Amateur trimaran design critique

[MrMillard]

Hi!

I'm a complete amateur who got the inspiration some time ago to design a boat I could one day realistically build should the circumstances allow it. After much doodling and many size and hull-shape changes I now have a complete sketch I am brave enough to share. I would like your honest opinions and constructive critique if you have the time and inclination.

I am a novice regarding all things boat related and certainly no naval architect, so no real engineering calculations have been done. Bear all this in mind and try to be nice, please!

First a little background:

Type:
I first decided on a sailboat for the some of the following reasons;
- environmental impact
- autonomy and endurance
- learning opportunities
- romantic charm

Size:
Having read how monumental a task it is to build, own and maintain a boat, especially a large one, I decided that small (subjective) would be most realistic. The smaller that I felt comfortable with, the better. However, I am quite tall and loathe being hunched over and would like to be able to cruise. These are my requirements;
- <11m/35 feet length
- at least 2m/6'6" headroom
- trailerable (<260cm/8'6" beam)
- fits 40 foot container for shipping
- at least 4 berths (preferably 6-8)
- good size galley
- on board shower and head

Hull form:
Except for the most obvious drawback of small boats, lack of space, I've read that they are almost agonizingly slow. This is especially true for cruising as opposed to racing designs.
So how do you make a small trailerable boat faster? You make it a trimaran! That is the conclusion I've found researching sailboats. Unfortunately trimarans tend to be even less spacious than their monohull siblings for a given length, so some out of the box design may be necessary.

Systems/equipment/rig:
A sailboat can be rather complex, and trimaran doubly so. Add in trailerability, easy shipping and cruising comfort and you have a lot to contend with. So whenever and wherever possible I've strived towards simplicity. Given my very basic knowledge, I may have misjudged what "simplicity" entails though. If I have perceived that there is a considerable gain in simplicity of use at the cost of adding some complexity of build, I may deem it a worthy compromise. This has led to the following requirements;
- sliding beam folding (not pretty, but simple and robust)
- all electric systems (cooking/heating/propulsion)
- outboard auxiliary propulsion
- freestanding mast
- Wharram wingsail
- self-tacking jib
- solent rig sloop
- integrated bowsprit
- daggerboards in amas/floats (debatable?)
- transom mounted, tiller controlled cassette or kick-up rudder

I'll go over some of the more unusual design elements here, and some of the reasoning behind them:

- very high freeboard. This is almost unavoidable if you want plenty of space and headroom. The deck is about 1.5m/5feet above the waterline.

- far forward mast-step. The idea is that you push the bottom of the mast through a tiltable collar mounted on the front crossbeams and then use a rope/line to force the bottom of the mast into a groove inside the main cabin/galley. Then you seal the hole left when the mast is in position with a form fitted board. So the mast slides from the bow into the hull through the front of the top structure and when it is ~2m/6'6" feet in you crank it upright in a 90° quarter circle motion. This lets the freestanding mast use the stiffness of the crossbeams for support, which wouldn't be possible if it was located further aft. Stepping a "keel-stepped" mast, freestanding or not, is nearly impossible without a crane if no tilt-mechanism is employed.

- combination wet head/corridor. A separate enclosed head on a small boat, and especially a trimaran with only standing room in the middle, is a waste of space. Having the area do double, or in this case quadruple duty saves a lot of precious space. The major drawback is that a lot of the boat becomes inaccessible when someone is using the head. That said, the same area now does all of this; gives access between the cockpit and inside, provides entry and dressing space for two mini quarter berth cabins, including stowage, acts as a wet room for showering and getting out of wet gear AND a slide out head. Having a wet capable compartment with drain between the cockpit and galley area also keeps the living area dry.

- three stage/section hull. The hull is made up of three distinct widths, like an upside down layer cake. The bottom provides a large length to beam ratio of 10:1 at the waterline and provides space for the demounted amas to lie down in a container. It is 80cm/31.5" in width. The middle part provides maximum beam while leaving enough space for the two amas to be pushed in for trailering without needing to be demounted. It is 180cm/6feet wide. The top part is the maximum beam you can fit in a standard container and provides additional room for stowage and deck space. It is 220cm/7'3" at its widest.

- full sitting height berths/cabins. Maybe not terribly unusual, but definitely not common on boats this size. By providing sufficient room to sit up, you gain a couple advantages; you don't risk hitting your head when walking up, you need a lot smaller lateral opening to get in or out and it is easier. You can use the berth for more than just sleeping, with a fold down worktable you can use your laptop or look at charts etc. The berth becomes an all day space for privacy, a much needed luxury on a small boat. In the front cabin, the ability to seat 6 adults in settee mode eliminates the need for a separate dining/lounge area, something that takes up most of the inside space in a typical small boat.

