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Weight Control

First of all, let me say that building a small wood trimaran down to weight IS a challenge.

As I've explained elsewhere, a trimaran initially (when launched) generally floats on just ONE slim hull and can sink noticeably lower if it's much overweight— generally more so than even a catamaran that has two hulls to share the load. Of course, at a certain point, the amas will touch and start to add their own buoyancy, but perhaps that's not what the designer intended at the initial loading. This challenge is increased if the boat is a small one, as it still needs most of the same parts, but now they are pushed closer together. So one needs to save ounces (or grams) on every part. Though this can apply to other wood trimarans as well, let's looks at this challenge as it might apply to the W17. (It is hoped that these options will not confuse the first-time builder. But while there are specific suggestions in the W17 Build Manual, we hope these thoughts will guide the new builder when deciding between possible options that can reduce weight and therefore enhance performance.)

W17 Ama


The first thing is to decide on ply thickness for each part. This can be a function of your own personal needs as if you're very heavy, you might for example, need a thicker deck and cockpit sole than someone much lighter. The plans and manual give proposed figures but this also depends on the quality of ply you purchase. Designers typically suggest that all plywood be of marine quality as this is a safe call. Some parts that will see a lot of water or exposure, will clearly need to be of marine quality and probably the lightest marine ply is Okoume (Gaboon). But for parts that are easy to sheath and enclose, you may be able to find some good quality Lauan ply that is still good for external use but not officially a marine ply. Often such ply will be even lighter and certainly less expensive. It depends what you're comfortable with. After all, your decision to build to any small boat is at your own risk so taking a decision over the materials used and potential life span, is also part of that.
ALL plywoods use 'wood of varying quality' though the variation is more limited and controlled for a marine ply. But if you can find local lauan with a dark, almost black, glue line, there's a good chance that at least the bond is waterproof. (You can try boiling a scrap piece for an hour to find out.) The other main differences with marine ply is that there are no voids and any defects in every veneer used are much smaller than for general-use plywood. But if we are sheathing both sides (as is highly recommended for all non-marine plywood), then the quality of the inner core layers becomes less important, PROVIDED the edges are well sealed and any veneer gaps are filled. So the thing you will need to do with non-marine ply is, after cutting the part out, check the edges to find any voids and then close these by pushing a long pointed stick down into any visible gap after coating 1t with epoxy, to seal off water penetration to the inside. (The small wood skewers sold for shish-ka-bob's generally works well.) Using such ply for small hulls under 16ft (and that includes W17 amas) is a real possibility and even for the sides of the small tri main hull that generally do not take high loads. But you will want to select your plywood carefully, with the best pieces saved for more loaded parts. For all plywood, weigh each sheet before marking them out and for the W17, allocate the lightest for the after end parts, as there is not only less reserve buoyancy there, but also less stress.
Local areas can always be beefed up with a little added glass or even carbon fiber tows. 'Tows' are strands of non-woven fibers that can be resin-impregnated and laid in the best direction to carry a particular load and when the fibers are well spread out at the ends, distribute that load out over the surface they are bonded to. The main strands of glass woven roving are also tows and can be used the same way, but keep in mind that glass has far more elasticity than carbon (with Kevlar® between the two). I use tows quite often, as they are one of the lightest ways to make a strong joint. If you still want the wood to take some of the load, then using glass tow makes sense to me as its elasticity is close to that of wood. The combination will flex more than when CF is used, but then the wood will carry a greater share of the load.

If using lightweight non-marine ply for side panels, consider adding an extra (cedar) stringer to break up the vertical span. Non marine (but waterproof bonded) ply can also work for bulkheads, unless they are under high local loads (from mast or beams) or for the forepeak 'collision' bulkhead, or for mounting some high load attachment such as for the forestay, when they certainly MUST be of good marine quality. When you sheath, use your squeegee fairly vertical for the final passes in order to remove all excess resin (pic).

Plywood for the bottom, dagger-board case, transom, foredeck, cockpit sole, and main beams all needs to be of marine quality, and for the cockpit sole and decks, pre-sheathed on the underside to take loads from above.


