Cruising power trimaran amphibious design – soliciting opinions

I have built and operated small boats for 30+ years, but have no experience designing one. I have been thinking about a trimaran power boat for long range expedition type cruising. I’m tossing my ideas out here, to get some feedback and ideas before I look into some professional design help.

I want maximum range for minimum fuel, so a long skinny hull is required. I would be operating in remote locations with limited anchorages or harbors, so I want to be able to drive it up on any kind of remotely flat beach, quickly and in rough surf. With the engines, people, gear and fuel I am looking to float about 2000 lbs. Dual engine redundancy will be required. The cockpit will be enclosed and the deck covered so that a large wave going over the boat won’t swamp it.

Hull design:
A tri-hull is what I want, so unless the design is completely un-workable other hull configurations are not going to be considered. I am thinking a wave-piercing main hull, overall length 40-foot by 3, with 20-foot by 1.5 almas positioned to the rear; the stern of the almas parallel or even behind the stern of the main hull. A lot of that length is to span waves and increase hull speed, so perhaps the front 10 feet or so could be a detachable section that is basically a float, removable for trailering. The almas could be sealed floats as well. I would want them to be at least 5 feet from the main hull, as the trampoline space is a major design consideration.

I am assuming that I should have an efficient max speed of around 8 knots.
The almas will have to be able to swing forward for trailering, or be removable.

I have been stuck on aluminum for the main hull for a while now, because I like it for strength and the ability to weld stuff to it. The almas and nose cone could be fiberglass over foam and plywood, a construction method I am very familiar with. One of my first questions to put to someone with experience with this type of hull design; is there any advantage to shaping the bottom of the main hull or just keep it flat?

Power:
I am thinking 2 Honda 20-hp outboards, at 110 lbs each, on the main hull. I want the land wheels to be powered also, probably a 5.5 hp Honda engine, with a good gear reduction ratio.

Amphibious:
The obvious attachment points for the landing gear would be the alma supports, but once the consideration of powering the front 2 wheels is included, it might be easier to attach them to the sides of the main hull. There would have to be some kind of strengthener between the alma attachment points anyway that could then also support the landing gear. Currently within my skillset, the design would be basically the driveshaft and wheel attachment structure (with the suspension components removed) out of a side by side. 4-6 inches of hull clearance from the beach would be enough, and the driveshaft penetration of the hull would be above the loaded water line to minimize waterproofing requirements.

 

Just a thought that I'd been entertaining myself for crawling out of the water part of an amphibian: using a truck hoist.

As I see it part of the problem with an amphibian is the questionable traction and leaving potentially unwanted tracks on someone's beach. By using a truck hoist you can tie off to a sturdy tree or other suitable connection point and not have to worry that a spinning wheel or tread digging a deeper rut than is absolutely necessary.

With an anchor rowed out and set it may also be a way to get into the water sans the ruts.

As for the wheels, I was pondering adapting the sort of "landing gear" employed by seaplanes like the Grumman Duck as being well suited to the task with a monohull ... though with your trimaran's struts you could use something like a PBY's gear.

These may not be best suited for rolling down the road at speed (it wasn't clear if you wanted to do that) but otherwise they are well suited to a basic amphibian.

 

A heavy duty winch works great, IF you can get to shore ahead of your boat and find something suitable to hook onto. Not an option for me most of the time, or in any kind of rough weather.

The landing gear from any boat hull seaplane would be the best for sure, the problem is that such an item might cost more than your boat, assuming for a second you could find one for sale.

 

Actually, the designs were reasonably mechanically simple and your use would not need to withstand landing forces. So it might be practical to build your own.

 

IIRC such gear was only for rolling up on a hard paved ramp, not any beach, AND they'd have the prop as power so they wouldn't need an traction on the ramp.

 

The idea of coming ashore and up the beach in a "rough surf" is asking a lot.

 

OK I can see everyone is off in the weeds on the anphib portion of the idea...I am pretty confident I understand the limitations and options there already though. Anyone have experience to give me input on dimensions, seaworthiness, and/or strengths needed for a tri hull with these design parameters?

Thanks!

 

"I want maximum range for minimum fuel, so a long skinny hull is required. "

This causes a very reduced load as weight is critical.

Give up 1 K of speed and a simple monohull will do what you require and carry 3-4x as much.

 

Based on the specs you mentioned I can build it in VR. It would work amazingly well. I can even change the backgrounds to those of fare away exciting places. I can even put a 3d version of you on it. But in real life the laws of physics are deadly set against you. Get rid of amphibious part and may be you have a chance. If you build a boat well, you can always beach it. Taking it out and finding a suitable place is another story.