Fuel Economy Differences- Different Motors , Same Boat

[shawnmd]

I am looking at 25-30 ft cruisers similar to ther below. I was leaning towards a 5.0L engine as opposed to the 5.7 or 7.2L due to fuel mileage. The boat will be used as a cruiser, so getting up on plane and blasting across a lake are of no concern. We like to cruise around the lakes slowly. We are coming from a fishing boat type trawler (with an old 250 chev 6 cyl, mizer on gas), that might have done 6 knotts on a good day.

Our main concern is with fuel mileage. We would like to find the best motor for getting the best mileage, (twin V6's, 5.0, 5.7, 7.2). In your opinion, based on the info above and the size and type of boat we want, what would be the best engine?



http://www.boats.com/boat-details/Regal-260-Valanti/122926641

 

[Philster]

Re: Fuel Economy Differences- Different Motors , Same Boat

25' = one engine

30' = two engines

Those five feet are are literally a massive difference.

From research I've seen over the years, 80% of all vehicle buyers (car, trucks, boats) regret not getting the more powerful engine option.

Also, a hard-working smaller engine, which initially shows better mileage lightly loaded, will lose out to a bigger engine that relies on a better torque curve and won't over rev to compensate for heavier loads.

At 25', the bigger V8 is right in my book. At 30'.... the dynamics change, because that is twin-engine territory, and going bigger at 30' with twins is something that gets expensive to the point of tears and broken dreams if you don't have 30,000 dollars of play/reserve money for the boat AFTER you buy it. Getting the TWO motors to be the biggest offerings has diminishing returns. It might make more sense with twin setups to get two 5.7 engines instead of two 7.4 engines (these are just examples). At this point, there are numerous dynamics at play, and it might make more sense to get smaller engines to save weight and have the best drives/props combo on the market.

This is gonna be choc full o' opinions, so keep it light fellow posters.

(If my boat was 3' shorter and ran a 454 (7.4) instead of a 502 (8.2), I don't think that in the grand scheme of all costs I would notice a real change in my bank account, but I have more boat now at 27' and a 502 and more engine and no regrets.) I know you are looking at a cruiser, but this info is still relevant -- maybe even more so, because mis-powering a cruiser can really hurt the performance, and it's all relative.

 

[steelespike]

Re: Fuel Economy Differences- Different Motors , Same Boat

A marine engine will use about 10% of its horsepower in fuel at wot.
your 250 if rated at about 165hp would use about 16.5 gallons at wot.
I think you can see that the various engines you mention will use much more at wot.
The trick is to find a combination that makes the most speed for the least throttle.
If your figuring to cruise at say 12 or 15 mph this could be the worse speed for mpg.
because the boat hasn't planed, but your throttle setting is likely pretty high.
Usually the best practical cruising speed is just on plane up to about 500 rpm more.
Your trawler might have done pretty well at just slightly off wot. As displacement speeds
can produce nice economy,but when pushed beyond its best speed could really use a lot more
fuel for little return.I guess the trick is to find a boat that gets on plane easily without having to use a lot of tabs to get there While tabs can help getting on plane and stay on plane they will use slightly more fuel because of the drag.You probably would need a flow meter to tell the difference.If you did have a flow meter at least you could tell the best throttle setting
for that particular boat.least fuel for the most speed or the least fuel for a speed you prefer.
Avoid twins as they present twice maintainence,and will use more fuel for less speed than a single of same hp.Twins are nice for off shore incase one fails.

 

[sw33ttooth]

Re: Fuel Economy Differences- Different Motors , Same Boat

twins is something that gets expensive to the point of tears and broken dreams if you don't have 30,000 dollars of play/reserve money for the boat AFTER you buy it.

even going with smaller engines say twin 4.3l or something simlilar. your playing with fire imagine that your single engime may have a problem here nor there now you have 2 engines to deal with, and what comes from that? two times the cost.
i have a 23 fot cuddy weighs in at 6250 on the trailer so about 5,000 lbs with a 5.8l and trim tabs with 1-2 people it moves just fine, if i load the boat up with 4-5 people i need to use the tabs to plan out.

Avoid twins as they present twice maintainence,and will use more fuel for less speed than a single of same hp.Twins are nice for off shore incase one fails.

back in the late 70's tiarra put a twin set-up into a hull and at wot your burnd so much fuel that you litterally could only go about an hour on a full tank.

