1.2 m autonomous boat hull ideas?

I'm in need of a cheap, easy to build, boat hull for an autonomous boat prototype I'm working on so where better to ask advice than here? This boat will serve as a test bed for a number of electronics projects I'm working on and will operate mostly in smaller bodies of water: lakes, streams, inlets, etc. I'm looking for ideas, designs or even free plans if you know of any that might work.

Here are the mandatory criteria:
- LOA roughly 1.2m / 4ft (so it will fit in the bed of my mini pickup truck).
- Must be easy / fast to fabricate
- Must be cheap to build from readily available materials
- Must be easily propelled by a small electric trolling motor (40 lbs of thrust or less) at a speed of 3 knots.
- Must be capable of carrying a payload of roughly 23 kg / 50 lbs
- All up weight of the hull should be 20 lbs or less. I'd like it to be light enough, when loaded, for one person to lift.
- The boat will need to have a fully covered deck (it will have no passengers or crew).

And the desired criteria:
- I'm leaning towards a one-sheet plywood "stitch and glue" design which I will cover with epoxy and fiberglass.
- I'd prefer the boat have a fairly straight keel to help it sail a straight course.
- A plumb stern will make mounting the trolling motor easier.
- Stability in small chop and waves is a plus.

I rather like the looks of this design:
http://forum.woodenboat.com/showthread.php?134124-Not-another-one-sheet-boat
But I'd have to scale it down a bit and I'm not sure about the rocker of that keel.

Any ideas?

 

Geez? Why don't you just put "a cheesebox on a raft"?

 

BK you are asking for more than you can have unless you are up to building a geodesic skin on frame type boat or some other exotic vessel. You need something on the order of 135 pounds of displacement........A 20 pound boat that will support that much weight will need to be carefully done. It is doable but it may get expensive.

Trolling motor 20 pounds
Payload 50
battery 45
boat 20
total............135 pounds.

It will be difficult to get the boat down to 20 pounds if it is to be decked. You could use 4 millimeter Okumee ply which will end up at about seven tenths of a pound per square foot after minimal framing is applied. That ply is not cheap and may get real expensive if you have to have it shipped any distance. In the raw a sheet of 4mm weighs about 14 pounds.

I have some doubts about making the boat go 3 mph if it is a four foot LOA, 135 pound boat. Maybe so but not likely with a "simple" design. You could scale the PDR down to 6 ft LOA and get a lot closer to the speed you need. Google PD racer to see what that thing looks like. Never mind that it is ugly. It is a pretty good little boat if simple and cheap is the aim. So let the end of the boat hang out from the back of the pickup. Six feet is better than 4 feet and you can load and unload it without too much risk of hernia. Many kayaks weigh 30 to 40 pounds and the users have no particular problem with carrying them. The shrunken PDR could be decked with visqueen or some other very light material like shrunken nylon. That will save some weight.

The Sampan is as simple as it gets but the one shown is close to eight feet long. At four feet long it will be pretty chancy about supporting the load. Scaling it down to half size will diminish the capacity by the cube of the ratio, or one eighth the capacity of the 8 footer.

Sorry to be a writer of discouraging remarks. Your project is not impossible, but it is challenging.

 

Thanks messabout. I didn't really work out the numbers for hull weights, just saw that some of the 8' single sheet boats were weighing in at 35 lbs so I guessed mine might come in at 20. I was thinking about skinning with 1/8" Luan and then a single layer of 6 oz glass on top of that. I know epoxy really bumps up the weight so what do you think a more reasonable weight might be?

Regarding payload weight, I included batteries in my original 50 lbs estimate and I hope to trim that trolling motor to less than the 20 lbs you estimated.

I can flex on boat speed also. Although I'd like it to maintain 3 kts, 2 kts will be OK.

This is a prototype and although it doesn't have to be gorgeous, it shouldn't be butt ugly either. Guess I might have to scrap the 4' idea and go with 6 or 8 foot instead. Hmmm....

 

I could help but I'd need to know more about it's mission, your budget, who's building it and your time frame.

If these are sensitive issues, PM me and we can email.

-Tom

 

How about making it a catamarran? Eight feet of hull in a four foot length.

 

Thanks Tom. Mission is to be a test platform for sonar and electronics testing. Meager budget for design (if I can't find an existing design I can scale I'll just design it myself). I'll build it myself so don't want to invest more than a week or two (3 weeks tops) into it's construction (hull and deck only).

Found some info here for another company doing pretty much the same thing I am:

http://www.oceanscience.com/Products/Q-Boats/Q-Boat-1800D.aspx

I like their boat, but I'm not real happy about it's lack of freeboard. Too much and it's unnecessary windage, too little (which looks to be the case here) and it lacks reserve buoyancy and flexibility to carry more payload.

 

@Franklin - Yes, I would consider building a cat, I can think of some advantages to do it that way, but I might loose some payload carrying ability and mounting a single motor would be tricky.

 

Thing is, if it flips you're screwed as it's autonomous...

I'd build it like a submersible that wont submerge. Lots of righting moment (easy to do with batteries in the bottom), hydro-jet propelled (no prop to foul), hugely variable payload (your free board height doesn't really matter as much if it's a covered deck). You'll get an excellent surface area to volume ratio, low drag at your target velocities, and they're easy to build (well, for me 'cause I've made so many).

-Tom

 

@Tom - I'm with you on all points except the hydro-jet. Where might I find a cheap one for something like this (and by cheap I mean roughly the same cost as a trolling motor).

 

Bilge pump.

-Tom

 

Not enough thrust, in fact a 1000 gph bilge pump has hardly any thrust compared to the same motor with a standard 4 blade prop.

 

I've done a similar set-up with a Rule 1500. It'll do the job for your drag and velocity.

Mounting a trolling motor wouldn't be difficult and you could run it on way less juice than full throttle.

If you're okay with a prop I am.

-Tom

 

I'm ok with a prop, too.

 

I'd advocate directable thrust then for steerage. Put the whole motor assembly in a horizontal pivot pod.

I have a better idea of what you're doing now, I just read through your posts in the "Auto Pilot" thread.

-Tom