Amateur trimaran design critique

For the hull to float on her lines it will need about 32 cu. ft of submerged volume. However, since the amas are not submerged, their weight and that of the beams have to be added. I understand the allure of designing your own boat. However, experienced Naval Architects have several years of design experience, plus four years of University, before they would tackle a design of this complexity. Building the boat alone will take you at least a decade. Why add another few years to end up with a design that will probably be mediocre. As a test, design and build a 3.5 meter skiff and take it sailing. That will give you a better idea of what you are getting into. As a bonus, that will be the tender for your trimaran.

 

I appreciate your concern, but I started making a design of my own more for my own entertainment and as an opportunity to learn than any practical reasons. I won't be able to start building a project like this for many years to come anyways, so I may as well fiddle with the design to the best of my abilities in the meantime.

In one of my previous responses I think I did point out; if I ever do build a boat, it is not at all unlikely that I end up deciding to buy a set of plans to use instead. Like you and the others wrote; it is realistically going to be the best option.

Regarding the 32 cubic feet; if my understanding and conversion is correct, the submerged volume would actually have to be more than double that when the boat is floating at full load, a.k.a. full displacement? Or I have I misunderstood some of the terminology? English is not my first language, and on top of that, this is a technical discussion, not conversational chit chat.

The size of the submerged hull at my estimated waterline is 8m×0,8m×0,4m (~26'×2'7"×1'4") but the ends are obviously tapered,with a crescent shaped bottom which rounds up towards the sides. This ends up being ~1810liters/64cu.ft. according to the CAD-model.

 

Sorry, I'll use metric . However, multihulls are usually very weight sensitive because of the narrow hulls. The exercise of calculating weights will serve you well if you ever build a boat. It is common to see boats perform poorly because they had too much weight added during construction. A careful account of everything that goes in assures the boat will float on her lines.

 

While I agree with those who say buy a plan already designed by an NA if you want to build , just in case you are another teenage Lock Crowther I will also say keep designing . It would have been a great loss to the multihull world if someone had told Mr Crowther to buy a plan from an NA when he was a teenager . There is enough experience on this forum to help you , and allow you to design a boat , people here may not always agree with each other , they may not always be encouraging , but if you are persistent they will help . Money and balls to build your own design is another matter .

 

Mr M, some years ago a gentleman here designed and built his own power catamaran for commercial use - he intended to take passengers on coastal cruises with it.
He put a huge amount of effort into building it, with high quality workmanship - however he did not know how to calculate the volume of displacement, and he never drew a lines plan - he pretty much built the boat by eye.
And needless to say, he never did an accurate tally of all the weights that went into this boat.
He already had bookings for cruises at Christmas, and the boat was launched amidst much fan fare a week before - and that was when he found out that he did not have enough reserve buoyancy in his hulls.
Indeed, there was only a few inches of freeboard on the hulls, and the deck was almost awash.
He had to quickly haul the boat out again, and totally rebuild the hulls to significantly increase the buoyancy available.
I am only posting this to help illustrate the importance of having accurate figures for your displacement from the lines plan, and an accurate estimate of your weights.

 

Thank you for the story. Don't worry, I never had the intention of beginning a build without doing calculations. I just wanted input on the general concept, shape, lay out, look and hull form before I put more effort into it.

 

@MrMillard, after some comments that have been, I think, a bit discouraging, I would like to support you in your endeavors. Everything you have done so far is the same as what a professional designer would do on his very first step. Your General Arrangement plan is as good or better than many that I have seen in my life as a professional. You simply have to carry out some calculations and clarify or complete the SOR of the boat to correct and improve your project. So take heart and move on. If there is some calculation that you do and you want some of the "nice" members of this forum to help you, I am sure some will do it, if you raise specific questions.
The first thing you should do to continue is define the body lines plan and calculate the submerged volume for the water line you drew in your GA plan.

 

Good attitude. Here are three things that stand out to me looking at your design.