[Added 2022]  Sheathing can be a large debate.    But if the lightest boat is required for race performance, you will have to cut back on sheathing and accept to adopt a higher standard for 'maintenance & ventilation'.   Wood NEEDS both, there is no avoiding that, though sheathing does better assure a minimal thickness of epoxy and protection.    But I have seen poorly built 10 year old boats with multiple coats (4) of epoxy that were totally rotted-out due to lack of drainage and ventilation, yet 25 year-old ply boats with no epoxy at all, that were still in great shape due to surface wood preservatives and great ventilation & drainage.   Where the internals do not need glass for strength, the use of 2-3 coats of aluminum paint is very effective for both rot protection and low weight, but areas that will see water sitting there for hours on end, must be pre-coated with at least 2 coats of epoxy (after first filling and fairing to avoid all sharp corners) or the boat will soak up way more water weight than the extra epoxy and cloth weight you thought you were saving!

In general, vertical surfaces drain immediately and survive well without sheathing and that includes bulkheads and upper sides.    But the exception is for the enclosed spaces inside a non-accessible ama, where most of the future problems will develop. For this reason and also for lifting on a folded boat, I think the use of rigid foam core (like Corecell) to be a great idea, with skins of fiberglass or carbon fiber (CF) if your budget allows.   But CF is a brittle material and not always the best choice over the heavier but more flexible fiberglass, so the potential loading at a specific location on the boat, needs to be taken into account.   Fiberglass may take a collision without shattering, while CF can puncture and shear like an eggshell. However, if protected from that, CF is lighter with far less stretch .. but not always that much stronger.    It's from this type of analysis that I chose to specify fiberglass for the hinges and latches, as the extra (triple) give of fiberglass within a thick laminate allows ALL the fibers to do some work and not risk any brittle failure, whereas with CF, there's a rish that some initially tight fibers can take an overload and prematurely fail.   

If there is a surface with a potentially high loading on one side, then I like to use a light but strong fiber sheathing on the reverse side as a tension sheet.  A deck or cockpit floor is a prime example .. but can also apply to hull sides or bottom that are at risk from exterior load .. such as from a collision or sitting on a rough seabed.     Areas away from collision risk may not need such protection and can be built lighter ... such as the sides between a main hull and the ama.  But amas ARE at high risk of being flooded at some point in their life and as there is little access, must have surfaces well sealed but also allow full drainage to some accessible point(s) that can be pumped and sponged out.    Provision for ama ventilation is a MUST ... especially important when laid up off-season.   I use hatch covers that I modify with mosquito screens, so that air can move in and out, without wasps, ants or mice taking the same path!   (Why on earth, hatch makers do not supply this important winter option, is beyond me!)

Later, you 'can' externally sheath the whole boat and this will add a little extra strength to the whole panel for load carrying, wave effects and mast loads.   This sheathing is typically 'the 6oz boat cloth' that gives goodlife, but a compromise can be a 40z that takes less epoxy weight to cover.     The use of Dynel cloth should also be considered as its quite resistant to abrasion on the outside and lighter too, but, like kevlar, this requires some care and attention in application, to overcome its tendency to float.   

But personally, I generally only sheath externally over the bottom and up to about 50mm above the waterline.  Above that, I epoxy coat only, unless I also selected a ply thickness below specs that needs the sheathing.

As fiberglass IS pretty heavy as fibers go and at least doubles with epoxy, its not something for every surface IF you want the lightest boat.  Performance-oriented Dudley Dix for example, does not often specify FG sheathing even on the outside of most of his large monohulls, but simply goes with epoxy coating and paint.    But not all will want to accept this for a boat you cannot easily inspect every day.    But excessive weight DOES factor into performance, as I have pointed out here.

By the way, when preparing to pre-sheath plywood, it's a good idea to cut the cloth fairly closely to shape and then weigh it on say a kitchen scale. Then, when you mix up your resin, keep track of what weight you use as you should be able to wet-out your cloth with a weight of resin that's only slightly more than your cloth weight. It will be a challenge, but a 50/50 mix with a hand layup like this is 'just' possible and most desirable.

