Advice needed on trimaran

Thank you for your critique. I'm sorry if my questions are so amateur. I'm doing the best I can to correct it.

About opening in amas: if it is zero, I suppose I'll have to make it foldable with zipper in skin? (this idea is from YostWorks).

You mean that I'm still aiming too high? That's what I'm doing: roughly sketch length and beam -> find approximate displacement -> based on it, roughly design amas and main hull -> find appropriate sail plan and size -> fine tune everything.
I'm not sure if I understood you correctly. Are you saying that I'm making too much displacement reserve, that boat will be too large and will break?
Sorry if I misunderstood, my English is not perfect with expressions.

Thank you for your patience.

 

I am trying to NOT tell you how to design your boat. I am trying to give you the reasons I know for why these boats are designed the way they are, point out the best designs I have seen, and let you choose how you will build yours. I have no intention of telling you how high to aim. Engineers can only tell how something will perform (fail) based on adequate input.

I think we could work most efficiently if you were to create a spreadsheet. We have two languages but one math.

Start with the design brief; how many crew, what skill levels, how much do they weigh max and min, how fast do you expect to go and in what conditions (back and forth, in front of your car is easy, across cold open ocean is a different world). You expect to break this down for storage and travel, how small must it get? And of course the budget.

Also make a bill of materials. There will be several for different designs considered. When one looks promising it gets more detail.

And on all of these requirements, are they fixed or might they be compromised to achieve a more important goal.

If you do this I can easily improve your plan and show you how these parameters relate. I think you will get even more input from others, including professionals.

 

Thanks, Skyak. I understand what you mean. I wrote a total summary of this plan for trimaran, hope it is like what you had in mind with a spreadsheet. Here goes:


Max displacement: 250-350, the more, the better
Amas displacement: 1.2 to 2 of total displacement (probably 1.2)
Length overall: ~ 6 meters
Beam 70% to 80% of length (4.2 to 4.8 meters)
Length main hull ~6 meters
Length amas 80% to 100% of main hull (4.8 to 6 meters)
Sail plan Bermudian/gaff rigged Sloop (no spinnaker)
Mainsail area 6 to 12 m2 (not yet calculated, dependent on other parameters)
Jib area 4-5 m2 (same)
Mast height 1.2 to 1.5 of LOA (7.2 to 9 meters)
Build time expected less than 2 years

Other:
Hulls – displacement, hard chinned, V-bottom (separate hull stability is not an issue since it’s multihull, numbers aimed for - maximal displacement and optimal hydrodynamics);
Will feature a net between hulls, so it won’t be sit-in trimaran, but rather sit-on tent (more comfortable, helps with reducing heel)
Mast shrouded, forestay, possible backstay (if mainsail Bermudian);
Rudder may be single, attached to main hull, or two rudders, attached to amas, same with centerboard(s)/daggeboard(s);
No winches.


Expected use:
Day sailing in Lithuanian lakes, lagoons. Cruising, few day passages, maybe participating in local regattas. All during warm seasons. With 2 people crew skipper would control rudder and mainsheet, while another crew member would handle jib and daggerboards. Also ability to sail single-handedly.

Expected qualities:
1. Performance with 220kg displacement (1/2 persons) (maximum speed – 6 to 8 knots)
2. Weight under 60kg (empty)
3. Foldable to parts no longer than 1.25 meters , disassembled should fit into car trunk;
4. Raisable centerboard(s)/daggerboard(s)
5. Life time at least 10 years without serious maintenance (such as replacing skin, sails or carcass);
6. Hull cost (including centerboards and rudder) no more than 400$, total – 600$
7. Doesn't have to be pretty. As long as it does it's job, it can look like a Waterworld (movie) trimaran.

Secondary qualities:
1. Reasonable windward sailing capability with 350-400kg displacement (3 persons + cargo);
2. Ability to raise 1 or 2 hulls for increased performance
3. Comfortable paddling capability
4. Some in-built cover for main hull opening (protection from rain)

Tertiary qualities (not much hope with these, but who knows?):
1. Sleeping place inside main hull for 2 people (expected uncomfortable)
2. Seaworthiness (starting with salt resistance)
3. Small anchor (only lake use)

Planned building methods and materials:
Hulls – skin on frame, frame may be aluminum tubes (10mm, joined with in-built inner joints of 8mm), or wooden stringers (dimensions unknown, join method – metal plates and screws), cross sections – 1.5-2cm HDPE or plywood (better marine, if out of budget – simple).
Skin will be PVC material (area not known, depends on hull dimensions and shape)
Crossbeams for joining hulls – laminated wood strips (with epoxy), breaks into two parts (around 2 meters, one of two things which will travel on car’s roof).
Rudder and daggerboards will probably have to be marine plywood, since they will spend most of their time in the water. Or leftover HDPE if I’ll use with it for the frame.
Sails – Tyvek or Dacron (probably will be Dacron). If Tyvek – self built, if Dacron – professionally sewn.
I plan to make a net for between hulls myself, from rope, since I red enough instructions to perform it, and it’s way cheaper than any commercially sold tent.
Mast will be made out of foldable metal tubes (up to 3 meters) which will partly fit in one another when assembled. Also travels on car roof. Mast diameter not known, depends on sail area, probably around 10cm.