Some additional numbers (estimates and suggestions):
- waterline length 8m/26feet
- total length including integrated bowsprit 10m/32feet
- beam sailing 6,8m/22'4"
- beam folded 2,6m/8'6"
- main hull total height 215cm/7'1"
- draft 40cm/1'4"
- Ama/float length 8m/26feet
- Ama/float beam 40cm/16"
- Ama/float max height 110cm/3'7"
- Aka/crossbeam diameter 20cm/8"
- total mast length 12m/40feet
- mast above deck level ~10m/32feet
- mast max diameter 20cm/8"
- berth lengths 2m/6'6"
- weight ~1000kg/2200lbs
- displacement ~2000kg/4400lbs

Suggested basic building info:
- 25mm/1" foam sandwich hull
- aluminium/carbon fiber tube crossbeams
- carbon fiber tube mast

That's all I can think of right now! Fell free to ask questions and I'll try to update/answer accordingly. Remember, this is the pipe dream of an amateur, nothing more, nothing less!

P.S. I've drawn a lot of inspiration from other designs and/or designers and I've greatly appreciated this forum. If I were to single one influential source out, I would have to say Kurt Hughes.

 

[oldmulti]

Accept the following as a suggestion. Kurt Hughes made a comment one day the cost of a professional plan will be saved in the reduction of material costs alone because the professional knows what structure is required where. I fully agree. I suspect you have been inspired by: Kurt Hughes Multihull Design - Catamarans and Trimarans for Cruising and Charter - 32' D-32 Trailerable Trimaran http://www.multihulldesigns.com/designs_stock/d32tri-rapido.html

This is an excellent design and is fast effective and trailable (with a lot of people or a crane). The engineering in tris like this is not a joke. Unless you are strong at maths and engineering do not underestimate the loads. The hydrodynamics is also a real talent. It is not about the best guess, good understanding is required. Have fun.

 

[bajansailor]

Welcome to the forum Mr M.

I appreciate that you have put an awful lot of work into your design already - Oldmulti is being very polite re his comments above but I will be a bit more blunt.
Your best bet will be to simply buy a set of plans for a boat that appeals to you - and Kurt Hughes' D-32 appears to come pretty close to what you are after.
You want us to be nice - and I think that this is the nicest thing that I can say about your design.

I have a naval architecture background, and have been messing around with boats for 50 years, but if I wanted to build myself a trimaran, I would not even think about trying to design it myself - I would buy a set of plans for a boat that is guaranteed to work and perform as advertised.

 

[gonzo]

There is probably too much wrong with the design for a single post. However, the most typical amateur mistake I see, is to draw the waterline where it looks pretty. The weight of displaced volume of water, equals the submerged volume. I challenge you to calculate the weight of everything in the boat, including the hulls, structure, etc. , then calculate the submerged volume you drew and prove they match. That includes calculating the center of gravity and center of buoyancy.

 

[MrMillard]

Thank you all for taking the time to reply to this thread. I appreciate your concerns and sincerity explaining the feasibility of designing my own boat. Yes, I know that realistically buying an existing design is what I should do if I ever build a boat. Yes, I am a huge fan of Kurt Hughes d-32. This is a dream of mine and as such it may never even come to fruition. I was hoping to get feedback on my design even if it never amounts to more than a mental exercise. See it as a potential concept and tell me what you think are good ideas or bad. If I ever get the funds and time to actually build a boat, I may even consider hiring a naval architect to develop it.

That said; my rough estimates for displacement tell me that the shape of the bottom of the hull and a draft of 40cm/16" should be sufficient.

 

[TANSL]

However, the most typical amateur mistake I see, is to draw the waterline where it looks pretty.

I don't know what others do but I always start by drawing a GA plan with a water line where I would like the fully loaded ship to be.

The weight of displaced volume of water, equals the submerged volume.

I think that is confusing and not correct.

 

[bajansailor]

That said; my rough estimates for displacement tell me that the shape of the bottom of the hull and a draft of 40cm/16" should be sufficient.

Mr M, this sounds very optimistic to me.
How 'rough' is your estimate for the displacement, and what have you calculated it to be?
Is your waterline the line shown in the side profile sketch in your first post?
Do you have even a very rudimentary / rough lines plan for the hull?
Have you done a rough weights estimate for the weight of all the materials used for the construction, along with the weight of all the outfit items required?

I don't know what others do but I always start by drawing a GA plan with a water line where I would like the fully loaded ship to be.

Oh dear Tansl, you are getting into semantics again - that is what Gonzo effectively said, except that he used different words.

 

[TANSL]

Oh dear @bajansailor, then why does Gonzo says the OP has the typical amateur mistake?.
It seems clear that I have not understood something well. It is not a matter of semantics, it is a matter of Gonzo or I (or both) talking about what we don't know.
No, Gonzo and I are NOT saying the same thing. In any case, oh dear @bajansailor, why don't you let him explain?. I suppose that this should be a dialogue between him and me in which third parties cannot know what each one has wanted to say.

