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Comparing different chine hulls

Please comment on a comparison of the Crowther multi chine and similar wider V-hulls such as Crowther amas and other hard chine boats with approx. 45 degree chine hull.  As the waterline is below the chine, I assume turbulence should not be a factor in fairly smooth seas.  I have also heard of monohull aficionados preaching the value of chines.

Without having the actual lines plans in front of me, it's hard to make a direct comparison, but please refer to my article on 'Basic Ways to Use Plywood' for general trends and relative complexities.

As far as chine-built boats in general—yes, I am also somewhat of a fan. IF well designed—and they not all are!—they can perform surprisingly well and look good too, as the chine lines (if kept fairly straight) can really help to make a boat 'look and move' fast. They also work well for monohull planing forms and sharpies but add resistance for most multihulls if the chine is allowed to become 'too banana shaped' or if the hulls end up too Vee'd, giving a significant increase in frictional resistance compared to a round bilge boat. Vee'd hulls tend to pump surface water out horizontally, both as waves pass by and as they pitch, adding resistance. The banana chine (or buttock) shape also offers less resistance to pitching, which further spoils performance.   (As Constant Camber boats tend to be very vee'd and combined with a somewhat 'banana profile', I find it hard to justify this shape for either performance or interior space, despite their relatively smooth ride).    In my personal view, chines are the guidelines for how the water is divided by a hull in forward motion.   If this is done BELOW the water surface, there is far less surface wave, so I like chines to start as low as practical at the bow, and be as straight as possible. Designers from the past, such as Van de Stadt and Phil Bolger knew this well, but in what might be considered 'misguided efforts to simulate round bilge bows', this now often seems to be forgotten.  Veed hulls do 'track' well and typically slip less to leeward, but as noted, there is a price to pay.    Perhaps a comparison of these two different bow photos will help.  The vee'd hull of this mono is pushing a lot of water out horizontally at 7kts, not only showing the resistance there is but also creating surface foam that will be blown in the air and create spray to wet the crew, while the vertical-sided tri is slicing through at nearly 9kts with so little surface disturbance that the keel 200mm down can still be seen.   Sure, the tri is narrower, but the sectional shape is still very much part of this visual difference.

    

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Multi-chines can help to gently break up waves, though this generally makes the boat slightly slower in the process, just like a Lapstrake boat can have a more kindly ride in waves—but the 'work done' on the waves generally making the boat slower. The lapstrake Folkboats were one example, when compared to their carvel-built cousins.   Interestingly, light weight dinghies with lapstrake construction only at the bilge can actually benefit from this by helping the underwater boat surface trap air to show small speed gains.  The lovely 17.5ft Osprey dinghy by Ian Proctor (1952) was one of these exceptions and one reason I used the same concept on my 13ft Canadian Beaver design in the late 1960's.  Added structural rigidity was another plus.

Today, there are several interesting round bilge alternatives that are no harder to build than the early chine ply boats (see the various METHODS articles—particularly the Radius Chine, Strip Hulls, KSS and Hybrid.   Round bilged boats have the advantage of lower frictional resistance plus, composite hulls offer the designer the ability to more easily mould an acceptable shape around the needs of greater internal space, (and Ian Farrier for one, was a master at this).  Another aspect of round bilge is the common association with professional boatbuilding.  If you build to high quality, a round bilge boat will typically have greater resale value than one with a hard chine - and, will be faster in really light-wind conditions.  One performance negative of round bilge is that leeway is often more than for a chined hull (demanding a deeper board) and the construction typically more complex, costly and time consuming.  

Added Dec 2017:

This cost & complexity does leave space for options and after the positive experience with the W17, the simple but efficient hull shapes now offer a design option that would be easier to build, perform great upwind and sit comfortably on its bottom.  IF the main hull can be kept narrow at the waterline (as for a multihull), the vertical flat sides of a deep 'U' or box section can be remarkably efficient in the mid-speed range due to low wavemaking resistance. (For more explanation and detail, see the Professional Boatbuilder article on the  "Science of Simple Shapes" now posted on this website under "Published Articles").

Regards,

Mike Waters NA

 

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