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Comparing the Cross 18 with the newer W17

QUES:  As your W17 appears to be ‘not unlike the older Cross 18’ trimaran design, would you care to comment on any differences, and why you felt it necessary to design the W17, with the Cross 18 design still being available?   .......   Harvey, CA 2019

ANSWER:  Be pleased to - being a timely excuse to share some subtleties of design.   But first, let me say that back in the 1980’s when I first got hooked on Trimarans, I was quite a fan of Norman Cross’s designs.   I thought they were rightly ‘conservative in design’ but also that they looked better than most of the competition of that era. Around that time, I also made a point of meeting Mr Cross at one of the early World Multihull Symposiums organized by the late Editor/Founder of Multihulls Magazine Charles Chiodi, along with designers Brown, Crowther & Wharram.  It was to be all part of my early Multihull learning experience and I am still very thankful for all that we can learn from these early pioneers.

But there’s been ‘a lot of water under the bridge’ since then.   His Cross 18 was probably his smallest boat and designed back in the late 1960’s .. now some 50 years back!     Despite that, I would not dismiss a design based on age alone - (still too many Herreshoff beauties around ;) – but for me, I later become aware of a sufficient number of potential design-upgrades, that I felt a new boat could take things to another level.

I will go through these briefly and explain what they offer … as always, from my personal study, tests, knowledge & experience, but acknowledging that others may come to somewhat different conclusions.

Perhaps the most significant are the hull shapes, so I will spend some time explaining this. 

Most hull designs to be built of plywood, from the 50’s right up to the end of the century, attempted to replicate the round bilge hulls of carvel, moulded ply or fiberglass construction, by using multiple chines.   The Brown, Crowther and Cross boats were early examples of that while the Scarab’s are more recent ones.   (We also have Constant Camber (CC) that has no chines, though this method is more limited in what shape can be produced.  see Construction Methods).   A system using multiple chines works very well for a hull with a fair amount of rocker or what I call ‘a banana hull’, as the lay of each plywood strake tends to be fairer if the ends rise up.  This also gives a fairly Vee'd section up forward that blends most naturally into a rounded, cutaway forefoot.   The majority of boats designed before 2000 show these features, so many have become conditioned into thinking this is generally the best shape for a boat.   But I will ‘stick my neck out’ and say, my studies have shown this not to be entirely true.

I worked for a while on submarines and was reminded that a volume of displacement well underwater creates far less residuary (wave) resistance than one that disturbs the surface water, so when I am looking for a specific volume of displacement to support a given weight, I am looking to see how I can place this low under the surface, with the least disturbance of the water surface.    What NA’s call SWATH vessels are an ultimate example of this with a submarine form under the water to support the weight but with a very fine part passing up through the water surface, offering minimal wave making.   While this is not practical for a heeling sailboat, it shows a direction we may want to consider. 

Sidenote: an underwater bulbous bow can also work the same way, moving buoyancy below the surface, and also helping to reduce pitching. Lock Crowther tried this with some success on some of his catamarans but then dropped it due to picking up too many lobster/anchor lines and weeds etc.  But Nova-Scotian NA Iain Tulloch still thinks it’s worth another try for his new cruising monohull.  (He says he may add a cheese wire to cut any weeds ;)

Also, another thing to consider is that the vee’d hulls of the multi-chine (or CC) construction, also give a rapid increase in buoyancy at and above the waterline.   ‘This is great’ the old guard may say, but it’s been my observation that this rapid increase in buoyancy has a couple of concerning negative effects.

It not only disturbs the surface water, throwing it outward as white spume that flies in the air and back into the boat making it pretty wet, but it also overacts on the bow, throwing it way up in the air, starting a pendulum pitching that can rapidly increase, especially if the wave spacing & speed are ‘just right’ (or perhaps I should say, 'just wrong' ;).   This is further enhanced when the hull profile is of a banana shape, making a see-saw out of the boat.    It’s now possible to look at the banana shape of an older trimaran AMA and say with some certainty, ‘she will pitch a lot’.   This is aggravated when the center of flotation of the ama is longitudinally close to that of the main hull, making a too-perfect pivot point right across the boat.

So, when designing the W17, I have done all I can to get away from both a flared vee’d in section and a banana hull in profile, as well as designing the two hulls with a center of flotation as far apart as practical to dampen out any synchronous pitching.  This has required nearly vertical hull sides, plus hulls with minimal rocker, and with any chine, as straight (‘non-banana’) as physically possible.   Center of flotation was pushed forward on the amas but kept back on the main hull.

But this is not the end of the hull form difference.   We must also consider the cross section.    As most boats, including the multi-chine of the Cross, have a rounded cross section, we must acknowledge that this will permit some transverse flow under the hull.  This will occur when lateral wind force presses on the boat, causing what we define as ‘leeway’.    On a keel boat, the deep continuous side skin greatly limits that side flow, but on a hull without a keel, it takes a very efficient foil to limit that side slip … typically a deep aero-foil dagger (or center) board.    But just pull up that board when sailing to windward and see what happens!  The boat crabs sideways, slows down and steering control is much reduced.  So despite adding its own wetted surface, it has to stay down just to get to windward..

But what if we change the sections of the main hull so that it’s NOT round and make it into a box section with very small-radius corners that will discourage any transverse flow?  Now most of the flow will be along its length and the bonus is that we’ve now also moved some displacement lower down, well under the water surface, enabling the section to have a narrower waterline for the same displacement.

