8m trailerable displacement powercat design

Have you considered a trimaran? A central hull would allow a corridor with standing headroom, desk and bunk space to the sides and you could be bridge it for a big sleeping area. Sorry, I'll shut up now haha.

Ok, one more idea, you could do something similar with a catamaran:

Doesn't solve the sliding problem but removes the complication of a central cabin.

 

Yes Dejay,
I have considered a tri with fold out ama's. Means going from a huge rear deck to a tiny one. Great if seaworthiness is foremost, but this build will be a coastal boat.
Tri was my favourite last year in my neverending circle of what should I build. If was to build something 12 meters long capable of heading to the islands for winter - that would be it!

That cat is nice, good idea, but a bit large.

My way of removing the complication of a central cabin was to use a heavy duty tent, I'm not a sailor so when travelling or anchored it acts as a nice wind break

I've built a few cats over the years and wouldn't consider using the hulls as accommodation until about 11 meters.

It's a fishing boat so I have to keep the windage down where possible, also on road as well.

 

First post noob so take this with salt... I've pondered a trailerable houseboat with two trailers, boats launched independant, then joined on the water. With your proto:

Interlock beams slide to span the middle section. Cabin designed to join at center. Center sponsons could be separated slightly for a single center motor. etc..
You might even make each half only 2m wide and really easy to trailer on narrow roads. I'm just so jealous of the big >20m ocean going solar catamaran beam widths...

 

I just stumbled on this thread and have not had time to read it all so can not offer much except that for perspective i used to own a Macgregor 36 sailing cat with 18 ft beam which had three beams in similar locations as those in the first post. The middle and aft beams were 6.625" od with 1/4" wall thickness, the fwd beam is the same od but 1/8" wall thickness. The aft beam has the traveler on it so has to take significant mainsheet loads loads which a powercat does not. The fwd and aft beams sit in troughs molded into the deck with ss saddles beach cat style with one bolt through the trough and beam wall to prevent them from rotating whole the center beam has just 2 x large bolts horizontally all the way though the trough and beam. There were over 300 of these boats built and have been raced hard at speeds into the 20 knot range, carried up to 16 passengers as day charter boats and one has crossed the north atlantic solo so are very well proven so I would think your 150mm beams suggested in the first post would be more than adequate. I built a telescoping trailer and was able to slide the boat closed to trailering width by myself relatively easily. It did require walking around the boat and working the ends in a bit at a time rather than pushing from the center but not a lot of effort involved.

 

Have you considered hinging instead of sliding?
Would give you the option to increase stiffness of the connecting beams, by using space frame sections (structural frame)..



Only problem there is that it gets longer when you fold it to put it on a trailer.
To solve that one could add another hinge point - which naturally makes the whole thing more complicated, heavier and adds more play - but maybe worthwhile:

 

Hi all,
Thanks for all the new input, there's some interesting ideas in there, Joan I've contemplated centre hinged beams before, they have been used before with good success.
Thanks Steve W for measuring those beams.
I got frustrated with the sliding beam cat option , and descended into a spiral that ended back at the Trimaran folder option again.
But now I'm back with a fresh idea.
I pinned down some requirements...

• Single larger central outboard instead of two smaller ones
• Aluminum construction for toughness / faster build time
• Preferably a hard cabin and fixed decking which was problematic with the sliding cat idea.

New idea is a folding cat, 2.5m folded beam, where the hulls tuck under for trailering, but rotate out 90 deg to increase the beam to 3.35m.

(Rough) drawings to follow....

 

Hopefully that make sense.
Apart from #3 that was weakness on my part.
Cat has pontoon style hulls, fully sealed, pressed ribs to minimize welding where possible. Very stiff.
Centre hull, with inbuilt cabin for diagonal stiffness, 4 x hinges per side, one at each structural crossmember. A locking system and some sort of diagonal cross brace to lock the transom end in.
It ends up a little heavier, so have widened hulls to 0.55m beam giving a loaded weight of between 1350kg - 1600kg at 0.25 - 0.3m draught.
Bonus is it drops the boat height on the trailer
Negative is the hull side height is limited by the the folding geometry, I'm still working on this, but have added 0.5m extra hull fwd of the cabin to add more buoyancy

 

Pic #4
Shows the hull in the folded position (in red)
Dotted red line shows the hinge / fold line.

