semi barge cruiser

[narsil]

i am looking into building a barge to cruise around our lake at low speeds. The barge will serve as a low speed cruiser, that will allow one to say put a few chairs on it or launch a windsurfer off the deck .
i am thinking something around 20 feet long , 8 feet wide max with very low freeboard so one can get off on on easily without it being too hard say a 8 inches hull thickness.
kinda like a moving dock
The width is for stability, so you could sit in a lawn chair or put a windsurfer and rig on the deck. If one gets near the edge of the deck the Bouyancy of the deck underfoot displaced plus the weight of the deck awthwartships that is now lifting up should add to the stability kinda the wide floating dock concept of stability . ( unless of course you load up one side with 10 people but thats not the idea , i then i hope the hull heels drops the unwitting souls into the drink and slides out under the load )
Up front it will have a cuddy that two abeam could sit in and shelter steer or have a snack . maybe 4.5 feet high, with extendable side skirts that could slide back in say cold or severe weather.
Hull design in plan view would be square aft and keeping square until 3/4 the way to the bow , then at this point gradual taper to a point, blunt.kinda like a bullet shape .
In side view hull rocker would be dead flat aft up to a station 3/4 the way to the bow here the ply bottom would bend up to gradually meet the deckline. The idea with the hull tapering is with waves, (when i do take it out in them) the bow gradually builds up floatation as a waves train hits it. And at full power , water her will act to semi plane up the hull hitting this inclined surface ( again the idea is not to actually plane up i am thinking the horsepower needed to do this will negate the whole idea and a simple structure). Unlike a blunt bow the whole 8 feet across which would really pitch if hit by waves .
Building material would be ply top and bottom with two long stringers running from stern to bow, and width wise stringers.
I am not worried about weight as i am thinking this will add to its stability to a certain extent.


Again the lake it will be on is NOT big , wave height max two feet wave train length maybe 8 .
This will be a wet boat when windy and wavy but thats when i will use it as a lauching platform for windsurfing. In little or no wind it will be a slow lake cruiser that the dogs can run about on deck and four people can lounge. Not play volleyball on but can sit and it ont do any sudden jerking like an open fishing boat can. the flat bottom and width will help here.
ultimately i need to have a idea whether this concept is just trash or it can do what it means to. and that NOT a high speed vessel.
i have drawn many concept drawing and now need to scale the boat.
the idea is not to be a speedster and easy build and stable platform.
So in designing this where do i start? do i need to think of a live displacement with cargo 9 coolers chairs ) then design from there?? Say 4 persons at 150 each, the weight of 5 sheets of 3/4 inch plywood top and bottom for a crude estimate?
maybe i am getting ahead of myeslf i will hopefull post pics you can see of the plan view and side view including the structure inside the thin hull and then we can go from there.
give me a few day to ge the pic online.
thanks in advance for reading my query.
J E

[gonzo]

Loaded displacement is critical to the design of freeboard. 8 inches seems inadequate. Waves will wash over the deck and flood the interior. You may compromise with a freeboard of about 18" and a step or ramp on the stern.

[narsil]

good point , and understood, the cuddy entrance was going to have a considerable lip, approx 6 inches on it to prevent flooding, with drop in hatch boards.the hull itself is going to be sealed aft and forward with ( inspection hatches only )to prevent such a thing,
the idea is not to be washed by wave action but pretty close.
thanks for the reply the point on loaded displacement is well taken.
J.E.

[sharpii2]

There is a way to have your cake and eat it too. You can have a great deal of sweep to your shear. The ends could be, say, 20" high, but the amidships only 8" high. This way, less water will come aboard over the ends and you will still be able to launch your windsurfer over the side without too much hassle. It will look kind of shippy too. This sheer can be nothing more than a bulwark build around a flat deck that has an inch or two of side to side crown.
The idea of having hatches in the centerline that seal up is a good one too.

I would also advise having a slight 'V' shape for your bottom. It will use less power to move it around and it will need less framing to hold its shape.
The run of the bottom can be flat and sweep upward at the last few feet at both ends. the ratio of run to sweep should be at least 2 to1. The bow transom should angle out over the water at a ratio of at least 1 to 2.

I would start my preliminary calculations at a loaded displacement of roughly one ton. Hopefully you will end up shy of that, but more likely you will end up with more. 150lb people are hard to find these days. It seems that most men are pushing 200. And really hard. Besides, you will end up with a lot more stuff on board than you first figured. Everything weighs something. Even paperclips. Dog food, lounge chairs, barbeques, windsurfers, and coolers full of beer, not to mention your engine and fuel, all add up.

A 6 to 10hp outboard that has a prop designed for pushing sailboats will do well to push it at a stately 4 to 6kts while burning through a modest amount of fuel.