If you do select to use marine plywood for longer life and less risk of failure, then the thickness you select should affect your decision about pre-sheathing. In the case of the W17, if you use 4-5mm (3-ply) for the sides, then a thin internal sheathing is still recommended but if you use 6mm (typically a 5-ply), the sheathing can be dispensed with and the surface simply epoxy-sealed or later painted with 2–3 coats of light-weight aluminum paint that protects almost as well. All these solutions are workable and their combination can permit you to meet a tight weight target, but if you buy 6mm ply and then ALSO sheath both sides, you can expect to take your weight somewhat over the target.

The loading and function of each and every piece needs to be well considered before deciding on its final thickness and what sheathing if any, is to be applied. UNI cloth is best for one-directional loads, tension or compression, while diagonally laid bi-directional or bi-axial cloth is best for areas under shear. General external sheathing is most commonly done with a so-called 6 oz bi-directional 'boatcloth' or sometimes even with a light-weight twill, and its laid direction depends more on how best to lay the cloth for good contact with the surface. Laying diagonally on the bias is best for the sides as the diagonal bias lays better over any corner and is slightly stronger too. but takes a little more material. It's also harder to pull flat, compared to an end-to-end layup.

Joints & taping

Quite a lot of filled epoxy is used for this and as this is quite heavy we need to keep this to the minimum, consistent with adequate strength. First of all, the filler material used will dictate the required size of fillets. Strong (WEST404) filler can be a little thinner than say one using 'wood flour', but the addition of a glass tape over the top will add even more strength and endurance. Using microspheres (or microballoons) is a good all-round filler of medium strength that sands acceptably. Fillets are sized by the radius of their internal curve and if that radius is fairly large, say 3 times the ply thickness it is joining, then often no further glass taping is needed provided the filler used is at least of medium strength. (ie: I'd be reluctant to use 100% wood flour without also taping.) For non watertight bulkheads, I typically do not tape these if weight is critical, and I've used fillet radii down to 2 x ply thickness without problem. But just to give an example of this, if you are joining two pieces of 6mm ply at 90 degrees, then the fillet needs to be at least 12mm radius so that means using a 25mm (1") diameter dowel (or 1" wide tongue-depressor) to get that radius perhaps larger than you first envisaged when I said 2 x ply thickness.

For a weight conscious project, here are some limits that I commonly work to, that have proven adequate with the respectful way I treat my boats. (I drive them hard but always distribute loads to avoid those brutal peaks.)
If you are joining a support frame that will take beam loads or some other force, then that frame needs to be taped both sides after you have a clean, strong fillet in place. I commonly use 8.75oz tapes of 1.5", 2" or 3" widths. I typically go for a tape width of about 10-12 times the thickness of the ply I am joining and find this works well. So 2" max for 4.5mm and 3" for 6.5mm and even less in non stressed locations, as long as the glass can lay on the adjacent wood clear of the fillet edge, for about 3 times the ply thickness in width. ie: for 18mm for 6mm ply .. so even a 2" tape can work for 6mm ply in low stress areas.
Pre-woven tapes have a selvedge edge that becomes quite pronounced once cured. Rather than fill to this level and add weight for nothing, best to feather the tape edge down with a sanding block so that it's not apparent to your touch, then whatever filler is used (suggest microspheres again), it will be so thin to be almost transparent. (Some builders cut their 'tape' from cloth to avoid this issue but fraying can then be a real problem.)
Once taped and smoothed down, coat with epoxy and sand between coats. You ultimately DO need a good film over all surfaces, but the tricky thing is to get it EVEN in thickness. A good level sanding via a large disc surface or sanding planks will help achieve that. Like weak links of a chain, a protection surface is little better than its thinnest spot, so rounding everything will help achieve that.
When taping a deck edge, I find it helps to first draw a line on the deck to work to and then wrap the taped edge with wax paper or peel ply to keep things neat and tidy while it cures.

