Amphibious Atlantic Cruiser

Hello! I have literally no idea what I am doing, hence my name.

I have never built a boat. I have never worked with fiberglass, or carbon. I don't know how to calculate an object's buoyancy, and no, I have never actually worked a sailboat, though I have been on them while they're out on the water.

That said, I have long been inspired by Ben Carlin and the story of the Half-safe. For those who haven't heard that one, Ben Carlin is an absolute mad man, who circumnavigated the globe in an amphibious jeep. So far the ONLY person to circle the globe in an amphibious vehicle.

I doubt I'll ever be able to do this myself, but that doesn't stop me from theoretical boat building, and I have several sketched out designs. The problem is, I am An_Idiot, and I have no idea what I'm doing. I'm not sure what hull shape or material would be best, though I have some ideas. My biggest worry is stability in the water. In order to be legally driven on U.S. roads, it has to meet certain dimensions. It can be a maximum of 45ft long, 13.5ft tall, and only 8.5ft wide. (13.5m long, 4m tall, 2.5m wide if we are using metric.) Sideways stability seems like it'd be a real problem on a boat like that.

I was wondering if I could get any thoughts or suggestions from people who know a boat actually works, and maybe ask if someone could show me how to calculate my waterline?

 

Welcome to the Forum Mr Smart (not idiot) - you are smart because you decided to come on here and seek some advice before you start to chuck truck loads of $$'s at your project.

Google came up with these interesting links about Ben Carlin -
Ben Carlin and the World's only circumnavigation by amphibious vehicle • MotorPunk https://www.motorpunk.co.uk/articles/ben-carlin-worlds-circumnavigation-amphibious-vehicle/

Ben Carlin - Wikipedia https://en.wikipedia.org/wiki/Ben_Carlin



Could you maybe take some photos of (or scan) the sketches that you have produced so far, and post them on here?

 

Yeah I can do that.

The first one was intended to be kind of a catamaran design, but I ran into the problem of the drive axles needing to be enclosed, so it ended up as some kind of trimaran? Each square is intended to be 2ft², and I'm leaving 4ft at the bottom of the vehicle solely intended for internal systems. Fuel and water tanks, battery banks, engines (which may require more room?), Etc.

The second one is so far my favorite. Not exactly a deep V, but should provide some space for storage and displacement, but again, I have no idea how to calculate that.

Last is a little set of sketches I did looking for a more exact approximation. The scale changes to 1ft², but I also did it in pen while I was at work, so these are really rough, even compared to the first two. I'm not considering rudders or props yet, I think the first thing I need to do is settle on a hull shape. I'm also considering a deep V hull shape with a drop down keel, but I don't know how big those are, and I think the drivetrain would be in the way, unless there were two drop downs, one on either side.

Edit: Wait! I could put a drop down keel at the far back, after the drivetrain ends! I didn't even think of that til I was looking at those sketches.

 

For your inspiration the smartest amphibious vehicle I know. It's probably not the most efficient though and the presenter mixes up some terminology.

 

That M3 video DID give me several ideas. I had thought of expanding the deck with a couple inflatable pontoons and folding platforms, but I would be worried about blowing out the floats.

I actually really like the idea of waterjet propulsion. A high cruising speed would be ideal for crossing the Atlantic. I was thinking about having either a collapsible sail, or an electric fallback system, in case I run out of fuel while in the middle of the ocean.

I had thought of expanding the deck with folding platforms and/or inflatable pontoons, but I would be worried about structural integrity of folding platforms. I don't really have a good argument against inflatables, but I am iffy about them.

 

Slow down old chap, and re-assess what your priorities are here.
If you really want to cross the Atlantic, then build a yacht suitable for this task. Or (even easier and much cheaper in the long run), buy a second hand yacht in good condition that is well equipped, so that you could set off next week.

They talk about high speed in the video that Floatation linked to, but those bridge units can only spin around in circles fairly quickly - I would not say that they are 'high speed' when going from A to B.
If you want to cross the Atlantic under power, then you ideally need a much more efficient hull form than an amphibious vehicle. I am impressed that Ben Carlin's jeep managed to cross the North Atlantic under power, and that he had the fuel capacity (I presume petrol?) to do it. In the motorpunk article in my previous post they note that he had 220 gallons of fuel, and that with this they were able to motor 1,700 miles from Nova Scotia to the Azores - that is almost 8 miles to a gallon, which sounds rather optimistic to me.

Here is another video about his craft - in it they mention that he was now able to carry 800 gallons of fuel - maybe he did have 800 gallons of petrol on that North Atlantic leg?

 

The biggest problem I see with your idea is size.
To motor across an ocean in a small boat requires two things:
1.) a very small engine, and
2.) a very large fuel tank.
IIRC, Half-Safe towed a submerged fuel tank behind her.

With a displacement powerboat, the slower you go, the greater your range. And when I talk about speed, I'm talkin Speed/Length ratio (S/L). This is the square root of your waterline, in feet, in knots. So, if your waterline is say 20 ft long, an S/L of 1.0 is 4.47 knots, or about 5.14 mph. But to get this, you need about 1.0 hp per ton of vessel weight.
If you had a really efficient small diesel, you may be able to get maybe 20 horsepower hours per gallon (hp/gal). This could give you a theoretical range of maybe 89.2 miles per gallon, if your total weight is 1.0 ton, but you would need a very hydrodynamically cleen boat to do this well. Plus you would need a very small engine. Bigger engines use more fuel at slower speeds than smaller ones. The wheels hanging out there create a lot of drag. Also, your boat is likely to weigh much more than a ton, especially with all that fuel aboard. So, with a much heavier vessel, plus the drag of the wheels, you might get 20 mpg at that speed. And this is probably a stretch.

But even at 20 mpg, to get across 1,000 miles of ocean, you're going to need at least 50 gallons. And this is throgh flat water with no wind resistence and no unfavorable currents. To deal with those, you're going to need more hp and much more fuel.

Suppose you end up against a 2.0 knot current. This will have to be subtracted from your 4.47 knot speed. Then you will be traveling at only 2.47 knots (2.84 mph). But your fuel consumption is measured by the hour, not by the mile. So now you're getting only 12.7 mpg.

This is one of the reasons Steam ships got so huge. The bigger the ship, the longer the waterline. And the greater the capacity to carry cargo and fuel. The s/l speed goes up with the length.
For instance, a ship with a 100 ft waterline would have an s/l 1.0 of 10 knots. Assuming its engine had the same effiencey, and it weighed 1,000 tons, it would burn about 1,000 gallons per hour. But it would travel 1o nautical miles (11.5 miles) per hour, not 4.47 nautical miles (5.14 miles). Now, when you divide the hp by the weight, 1,000 tons/ 1,000 hp, you end up with the same hp per ton. But the bigger vessel is going more than twice as far for the same rate of fuel usage ( 1.0 gal per ton). And if it encounters the same 2.0 knot contrary current, it will be slowed down to 8.0 knots, or 9.2 mph.

I hope this gives you some idea of what you'd be up against.