Advice needed on trimaran

It's not a problem to lessen the beam, then... I just thought larger beam would help with heel.

Well, actually, the only place where I will be able to storage this trimaran (folded) is small basement, about 2.5 meters ceiling. I didn't think of this earlier. So don't mind what I said about 3 meter parts before, they wouldn't fit. So mast could be 2.5 meters. I don't think that qualifies as mast...

Are you sure that would be cheaper? Because I would have to buy some special PVC material which is airtight, there will be need for special glues, valves... Not to mention that if I make a mistake (even a smallest single gap) it will leak air, or worse, water. Could you comment, please?
I'm ok with fixed keels on amas.

Okay, I was off in my calculations, then. I'm ok with increasing weight as much as needed, but then again, I need to increase displacement...

Well... Then I'll have to find another supplier of tubes, since that which I mentioned doesn't have larger tubes which are close in diameter (for fitting).
What would you say could be enough diameter for main hull in this cost range, considering amas inflatable?

As I mentioned before in this post, I was mistaken. I hoped that most parts apart from vakas and mast could go into car's trunk, but since I'm unrealistic with budget, I guess all parts will have to be longer and travel on car's roof. So they may be up to 2.5 meters... But I guess something like 2 meters would be more reasonable.

Well, I think I could manage another 100$ for this... And I forgot to mention, I have a sewing machine. Old, but does it's job.



So, considering I leave the same length (for now), make amas inflatable, increase main hull stringers to, let's say, 2.5cm in diameter, make all parts breakable no to 1.25 as before, but to 2-2.5 meters, use fixed keels on amas, how closer I am to the 400$ budget? (If I'm still off a lot, maybe I can sacrifice main hull - make it inflatable, although that means loss of a lot of space, and next thing will be reducing length).
I know that exact numbers are impossible to tell, but even approximate would be a lot of help.

I'll try to make that spreadsheet. Not an easy thing when so many numbers (like prices) are missing.

 

Alright, after 4 hours of work (first time in Excel) I'm now attaching spreadsheet to this post. It is far from complete - but I managed to program several important calculators: stringer number, joints, diameters, with all hulls - or just main (considering amas inflatable). All numbers react to each other, and calculate total weight, amount of parts, and cost.
There is also a calculator for PVC skin. It is very approximate, though, because exact hull shape is unknown. It reacts to hull length, depth, max beam. It assumes that chine at beam is in exactly between box shape and V shape. It also checks if design doesn't exceed PVC fabric roll width.

Please check it out. All yellow cells are for user input, and EVERY number (as much as I tested it) is included in some calculation. Try out different combinations, like increasing ship length, number of skin-on frame hulls, foldable parts length, hull dimensions and so on. It should affect total cost (at the bottom of the page), and warn if some things are out of limits.

The only thing I can't figure out is how to calculate displacement knowing hull length, beam, Block coefficient, top-down and side-view fullness of the ship (by the way, I thought this up, it seems a very good way to calculate things approximately: take a ship's design, top-down view, draw rectangle around which touches all sides of the ship, and compare it's area with area of actual ship silhouette. The same with side-view. It helped me a lot with PVC calculations). If I could figure out a way to calculate displacement, I could see on-the-spot how it affects final cost.

I know that there are a lot of things missing, like crossbeams, cross-sections, screws and joints, but I didn't figure out a way to calculate these... yet.

Please comment if I'm going in the right direction.

P.S. Oh, and don't mind currency calculator.

 

My intent in questioning many of your requirements is to find some that might give a little so that other can be met (mostly cost). Structurally, I can do a lot with 2.5M pieces. 2.5M might not be tall enough for a mast, but a 2 piece mast is much simpler than a 3 piece. With 2.5M pieces I can make a 5M folding hull and only join it once in the center.

I think inflatable amas with external frame will be cheaper and simpler because the frame can be just 2 sticks that don't have to be smooth, and they don't have to be assembled inside a sealed space. You are right that there are quality concerns with inflatables. My intent was to have an internal liner so that you would not have a problem with small leaks.

The truth is I have never seen a folding ama. I don't know how one could be made with less than 4 stringers and 3 frames. I don't like the idea of a folding hull that can't be bailed.

I can think of a design that would carry 250Kg of crew and cargo at over 6 knots, break down to 2.5M max, and cost less than $400 (based on your $100 8Msq dacron sail). The Vaka would be a folding kayak a little over 5M long. It would use 4mm marine ply for the floors 2.5M long fore and aft. I will try cutting a model out of cardboard.

