Advice needed on trimaran

Here's information on materials:
http://www.thewoodshop.20m.com/kayak6.htm
http://www.stolaf.edu/people/becker/baidarka/index.htm#Suppliers
http://shop.skinboats.com/

Dyson, Baidarka & Co.
435 West Holly St.
Bellingham, WA 98225
(360)-734-9226​

 

If you have a good source for tubing it will save you lots of time and work on the joints. If you can't get good dry wood the case for tube is even stronger.

Yost uses 3/4 inch OD 5/8 ID tubes from T6061. The alloy might not be important if you are not using it in salt water. The important thing is you need a connecting tube that fits tight and slides smoothly inside your main tubing. 3/4" is about 19mm, 5/8 is a little under 16mm. The price of the tube is proportional to the weight but the stiffness is proportional to the radius cubed. If you could get larger thinner tube I think you would be better off. Before you buy small diameter tube be sure to feel how hard it is to flex one that is as long as your boat. You should also know that some alloys of aluminum can be quite soft.

For my kayak I am going with 1" telescoping down to 3/4" in the keel, and 3/4" in the sides, but I only have 3 tubes so I want them stiff and strong.

Plywood will be fine for frames. I was thinking about using spring pins to snap them to the stringers. I don't think an interference fit would be reliable.

 

If you have a sours for HDPE cutting boards, used in the kitchen, you can use those to make frames. Sometimes buying the cutting boards that are imported from china is cheaper than buying HDPE sheets retail. I have found some used ones at garage sales and second hand stores for only $1 each, the larger ones are big enough to make kayak frames, so they should be large enough to make hull frames for your tri. As already mentioned the inside of the larger frames is where you cut the smaller frames from as well, typically if you need six frames, you will only need two or three large cutting boards.

 

FWIW - I just remembered, I went to a boatshow some years a gom where they built kayaks using ballistic nylon (bullet vest material)

They stretched it over a frame, and saturated it with some sort of clear 'varnish'

It was really tough, and resists sharp objects really well, as well as being really light.

Just found a source

"ballistic nylon sourced from Corey Freedman at skinboats.org "

http://www.seawolfkayak.com/about-us-at-seawolf-kayak

 

Dyson, Baidarka & Co.
435 West Holly St.
Bellingham, WA 98225
(360)-734-9226

and
http://shop.skinboats.com/Skin-Boat-Fabric_c3.htm

are also sources.
Note that Nylon will shrink when dry and expand when wet.
Dacron/polyester can be heat shrunk.
Nylon is slightly stronger for a given weight, but both are reasonably tough.

 

Stephen, rwatson, thanks for the information, I'll investigate it.

Petros, I wasn't been able to find any cutting boards online (well, there are a lot of cuttings boards sold in Lithuania, but no one writes if it is made of HDPE ). I'll investigate it further. I have also emailed the only HDPE seller in Lithuania, asking for HDPE sheet dimensions and prices. If they even sell retail. Worth a try.

Skyak, well, there aren't many options: I can use 8x6 tubes with short 10x8 pieces for connections, or 10x8x1 tubes with 12x10x1 for connections. Any larger diameters - there isn't a smooth fit combination, only like 16x13x1 and 25x21x1, so since diameters don't match even close, I guess I'll have to go with either 8 or 10 millimeters for tubing. Probably 10. It ain't much, but little choice here... There are few sellers in Lithuania, and even fewer who give information on diameters and prices.
I have worked with 25mm aluminum tube before, used it for mast connector (mast was 30mm diameter, shrouded with nylon rope, 3 square meters mainsail) in my bottle raft I wrote about earlier - well, mast broke... In the aluminum part

But if I make enough outwards bend in my construction, I guess these aluminum tubes won't bend inwards from water pressure against hull. Can't say for sure, it would be good to test these 10mm tubes... Maybe I'll get some.

 

Of the tubes you mentioned, the 12x10 would be my choice with 10x8 inside for connectors. If you use smaller tube on smaller centers you might do well.

HDPE seems to be priced around $10/sqft 3/8" thk in sheets or cutting boards (new).

http://www.usplastic.com/catalog/item.aspx?itemid=23869&catid=705

 

That ain't cheap... But considering it has superior strength, ease of work, no maintenance...

I didn't think of inside connector! Thanks! How would you recommend one of two connecting tubes with connector? Welding (have no welder)? Screwing? (I understand that another tube will fit in freely, but one of tubes has to be permanently connected with connector)

Could you please remind me of disadvantages of HDPE + aluminum versus plywood + wood? Because right now, HDPE + aluminum seems superior in every way. Too good to be true.

 

Just a second... I accidentally found some prices on one Lithuanian sites... It says that 1 square meter of HDPE 300 (extruded) 1cm thick costs 54 dollars... Comes in 1x1m and 1.5x3m. Wait... That is.... Well, "expensive" just doesn't quite cut it.
Is it possible?

 

Tube and frame work is well documented with pictures on Yostworks site.

http://www.yostwerks.com/

The inserts are 6 inches long, inserted 3 inch into one of the tubes with 2 holes drilled and riveted. We have a filled epoxy here called 'JB weld' I might try.

Other than the cost I don't think HDPE has any disadvantages in frames as long as you don't cut it too thin. And as I said, you can just use it for the lower part of the frame to cut the cost. Petro was the one talking you out of plastic frames.

 

$54 for 1sqM x 10mm thk sounds about right. Now the question is how much do you need?