 

Just buy Skoota 24 plans and find someone to adapt them to ally.

 

Still going around this never ending loop in my brain.

Revisited this concept - selling the beach house and moving further south, to get a bit more land, bigger shed, reduce debts and prepare for Covid induced fallout.

Boat project will now be a moored mobile holiday location (4 hours drive away) as opposed to a trailer boat

Same criteria apply, but includes accommodations for 4 persons for (up to) week long coastal cruising and fishing.

Outboards - As much as I love diesel, the difference in fuel cost over a year will probably be less than $1000, and so much less work to fit outboards

I am drawn to the unique, so still bouncing between;

• Catamaran (9m)
• Stabilized Mono (10m)
• Power Outrigger (10m)
• Power Outrigger (12m offshore capability)

Hence my other thread - Minimal Offshore Power Proa

Back to the requirements;

• 10knot cruise
• Economical
• Stable
• aluminum construction
• Simple
• Rough water capability

Catamaran is probably my favorite currently, with nice thin hulls of 15:1 ratio or so, twin Honda 40hp should be economical and reasonably quick if needed
Low profile cabin with two doubles, about 2000kg loaded

 

Where are you coming up with 15:1?

I can't imagine finding that.

The Skoota 24 is a nice double cabin cruiser, but you may not want the trailerable aspect. And RW probably doesn't mess with the aluminum.

 

Hi Brendan, after considering a motor outrigger and having read your other thread I'm curious what your current thoughts are?

I'm similarly bouncing around with ideas and design in my head although my requirements are different (lighter composite boat, going slower and shorter trips with solar electric). Here are my (still uneducated, novice) thoughts:

For ultimate stability you'd want reserve buoyancy in both directions, so a trimaran. Stability from buoyancy is going to be higher than from ballast in an outrigger.

One way to increase ballast for a power outrigger would be to just shift the slim hull to the right side while having the bridge deck more asymmetrical. Instead of having the 1.2m wide hull / corridor in the middle of a 2.5m sheer width hull you position the hull it closer to the right side. That weights down the ama without having to put ballast there. So closer to a asymmetrical catamaran that has a minimal and a bit shorter left hull. The asymmetrical interior could also be advantageous for e.g. a wider bed on the left and shorter shelf space on the right.

For efficiency I believe a trimaran would be better also. In a power trimaran the amas only need to be very slightly submerged to provide stability. Also I've looked at the paper that showed that if "placed right" and at the right speed the amas compensate for their increased friction drag by reducing wave drag by recycling the bow wave of the main hull. Not sure how a motor outrigger would compare in non calm waters. Compared to a catamaran a power trimaran of same weight / displacement can be significantly longer.

For seakeeping I believe very plum hulls up to the "bridgedeck" / interior shelf should give gentler responses. This is going by what I understood from what Shuttleworth wrote about his Adastra. Flaring the hull provides reserve buoyancy but it also gives faster movement moving up a wave and then also faster sinking down. He also argues that cats have more hobbyhorsing than power tris.

Maybe a well balanced motor outriggers can provide the same things with shorter and properly positioned outrigger. But so far it seems a power trimaran (or stabilized monohull) wins in all 3 categories.

Not sure where you are with building material, but if you want fast and efficient I'd think about ditching aluminium and build similar to the "ultralight" harry proa cruisers. Some nice experiments ongoing for his 80' cargo ferry. Adding something like 3m "empty bow" to go to 15m might only cost you something like 300kg and similar in money but increase speed/efficiency. Maybe this is madness or maybe the same could be done with aluminium as well.
Alscal posted some info about his 20m caraboa, which apparently weights only about 2t empty and can carry twice that. The 21.3m ILAN voyager weighted 4t with plywood.
Lighter building material would also allow you to make the amas higher for more reserve buoyancy and a straighter bridgedeck you can walk on and fish from.

Hope covid hasn't destroyed your building plans.