small cruising tri design

So let me try this again. as I get the feeling I'm peeving people off by asking unwarranted questions on their threads.

I'm in the highly confusing process of designing a small cruising trimaran.

I guess the problem with this type of design is that one tries to do too many things with the same boat.

I'm trying to design and build a boat on a budget stretched impossibly thin, as surely many others are doing.

I would like to go fast (well, quick at least....)

The boat has to be seaworthy. To me this is one of the biggest issues, because precisely how do you quantify what the sea will throw at you. And looking at the track record of great designers, and with humble apologies, they don't get it right either. Keel and boards failing, rigs breaking etc.

I sometimes sit and watch the sea when a real squall is blowing dogs off chains, and try to remember that feeling of awe when next I'm trying to decide on a structure. Unfortunately, this inevitably happens in cosy surroundings. To summarise, imagine doing a quick dash between Durban and Richards Bay on the South African East coast, and within a few minutes a squall of seventy or eighty knots starts to bully you. I have to be able to survive this.

I also have constraints imposed in the form of the rig which I have already (Prindle 18 beachcat.)

The building method is hard chine plywood, because I enjoy this type of construction, (and I have to justify having designed and built a 8x4 foot cnc router.....)

There you go, and oh yes, I would like to be able to sleep inside the thing and sit upright like a remotely civilsed human being ( ie a builder whose own structure is becoming stiffer and more brittle and will be even more so on launch day, given the glacial pace at which events are currently unfolding.)

So the point is that I have one shot at doing this properly. I sincerely hope that some of you will assist by commenting and guiding me on my way, as I am serious about building this boat.

 

Hi , I have been thinking about the same thing. The way I see it you first have to collect a lot of info , then figure what to do with it . Most of the designers have gone through this already and then tested there thinking and built on a knowledge base created over many years. You could design a boat , but why ? Would it perform the way you want ? Check out the Bermuda trimaran race videos , and see the the development of the type.
The amount of things to consider stagger my mind. There has been an evolution of the
structure as well as the technical aspects of their design. It is interesting , but why try to get 30 years of knowledge for one small project .

Anyway best of luck .

F

 

I like to design, but I think you are out of your depth. No insult. My observation is the kind of people who should be designing boats, have typically had to do at least one trial run. The combination of you budget limitations and needing an ultimate survival vessel, is really going to push that process, as would your apparent slow process.

What is small? Are we talking 20 feet, 24 feet. I think the Bucc is pretty much as good as it gets in hard chine. It is a good boat generally, but there are lots of little things that make it great. I don't know how ultimate survival they are.

Survival wise, I think a boat needs to be really well thought out to deploy drogues and chutes over the bow or transom, and I wouldn't want to be much under 30 feet. A lot of details start to get super critical at that point, how the boards work, do they flood the boat, how strong are the rudders, particularly if the waves are working under the rear of the boat, The transom design. A rig that won't tear itself apart. Rotating beachcat rigs get real active even tied down. Most of those rigs need a lot of modification to reef well. It is like the stuff about the best war plans not surviving first contact with the enemy.

Survival wise, I would want to go with a proven design concept. To my way of thinking that is stuff like the Wharram cats, which don't really meet your other criteria, and the Newick Val. Another boat that I would study is the Jones Hummingbird. His approach to making it an ocean boat. Jones crossed the Atlantic many times, and did a huge hurricane in the smallest Wharram. Jones really hated the Val though.

You have sensibly mentioned that you understand that part of the design problem can be getting too much stuff in the tent. But like most of us, you turn around and ask for speed, safety, accommodation, economy, and then throw on some constraints. If I was in your situation I would consider the safety side of the thing and subordinate all other claims. Luckily budget and safety do to some extent go together.

 

Having had a look at the Val 31, one things that struck me was the very large clearance under the beams. There is enough different about this boat that one can't help but get the feeling that the designer knew a few things, and maybe thought about things differently than other people.

I have always been an admirer of the Jones designs, and it is certainly food for thought to look at his boats again.

Maybe it is too ambitious to describe horrid storms of eighty knots, but the point I try to make is, how do you motivate your structural design approach. What does it really mean when a boat is described as a coastal vs a bluewater boat? Do you look at the worst case scenario, and if so, how much worse does it get 100 miles from shore compared with one mile from shore. From my limited personal experience, it can get pretty damn hairy one mile from shore.

I will first have to study typical survival strategies employed so form some sort of idea.

