Wouter, why exactly do we need a max luff dimension ? to me that seems like just having a rule for the sake of having one and I can't think of a good reason for having it unless you want to rule out boomless rigs?
Setting the max luff length has nothing to do with ruling out boomless rigs. I actually don't even see how those two are linked in any meaning full way. Please explain to me how you think that they are, maybe I'm overlooking something here ?
The true reason for the max luff length is equality in performance, equality in perception, equality to Laser standard rig, good view to the leeward side of the boat, ease of tacking and with respect to my design it is a critical measurement to stiffen up the unstayed mast without increasing mast and boat weight.
Without a luff max limit I strongly expect a run for the lowest Boom/Foot height possible (see A-cats) to gain maximal (perceived) advantage. I think this to be undesireable from several aspects while fixing a max luff length doesn't really have any significant drawbacks.
A more detailed analysis follows now :
-1- Performance
Sails that share the same overall area and luff length will have largely an equal potential for producing sail drive forced on them. Only variables left will be amount of draft, position of draft and overall shape. Our proposed sail area with that luff length will already force a squaretop design as sailshape to such an extend that the variable of sail shape is not a significant discriminator anymore. That leaves draft and position of draft. I would like to get this open in order to have crew suit the sails to their body make-up and sailing style. I think this has been proven to work well other cat classes; in fact this seems to further equalized performance when only one sail per event is allowed to be used. But maybe more importantly the sails will now look very much the same from design to design, with only very small differences that are difficult to spot. I feel this will install alot more confidence in equality of performance in persons who are otherwise not educated technically = youths and most parents.
Basically, the less obvious the differences between rigs are the less cause for parent or crew related "concerns for inequality". And rigs are very important in this respect, even more so then hullshapes.
Drawbacks of a fixed MAX luff length ? The only one I can name are loose some very small amount of performance relative to say a luff the length of the mast. But this is not really a disadvantage as long as everybody is limited to a single luff length (=equality). I just prefer to fix this limit to a measurement smaller then the full mast length and I think the above benefits are sufficiently prefering 5.30 mtr as this will allow sail with aspect ratio of 4 which is already high.
F18/F16 have 4.25 ; hobie 14/hobie wave = 3.2 and 3.5
-2- Equality in perception
This is already partly covered in the paragraph above. Differences in rigs and sails are very easily spotted. Alot more so then any other difference between boats like hulls. Sail that are fixed both in area and luff length will look alot less different, even when fitted to masts of different lengths or when their drafts are significantly different. I also think that to take out this easily spotted difference is good for younger sailors as I expect them to feel more equal to eachother.
-3- Equality to Laser standard rig
I'm intending to have the F12 be very similar to the Laser dinghy in important specs like the waterline length (which is equal now with our max 3.8 mtr hull length limit). Same with respect to masts (5.9 mtr in laser although class allows 6.16 mtr) and sail area (=equal already) and luff length (laser = 5.260 mtr.) The difference in laser mast specs in mostly likely due to some people including the part that is in the pod and below decks. Our 6.00 mtr mast length above "deck" = beam would be highly similar.
Basically I just rounded off the number to these or what I had determined to be very attractive specs. Again, sail aspect ratio of 4 with a 7.00 sq. mtr. rig requires a 5.29 mtr luff length that I rounded off to 5.300 mtr.
I'm after laser equality in these specs for marketing purposes. F12 will definately beat the laser, especially the youth versions of the laser, and by keeping the specs pretty equal we avoid this performance difference being explained away by mono fanatics by refering to some "significant" differs like a "taller sail". I want the F12 to proof the cat design strong points in relation to a mono and partly increase acceptance of all catamarans in the larger fraternaty of sailing.
Again the benefits of fixing the luff length to another limit (like mast mast length) too small to outweight these benefits.
-4- Good view to the leeward side of the boat
I do indeed think this to be a safety issue. I also want to avoid sails with windows in them. The laminate that these windows are made off is sensitive to abuse and rips easily. It is also another thing to do to a sail by a sailmaker and will add cost. I think it is better to be able to look under the boom/foot-of-sail. Mostly I'm unimpressed by the view through these windows anyway, especially in rough weather when lots of water droplets deteriorate the view.
Having a minimal boom/foot height that is sufficiently high (at least shorter then the mast length)is an easy and cheap solution. I know from measurements that 0.5 mtr is pretty much a minimum and the 0.7 mtr boom/foot clearance given by the laser-equality and sail-aspect=4 criteria would therefor be sufficient too.
I also think that having this clearance near the mast is important as boats travelling upwind at equal speeds and equal relative positions (= collision) are to be spotted in this area. Boats that are to be seen further back along the boom will pass behind and boats seen in front of the mast will pass in front.
I don't think this aspect of the design should be discarded without a careful analysis.
-5- Ease of tacking
Low booms are a pain in the neck on a catamaran especially when their hulls are boardless. In order to come close to tacking the F12 much like a laser the F12 crew needs to roll tack. And you'll need to be very quick but smooth in crossing the trampoline as bobbing the boat around slows a lightweight boat down very quickly and being to slow capsizes you. I fear that on the short F12 there is not alot of room to have the boom/foot angle upwards quickly enough.
I want the F12 to be a really well behaved boat, after all it will be compared to modern dinghies which have a few significant strong points here, boom clearance on the laser and Open Bic is one of these. I don't think a very low boom is any benefit that isn't outweighted heavily by its drawbacks.
So for this reason a mas luff length shorter then the max mast length is to be prefered. I know from many measurements and test sailing on a score of boats that 0.50 mtr is very much the minimum.
-6- It is a critical measurement for my own design.
I shall make no secret off it. The push rod setup needs a max luff length shorter then max mast length for ease of raising the mast, stiffening up the mast sufficiently, lowering the stresses on the parts and hulls AND safety !
Safety because I'm counting on my design being able to completely weathervane the rig. When a kid gets surprised by a squal he just unthreads the mainsheet and the rig will completely weathervane itself. If he needs to sail downwind to safety in a blow he can over rotate the rig so the boom is angled forward by 45 degrees and loose all capsize moment (!) and most of the pitchpole moment. For upwind and reaching legs he can of course let out the sail with the same results. As such a kid can always sail to safety on any course with a very calm and easily controlled boat.
On the beach the rig can also weathervane which makes boats tipping over a thing of the past as well as dumb mistakes like slam gybing the boat on the beach by turning it the wrong way.
For the weathervane property the boom/foot needs to be above the push rods. And I'd like to have the push rods as high as I possibly can without compromising looks and performance too much. 5.3 mtr luff length on a 6 mtr mast does the trick for me.
Stiffness of the unstayed rig. The difference in the top of the mast flexing off between my push rod design and Phill pod design, when using the same mast, is 40%.
In effect, the difference is between the top flexing off 0.85 mtr or 1.20 mtr; a difference of 0.35 mtr. and this is alot. (Phill, remember my "feel the need to tell ..." moment ? That was this (and some more I may tell later).
With a max luff length of 5.3 mtr on a 6.0 mtr mast I can have a homebuild mast from basic alu tubing that weights less then 9.00 kg and has sufficient stiffness to not look to bendy and not pump to much. Without it we can probably kiss the unstayed mast principle good-bye.
Seen that way NOT having a max luff length rule (or minimum boom/foot clearance rule) is the same as effectively ruling out unstayed rigs. These already have a performance disadvantage to stayed rigs, with no such limit the disadvantage is getting worse.
Having an unstayed rig is also a critical element in my design; the whole concept of simplicity is designed around it. From the free hanging boom (no fittings) and cheap mast production to the ease of car roof transport and shed storage.
Wouter