Multihull Structure Thoughts

T.B.
The only designs I know that come close to fitting your description are the larger Horstman trimarans, there are numerous designs from 50-80’
Accommodation in the floats is a Horstman speciality !

 

I'm familiar with Horstmanns designs! They aren't really my aesthetic, unfortunately.

I'm going to be commisioning this with an NA once I get my preferences right, so I'm not looking for a stock design. I'm looking for creative commentary, criticism, etc from the salts. Not doing a powered vessel. Not doing luxury yacht per se. I'm looking for anyone who might tell me "you forgot this one thing" I only have about 10k nautical miles, mostly on 40+ Mono's, so I guess I'm looking for salty multihull sailors to tell me I'm crazy, but if that is good crazy...or bad crazy.

Basically, I am trying to put together an expeditionary trimaran that can handle big seas, and berth up to 6 comfortably, but can be handled by one, in emergencies. There are alloy catamarans that do this work, but I just do not like the rigs on those, due to their complexity. Its also not going to be built by a third party yard, as although I'm fairly comfortable, I'm not wealthy! So I might have bits of it helped along by a yard, but that'll be it.

I'm sorry if this is a confusing request...ive been looking at this for some time and I'm at the last decision point before I start committing resources and I guess I just wanted a sanity check on the basic idea.

 

"I'm not wealthy!"
in view of the planned size of the vessel this could be a problem...
(make a not-favourable estimate of labour & material costs -
...
then triple it!)

 

Your breaking one of the basic tenants of why a trimaran is superior to a cat[ha], don't add weight to the floats/amas a trimaran is a bird, I don't understand why after much study you would arrive where you have. This isn't the thread for this discussion you would need to start your own.

 

T.B. if I understand the concept, you want a 65 x 40 foot tri with accommodation in all 3 hulls but without wings on the main hull. The main hull is to be about 65 x 16 foot with flair after the first 33%. You would like an aluminum main hull and wood floats. Deck is either plywood or aluminum. Speed is secondary to seakeeping and comfort.

Let’s get into this. Design. The effective lowest length to beam at the waterline for the main hull of a tri to get reasonable performance and comfort is 7 to1 which means a waterline beam of about 9,2 foot and about 3.4 foot of flair above the waterline. This implies a displacement of about 70,000 lbs. For comfort the floats would also probably add some additional buoyancy at rest. The floats would need to be about 55 foot with a waterline beam of 6 foot when heeled to a float draft of 4 foot. This is getting a big boat and will need 2 x 80 foot high free standing masts with about a minimum 1000 square foot of mainsail on each mast.

The build. Aluminum or wood is OK, Bret Swain or constant chamber OK plywood OK for the hull shells or decks, Cross beams get a Professional designer. You are building a tri that has 1.2 million foot pounds of righting moment. This means you could carry full sail in over 60 knot winds. Your masts now have to be very strong. Professional designer again.

Time and cost to build. The killer. I know 3 people who have built 60 foot plus boats by themselves from start to finish. One spent 4 years with about 10 days off during the entire 4 years. He built a 60 foot aluminum charter cat. The second guy built a 100 foot aluminum power cat and invented many tools to help him. No fancy curves here, much flat panels with a lot of stringer support. You have read about the tri That. Translation 20,000 build hours minimum. Hope you have access to a build yard that you can launch the boat from, preferably with a shed that can house the build. Next you now need multiple sets of tools to build. Timber tools can cut aluminum but not steel. MIG welders are required for aluminum. Brett Swain ideas give you a hull, not the internal structure or a basic fitout. For comparison full production 40 foot cats with molds for everything take 2500 hours to build, 48 foot semi custom Dolphin cats take 10,000 hours inside full production factories. 65 foot multihulls are twice the work of a 48 footer (exception Wharram 63 foot cats but the accommodation would not suit).

The gear required. The engine in this would need at least a 100 HP diesel. The plumbing runs with tankage will be substantial, The electrics, batteries, solar, lights, navigation equipment etc will not be cheap. The anchoring gear, ropes, sail controls, winches, deck gear, dinghies, safety gear etc will amaze you.

Finally check if you will be able to get any insurance while you build and when afloat. Cost, who knows but think of a number then double it.

Translation. Think secondhand first, a smaller boat next, a professional design next (you will save the plan cost in reduced material costs alone) and finally do your own design. If you do your own design build a reduced size test model first to find out if it will work.

I am not saying this tri is not possible but please think long and hard about attempting it.

