Determining outrigger size

I am building a small trimaran, and I need to determine the outrigger volume. Basically I am modifying a known design of a proa (larger Vaka and double outrigger).

I want to minimize the outrigger size but something doesn't seem right when I work the numbers.

I have determined that in an 18 knot wind, with a sail area of 180sf, and a 200lb person standing on the outrigger to be submerged, a bouancy force of approx 850lb is needed to keep the tri from roling over.

Assuming density of saltwater of 64lb/cf then the required volume would be:

Vreq = 850lb/64lb/cf = 13.3 cf.

My LOA is 20ft, therefore if I assume the outrigger is 14ft, then the required cross sectional area of the outrigger(assuming constant shape) would be 0.95sf or ~140 square inches.

That would give me a 12" square or 13" diameter, by 14 ft long outrigger. This seems too large for such a small craft.

Loa: 20'
Beam: 12'
hull wt: 175lb

Is this the correct thought, or am I totally off the mark?

 

I can't help with the numbers, but my Ulua with 128 square feet of sail and amas of 16'x8"max tapering to about 4" on the stern can't quite support my weight at rest. I would assume that more sail area will drive the amas quite hard. I doubt you would find yourself on the lee ama while the boat is pressing the ama to it's max, however.

I was surprised how much buoyancy I needed though, and will be building larger amas for my Ulua. Look at the Holopuni canoe as tri. His amas are 14' and HUGE in cross section. His canoe only has 100 sf as well.

Dan

 

For a modern type tri that can fly the main hull, the amas should be ~150% the volume of the main hull. ie, very large.
For a canoe-type trimaran, like Dierking's, the amas can be much smaller, but you rely on crew weight (and/or reefing) to keep heeling moment under control. And the vaka stays in the water.
It's common in the latter type to make amas that are just big enough to stand on at rest, but not big enough to stand on the lee ama underway. Another benefit of small amas is that the beams only need be strong enough to support the buoyancy of the amas (plus a safety factor) instead of the max displacement of the boat, plus rig loads. So you get lighter components and a lighter boat that goes well, you just have to keep it flat.
Go the other way and you get a more robust boat, possibly higher performance sailing, but a heavier and much bigger boat, that may be much harder to right if it goes over.

 

Thanks for the info. BWD, I am a little confused. You say that modern tri's that fly the main hull, the amas should be about 150% vol of main hull.

1) What does fly the main hull mean?
2) Having studied various commercially available tri's such as Corsair, Farrier and others it does not seem that the amas are 150% vol of the main hull.

Although I do think that I understand the intent behind what you said. Basically the amas can be designed, at minimum to float a person for a smaller tri, to the other end of the performance spectrum, where each ama is supporting 33-50% of the main hulls volume.

If this is correct then there should be a happy medium for a particular tri where the ammas are not too small or large.

 

The measurement is based on the submerged displacement of the float against the sailing displacement of the boat which in a modern tri takes all the weight on the main hull at rest. So if you have a designed displacement of 2000lbs and a submerged float disp of 3000 lbs you have a 150% float. This changes depending on payload and crew weight of course. But you get the idea. A 100% float tends to sink when overpressed and won't lift the main hull much. 150% or 200% floats give more stability and allow more sail to be utilised hence a more powerful boat which will lift the centrehull if conditions are appropriate. Some would argue that low disp floats are more forgiving.

 

flying hull =hull not in the water due to sailing conditions.

I don't know all the models you mention but some of those do have fairly high vol amas I think. Remember "volume" here means the volume underwater, not all the space there may or may not be inside the hull above the waterline. Forgive me if I am stating something obvious, but I don't know exactly where the confusion is.

If the main hull is touching the water it is probably supporting its own volume! Performance multis usually only keep one ama in the water at a time, due to their shape.

No. in a big ama design, whichever ama is leeward can support all the weight, and needs more than 100% volume so it will float up to the sea surface if for example you crash down off a wave. Picture explains the rationale, either to hike or reef on a low vol tri, or to have big amas.

The conclusion is right, but there is a lot more to consider to get to it.... There is a lot of cool stuff to read on the subject of multi design.
?You might want to read a book that covers all this type of stuff in some depth, just for fun, as well as background info. For example "Multihulls Offshore" by Rob James. It's old but good.



Another thought:
The design process just goes round and round, but if you start with overall size/weight/displacement, then figure out ama size, you can then find the loads the boat will put on the beams, and determine how to build them. Then you have gotten a handle on
boat type
performance goals
displacement/weight
loads
strength requirements
materials
cost
and can move on to other things like hull shapes, rig choices etc.

Or do as I did and just build a canoe hull and think about all this stuff as you go =the dumb way. I'm pretty sure it would float if I put it in the water though!

 

Thanks BWD. I have a few books on boat design. Actually I just bought 2 good books but there is next to nothing about multihulls. One is by Norman Skene, "Elements of Yacht Design" and the other if by Dave Gerr, "Elements of Boat Strength".

Both good books with a lot of usefull info, although much of it can't always be applied to multi's.

