Amateur trimaran design critique

@peterAustralia

Was the "sigh" really necessary? Or am I misinterpreting its meaning?

Anyways, I'm grateful for critique with specific pointers (or A point at least). I know that the flat chine/flare is a potential issue/downside, but it is unfortunately a must to make the interior layout work as planned. Most trimarans have way steeper hull sides, true. This concept is a little different, maybe downright ugly from some angles, but the design is utilitarian first and foremost. After all, there are plenty of slender, sleek trimarans in this size available, so what need would there be to design another one if that was the boat I wanted. You'd probably want to slow down in a seaway in a cruiser anyway, so the extra drag isn't that terrible, but maybe I'm thinking about it wrong.

If you have anything else to add I'd be glad to hear it.

 

sorry about the sigh, I work in construction and today was very hot and humid and we were working outside and I got very tired.

Yes I know you want the extra space and beam of the wider cabin. My issue is that the wider cabin is only a few inches above the waterline. In any waves, the wider hull will be going through waves all the time, I think there was a large hobie trimaran (could be wrong about the name),, that used a wide hull, and trussed folding crossbeams,

The way they married the wider top part of the hull with the narrow waterline was to use an angled topsides to mate the two different widths

As to trimarans that use the hard chine method, with the use of an overhang to combine narrow width at waterline, and the wide beam at topsides, the only one that comes to mind from a quick think is Nicky Cruz. That one has the overhang much much higher above the waterline

If I am serious about the design, say your are sailing along in a medium sea, nothing huge, maybe a three foot swell, that wide cabin is going to have waves passing around it on every wave, slowing it down.

Every trimaran I am aware of tries to keep the hull narrow above the waterline, for a good 14 inches at least, simply in order to allow the boat to travel at a reasonable speed. I have attached below a photo of a 12m trimaran, and how they attempted to match narrow waterline beam with large width at deck height

 

Also I am a little confused, why such high freeboard at the bow and stern, why is the stern so wide? why the step thingies where the crossbeams meet the outriggers, surely they just add weight and windage. The hull is so high, windage will he huge, making it a slow boat. You still have a full underwater section only five feet from the transom, thus water has to move from its full width to zero in only five feet of length, thus more drag. It might work very well as a power boat on lakes and rivers. A rule of thumb is that a catamaran needs at least 6 percent waterline length as clearance. You are a tri, not a cat, but it gives you a rough starting point... well good luck, but i have to go to sleep now

 

I had a look at your diagrams

From keel to deck at the bow is 6.8ft. Why do you need standing headroom at the bow?

At midpoint you have 8ft from keel to deck!! You have a flat bottom, your feet can be placed there. All you need is say 5 inches tops, realistically 3 inches, as thickness for your hull bottom and that is being generous, add say 6ft 1 headroom, that gives 6ft 6 inches. So you can still have standing headroom with the cabin being 18 inches lower. Whilst at it, lower the bow deck and stern deck by 18 inches. Doing this will decrease weight and windage, I see that ou have a stowage compartment under the companion way, why not eliminate that and have the companion way located as low as possible

Also the line where you have the hull at full width, that is at the same height all along the hull, why not have it curved, so that at the bow you have more clearance

 

I think you are going to have to make some compromise,s unless sailing ability is bottom of your list . Ed Horstman's designs are sometimes called Roomerans , the small cruisers are at about 8/1 on their beam . 8/1 beam is still going to get you there faster than most mono,s , go below that and you are starting to look apples to apples to a mono as far as speed . The closet design I see to compare your hull form to would be a seabright skiff , or something with a box keel . A seabright skiff hull form on a tri might give you a wider bow with more room , it might make for a good cruising tri , but you will still have problems with wind if you don't compromise on your upper structure . I think you should refine the hull form and start looking at where you can give up head room and space . Look at Hartley trimarans , they have wide beams on the center hulls for a tri , the 30 footer is I think 10' . You may need to think of a mono concept with amas's more like the Hartley .

 

Have you sailed one ? In my experience the condemnation of the Horstmans come either from badly modified boats or from people with no direct experience of them. They aren't racers but everyone I've spoken to who has really sailed one says they are ok boats.

EH is supposed to be an aeronautical engineer. From what I can see of his boats he's done some clever things to combine high accommodation volume and low drag.