The big advantage to HDPE is that you can cut precise snap fit holes for the tubes so that the outer skin only touches the tube stringers below the waterline. Above the waterline it doesn't mater.

 

Yeah, I know... But probably it is better to one-piece frames, right?

I believe you mentioned that it is better to cut now whole frame, but two C shapes, because then it is possible put smaller C's "inside" larger C's, and save a lot of material. But then again, how to connect these two C shapes of HDPE into one strong shape? Overlapping? Metal connectors?

And that snap fit sounds very promising. But I'll probably need at least 3-4 square meters of HDPE for this. At least 4 frames for main hull, and for amas... 3, maybe? So about 200 dollars for HDPE only. Wow, I need to consider this a lot...

 

the best way to secure the connectors to the tubes is to glue them in with loc-tight glue or epoxy, no need to rivet them if the fit is tight. The Easton aluminum tent poles hold their connectors in this way.

HDPE has the advantage of being tough, rot/corrosion resistant, holds shape, no finish required. Disadvantage over plywood is weight and cost. plywood frames likely have better strength to weight ratio, and is easier to hand work as well. Either will work, not trying to talk you out of, or into either. I like working with wood, but I have used both (I have also made folding aluminum kayak frame as well). Finished wood is more attractive, but also looks "traditional" (low tech), in your case a high tech look might fit the design better.

Aluminum tube has good strength to weight ratio, but it cost more, prone to corrosion/electrolysis, easy to damage ends at connectors (difficult to assemble if ends get beat up). It is more difficult to field repair than wood. Wood would be heavier and bulkier, more traditional material ("low tech" look, unless laminated), and likely cost a lot less. connectors would be more difficult to make, and likely assembly time for a folding hull would be more time consuming with an all wood frame, requires special fasteners to hold the stringers on the frame (as compared to the HDPE w/ AL tube).

AL tube with HDPE frames would likely make a much simpler frame (fewer detail parts), light, compact and relatively maintenance free. THe only advantages you have with wood is it is likely less costly, and the nice wood appearance of the frame. I also like working with wood, but it is more time consuming.

Also consider you need some means of putting tension on the skin after it is slid over the frame. There are a number of ways to do this, the simplest is to have some inflatable tubes run down each side (which also gives you extra floatation). EAsy to make by cutting up and gluing inexpensive air mattresses. The sponsors can use a simple lace-on way to tension skin, like a large shoe lace the full length.

Not totally necessary, but it would also be ideal if you can have a way to also adjust lengthwise tension in the skin. The fabric skin usually changes shape with temp and moisture so you will get minor puckers or wrinkles, it will also stretch over time. With ability to put lengthwise tension (along with width-wise tension with inflation chambers) you can eliminate all wrinkles. Not as easy to get adjustable lengthwise tension in the frame, but I think it is necessary for proper skin fit.

 

Okay... So I'll definitely need to work with Epoxy either way.

If you say that plywood has better strength/weight ratio, so if I would use plywood instead of 3/8'' HDPE, how thick would plywood need to be for the same strength?
And isn't it possible to make snap-fit with plywood too? Why not?

And you are sure that wood stringers are cheaper? That aluminum tubes (10mm) cost around 50 cents per meter, no finish or maintenance required. You mean that you can make wood stringers even cheaper, even with all the finish required for it to be waterproof?

As for the tensioning, RWatson has shown a way which looks very ideal to me. What do you think? Here is the video he posted.

By the way, I am now designing the amas, and I'm not sure about two things:
1. How much flotation should they support? One friend of mine said that total boat weight should never exceed 1/2 the volume of the hull, or it will become too submerged. I calculate that my trimaran weight (fully loaded with people) will be no more than 400kg (this is worst case scenario: trimaran weighting 100kg, 3 people (80kg) with 60 kg cargo. Of course, it should never happen, but I'd like to be on the safe side). Meaning twice that is 800kg or liters of volume in single ama (flying other two hulls).
I of course realize that no one will fly two hulls with 3 people and 60kg cargo in 20ft trimaran, but as I said, I wanted to be on a safe side. Reasonable estimation could be 230kg (2 people with 10kg cargo, trimaran weighting 60kg), so ama could have volume of 460 liters. But if I'm wrong with such estimates, trimaran can go nose diving or sinking.
Could you comment on my calculations, are they reasonable?

2. How big should be opening in amas? I realize that the lesser it is, the less water may enter the hull from waves/nose diving. But that means more PVC to buy, and more complex assembling.

 

Ama displacement of 2 times the all up weight of the vessel is in line with modern high performance trimarans like the ones you were looking at. Those tris also have a beam=length and mast heights of 1.5xlength or more. There are lower performance designs but generally all real trimarans have ama displacements over 1.2.

How big an opening in the amas? zero! The amas are to be sealed to depths equal to the beam. This gets more important the smaller the displacement of the amas because they will spend more time under water.

This is where you need to give serious thought to cost and performance. You are still thinking of a raft with three hulls and a sail. A trimaran is a very efficient main hull powered by two amas, a mast and rigging to support a large efficient sail plan that sweeps lots of power from the sky. Ama size is more closely related to sail size and strength. If your sail is not big and tall, you don't need large amas and large beam.

It appears to me that you are weak in your design goals. Your boat must be designed to a displacement. It will float much more weight than it's design displacement. If it is high performance, in operation (sailing), it will not have much capability to spare without breaking.

I have no intent to be harsh. But it is pointless for me or anyone to advise you if the inputs are wrong. If anything, my goal is just to get you educated enough to ask professionals.