As far as the design, I am building the modified Ulua. Larger, more displacement and as a tri. I think I will keep this project relatively simple since it is my 1st build. The NEXT tri I will build will almost be entirely engineered before the building process.

So do you have any photos of you tri? I will try and post a few of mine. I just finished putting the interior gunwales in and now I will be installing floor, bulkheads and interior hull bracing.

 

You're quite welcome.
Mine is not a tri, yet. Plan to make it a single outrigger for starters, maybe "upgrade" later.,..
My decision was to stay with single ama canoe style for minimum weight and complexity.
And convenient beam <8ft.
With only one ama of ~200 lbs vol, I can get away with aluminum beams that weigh 9-10 lbs each, or wood ones under 15 lbs each, based on loads anticipated. And beams are short for easy storage.
Also I skip the weight of the second ama and beams extended to it. This way maybe the total weight without rig will stick around 130lbs (vaka weighs ~75lbs).

I have an old thread on this board "outrigger canoe plan." You posted in it a while ago, I think?
It partially documents the build, with pictures. Info about the beams on it may not be useful though. I think I said 3"x.125 alum. tube, really it may want 4"!! A stretched tri ulua with a lot of sail sure would....

Since you are going with ulua-based design, I think you may want to stick with fairly small amas, 200-250# floatation (each), and plan on hiking to keep it level.
You will also find you might want a few strong thwarts in it or a deck to resist torsion, if you plan to carry 180sqft in 18kts. And stronger gunwales.

I sometimes think I should have just built an ulua, per directions, but wanted to learn the design process as well as how to build. Certainly would have been faster to complete! Gary Dierking did kindly give me a little advice though, which you can see in my thread.

"Boat Strength" is supposed to be good, definitely goes on the list. Skene is a classic and teaches some lessons....
That Rob James book may be out of print but helps with concepts a bit. Try a YC/sailing club library....

Multi design if you think about it too much, is the same as mono design, raised to the power of the number of hulls.
So by the time you read 9 mono design books, maybe you will know how to design a tri.

My problem is I probably need to read a couple more books yet myself!! And find a pot of gold.

It would be great to see some pictures!

 

BWD said:-

 

oldsailor, that was a joke I was making,

as in TOO much is not neccessarily enough, or correct...
lead? not going there, it's for other boats altogether....

 

BWD. Sorry I mis-interpreted the joke.

I thought about this L**D business last evening as I watched Quickstep and Indian Chief slicing their way thru the smaller monohull fleet --have already cleaned up the whole of the big mono division, in the Monday evening races on Pittwater here.

 

I'm no expert on what it takes to fly a main hull. That sort of performance is foreign to my cruising interests. Before the sport moved in that direction the floats were already at 150%, and I would imagine the floats on boats flying the main hull would be even more bouyant, the 200% mentioned.

I have always been interested in boats that went with minimal amas. It seems to me that at the small end of the spectrum, if one is mostly a cruiser, a small displacement float would make a lot of things easier, particularly on folding boats. The Gougeons have designed many boats of that type, and one could argue the early Tremolino was of that type. Of course if one wants to press on the sail, or for extreme conditions, for fixed beam boats that a moored, there is little downside to large floats. At this end of the spectrum 100% is probably the minimum.

There is more than just the volume at play. Sectional and longitudinal distribution of the volume will determine how much the boat heels for a given healing moment.

 

Flying the hull means the main hull is just out of the water. Although extreme racing trimarans actually do the, the significance for other designs is this condition determines the maximum loads that can be applied. Once the main hull is flying, the loads on the rig will be relieved by the boat capsizing.

Although the ama may be supporting 50% of the hull's weight, the question becomes, "Which 50%?" This is why the amas are typically 150% in volume or more.

When sailing to windward, the pitch trim of the boat may be level and the 50% volume carried by the ama is evenly spread over the length of the ama. But when sailing on a broad reach, the thrust of the rig will depress the bow, and the 50% encompasses the entire bow and everything below a diagonal line running back and down along the ama, with the stern perhaps completely out of the water. The submerged volume for the upwind case is different from the submerged volume for the downwind case.

So you have to consider the most critical loadings and make sure you have adequate buoyancy in the ama for each load case. There will be some part of the ama that is immersed for a particular case, but not not immersed for others. When you add up the submerged volume for every case, you get a number that is greater than 100%, even if you never approach 100% for any given case.

Typically, diagonal capsize is the Achilles heel of a trimaran. Upwind, the sails can be luffed if the boat is overpowered. But on a broad reach, when the boat speed is reducing the apparent wind, anything that slows the boat down will also result in increasing the apparent wind. And the mainsail may not be capable of being luffed. If the bow of the ama is under water, the extra buoyancy that is added with further ama submersion is added progressively aft, where it has little effect on stopping the nosedive.

 

I know Kurt Hughes is a big fan of 200% amas way out there. Many of his Trimarans are almost as wide as the are long. Lot of good material here.

http://www.multihulldesigns.com/