Trimaran Design Concept with longer foils than the main hull

Thanks guys. I appreciate the help

That's a pretty cool way of getting the displacement Oldsailor7 . What I wish is that there was something like that for finding surface area, but the shape is too irregular, so it has to be calculated.

I just watched an MIT Courseware video on The Simpson Rule, and that helped me solidify how to get the transverse areas of the cross sections per station, but I am still trying to find something that better explains Taylor's Secant for WSA.

I did find this cool Naval Architecture Course online from India, here.

It is actually explaining a lot of terms in the first lecture that I didn't understand!!!

 

Richard, thanks - I was wondering how relevant leaving out the calculation for the bow plane perpendicular to the longitudinal axis would be for calculating the WSA, since the corresponding area at the stern is of little value. I wonder if instead an oblique nose would have positive effect on wave breaking in a cat or tri hull.

 

That's not really a rule of thumb, rather it's the definition of Cp. Ie the proportion of 'Boat" in a rectangular box

I suspect that many people use a Cp nearer 0.6

Richard Woods

 

Wan Hu launches from his rocket arm chair

1...2...3...

 

Being pedantic, this is the definition of the block coefficient, Cb, which is volume divided by (Length x Beam x draft) not the prismatic coefficient, Cp, which is the volume divided by (Length x Area of largest transverse section).

Generally large values of either indicate a full form, and hence high wave making resistance, but for very slender hulls higher values are achievable with good resistance characteristics, so typical multihull forms tend to have values a little higher than mono's.

 

That's an outdated picture. You want the version as she was in Valencia, with the newer, longer amas:


Of course, a design rule that only limited the waterline length may have something to do with the proportion of ama length over all to main hull waterline length...
(The length of the amas was unbounded, since they barely kissed the water in the measurement condition.)

 

When designing the size and position of the amas, you should construct a mulithull footprint diagram like this:


It shows the virtual center of gravity location (the c.g. location that would provide the same pitch and roll moments as the sail rig) (black lines). It also shows the combined center of buoyancy of the two hulls as a function of pitch and heel angles, with the total displacement equal to the weight (blue lines). (You can include the down-force from the rig if you want to get really fancy.) Since the virtual c.g. and the center of buoyancy have to coincide, you can read off the heel and trim angles for each sailing condition.

You can also convert stability indices, such as from Shuttleworth, and do the same thing.


Diagonal capsize is generally the most critical condition for a trimaran. The footprint plot helps to strike the right balance between pitchpole, diagonal, and sideways stability.

 

I would think the most important thing would be its intended use.

From what I understand the Malcom Tennant Wild Thing design is measured by overall length for racing which has put it at a disadvantage to other boats. from what I understand Malcom Tennant came up with the flying circus design to build upon the Wild thing design for semi off shore use.

What you have is basically a big little boat, rather than an equal/ full size boat.

It might be better to go with equal length floats, for a similar level of fore and aft power. Based upon what has been learnt from other designs.

 

Thank you & some questions about hull stability

Tom, thanks much! I appreciate your response and looking me up here. I am not sure that I am quite capable as yet of understanding these diagrams other than intuitively; however, I am making progress in learning about design, albeit a little slowly. I am just getting a grip on bouyancy, and displacement from this online course in naval architecture, but your diagram brings up some questions that I have regarding hydrostatic stability.

Given (using a monohull as an example initially, simply because it is easier to understand right now) that we can calculate for the distance GM being greater than zero using the hull, I was wondering if it is likewise possible to scale one's sail plan accordingly, and if in so doing, that there is a particular angle at which the area of the sail from the topmast will taper off so as to minimize the greater effect of leverage by the wind at its highest altitude on the sail. I am not to the mechanics of sail plans yet, but I hope to be there in a few weeks or so. Books are on order while I am learning from what has been made available online.

My second question has more specifically to do with trimaran hull design and bouyancy. On a catamaran, it is somewhat obvious to me that displacement and bouyancy are quite similar to monohull design in that there is essentially one overall level of displacement to be considered. But on a trimaran, there seems to be a greater range of variability in selecting the amount of bouyancy and displacement with respect to the amas versus the main hull. At heel, it would seem that the mechanics would be similar to a catamaran in that some equilibrium would seem to be established between the main hull and one of the amas, except that the mast would already be at an angle. I suppose my question would then be, what are the design limits normally observed for the amount of displacement and bouyancy in the form of a ratio of main hull to ama? Another way of putting this might be to ask more specifically what the preferred ratio should be. At what angle of heel should they bear a symmetric mass distribution, and how does this have bearing on the designing for the maximum amount of heel where the distance GM is still positive?

Last, is there a certain margin between GM = 0 and GM > 0 where design specifications should fall for practical considerations (non-racing vessels)?

 

Richard, nice Banshee BTW. I would love to see what the inside cabins and navigation room look like. I know she is supposed to be roomy, but is she fast? How did you accomplish that? Skene says you can rarely come off well with even two design parameters on a yacht, so it sounds like quite a design feat!!

