I would appreciate any input on initial mast rake for IOM’s?
I never seem to be able to get it right.
TF
Hi TF
As far as I know, mast rake is just shorthand for balancing your boat. Put the mast wherever it needs to go so that the boat balances according to your preference (neutral, weather helm, etc). Then given its position you need to tune its bend to match your main luff curve using tensions in the backstay, shrouds, and jibstay acting against spreader ‘V’ and mast ram…
In an ‘open’ class, such as IOM, with different hull designs, there is no ‘majic’ number. You have to find it by trial and error. However it is easy to get close.
First you need suitable conditions, say 6 to 8 knots of breeze. Set up the boat and set the rudder to centre. Make sure that the mast is centered, side to side. Now sail the boat close hauled on one tack, hands off. Does the boat round up or fall off? Does it do the same on the other tack?
What you want is for the boat to sail about ten boat lengths before rounding up a bit so the sails begin to luff. If the boat rounds up quickly, bring the mast head forward by shortening the headstay by a quarter of an inch,ease the backstay accordingly. Repeat the test.
If the boat fall off to leeward, do the opposite. . . ease the forestay by a quarter of an inch. Repeat the test.
If the boat behaves differently on opposite tacks, then something is out of true, eg mast not centered, sail trim slightly different (off centre), or rudder or keel not true.
John
Ok, so I loosened the jibstay just a little and tightened the backstay… This should decrease the “Lee Helm” that turns the boat downwind?
Yep. That,s it!
I’ve wondered sometimes if I have my jib too tight with respect to the main. Sould it go out farther, not as far, just the same, or is it something you have to fiddle with for each boat in each circumstance?
I’m basically a big boat sailor, but models shouldn’t be much different. Main and jib should start to lift at the same time +/-. If the jib is in too tight, this well tend to cause lee helm. Sheeting the main too hard will cause weather helm.
A.
That’s how I thought it would work. I’m headed out to sail, so we’ll soon see.
Well… It’s a most beautiful day for sailing, but my radio isn’t working. I’ll try changing receiver batteries. I wonder if it got some moisture in the receiver.
Boat Balanced means the “Center of Effect” of the sails (the point representing the sum of all force vectors from the sail) and the “Center of Laterial Resistance” (the center of resistance of the hull and keel to resist leeward movement) lie in the same vertical plane.
So the easy way to remember is hold one hand out, point with your fingers, on your other hand with one above your hand to represent the drive of the sails, another finger below your hand to represent the center of laterial Resistance, so you will then recoginze the only way the boat will fall off, have lee helm, is if the center of Effect, the driving force is ahead of the resistaning force. If, so, you need to move the center of the sail drive, aft. Weather helm, the opposite. In a real boat you can fool around with the center of laterial resistance by moving crew fore or aft and if swing keel by retracting the keel somewhat, neither possible with a model, so balancing is restricted to moving the mast or changing haylard tensions.
Hi Ernie
There is something called “lead” which modifies your discussion somewhat. To ensure dynamic balance of the boat, the static set-up requires the Centre of Effort to lead the Centre of Lateral Resistance by some amount, called “lead”. Typically 2% or 3% of LWL.
Thanks Lester, but I was more trying to talk about visulizing balance in a dynamic environment as a simple way to decide what to do if the boat is sailing unbalanced.
I guess I don’t know how to address the subject in a static setup enviroment.
Doesn’t sail trim and wind force affect the “center of drive”? For example, if suffering weather helm, easing the mainsail will cause the resultant force vector to move forward, reducing inbalance and hence weather helm. Too bad we can’t adjust halyard and cunningham tension while sailing. Yesterday I saw a Santa Barbara with five servos, just what I would need (I have a hard enough time getting my thumbs to work with two).
It is conceiveable that you could steer your boat without using your rudder. Mast forward, mast aft, sails sheeted in and sails sheeted out are you basic no-rudder steering methods. Think “sailboard” - since they manage around a course just fine without rudder or sheets - and quite rapidly I must add. Just using mast position for or aft and how much they pull in (or let out) their sail.
To balance your boat, moving the mast will emulate the sailboard and can correct lee or weather helm.
Sure. In some ways one of the most sensible and in some ways the least sensible thing the late, lamented IOR did was to restrict the use of hydraulics for mast control. Structural integrity got dodgy but the ability to control the hull-rig relationship was amazing.
As far as I know, the most extreme/sophisticated expression of the art was the C&C designed Canadian Canada’s cup defender Evergreen, which had no less then 13 hydraulic rams controlling the rig (the odd number was the vang). It needed the guy who designed the mast to twiddle the taps but it is reported that on a close spinnaker reach she was half a knot faster with the mast in the optimum position compared with its ‘vanilla’ state.
Go buy some more servos!
A.
A bit off topic, but…
When we were kids sailing in dinghies, there were a couple of games we used to play - one of which was to take the rudders off our boats completely and sail without them (we’d only play this game in light breezes). Another was to play “tag” with our boats - where you got somebody “in” by capsizing them - an act achieved by sailing up alongside your victim and pulling down sharply on the end of their boom. The boats were were sailing were native to New Zealand - something called the P Class, a 7’ dinghy, and the Starling, a 9’6" dinghy. Both are monotypes.
The outcome of these games was that we became very good at understanding and managing the forces in our rigs and the importance of our centerboards - very valuable skills during mark roundings and pre-start maneuvering.
Watching a PBS travel show they were in Bermuda. There was a bunch of kids sailing little boats. Looked like a plastic boat-shaped tub with a sail and a rudder about 10ft long and maybe 6ft wide. They seemed to be playing some game that ended with lots of them in the water pulling their swamped boats back to the bank to dump all the water out. Looked like fun!