Is the way the masts taper the same for the sdm and the rdm? If say one was straight and the other curved like a Doric column this would be like having more mast bend. Even a few mm of luff curve changes the shape of a sail so I would expect the way the mast tapers might also affect it.
I think rdm on smaller sails might help because rdm might be a more optimum leading edge radius. I did some thinking and research a while ago that suggested that smaller diameter might be better. Haven't got the books with me. All to do with low Reynolds number airfoils.
Select to expand quotePiv said..
Is the way the masts taper the same for the sdm and the rdm? If say one was straight and the other curved like a Doric column this would be like having more mast bend. Even a few mm of luff curve changes the shape of a sail so I would expect the way the mast tapers might also affect it.
Totally different, RDMs are almost straight.
So I think the difference is because of the way the masts taper and the effect that has on the apparent shape of the mast curve. Imcs measures deflection so plots the centre of the mast but it is the front of the mast that mates with the luff pocket. That will be a slightly different shape
When talking about SDM masts vs RDM masts you have first to establish what size of sail you want to use.
The key issue in the SDM vs RDM debate is that each different length mast has an optimum diameter for strength to weight ratio, and then for bend.
Once you get to 6m sails or bigger, you will be on a 430 mast or longer, and after that crossover 430 mast length the larger diameter tubing works better.
For sails of 5.7 or less you can be on a 4m mast or less, and the bend and strength and weight characteristics for shorter masts can be better if tube diameter is reduced to the RDM sizes we now use.
With bigger sails, set on longer masts of 460 upwards, the ability to let the top of the sail twist becomes more critical due to apparent wind issues, and so the larger diameter tube is important as it's actually easier to make a flex top mast with an SDM than it is with an RDM.
At the end of the day, the sailmaker has to make his sail work with the mast he's selling or recommending. And it's mast match and rig performance that we care about.
Select to expand quotePiv said..
So I think the difference is because of the way the masts taper and the effect that has on the apparent shape of the mast curve. Imcs measures deflection so plots the centre of the mast but it is the front of the mast that mates with the luff pocket. That will be a slightly different shape
That's quite interesting!
You have tension being applied from the foot and head of the sailing bending the mast, and at the same time an opposing force from the front of the luff tube trying to straighten the mast, by changing this interaction with a mast that tappers i.e. using an SDM you would be changing this relationship.
Of course if this is the primary reason then this similar difference/effect between the two types of mast could be achieved by just cutting the luff tube to suit the RDM in the first place...
Select to expand quotePiv said..
So I think the difference is because of the way the masts taper and the effect that has on the apparent shape of the mast curve. Imcs measures deflection so plots the centre of the mast but it is the front of the mast that mates with the luff pocket. That will be a slightly different shape
The difference in taper could very well be a factor in what they feel different.
However, IMCS measures to the top surface of the downwardly flexed mast. In any case, that does not alter the substance of your musing.The bottom surface that in at the front of the mast sleeve will indeed be in a different place.
Yep ideally the sail luff pocket would be cut to suit the mast and they are but then people go and use a different mast brand or size and wonder why things change. But then some like severne overdrive and the smaller Ka concepts are supposed to work on sdm and rdm and they do but there are slight differences that are there due to the difference in the way the masts taper
Select to expand quoteBasher said..
When talking about SDM masts vs RDM masts you have first to establish what size of sail you want to use.
The key issue in the SDM vs RDM debate is that each different length mast has an optimum diameter for strength to weight ratio, and then for bend.
Once you get to 6m sails or bigger, you will be on a 430 mast or longer, and after that crossover 430 mast length the larger diameter tubing works better.
For sails of 5.7 or less you can be on a 4m mast or less, and the bend and strength and weight characteristics for shorter masts can be better if tube diameter is reduced to the RDM sizes we now use.
With bigger sails, set on longer masts of 460 upwards, the ability to let the top of the sail twist becomes more critical due to apparent wind issues, and so the larger diameter tube is important as it's actually easier to make a flex top mast with an SDM than it is with an RDM.
At the end of the day, the sailmaker has to make his sail work with the mast he's selling or recommending. And it's mast match and rig performance that we care about.
