# Calculating expected heel for trimaran

Discussion in 'Multihulls' started by Derek_9103, Apr 29, 2020.

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### Derek_9103Derek

I'm designing a 32' trimaran to build myself and with the help of friends. It will be cruiser, but trying to stay sporty and decent performance.

I'll keep things bare bones to save weight. It will have a 0.8 beam to length ratio to allow a little more sail power than something narrower.

An issue I cannot find good guidance on with internet searches, or the searches I tried in this forum,
is the relationship of trimaran ama design and the boat's heel.

There must be equations for that? Theoretical principles?

What I know so far:

If I understand correctly, if the ama volumes are each about 150% of the boat's displacement, then the leeward ama won't submerge when heeling under sail. And obviously, there's a "danger point" when the ama would be submerged near 100% displacement, because the main hull will be close to flying, with the difficulty to control that is not feasible for cruising if the normal and efficient sail plan would have the main hull flying in winds that would be seen with any frequency. (And I understand some designer's preferred approach is to go with, let's say, amas with 80% of the boat's displacement volume, intentionally, as a more intuitive sailing experience to limit chance of capsize because the ama submerging is your warning to reduce sail -- but I also think I understand the 150% ama is a different design path, with different benefits and drawbacks and limits.)

Bottom line: since this will be a cruising boat, the sail plan should be sized such that during normal sailing in normal winds, the main hull doesn't fly. I'm looking for knowledge to take the design to the ballpark of that "almost flying" limit, which would be impacted by the wider than normal beam, and the height of the amas. (I'll get professional help to check it over.)

To not start from scratch with a naval architect... what are the limits for this area?

I'm just guessing what kind of factors I'd have to consider... for instance, Should I expect 60% (my guess) of the boat's weight to be on the leeward ama in a certain reach point of sail, with certain wind speed and full sails up? And when that's the case, how do I calculate the relative height of ama's compared to the main hull waterline, and the resulting heel angle that works the best for overall design?

I think I remember that allowing at least some designed heel is good, i.e. amas intentionally up a little relative to the main hull height, because it allows wind to spill from the sails if the boat is in danger of capsize. Correct? What are the equations and thumbrules?

Another factor I'm not seeing much information about is how wide an ama is vs. how deep it is for a certain volume and length. I'm seeing other designs are taller than they are wide. I would guess a narrower and deeper ama would give more intuitive warning of a potential capsize? The reason I need to know instead of just copying someone else - I want to keep a "long and narrow" amas, but how to balance this with getting to the desired displacement? I'm guessing at a certain point of "too tall", you increase surface area, and therefore drag resistance, correct? Equations and thumbrules?

Last edited: Apr 30, 2020
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### jamezSenior Member

When you say 'decent performance' what sort of figures do you have in mind?

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### gonzoSenior Member

You should read Professional Boatbuilder. There are a couple of articles in structural design of multihulls. If you are designing anything that will fly a hull, or is capable of, the proper engineering will help you stay safe. What you are describing are requirements that make sense to consider. They should be part of the statement of requirements (SOR). However, multihulls have more complex structures than monohulls and it would benefit you to pay a naval architect or other engineer to run the numbers for you. At the end, the plans are one of the cheapest but most important part of the boat construction project. This is a question I often ask: why do you want to design the boat? To properly learn boat design, particularly high performance, will take a long time and a lot of math.

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### Derek_9103Derek

Jamez,

"Performance: As much as I can, given other constraints."

I'm looking at Farrier/Corsair, Rapido 40, Grainger, Shuttleworth, and Chris White tris and sail plans as sources of ideas and general shape.
It seems there's a sweet spot at 80% beam to length ratio, I've seen several well-regarded tris in that ballpark, including Shuttle 39 - that should allow more sail for the same length than something in the 62% ratio, but I know there's trade-offs, I'll keep that in mind.
I'll keep things a little Spartan to keep weight down.
I'll use stays for ama support to push in the direction of overkill so it doesn't cost an arm and a leg to make sure my ama design is safe enough. Low tech, but if it's good enough for Dragonfly and a bunch of "this is a good trimaran" reviewers, it's good enough for me.
I'm tall, and I don't want to stoop too much, so I'll try to get decent headroom. To cut down on resulting windage, I'll go for a very rounded aerodynamic hull, and to counter the increased weight from height, I'll reduce cabin WIDTH to keep the whole hull relatively narrow, probably 6'3" or so at the widest, i.e. narrower than a Farrier/Corsair of the same length.
I'll consider carbon fiber if I can afford it, all things considered.
I'm shooting for a 9:1 length to beam ratio at the waterline. If I get ready to build and determine I can narrow the waterline beam without sitting too low, I'll maybe shoot for a higher L/B ratio (which may also help windage, as long as I'm not too low to slap waves too much above the chine and get wet and knocked around going to windward). I'll have to see what the trade-offs are.
I'm thinking a ketch sail plan, specifically, two wing sails, with inspiration from Chris White's Mastfoil - there are a few wing mast plans out there, I'll see where that goes.
I think Mike Waters has a few ideas to fold in regarding leeway control, keeping things simple, and overall general knowledge.