 

guzzis3, I have not sailed on one, but I have bought the plans for the 31 and enough material to build it, but I won't, life changes. In my view Ed's designs are the best cruising tri designs around, so I'm not knocking Ed. I was trying to show where Ed's cruising designs are at the low end of speed and high end for room, the flair on the Op's design will most likely slam or become the bottom of the hull if overloaded; it will be much greater than 8/1. If the flair were re-designed into a wide hull with a box beam, or a seabright skiff design, I think it would preform as good or better than the Hartley, a mono with amas.

 

@MrMillard have a look at this trimaran design posted by @oldmulti on his long running thread Multihull Structure thoughts.
Multihull Structure Thoughts https://www.boatdesign.net/threads/multihull-structure-thoughts.62361/page-160#post-924408

OK, this trimaran has much less accommodation than what you want, but the hull design is a lot better really - maybe you could modify your design more like this, with the flared width higher above the waterline?.
@peterAustralia was justified in sighing, as he was worried about the design that you initially proposed, and with good reason.

 

Hi again!

First I'd like to respond to some of the recent posts:

@peterAustralia
First post:
The 12m trimaran you posted a picture of seems to share design philosophy with mine. I'm having trouble finding any info or other pictures of it, could you tell me where you found it or any info you have on it? The Nicki Cruz designs I could find through a Google search seem to feature a flare parallel to the water's surface. That might avoid some additional wetted surface, but wouldn't it make slamming worse? In addition, as soon as the boat heels (which is often), the flare would create a dipping "V"-shape with a higher air pocket between it and the hull, that seems inefficient to me as the wave would hit 3 surfaces instead of 1 if it was at an angle?
Second post:
The high freeboard was to enable generous accommodation below, in the bow to provide sitting headroom in the front cabin and space for stowage and sails, and in the stern to make room for a quarterberth and big lockers. The stern is wide to provide maximum support to the telescoping aft crossbeams and to provide a spacious cockpit. The steps I added to the ama supports to facilitate easy access, but yes, they are exceedingly bulky, I will change that. Windage will be unusually high, yes, but not more so than a catamaran. It is unfortunate, but without access to a pocket dimension, interior space has to come at the expense of something. And it is a cruiser with the main goal of being faster than the equivalent monohull, so high speed is not a priority. The back end of the submerged hull may need some adjustment to improve flow/reduce drag, yes. Thank you for the rule of thumb.
Third post:
The bow height came about because I wanted consistent deck height along the entire boat, and I wanted sitting headroom in the front cabin without having to move the cabin much lower and further back. And yes, the boat is very tall, but I disagree with your statement that 18" less would be fine. First of all, I'm 6'4", which I think I stated in the first post, so anything lower than that would drive me nuts for anything but day sailing, and secondly, the point was to be able to have tanks, batteries and other heavy things in the bottom, beneath the floor. The companionway can't be lowered without making the cockpit sole lower, which in turn would require lower seats. That won't work with the current layout as there is a quarterberth under one seat and a washing machine under the other. Your point about having the hull curve up at the ends has merit, the only downside I've seen is that it interferes with the aesthetic, but that may be fixable.
Conclusion:
Most of these concerns will be somewhat addressed with the latest redesign, which I will write about at the end of this post. But it will still look rather square and clunky compared to most trimarans.

@rberrey
First post:
Sailing ability isn't bottom of my list, but perhaps near it. A normal cruising monohull keelboat of this length has a cruising speed of what, 5-6 knots? If I can provide the same comforts in a trimaran but cruise at 8-10 knots, that is a win in my book. Getting to 20+ knots like in an equivalent corsair/farrier/dragonfly is probably not feasible (even if that is hardly cruising speed for those examples).
The L/B on my design is actually 10/1 for the main hull and 16+/1 for the amas, so on paper it is sleeker than the Horstmans. I checked the Hartleys, nice looking boats, and you were right about ~10' for the 35', but my design is only 7'4" on the main hull to fit in a container, so I don't know what I'm supposed to take away from the Hartley design, which doesn't even look demountable.
Second post:
As I've stated before; the submerged hull with a draft of 45cm/1'6" under the waterline gives a buoyancy of ~2500l/88cu.ft. and that is without the amas providing lift. The amount of gear you'd have to load down the boat with for it to submerge the flare is ridiculous. I don't understand your redesign suggestion of the flare, could you provide a sketch or something? Thanks in advance.