I'm not sure there is a 'Debate'. I thought it was more of a discussion. Can you share any insights as to why RDM's and SDM's impart a different 'feel' in use in the same sails?
RDMs and SDMs will feel different in the same sail because the sails set differently.
For a start, the luffsleeve may not accommodate both diameters of tube, but if it does, then the RDM will have a more loose luff sleeve in the boom area. That in turn has implications for the luff round cut into the sail.
You can get bogged down on discussing what happens technically with the change from SDM to an RDM but the key thing is to try any sail with the mast you have – and then if it works, that's fine. If you have the money, then buy the recommended mast, or a suitable, cheaper, after-market, one. Most RDMs are actually constant curve nowadays, and it's the carbon content and pre-preg quality build you pay for.
Generally however, we like soft, forgiving, flexible rigs, and the lesser diameter of an RDM gives you a bendier and softer feel for smaller rigs – and for any wave sail. People also like the 'grabability' of the smaller diameter RDM tube when sailing on small kit – because we are more likely to be changing direction, then sailing fast in a straight line.
Select to expand quoteBasher said..
RDMs and SDMs will feel different in the same sail because the sails set differently.
For a start, the luffsleeve may not accommodate both diameters of tube, but if it does, then the RDM will have a more loose luff sleeve in the boom area. That in turn has implications for the luff round cut into the sail.
You can get bogged down on discussing what happens technically with the change from SDM to an RDM but the key thing is to try any sail with the mast you have – and then if it works, that's fine. If you have the money, then buy the recommended mast, or a suitable, cheaper, after-market, one. Most RDMs are actually constant curve nowadays, and it's the carbon content and pre-preg quality build you pay for.
Generally however, we like soft, forgiving, flexible rigs, and the lesser diameter of an RDM gives you a bendier and softer feel for smaller rigs – and for any wave sail. People also like the 'grabability' of the smaller diameter RDM tube when sailing on small kit – because we are more likely to be changing direction, then sailing fast in a straight line.
It's not a matter of getting bogged down in technical side of things. There is a place for both theory as well as practical experience (I.e. Experimentation). This combination is the basis of scientific method, which humanity has successfully used for thousands of years to understand the world around us. Besides it gives grumpy old windsurfers something to do when the wind doesn't blow.
M
There you go. Those guys have the perfect rig to test just what we are discussing. I wonder what the bending pressure was when it failed?
I have contacted them to see if they can enlighten us from their test rig.
Of interest to me in the video is that the bend curve difference top to bottom looks similar the whole way.
Select to expand quotesailquik said..
>>>>>>
Of interest to me in the video is that the bend curve difference top to bottom looks similar the whole way.
I disagree Andrew, the 2 intermediate side posts look to have a scale on them, the right side of the mast bends markedly less than the left, it snaps at the point of maximum bend, (where you'd expect) to the left of center. The pic isn't taken quite square on, so there is a bit of distortion involved in how the bend looks.
Select to expand quotedecrepit said..sailquik said..
>>>>>>
Of interest to me in the video is that the bend curve difference top to bottom looks similar the whole way.
I disagree Andrew, the 2 intermediate side posts look to have a scale on them, the right side of the mast bends markedly less than the left, it snaps at the point of maximum bend, (where you'd expect) to the left of center. The pic isn't taken quite square on, so there is a bit of distortion involved in how the bend looks.
Ah, I don't think I was very clear about what I meant. I mean that the % difference in the bend shape (Bottom - right side- bends less than top - left side) remains a similar difference. But thats a very rough observation. Actual % difference measurements could say otherwise. The 2 intermediate side posts with the scales on them are what is used to measure the deflection of the top half and the bottom half when the mast is weighted to 30KG. The % difference is the 'bend curve'
Quote from Boards Magazine article: boards.co.uk/features/masts-what-does-it-all-mean.html
"The industry standard terms describing windsurfing mast bend curves are:
0-6 = 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
The higher the number the more flexible the mast is in the upper half relative to the bottom"
Looking at the bend scale, I would guess that the starting bend in the video is close to the bend with 30KG. I wonder what the KG was at the end when it failed?
Sorry Andrew, Dot said as she looked over my shoulder, "what disagreeing with Daffy?" I should have known better and read more closely!
