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Can I use a monohull mast and sail on a trimaran ?

QUESTION:  If in good shape, can I use a mast and sail coming from an old monohull on a trimaran like your W17 or W22?

 Brad-W, San Diego, USA

ANSWER:  With its many nuances, this just maybe the most frequent question I get ;)

While most of the answers are already on my website, they are somewhat scattered about as they deal with different issues in different sections, so let me bring it all together here.      Unless you are quite ok to sail with a boat cover or bedsheet, then the short answer is ‘NO’.  But of course, it’s reasonable that you will then want to know ‘Why?’.  So here goes.

There are many differences between a monohull and a trimaran, but two stand out relative to this question.   One is stability (re: the mast) and the other is boat speed (re: the whole rig).

Mast loading is a direct function of the boats stability. The more the stable boat can resist heeling, the more load the mast has to take. Heeling relieves the load.

Except for large catamarans, the trimaran in general has the most inherent stability of any of the small boats, be they monohull, catamaran, proa or trimaran.   (Very wide heavy catamarans can also have high stability until the windward hull lifts out).

The monohull starts off with mostly only stability from its waterline beam, but this picks up as it heels due to its weighted keel gaining leverage, to offer a maximum when laying over at 90 degrees (assuming the cabin is kept watertight).  But at that point, there is very little load on the mast, sails or rigging, as it is now parallel to the wind, more like a flag.  This limits the maximum load on the mast and sails to the initial gust.

Catamarans have maximum stability at about 10 degrees only, when the windward hull comes just clear of the water, acting as a dry weight to windward (plus crew weight) that creates this maximum righting force.   After that, with increasing angle of heel, the stability ‘goes downhill’ as there is no additional source of counterbalance available.    Generalizing,  once the catamaran gets passed about 30 degrees, once starts to sail ‘on a knife blade’ that is the leeward hull.    If the design also has a centerboard in the leeward hull, that serves as a tripping point and many boats will capsize.    (So having a centerboard on the centerhull or windward hull only, is a safety feature, as this potential ‘tripper’ is lifted out as it heels). 

But again, as for the monohull, the high heeling catamaran unloads the sail force and load on the mast, so masts can be lighter in weight than for the more stable trimaran.  This is particularly true of light beach-cats that heel easily.

Depending on whether it’s an Atlantic or Pacific proa, the stability of a Proa can vary. The Atlantic proa is like a trimaran without the windward ama, so depending on how far apart the leeward buoyancy hull is, this may have a greater stability than a catamaran and can be close to that of the trimaran … except that with the dry windward hull-weight missing, it will always be somewhat less when all other dimensions are similar.   (The first Cheers designed by Dick Newick that successfully raced across the Atlantic in 1960, was like this (see photo).  Splitting the sail area into two lower sails, also reduced the heeling moment.

The Pacific Proa is a potential speedster as it can balance on one hull much of the time as the 2nd hull is now to windward and could house a little accommodation or at least crew members for weight, so that the ‘basic’ stability is generally even less than a catamaran, if the distance between the hulls is less.    Of course, to totally prevent a capsize, the Pacific proa typically has a short emergency pod of buoyancy that only goes in the water at high heel angle to spill wind from the sail(s) and in the case of the 31ft Pacific Proa Madness shown here, this leeward pod serves as low headroom bunk space.

So mast loading on these boats is not high, which is fortunate, as particularly on the Atlantic Proa, there is no windward ama to attach a windward shroud.  So these boats quite often use unstayed masts and split the sail area  between two masts which also lowers the load on each.

(Although not pertinent to the question at hand, I will add that proas are unique to sail, as they stop and reverse to tack .. so for that, they are always double-ended.   Although far in the minority, enthusiasts like the relaxed pause with a feathered mainsail, as its like being briefly hove-to at each tack, when you can do odd jobs or even take a quick snack).

So what does this all mean for mast movement from one boat type to another ?

It means that the section of a beach-catamaran mast for the same height is too light for the more stable trimaran and the same would generally apply coming from a monohull, but in the later case, monohull masts are also typically non-rotating, whereas most multihull masts are, since they work efficiently with the required flatter sail .. see more below re Sails.

Trimarans are stable primarily due the buoyancy of their amas.   This stability acts automatically and requires no extra weight. and is a prime reason these boats can be comfortably sailed by both the elderly or those physically challenged.

So when using a beach cat mast on say a W17 trimaran, one needs to at least use the heavier one from an 18 footer and then, to make it relatively stronger (or less stressed), a shorter length is also used …. taking off perhaps 2ft from the bottom and 1ft from the top.  Even then, the diamond stays may be marginal, as these have failed before on such a substitution, underlining the extra load put on the mast by the more stable trimaran.   

Using a non-rotating monohull mast on a trimaran is really a major mis-match as at the added speed, much efficiency will be lost due to the inferior aerodynamic flow at the higher relative wind speed.   A multihull mast is also ideally a non-flexing wing, whereas masts on monohulls typically have some fore & aft flexability to flatten their fuller sail.

Now to the Sails …

All sails are (well, should be) cut to suit the relative wind speed they are to work in and in addition, most monohull masts have a chord to width ratio of 1.6 or less, which means that it will/can readily bend fore and aft.   This is a useful feature on a monohull as the mast can be straight when in light winds giving a very full sail shape for the slow air flow, yet can be bent in high winds, to bow forward and pull out most of that extra sailcloth to flatten the sail for raised efficiency and less heeling force.

However, multihulls sail faster so the relative apparent wind is greater.   This requires sails that are cut flatter by the sailmaker and are very different from the fuller sails on most monohulls (unless from a boat of very high performance).     But because they are cut flatter for the higher speeds, they would be too flat in light airs, so they ideally need to work behind a rotating mast .. with a wingmast (chord/width over 2) being the more efficient.   With the higher chord, the mast is always designed to be rotating to be efficient and effective on both tacks.

If you look at this sketch, you will see how the rotating mast replaces the deep camber of the soft sail with a non-rotating mast, illustrating the considerable difference in sail shape between that for a rotating wingmast and that for a fixed mast.


This is why the W17 trimaran is designed to take advantage of all these attributes, namely with a full rotating wing mast and a fairly flat sail … with the higher race rig, the wingmast is of carbon fiber.   Because such a mast pro-built is extremely expensive (double the cost of the rest of the boat!), I have developed a now well-proven design for the Do-it-Yourself builder.   Different procedures but less complex than even building the boat, the total materials for the W17 carbon fiber wing mast are around $1000 in the US and this is by far the most efficient way to rig such a boat.   (If interested, read what a recent builder had to say about it HERE).   l

Yes, the rig (cut down to suit) from a larger catamaran can work, but as the stability of the typical beach cat is SO much less, the lightweight sail material used (often only 4 or 5oz) will only last 2-3 seasons on the more stable trimaran, so IMHO this is a false economy.    And as (*I hope) was clarified above, the rig from a monohull is a real mismatch which would not allow anything close to the real potential performance of the W17 to be achieved.

Hope this is all clear now  ;-)

My advice, if you want to sail efficiently and will be keeping the boat for more than say 3 years, is to go for the rig designed for the boat .. with a rotating wingmast and matching flat mainsail of heavier material, be it either the cruising or race version.  These comments equally apply to the W19 and W22, as well as to most other performance trimarans.

Mike   …. July 2021

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