From my understanding a sail made for CC mast rigged with a "soft top mast" the top of the leech gets very loose before correct downhaul is achieved and there's heaps more shape down low.
The same sail rigged on a hard top mast has the opposite effect ie takes more downhaul to get the leech to loosen and the sail is a lot flatter down low.
In saying that I've seen maui sails rigged on CC / Pryde masts and they looked quite good, so there mast be variations in how masts are built.

Choco's explanation is what I have found in general when testing different bend masts in sails

But there could be differences in the effect of changing masts in different sails depending on the way in which sail designers achieve the shaping of the sail. i.e. the amount of shaping from the luff curve and the seam shaping and the combination of both.

RDM v's SDM sail shaping is a bit different. In my experience is that RDM's bend differently from SDM's. generally, RDM's naturally tend to be more CC because of their shape (far less taper from bottom to top) which means they rely more on layup construction to vary the bend curve. But also the stiffness and curve sems to vary different as the mast is actually loaded. Two masts, one SDM, and One RDM, which test the same on the IMCS test, can have quite different dynamic flex characteristics, and can shape a sail differently. In my experience, with the sails i have used and tested, I never get exactly the same shape from a sail when I switch between RDM and SDM, even if the UMCS test data is the same. It may be that Loft Sails hove found some design ways to minimise this, but there must be a difference.

What I suspect is, that the larger diameter of the SDM carbon masts more progressively resists bending as it is loaded and bent further. This is why RDM's often give a 'softer' dynamic flex feel in sails, and SDM's often rig race and slalom sails deeper in the lower half (and often with more forward draught) than SDM's with the same IMCS test curve and stiffness values, and the same amount of downhaul.

This is something i have been meaning to test with an experiment for quite a while now. In this respect, it should be enlightening to test masts with more and less deflection weight in an IMCS type test.

I have said it here before. My most powerful tuning tool for sails is trying different masts. A sails characteristics can be tuned quite well this way to suit a particular sailor weight or purpose. i have a set od RDM masts that have interchangeable tops and bottoms between sizes, and this is something I do use to experiment with sail tuning as well. Shorter and longer masts can also be used for tuning. Shorter masts are softer in the stiffness when extended, but when you use them in a sail that specifies a longer mast it also moves the mast further up the luff sleeve which can change the overall bend curve enough to make a difference, especailly with SDM masts.

Thanks guys.

Choco: everything you said matches what I've seen. I have also seen plenty of MS sails on constant or soft top masts that looked great.

Decrepit: I find the entry angle part a bit confusing. Not sure how that relates to differences in wind speed by height, or the advice to have little tension in the top battens, or that the loose in the top part of the sail acts to minimize turbulences. But since Bugs knows a lot more about sails than I do, I'll have to think about this some more. The pictures from Liptons seem to show quite a bit of twist difference, although the angles are also a bit different.

Sailquik: The Loft sails seem to rig quite differently on RDM vs. SDM. I think you got a very good point about the bend curve changing as the load increases. The typical number used to describe bend curves relies on just one measurement, with a force that's probably a lot lower than what a typical race sail needs. From the shape, I can see how a SDM mast might measure as "constant curve" under light load, but behave more like a soft top mast under increasing load. In contrast, an RDM mast with more even diameters would have a bend curve that changes less under increasing loads.
For the Racing Blades, my impression is that they rig with a deeper profile and therefore more power on SDM masts. If the top in the SDM masts is effectively softer, that would allow the sail top to "breathe" more in gusts, and keep it controllable. For a given sail size, the RDM would give a bit less power at the low end.
I agree that trying mast-sail combos on the water is what ultimately is most useful. But practically, the wind changes by several knots in most sessions. If I stop in the middle to re-rig, it's hard to tell the differences from changes in the rig and changes in the wind apart. Plus, who wants to stop in the middle of a great session to re-rig - especially in the winter when the days are short, anyway?

Peter, I think there is a difference between the speed sails and the race sails.
There is less wind pressure going hard downwind, I have the feeling that turbulence is less of an issue than going hard upwind with heaps of sail pressure.
And yes the angle of attack should be relative to air flow, not the rig.
I haven't heard the advice to leave upper battens loose, I guess that could allow some shape near the mast.

Just rig it and try it. Shape on the beach is not as relevant as shape on the water when loaded.

The top battens should be straight and tightened only enough to take out sleeve wrinkles. The battens get more curvature, and more tension, as you go down. That is the oversimplified ideal, but, again, rig it and try it.

segler said..
Just rig it and try it. Shape on the beach is not as relevant as shape on the water when loaded.

The top battens should be straight and tightened only enough to take out sleeve wrinkles. The battens get more curvature, and more tension, as you go down. That is the oversimplified ideal, but, again, rig it and try it.

Watching s few rigging videos most PWA sailors crank heaps of pressure on the top head batten.

choco said..

segler said..
Just rig it and try it. Shape on the beach is not as relevant as shape on the water when loaded.

The top battens should be straight and tightened only enough to take out sleeve wrinkles. The battens get more curvature, and more tension, as you go down. That is the oversimplified ideal, but, again, rig it and try it.

Watching s few rigging videos most PWA sailors crank heaps of pressure on the top head batten.

I tried this choc didn't work for me at all.

