Amateur trimaran design critique

@gonzo
Well, you'll have to forgive me for getting the terminology wrong. English is not my first language and I'm not an engineer or naval architect. No calculations have been done, I wanted input before I started pouring more energy into a specific concept. Besides, I'm not sure I could calculate it to an acceptable standard yet. And regarding trimarans with a wide flare above a narrow hull, there are plenty? Already on the first page Kurt Hughes d-32 was referenced, Nicki Cruz was mentioned just recently (I didn't even know about him) and there is the 12m trimaran in the photo above. I'm sure there are plenty more?

 

MrMillard, gonzo is right when he says you are trying to backwards engineer, at least from a conventional view point, but there is nothing wrong with that. It is what you have to do if the interior is priority, it must be the starting point of your design. Once you tweak the interior you need a purpose driven hull that will support it. If the one you have in your design works then your good, if it won't work you have to be willing to design another hull if you wont compromise on the interior. I am waiting for an Engineer to complete my house plans. The house is non typical residential construction; I designed it from the roof down, footers last . I have to have stamped plans, so finding a residential Architect that was willing or able took a long time, same with an Engineer. Because I couldn't readily find professionals willing or able to work on the plans did not mean I had a bad design; on the contrary, it spoke more to their abilities. It is true that boat designers look to other designs when designing, same as a house designers; it is a tried, true and safe practice, and that is one reason why you don't see outside the box boat designs like yours, not that they are not possible.

 

@rberrey
Thank you for your encouraging words. I know my concept isn't the best thing since sliced bread, but I do think it could fill a niche and fulfill its purpose. Then again, there may be many revisions yet before I'm satisfied, if ever.

 

MrMillard, if it fits your needs and wants, and you can live with the compromises, it is the best thing. Ed Horstman wrote at least 3 books; in Trimaran sailing page 2.6 he writes about Trimaran stability provided by the amas. His designs have amas with what he calls dockside stability. The amas are touching the water at rest. His designs allow you to carry the useful load either divided between the two amas, or in the center hull. I don't know if you have high buoyancy amas in your design; if not, they may give you some more clearance on your flare. Ed's sailing trimaran and foam construction books would be a good read for you, and give you some insight into his thinking on a cruising tri concept.

 

I know we are not being super polite, but sometimes the images makes being polite difficult. The design does look better, not sure what changes you made, perhaps raised the step-out position (the height where the hull beam more than doubles) ?

The outriggers have high volume at the bow, my understanding is that such high bow volume outriggers are used by very fast high performance trimarans to give great stability when travelling very very fast through waves. My guess is that this one is designed to be sailed more sedately, thus more normal, simpler outrigger shape might be better. Example cutting down the top at the ama bow to make it slice through waves, not really sure you need something like that. This is a small thing, subjective.

It would be nice to see a profile view (side view) of the main hull with the ama not there. My guess is that the line where the hull massively increases in beam, is the same height all along the length. If this line was curved, instead of flat, it would greatly assist motion in waves. Yes you would lose some interior volume, but a modest amount.

I tried to follow, but could not understand, as to why the cabin sole, cannot be just above the keel. You have a box keel, is it not possible to have the inside of that box keel a place to walk through the boat? So you would have keel,, keel structure of 3 or 4 inches of material. Then a cabin of 6ft 5 inches, giviing 1 inch head clearance, then say add 2 inches for roof structure thickness including some frames and stringers, That is 4 inches, plus 6ft plus 5 inches, plus 2 inches, giving 6ft 11 inches between keel and top of cabin roof. Maybe you did this already?

Do you really need a washing machine? Did I read that correctly?

I think Chris White(?) wrote a book on multihull design, must be a good 20 years old now, it was a quite good read, though I have not looked at it in ten years

The Nicky Cruz uses a similar method of increasing main hull beam, by having a horizontal step out. In that trimaran the width of the step out is less, also the step out it placed higher, so less likely to be hit by waves. If the step out is placed too low, the overhang will catch on every wave, slowing the boat down. Even going a few knots the boat will pitch and the bow will reguarly pitch down into waves, and at the bow, and also all along the hull, the stepped out hull with catch on waves

If you look at the end view of the Nicy Cruz, you will notice how the hull sides flare out below the step out position. Not only does this allow for some more volume, it also reduces the width of the step out, it also provides more buoynacy so that the hull is lifted. I looked at the much smaller 6 meter Nicky Cruz trimaran, on that boat the step out is located `13 to 14 inches by my measurement, above the waterline. Your location of step out is 4 inches or 5 inches? above the waterline of a boat of 30ft length vs 20ft

I think amas (outriggers) like the Nicky Cruz look nicer than yours, maybe just copy those?