I am not intimidated by the prospect of designing a boat. Surely there are lots of things to consider, on the other hand, there is a very large amount of information readily available via the internyet.

Philosophically speaking, there are three ways to acquire a boat. Buy it from a builder, build an existing design, or design and build it yourself.
All three lead to the complete irrational outcome of owning a boat. Only a very lucky few of us can honestly say we actually need a boat.

I take as much from thinking about and building boats as I do from using them.
I admit I have only designed small boats (paddling craft and dinghies) but I am lucky that my building experience includes composite monohulls up to 47 feet.

 

Design Categories
Category ...........Significant Wave Height ...Beaufort
A - Ocean ........ Exceeding 4m.................. Exceeding 8
B - Offshore ......Up to & including 4m ....... Up to & including 8
C - Inshore .......Up to & including 2m ....... Up to & including 6
D - Sheltered ....Up to & including 0.5m .... Up to & including 4

A category "B" small trimaran, are there any with such certification? What is the F24/Corsair24 a C? We sail in 2-4m waves offshore Beaufort 6, a wet ride but never really worry about safety. Don't reef but keep sheets handy, certainly not willing to try Beaufort 8 though! A C+ then?

Most of the kit jobs are Ds, you're asking a lot of a small tri, need some min mass to offset bare windage; trampoline, mast, etc. or she may flip in a gale/storm.

 

18' tri

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DriesLass, in one of your original posts you suggested using a Prindle 18 rig on the new boat. You said your target weight was 550 lb and that there would be a racing mode and cruising mode.
Some things to ponder:
--Max RM of Prindle 18 with two 175lb people on trapeze= 5011ft.lb..
--Max RM of your boat with 305lb of crew sitting on the windward ama and the main hull just flying: 8700 ft.lb.
--Max RM of your boat in "cruising mode" (@1000lb displ)= 7500ft.lb. with crew sitting in the center
======
The Prindle rig is probably not strong enough for this at "hull flying" RM. The Prindle rig with the CE at about 12' is designed for a max wind pressure of about 1.9lb. per sq.ft-about the same as the 1.8lb.sq.ft of an F18 for a rig that is not depowered or reefed. Your boat would require 3.29 lb per sq.ft. in race mode to just fly the main hull with the crew sitting on the windward ama.
With the crew in center the @Max RM(main hull just flying) the boat would require 2.8lb. per sq.ft. of wind pressure at the CE. It is convenient to express this in wind pressure because accurate boat speed and apparent wind angle is impossible to determine with the available info. However, you can see where you are with reference to the P18 and F18.

You mentioned a L/B for the main hull of 8.2/1-thats probably ok if the main hull is designed as a planing hull(and is capable of planing). I think it's fair to say with the results above that it will take a fairly strong wind to fly the main hull as you seemed to suggest you wanted to when you first brought this up. That's ok if the main hull can plane because if not it will be severely over pressed. A L/B of 8.2/1 as a displacement hull is not a good idea for this size and power.
An old rule of thumb was 500sq.ft. of sail per ton of total displacement to be able to plane. The best example of a planing small tri is the Weta: 607 sq.ft. per ton. Your boat in race mode: 500 sq.ft. per ton ; in "cruise mode" 440 lb per ton. Probably close enough for jazz .
--
Hope this helps a little and I hope you'll consult with an NA on the beams and other engineering problems. Good luck.

pictures: 40 year old(!) pictures of my 20' tri with a planing main hull, the Weta planing.

 

I would consider finding a proven design, buying some plans and than beefing up the strength and perhaps altering the cabin to suit your needs. At least then you will have some known handling qualities, and perhaps a builder group for support during the building stage. It will save you a lot building costs, and perhaps a lot of rework or modifications to get your own design manageable. Start with a known design, and make the alterations to suit your needs. It will also come together faster.

sounds like a fun project. Good luck

 

I agree with Petros, this is the right way. It is no easy task to design a well balanced multihull with that small length.

Michel

 

While i do agree with Lucky and Petros i also dont.
If you want a boat that is realy realy fast, than it would be better to go for a well known fast design.
But if you love to design your own stuff, and you are going to spend time and money on building your own boat, then i would design it myself with the help of this forum, internet and when you feel confident in your design, contact and pay a naval architect to have a look on your drawings for fine adjustments.