Overall it appears that you see a trimaran as a bunch of attributes -that it will be good if you have all the right things. The truth is that a trimaran is just an opportunity to get higher performance if you do loads of engineering work. If you don't do the engineering you will end up with a boat that breaks, flexes too much, is too heavy to perform well, or some combination of all of these. Keel boats have limited righting that is well defined and lower sensitivity to weight. Trimarans get their power from large form stability and the large distance between the forces mean that you can't ignore dynamic effects. It's not just strength through every load path, you need to know where all the weights are and the stiffness of those paths

The unstayed mast is a bad choice for a trimaran of significant size and it completely changes the load path, so any ideas you have about size and weight based on successful rigged trimarans don't apply. The problem I see is you have jibs but no way of generating luff tension. Even the mainsail luff is a challenge with the forces a tri can generate -the tip deflection will be large. Do some calculations an tell me what you think the material will be and the diameter where it passes through the roof.

The last thing I see is more accommodation volume than boat volume and complete absence of structure. This is a 30ft boat with more accommodations than a 32 -why do you cut the bow volume away to make it 26? Normally there needs to be a bulkhead at the forward crossbeams. I don't see or understand your plan to keel step the mast, but then there is no reason to do so. The floor has nothing to offer. The lateral and bending support all comes from the ama through the aka.

I hope you can see this as 'constructive' more than as 'criticism'. Look up "free body diagram", "simply supported beam formulas" and "catenary curve". Is this the kind of "mental exercise" you would enjoy doing for a few years? Or do you just like to doodle boats around a nice kitchen so you can dream about nice views from this happy place? If it's the later you would be better off drawing keel boats or trawlers.

 

Hi MrMillard,

I am afraid that people have forgotten the inner fire they once had (some dont even remember they had one) .
I understand your dream and this was my dream as well. It is hard to find people who arent negative about new things, who like to experiment and who want to explore new horizons.
If it wasnt for the dreamers explorers and pioneers, we would be sailing/building monohulls.....
I would imagine that when the first pioneers trying the catamarans and trimarans were ridiculed by the "traditionalists" and monohull builders. Only after they accomplished and succeded their vision, their ideas were adopted.
Its the 100th monkey effect. If enough people try it then everyone accepts it.
What really grinds my gears is when those dreamers explorers and pioneers become the espablishment and then they do not think outside of the box anymore, they become those who ridicule and try to oppress people who would like to make changes or try something or experiment .
What i would suggest is to either find some naval architect and help you with your quest, or search for an equally dreamer and passionate naver architec student who also likes to experiment. Try a small scale and then go bigger.
If time doesnt allow it then go for the naval architect because i dont expect any cooperation from some designers especially some their cult zealot followers.

They dont understand that every boat has its pros and cons, and what is a pro for me is a con for thee. People dont understand that, they tent to be fanatic zealots arguing "our boat is better" blablabla.
What i also really cant understand is the fact that if someone builds something that looks or has similarities with something that an other designer has build, is considered "imitation-copy cat" etc.
I mean, i see the evolution and inspiration between the desingers , many of them have either worked together one way or the other and i am sure deep inside they enjoy other designers bulds and not only that, but that (not always) triggers their imagination and they build something that is inspired by that idea. We are evolving species after all.... if you dont evolve, you disapear. But ego is hard thing to keep in check.

I cant imagine life without bicycle, airplane, cars, electric tools....i mean we use epoxy instead of oil and tar for Gods sake....
Keep your dreams up, and try to find people who share the same flame. Listen to what those critics because that is essential to keep your feet on the ground, just dont let them rip your wings off.

cheers

 

The laws of physics are what they are. Wishful thinking does not supersede them. If you believe we are fanatics, give the OP some structural and hydrostatic advice to improve his design. Maybe, if as you say, his dream design is fine, help him build it. Talk is cheap. We build and design boats that operate successfully. However, I will retract my statement if you show any of your designs or construction examples.

 

Reality is what it is, not what everyone wants to tell.

