|Keith Richardson||21/03/2009 04:47:22|
30 forum posts
Dhow Construction Project
My Hobbies for many years have been computing, sailing and model airplane construction. Sailing to escape from a high-pressure job, model airplanes were with my grandchildren and science students, and computing was a passion. Now retirement has arrived and I have the time to do the things I always wanted, but did not have the time. But what? A long-term project, yes, and something that would both indulge interests and stretch imagination, knowledge and skills. Yes: design and build a… sailing boat! But, where lay challenge, as many such designs were available, and I had for years sailed a 14ft catamaran so knew about conventional sails and how to use them. I needed to shift out of my comfort zone.
A glance around the nautical world took me to the “Dhow”, from India, Arabia and East Africa, beautiful and with a sail configuration foreign to me. Decision made: I would design, build and sail a dhow, with radio control and an electric motor to safely bring the model home when all else failed.
And so the following questions arose:
1. Were there any existing dhow design plans out there that I could use? None found. Why? It appeared the craftsman skills of dhow design and construction were handed down from father to son over countless generations. Priding themselves on not requiring plans, they follow dhow-construction tradition and meet the capacity, produce and trading route requirements of each sponsoring merchant.
2. Were there any photos of dhows? Yes, plenty, so I collected over 100 and studied them closely for hull shape: side-view, plan-view and cross-section. Rigging was explored, mast/s (size and angle), yards (like booms but at the top), sail shape and sheet arrangements, and standing rig to support mast etc.
3. How does one control a dhow’s sailing direction, trim for running and tacking, and go about? I foresaw a problem here, as the way the mainsail hung beneath the yard on the port side, for example, free of the mast, would have it bearing against the mast on the opposite tack – not a good look as the sail-shape would be compromised. Many days of searching the web brought scant results, asking at the local sailing club drew a blank, then I located a reference to the out-of-print book “The Dhow – An Illustrated History of the Dhow and its World”, by Clifford W. Hawkins. Eureka! Solid gold. Answers found, and much more.
4. How to create a set of plans from which to scratch-build? First I explored available 3D boat design software, finally selecting FreeShip2.6 from sourceforge.net for its availability, strong support base of users, low price (free), and relative ease-of-use. (With previous experience with 3D programs I felt confident, and this proved to be the case.) I chose non-chine (chines seemed to be absent in dhows), then adjusted to 20 stations, a healthy beam size, and significantly raised bulwarks forward and starboard (as many dhows displayed). Mast position, size and slope was approximated from many photographs, as was yard length and position of tilt-attachment point.
One problem I encountered was how to enlarge the printed product from 20 cms to 100 cms long retaining a high degree of accuracy. Using Photoshop, I created a sequence of vector pathways following each of the lines on the plan, particularly the bulkhead shapes and the side-view. Each pathway was then precisely enlarged by the same factor. And from them 1 pixel wide lines were drawn, each on its own separate layer. Thus sharp, accurate plans were produced. To ensure exact symmetry of the bulkheads, I worked initially with only the port side half, and once enlargement was completed, copied the half-pathway, flipped horizontally the new bit, moved it and aligned to the centerline of the first.
5. How to control the dhow via RC? I drew up a list of changes I would need to be able to make (motor speed/direction), rudder changes, mainsheet setting, forward-sheet setting, yard-foot control via back-sheet, yard elevation adjustment up and down mast. These I then translated to the number of channels required, and put it to one side for later consideration. Not an area of expertise, I trust that the space available will be plenty, and the hull construction sufficiently strong to cope with a variety of servo locations.
6. How to build-in propeller and driveshaft, and electric motor of what suitable size? Allowance made based on rough sizes located on Internet, and final decisions left till later. This is not a strong area for me, so I will try to work generically here and refine details later during installation phase.
There seems to be a length limit for each posting, so I will work further on creating this thread. Keith.