 

[shawnmd]

Re: Fuel Economy Differences- Different Motors , Same Boat

A marine engine will use about 10% of its horsepower in fuel at wot.
your 250 if rated at about 165hp would use about 16.5 gallons at wot.
I think you can see that the various engines you mention will use much more at wot.
The trick is to find a combination that makes the most speed for the least throttle.
If your figuring to cruise at say 12 or 15 mph this could be the worse speed for mpg.
because the boat hasn't planed, but your throttle setting is likely pretty high.
Usually the best practical cruising speed is just on plane up to about 500 rpm more.
Your trawler might have done pretty well at just slightly off wot. As displacement speeds
can produce nice economy,but when pushed beyond its best speed could really use a lot more
fuel for little return.I guess the trick is to find a boat that gets on plane easily without having to use a lot of tabs to get there While tabs can help getting on plane and stay on plane they will use slightly more fuel because of the drag.You probably would need a flow meter to tell the difference.If you did have a flow meter at least you could tell the best throttle setting
for that particular boat.least fuel for the most speed or the least fuel for a speed you prefer.
Avoid twins as they present twice maintainence,and will use more fuel for less speed than a single of same hp.Twins are nice for off shore incase one fails.

\

The 250 in the trawler would do 6 knotts at about 1500 rpm, very good on gas.. However, that was the best speed you could get. We could bring the throttle up as high as we wanted, it wouldn't go any faster.

 

[Tahorover]

Re: Fuel Economy Differences- Different Motors , Same Boat

If fuel economy is a large concern then you do NOT want the big block (7.4 or larger) engine. They are great power plants but they are NOT economic in any sense and use a LOT of fuel just idling. The flip side is that its a heavy boat so I would not want anything smaller than a 350 (5.7) unless you go to dual engines. I think even the 305 (5.0) would be underpowered and it would have to work that much harder (more throttle) that there wouldn't be any fuel savings compared to the 350 (5.7).

In most cases fuel effeciency really comes down to to the hand on the throttle.

A 30ft cruiser will be 10-11k pounds. IMO 260hp moving 5500lbs is way under powered!

 

[rallyart]

Re: Fuel Economy Differences- Different Motors , Same Boat

To move any boat through the water at any particular speed requires X amount of horsepower. It does not matter what the engine is. It just takes that much power to move it at that speed. Any engine you put in will be producing that much horsepower to make that boat go that speed. To make X horsepower you need Y fuel. Unless there is a reason to be running at some inefficient rpm almost any engine will use the same fuel. There is a difference between engine designs that can make one more efficient than another but for marine gas engines available here the efficiency is very similar. The engine weight between a 5.0 and an 8.1 is not significant in a 25' boat.
Any engine will get about the same economy at a particular cruise speed. WOT, idle, and getting on plane use different amounts of fuel.

 

[109jb]

Re: Fuel Economy Differences- Different Motors , Same Boat

I'm by no means any kind of expert. Rather trying to learn, so bear with me.

With a bigger boat powered by I/O, you run into prop limitations too. Don't you? I mean you can only put so big a diameter prop on these outdrives which limits you to pitch changes. At some point the pitch becomes limiting. So, a big enough boat with only one prop may never get on plane, or take too long to get there, right? Hence the need for multiple drives on bigger boats. So there has to be a point where 2 outdrives are better than one no matter how much horsepower you can get out of an engine. Right?? That number seems to be around 25' to 30' if what Philster says is right.

So Now lets say you have a boat that is right on the edge of needing 2 outdrives. Then going with the 2 outdrives increases the weight of the boat because of the extra motor/drive. So the weight increase alone will require a little more horsepower to go a certain speed. Right?? Maybe not much when talking about a 25 to 30 foot boat, but at least a little more. So, for economy it would seem in this case that if the efficiencies of the engines are about the same a single drive with a bigger engine would be better. Right?? Maintenance costs would be lower too because of one less drive/engine to maintain. Right?? But then there is the benefit of having 2 engines if one goes belly-up out on the water. Gotta value this on your own.

Also, I read that the OP is coming from a 6 knot boat and that he didn't necessarily want to go "fast", but not that he didn't want to go faster than 6 knots. If it is true that he is happy with a slow boat and isn't interested in planing speeds, then it seems to me that a displacement hull boat would be his best choice for efficiency, not a planing hull. Right?

Again, just trying to learn a little something about the bigger boats.