Model Boat journal
Lester Gilbert on understanding of sail making for model sail boats
Lester Gilbert wrote:
For anyone interested in sailmaking, I've just finished editing Larry Robinson's "Making Model Yacht Sails" (part 1 only) booklet and have published it as an "international" edition. I've done this, not to get rich ('cos this isn't going to happen to either me or Larry or anyone else connected with this enterprise!), but because I've spoken to a lot of sailors who want to make sails but don't know the "right" techniques, and who are being misled by incorrect accounts of how this might be done.
A sail block
(Illustration from Larry Robinson's "Making Model Yacht Sails")
From my understanding of sail making, there are two ideas I want to contradict.
The first idea is that you can make sails by accurately cutting a curve on a panel, and then attaching it (stitching, gluing) to another panel. Well, while you might be able to cut a good curve some of the time, your fingers just don't have laser accuracy in them to stick A to B and you'll hardly ever obtain reproducible or reliable results. (It might be possible to butt-join the curved edge to another curved edge with a little more reliability, but this doesn't yield what the Equipment Rules of Sailing define to be a seam. Such a sail couldn't be used in sanctioned IOM competition, though it would be OK in a development class.)
The second idea is that you can drape your panels over a "camber board" and get a nice shape that way. Well, let me be clear about what I'm knocking here. I take a "camber board" to be a length of curved surface, where the curve is like the surface of a cylinder. In this case, your panelled sail will have exactly the same shape as a single un-panelled sail and, if you wanted a three-dimensional shape, you've wasted your time (though the result certainly looks the part).
Larry's booklet is the only source I know which carefully explains the use and construction of a sail block. I am sure that this is really the only way (in your garage, please, not in some specialist workshop!) to make professional sails, to obtain reliable and reproducible three-dimensional shaping, and to be able to tweak and change your shaping as you learn about the whole business.
(Warning: Y'all should know I have ten thumbs and have never made a sail yet. What I have done carefully is to watch and talk to those who do, both professionally and as home builders, and measure the results. Making Larry's booklet available internationally is my way of telling you what I've learned.)
Bob Wells will be able to ship this within the USA, and I expect that it will also be available from Don Ginthner at GBMY. For worldwide sales, contact SAILSetc. Bob Wells' e-mail is "bob" at "islandinet.com", GBMY is "rcsailing" at "gbmy.com", and SAILSetc can be contacted through "sales" at "sailsetc.com".
I've attempted an analysis of how blocks work on a new page, Sail blocks analysis, and have a new spreadsheet there to help.
Model Boat journal
Lester Gilbert on boat balance
The boat is balanced by either moving the sail plan forward to reduce weather helm/increase lee helm, or moving the sail plan back to reduce lee helm/increase weather helm. Depending upon the mast step you have in your hull, you'll do this by raking the mast (pivoting it on its heel), or moving it fore and aft in a mast slot.
For any given hull/rig combination, Graham Bantock feels the degree of weather helm is dictated by three basic factors:
* mast rake/mast position
* relative twist in main and jib
* relative sheeting angle of main and jib
with the minor factors:
* relative camber of main and jib
* relative shape of main and jib.
He goes on to say:
There is a fairly small range of successful pairings of twist and sheeting angle. The sheeting angle ranges are covered by the range 8 to 15 degrees for the jib, and 2 to 8 degrees for the main. Keep the twist in the sails so they look much the same from astern. Use mast rake/mast position and relative sheet angle to tune the balance. Use sail camber primarily as a throttle - fairly flat in very light airs, fuller as wind speed increases up to the point where you need to keep the boat more upright for best speed and pointing when you flatten sharply. Camber can also be used to adjust the helm of the boat. Experiment.
As far as I can tell, my Ikon and Italiko tune up just like any other IOM, the major issue being to get their balance right -- ie, appropriate weather helm. So far, I've found both work best for me with a touch of weather helm, that is, when they reliably round up into the wind rather than sailing "neutrally" with my finger off the rudder stick. If you have your rudder trim calibrated, I'd suggest going for that amount of mast rake which gives you about 2 or 3 degrees of weather helm as the "ideal".
As an aside, some sailors feel that mast rake does other things for you, perhaps improving the lift characteristics if the mast is raked well back to give some sort of "sweepback". As far as I can see, mast rake in an IOM is exclusively concerned with boat balance and helm, and has negligible other effects on any other characteristics of the boat.
There is an aspect of helm that has puzzled me for quite a while, and that is the phenomenon called "snap" weather helm. This happens when an otherwise well-behaved boat luffs violently into wind when a puff hits, and I had this problem with my Ikon. When I started sailing my Italiko and until recently, I never encountered it, and began thinking that snap weather helm might be a characteristic of hull shape. I realised I was wrong about hull shape when my Italiko snapped back at me, and I spent a little time investigating. It now seems to me that I had inadequate rig tune.
Model Boat journal
Lester Gilbert on Boat weight and Boat speed
I was wondering whether it really mattered if a yacht was a little overweight. I've taken the "Acceleration" page spreadsheet and produced a whole new version. It calculates drag and acceleration against a time line, rather than against a speed line (spreadsheet here, approx 96 kb), in order to yield relatively stable calculations. More importantly, it now factors in the yacht displacement in order to calculate acceleration, and calculates the distance travelled during acceleration. For a given wind speed and displacement, the sorts of results it gives are shown below, plotting speed and distance against time while reaching and while running. These particular graphs are for a wind speed of around "1", showing the yacht reaches maximum reaching speed after about "6" units of time. These are arbitrary units, because the spreadsheet is not calibrated. However, if you think "1 metre per second" for the wind speed, and "6 seconds" for the time, you'll not be too far out.
http://www.onemetre.net/Build/Accel/Accel2.htm
Model Sail Boat Building, How To Build A Wooden Star45 R/C Sailing Modelstar 45 construction | double diagonal planked hull
From: "John Fisher"
Here are some photo's of Sherwood Jones Star 45 with double diagonal bottom planking. He used two layers of 1/16 planking. The planks are 1" wide. He then covered it with 1 �oz glass. Weight is about the same as mine with the 1/16 longitudinal planks and two layers of 3.2 oz glass. Just goes to show that there are multiple ways to solve a problem.
John
