Polynesian Flat Boats

I have been toying around with the idea of building my own sit-on top flat boat like the one in the picture. Being an avid spearfisher, I would obviously prefer to NOT use PVC or any other material that presents the risk of punctures. I dread the idea of landing a 50lb. White Sea Bass and having the thing start to fizzle out on me while in route towards the shore. Anyone have any alternative ideas, designs, or suggestions that might make the construction process go a little smoother? Typically, kayaks are used, but I am looking for something that allows me the freedom of boarding and unboarding with very little effort. Mahalos for the help.

 

Hi kanakafari, welcome aboard boatdesign.net

So you want a small sit-on-top, not an inflatable, that you can fish from....

For inspiration, here are a few boats we've been seeing around here that might have some of the features you're looking for.

Ocean Kayaks, polymer sit-on-top boats for general recreational use (kids, fishing, etc.) and they have some that are designed for fishing: http://www.oceankayak.com/kayaks/angler_editions/

Hobie Mirage, which is usually equipped with their (rather interesting) foot-pedal drive, but there are more conventional variants too: http://www.hobiecat.com/kayaking/index.html

These two lines are both factory-built plastic, but I see no reason why similar concepts couldn't be extended to a one-off construction method such as taped-seam plywood or strip planking.

 

I saw a guy at our coast that had a similar flat er... surfboard if you want. It was about 1m wide and about 5m long, the nose bent upward, shaped like a surfboard. I'm sure it was made from foam and fiberglass covered, the guy single handedly dragged it into the water without much effort.

I was amazed at how easy this guy rowed through the waves, thence it being so long. He used it for fishing behind the breakers. It was probably about 100mm thick.

When I saw this my thoughts were if I am ever going to row to fish, then that would be the thing to have. I specifically remember because it was so long.

 

Thanks for the replies everyone. I am no stranger to marine kayaks however, I am just looking to row out and dive straight into the kelp without the confines of a dug-out seating area. Even the sit-on tops have a recessed area. I basically want to construct a raft/boat/board with as little cost as possible and in the smartest manner. In fact, I was just thinking about utilizing approx 3" - 5" PVC and capping the ends to serve as the flotation with elbows towards the hull to give it an upturned nose and applying a deck of some sort much like a catamaran sailboat. I currently use 1 1/2" PVC filled with expandable foam as a float when hunting White Sea Bass and Yellowtail. It seems to have a ridiculous amount of bouyancy so I figured I would apply the same theory towards something larger scale i.e. flat boat. Any thoughts? Mahalo

 

Do a search for paddleboard plans on google. There is a bunch of stuff related and you might find some inspiration there.

Steve

 

Thanks Steve. I took your advice and searched Google over the past couple of days, and unfortunately, I did not find much inspiration aside from what was originally conjured up in my head. Don't get me wrong though, I did find some pretty cool stuff. Knowing nothing about water displacement calculations and wood construction, plus the lack of tools available, I was thinking of using (2) x 4" PVC @ 6' L with end caps on the rear and then utilize a 22 degree hub/elbow on the opposite front end with another 12" section joined and capped so that it resembles a Hawaiian outrigger or perhaps the gondolas of Spain. I was thinking of a simple aluminum constructed frame that will serve as the deck then just lay down some sort of deck material (preferably not plywood). The intent behind the project is to simply navigate it out to some offshore kelp, jump in, and have it be able to carry my weight, plus an additional 25-70 lb. fish. My only question is if it is legal to launch something like this in Southern California or are their boating laws that apply. In my opinion, it is no different than a paddleboard except that it is a DIY project and it definitely looks out of the norm as fas as vessels are concerned. I figured that if I were to take a 6 ft. piece of 4" PVC, capped the ends (airtight) and then swam out with it, then it would surely float my 200 lb. frame with no problem. Now by adding another piece and constructing a deck, theoretically it should provide more than ample bouyancy. Am I wrong in my assumptions?

 

You don't have nearly enough buoyancy in 2 pieces of 6' x 4" dia PVC to support you. Not even close. Many try to make PVC pipe into a boat like object but it is never the best choice. PVC pipe works OK for a few boating things like stabilizing amas, but never as the primary buoyancy. The advice given to make a float out of fiberglass covered foam is the best choice for this.

 

Thanks for the info Tom. I threw together some diagrams of the design I was planning on implementing buit now you have me second guessing the whole thing. What if I were to fill the PVC with closed cell expandable foam? Or would it work out to be the same cost (approx) if I were to do the foam/glassing. Foam is not cheap and I can only imagine that I would need quite a bit of foam to fill 6' of PVC. Any thoughts on the likelihood of either of the diagrams actually working? They clearly demonstrate the purpose of the project. Thanks again for everyone's input, it is truly appreciated. As you can all see, I am a hunter and in no way, shape or form, a boat builder.