Peter. I have the same little 4-9 loft racing blade as Nina. Always ran a unifibre rdm with same results fairly flat on the bottom ( daffy always commented on this )and plenty of twist on top but still great speed and nice across the wind had spare sdm cams from my new KA used them with old 100% arrows Sdm completely different sail with full bottom and very even great looking leach fantastic speed and control. Great rotation both masts. I also have some ezzy masts they seem really soft on my KA,s and not as much belly. usually use hotsails hot rods the ezzy has better bottom end to get planning early but just gets overpowered early on the top end. Way better sails on the hotrods.

jimbob SA said..
Peter. I have the same little 4-9 loft racing blade as Nina. Always ran a unifibre rdm with same results fairly flat on the bottom ( daffy always commented on this )and plenty of twist on top but still great speed and nice across the wind had spare sdm cams from my new KA used them with old 100% arrows Sdm completely different sail with full bottom and very even great looking leach fantastic speed and control. Great rotation both masts. I also have some ezzy masts they seem really soft on my KA,s and not as much belly. usually use hotsails hot rods the ezzy has better bottom end to get planning early but just gets overpowered early on the top end. Way better sails on the hotrods.

Jimbob, Nina had an opportunity to try one of our small RBs (5.0 or 5.6) with the correct SDM, and ended up buying an SDM. Just like you, she liked the sail before, but loves it even more with the SDM. When I had to replace a 430 mast for the larger sails recently, the store did not have any SDMs. I use an Ezzy 460 in my Switchblade 7.8, and always was quite happy with it, so I got an Ezzy 430. I think some of my freestyle/foil sails need an RDM, anyway. But when I rigged one of these, it seemed quite clear that the Ezzy has a softer top than the older KA RDM that it replaced (and also than the Loft RDM).

The KA RDM mast that broke worked perfectly in my KA sail, and the Racing Blades looked just like they did with the Loft RDM. They worked plenty well (especially on Lake George and other top Oz spots ), but the sails don't look 100% right, and I think they'll work even better if the bend curve is exactly right. Not sure I'll be able to get that with the Ezzies, but I have a gut feeling that the softer top might help. If mix & match of tops and bottoms allows some fine tuning, even better.

What you say about the shape confirms what others have said. I'll check if I can find an Ezzy combo gives a fuller bottom and a leech that looks right. I got the Loft RDM for comparison. Maybe I can even use the MS hard top SDM to get a good idea what the sail should not look like .

After a great deal of experimenting, my experience is that sanding the batten tips thinner or to a different taper in Slalom/Race/Speed sails does NOT significantly change the position or depth of draught in the sails i have experiemented with. In my sails, the shape and position and depth of the draught is almost completely dictated by the sail shaping and the mast characteristics. Once the sail is under full tension the batten tip shape and stiffness has little, or no effect, being completely overpowered by the sail tension, mast curve and seam shape. The downside of sanding batten tips is that you make them thinner, softer and weaker which can adversly affect batten tip durability and cam rotation.

Mast specs on the other hand, can have a significant effect on the sail shape.

Your sails may be different, and you milage may vary, but dont assume you will get the effect you are after by sanding and reshaping battens tips.

There are however, noticeable gains to be had in stability, by stiffening up the back half of main lower/midde sail battens, usually by using higher carbon content, stiffer batten tubes.

boardsurfr said..
......I agree that trying mast-sail combos on the water is what ultimately is most useful. But practically, the wind changes by several knots in most sessions. If I stop in the middle to re-rig, it's hard to tell the differences from changes in the rig and changes in the wind apart. Plus, who wants to stop in the middle of a great session to re-rig - especially in the winter when the days are short, anyway?

Yes, I agree. It is most often not at all practical, informative or desirable to change masts during a session. I have rarely done that unless there was a really obvious mis-match from a rigging experiment gone wrong.

But a lot can be learned from a rigging session at home on a non sailing day. If I am looking for something specific in the way a sail is shaped, if it looks better on the grass, it may well work better on the water. Of course, I have had some false starts with that as well, where a sail looked very promising when rigged, but didn't perform as hoped on the water. Those are probably the best learning experiences.

boardsurfr said..

I use an Ezzy 460 in my Switchblade 7.8, and always was quite happy with it,

Thats interesting, as I have 5 1/2 year old Ezzy Lions (6.5/7.5/8.5) which I'm going to replace. I thought about Loft Switchblade as they would be a bit faster (probably) than the Lions, but didnt want to run to the expense of a new set of masts as well. How did the cams fit on the Loft with the Ezzy mast, did you have to do a lot of fiddling with spacers as the masts are different? Is the rotation as good?

choco said..
From my understanding a sail made for CC mast rigged with a "soft top mast" the top of the leech gets very loose before correct downhaul is achieved and there's heaps more shape down low.

My experience of slightly different mast curves in sails the same as Choco's above.

So did the flatter profile low down from RDM masts balance out any extra shape low down from using a slightly more flextop Ezzy mast than the Loft mast?


In the end I decided to stick with Lions as they are easy handling and rigging and good all-round performance so have ordered another set. But its interesting reading anyway. Thanks.

Good info. As well as the Lions, I was thinking of getting a second hand 6-6.5 race sail to use as a speed sail every now and then so if the Loft works out I might get one of those.

paddymac said..
Hard top vs soft top has not shown any material performance advantage so I wish the industry would just standardise on constant curve.

To some extend, you've gotten your wish. Maui Sails went to constant curve in their last years, before they stopped developing new sails. Gaastra was the other big hard top brand, and they went constant curve, too. I'm not sure about the soft top guys like NP (and Hot Sails?).
Of course, there are some differences within the "constant curve" range that may still matter for race sails, plus additional differences between RDM and SDM.