Another thing to watch out for is the tendency to 'just fix things' with lots of epoxy filler—commonly called 'putty'. It's just 'so much easier to fix' difficult areas and joints with putty. But this is a sure way to add weight, so try to do good workmanship so that your putty work is kept to a minimum. Remember that epoxy putty typically weighs double what wood weighs for the same volume.

Solid timber

Selection and choice of solid timber also has an important role in achieving overall light weight.
This seems to be very effected by the local availability of certain woods. In the Philippines for example, local woods are certainly heavier than say the cedars of North America that in many cases, would be strong enough to make the needed connection between two plywood parts. I personally try to use cedar as much as possible though its relatively low strength can mean more work. This is because I often laminate on a stronger 'tension face' to it to compensate. This might be a strip of mahogany for say the lower edge of a beam—20–30% of the total depth. Alternatively, I'll lay on some straight UNI fibres of glass in epoxy over edges that are most stressed. Both work.
One first needs to think about (call this 'analyzing' ;-) what is the role of the solid timber. This is particular so with cedar. Sometimes it's used in tension and compression and in that role, works fine without added glass if there's enough of it for the load—the cross beams of the W17 being one such example. But as a beam, that lower edge is vulnerable and needs something stronger along the lower edge, as in bending, cedar splits easily. Like balsa, it's pretty good in compression—always relative to its weight.
End-grain balsa itself can be used effectively in compression where the akas bolt to the amas (photo above).
But for parts that will be highly stressed in many directions, it's hard to beat a good piece of African mahogany in strength per lb or kilo. As far as a choice softwood for stressed parts, sitka spruce is still the 'gold standard' for strength versus weight and therefore the wood most commonly specified for light aircraft frames. However, like other choice woods, it's becoming increasingly difficult to obtain and price is constantly rising—typically from Norway or Alaska. For that reason, I look to the lightweight cedars, though one must remain aware that only the denser Port Oxford variety matches or exceeds sitka (silver) spruce in strength and that seems even harder to find. For boats over 20–25 ft, though I still think it's worth shopping for the best strength-to-weight woods, as the investment in labor and other equipment materials better justifies the higher timber cost, especially as not too much of it would be needed anyway. But for small boats under 20 ft, I feel the more common western red cedar can do a good job, as the section used is proportionally larger on a small boat.

Here's one table of wood strengths for reference and it's interesting to compare woods by dividing strength values by their density and choosing from highest value woods that are available to you:

Keep in mind though, that some woods are more water resistant than others (like mahogany, cedar and teak) while others split more easily (like cedar and teak etc).

If solid blocks are sandwiched between plywood skins, drill lightening holes before closing (pic). Avoid lightening holes that will not be closed over, as extra epoxy in these cavities will be at least as heavy as the wood removed. Also, consider adjusting scantlings (sectional dimensions) of stringers, gunwales, beams etc in line with wood chosen so that if stronger, heavier woods must be used, the section is slightly reduced. Cut or drill lightening holes where core material is not required for strength. (See W17 plans for example of this in outer beam ends.)

Metal Parts

While the wood parts make up the physical bulk of any boat, it's the metal parts and even paint, that contribute a significant part of the final weight. So metal parts should be lightened as much as possible—either using aluminum where possible or by drilling holes in heavier stainless steel parts. Building a boat for salt water use can mean that aluminum is not the best material as corrosion can be a real issue. But if parts can be made of a more resistant alloy (like 6061) and then installed with stainless steel bolts that slip through a nylon isolating collar, the galvanic action can be minimized. Such corrosion is rare in fresh water so just being able to wash off the salt as soon as possible with fresh water, is another approach. More and more, we are also seeing ways to replace all metal parts with composites such as epoxy and glass or carbon fiber. These are lighter and corrosion free. Also, if the metal parts required much forming or welding, then the replacement composite parts are now often cheaper too, especially if your own labor is priced cheap ;-)
This will not apply to things like a standard eyebolt or even a gooseneck fitting, but certainly for custom items like folding systems or special-purpose attachments. Hopefully, designers will help builders to make this switch and come up with inventive solutions to keep both weight and corrosion down. Just by avoiding welds, often part thickness can be reduced as local stresses can be less and that, is progress.  The custom folded steel hinge-latches for the W17 are a good example of this, though the DIY fiberglass hinges and latches are even more weight & cost effective if your time is free.