Note the use of wire stays on the Nicky Cruz to support the ama. Note also the lower foredeck and cockpit, done that way to reduce windage and weight

Having said all that, I cant see exactly what you have done, but whatever it is, it does look better

 

I had a go at a doodle, I lowered the deck along the entire boat by about 5 inches. Still gives 7.5ft between keel and cabin top, should be enough height to fit 6ft 6 inches of headroom there somwhere.

The other change was to put a curve on the step-out line

 

total load, 2.25 metric tonnes... maybe,, my guess is real world will be heavier, 2.5t min boat, fuel,, water, food, books,,, just my guess

 

Hi again! New update incoming!

@peterAustralia
First I'd like to say how grateful I am for your most recent replies, they are some of the most constructive posts in this thread so far, so thank you for taking your time. I will try to address some of the points you made:

1. I have simplified the amas. There is no inverted wave-piercing bow now, just a straight entry, perpendicular to the waterline. Some of the other decisions I've made have also made me able to have a more balanced fore-aft buoyancy configuration of the amas. What I mean is, it isn't as heavily biased towards the bow.
2. In this update I have included a side shot of the main hull without the ama obstructing the view. In the newest iteration the line were the flare begins is still not curved like you describe, but the flare has been heavily "hollowed" out, at least towards the bow. The aft I have left pretty untouched for aft cabin space.
3. The cabin sole has now been lowered closer to the keel, it is only 15cm/6" above it, just enough to fit cables and some shallow water bladders/tanks. I will explain how the entire height has changed further down.
4. The need for a washing machine is highly subjective, but most "real" cruisers I've seen have space for one. Granted, most of theses yachts were a lot bigger. In the latest model there are 4 (!) spaces big enough to fit large appliances. But you have to decide how much refrigerator/freezer/stowage/oven size you want and plan accordingly.
5. I have gotten a few book suggestions so far, I'll have to see if I can get my hands on any of them, not living in an English speaking country and all.
6. I have put the flare out a lot higher on the latest model, but it is still at an angle, it begins 10cm/4" above the max waterline and it terminates 60cm/2 feet above the waterline with significant "hollowing out" at the bow (easier to understand when you see the pictures).
7. I think I get what you're saying with the Nicky Cruz design. You'll have to judge yourself if the latest hull design is more like that or not.
8. As previously stated, the amas have been redesigned. Tell me what you think.
9. My design will most likely use wire stays, but they are so small that I haven't bothered modeling them yet, like with a lot of other smaller details. Windage has been significantly reduced with this latest update, which you will see.
10. Thank you for the doodle, I have lowered the boat and I addressed the curvature thing in a paragraph above.
11. I have increased maximum displacement at a waterline/draft of 50cm/1'8" to ~3600kg/liters or 127cu.ft. If that isn't enough, god help me...

Now to the actual update, model and pictures!:

1. To ship the trimaran in a high cube 40ft. container, you now would have to tilt it 90 degrees. A worthwhile compromise seeing as this will be a rare occurrence.
2. This has increased main hull beam by 30cm/1 foot to 250cm/8'2" and decreased cabin height above waterline by the same amount (30cm/1 foot) to 5'7".
3. The flare now begins further up the sides and isn't as drastic. It is also "hollowed out" in the bow (see pictures).
4. The amas have been simplified, the wave-piercing has been removed and the distribution of buoyancy has been "normalized" to be less front biased. The amas are now more narrow again with a L/B of 19 (main hull L/B is 10).
5. The extra quarterberth is now beneath the front cabin, but still has sitting height of about 1m/3'3".
6. The settee is now enormous and is practically a whole circle with the exception of the entrance.
7. There are now 4(!) huge standard 60cm/2 feet wide appliance bays, for refrigerator/freezer/oven/dish washer/washing machine/stowage in whatever combination you may want.
8. There is now a bridge deck and seating all around the cockpit, with a huge hole under the crossbeam support to quickly drain the cockpit in case of big waves.
9. The sink is now opposite the port galley in the literal middle of the boat to minimize movement of the water inside.
10. Mast has preliminary been moved to 50% of hull length, this may change.
11. There is probably more, but I can't think of specifics.

I'm looking forward to your inputs!

 

Have you done a rough weight estimate of all the materials that will go into the construction of the boat, along with all of the outfit items (including that washing machine.......) and stores / provisions, and not forgetting the weight of the crew members?
If not, then that should maybe be your next step - it is tedious, boring and often frustrating, but it is a vital thing to do before you get too involved in detail design.
It helps if you work up a spreadsheet for doing it on.
Apart from the final total weight, you also want to have a column on your spreadsheet for the longitudinal centre of gravity of each item on the list - you can take moments about either the stem or the stern to find out where the overall LCG of the vessel will be.
And you ideally want this to be in the same position as the LCB of the main hull at the load waterline. Do your hydrostatics give you stuff like LCB as well?
If the LCG is forward of the LCB, then she will trim down by the bow, and vice versa.