Talk is cheap, there I give you the reason. Who is "We", if it can be known?. Are we to believe that you are included among those "we"?

I will change the image I have of you if you show any of your designs or construction examples.

 

I dont know if the shoe fits but you clearly havent read the following.

I also have to add that the indonesians and all previous civilisations and cultures didnt had naval architects when they were crossing oceans, they had people who had knowleadge on building boats who had experience.
Personally i went to a naval architect and aske them to check if the changes i want to do are safe since i didnt got the answers/help i asked for from the designer. But when the plans cost a couple of hundred dollars, and the naval architect asks 12k $ yeah well hm.... so i guess i am going to have to put my life at risk and follow the way of our ancestors. As we change modify and personilize our cars , same thing applies with boats.

If i was a designer, i would revisit rethink redesign (older designs) and most of all...LISTEN to what the market wants and needs. Maybe that will bring more clients to your business.
I see lots of people who are interested in small trailable cats/tris and also lots of interest in the 28-31 ft length. Of course you cant please all people, but for example, i love the fact that Kelsall has different variations on his boats, and i also like the ed horstman has version with vertical and normal flared bows, woods has hard chined and round bilge options, and Wharram has different sail configurations and beams . I also really like the fact that Wharrams desinged a new boat, Mana 24.
Hope it does well. Its doesnt fit my needs but im really happy of the new kid in the wharram block.

I will never ask nobody to retract their comments. I stand behind my statements . I might explain something if my meaning isnt clear but will never delete or retract. I also dont have the need to flash my credentials , nor have i the need to see yours and frankly i dont see why you should be bothered with my opinion. I thought when people like me fail, we are used as means for designers and naval architects to show(and scare tactics) to their potential clients what happens if people dont ask the help of specialists.

There is a great channel on youtube called Sailing Ruby Rose were they talk with a french naval architect who gives answers to lots and lots of questions.
Also there is an other channel called DMS | Marine Consultant were he explains various desings and other stuff.
You wont see Sven Yrvind boats anywere in the market, but he did sailed around the world with them.

Here is an idea, how about you make a DIY got talent thing where people come with their designs and you see if that would make it to the naval architect drawing board ? How about you pioneer a bit with your years of studying and working as a NA ?

 

To design a boat one must first develop the skill,s to do so , I think everyone agrees with that , but the route you take to do so doesn't have to go though a NA degree . George Buehler is just one designer among many that designed and built boats , big boats , and all of those people that spent big money to build them , spend countless hours of enjoyment sailing them , all must have been fools to have built boats in that size range not designed by a NA . I like to read the bio,s of some of the designers whose boats I admire , they range from self taught to Collage educated NA,s . the one thing all the great designers have in common is a love of boats , and the dream of boats . The OP can learn the math online , he can learn much from this forum , both the positive and negative , as sometimes it,s better to learn how not to do something , or what thought process not to pay attention to . Professional,s such as NA,s and Engineer,s are an irreplaceable asset when you need them , but how much of that asset you require depends on the skill sets you develop and risk,s you are willing to take on your skill sets . There are God like Professionals who think they can do no wrong , and there are those that will take the time to explain to a Carpenter the moment of a beam , or why a bulkhead is where it is . Those Professional,s who are not God like will answer the phone 20 years later when the Carpenter turned Superintendent call,s and tell,s him something is not right , and say good catch , how do we fix it .

 

First of all; I didn't want to begin with mentioning this, because I feared the discussion would quickly become more technical than I had intended and am capable of following, but since it turned out that way anyhow, here it is; I'm studying to become a mechanical engineer. So while I don't know much about the physics and mathematics involved in boat building yet, in time, the goal is that I will.

@Skyak, it may not have been your intention, but your post comes across as somewhat condescending, especially the last part. Of course trimarans are generally more complicated than monohulls, but they are nevertheless pretty well understood. As I have stated earlier, I never had the intention of building a boat without having proper calculations done.