Most paint is surprisingly heavy and we tend to forget this adds up on any boat. As I've mentioned in several articles, the use of aluminum paint is one effective solution to preserve wood without excessive weight. The other is to make sure that the epoxy barrier over the wood is really intact before any paint goes on, so potentially reducing the number of coats required. This goes back to the filleting and rounding of all joints and is addressed in the first part of this article.
If that is done, then the finish coats are primarily for color and to prevent ultra-violet rays from reaching the epoxy. Two coats should do that adequately if the epoxy barrier was really made watertight first. I also found that using a squeegee to spread the final paint coats seemed to result in using slightly less paint than the traditional roller and tipping brush. The latter comes from days when a minimum paint film coat was required for watertightness but that argument weakens over a good epoxy barrier—except when paint companies want to see you using more of their paint rather than less ;-)
Whatever your leaning, be conscious that paint weight adds up and it's not a bad idea to actually pre-weigh the paint of your choice to know what you're putting on. It's surprising how different types, and even different brands, vary in weight.

Wrap up

So in conclusion, one needs to watch EVERY ounce or gram to really keep the weight down and that takes real discipline to do consistently. The advantage is a more sprightly boat performance and greater ease of handling ashore.

But then the question comes up, can one go too far? Personally, I think so, though it's hard to get there and almost impossible without a fair use of carbon fiber.
Based only on experiences that others have shared with me, I've come up with this limiting empirical formula of L4/215 (L = waterline length in feet) —always referring to small trimarans of course. If the boat weight, less crew (in pounds), goes lower than this, there could well be times when the boat will become a handful to manage and be too light to punch upwind in a good sea without an exceptional amount of attention. Of course, the boat will perform above average in lighter weather if the appropriate sail area is set.
The only production tri I know that had a boat weight below L4/215 was the F25C, and I've certainly heard the noted issues voiced about this racer in rough weather.
The WETA gets close, with an all-up weight of 200lbs, compared to 189lbs by formula, but still ok. The all-up W17 would have to go below 370lbs for excessive lightness to be a potential issue and anyway, my little formula may well prove just too simple.

Of course, making parts too light and weak to take the loading imposed on them, would be another example of 'going too far'. As much as anything, this can depend on HOW your boat is sailed and loaded, as some sailors can be more brutal in the way they load their boats than others. Best to learn about where the high stresses are and then do what you can to see that loads are applied as gently as possible as shock loads typically require at least double the strength.


Another way to save all-up weight is to pick lightweight fittings to replace heavy metal ones. Start with the heaviest items like the mast and boom and then look at the rigging and blocks etc. Look at the options and weigh generally higher prices for lightweight parts against the actual weight saving. The wood/glass wing mast should be a little lighter than an alloy spar and the use of Dyneema/Spectra line instead of steel wire rope for standing (fixed) rigging will save weight too. There is nearly always a trade-off with these changes and in this case, the new ropes are not as abrasion resistant as is SWR and they will require frequent adjustment until they've reached their operating length for the load they will carry—ie: an initial stretching will likely occur.
There's quite a range of weights for blocks that can be used for the mainsheet too, so shop around and buy the lightest you can afford. Sheave blocks for the W17 should not be less than about 55mm for easy running while 40mm sheaves should work ok for the foresail sheets.
Weight can also be saved by building the rudder stock-housing, tiller and bowsprit with cores of lightweight woods and minimum of glass. The plus thing is that the mix of woods can also make them look attractive as well as being of lighter weight.

Check list

For every part, ask yourself where a few grams can be saved.

Wishing you successful weight saving ;-)

Mike Nov 2011   [expanded May 2022 ]


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