Question about hull speed fuel usage VS fuel usage while on a plane

[RobATX]

I am new to mid-sized power boats. I am considering a 46C. I was very surprised at the amount of fuel these diesel engines consume, particularly while on a plane (twice as much / 2gal per mile!) I have been on center consoles most of my life and generally you get better fuel economy when you get on a plane and reduce surface contact with the water... Is it just that those big engines burn so much more fuel at higher rpm's that friction doesn't even factor in? If these numbers are accurate, is there an engine for the 46C you can recommend that will provide better fuel economy?

Thanks,

Robert

[jim rosenthal]

46Cs were built with mostly two series of engines- 6v92s and 8v92s for the high performance version. All 2-cycle Detroits are thirsty engines- the 8v92s will burn proportionately more, with a higher cruise and top speed. Getting a big heavy boat like a Hatteras 46 on plane and keeping it there while driving it over the water requires a lot of thrust from the propellers, and that thrust is achieved by turning a lot of diesel fuel into revolutions of the props. The amount of mass to lift and keep elevated, as well as the friction with the water surface, is considerable. It's kind of amazing that it all happens as it does.

That said, four-cycle diesels are more efficient and will burn less fuel for a given level of performance. For those boats, the best engines are probably Cummins M11s and Caterpillar 3406Es or (I think) C12s or C18s- anyway the larger Caterpillar inline sixes. They will still burn a lot of fuel but they are more efficient and will push the boat to quite respectable cruise speeds.

Hot rodders used to say "speed costs money. How fast do you want to go?" The corollary statement to that with "big" yachts (by modern standards the boats we are talking about aren't considered that big) is "size costs money- how big a boat do you want?" The 43, 45, and 46 Hatteras boats all have their proponents- in particular there has been a lively discussion here lately about the merits of the original 46 boats, to the point where I would carefully look at good examples of all of them to see which would suit you best. They were all popular boats so finding one that suits you ought not to be impossible. You may even luck out and find one repowered with one of the engine choices noted above, and since all these boats are selling for a lot less than they would have a few years ago, you may well get lucky. The later 46s appear maybe to be the best bargain, especially if you can find an updated one.

[RobATX]

46Cs were built with mostly two series of engines- 6v92s and 8v92s for the high performance version. All 2-cycle Detroits are thirsty engines- the 8v92s will burn proportionately more, with a higher cruise and top speed. Getting a big heavy boat like a Hatteras 46 on plane and keeping it there while driving it over the water requires a lot of thrust from the propellers, and that thrust is achieved by turning a lot of diesel fuel into revolutions of the props. The amount of mass to lift and keep elevated, as well as the friction with the water surface, is considerable. It's kind of amazing that it all happens as it does.

That said, four-cycle diesels are more efficient and will burn less fuel for a given level of performance. For those boats, the best engines are probably Cummins M11s and Caterpillar 3406Es or (I think) C12s or C18s- anyway the larger Caterpillar inline sixes. They will still burn a lot of fuel but they are more efficient and will push the boat to quite respectable cruise speeds.

Hot rodders used to say "speed costs money. How fast do you want to go?" The corollary statement to that with "big" yachts (by modern standards the boats we are talking about aren't considered that big) is "size costs money- how big a boat do you want?" The 43, 45, and 46 Hatteras boats all have their proponents- in particular there has been a lively discussion here lately about the merits of the original 46 boats, to the point where I would carefully look at good examples of all of them to see which would suit you best. They were all popular boats so finding one that suits you ought not to be impossible. You may even luck out and find one repowered with one of the engine choices noted above, and since all these boats are selling for a lot less than they would have a few years ago, you may well get lucky. The later 46s appear maybe to be the best bargain, especially if you can find an updated one.

Thanks Jim. My wife and I enjoy fishing but in the Gulf you need to go 40-50 miles before you hit the blue water, and it's hard to imagine heading all that way doing 9kts (about 4 hours out and 4 hours back!) So I am definitely interested in engines that can at least plane the boat and maybe even a little faster while burning the least fuel possible. Great explanation. Thanks again.

[seasalt]

Jim,

Do you have any comments on repowering 45 or 46C's with higher hp 4 strokes? I have heard that these style hulls may not handle it well. I have heard the term "keel walking" and have heard of a 45C with Cummins that had the keel cut down for some reason.