Materials I already have:
A lot of screws;
11 line blocks, usable for lines of less than 6mm diameter (non-professional, but do their job)
A lot of rope of various diameters. Most if it is capron, but I have 3 pieces of 6 meters of professional climbers rope (8mm);
Tools I have:
Very good screwdrivers, hammers;
Made-in-China electrical drill;
Poor, poor wood plane which someone tried to use on a rock.
Blunt hand saw.

That means, no table, no saws, no clamps or press, even no indoors workshop.






Now I tried creating a bill of materials, but found it absolutely impossible at this stage. For example, I have no idea how many square meters of PVC I'll need for hulls, since I haven't designed their size, volume, shape... Same goes with aluminum stringers, I can' tell how many meters of them there will be, and what diameter will be enough. And price can change radically. I don't know whenever I'll use HDPE or plywood (marine or simple). I don't know how much epoxy will be needed, since I have to calculate the the area of all surfaces that are to be glued. Can't tell exact sail area, since I don't know how to calculate heel forces. That also affects mast diameter...
I really can't even speculate, I have no idea. I must make a plan in which all these things are decided and tested that they work in that combination, then calculate how much that combination would cost, and then modify it accordingly...

Sorry if this spreadsheet ain't professional enough, but I told as much as I know at this point.

P.S. If you have questions why I planned something that way or another, I can explain, just didn't want to expand too much on a summary.

 

a couple of thoughts: the min amount of stringers you will need for a skin on frame hull is 4 (two chine, two guwale), a 5 stringer hull makes a better shape (two chiine, two gunwale, one keel), and it is helpful to have one on top center line at the bow and stern so it sheds water (assuming the center area will be open cockpit). the amas can get by with only three stringers for a triangluar cross section (two gunwale, one keel), but again a 5 stringer ama would make a more pleasing shape. But this adds parts, and assembly time. I would keep the number of parts to a minimum.

 

I think this might be a helpful reference point:
http://www.watertribe.com/TridarkaRaider/
http://www.platypusboats.com/tridarka.html

Also:
http://www.multihulldesigns.com/designs_stock/23tri.html
http://www.multihulldesigns.com/daysailer.html
http://www.sailingcatamarans.com/Strike 18.htm
http://www.sailingcatamarans.com/wiz.htm

Plenty more at:
http://smalltrimarans.com
Including:
http://smalltrimarans.com/blog/?p=8343#more-8343

Looking through these links I think you would do very well to purchase plans for Richard Woods' STRIKE 18 trimaran. Many of Richard's plans are downloadable. You'd be getting the benefit of a lot of experience at a reasonable price. Also, Woods sometimes posts to these forums. Here's his profile.

But these are all plywood boats, not skin-on-frame. So perhaps I got confused about what you really want somewhere along the way.

 

Petros, would you be so kind to provide some photos of skin-on-frame hulls which have 4 and 5 stringers? Would be interesting to see how they make these.

Stephen, that Tridarka looks absolutely gorgeous. Very similar to what I have in mind. Thank you for the links!

 

another thought: forget the gaff rig, go with full battened main, fewer parts and faster/easier to rig.

 

Well, I really like to mess around with complex rigs... Like very complex. But if it is cheaper and more reliable, I'll go Bermudian. But what about battens - I red in "Sailmaker's apprentice" book that battens, although increasing performance a lot, reduce sail life dramatically due to chafe. And they cost a lot more. Is that true?

 

But... All these have a keel stringer, so that's 5 total, right? Or did any of them have 4 stringers that I missed? (Because I wanted to see how do they make them without keel, I can't imagine really)

 

they look similar to the pictures but have a flat bottom. the problem with a skin-on-frame is you will end up with a slight concave bottom if you only use a four stringer hull, no matter how tight you manage to get the skin. Though this is not such a problem, it is just not as efficient a hull shape. If you have a plywood floor you can likely do away with the keel stringer (allow the floor to support the skin for a flat bottom hull), but that would also add more parts and complications on assembly. Unless you were going to make a floor anyway.

Do not over think this, gets some hull lines you like and start building it. start with one of the out board floats, it will go fast and you work out details as you go. You can even just buy some tube stringers, and use cheap salvaged plywood for the frames to work out the shape and details, once you are happy with it you can than cut the HDPE frames. The second one will go faster, after that you will be ready to do the main hull and how it will come together will be much easier to understand after you have the two amas finished. You will learn much more by doing with your own hands than all the posts we can give you here.