 

Thank you both

I'm not a newbie to sailing and its costs. The most helpful part of this is your point that a pro engineer needs to design her, structurally, which is what I'm planning for. Also, your point about costs is well taken. Let me put it this way: if this is an impossible, or too expensive thing for me to do, I'll just go with what I know, and buy a 70' steel mono, rebuild, refit and be done. I dont want to do that, as I dont like heeling and rolling, and Id like to have shallow draft, but that's my fallback. I need a rig I can sail and maintain when im 80, in good health, but waning, so that's the one non negotiable. Unstayed masts and a low aspect rig. Think Freedom cat ketch. Most likely she'll be schooner rigged, with perhaps 3 masts, for that displacement of mono. On a multi, less sail would be needed, which is one reason I'm looking at multis.

See, this is basically a "what if?" Scenario. The numbers dont prevent it, and the only objection that isnt surmountable with time and effort is member and structural strength. And that has already been demonstrated plausible ( if expensive ) with various examples.

When I say I'm not using a third party to build, I dont mean I won't have pro help or paid workers. I mean I'm not paying a luxury yacht maker a 100% premium to build it for me. Im going to be my own general contractor. I have some money, but I'd like to have a stable income when I retire

Your warnings are well taken. They are precisely the same ones ive heard before, and have tried to design for. So, given your guys passion for this, I'm going to assume I haven't missed anything huge, and commission that NA.

Ill say, I disagree with your 7:1 statement on the main hull. This is a standard developed for racing. There is nothing stopping having wider beam on a main hull if you dont mind paying for it with speed, and comfort on a Tri is, from what I can determine, mostly a matter of minimal dihedral and a wave piercing bow and aka/ama design. In other words, if you basically just design a wide cat, but put a fat main hull in the middle of it. Why? The central structure is needed for my one non negotiable. The thing that has become more apparent here is that the tri might have to have equal length and larger amas, and the main might need to be downsized.

Again, thanks very much!

 

T. B. You have thought about this and understand many issues. Yes there is trade off between speed and main hull beam (look at the resistance chart below) but if you are happy with that trade off OK. A length to beam of 7 to 1 is a cruising hull. Length to beams of 10 to 1 is a fast cruising design, 12 to 1 is the start of performance cats and tris. Older Piver trimaran designs had 6 to 1 main hulls and could achieve 10 knots but could not sail well and due to float shape were uncomfortable. Jim Brown Searunner designs are the best compromise of earlier designs.

Next issue is fat main hulls with fat floats can have wave interference between the hulls. Again no major problem but it will again effect performance. Translation, if your 65 footer sails like the 40 foot mono's you sail on you should be OK. Also consider the hull shapes for pitching resistance. In tris the placement of the centre of buoyancy of the floats versus the centre of bouyancy of the main hull can minimize pitching while under sail. Again a good designer can advise.

Can others please provide input to help T. B. get a wider set of views.

 

I'm not doing the hydrodynamics or etc. I'm working up a proposal to submit to an NA. I understand that, for example, ama shape needs to flare up foward in a way that moves her center of flotation forwards as it submerges, in order to prevent hobby horsing. I understand that the "tunnel" in between hulls can, if not properly designed, create transverse wave drag. Etc.

Pivers, at 6:1, were deep vee hulls. That had a lot more to do with their speed than fatness. You're assuming a standard hull arrangement as well, and so a shallow central hull relative to the amas MIGHT do the trick, depending on other factors.

I'm putting together a design and build proposal. I was only looking for macro issues that might immediately doom the project. You have helped me see that there are no macro issues, only design and build particulars that need to be sussed out.

I am paying a Naval Architect to do the detailed design, maths, and engineering. I just wanted to know if any huge, glaring issues existed.

Thanks so much for your help.

 

As a suggestion look at some of the Navy powered trimaran shapes. With a wider main hull like 5 to 1 you are getting some of it's form stability. I'd suggest narrower amas to cut down the wake issue, in that size range you can still have room inside. Some deck to cut down the spray would be nice, especially if you are outdoor commuting to the amas.

 

France is one of the few countries that would build an 80 foot charter cat in plywood in 2004. The builder was Etoile Marine to a design by Giddas Plessis & Associates. The cat features a large saloon able to entertain up to 40 guests when in port and she comes complete with a home cinema system and satellite televisions. It also has many cabins for charter.

“ATARAXIE” Is an 80.6 x 37.7 foot of unknown displacement (estimate 130,000 lbs, 58 ton). The schooner rig has a 68.6 aft mast and a 65.5 foot foremast. The sail area is approximately 2,100 square foot in 2 mainsails and fore triangle. The hull length to beam is about 10.5 to 1. The draft over the fixed fin keels is 7 foot. The engines are two 115 HP diesel engines. The cruise speed is 8 knots with a peak of 10 knots under power.

The accommodation is vast. There are 8 triple bunk cabins in the hulls each with its own toilet suite for a total sleeping accommodation for 24 guests and 2 crew cabins in the bows. The main saloon is large with 3 moveable dining tables which allows the space to be made into a 40 person dance floor in port. The very large galley is across the forward end of the main saloon along with the internal helming position. The cockpit is also large. This cat is designed for charter work.

