Trimaran Design Concept with longer foils than the main hull

Hi, I am new to these forums, although I have already met a few of you on Gary B's Sid forum. I am trying to pick up as much as I can about boat design, and specifically about multi-hulls.

Has anyone designed a trimaran with a shorter main hull than its foils? If so, what would the pros and cons be for such a concept?

My thoughts were along the following:

1) Cheaper construction for a longer LOA (the foils are less complex to build than a main hull with cockpit, tiller, keel/dagger-board, cabin, etc)
2) Detachable for carrying on a trailer with the main hull, foils can break in the middle with a grooved hinge at the bilge and double bolt lock on top
3) Overall lighter construction and less wet area
4) Potentially greater longitudinal stability if the center of mass with the main hull is set more aft
5) Greater BOA and lateral stability and righting moment if the amas are long and able to fold parallel to the foils after detaching from the main hull.
6) Smaller frontal cross sectional area aerodynamically - which I suppose would mean less drag as well for the wetted surface area.

Thoughts, ideas, criticisms anyone?

Thank you much in advance!

 

All this talk about foils, I think you mean outriggers? (or amas depending on the language you speak)

yes it has been done, Derek Kelsall and Malcolm Tennant spring to mind

Remember that the hull(s) in the water are still making waves, and generally the longer the hull the "faster" it is

Take your idea to the limit of course and you end up with a catamaran

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Richard, thanks! I am learning what things are called. I have seen some refer to the side hulls as foils and have seen some sites that talk about proas use the term outrigger - so I thought it was perhaps more geared toward proas. Anyhow, I will certainly take a look at the designers' work that you mentioned. Did you have any particular models of theirs in mind to take a look at? Thanks again.

 

I was thinking, for example, that the outriggers could be 10M LOA, while the main hull would be 5M LOA. I would like to have the main hull in the water so as to keep the center of gravity low. I wonder if there would be significant structural problems to this design as well as potential challenges to steering the vessel. With such a large discrepancy between the hull length and the outriggers, it would seem that the outriggers might be best outfitted with dual tillers as on catamarans. I am wondering, however, if it would be more beneficial to have the main hull in the dead center of the outriggers rather than more to the aft. This seems like it would be more longitudinally stable when heaved to. Additionally, there should be still be some advantage to having the center of mass further behind the sail rigging when moving forward.

I looked on both desingers' sites, but I could not find anything that looks remotely like what I am envisioning. I think, perhaps, the closest design was tennant's Firebird, but only because the hull was set slightly aft of the outriggers; in actuality the main hull and the outriggers appear to be the same length.

I saw on another thread where you and some others were discussing the forces involved with telescoping amas. I thought that with the addition of length to the outriggers (as opposed to breadth - although that would naturally be also desirable, in order to stay within a preferable LOA to BOA ratio) that careful consideration should be given to the structural integrity of the tubing connecting the outriggers to the main hull.

I would like to find out more about boat design in general, and any resources that discuss the engineering aspects of hull design, righting moment, and the various forces addressed in boat design, and especially with respect to multi-hull desing would be most helpful to me. I am not an engineer, but I do understand some basic physics, and I am eager to learn.

Thank you in advance for any advice or help you can give to me.

 

You could ask gary about his three devils concept that he is working on, it has long floats with foils. It might be a bit large overall but might be a staring point.

 

Thank you, Warwick! Also, I just realized that I probably have mistakenly been using the term foils for amas, thinking that amas were the poles that attach the outriggers to the main hull, and not realizing that foils probably refers to hydrofoils. I can see that I have much to learn! Thanks everyone for being so kind and instructive!

 

Foils are foils ... and this means dagger and centreboard, lifting hydrofoil and rudder - and any balancing boards too, although the latter are not common on multihulls. Ron Given put balancing boards on the original version of 40 foot catamaran Split Enz, (fastest 40 is Southern Hemisphere for many years) two daggers and a rudder to each hull and spread apart so that by lifting or lowering the board combinations, the boat would be perfectly balanced and virtually steer itself. However it was complicated, another thing for crew to look after and later was changed to more conventional, one board, one rudder per hull setup. The NZ working scow of 19th Century also had two boards, sometimes three, plus rudder to the wide and long, chined and flat bottomed hull. You can say that all balancing, steering appendages below water are foils.
Amas are the floats, akas the connecting beams; the aka term is not common, people seem to prefer the word beam.
The Decision 35 of the European lakes are close to a short main hull, long ama trimaran configuration, but they are really catamarans because the central hull/pod? rarely touches water. As Richard says, Kelsall and Tennant had true trimaran designs like so.