It is not that complicated, you are just using frames to hold the stringers in the shape of the hull you want. I have built them in wood, fiberglass, and aluminum, both folding and none folding. I do not even use plans anymore, just sketches of the shape I want and some scribbles to work out any new details. I usually work out the fit and connections as I build it because when you have the parts in front of you what needs to be done next becomes a lot more obvious than when working out plans (which can take as long to work out as actually building it). If I do ever make proper plans, I do it after I have built the first ones. No matter what you do on paper will likely end up being changed once or twice (or many times) after it is built because you find a better way to do it. So I just went straight to the first build and saved the time of drawing up detailed plans until after I am happy with what I built.

Time to stop thinking about, make your choices, and gets some materials together and start cutting wood (or metal, or plastic).

 

Sorry for interfering in skin-on-frame project, but I think this boat better fill the bill.
http://gik.fordak.ru/index.php?PHPSESSID=8locvmktnudj37207tsc48dq71&topic=9652.0
AFAIK, owner sells second central hull. He lives not far from you, in St-Petersburg. Before becoming trimaran, this boat was catamaran, so there is spare central hull now.
Sailing trimarans, based on folding kayak as central hull, passed a long way of evolution in USSR. Last models use robust A-shape frame from Al tubes, which takes all loads from floats and rigging, kayak was simply suspended under this frame. But they still could not compete with inflatable constructions and practically disappeared 15-20 years ago. Survived only simplest and lightest boats with 3-5 sq.m. sails.
Down the link you can find some additional 1986-vintage trimaran ideas
http://parusa.narod.ru/bib/katy/1986/0666.htm

 

Well, that's what I thought. I'll definitely go with keel stringer. Don't know if I'll make a floor, only if there will be spare part of budget left.

About going and building - well... You are a professional, you can allow yourself that because of the experience. It might end messed up and expensive for me. I approached my last project (that catamaran with PVC tubes) with similar attitude - made some plans, but in the end, changed many things as I built. And soon I realized that I can't change something because it's too late, some parts are already bought and cut, something is not compatible anymore, I have to work-around some things, or remake something that doesn't work because I planned it poorly. For example, I wanted to make hydrodynamic shapes to seal PVC pontoons. I thought I could make them with glass fiber with polyester - so called glass composite, used to make contemporary yachts. Only to find out that it leaks water like mad. Tried making simple non-hydrodynamic plugs for PVC pontoons with 4 layers of glass fiber - still leaked water. So I threw out like 100$ due to misinformation and poor planning. Or Tyvek sails. I believed that one kind of double stick tape will work on it, only to find out 3 days later that it unsticks after some time. I had to add more tape of different kind, but in the end, sails required pressing joints with hands every time before I sailed out, so it wouldn't unstick while underway.
Some mistakes cost me a lot of time. For example, I spent too little time calculating forces that will affect pontoons, and reinforcements that join them to the deck were in wrong place, and everything broke when I tried to turn catamaran upside down. Had to buy replacement materials, lost 2 days of work.

My point is, if I learned something from this, is that time spent thinking, planning and testing on small scale doesn't cost, and doesn't bind me to compensating and living-with consequences of poor decisions.
Having that in mind, I can say that poor planning cost me my ship. If you red my description of that ship, I had to demolish it, only after last 2 days when I finally managed to get it in shape to sail to windward. I realize now that if I would have considered everything more in advance, I could have saved it. Made it in a way that is practical to re-use, that is, break down into parts and transport. And I could have saved like 40% of the final budget.



But I think I can test things on smaller scale. What if I made small sized model, like 1:20? Of course, I couldn't use the right materials, but I would be able to see if there are some serious mistakes, like if sail plan is too small or too large, if some parts tend to break, if it sails at all, can it raise hulls without flipping over, if center-of-effort and center-of-lateral-resistance are aligned, and so on.

Am I wrong in this thinking?


Nnnnnnnn, (how may n's are there?) thanks for the link, that looks like a very serious ship. However, there any many problems traveling to Russia (getting a visa, transporting cargo), not to mention trip cost - whenever by car or by train, it would cost no less than half the budget of my ship...

 

What I had in mind was a spreadsheet that calculates cost and performance so that you could consider tradeoffs. I mention some major tradeoffs above. I prefer to work in excel, but if you don't have excel we could work in

http://www.google.com/google-d-s/spreadsheets/

 

There is something to learn from this evolution of design, particularly the structure. Three aka tubes,each at least 5cm diam. and an aluminum ladder in the center to take the torque. Notice the picture where one man sits on the front of one ama and another sits on the back of the one on the other side -the deflection is reasonable. Now think about the folding kayak you plan to build, what would it do under the same load? Twist and crumple? This is why the Russians don't use folding kayaks in their trimarans.