 

Using airtight inserts is real PIA, because of problems with sand, water etc. getting between two layers.
It has some sense if you choose the cheapest construction for inflatable hulls: not-airtight outer shell from strong fabric, which creates form of the hull, and inflatable insert made from doubled PE-film tube.
http://eng.accon.se/Products/Film-Foil/PE-Tube
You will be plagued by constant small leaks, ugly hydrodynamics, but this construction is cheap (at least in Russia) and fast to build.

 

I appreciate the advice. My recommendation is to use PVC impregnated polyester weave ~1000 denier. This material is common in mid-range RIBs without a liner. We can use it without a liner, but I anticipate that after a few years, tiny leaks will develop due to folding stresses in a composite material and some UV hardening. A PE liner could be fitted later, after this happens (or not if it doesn't). This liner will not be composite, will not have UV exposure, and will not be stressed by inflation. I don't see a problem with sand getting between these layers, since the outer is essentially sealed.

These concerns do influence my preference for folding, not inflating, the vaka.

laukejas,
I am looking your spreadsheet over. I was afraid you would not have excel. It turns out your copy is newer than mine.

I will try to add some force and deflection calculations, and may add some materials with the prices I get here.

 

Well, as I said before, 2.5m is absolute maximum, I would prefer not to go over 2m parts unless it turns out absolutely necessary.

I don't get what is the most expensive part of my design. As you may see in spreadsheet, currently I came up under 160$ in all hulls skin-on-frame scenario - that is stringers, PVC skin, and epoxy for joints (some will be left over for laminating crossbeams, I think). I know that cross sections are missing, well, add another 50$ - and then there're screws, joints, and so on - another 50$ at most. Then there are crossbeams and net, maybe 70$, and rudder + daggerboards - let's call it another 70$. I'll chose exact stringer lengths so that there won't be any waste. There will be some waste with PVC, maybe 20%. But all that is still a bit under 400$. Of course, I'm guessing these numbers based on experience, so I might be wrong, I'll try to calculate more reliable numbers in spreadsheet, but what I mean is,
What am I missing? You said that my design is factor 10 off my budget limit. I imagined that stringers and PVC will take most of the price, but even if I use stringers that are double this price (larger diameter) it's still just a bit out of budget.
So what is the cost-hungry part that I'm missing here?

I'm not sure how I would achieve any kind of hull shape with that tube inside outer skin. I'll check Yostwerks again. Thanks for the suggestion!

I'm not sure if I mentioned it before, but that 400$ is for hull + rudder + daggerboards only. Mast, sails, rigging has separate budget - around 200$ (not sure yet, need to calculate how realistic is that).
Thank you for your time!

 

The work is going very well, at current state it seems that I will be able to achieve all hulls-folding and two hull flying condition under my budget. Right now I'm modeling outboard hulls, and I came up with these numbers:

in one sailor scenario (total displacement 180kg), with two hulls flying this trimaran will achieve a maximum hull speed of 6.6 knots IF sails generate forward force of 1.28kN.
I'm not sure what kind of sail area do I need to generate that kind of power, maybe someone could give me some reference in similar designs? Is that much?

There is one thing I'm considering. Since outboard hulls hydrodynamics are best with heel angle of 0, but to fly two hulls ship needs heel angle of 9 degrees (maybe 10 to get main hull clear of waves), I'm considering attaching outboard hulls at an angle of 9 or 10 degrees, so that when trimaran is heeled, the leeward hull isn't heeled, and performs with best hydrodynamics (least forward and most lateral resistance). I've seen this design in larger trimarans, but I'm not sure if there some draw-backs. Could someone comment of this idea?

And one more question: I red everything in Yostwerks site, and Yost insists on using vinyl cement HH-66 for gluing PVC fabric. However, I googled, wrote some e-mails to various companies, and found out that there nothing even close to vinyl cement in Lithuania.
Which leaves me with two options: buy from e-bay/amazon (which costs a lot), or use different glue.
What about epoxy? Would it work with PVC?

Maybe someone has Yost's e-mail? I could ask him about this, but I couldn't find his contacts anywhere.

 

Don't use epoxy, use localy available glues for PVC. If you can not find glue in departmment store, ask in your local shoes-repair shop. After some training you glue joint will be stronger than PVC film on fabric.

 

Well, I wouldn't even know where to look for these glues, because even the PVC is only sold in one store in whole country. So why would someone sell these glues just like that.
Maybe you could be a little more specific on brands and models which I should look for? I don't want to end up with choosing something untested.