 

The flare looks good , I would sleep on that boat . They make a small hand cranked washing machine and if you stay at a marina most should have a washing machine . Weight is weight and you will have to pick needs first , then your want,s . A boat full of nice amenities that can't carry a cruising load will set in the marina , and that's ok if you want a nice counter top and a wine sipping boat . A cruising boat of that size and displacement is a " go lite boat " , you are in the lowest range size wise for a cruising tri , lets trim a little more fat .

 

You should avoid running wires in the bilges. The higher up they are the better. Insulation, unless rated for underwater, is not 100% waterproof. If the wires are submerged, moisture will eventually get through.

 

It does look a lot better. I know we have not been super nice in previous posts. As to whether building this boat is a good idea or not is a thing for you to work out. It will probably cost a lot of money, and take a lot of time. There may be some off the shelf plans that are of similar size. Personally I am impressed by Jim Brown/ James Marples Seaclipper trimarans, the 34ft one looks of similar size... your modification would be to raise the deck by 5 inches, because u like me, chose tall parents

 

Hi again! Here to answer to some more replies and to give a minor update:

@bajansailor
I did do a rough weight estimate for the first concept, that will have to be updated to match the current iteration. I knew it was a crude estimate and stated such, but as was pointed out by @Ad Hoc and @TANSL, it was even cruder than I thought. But I don't want to put more work into that aspect before the community deem the concept worthwhile (which is still up for debate). There have been more positive reactions to the latest changes though.

I had my modeling software spit out some numbers, The center of buoyancy should be accurate (just give the hulls the density of water), but the center of gravity is very preliminary; it only shows the current structure, no details, loading or provisions, and everything is of uniform density. The numbers are relative to the middle of the stern, so the midpoint of the boat is about (x=4500mm, y=0mm, z=1100mm);

Center of buoyancy = x,y,z (4365.82mm, 0, 298.643mm)

Center of gravity = x,y,z, (3945.116mm, -22.211mm, 1219.926mm)

Sidenote; this means that CB at least is only ~13cm/5" aft from the absolute middle!

@rberrey
I know that a washing machine is a heavy luxury, but it is easier to design space for one and not install it than vice versa
Time will tell if I ever have to face that dilemma.

@gonzo
Thank you for the heads up. No wiring in the bilges if I can help it. I had an idea about glassing in electrical piping and some plumbing directly into the hull when doing the lay-up, and then drilling into them to draw wires and install sink etc. What do you think about that idea? PVC piping should bond well, right?

@peterAustralia
Yes, as I have stated before, this idea may never reach the water, and if I do build a boat in my lifetime it may be a design made by someone else. The pictures that came up when i googled the Seaclipper 34 didn't give very much, but they are not foldable or trailerable, right? I think the Hughes's d-32 mentioned earlier is a more likely candidate. By the way, the 12m trimaran you posted a picture of earlier, I still can't find it. Do you have any more info on it?

To the update:

1. I have redesigned the coamings for the cockpit. This was a purely aesthetic decision, but the positive side effects are easier line handling to secondary winches, negatives are less wind and water protection. See pictures.
2. I did minor adjustments to the bulkheads for the appliance shelves. This resulted in more symmetry (better looking in my opinion) and gives extra room in the head compartment for a proper sink and bench area by the window.
3. I lowered the cutouts on either side of the companionway. If you have been wondering what these are for, it is to reduce the height of winches and clutches so that they don't have to be demounted for shipping. It also should improve aerodynamics. I also added a hole on each side to stuff excess lines into.

Lastly, a question:

How do sailors and designers deal with wet gear and or rain and spray when transitioning to inside the boat? Most designs I have seen have the companionway end directly into the main cabin onto a wood floor? Doesn't that leave a mess? I know some boats have wet lockers, but not all of them are close by. And what about wet footgear? Would it make sense to have the companionway enter into a wet area, say the shower? Of course you'd have to be able to enter end exit even when the shower is in actual use, so some secondary opening must exist. But that way you could get out of your wet gear without soiling the warm dry interior of the cabin? Especially if the shower has direct access too or doubles as a wet locker. In Scandinavia all private residences have a small entrance hall for just such bad weather occasions. With accompanying hangers for jackets and a shoe rack to dry your shoes. Are there shoe racks in any sailboats? My sailing experience is very limited, so I'd love some perspective here.

Til next time!

 

If you have your shower next to the entrance way you can tie in a drain from the entrance way into the shower drain . Build your wet locker in the shower room . You can tab pvc pipe with glass tape to your structure .

 

I dont know the name of the trimaran I posted an image of. I saved the photo to my laptop many years ago and have forgotten all details