Bob

[Deering]

Rather than a barge hull, I'd suggest a pontoon boat. Your floats could be the polyethylene-encased foam floats made for docks. Here's one supplier of many: http://www.formex.com/floats.html

Build a standard wood deck on say 2x8 joists, then strap a row of floats down each side. If worried about shipping waves onboard , add a rail/curb across the bow and stern (as such) maybe a foot high to block waves. Most of the waves would pass underneath in the tunnel. It won't be the fastest or most graceful thing afloat, but your descriptive title pretty much says that anyway.

Lots of advantages over a plywood hull. No worries about waterproofing/sealing plywood. Use standard construction materials so inexpensive. No exotic or unfamiliar construction techniques.

From a design perspective on an 8x20 raft, you just might as well plan for 10 people and supplies. Once your friends/family get a load of your cool party raft, well...that's not the time you want to be worrying if one more person will swamp the whole shebang. If you assume each person (with beer - you're Canadian, right?) weighs 200 lbs, add the deck weighs maybe 1000 lbs, that's 3,000. To support 3000 lbs, you need to displace that much in water (Archimedes figured that one out). Water weighs about 62 lbs/ cu ft. 3000/62=48 cu ft of water - call it 50 cu ft of floatation. Then add another 20 cu ft to maintain freeboard and just be safe. Two 20 ft rows of the 2'x4'x12" Formex floats (5 floats per row) would give you 80 cu ft.

When you get it done, let me know. I'll bring the beer.

[marshmat]

Hi Narsil,

There's a similar hull (without the cuddy, and built in steel) cruising one lake that I frequent. Great at what you describe.

I'd suggest two routes, equally feasible but with somewhat different outcomes. One is Deering's pontoon barge, which might not be particularly pretty but will be extremely stable and easy to build. The other is the box-barge idea you put forward at first. If you go barge, definitely build in more freeboard than you think you need. Can always have a transom step or, as the Coast Guard buoy tenders do, a low cutout midships with watertight bulkheads fore and aft. Note that when you submit the dimensions to the government for your capacity plate, which you WILL do, they'll calculate your load based on the static float plane- ie. how far you can push the hull down before water comes in. Higher freeboard before water comes in, means you get rated for more people. Also makes it much more comfortable and easier to handle.

I'd redo Deering's calculation with the numbers you intend to have on it. Figure on everybody being on one side. If you want to be a tad scientific about it, do a sketch, dig out the Grade 11 physics book and take moments about the centre of the submerged part of the pontoon. Then do the same with the box hull. A few rough numbers will give you some idea how the ideas compare stability-wise and in terms of actual capacity. Also ask the Coast Guard (or is it Transport Canada now...) for a copy of TP1332 (1999), the Construction Standards for Small Vessels- got a lot of good tips in there for maximizing load, stability, safety etc. And let me know when it's done too, I'll try to one-up Deering and bring a keg

[narsil]

wrote a long reply......... then it didnt let me post
Ok here we go again.
thank you for the ideas.
freeboard ...yup your right I think i need more . As such I am rethinking the idea of how to launch and land a windsurfer or a swimmer .
Maybe a transom hung ramp?? hinged off the stern with foam inside for flotation that way it will ride lower as the board runs onto it ( with indoor outdoor carpeting) or as a swimmer climbs up. A rubber or plastic gasket below screwed to the main hull and overlapping the ramp, to smooth out water flow and also above so feet dont get pinched.
Oh and i havent forgotton about power, the engine would be mounted behind the cuddy, so the prop thrust would be approximatley centre (it was never meant to be on the transom...so that why i thought of a the transom ramp.) Ok directional stabilty under power would be a problem i know.
But would long shallow skegs help? As soon as the boat gets under way the shallow skegs would make it track better and act as grounding strips . ( the whole idea of the motor in this location was that one could use the motor from the cuddy and be sheltered.

I did think of the pontoon boat idea, and using dock flotation but its now the look i am wanting.
the lake has some shallow areas and never gets very rough, ( i do sail on lake superior as well)
as to the wheel hosue ahh er cuddy design well heres what i am thinking .. smaller then this but to scale ,so one can sit inside ....a tribute of sorts and ah....

[narsil]

try not to be too hard on me its only a crude sketch i made, now its just a concept sketch the beam may be off, ie: too narrow.

the interior is cutaway sligtly to see the interior enough room to sit two and have a beverage or in incliement weather look out thru the big bugeye windows.
the bow is cut off , paper ended
i will get a better drawing this took me 5 minutes

[gonzo]

They make plexyglass bubble skylights of different sizes. It looks OK but a bit low on the freeboard. It would certainly be different.
Ahoy, Capt. Nemo

[narsil]

thanks for the input gonzo yes freeboard will get higher