 

Thanks Gary! I appreciate the help with the proper words for things. Having the right vocabulary changes everything I will take a look at Decision 35 and look again for Kelsall's and Tennant's trimaran designs. I don't know if I did not look thoroughly enough, if the designs are a bit older or less popular or if maybe I was looking in the wrong place on their websites, but I actually couldn't find any of them. I didn't look under power boats, though, just under sailing boat designs, and I glanced over the images associated with each design to look at the hulls. I probably missed something somehow. Thanks for encouraging me to take another look!

 

For what purpose?

A tri or cat will be very stiff (owing to the size and spacing of the hulls) and the VCG is of "minor" importance in this regard since their GMs are invariably very large owing to the large waterplane inertia for their size.

 

Here's a couple of long float Tennants, a Wild Thing and Demon Tricycle design from 20 plus years ago.

 

Low Center of Mass

Ad Hoc,

Thanks for the question! I am not necessarily set on the idea, but my initial thoughts were along these lines:

1) I read somewhere on the forum that catamarans have a weight limitation that a mono hull does not inherently have topside, and because I am thinking in terms of using a cabin on the main hull that would necessarily seem to imply the need for more floatation in the amas and a deeper draught, or reducing the cockpit of what would have been the main hull to increase the size of the amas to include water-tight cabins.

2) I also read on the forum that trimarans have a shallower draught, and that they would be easier to haul up on to a beach - which seems quite practical for a single hander to be able to do.

3) I read a some conflicted discussions about trimarans being faster than catamarans as well, although that seemed to pertain more to conditions with lighter winds. Here, I sort of would like to apply my strategy for freeway driving that I developed from living in the metropolitan Houston area. Having power and speed make all the difference in being more manoeverable, and therefore "safe" (when properly used) in my opinion. Having already experienced being drawn out to sea because of too little power against the Pacific current on my way from LA harbor toward Catalina Island, I would prefer to err on the side of more speed and power in the future - at least as a rule, generally speaking, that is.

4) I had also read somewhere (maybe not here) that a lower center of mass is desirable for one or more of the following (which, unfortunately, I have forgotten - maybe someone can help clarify this for me) - capsizing, hobby horsing, pitch poling.

Additionally, most of the trimaran designs that I have seen seem to have beams that slope in downward from their amas toward their main hulls.

Thanks again!

 

Thanks, Gary. Loading the pictures up as I type...

 

Wow. That tricycle has an interesting cockpit! What is truly interesting here is that the beams do not seem to slope downward toward the main hull but to go straight across, rather, and the expanded cockpit above the main hull looks like it was designed so as to not increase the beam of the central hull. I wonder, on his other tri, what that thing is that is projecting slightly below the right ama? It looks kind of bulbous.

 

I agree with Adhoc, the vertical CofG is less important on a multihull than on a monohull. From a stability viewpoint it only becomes important at high heel angles. Mast height and weight have a big effect on the VCG

Of the three you mention, having the CofG central will help reduce hobbyhorsing, but that is a minor effect compared to changing the hull shape

I think you should look at more videos of sailing multihulls and also try to get sailing on one. Its really hard to learn from books!

Also consider that if a few world class designers, like Kelsall and Tennant have tried the concept you suggest but didn't pursue it - should you?

Adhoc is very keen on saying "what is your SOR" meaning Statement of Requirements or basically, "what do you want the boat for?"

So you need to clarify that before doing any thinking about design. Most designers design the complete boat in their head before putting cursor to paper or pencil to screen. But you can only do that with a clear idea of what you want

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

I really like the design of Wild Thing. I can see how setting the main hull back allows for greater sail area, but one thing I wonder about is how stable she is in less of a racing situation, say cruising at sea at a slower pace, or even hove to. It worries me a little.