 

For example, you can look for Desmocoll glue with Desmodur hardener by Bayer or others, or some mono-component PU-glue like HH-66? you mentioned, or UR-mono (УР-моно in russian, don't know is it available in Lithuania).
Ask in local shoe-repair shops, PU-glues, used to attach soles, are the same as for PVC fabric.
And before going to skins for hulls make some training on smaller items. Something like flotation air-bags etc.

 

I'd look for Methacrylate. It's similar to epoxy, but bonds better to other plastics than epoxy (better than epoxy to fully cured epoxy, but not better than an epoxy primary bond) and is already somewhat thick, so it's not necessary to add thickeners.

Near me the big manufacturer is ITW, which makes two brands, Devcon and Plexus. I think the Plexus MA310 is pretty much the same as Devcon Plastic Weld. Devcon is available in smaller quantities (such as small tubes) and is carried by many hardware stores. Plexus sells to businesses like established boat builders. There are probably European companies making it and boat builders using it. Try this link: http://www.sika.com/dms/getdocument...ized_Structural_Adhesives_Boat_Production.pdf

 

Hello again,

The project is going well, but again I'm holding back because I lack some knowledge.

I was hoping that someone could tell me how I could solve these few problems:

1) Center of Lateral Resistance and Center of Effort considerations. I'm not sure which one should be aft, and by how much. Most sources say that CE should be aft of CLR, but some say otherwise, and none say by how much.
I know the basic relationship, that if CE is aft of CLR, then the boat will tend to turn into the wind, and vice versa. But I also heard that due to heeling, additional rotational forces influence this.
Is there some kind of formula or something by which I could plot where should I put CE and CLR in trimaran?

2) Beam, outrigger volume, sail area, and wind speed relationship. There are many explanations with monohulls, and some with catamarans, but I can't find any formula that could tell me the relationship between these with 3 hulls.
I know that I have to decide at least on some of these values: I know the displacement of outriggers, I know the wind speeds that I'll usually sail in, I know the beam length, but how do I convert that into sail area?
If I knew some kind of formula, I could also decide on reef points.

3) If outriggers are significantly shorter than main hull (and are only to help with stability), won't trimaran actually be slower? Since hull speed is dependent on hull length, and there are 2 or 3 hulls in the water at the same time, the shortest hull will limit the total speed, right?
If I'm wrong, and trimaran can sail past outrigger hull speed, what is proper way to determine maximum trimaran hull speed?


I have searched for weeks for this information, and I'm still not sure about anything, so if anybody could provide some information (link to some articles which I could dig into), or explain in own words, I would have overcame one of the last problems of my project.

Thank you for your time.

 

for item 3 the issue is heeling moment. I suppose similar sized hulls dragged through the water, the tri would have more drag, but you can carry a lot more sail on the tri. the wider stance means you can have larger sails and the tri design can better resist the heeling of the larger sail, so the combo goes faster. A mono-hull will heel over and spill all of the wind and have less drive. The wider the stand, or the lager the ama, the more resistance to heel you will have, the larger a sail you can install and the faster it will go in the same wind.

As far as item 1), I have always understood for best performance you want to match the location of the CE and the CLR, this puts less lateral load on the rudder, reduces trim drag. with the CE and CLR in different locations you will either have lee helm or weather helm, and some pressure on the tiller. Some feel a slight amount of weather helm is desirable so if you drop the tiller the boat will round up into the wind, reducing speed and heel, the other way could get out of control and result in an uncontrolled jibe (very bad).

That is the way I understand it, other may have different opinions. So take your pick.

 

Thank you, but I kinda understand these basic principles... It's just that I don't know how to convert them into numbers...

 

simplest method, is approximate but close enough for small boats, is to accurately draw the profile of the boat, with keel/dagger board, and sail plan. find the centroid of the area above the water line (including sails and hull), and the centroid of the area below the water line. You want the centroid of the areas both above and below to match, or to have the one below the water line slightly ahead of the one above (2 percent of length).

That is all the calculations you need to do. you can even do it without any calculations, cut these shapes out of cardboard or similar material, and find the balance point for each.

If you want minimal rudder forces you can leave the rudder off the centroid calculation, so the rudder does not take any lateral forces except when using it to turn the boat.

Heeling moment is even easier, the max righting moment you can get is the displacement of the ama times the beam length (from the center line). And the sail force you just have to adjust as you sail, large wind reef and reduce the amount/size of sails up, low winds, you put as many sails up as you can. all the forces are based on the max righting moment you can achieve with the hull. for detail design you can assume your sails can generate about 1 lb per square foot of sail area (plus a 1.5 safety factor), should give you a pretty good boat that is not too heavy.