Ok after spending a week studying different rig set-ups I have boiled it down to what I will “try to summarise” into what I think are two systems, which have natural variations:
1) Flexi masts with two different rig set-ups for i) aluminium round or foil shape mast or ii) or carbon round mast shape, only because aluminium can be pre-bent and generally carbon masts are not.
2) Stiff masts which I see as being only a carbon foil shape.
Starting with flexi aluminium mast I understand two options A) “forward” pre-bent flexi mast where the tension applied to backstay pulls the pre-bend out. To set mast shape to match the sail luff curve you tighten the backstay for adjusting the mast/sail shape up and down the wind range the back stay tension either pushes fullness into the body of the mainsail mast straightens, or pulls fullness out of the mainsail when the mast bends more.
Pre-bent mast backstay adjustment directly transfers to jib stay tension, high back stay tension = high jib stay tension and conversely low backstay tension = low Jib stay tension. Back stay tension controls jib luff sag via pre-bent mast therefore you need to have very little jib luff allowance cut into your jib sail.
This seems to work fine in mid to upper wind ranges, The downside is that it is questionable if jib stay can be set-up “slack” in very light airs, otherwise the jib does would not develop sufficient draft. Jib stay tension can be changed by using topping lift according to the position of the jib pivot. If the jib pivot offset is closer to the jib luff, less tension feeds in to the topping lift, and vice versa as the pivot offset increases.
If the mast is positioned so that it cannot step fore & aft then the shroud base is fixed abreast the mast base which allows for minimal shroud tension adjustment then the sail plan centre of effort is balanced by raking the mast forward or aft and then fine tuned is done by adjusting jib stay length and the jib boom pivot point.
At the top of the wind range for this rig, the backstay can be tensioned further to increase twist at the head of the mainsail and to flatten the head of the mainsail. The head will flatten only if the spreaders and a “high” mast ram control the mid- and lower sections of the mast, otherwise increasing backstay tension will change the whole of the mast bend in ugly ways.
This rig set-up is seems to be the most difficult to fine tune and is used predominately by IOM class which limit class rules to round aluminium masts, mast ram, only one shroud and no jump strut…Then we have
B)]A straight flexi mast, here you can use any material or mast shape where you do not need to crank in as much backstay tension. This rig set-up has two shrouds (upper & lower), swept back “V” shaped spreaders and a jumper strut at the mast head, to keep the top of the mast straight, compared to a pre-bent mast.
Fine tuning of main shape is done by adjustment backstay tension along with upper shroud using spreader “V” together with the mast ram which pushes a little body into the mainsail, or takes some draft out. The mast ram continues to control the lower section of the mast and V spreaders mainly control mid section of the mast with shroud tension, which pushes the mast forward.
Shroud tension on this rig does a number of things, most obvious is that they keep the mast upright, and needs to be tightened as the wind picks up in order to hold the mast upright, but most important on this rig is that the swept back V spreaders become effective by also feeding tension into the jib stay.
The amount of tension that feeds in to the jib stay is partly related to the shroud base. The further aft attachment of the upper shroud from the mast step, the more tension is put into the jib stay. Conversely, if the upper shroud base is abeam the mast step, no shroud tension finds its way into the jib stay.
The amount of spreader “V” you need simply depends upon your shroud base offset. The further aft the shroud base, the greater the spreader “V” angle required. Lester Gilbert says that between 0% and 30% of the shroud tension is transferred into the jib stay depending as the shroud base can vary from 0 to 50 mm aft of the mast step on an IOM “A” rig.
The upper shroud aft base position (using V spreaders) means that backstay tension shares the jib stay tension with the upper shroud tension. Jib control stay tension depends much more upon adjustments to your shroud tension, depending on how far aft the upper shroud base is.
My assumption with this rig set-up is: you have more control over jib stay tension and mast bend and the up side is sailing in very light air, where you don’t want very much jib stay tension, you can better fine tune jib stay sag.
Remembering that Jib tension at the tip of the boom also increases when using topping lift tension, according to the position of the jib pivot. If the jib pivot offset is closer to the jib luff, less tension feeds in to the topping lift, and vice versa as the pivot offset increases.
The only difference between a “round” aluminium or carbon mast using this rig set-up would be the amount of stiffness vs weight of both options.
On my original rig I was using this rig set-up using aluminium foil shaped mast “without” 1) mast ram 2) jumper strut 3) aft swept spreaders …can you imagine how pretty my sail plan “was not” in upper wind range?
I’m now beginning to believe a straight aluminium foil mast with these “additional” fine tuning fittings will bring this rig set-up closer to a foil shaped carbon mast.
C – Straight stiff carbon foil mast “tapered” I emphasise a “tapered” carbon foil mast as “without the mast taper” I can only imagine would be very difficult to control any mast curve… it would be rock solid !
The characteristics of a “tapered” foil shaped carbon mast lend itself to minimal lower mast bend (hence low mast ram is possible) to control lower mast shape and as the mast tapers to the mid section of the mast, the curve here is directly tuned by backstay tension, while the jumper strut maintains straight upper mast head.
With this rig set-up you can easily fine tune COE by raking the mast forward or aft as needed (once final centre mast setting has been found) and the shroud tension does not affect jib stay tension therefore you can use straight spreaders and one single shroud base for the upper & lower shrouds.
With this rig set-up you use your backstay purely to manage fore-and-aft mast bend which means that backstay tension is more significant in affecting jib stay tension.
Jib control depends much more upon the jib stay by varying the tension at the hounds and length of your jib stay plus using topping lift tension according to the position of the jib pivot. If the jib pivot offset is closer to the jib luff, less tension feeds in to the topping lift (light air) and vice versa as the pivot offset increases jib stay tension (heavier airs)
Finally the other role spreaders play (straight or V shaped) is the control of the mid mast “side bend” which works off shroud tension. Sideways mast bend influences two things 1) it changes the slot, the gap between the jib and the main. If the mid-mast bends to leeward, the slot closes, and if it bends to windward, the slot opens. 2) the sideways mast bend also influences the effective twist of the mid-main. If the mast bends to leeward, the twist in the middle of the mainsail reduces, and vice versa if the mast bends to windward.
So there is my current mind set which is open to change if I’ve got it wrong (hope it was not too much in 1 hit) …comments :rolleyes:
Cheers Alan