Edited By Keith Richardson on 21/03/2009 04:49:32
|Keith Richardson||21/03/2009 04:51:06|
30 forum posts
7. Construction approach? Several model boat construction forums were consulted – wonderful! Planking would be employed, triple layer for strength with the middle layer running across the inner and outer layers’ grain. I would use balsa (selection for hard and strong) gluing with hot glue gun, the wood being heated and compressed to maximize spread and penetration before its rapid setting. Bulkheads – 20, made of double and triple layered balsa. Keel, stem and stern posts from 2cm pine with long straight grain pinned with dowell and hot glued. A horizontal deck was established so that each bulkhead would be supported at three points for maximum strength and warp resistance. Outer skin may be fiberglass, but not yet decided. Mast, spars, yard would be of layered straight-grain wood, possibly bamboo, but not yet decided. The mast needs to be rigid, the yard tapered with controlled flexibility, spars relatively inflexible. The mast would be seated in a fixed support tube so that it can be easily removed for transport/storage/repair. I am exploring the idea of a pivoting tube for mast rake adjustment, a feature used by some dhows to allow trim adjustment of centre-of-force location along the boat’s hull.
Orientation? Some plank upside-down and others right-side up. I will build a support chassis to allow for both, holding the model rigid yet able to be rotated on my workbench.
A removable false keel was suggested by Colin Bishop of Model-Boats Forum for the following reason: "when you scale down a sailing boat, the sail area reduces by the square root of the original but the displacement reduces by the cube root so the relative proportions of the model are very different to the full size ship and it is relatively over-canvassed". He also suggested how it might be attached, so I will definitely go along with his advice. More designing needed here.
Images shot during design and construction with comments.
Next I will upload some photos with comments. Keith
|Keith Richardson||21/03/2009 04:53:32|
30 forum posts
Removal of excess wood is by cross-grain cuts down to finish-line for future chiseling. Oh for a bandsaw!
More to come...
|Keith Richardson||21/03/2009 04:55:24|
30 forum posts
With keel held firmly in vice, the excess wood is chiseled away prior to final finish with spokeshave.
A relatively deep stem-post was cut for a diagonal join with the keel, to maximize strength.
Stem is shaped roughly with saw following gluing then rounded with plane. This joint will be strengthened by pinning with a dowel glued in.
The keel is stepped up at the stern to allow for future installation of propeller and shaft. The step-keel joint is glued then double-pinned with dowels at different angles. The pencil lines show the direction for each dowel-hole. Using hot-glue, that sets rapidly, requires speed and sureness, aided by the work being held firmly in a vice.
Here the double-pinned joint can be clearly seen.
|Keith Richardson||21/03/2009 04:57:23|
30 forum posts
|Keith Richardson||21/03/2009 04:59:03|
30 forum posts
Aft bulkhead pile viewed from aft-port lower quarter. The regularity of the shape change can be observed – so far so good.
That's all for now. Will add more when progrewss has been achieved.
|Paul T||21/03/2009 08:55:11|
7340 forum posts
All I can say is fantastic and what a super build this is going to be.
How do you find working with FreeShip2.6?
|Keith Richardson||23/03/2009 10:04:47|
30 forum posts
19 completed bulkheads stacked, viewed from below, forward aspect. The broad shape of the hull may be perceived, as can the sweeping curve of the hull shape. Next task will be to attach all bulkheads to the keel-stem-stern skeleton. My challenge is to devise a workable set of guides to ensure accuracy and squareness.
The same stack of bulkheads viewed from above-forward. The level deck can be seen, as can the bulwarks, high forward and aft, and low amidships.
Bulkhead stack viewed from directly overhead – its symmetric accuracy may be judged here, as can my somewhat crude cutting-out. Most of these surfaces will be covered once construction is complete. I plan to cut away parts of the bulkheads and deck for the fitting of RC gear, electric motor & battery, and drive shaft & propeller. I would like to keep the structure relatively rigid during skeleton assembly and planking, to reduce warping under stress. It will be interesting to see how it progresses.
|Bob Abell||23/03/2009 10:12:59|
9337 forum posts
I had the same problem when building my Louis Heloise!
I temporarily glued cross struts across the top edge with the centreline maked and used a flexible straightedge to line them up...............and that solved the problem........Bob
|Keith Richardson||25/03/2009 00:59:42|
30 forum posts
Bob, that is so beautiful! No wonder we give boats female names (not a practice in the Arabian Dhows, by the way!). BTW - what wood have you used for each of these bulkheads?
Many thanks for posting this great photo!
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