 

[MrMillard]

This design is still just a concept, that is why I only have hand drawn sketches. No, I don't have a lines plan. I have however started to make a CAD model (mostly for fun). The estimates I have made have been simple calculator calculations, but I can hopefully measure more accurately on the model soon. The lowest line on the side sketch is the flaring out of the middle part of the hull at 60cm/2feet from the absolute lowest point on the hull. The waterline I have imagined would be at ~40cm/16" from the lowest point, so about 20cm/8" beneath the flare out. The waterline is not drawn on any of the sketches, I'm afraid. I hope to have a weight of ~1000kg/2200lbs and a displacement of max ~2000kg/4400lbs, which I wrote in the first post. The fitout will probably have to be pretty spartan and lightweight to achieve this.

 

[MrMillard]

My CAD model is still very rudimentary (it lacks interior, details and superstructure/cabin top), but I have taken some measurements.

Hull volume (displacement) beneath the waterline on the main hull is 1810 liters (1,81m3) assuming a draft of 40cm/16". Together with the ama volume beneath the waterline, 86 liters (0,086m3) each, that is a total displacement of 1982 liters (1,982m3). That is fantastically close to my 2000kg/4400lbs guesstimate.

Total Ama volume is 2429 liters (2,429m3) each.

Total hull volume (excluding superstructure and cockpit, which will somewhat cancel each other out) is about 21,5m3. Total hull surface is about 64m2. With a hull thickness of 25mm/1" everywhere I get a total hull skin volume of 1568 liters. Having read about typical foam sandwich weights this puts the main hull structure/skin at around 250kg/550lbs. This to me feels like the total weight goal of ~1000kg/2200lbs isn't totally unobtainable.

Center of gravity ends up about ~30cm behind the middle of the boat. Bear in mind this is calculated solely on outer hull structure, not interior fitout.

Center of buoyancy I don't know how to reliably calculate at this point.

 

[gonzo]

The main hull, deck and structure at 550 lbs would be really hard and expensive to achieve with exotic materials and highly skilled workmanship. For really light weight, the hull will have varying thicknesses depending on the stress expected on each area. How did you calculated the weight of all the interior, hardware, water, fuel, etc.? Also, what do you mean by total hull volume will cancel each other out?

 

[MrMillard]

No, I haven't calculated the weight of interior, hardware, water, fuel, etc. It is still in concept stage. I have looked at similarly sized boats and their weights and have taken a best guess at where this might land. As for the 550lbs figure, it may not be achievable, but that is just the outer main hull estimate using the total area and a suggested foam core sandwich panel weight of about ~4kg/m2. The hull volume I mentioned may not be important from a construction perspective. It is just the total enclosed volume inside the main hull. The inside livable/useful space should be roughly the same, as it is the main hull plus the superstructure (the top part with wrap-around windows and roof) but minus the space taken up in the main hull by the cockpit, which is essentially "scooped" out of the back part of the hull.

Forgive me, but the question posed by this thread was "what do you think about this potential concept?", not "what is the best/easiest way to build a trimaran?" or "here is a complete design from my engineering firm, please check and confirm my work/math?". Because the responses so far imply the latter more than the former...

 

[bajansailor]

Forgive me, but the question posed by this thread was "what do you think about this potential concept?", not "what is the best/easiest way to build a trimaran?" or "here is a complete design from my engineering firm, please check and confirm my work/math?".

Re this concept, it should work if you closely follow the example set by Mr Hughes.

But looking at the end elevation sketch, notice where the bottom of the outriggers are in relation to the main hull. If they are even kissing the water, then the main hull is immersed in the wide area - but you do not want it to have so much draft, you want it to be like the Hughes tri, where only about half of the skinny section of hull is immersed (as per @oldmulti lines plan in post#2) .

 

[gonzo]

Many people make a drawing with the expectation that a boat can be built out of it. There is a difference between a drawing and a design. A design starts with stating what the purpose, requirements and constraints are. For example, the drawing shows a huge volume above the waterline that needs to be supported by a tiny submerged volume. The weights of everything in the boat will drive the design. Other constraints may be maximum draft, minimum headroom, minimum beam at shoulder level, maximum beam, etc. The laminate also is a function of those. Your original post lists quite a few of them. The first step, before drawing anything, is to determine any conflict between your requirements. The main conflict is the unusual light weight you want. Add the weights of all the stuff you want inside and see what you come up with.

 

[MrMillard]

Thank you for the two last posts, now we are getting somewhere. I appreciate your patience. Sadly, a sketch with an obvious waterline and the amas in the right position did not fit on the single piece of paper in 1:50 scale. I will make one at earliest possible convenience. I will also make a list of all the interior structures and fittings to the best of my abilities and try to add it up.

Meanwhile; the amas are tilted backwards, at the same angle as the top part of the hull. That said, the "backplate" should still be a good distance above the water when the trimaran is floating level in calm water. The end of the ama bottom curve should be about water level only when the trimaran is aggressively heeling. I think that is how amas should be shaped? If I have misunderstood your point or got this wrong, please explain.

Regarding the light weight. Even if I am way too optimistic, and the weight ends up being let's say 1200kg/2640lbs (which is as much as the d-32 at two meters longer) that still leaves 800kg/1760lbs available as payload at 2000kg/4400lbs displacement. I can live with that. Is that a correct understanding of your concerns?
Thanks again.