The construction is sheet plywood for the hulls supported by ply bulkheads, frames and stringers. She also has a composite (foam glass) deck and composite with plywood superstructure. In 2017 the cat started a significant refit which lasted until 2020 with engines, electric, and plumbing replaced and the internal fitout upgraded. During this time the courts seized the cat due to some unpaid bills. It was listed for sale at 800,000 Euro and eventually sold for 300,000 euro (plus a lot of unpaid bills).

Performance is listed as capable of sailing at 12 knot averages (in the right conditions) under sail and peaks of 15 knots. This cat has crossed the Atlantic at least 8 times and was used in racing occasionally (jpegs show a sponsor occasionally). The chine hull shape is relatively good and would have made for easier construction without having any real impact on performance. The rig and weight would be the limitation on a higher top speed.

The jpegs give the idea.

 

...would be interesting how buildt in aluminium would compare in weight, performance & costs in material & labour

 

I have. They are definitely interesting. But I'm looking at a sail powered vessel with auxiliary propulsion. I think Neel has proven the point I'm making, in terms of an equal length hull Tri being a viable platform for cruising, and many are currently enjoying those vessels. If I wanted a standard rig, and modern aesthetics, would just buy a production Neel. They are great boats.

 

Sorry if I have done this cat before but I am not updating the index at the moment. The Adventura catamaran group started small but listened to their customers and have expanded their line of cats according to there requests. The Adventura 37 is aimed at cruising cat market providing space, reasonable performance in a compact size.

The Aventura 37 catamaran is 35.4 x 19.5 foot with a weight of 17,700 lbs. The fixed 53 foot mast carries a 624 square foot battened mainsail, 387 square foot genoa, an optional 645 square foot gennaker, an asymmetric spinnaker of 914 square foot and a symmetrical spinnaker of 968 square foot, The draft is 4 foot over the fixed fin keels. The engines can be 2×20 or 2×30 hp diesels.

The accommodation is in the standard configuration:3 double cabins / 2 bathrooms. Optional configuration:3 cabins / 3 bathrooms or 4 cabins / 2 bathrooms. The main saloon has the galley, seating and entertainment artea connected to the aft cockpit. The flybridge on this size of cat is a bit of an achievement. The helming and sail control cockpit is an independent area in the aft corner of the main deck structure.

The architects and designers are Lasta Design Studio who specified the boat for Adventura Catamarans. The cat is basically Airex foam, e-glass with polyester resin throughout. The build is done in molds with resin infusion. The fin keels are solid glass to minimise maintenance.

Performance is reasonable. One test said: “For our test, we saw good conditions (fine weather, 10 to 16 knots of ESE wind, but a large cross sea with a chop of 3 to 5 feet), the boat speed on a close reach was quite satisfactory (6.8 to 8.2 knots), as was the passage through the waves in choppier seas. It was obvious that in these conditions a code 0 (recommended!) would have reduced the platform movements by better supporting the catamaran while providing an extra knot of speed. With the breeze oscillating quite a bit, we noticed good acceleration in the 13/14 knots of wind. Sail manoeuvres aboard the 37 are child’s play under furling genoa and mainsail thanks to a neat console separated from the helm which groups together the clutches and winches. The visibility over the whole deck plan is appreciable and the helmsman placed at the back can observe the action and advise the crew.”

We have a well designed, well built cat that can be cruised by a couple and single handed if required. It can be a personal cat or configured as a charter cat if required. Yes, it has high freeboard, puts a lot of accommodation into a short cat but it can still sail well. A good addition to the market.

The jpegs give the idea.

 

...I have read to the contrary:
trimaran neel , bad news,,, - Cruisers & Sailing Forums https://www.cruisersforum.com/forums/f48/trimaran-neel-bad-news-283949.html

(apart from godawful aesthetics...)

 

I had planned to post about this if/when the build video blog was updated, but it’s been a while now and I have no idea what happened, but the video posts just seem to have stopped until recently. I know that the videos that were being posted were post dated leading me to believe the boat was complete and being sailed, so it’s strange that the channel went dead. I assumed either the worst, or that the boat was simply being enjoyed.

Anyway, OldMulti often posts about ambitious sailors who figure out ways to achieve their dream/goals on a budget and with the recent posts about Trimarans with lots of room, the Neel concept, etc., I think this is/was an interesting example of what can be done to an older Tri to completely transform it into a more more modern live aboard.

Some of the build techniques and decisions might be controversial, but what I really liked about the videos was seeing them work with Nomex ridged panels to quickly build strong lightweight structure.

Here’s a direct link to video to give you idea of the concept that has some time lapse clips showing the start to finish and a link to the channel where you can see the step by step progress of the build out.





https://youtube.com/@seeweedboatlife?si=Rwhrrk2oNyHBWBhY