All that plus and minus (especially weight savings, wide beam and more sail, and aerodynamic and narrow, but higher hulls) comes with a performance that will be what it will be. A decent balance of sporty and safe, I hope.

After I finish my design and am ready to build, I'll hire a designer to check things over.
And for performance, a cruiser's sail plan, amas, heel, etc. should minimize the danger of tipping over over the long haul.

So... with the above factors, plus or minus, taken to a designer who knows how to size sails and tweak hulls, given those constraints... and I'll push the limit of "cruiser safe" about as far as Rapido, Chris White, or Farrier, plus or minus a bit given the other tweaks above, and/or, whatever a designer suggests within reason.

Last edited: Apr 30, 2020
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### oldmultiSenior Member

Derek 9103. Spend \$150 Australian or about \$100 US and buy the following 32 foot tri plans. They are in PDF format and are fully detailed that will allow you to build a complete boat. This guy produces good boats and the plan will be an excellent guide and education tool. The boat is 32 foot LOA (9.750 metres). 24 feet 3 inch BOA (7.4 metres). It will fold for trailering with an oversize permit or for berthing in a marina. She will have full standing head room and sleep six although it has been designed for a couple. The main hull has a displacement of 5500 pounds (2500 Kg) and the floats have a total displacement of 3000 kg each. This gives a 120% float displacement. The sail area will be 432 sq ft (40 sqm) in the main with 205 sq ft (19 sqm) in the jib. Mast length 44 feet (13.4m). The plans are detailed for foam construction or plywood. The hulls will be built from flat panel shapes similar to the Scarab 18 and 650. His web site is Plans for the Scarab 32 folding trimaran http://www.teamscarab.com.au/Scarab%2032/design.html

Good luck.

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6. Joined: Oct 2019
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### Derek_9103Derek

Gonzo,

"Professional Boat Builder" - great idea. Thanks!

I'm shooting for a little short of a "flyable" hull... but how to push that limit is a decent part of what I'm asking.

"Pay a naval architect"... yes, I plan to hire a good one before building. I haven't done a lot of research yet, but if I had to pick one today, I'd go with Paolo Bisol, his Tritium 720 is what I'd build if that wanted something a little shorter and out of the box, and my initial impression is the ideas I'm playing with are in his wheelhouse (foam sandwich, Farrier-ish profile, balance of performance and accommodation). When I asked him if he sold study plans for the 720, his answer was no, but he answered in a "decent fellow" kind of way, and from another article I read about him it seems like he gets working with people who have ideas and priorities, but still bring his own knowledge to the table in a constructive way. (If I could afford Chris White, he'd probably be my choice, my current hodgepodge plan is pretty close to a Hammerhead 34 with 6 more inches of headroom, and I'm willing to trade off the \$ of a wider ama beam, etc., and other performance considerations... to be able to stand up straighter and still go REASONABLY fast.)

"Why do I want to design the boat?" Good question.
• I want more headroom than almost all boats in this length offer.
• And, I've noticed my preference compared to almost all designs out there is I'd like fewer berths, but everything else more "spacious", yet simple.
• Combine this with the previous requirement, and I need to change "the lines" also.
And, since the headroom requirement comes with a performance cost... what am I willing to sacrifice or beef up to get the performance (or most of it) back?

And once I start playing with those ideas, and doing research on how to juggle THOSE competing priorities... in the process I come up with QUITE A FEW ideas from quite a few sources that seem pretty cool, and no one boat out there uses them all. (Slightly toed in amas, anybody? Thanks, Mike Waters and Magic Hempel/W17!)

Re: above, and "a lot of math", I'm putting a decent amount of effort into "interpolating" a bunch of existing plans and ideas, especially when those plans and ideas are already well-regarded and already along the lines of fitting my overall theme, staying in the ballpark of similar boats in a popular length, well-trodden ground... HOPEFULLY that will get me in the ballpark of something workable, especially if I err on the side of safety as I understand it, then I'll have a designer take my money to tell me where I'm out to lunch, and where I can improve performance, for example, I'm designing the main hull a little wider at the waterline than I think it needs to be, but if I err on the side of caution, I think it will be easier for a designer to straighten me out than if I erred the other way. Maybe I spend a few thousand extra to juggle so many balls, so be it. It will be fun. I like a good challenge.

And... as this gets to the stage I have drawings, etc., I'll share those... hopefully the community gets something back from me.

Even in the rest of my life, juggling competing priorities is something I enjoy, so designing a boat is my kind of fun.

And, I just like to create things, and learn new stuff.

Farrier's F-9AX is probably the closest production (already designed) boat out there compared to how they resolved all the competing priorities in a way that suits me.
If I could find one of those, I'd consider buying it instead of building.
I want to play with two wing sails, a la Chris White, so I'd want to retrofit those onto the Farrier eventually... and structurally, that boat is just not designed for that... so... what the heck, let me take a whack myself.