What you have to be prepared for is that it may not act like you intended, and you will for sure not get the money back if you try to sell it.
Maybe buy plans for a boat with similar building method, shape and size, just to get hints of the design, and help with the building instructions (even though you use your own design)

 

"Maybe it is too ambitious to describe horrid storms of eighty knots, but the point I try to make is, how do you motivate your structural design approach."

I work from a weight budget. Decide on the weight as part of a design spiral, and then just keep moving it around until you have an efficient structure. That method combines your needs with a boatbuilding approach. Most people who ever lifted a pen to design a multi, until very recently couldn't have begun to engineer one. I know a goodly number didn't model weight distribution, stability etc...


"What does it really mean when a boat is described as a coastal vs a bluewater boat? Do you look at the worst case scenario, and if so, how much worse does it get 100 miles from shore compared with one mile from shore. From my limited personal experience, it can get pretty damn hairy one mile from shore."

The terms are in a sense meaningless as regards boats. Seems to be more a mater of human aspiration. You have your coastal sailors and you have your offshore passage makers, racers, cruisers etc... These people may choose different boats. But one can also re-purpose designs and refit them to different conditions.

"I am not intimidated by the prospect of designing a boat. Surely there are lots of things to consider, on the other hand, there is a very large amount of information readily available via the internyet."

Really? There is a lot of talk. I see very little real design info or even discussion. You can get an infinite number of opinions, but at some point you have to start making decision etc...

"Philosophically speaking, there are three ways to acquire a boat. Buy it from a builder, build an existing design, or design and build it yourself.
All three lead to the complete irrational outcome of owning a boat."

You wish. Buying a boat sounds pretty good. Making it yourself is already probably an 80% drop out rate. Designing it and building it yourself, while starting from a position of no knowledge, is exceedingly unlikely to get you a boat. And I have done it several times so I know how rare it is. But as others have said, if that is what you want to do go for it. Just so long as you have your priorities right.

"Only a very lucky few of us can honestly say we actually need a boat."

"oh reason not the need" to quote the bard. Most of us need a boat as much as most people need anything. There are a relatively short list of things people live to own, and boats are certainly on the list.

 

I have considered all the input received, and somehow my ideas are changing slowly.
It seems I will be reducing the upwind sail area to 20 sqm.
My mass budget looks to be creeping towards the goal.
The volume of the boat has now shrunk to what I consider an absolute minimum.
The main hull shape has changed towards a planing hull.

I attach a profile view to illustrate. Will all of you please be so kind as to comment?

Regards,

 

=================
DriesLaas, glad you're making progress! Boat design is fun and can be enlightening and very fulfilling. As a suggestion, it might be appropo if you consider posting some of the comparative ratios. If you don't have quick access to them, and their appropriate ranges, I've posted Eric Sponbergs excellent summation paper below. If more of us used these ratios it would probably help all of us to build a database of designs. One thing I try to to with an experimental design is to post some of the most important comparative ratios for my design and another well known boat that I have researched so that I'm not just tossing out numbers. Comparing an existing boat(or two) to your own design using these ratios can help any designer. On a design forum such as this its disappointing how few people take the time and effort to do these numbers-they are the "language" of design and we could all benefit from sharing this kind of info.
Best of luck in pulling your project together!

 

Hi Doug, I've had a read through Sponberg's article a while ago. What a nice summation of the ratio's! I agree that this is a good suggestion. I have a spreadsheet detailing the basic ratios, which can be deduced from published data on comparable boats (of which there is not all that much and which presents a problem in its' own right.)

I will drag that open and try to populate it a bit further.

Kind regards,

 

I have started doing some research on the ratios as suggested by Doug, and would like to post the results of this. Keep in mind that the numbers of the boat I am busy with ( SR18 and SR18 Boost) is estimated, and probably on the optimistic side. It is always a temptation to tweak these numbers until the boat appears favorable compared to the others, but I suppose part of the art of design is to know where to stop with this silliness.
By the way, the idea behind the SR 18 Boost is to create a boat with the same floats, beams, rig and foils as the cruising version, and a minimalised main hull, for purely racing purposes.
If you spot any glaring mistakes, please let me know so I can correct them.

The next step is then to do a preliminary design and determine if I can build the boat strong enough to the hoped-for mass.

If that looks OK, I will do a scale model to finalise lay-out and ergonomics.

Regards,

 

Boost

=======================
Dries, that sure is a thorough job-well done! I haven't had a chance to study it yet but you can bet I will.
Will the "Boost" be designed to fly the main hull? Will it use trapezes?