Inboard boat fuel mileage vs engine size

[444]

I've been doing some reading on boattest.com and looking at reported fuel mileage figures. What I'm seeing is hull weight and shape seems to make more difference to fuel mileage where engine displacement doesn't seem to make as much of a difference at around a 25mph cruising speed, assuming everything is geared and propped correctly. Do you guys find this to hold true in the real world? Reason I ask is I'm thinking about selling my outboard 15'er for something inboard in a 17-19' and I am seeing a variety of choice in engines.

 

[smclear]

Re: Inboard boat fuel mileage vs engine size

I believe you are correct in that hull design (shape) and weight are huge factors with overall efficiency. Engine displacement may or may not be more of a factor, but it is a factor. When you say inboard, do you mean an inboard, or an inboard-outboard? I believe they each have their influences as well. I/O disturbs center of gravity but allows trim ability. Straight inboards give you better center of gravity but you loose the trim. All of those things affect efficiency. Personally, I don't much pay attention to fuel efficiency. Boating is my passion and my means of escaping the daily routine and monotony. I'm not going to let a little gas get in the way of that.

As you see in my signature, I have a Century Resorter. It's a straight inboard powered by a Chrysler 440. Fuel economy sucks big time. Very easy to go through 60 gallons in a weekend. That includes spending MOST of the time on the pontoon. The pontoon with a Honda 135 outboard, sips the fuel. The Sea-Doo with a supercharged/intercooled 3 cylinder doesn't do too bad but will still go through 20-30 gallons in a typical weekend. At one point in time, the family had a 15' I/O powered by a small 4 cylinder. Somewhere in the neighborhood of 130 HP. Darn efficient but way underpowered in my opinion. Prior to the the Century we had a 21' I/O with a chevy 350 in it. Probably the best combo in terms of efficiency (excluding the pontoon).

Hope I didn't add to your confusion.

 

[QC]

Re: Inboard boat fuel mileage vs engine size

You mention Inboard, but I assume you really mean I/O. With that understood, you are still correct, and the question is indicative of it too. True inboards, as smclear mentions, are the worst on fuel. Old OBs too. But modern OBs and I/Os with typical planing hulls, are pretty close regardless of engine displacement. Weight is the next factor, and speed.

You've already figured out the best way to understand this. Data, data, data . . . BoatTest.com is a great place to pick a lot up in a short time. Be careful I have found mistakes, but generally speaking, it is the best source around. Beware!! New test boats always run best in those tests. I'd subtract 10% in top speed and fuel efficiency for the same once it is home and loaded up . . .

 

[drrpm]

Re: Inboard boat fuel mileage vs engine size

Fuel is one of the lower costs of owning a boat. There may be some differences is fuel consumption in the same hull with different engines but it won't be as significant as the differences in performance. For water sports use, the biggest engine available in a given hull is the most desirable.

 

[QC]

Re: Inboard boat fuel mileage vs engine size

Oh, one more point . . . when I was shopping for my first boat 30+ years ago I was talking with our marine specialist (I'm in the engine biz). I was stressing about economy and choices and he said "do NOT make a decision based on fuel efficiency. You are talking about a swing of $5 a weekend.". I believe that is true still today and he is soooooooo right.

Let's say you use $50 a weekend on fuel. Some use less, some more, but bear with me here. And let's say you find one boat with 10% better fuel efficiency than another. There's the $5 . . . and let's say you boat 20 times a season. That's $100 a year. In the grand scheme of this addiction, PEANUTS!!!!

 

[Chris1956]

Re: Inboard boat fuel mileage vs engine size

I/Os with their 4 cycle motors are much better on fuel than carb 2 cycle outboards. Their fuel also costs less, since the 2 cycle oil is not added. However, you will spend much more maintaining that I/O. In addition the weight of the I/O powerplant(1000+#) adds to trailering/launching/recovery issues, vs a 400# OB.

 

[Bondo]

Re: Inboard boat fuel mileage vs engine size

Do you guys find this to hold true in the real world?

Ayuh,...Hull weight and shape, along with the Captain's frame of mind, 'n his right hand have Everything to do with fuel efficency....

If the Captain spends the day with his right hand, Fully Extended,...
He's gonna go through a boatload of Fuel, regardless the motor......

 

[rallyart]

Re: Inboard boat fuel mileage vs engine size

There are very few people who complain that anything is over powered. It's like complaining that your garage is too big. Just saying...

 

[Fireman431]

Re: Inboard boat fuel mileage vs engine size

I/Os with their 4 cycle motors are much better on fuel than carb 2 cycle outboards.

I'm sure there is data to back this up, but I had found the opposite in the boats I've owned. But then again, if you're comparing HP to weight, you're probably correct.

The other thing to consider is that the larger, higher HP engine is going to work less than a smaller displacement engine to maintain the same speed and performance. That being said, the bigger engine may burn more fuel between 0-30 MPH, but will burn less maintaining that same speed than a smaller, more fuel efficient engine would.

And QC said it right...as long as you're in the proper range of HP to weight, don't worry about the nickels involved in boating. If you want to save your money, don't buy a boat.

 

[444]

Re: Inboard boat fuel mileage vs engine size

Yes I meant an I/O. Boating is not my #1 hobby but I do enjoy it. My main hobby has always been muscle cars, working on restoring my roadrunner with the 440 in it is my main focus right now. In a car, I find the 440 mileage around town to be very reasonable for what it is. Maybe not so much in a boat...

Fuel economy isn't my #1 concern for picking a boat, but it is a definite factor. If boat A gets 50% of the mileage of boat B, that will have an impact on a weekend fuel bill. But no, I'm not worried about that last 10% mileage difference. My main reason for wanting to transition from 2 stroke out board to a 4 stroke I/O is I'm tired of dealing with all the marine specific outboard stuff. If a 4-stroke car based engine in a boat starts acting up and not running right, I can diagnose it and repair it myself fairly easily. When my 2-stroke outboard starts to act up, it's a lot harder to diagnose and the parts are a lot more money. I always do all my own work on everything so there is no such thing as shop labor to worry about.

Most of my boating is done at medium speeds, I do hit the throttle every now and then but most of the time keep it under control. I've been doing some reading on boats with the volvo penta 4cyls, and the GM 4.3 v6 and small block v8's. I'm not seeing a heck of a lot of difference in claimed fuel mileage between comparable hulls/weights at ~25mph cruising speed. Most of my boating is done around that speed. I'd like to be able to pull a couple tubes or skiers. I've been thinking for what I want to, I might just be better off picking a boat with a hull on the smaller/lighter size with something like a v6.

 

[smclear]

Re: Inboard boat fuel mileage vs engine size

In a car, I find the 440 mileage around town to be very reasonable for what it is. Maybe not so much in a boat...

I've never put a meter on my fuel line, but my closest estimate is ~10 - 12 gallons per hour @ 40 mph.

But, there's nothing quite like the sound of 440 cubic inches.

 

[45Auto]

Re: Inboard boat fuel mileage vs engine size

Congratulations on being one of the few people capable of looking at real data and making an intelligent decision regarding engine size!

Most people just assume (or listen to a salesman who knows even less than they do) that a smaller engine means better economy. As you've discovered, displacement is NOT the primary driver of fuel economy. There's a reason they don't put Honda Accord engines into Mack trucks .....

The article below from Boating magazine last summer covers the same thing.

 

[NYBo]

Re: Inboard boat fuel mileage vs engine size

Here we go again!