggle to plane, say 10 - 15 MPH, then above 20 MPH they start to flatten out and fuel use rises with speed, until you are at wide open and then efficiency goes down again as the higher speed is less efficient for both the hull and the engine.
 
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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

Thanks QC, I just finished my first browsing of the boat tests site. The test result chart was nice and I was trying to see comparitave boats/engines and there wasn't much difference.

Your explaination was very helpful as well. Even though we're still several years off of this type of potential purchase, it's still fun to explore the nuances of boating.
 

emoney

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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

It's totally out in left field, battleshipgrey, but have you guys considered a sailboat? I know that's an entirely different ballgame, but for a "condo on water", nothing beats the efficiency of 28-30 sailboat, and I think they're roomier. Lots of folks don't consider sailboats as they're hung up on the "work" involved, but they're not that difficult to learn. My first sailboat I purchased, btw, was sailed down the Mississippi making it's way from Ky/TN as a starting point. The one huge benefit to a sailboat is that their GPH is ZERO when the wind has filled the sails. Just a thought.
 
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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

Actually I had thought about it, but haven't seriously entertained the thought. It's not really the "work" that concerns me, but the fact that I've never learned to use a sail boat is a little daunting. My dad has a small sail boat (about 10') that I've told him a couple times I'd like him to show me how to run. I've been on it many-a-time but at the time I guess I never asked to give it a go. I told him just a few weeks ago that we'll have to get it out. However, we've only been able to get together and ski on his speed boat about once a summer for the last few years, though now he's retired it should be a little easier. Perhaps if I learn on his sail boat I'll have a little more confidence when comes the time to look at buying. Thanks for the suggestion.
 

JoLin

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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

If all you care about is GPH, it doesn't really matter about boat weight. Propeller load curves are pretty linear, and you will use about 1 GPH at idle to 30ish at full load speed (4400ish?). Straight line that up and you'll see with a cruiser (it is heavy) it will use between 15 and 20 at Cruise speeds. say 3800 - 4000 RPM.

I'm going to differ with you just a bit here, QC. 4400 rpm's at WOT is a good estimate, but I'm betting no more than 25 gph, with 15-16 gph max at optimal cruise speed (32-3400 rpm and 22-26 mph). That particular Searay is both narrow (8.5') and light (5400 lbs) for its length.

As for the seller's 18 knot 'cruise' speed, that's too low. The boat is barely on plane at 18 knots (20.7 mph) and IMO, turning no more than 3000 rpm. Not a very efficient speed.

OP- you can look here for additional info on that boat:

http://www.searay.com/boat_graphics/electronic_brochure/company1729/1C1_27_74DERNL5E579.pdf

My .02
 

QC

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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

I think we're actually pretty close without any real data, Jolin. You have more experience with this size boat for sure. My point in that first post was that GPH is pretty easy to find from other engine test data, and is somewhat linear except at the point of climbing the bow wave. It doesn't matter if it is a 70 MPH 20 footer or a Cruiser, a 330 big block (300 propshaft) will use around that amount. I also agree closer to 25 than 30, but I was trying to be conservative.

I also agree that 18 MPH is too slow. 22 - 25 would probably be decent, but without any data I was going with what I had . . .
 

Thalasso

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Re: Approximate GPH on aft cabin cruiser (example boat enclosed)

30ft boat weighing more then 10000lbs. 2- 5.0's running 3200rpms burning approx 7.9 gph (each engine) for a speed of 28mph.Approx 1.7 mpg ( each engine). Speed is GPS and fuel burn is with EPR 60's. All above calculations is off the top of my head because i can't remember exactly but this is close.
 
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