economical coastal cruiser

Sandy,
If you're satisfied with a top speed less than 20 knots I think that you have the best chance to approach your requirements if you go for long and narrow displacement hulls. "Hull speed" is no absolute limit for a displacement hull if it is long and narrow. In the range of 10 - 15 knots it's more efficient to use a long and narrow hull and force it past "hull speed" than to go for a planing hull. A long and narrow displacement hull will be very efficient at lower speeds, like 8 knots, which a planing hull will not.

Regardless of hull type, I think you should consider to reduce your accomodation requirements so it fits within a boat displacement of 2 - 3 (metric) tons. The alternative is to go for a conventional full displacement hull restricted to "hull speed", which for a trailable length boat would mean 7 - 9 knots. Assuming of course that fuel economy is high on the priority list.

A power catamaran would use two long and very narrow hulls, and would reach your target spec of 12 knots and 1 gallon/hour for a displacement that might be close to your needs for accomodation. A cat would need two outboards (or inboards) and I believe the beam restriction to 8.5 ft would make stability problematic. A scheme to reduce the beam for trailering would in that case be needed.

Another possible route would be to use a stabilised monohull, essentially a trimaran hull, where the central hull is maybe 10 m long and 1 m wide at the waterline. Above the waterline the beam can wider, and stability is provided by two small, long and narrow hulls. There are several threads on this type of hull, mainly for smaller and lighter application like pedal powered boats and electric boats. Look at the threads "Pedal Powered Boats" and "Coastal Cruiser" in this forum, both started by Rick Willougby. Rick has named the hull type "faux tri".

There is however a succesfull scale (up) model of a "faux tri" with a length to displacement ratio that would scale close to your needs. The US Navy has ordered trimaran combat ships that are very interesting. Look at the thread "Ultimate Multihull - a Trimaran" in the Multihulls forum. The trimaran in question is 127m long and displaces 2784 tons. This is more or less a 10:1 scale modell of the boat you need. It would be interesting to get data on power vs speed for that one and see how it scales to a 12m boat.

The problem with the "faux tri" (trimaran type stabilised monohull) is that as far as I know there are no ready plans to be bought. The suspicious mind asks why, but maybe the simple answer is that the market hasn't asked for it yet. There might be plans for power cats with reducible beam to allow trailering, I don't know.

Erik

 

This IS getting pretty exciting, Boston!

So now we have a @ 30' boat, longer if possible, less than 7800# and 8.5' or narrower beam, that will cruise 8 knots on a 20 to 35 hp diesel. Or (since there just aren't any very efficient FWC gas inboards in that range,*) an outboard or two. Something closer to 34' would have a better ride in the chop found on coastal bays.

I think the head and shower are the only things that need to be enclosed, so they might be positioned forward. The pilot house likely needs to be over the engine and mechanical spaces which hopefully won't be wrapped too tightly, for very accessible maintenance (in addition to being old, I'm wide and don't bend voluntarily.)

* Or are there? And we are definitely not talking Atomic Four here.
If diesel, I would like to stick to Yanmar.

Thanks Erik; thats rich food for thought. Another thought is a folding tri, ala' Farrier or Gemini, for added deck space afloat as well as stability. Maybe a "condo-ized" Corsair 30?

 

Sandy
Can I suggest that you wait until the results of the latest Proffessional Boatbuilder Magazine are released ( www.proboat.com ). The design brief for that comp will almost certainly fit your needs.

One of the biggest problems, as I see it with a stabilised monohull / trimaran, is that they can be a real pain to berth. This has to be considered right from the outset. the one that Marshmatt is developing is likely to be better than most in this regard ( see http://www.boatdesign.net/forums/projects-proposals/trailer-cruiser-revisited-trimaran-27032.html ). My most recent experience of this was on board the Ady Gill (formerly Earthrace, and now lying on the bottom...) where it took a team of 4 of us almost an hour to trun the boat around 90 degrees in the berth it was in!!

 

I would also question eriks assertion that a displacement hull that is 'pushed' beyond its theoretical hull speed - and that is designed to do so - is a better proposition than a planing hull that is designed to operate at the lower end of the speed spectrum. Depending on circumstance, one may well be favoured over another, but it's certainly not a hard and fast rule.
Clearly, at true displacement speeds the displacement hull is likely to be better, but beyond this, an argument can be made for either solution IMHO

 

Illustration

A pic to illustrate "faux tri"

500 to 600 kg for the hull / 12m long / 30 hp for 15 knots.

 

Will,
What is it that makes such a vessel so inherently hard to manuever?

 

Uulp! I just started reading the Faux Tri string. I wish I had studied it before I jumped in here. Many answers are now questioned!

 

Well, to be fair, it depends a great deal on the vessel itself. Mostly it stems from an inability to get near the bow. Obviously, the longer the ama's are, the less of an issue this is. And Earthrace was about as bad an example as I've seen - movement aboard was about as difficult as it could get!

 

Wow,
If I'm understanding you (IF) it's because the crew can't physically get to where it would be convenient to do their jobs?.

 

Yep... that's much of the problem. But then you have to remeber that this particular boat was made to circumnavigate the worlds flatter oceans to publicise the use of biofuels... not to be managebale at the jetty.

 

LOL,
I realize the boat's purpose was to cirumnavigate the world without using any demon oil. But me, after a long voyage around the globe, I'll be a little tired and cranky and don't want a bunch of hassles tieing up at the dock keeping me from getting off that boat!

 

funny thing is the whale wars folks converted it to that demon oil if I remember it correctly

which I may not

 

Just a note , wide beam , over 8ft 6 inches can mostly be overcome with a permit.

TOO LONG , usually over 65 ft from front of vehicle to end can not be permitted in most states.

Yes there are some states that will allow longer , but keeping the tow vehicle and trailer down under 65 is required.

Towing with a motor home would make loads of sense 30 -35ft being the most common with an engine that can drag a 10,000 load at highway speeds.

Using this concept might shrink the boats LOA a bit .

FF

 

Does anyone know of a production boat that approaches this goal? I mentioned the Nimbles, and have discovered the Rosborough 246, which are very close, but I have to ask myself whether a small, narrow displacement diesel coastal cruiser is all that practical or desireable, if none have been built?

Yes, I err to ask here about conventional market approval....

 

How much money do you have to build and outfit this boat you want to build?
Once that is known then you can weigh the plus and minus and know what you have to settle for. Were all kinda stabbing in the dark right now.