If I get tired of all the work (it's a lot of work, and THANKS to everybody that's given me input so far here and elsewhere!)... I might just settle for an F-9AX when one becomes available.

But this IS fun to play with. And my wife is RELATIVELY understanding, if I sell her on it now and then.

Thanks!

Last edited: Apr 30, 2020
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### Derek_9103Derek

oldmulti,

THANKS for the link. I saw this boat maybe a year ago, and I did like it.

I'm going for the rounded hull above and below the water, for better or worse.
Two chines/knuckles, like a Chris White and/or Rapido boat.

And I'm looking for more headroom, although I know I need to give up other stuff to get it.

But overall, yeah, if I beefed up the amas, this is in the ballpark of my sweet spot. Thanks!

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### gonzoSenior Member

I think you are in the right track. How much headroom do you need? For all I know you are an incognito NBA star.

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### Derek_9103Derek

I'm 6'2", 6'3" would be ideal.
(And I know I'll give up a decent amount of other stuff to get that.)

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### gonzoSenior Member

Assuming you are wearing shoes, probably 6"5" would be ideal. It doesn't seem excessive on a 32 footer. You could consider what areas need full headroom, like the galley and main salon, and leave others lower. For example, bunks may only needing seating headroom.

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### jamezSenior Member

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### guzzis3Senior Member

Floats should be 15-20:1 so say 18 times longer than beam at the design waterline. You want lots of buoyancy forward and near circular sections below the waterline. Main hull should be better than 8:1, slimmer is faster up to a point but you lose carrying capacity which is critical in a tri.

Heel is determined by sinkage and diahedral. So lets say your main hull skims the water when the lee float is drawing 1' or 305mm, at that point what is the angle of the boat ? It's a simple geometry problem, but you have to calculate sinkage on your float design and the angle across your beams. It usually comes out about 15 degrees but you can make it anything you want. A horstman for example is probably much less. I remember one designer loved a lot of diahedral but I can't remember who.

The overriding factor in safety is the person sailing the boat. Good crews make passages on dodgy boats, bad crews sink good boats. Sensible sailors reduce big rigs in any sort of wind early and don't fall over. On the other hand an under rigged boat is useless in light winds.

There are very cheap plans available for tris in that size. The scarab is one option although you don't mention trailerability. Butcher it to suit your requirements. If you insist on designing from scratch you really need to learn a lot more. If you don't understand the basics like heel calcs and length to beam how are you going to get the structural calculations done ? These are important. Get them wrong and your boat will either be to heavy an sail like a dog with no carrying capacity or it'll break.

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### trip the light fandangoSenior Member

Pop tops are a useful compromise, although the standing area may be fairly small, a chook egg shape profile for the cabin space is also an option,, long low and round amas really tick a lot of boxes, check out giant racing tri's. Buoyancy built into the beams and a strong sealed mast can add peace of mind.
More beam adds speed and stability but crank up the engineering,.
.Have a look at Gary Baigents beautiful main hulls , how he works flare and his ring frames.
... sugar scoop wide sterns help surfing, and squirt out if a wave dumps on it bow up+ handy to get back on, except the mast or rake may move aft to compensate. a wide main hull beam is possibly a bumpier ride, but practical..
Your sailing grounds ,and how you mainly plan to use it create the shape to a large degree.
A carbon wing mast ,ie how much money do you want to spend on getting the boat light? Lots of diahedral is uncomfortable at anchor , but you can hang buckets etc., sailing flatter is nice., just reef early. It sounds a little like copying the boat you can't afford is your plan , you have to enjoy building for this to be worth it.
Will the cost, resale and time to build balance out against getting a Farrier 2nd hand ?
Going through the motions of what you would build is worth the exercise in itself. , my amateur 2 cents worth, good luck

Last edited: May 2, 2020
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### Derek_9103Derek

I hadn't seen Erick Lerouge before, and I do like his Pulsar 33
Erik Lerouge http://erik.lerouge.pagesperso-orange.fr/tri_10.htm
I'd go narrower to keep weight down / it is similar to the lines I have in mind for the main hull, though.

To me, most of his boats are not pretty
but I do like things about www.multihulldesigns.com/designs_stock/d38tri.html

One thing I got out of your links, although you didn't mention it, is committing to use wire stays for ama support.

I just started a list on paper (instead of in my head) for designers to finalize a design with, and I added Kurt and Erik to it, so thanks overall.

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### Derek_9103Derek

By saying the word "diahedral" I think you've come closest to answering the question - now I have something to search on my own.

I did hours and hours searching on my own, and all my searches without that word were coming up with almost all chaff and no wheat.
But I think searches with this new keyword won't have so much chaff, so thank you for that.

I believe there are two components to sinkage - wind pushing to the side translates pushing down on an ama, and displacement, correct?
I know my terms may not be accurate, but I think I understand the idea.

I also need a way to calculate sinkage based on design and materials = displacement - I'll save that for another day.

I did buy study plans for a Scarab maybe 6 months ago.

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