1998 27 Full Cabin Inboard Fuel Consumption

Optimizing the performance of Boston Whaler boats
sandbox9@hotmail.com
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1998 27 Full Cabin Inboard Fuel Consumption

Postby sandbox9@hotmail.com » Wed Jul 11, 2018 9:43 pm

What is the fuel consumption of a 1988 27 Full Cabin powered with twin Mercruiser 5.0 litre inboard engines.

jimh
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Re: 1998 27 Full Cabin Inboard Fuel Consumption

Postby jimh » Thu Jul 12, 2018 7:53 am

I suggest the gasoline engines will have a BSFC of about 0.55-lbs/HP-hour.

Assuming each engine produces 250-HP, then the pair of engine at a total of 500-HP will in one hour consume

500-HP x 0.55-lbs/HP-hour = 275-lbs of gasoline/hour

Assuming gasoline gas a density of 6.25-lbs per gallon, 275-lbs/hour will be

275-lbs/hour / 6.25-lbs/gallon = 44-gallons-per-hour

That’s about the best that can be hoped, and the actual fuel consumption could be higher.

Mr 88
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Re: 1998 27 Full Cabin Inboard Fuel Consumption

Postby Mr 88 » Fri Aug 10, 2018 9:41 pm

At cruise [a 1998 Boston Whaler 27 FULL CABIN will have a fuel economy of] about 1.1 to 1.3-MPG.

Variables are sea conditions, correct propeller, on-board loads such as one person versus five persons, fishing gear, full or half tank of fuel, amount of fresh water in tank, a tower, a hard top, the hull being smooth and free of any buildup of moss or barnacles, and so on.

Generally most twin inboards in the 27 to 29 [foot] range and sport fish configuration get around the same milage at cruise.

ASIDE on a different boat made by a different manufacturer:
On my 28-feet Rampage with less deadrise, and thus less drag and better fuel mileage, the hull has an 11-feet' beam and weighs about 700-lbs more; thus worse fuel milage. My boat has a tower with wind drag. [The] 350 engines burn about 28-GPH at cruise 24-MPH. I figure it should all even out and be about the same as the 27 FC.

I just picked up a 1987 27 FC with 305-engine, a factory tower, and a factory add on 8-inch keel to stabilize the tower on a drift. I am figuring the milage will be very close--within a tenth of a mile. Won't know for sure until next spring. I cannot wait to compare the two rides. I have high hopes for the Whaler.

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Re: 1998 27 Full Cabin Inboard Fuel Consumption

Postby jimh » Sat Aug 11, 2018 11:33 am

Mr 88 wrote:...Variables are sea conditions, correct propeller, on-board loads such as one person versus five persons, fishing gear, full or half tank of fuel, amount of fresh water in tank, a tower, a hard top, the hull being smooth and free of any buildup of moss or barnacles, and so on.


WIth all those variables, you only allow for a change in fuel economy of plus or minus 0.1-MPG. It seems like all those variable contribute hardly any effect on your prediction of fuel economy to be in the range of 1.1 to 1.3-MPG. A change of 0.1-MPG from the average 1.2-MPG you suggest, is only a variation of 0.1/1.2 - 0.08 or eight-percent.

Mr 88
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Re: 1998 27 Full Cabin Inboard Fuel Consumption

Postby Mr 88 » Sat Aug 11, 2018 10:03 pm

Since I don't know what [boat speed] the OP is cruising at or his actual load, all I can do is base my information on the fuel flow meter in my boat and the hundreds of articles I have read on fuel consumption of boats with approximately the same length, beam, weight, deadrise, and horsepower configuration.

How much of spread in MPG do you think [you are] going to get with little to no info from the OP's question?

Is he asking about consumption at idle?

At trolling speed, at pre planning speed, cruising speed, or WOT?

Does he run stock bronze or computer tuned cupped Nibral propeller?

I stand by my figures regardless of you trying to go scientific and mathematic versus my real life data on a 26 to 30 Sportcruiser with twin small blocks and what they use at a average cruise speed of 22 to 25-MPH.

Heck I even doubled checked my fuel flow meter today under calm.conditions with a somewhat average load of four people, a half tank of fuel (120-gallons). and few hundred pounds of fishing gear.

What does your 27 inboard consume per mile at cruise?

By the way [the engines under discussion are] rated at 230-HP per engine compared to the 350 at 260-HP in that vintage block, cams , stock intake manifold, and carburetor model.

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Re: 1998 27 Full Cabin Inboard Fuel Consumption

Postby jimh » Tue Aug 14, 2018 7:58 am

Mr 88 wrote:...How much of spread in MPG do you think [you are] going to get with little to no info from the OP's question?


