Searching for fuel efficient powerboat

Sabahcat
My understanding from several sources is that the deck/cabin is all fiberglass. There may be some encapsulated plywood for strength in certain areas. There are no teak decks and the windows are set in the fiberglass cabin sides whereas the typical Taiwanese boats have teak deck overlays screwed into plywood cored decks and the windows have wood frames both of which leak causing rot problems. That's the big differences that I am aware of. There are probably others too.
IMHO the problem with light weight cruising catamarans is that they don't have the load carrying capacity to carry what you need on a cruising boat. Cat's seem to have at least twice the structure of a monohull. Sure the cat's structure is lighter but there is more of it. I have not seen one catamaran cruising in Mexico that was not significantly down on it lines. Some as much as a foot or so. I think that a cruising catamaran needs at least as much displacement as a monohull if not a little more to allow for the extra hull and deck area. Now don't get me wrong. I generally like power catamarans because of their fuel efficiency at displacement speeds. I also think that the designers need to rethink their interior layouts and get away from cramming fore and aft state rooms with multiple heads in each hull. Maybe the hulls should only be used for engines, tankage and mechanical systems. That way they could be narrower and the bridge deck clearance could be higher. Then go with what the American RV industry discovered decades ago that it should sleep 2, dine 4 and party 6. One state room and head on the main level along with living area, dinette and galley. If they can do it in a 35' long, 8' wide RV it should certainly be possible in a longer and wider catamaran. By the way I have these same issues with most monohulls. Just some thoughts. And thanks for all your posts.
Chuck

 

Its a shame the older Bluewater Cruisers aren't more water capable and come with diesel-seem to fit your needs of s2/d4/p6 and are not pricey at all.
Saw off the flybridge and head out...

http://www.yachtworld.com/boats/1978/Bluewater-Yachts-Coastal-Cruiser-2049039/WA/United-States

http://www.yachtworld.com/boats/198...Cruiser-1615769/Madisonville/LA/United-States

 

Westvanhan
If I wanted to forget about fuel efficiency and go with the typical wide, fat powerboat Mainship made in my opinion a nice looking 40' x 14' pilothouse model that meets what I want in an interior layout and has twin diesels. My best estimate is that it would burn around 2.5 to 3 gph at 7 to 8 knots. I wouldn't have to do anything to that boat except make the swim step an extension of the hull. Thanks for the suggestions.
Chuck

 

The vessel I linked to earlier fits that description but I think she is quite economical on fuel



She gets

from a Gardner 6LX

http://cruisingunderpower.fastmail.net/Lifeline photos.html

 

Have a look at my build, http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962.html adding fixed blade propellers so should gain in efficiency when I relaunch...
http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962-114.html#post486348 for latest images posted...

 

I'd like to thank everyone for their posts. It has confirmed what I had believed about existing fuel efficient powerboats. They basically don't exist except in the power catamarans, which I can not afford. Nor can I afford to have a custom boat built. So I am stuck with the compromises that come with existing boats on the used boat market. Basically too wide and heavy with transoms that drag in the water. Can't do anything about beam or displacement/weight but maybe something can be done about the transom dragging in the water. I have read other threads that debate just how much additional resistance is caused caused by dragging the transom in the water at displacement speeds. The only consistency that I saw was that there was additional resistance.
Could this be solved be adding a swim step that becomes part of the hull. My basic idea is to remove a portion of the hull starting just aft of the keel and then fairing the hull out to the end of the swim step. The transom would no longer drag in the water and you would add a little bit of waterline. I think that the volume added in the swim step should equal the volume removed. Maybe not too clear verbally. I have attached a crude sketch of what I am thinking about. Let me know what you think.

 

Obviously 1 L/nm is not good enough for you?

Like I have shown, there are many wide transom dragging trawlers that get that now, which is in line with what a power cat will use.

The modification you propose, how much will it cost and how much fuel would that buy?
Perhaps it is enough money that you could just put it towards buying the right boat to start with.

 

You're welcome Chuck,no problem at all.

The only thing I didn't mention ( I assumed you knew of them) was the Buehler Ducks,they may not fit your bill but future seekers may stumble on this thread.


