I’m glad some-one finds this question as interesting as much as I do, I’m been researching the question on the net and am surprised at lack of information available considering sails are the “engine” of a sail boat.
Btw: I’m completely ignoring hull design /speed in this exercise & only focussing here in how to quantify the most efficient sail shape and area.
I should clarify that boat helm balance is accepted as the norm by mast stepping the rig with micro adjustment using mast tilting & the settings of boom angles.
Good point you make about being fractionally or masthead-rigged ? but the question still remains the same.
The difference being: a mast head rig fixes jib head to the top of the mast which greatly determines the CE height that you can only change with the foot length of the Jib whereas, the fractional rig provides the flexibility of lowering the CE height by having variable fixing point of the Jib luff (below 80% max of the main luff length) as well as a variable foot length and here we begin see the fundamental differences between the two rig types.
Further, a high aspect or taller sail plan results in a taller mast with increased weight of the mast, rigging and spreaders that contribute further to the heeling force as wind speed increases, therefore it’s great for light air but becomes more detrimental as wind speed increases.
The compromise catch with high aspect ratio - heeling moment goes up - puts a practical limit to the tallness of the rig and low aspect ratio performs better off wind, conclusion: Higher aspect with light air and low aspect with higher winds speeds ?
Then there is the influence square & pin head mains on RC boats and my limited experience so far, I’m beginning to drift toward having high aspect square head for light medium air and pin head for heavy air, which based on the amount of control you can have on main leech and twist that declines rapidly in higher winds speeds using square head main, unless you are prepared to support the head in less conventional ways, than just having traditional head crane and vang adjustment for sail settings.
With RC sail racing, when the sail suit (and the wind) causes too great a heel (slowing-up of the speed) it is time to put on the second suit, continuing to the third, if the second proves too large. There doesn’t seem to be any hard and fast rule as to how much smaller the second and third suit should be unless you test and trail different sail suits, that can be expensive. Experienced eyes use rule of thumb saying if you heeled more than 30 degree’s it’s time to rig down, but I ask myself again how much should I be rigging down 10-15-20% in sail area, to have best the engine (sail) efficiency for the conditions.
Working toward trying to define ratios between jib and main sail areas, I’m looking at the aspect and foot length and throwing into consideration sail shape with the pro’s and cons of square head vs pin head mains for a RC yacht, that provides maximum engine power in three winds conditions of:
- Light air 1-5 knts
- Medium air 5-10 knts
- Heavy air 10-15 knts
My wish is to gather information into illustrations and measurements and finally percentages that could be helpful to different RC classes, not just AC 120.
Finally a PM tells me, “Generally speaking the fore sail is increased with low winds force to get more power. It is important to know that a jib equivalent to 30% of the total sail area, is providing 50% of the total power” Now that’s a very interesting number to think about !?
Looking forward to reading more comments on this interesting topic.