You hit the nail right on the head Swoosh. But it
is very frustrating that many manufacturers and brands do not list the IMCS bend curve information for their masts.
I think there may be a number of reasons for this:
1. Their masts are not all the same model to model. I and others have tested masts of different models in the same brand and found different mast curves (see the Boards magazine tests referred to earlier and Petermans tests here:
www.peterman.dk/masts-all-imcs01.htmEven mast manufacturers that use the IMCS system (eg. Fiberspar) have inconsistencies in their range. The wave masts are different curve from their smaller slalom masts which are different from their large slalom/race masts. I have no idea if this is deliberate or if this is a limitation on materials control. I suspect it is a bit of both.
I guess brands may be reluctant to be too specific about their mast curves if they know they vary across their range.
2. Their masts vary within the same model and even the same length.
Quality control of exact bend curve seems to be a quite difficult thing. I have tested more than one of the same mast model, same size and same manufacturer in the past and got two different answers. The stiffness and curve seems to vary a little. In all my tests it was mostly less than 2% curve difference and 1.5 IMCS points but I would guess that that is fairly typical so again brands may be reluctant to specify exact numbers when they know that every mast might not be exactly that number.
I could be wrong here and I admit I have not tested many large batches of same size and model masts, just a few random pairs, and they could well have been anomalies.
3. RDM masts often have a different curve from SDM masts even though they are recommended for the same sail (and work perfectly well). This is because the reduced diameter itself has an effect on sail shape and set as it takes up less of the luff pocket space. I have found a 12 % RDM mast that rigged a sail I used in exactly the same shape as a 15% SDM mast.
One mast is Constant Curve and the other is almost at the Flex Top specification.
4. Some sail manufacturers claim that even though their own masts can be loosely described as, for example, Constant Curve, they have characteristics that suit their sails better than other Constant Curve masts. Whether this is real or just marketing spin to encourage you to buy their own masts I can't say. What I can say is that I have actually tested two brands of mast that came out almost identical in IMCS curve but did rig and work in my sail in a subtle but noticeably different way.
Here are the IMCS categories for mast curve:
Categories - 6 Hard Top
7 - 9 Hard Top - Constant Curve
10 - 12 Constant Curve
13 - 15 Constant Curve - Flex Top
16 - 18 Flex Top
19 - 21 Flex Top - Super Flex Top
22 - Super Flex Top
I can tell you that all the 4m SDM KA masts I have tested (5) were in the 14-15% category. This puts they squarely in the "towards the flex top end of constant curve" category as Martin said.
The RDM KA 370 and 4m masts I tested were between 12-14 % curve and the 4m masts work perfectly in the same KA Koncept 5m sails as the above SDM masts.
Without more extensive testing of the other sizes I am reluctant to categorically say they would be the same but from the way they rig the sails I suspect they are at least very similar.
I can say that all the Powerex SDM wave masts that I have tested in 4m and 4.3m lengths came out between 10% and 13% making them a little more CC than the KA masts. The interesting thing was that they actually rig my small Koncepts in a VERY similar way. I have also seen some of the smaller Koncepts rigged on the NP 'Progressive Flex' masts and they have looked and worked well too. These are examples that tend to support the claim that KA sails tend to be somewhat mast curve tolerant.
I found this terminology interesting: There is information on the NP website (
http://www.neilpryde.com/insiders-guide/insiders-guide/understanding-mcs-and-imcs.html ) that calls their masts "Progressive Flex". As far as I can see this is not a 'standard' IMCS category but the example figures given are 61% bottom, 77% top. This = 16% difference which I would call Flex Top (but at the lower end).
Let me add that IMCS mast testing is a time consuming and tedious business for ordinary sailors (like me) to do. Have a go at it yourself if you doubt me.
I assume that the mast factories are set up to do it in a much more efficient way. I agree it would be nice if they and their brands would publish more data.