@bajansailor
I have made some modifications that I will write about at the bottom of this post, hopefully those will be somewhat to your liking. That said, the design you linked to is supposed to be used in a race, no wonder that it is much sleeker.

To the main point:
I have made a slight redesign and I will post some pictures in this post to show them. The major changes are these:

1. Hull length has been increased to 9m/~30ft. This was mostly to have a sharper bow and has made little impact on the hull volume.
2. The flare has been raised up 2" and has been hollowed out to provide more clearance.
3. The submerged hull has been flattened out somewhat and even at a draft of 40cm/1'4" now provides ~2250l/79.5cu.ft. of lift, so when not loaded for long term cruising the boat should be higher in the water.
4. The freeboard has been decreased with 8" but the height of the cabin is unchanged. This reduces windage some, and makes boarding easier. It also gives the boat a different silhouette.
5. The cockpit now has protruding coamings and sits higher above the waterline.
6. The aft quarterberth is now a queen size.
7. The cabin has a more gentle slope up from the foredeck with the front cabin hatch integrated into it.
8. The ama supports are now smaller, sleeker and aerodynamic.
9. The amas are wider, but with a more rounded bottom and wave piercing bows.

I hope you like this iteration.

 

Here is one additional picture of the main hull seen from the side.

 

You are misunderstanding the concept of waves slamming on the wide flat flare. It is a dynamic condition different from the static waterline. The waves have crests and troughs. Even though the average waterline will be close to the static one, the trim of the boat will change (pitching) which will submerge the flare. Also, the crests of the waves will be much higher than the static waterline. The design has serious fundamental flaws. I understand your attachment; it is your baby. However, it is a bad design.

 

MrMillard , my point the Hartley's are examples of a designer using a mono hull for the center hull of tri , they are mono hulls with ama's . I don't have the plans for the 28 , but I think the center hull beam was in the 7' range , a basic V hull . The 30' with a 10' beam is wide for a tri concept , but from all accounts both Hartley designs sail well with good speed , and good accommodations space wise . The Hartley,s are very out dated , I would have to look at my plans to see how the 30' demount,s . Your flair may not be too low , it may an optical illusion , raising it 2" may have fixed any problem with slamming . The take away from the Hartley's is you are designing a non typical tri , if you can't get the desired results you want with a typical tri hull concept , why not do as Hartley did and design a 7'5" wide mono with ama,s ? If you have to look at taking the flair higher and loosing accommodation , why not also look at taking it lower and blend it into some kind of box beam keel , or sea bright skiff design hull and keep the accommodation ?

 

@gonzo
I mean, I understand that the sea isn't a mirror. Waves move up and down the hull side all the time. When you say that the flare will submerge; if you mean partially, you could say that, as wave tops will likely hit above the flare line on the hull, but completely, not unless the wave is huge and has a really short period. I mean, all trimarans with flared bodies will have parts of the flare submerged from time to time. Mine will likely have it more often because the flare is wide and low, but so what? Worst case scenario is that it behaves more like a monohull in rough water, which I don't see as a problem. When you say that it has fundamental flaws, is that the one thing you are referring to? That the flare out is to soon and too drastic? When you say that it is a bad design, that heavily depends on what the design goal is. What design goal do you think this concept won't fulfill? Or how would you tackle the state of requirements? Or do you think my wants are not all compatible?

@rberrey
Ok, after having looked a little more at some google searches, I think I know what you mean. By the way, the 10' wide Hartley is 35' long, not 30' so I think that is why you deemed it "very wide". You are right in that the goal is to be superior to a monohull keelboat, not to be a speed machine.

I'll wait for some more replies and see if I have any ideas in the meantime. Cheers!

 

Not sure why I was thinking 30' .

 

Slamming results in high vertical accelerations. As a result, the boat rises rapidly. When it reaches zero vertical velocity, it falls, slamming again. Have you calculated the dynamic forces created by the accelerations? There is also a matter of comfort. A drawing and a design, from the engineering perspective, are two different things. The drawing is a representation of the design. As such, there are calculations and verification of satisfied requirements that are in the background. You are trying to backwards engineer a drawing. For example, you developed an interior and now have a wide flat flare to make it fit on top of a narrow hull. Ask yourself why there aren't boats with the shape you are considering. A square wheel is novel, but hardly better than a round one. Even experienced Naval Architects use previous designs as a reference to start.