I also have seen with my own eyes either a 50C or a 52C that was repowered with hig hp engines and they broke the boat almost in half in rough seas. There was a crack (that you could stick your fingers in) at the intersection of the deck house and the cockpit that went from the gunnel to the water line. The boat was repaired and someone out there has it.

RC

[Boatnut]

For the best answers to your questions, ask someone who owns the particular boat you are interested. Opinions are nice, but first hand information is best. Good luck

[krush]

Let's keep it simple. In general on a boat in the size you are looking, it's going to be about 0.5 NM/gal at cruise.

If you are used to center console fuel burn and still expect to run out and back for fish, it may be wise to stay with one. The center consoles have taken over much of the 30-50 foot sportfish market due to fuel burn and speed.

Slower cruise around 8-10kts can be 1-2NMpg....depending on engines, setup, whom you talk with, etc.

These boats are 1000lb +/- a foot. It takes HP and fuel to push that

[capttonyf]

Let's keep it simple. In general on a boat in the size you are looking, it's going to be about 0.5 NM/gal at cruise.

If you are used to center console fuel burn and still expect to run out and back for fish, it may be wise to stay with one. The center consoles have taken over much of the 30-50 foot sportfish market due to fuel burn and speed.

Slower cruise around 8-10kts can be 1-2NMpg....depending on engines, setup, whom you talk with, etc.

These boats are 1000lb +/- a foot. It takes HP and fuel to push that

This comment is pretty spot on. I have a 1990 45C with 535HP DD6V92's. We cruise at 1950 RPM doing about 21Knts burning 35 to 40 Gal per hour. So 1/2 a mile per gallon is what I figure +/- depending on load and conditions. A 30 ft CC is going to be faster more efficiant and give you the ability to fish from anywhere on the boat BUT, it is also far less comfortable and I'm not talking about Staterooms. On the larger boat you have an air condition salon, Shaded Bridge, roomy heads, and a Dry comfortable ride out and back where you can actually sit and relax. On a CC Boat your typically standing, holding on, and dodging spay. Both types of boats have good and bad points, both get you where your going, and both catch fish they just do it in their own way. One way to look at it is if you plan on fishing two days in a row with the larger boat maybe you just spend the night offshore and only have to run in and out once.

[jim rosenthal]

Jim,

Do you have any comments on repowering 45 or 46C's with higher hp 4 strokes? I have heard that these style hulls may not handle it well. I have heard the term "keel walking" and have heard of a 45C with Cummins that had the keel cut down for some reason.

I also have seen with my own eyes either a 50C or a 52C that was repowered with hig hp engines and they broke the boat almost in half in rough seas. There was a crack (that you could stick your fingers in) at the intersection of the deck house and the cockpit that went from the gunnel to the water line. The boat was repaired and someone out there has it.

RC

We have at least two members here who have done that, probably more- Capt Coop, and George- both have Hatteras convertibles in the 45 foot range, one repowered with CAT 3196s, I think, and the other with Cummins M11s. (Coop's boat) I think Krush is spot-on about the fuel use- about a half nautical mile per gallon is right. That is at a planing cruise. As far as keel walking, while these hulls weren't designed to go that fast (I know Coop's boat did at least 34 knots on her trials) they seem stable if they are propped right. And you are not going to run at that speed- essentially you are going to get up on plane and throttle to a reasonable cruise, hopefully with the aid of boost gauges and exhaust pyrometers to tell you where the engines are happiest and working most efficiently. That will not be WOT, for the sake of your wallet, engine life, and wear on your boat. You are going to end up cruising your boat in the 20s, more than likely.

The best sources, as noted above, are people here who own these boats, or have built these boats. They will know what they are talking about. For me, I can tell you something about 36 convertibles because I own one. For real information, those in the know can tell you a lot more.

I suspect any planing hull with a keel will keel-walk if you make it go fast enough. I think we are talking about speeds in excess of what you are planning to run.

[RobATX]

Thanks to everyone for all your input. I have come to terms with the fact that I am going to be paying more for fuel and going slower in exchange for comfort... I just wanted to be sure that the numbers that I was hearing tossed around were correct, and it appears they are. I can live with it. They are such wonderful boats!