 

Easy and cheap..
I made one as a kid lot of fun..

 

Kanakafari,

A 6' piece of 4" dia. pipe is about 0.016 m^3, subtract its own self-weight and it can only really hold 13 kg or so at most- half that if you want it to be stable. You'd have to raft a bunch of them together, which would work, but would also have a lot of drag and thus be hard to paddle through the surf.

If you could get your hands on bigger pipe, you might be able to do it. A 6' piece of 8" is 0.064 m^3, a 6' piece of 10" is 0.10 m^3. For stability you don't want to submerge them more than halfway, of course, so two 6' pieces of 10" pipe would give 100 kg of useable buoyancy. Subtract 15-20 kg for the dead weight of the boat, and you still have 80+ kg for you and the fish. No need to foam-fill the pipes, they would just have to be sealed airtight with end caps and that foul-smelling goop they make for joining PVC.

This is one application where extruded Styrofoam billet might be suitable. A big block of the stuff (as sold for floating docks) or a few sheets (as sold for house insulation) laminated together would be an OK starting point, you could carve it to the shape you want, and give it a skin of a few layers of fibreglass set in epoxy (polyester would dissolve the foam). Nothing fancy, but it'd do what you want.... catching 40+ inch fish....

 

A couple of flotation basics are necessary to think about this project. Air is lighter than anything other than hydrogen gas that you could put in an enclosed space, so foam or anything else actually detracts from the buoyancy of your pipe. Even the nine pipes will not support you, your gear and the fish. The thing will be just submerged and totally unstable. Enough PVC pipes of any diameter will be heavier and less likely to result in a suitable float than either foam/fiberglass or plywood.

2" thick styrofoam from the local builder's supply house is cheap and plenty good enough for this project. Easy to work to any shape you like.

I guess we need to know what reason you have for not considering what the consensus here recommends.

 

I'd have to say that cost is the major deterrent. Aside from that, the lack of tools, know-how, and availability of materials would be in close second. You guys have given me a great deal of knowledge than I originally started with that's for sure. Perhaps I will take your advice and try to get my hands on some of these materials and play around with it. I will definitely showcase it once it is done, no matter what the final outcome is. Any suggestions on a material list?

On another note, I Googled and saw some plans on building a dock with 50 gal. barrels..........let's all hope I don't end up going in that direction. Haha

Thanks again all......mahalo nuil loa - a hui hou

 

None of us just go out and buy stuff to build things. Make drawings of what you want and do a bit of research. Find out how to use the materials. Save a couple here and there and before you know it you can build something decent.

The hollow tree stump you made with the $20 hammer and chissel may float but it is going to be frustrating, sluggish and a pain in the butt..

The well planned well built flatty is going to work much better and will in the long run bring much pleasure and comfort.

Forget the drain pipe thing, it is going to work ... well like drain pipe, use proper floatation foam and glass it over, paint. It will last forever.

 

I have taken all your advice and seriously considered the use of the billet foam/fiberglass construction method. I have a couple questions however...

What is the difference between polystyrene insulation, polyethylene (closed cell) foam? And what effect will glassing it have on the different materials?

I found STYROFOAM measuring 7" D x 20" W x 96" L and 10" D x 20" W x 96" L for reasonably inexpensive and it looks as if it can be easily shaped. Specifications indicate that the 7" has a capacity of 425 lbs. while the 10" holds 610 lbs. Any idea if I were to proceed with the original pontoon design, would (2) 96" lengths be sufficient buoyancy?

Also, are there any good resources online on how to customize color/graphics/etc. prior to the glassing process. I figured that if I am going to go this route then I might as well try to make it pretty. Haha

Thanks in advance for all the input.

- Elika

 

Forget polistyrene, the polyester resin will attack it and eat it away, and secondly it absorbs water.

The PE foam is fantastic, it is all I use. It is UV resistant (black), bonds very well with the resin and doesn't absorb any water. This will be my only choice.

Flotation you can calculate. 1m x 1m x 1m volume can float a ton. 100mm x 100mm x 100mm floats 1kg. So if you have 1m long and 1m wide and 100mm high you can float 100kg.

Remember you have to add the weight you carry to the weight of the material.

You also have to compensate for boyancy. Obviously you don't want the 100mm thick material to be flaoting 100mm deep, more like 20 or 30mm deep..

Another reason for the added floatation, I assume you will add cavities where you butt fits into and where the groceries you take with strap in place, then a nice to have would be holes right through and slanted backwards. This will automatic drain water that lands in any of the cavities. 20 to 30 mm holes should be ok.

Don't put the holes right under your butt, the water won't drain and the fish will laugh their arses off for the imprint it will leave