Sails are designed around masts with specific bend characteristics of which there are many as shown in
www.unifiber.net/masts-selector
www.peterman.dk/masts-all-imcs01.htm
I think I understand the concept of having a flex top - the stiffer more stable part of the mast is lower allowing good stability in the lower grunty part of the sail. The softer upper section allows the sail to exhaust well.
If that is the case why would you go to a hard top? What would be the benefit?
Maybe I'm confusing stiffness with curvature and the stiffness of the top section of a hard top and flex top is the same. Then I guess it becomes a sail shape design choice - you can carry more sail in the head, all other factors being equal, for a hard top.
I'm no expert, actually I have no idea what the advantages with any of the curves are. But I'm sure there is method to the madness..
But considering our minimal mechanical advantage over the rig, it seems that having all the bend at the very tip is putting all the action too far away.
This is my visualisation of what the curves look like.. The centre of effort on the sail is not far above the boom..
Left is stiff top, then flex top then crapstant curve..
i found this some years ago, makes an interesting read after you get past the bits about mums sewing machine.
www.sailrepair.co.uk/makingsail.htm
would say barns highly accurate diagram is correct, when you see the bend curves for top 3/4, bottom 1/4 this is what they describe. also explains why IMCS is not an accurate enough way of describing mast bend.
paddymac you may want a hard/stiff top mast if you want more tension/control over the top section of the sail, or if you want a tighter leech on the sail (e.g. gaastra severne etc)
One thing to remember with these mast database websites like the unifiber one is that for some brands their masts change from year to year, either intentionally or because they switch factory...
Al
Al, are Severne Code Reds any tighter in the leach than Pryde RSRs? They look pretty similar to me. Sure the Reflex is trying to get a more engaged head but is that directly related to mast curve - could they achieve that on a flex top? I haven't seen any Gasstra race sails up close.
In any design choice there are pros and cons. I'm keen to understand the pros and cons of the curve choice. Is it just a mechanism to try and lock you into a brand or is there some science behind it?
The mast will dynamically bend backwards, not just to the side. This backwards bend is just as important to the dynamics of the sail handling IMHO.
The shape of the luff curve pre-tensions the sail. It is designed to complement the bend curve characteristics of the mast so when tensioned/set the sail has the desired shape and dynamic flex.
In race/slalom/speed sails, where a lot of the shape of the sail is determined by seam-shaping, using a different bend curve mast can still make quite a difference to the way the sail sets and handles.
eg: Use a more flex top mast - deeper draught down low, more twist up top for the same down-haul tension. Use a less flex top - Less depth low, tighter upper leech with same down-haul tension. In the first instance the sail will feel more locked and stable and may not pump as nicely. In the second, it will have a great pumping feel but may have less low speed grunt and high speed stability.
I am sure the same applies to non cam sails.
It can be very useful when tuning a speed sail to have the option of trying different characteristic masts. In my case I can also switch tops and bottoms from different sizes as well to change the flex and bend of the mast. It can be a very interesting exercise and can sometime yield great benefits for a particular purpose. 
One of the difficulties some of us face is that some race sails are often designed and tested by gorillas that resemble humans. Since I am more like a chimp, I find a softer, or different flexing mast can often make it work for me in a way that closer resembles the intent of the designer.
But as to why some designers start with hard top, and some start with flex top, I am as in the dark as the OP.
asked the question of Barry Spanier - of MauiSails , Gaastra and NielPryde fame
MS masts are known to hard top - like Gaastra and Severne
their masts seem to have developed over time and it works !!
Barry says:
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i think our mast simply bends lower down and bends in more of an arc. every mast is softer in the top than the bottom. i think the mast bend should work off the area where the boom attaches, more than higher up. allows for the flex to 'work' more for you.
as far as evolution, i would take our mast bend all the way back to the serfiac aluminum masts, which we loved back then, had fiberspar imitate in carbon, developed sails for for many years, and found them to be always fast and easy to manage when the sail design fit. not sure exactly why or how the flex top thing got going, but it never seemed right to me when trying to match sails to it, so we stuck with something more even.
lots of people have experience with only one brand type and don't have the chance to compare. often what you start with is what you use for as long as you stick with the brand. i encourage trying other gear to find out what you might be missing.... or not.
aloha
barry