The above thinking is not totally new, even if this application may be.  Sharpie designs by the likes of Ralph Monroe, Howard Chapelle, Phil Bolger and others, share many of these attributes but there’s one very important difference.   Monohull sharpie’s are long and narrow and therefore limited in basic transverse stability, while with a trimaran, stability comes from the outer amas. That changes everything.

For the amas (outriggers), there are also several significant differences between those of the Cross 18 and the W17.   First, the W17 pushes its volume and center of flotation farther forward to reduce the pitching as noted above and also reduces nose diving too.  (In his WB review of the W17, Geoff Kerr noted "... saw no sign of that dreaded beach-cat tendency to bury the lee bow.".) 

The W17 also totally avoids any banana profile by having a deeper bow and straighter lines.   Most trimarans use symmetrical hulls port & starboard as does the Cross, but contrary to catamarans where symmetry makes good sense, trimaran amas are not both in the water at the same time and the one that is significantly immersed is typically always to leeward, so it presents an interesting opportunity to be asymmetrical, IF indeed there is seen to be any advantage.    

For the W17, advantages were identified so the amas are asymmetrical and here is what they offer for a W17.   When to leeward and down in a sailing position, the inner sides become vertical and straight, causing minimal disturbance of the passing water, without squeezing the flow against the main hull.   The outer face has one totally straight chine, with a lower panel that applies a slight force to windward up forward, with a twist as it moves aft, to finish narrow like a fish tail.    This shape offers an asymmetrical Vee to the bottom, that serves a different purpose on each side of the boat.   When immersed to leeward, the midship area does not dig in (à la Hobie 16), but has some ability to slide under any sudden side force (from wind or wave) so reducing capsize risk, while the angle on the windward side, is now looking vertically symmetric, so efficiently dividing any wave tops that reach it. In practice, such waves are split in virtual silence, indicating its low resistance.  

(for more on this, see:   https://smalltridesign.com/pdfs/W17ProBoatOctNov2017.pdf)

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Other design differences are noted here:

Cross beams (akas) on the Cross 18 are straight and slightly below deck level, whereas on the W17, they are raised higher above waves to rarely contact them, making the boat either drier or adding capability in the rough stuff.

Although both boats have split-aka pivot-hinges, the W17 design is flat and wider, so can be walked and sat on.    Today, both hinges & latches are home molded in fiberglass for the W17, making them lighter, stronger and corrosion free.  See Folding Systems Part 3 for more on this. The W17 beams are far more robust than those on the Cross, adding to its rough water capability as well as to its appearance.   As many have commented, the uniquely faired forward beam of the W17 gives it the look of a miniature ocean racer as well as less wind and water resistance. 

Overall beam is significantly higher (14ft vs 12ft), so adding to stability and power to drive the boat.  B/L ratio is 0.82 compared to 0.67 for the earlier Cross.    This increased stability allows more sail.  While the W17 Cruising rig is about the same as the Cross 18, the so-called Race Rig has nearly 20% more sail, which is much appreciated in light or moderate winds yet can still readily be reefed.

The conservative Norman Cross was known to not like rotating masts, but he was missing out, and today all top performing multihulls have them.   Rotating wing-masts are standard on the W17 and their Build-it-Yourself designs are offered to suit each rig, in wood or carbon fiber. These masts add significantly to upwind performance.

One weakness on many small boats is the commonly suspended rudder attachment.  On boats that cruise long distances, rudder damage is by far the most common.  (In the late 60's, a teenaged Peter Clutterbuck cruised his British 16ft open Wayfarer dinghy to wildly-unrealistic places such as Denmark, Sweden, France and even across the Bay of Biscay!  While he survived 2 capsizes he also reportedly suffered 9 failures of the rudder!).   On the W17 a totally different mount is employed, that spreads the load from the deck to the hull bottom, creating a very strong mount.  In addition, the W17 sports a balanced spade rudder totally under the hull, which still pops up with an auto-release in case of touching the bottom.  Its improved location permits it to have less depth than a transom-hung rudder and still be highly effective. With a raised daggerboard and the low hull-leeway, this permits the W17 to be sailed to windward in as little as 2ft of water if so needed.

The mainsheet arrangement is upgraded with a full width radiused traveler, permitting the mainsail to be sheeted down flat without use of a kicking strap (boom vang), adding to boat control and safety in windy conditions.  

The more efficient flat-top full-battened mainsail is also stowed on a boom that is rotated via a removable handle and this not only adds life to the sail, but also keeps the cockpit tidy ... (see photo below).

The cockpit seat backs contain 6 small deck lockers.

To reduce fatigue and give variety, the W17 can be sailed from at least 6 different areas on the boat as the helm is very light.  Two lightweight, 6ft extensions that reach well forward and to either ama, enable this.

The main hull is also totally self-draining, so even if it were to take a freak wave over the bow, it drains away in seconds without need of any bailing, adding a very important safety factor.   (It’s appreciated that this feature might also be possible to add to the old Cross design).

It’s also acknowledged that, while the hulls of the W17 are basically easier to build (using the ABC System) and retain a cleaner frame-free interior, the shaped beams of the W17 take a little more time, so the total construction period might be a little more.   W17 builders think it’s well worth it though, noting the advantages and improved esthetics that this newer design offers.

Yet having clarified all that, a few builders will still appreciate to build and own the old classic C-18 design so we hope its plans will still be available for years to come.   As someone who always tries to works with ‘facts’, it’s fair to add that the extra length of the Cross 18 should theoretically help speed and seaworthiness; the quasi-rounded bottom of the hulls should give a gentle ride and lower skin drag at slow speeds; and the more vee’d hull will sink slightly less with excess load.  But for other aspects, it will miss many of the W17’s perks.

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