Small Trimaran Design
Published Articles
A listing of published articles by Mike Waters (and others) as related to his designs, in magazines and other publications:
Posting of this article is by kind permission of WoodenBoat magazine and author Geoff Kerr. Mr Kerr has also approved the minor corrections that this copy contains (for refined technical accuracy), compared to the original Jan/Feb issue #254, two of which were covered in an Errata published in the following edition of WB.
Tribute to Charles Chiodi, founder and ex editor of Multihulls Magazine Jan 2015
SAIL
/ Multihull Sailor - Fall 2014 - "Take a Bow"
This article is based on an earlier
article entitled 'Reverse Bow' on this website, but edited
down by SAIL Magazine to suit available space and published
with different photos. If something is not clear, please
refer to the original in the Design section". (PDF
1.5Mb)
MULTIHULLS Magazine, July/August 2010 –
"Considering a Small Trimaran? — Part 1"
MULTIHULLS Magazine, Sept/October 2010 –
"Considering a Small Trimaran? — Part 2"
"Background
on W22 Design" (book article - Dec 2009/July 2010)
Bob
For a displacement hull, I have found with side-by-side comparison that flat panel is faster. For given design speed and displacement, the optimum shape flat panel hull will have a higher block coefficient making it narrower at maximum waterline beam and shorter in waterline length. Those factors mean it will be lighter to build for the same strength. The reduction in weight more than compensates for the tiny increase in wetted surface. The difference in wetted surface between a flared side flat panel section and a semi-circular bottom section is not much. A weight saving of 5% will nehate the difference in wetted surface area.
A displacement sailing boat will have extra drag from pitching so a longer hull may be better because it will pitch less than a shorter hull of the same displacement and bottom shape. A flat panel hull with a hard chine with some rocker in the ends will have better pitch damping than a round bottom hull. The bow of such a hull will also lift better in waves; even better if the foredeck is not overhanging and also peaked rather than flat. You can shift body weight in a small proa to alter the trim but with a large proa there is benefit in getting the bow to generate enough lift to get the nose to pitch up.
I originally optimised a flat panel hull with the view to accepting the extra drag but once built I determined it was noticeably lower drag at any given speed than an optimised sem-circular section for same weight and design speed. There are quite a few flat panel hulls that I have designed being pedalled or paddled in competition and doing well. Five of my flat panel hulls have had top 10 finishes in the MR340 - a 340 statute mile race on the Missouri River with hundreds of competitors. My V16-62 hull, seen in the linked video, indicates how easily they move :Only a couple of sailing boats of my design have been built but you see all fast sailing boats moving toward flatter sections.All fast, all foilers and all relatively flat sections. The flat sections on a slender hull enable nice transition to foiling. However the development of flatter sections was occurring before foiling became common.
Structurally, some radius on the chine is nice but with flat panel construction I make the chines the strong points using a good fillet and heavy reinforcing along the seam internally. I am not infusing hulls as I assess it to be risky without a lot of experience. I glass foam on one side as flat panel. I build the hull over a removable frame (or spider frame) by laying up the cut panels with the pre-glassed side inside then do the external sheathing in one go. Once the hull is rigid, I invert, remove the spider frame, fillet and sheath the chines then fit bulkheads.
With a round chine, the water flows easily up the side of the hull as attached flow so produces less lift than a hard chine hull of same dimensions, load and power:Notice how far out the spray is being propelled.
Generally flat section hulls are easier to build and perform better than round sections. I think rounder sections look sexier but I know they are slower. I have often said to a friend who likes sexy shapes - do want sexy and slow or flat and fast!
A lightweight sailing boat with good sails and appendages should be capable of boat speed exceeding 2x windspeed up to about 20kts on its fastest point of sail. So if you are aiming to sail in 8 to 10kts of wind then you could be contemplating speeds of 16 to 20kts. That is getting into the region where dynamic lift is beneficial.
Rick
On 29 Aug 2019, at 10:37 am, bobg3723@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
As to choosing between a radius bilge hull and a hard chine hull for an average speed of 8 kts winds would one design gain a definite hull speed advantage for the other? Just so you know, I'm not thrilled about surfing down the face of a swell into a trough, so my preference is for steady fast speed in all w inds below 10 kts and wave heights. Other factors I didn't mention, please fill me.
Bob
Posted by: StoneTool <owly@ttc-cmc.net>
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