The data provided by the original poster was only that there were two engines and the engine displacement was 5.0-liters. In my reply I made only one assumption: the engine horsepower would 250-HP per engine, which I clearly and immediately declared in my reply. I made no assumptions about boat speed. I made no estimate of miles-per-gallon. My answer was only a rate of fuel flow, and it was based on my one assumption, that the engines made 250-HP. From that assumption, I used a reasonable figure for brake specific fuel consumption (BSFC) for a modern internal combustion gasoline engine of four-stroke-power-cycle design (0.55-lbs/HP-hour). This figure is based on my general awareness of the efficiency of a modern four-stroke-power-cycle gasoline engine to convert its fuel to output power.

Your question to me asks what range of speed would I think the boat would be running at as the basis for an estimate of fuel consumption. My answer: I made no assumptions at all about boat speed in my reply. Apparently you made a number of assumptions about boat speeds and fuel flow rates in order to come up your with reply that the boat would obtain an fuel economy of 1.2 ± 0.1-MPG.

My remark about your estimate was only to express my surprise that with so little information, the speed range unspecified, and the load, running conditions, and the many other variables you mentioned that affect boat fuel economy unspecified, you were able to arrive at an estimate of MPG with a range of variation of only 8-percent.

There was no burden placed on you by me for precision of the answer you would provide. You seemed to take on the burden of deducing the boat speeds, hull characteristics, propeller efficiencies, etc., on your own. That you could deduce or infer or estimate all of those variables and arrive at estimate of fuel consumption with a variation of only 8-percent was, again, a surprise to me.

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Re: 1998 27 Full Cabin Inboard Fuel Consumption

Postby jimh » Tue Aug 14, 2018 8:14 am

Mr 88 wrote:What does your 27 inboard consume per mile at cruise?


Your question to me is an old debate technique of "begging the question". Your argument presumes two conditions that are not true, then uses them to support your position, that is, unless I owned a 27 FULL CABIN and personally recorded its fuel consumption, I cannot answer the initial question.

It was your response to the initial question that introduced the notion of fuel-consumption per-mile. The initial question only asked for the "fuel consumption" and did not specify how the fuel consumption was to be specified, that is, whether in terms of per-hour or per-mile. My reply specified an estimate in fuel consumption per-hour, and your reply specified fuel consumption per-mile.

Your question makes two bad inferences:

--that I own a 1998 27 FULL CABIN boat with twin inboard engines of 5-liter displacement. I do not; and,

--that my reply contained an answer in terms of fuel consumed per-mile; it did not.

Your question is inappropriate for me to respond, as it incorrectly suggests or implies that the estimate I provided was based on first-hand operation of the boat and having recorded its fuel consumption on a per-mile basis. The estimate of fuel consumption I provided was not based on a gallon-per-mile basis, and was not based in any way on the actual boat performance. My estimate was based entirely on one assumption: the engine horsepower would be 250-HP each for a total of 500-HP. This was the only reasonable assumption I thought could be made, owing to the vague question that was asked.

In any case, if the engine horsepower were different from my assumption of 500-HP total horsepower, my method of estimation was clear and could easily recalculate the estimated fuel flow rate in gallons-per-hour.

For example, if the engines under discussion have only 230-HP each, my estimate can be easily recalculated:

460-HP x 0.55-lbs/HP-hour = 253-lbs of gasoline/hour

Assuming gasoline gas a density of 6.25-lbs per gallon, 253-lbs/hour will be

253-lbs/hour / 6.25-lbs/gallon = 40.5-gallons-per-hour

Mr 88
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Re: 1998 27 Full Cabin Inboard Fuel Consumption

Postby Mr 88 » Tue Aug 14, 2018 7:34 pm

Sorry but I am getting 28 gph at 24-25 mph which I stated in a previous post on my 28 x11 sportfisherman that weighs a few hundred pounds more than the whaler and has twin 350's putting out 290 hp per. Nibral props , usually with 4-5 adults ,160 gallons of fuel a tuna tower and hundreds of pounds of fishing gear. These are real life figures and I stand by my mpg figures. Buddy has a 29 x11.5 Tiara with 454's and runs close to 31 gph ,his 31 x12 Tiara with 8.2s and NEMA 2000 gear hooked up to his ECM averages 36 gph at 26-28mph according to his data. Many people that ask there fuel burn numbers prefer MPG over GPH to give them a better idea knowing how many miles they might normally cruise in a day or weekend. Again most boats in this class, 27-30' with comparable beam,weight and deadrise will get fairly close burn numbers at there respective optimum cruise speed. 1 to 1.3 , unless diesel powered . Sure you can come up with 40 or 50 gph at WOT which no one in there right mind does. Haven't heard a thing from OP as well, time to move on.

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Re: 1998 27 Full Cabin Inboard Fuel Consumption

Postby jimh » Wed Aug 15, 2018 5:10 pm

Thanks for all the data about different boats that are not the boat under discussion.

And, yes, the OP seems to have abandoned this thread. Shame on him.