Anyway, I am slowing down a bit here with this concept while I try to dig in a bit and learn more. I'm still keen, but I may not have a lot to post during the next couple of weeks...

 

Help with the hull shape?

Ok, finally, I believe I have some basics down for being able to establish a mathematical relationship between hull displacement and wetted surface area that makes sense to me (hopefully I will broaden this understanding as I proceed). Although I have yet to understand Taylor's Secant Method and its pros and cons (although I do understand that various methods used for calculating WSA, from the WSA thread, do have pros and cons, and therefore leave something to be desired if the calculation can be done more accurately, for example by good CAD software), I do understand the primary method for obtaining surface area in that it is comprised of taking the product of two perpendicular coordinates between a set of descriptive bounds, such as the intersection of the LWL with that of the Keel with that of two stations.

Obviously, I then had to search for a way to calculate the length of a curve in order to obtain the length that outlines each station from LWL to keel. I believe that for the purpose of approximating the area for each section, that it is enough to take the mean length between any two stations to be as long as there are not any "critical points" on the keel line between them (no changes in direction, plateaus, etc). But in order to do this I still curve of the the keel (the LWL is a straight line).

Anyway, at this point, I am beginning to think in terms of the shape of the keel line and in terms of the shape of the transverse cross sections of each hull. I admit that my math skills leave something to be desired, yet I was able to track down the formula for obtaining the length of a curve. The problem is that it involves integration, which necessarily implies that I begin to think in terms of shapes.

That leaves me with basically two approaches:

a) designing the transverse (and also longitudinal) hull shapes by just taking some existing designs and playing around with them to come up with what I think would be reasonable shapes, and then integrating them to find the length of their curves to the half girth, and from there their perimeters, from which I would subtract the beam at the waterline (I could use software for this),

or

b) looking at shapes for which there are known mathematical solutions for their curves in order to both create a nice line, and to reduce the amount of integration should more than one shape be used below the waterline along any given axis.

I believe that the second method will yield the most pleasing, economical, and hopefully easily scalable results.

So that leaves me asking the following questions which hopefully more experienced members of the forum will be able to help me to decide the answers to:

1) What are the best transverse hull shapes, and why,

and

2) what are the best longitudinal hull shapes (including dagger boards, thin keels beyond the hull's displacement, etc)??? :?:

I read recently, that the streamlining of fish bodies is quite efficient for movement through water. Of course this makes sense, but one of the things about fish is that they move through the water, under water. Early ships seemed to be keen on a hull shape quite similar to that of a fish body, and used ellipsoid shapes for their boats. Viking boats, canoes, and kayaks all make use of the ellipsoid shape.

But I am not convinced that this would be the best shape to adopt, either transversely or longitudinally:

A) There are fineness (Cp), turbulence, wave cutting, and WSA all to be considered.

B) There are square versus round keels to be considered.


C) There are dagger boards versus fixed keels.

Can someone help with the trade offs for each of these designs? I would think that each design has at least one pro and at least one con...?

Thanks in advance!

 

Further thoughts on in differing hull lengths: drag

Ironically, I thought I would be posting less. Oh well!

Something occurred to me as I was browsing some of the older threads and saw an R/C trimaran zipping in and out between a pond full of slow mono hulled models. As I noticed how trimaran stability works, I became increasingly concerned about how a shorter main hull might affect stability when actually in motion.

For one, I noticed how the R/C model, like some of the sleeker trimaran models I have seen, all have very similarly thin hull shapes. As I watched the R/C model sail, it became more apparent to me how important it might be to have a similar hull configuration for all three hulls so that there is no drag to slow one hull down, and cause the tendency for the boat to want to turn toward the most resistant hull.

I have never sailed a trimaran, so I don't really know what I am up against here. But I have noticed that none of the models that had shorter main hulls had hulls that were as short as the one I have suggested (1/2 of the 10M amas = 5M).

Practically, from those of you who have actually sailed on trimarans of different kinds, and especially hats off to anyone who has ever sailed on something like the USA 17, in your experience, would there be a significant stability issue created by the difference in drag between the two hulls when heeling to one side? And if there is a problem with the drag, is it something that only acts significantly on the turning plane that is horizontal to the water surface, or can it act to pitch the trimaran diagonally with a increased risk of capsize?

Thanks.

 

It seems to be that it would be important to know whether the short main hull was designed to plane and/or how low a wind velocity will fly the main hull-or maybe the main hull would not fly?
The Farrier tri's use a planing main hull coupled with a high L/B ratio displacement ama. Same with Antrim, but he uses planing ama's as well-at least on one boat by his own description. The small Weta tri does not fly the main hull but the main hull planes. The amas are skinny displacement hulls.

 

Having planing outriggers are not ideal from a seakeeping/motions point of view. A planing surface outrigger produces a very noticeable "snap" motion, compared to a canoe/displacement type outrigger shape.

 

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Ad Hoc, just out of curiosity: are you familiar with Jim Antrims work?