Regarding the freestanding mast; you say that it is a bad idea, but give little explanation as to why. There are plenty of multihulls with freestanding masts, including trimarans. What is "significant size"? Without the bowsprit, my concept is only 26 feet, I'm only guessing, but I think most would classify that as small, possibly medium. I'm still a novice, but I've done some quick reading so far; jib luff tension may prove to be sub-optimal without any backstays, but that is a flaw of free standing masts in general, not this particular layout, no? The jib is a small part of the overall sail area in this configuration and its steep angle to the mast top should reduce bending of the tip, no? I may even consider tilting the mast a few degrees aft if that improves matters. As for the mainsail, it has a top haul, a peak haul and a down haul, shouldn't those be enough to generate luff tension? I don't know enough to calculate forces on a sail yet, but from studying other designs and dimensioning up until proper calculations can determine optimal sizing, I have used a 20cm/8" carbon fiber tube as reference. For another design using standard carbon fiber tubes, see "Troublemaker 9500".

My own arguments for having a freestanding mast are as follows;
- Simplicity. While the construction and installation may be more complex, maintenance and usability should improve significantly. Multiple small (but critical) failure points are traded for one large one. Most modern trimarans have a stayed rotating wing mast with spreaders and articulated boom. That is a pretty complex arrangement for a beginner designer/sailor. A freestanding mast with a luff pocket acting as a "soft wingsail" should give adequate performance with a lot fewer parts.
- Load paths. My understanding here is very basic, but the idea is this; usually a trimaran has sidestays connected to one or multiple points on the amas/outriggers. This increases bending loads on the akas/beams and requires additional calculations and reinforcements. The beams have to resist the buoyancy induced twist when the trimaran is heeling or dropping, and the stays would add additional forces acting in the same direction. In addition, when the structure in the main hull is strong enough to resist the bending loads from the beams, it should be a long way on the road to be stiff enough in the perpendicular direction, resisting the bending load from the mast. At least, that is my theory. It also eliminates chain plates and the necessary reinforcements to support them. Stays also present problems with folding geometries. How do you easily allow for adjustment between folded-out and folded-in mode without having the mast be unsupported in the transition?
- Safety. A trimaran is much worse at spilling wind in gusty or severe weather than a monohull as it resists heeling more. Having a mast that can bend somewhat mitigates this. It also makes sailing easier and more pleasant, because frequent and early reefing is less crucial.

I think I stated why I wanted the dimensions minimized where possible in the first post, but here are some more reasons:
- To fit in a 40ft container the mast can't be more than 40ft long. Rule of thumb regarding mast to hull length is about 3/2, which leaves the maximum hull length at about 26 feet unless you're willing to have a two masted rig (which I have considered, but would rather avoid).
- The hull is tall to provide ample headroom for me and still leave enough structural space for a strengthened bottom and roof. I may as well fill up the additional space with accommodation, which if unused weighs preciously little.

Bulkheads. I have never made a general sketch for a boat before, so I don't know how to properly indicate bulkheads. There IS a bulkhead at the forward crossbeams. In fact, all the partitions in the boat are meant to act as additional stiffeners/bulkheads, but that may not be obvious on my drawings. Assume at least 2,5cm/1" thick reinforced bulkheads.

As far as I know you have to keel step a freestanding mast. Your point about the floor only makes sense with a stayed mast, and since the mast in the drawing is unstayed, of course it is keel stepped.

The last paragraph I'm just going to ignore @Skyak.

 

MrMillard , I have the plans for a 31' Horstman tri , I wanted a cruising tri and it fit the bill for cruising -v- fast modern tri , I am no longer going to build that boat due to lost dock space . The problem with a cruising tri is lack of space in the center hull . Flare is one way to add space to the hull and you are showing flare in your design , but flare is limited to 8'6" in a trailer tri and the ama,s must be above or below the flare in a folded system and that brings in problems of a folding system . So my question to you would be why not a de-mountable beam system ? You are showing flares in the center hulls allowing for a fat center hull , why not take the top flare to the max 8'6" , design longer ama,s with de-mountable beams ?