The Idlewild at 57' is too large for you and she's for sale for far too much $ IMO here in Vancouver but she does seem to get good mileage,so I'd imagine the 40' would do well too.

http://www.trawlersandtrawlering.com/news/idlewildnonstop.html

 

7-8 lph at 7 knots works out to around 3 1/2 mpg. You are correct that is not good enough for me. I currently average 6 to 7 mpg at 6 knots with my sailboat. And you are correct that there are lots of power boats out there that average 3 1/2 mpg. That is not hard to do. I want to stay as close as possible to the fuel economy that I currently get. I don't know the cost of the stern modification but I don't think that it would be that much because you aren't changing any major components around. Whether the additional fuel economy if any would be enough to offset the cost I don't know. I'll look into that. I have seen a number of boats that have made their swim step an extension of the hull while still dragging the transom through the water. I have only seen one that tried to fair the extension into the existing hull. It wasn't faired very well. More like a triangular wedge tacked onto the back of the boat. It added a fair amount of volume to the hull aft and lifted the stern to the point where the boat looked light in the stern and down at the bow at anchor. Never saw it underway.
The stern modification was just a thought on how to solve the problem of dragging a transom through the water that everyone seems to talk about. Maybe it's a dumb idea. Maybe it's not. You'll never know unless you ask the question. I am not afraid to ask questions.

 

Chuck,

You need to turn your motor off when you're sailing, you'll get much better mileage!

-Tom

 

Tom,
The problem in the Sea of Cortez is that you either have little or no wind. Not enough to sail or you have to much wind and you have to hide out. We get a really nasty short period swell that is virtually impossible to sail into and can be very uncomfortable downwind. That is why I am looking to go back to a powerboat because you motor 80% to 90% of the time.
Chuck

 

I don't know what mpg they get,but you haven't mention any research into motorsailors..would that work for you?.

 

Chuck,
I like you're idea about modifying a submerged transom cruiser like a trawler to have 75 to 95% of the advantages of a full disp hull without building or buying a new boat. I've had that idea for several decades. A 32 Nordic Tug would (I think) be a good boat to operate on. A problem w it's economics is that it would need to be repowered and most other boats would need repowering too since builders and designers (through customer demand partly to mostly) 99% of the time overpower their boats so a repower is a given. I intended to cut away and reshape much more of the stern than your example. The best boats suited for the sawzall are narrower, lighter, single engine, are priced right (ideally due to engine problems) and float high in the stern like many Uniflites. One could move tanks fwd along w batteries and other heavy gear. Changing of the QBBL ideally should start just aft of amidships. One could leave a narrow part on CL to keep the original rudder configuration intact. One could make the new aft section of the bottom straight. Would'nt be as good but would be perhaps 80% as good and one could start the cutting and changing further aft. My original thought was to do an old Chris Craft wood boat as I thought wood would be easiest to do but now I think FG would'nt be that difficult. The CC would be very successful as far as fuel burn as they are quite narrow and light. In the end I bought a Willard but there are VERY few full disp boats available.

 

WestVanHan
I have close to a motorsailer right now. IMHO a motorsailer is just a sailboat with a slightly bigger engine and short rig. I want to get away from the hassle of sailing and all the additional maintenance and expense that goes with a sailboat. People seem to think that a sailboat is cheaper to operate than a powerboat. If you really add up all the costs of properly maintaining a sailboat I don't think that there is much difference, if any, in cost between a displacement powerboat and a sailboat. Even when you factor in additional cost of fuel for the powerboat. And the powerboat may even be cheaper. I know that will get an argument. I have a friend who did a very thorough cost analysis back in the 90's and came to the conclusion that a powerboat was cheaper to operate and maintain than a sailboat even factoring in the extra cost of fuel.

Easy Rider
We seem to think alike. I too thought to leave the rudder in place. For me working with fiberglass is no big deal. Fairly simple to make a mold off the existing hull on a hard chine boat. Much harder with a round bilge boat. I also agree that most boats are way over powered. I just don't know if the cost of the modification would far outweigh the fuel savings. As for where to start the modification I guess would depend on how deep the hull is to a certain extent. My thought from an ease of construction and cost stand point would be to start just aft of the last support for the prop shaft if the shaft is supported by a strut or the end of the keel for a keel supported shaft. All this may be just a pipe dream.
Chuck

 

Chuck,
I would start way fwd of what you describe. For ideal lines and lowest resistance I'd start where the curve from the fwd end of the boat straightens out ..basically amidships. I'd favor a bit more fullness aft and make the Quarter Beam Buttock Line slightly non-linear and terminate just below the WL so the boat won't be noisy at anchor. Cutting out so much of the bottom one will have much trouble figuring out where the WL is going to be but displacement figures at numerous cross sectional stations aft may deliver. On most conversions I would not leave the original rudder port. And Chuck ..I don't think it would be very expensive to mod the hull but the repower could be but may not be either. If the engine in the boat was sellable sell it and do just as good of a job buying the replacement. Could be a no cost event and since you're selling a 150hp engine and buying a 50hp engine you may even come out ahead on the transaction itself but need to spend considerable money on new engine mounts and mods to the boat such as engine beds and exhaust ect. Or if you have the money just repower. If you had a line on two 30hp engines a twin would be just as well ..just a bit more trouble. What do you think?