Looking at the vid again, the difference just may increase a tad if anything. I wonder if it's possible to do a stop motion and measure off the pic?
Think I'll try a screen shot while it's playing and see what I get.
OK
Here's what I've come up with, but don't vouch for it's accuracy.
min bend deflections, just used a ruler on the pic to mark a straight line between the two mast mount points, and measure from this to the closest mast edge.
base 145pixels tip 157pixels so is that 8%? sorry my heads not coping with this simple maths at the moment.
max bend
base 340pixels tip 380pixels and same thing is this 11%? If so it's changing from a hard to a C C with extra load.
I should have been clearer about my observation.
Thats very nice work, measuring from the screen! 
Not sure if that confirms my initial impression or not, but it does indicate the difference is not huge.
I got a great response and reply from unifiber. It is very much appreciated that they are so accommodating and helpful.
"On behalf of Unifiber I like to answer your question related to BC differences (both RDM and SDM) under different loads.
Very nice discussion on your forum ! Lots of useful info too.
From my background (Engineering R&D) I was in favour of linear behavior. But to verify that could be useful for your discussion.
It was not that difficult to perform a few measurements under different loads.
I measured with weights of 15, 30 and 45 Kg an RDM and SDM mast. Measurement errors of the set-up are around 0,25 BC points.
In short max BC's variation was +/- 0,3 BC points, both for the RDM and SDM. Hence for this simple static BC measurement both types of masts behave linear.
As discussed also on your forum, IMCS and BC are static figures and only intended to characterize in an easy way a mast. On the Unifiber WEB a lot more detail could be found.
But...Sailing is a 3D dynamic issue. As far as I know no serious modeling and verification has ever been performed on a windsurf rig.
Hope that above will answer your question.
Greetings Caspar Verhaagen"
So the Bend Curve behaviour is linear. Great information!
I have asked for confirmation that the bend stiffness is also linear.
Caspar added this link which is full of great information: www.unifiber.net/windsurf-gear/masts (info on lower part of the page)
my n/p rsr 4/6,5/0,5/8 feel great rigged on a 100% loft rdm, the same sails rigged on a n/p rdm feels terrible ,no power and way to much layoff, loft is constant curve and n/p is constant flextop
Select to expand quotekeef said..
my n/p rsr 4/6,5/0,5/8 feel great rigged on a 100% loft rdm, the same sails rigged on a n/p rdm feels terrible ,no power and way to much layoff, loft is constant curve and n/p is constant flextop
That makes sense according to my experience testing with lots of sails Would you say that your sail on the NP RDM is rigging flatter in the bottom and looser in the top with the same amount of downhaul. That is probably to do with the smaller diameter of the mast and the way the sail is cut to get it's shape. Do you give it less downhaul (measurement) in the CC mast?
I am speculating that more of the NP shape is derived from the mast bend/Luff curve relationship, than seam shaping in the body and luff of the sail, but I could be wrong.
Sails like the Koncept are a little less mast BC sensitive because they get a lot of their shape from the seam shape in the sail body and Luff. Having said that, you can certainly change their shape and character with different BC masts.
OK, so the bend curve behaves linear when "over-bent" so my theory is proven to be BS, great to know and thanks for doing that test Caspar.
I had a 2007 5/0 concept rigged perfect on the loft 400 rdm with very little downhaul where as the prides need heeps of downhaul
l
I just got confirmation form Caspar at Unifiber that the bend stiffness is indeed Linear:
"Yes, also the deflextion figures were fully linear for both the SDM and RDM"... (over the tested range of weight)
Engineers Rule! hey John?
Select to expand quoteIan K said...sailquik said..Shifu said..
Key factor is how fast the mast comes back into line after the rig has been distorted by a gust. SDMs are faster at that..
That old marketing chestnut of 'reflex response' is just that: A load of marketing BS.
Its all about weight with carbon masts.
Yes a load of marketing bs alright. The response of a bare mast, the frequency with which it vibrates when you put it in a vice and give the end a good flick, is nothing more than the stiffness divided by the mass. So a lighter mast of the same stiffness will respond quicker by the percentage that the mass is reduced. But when you wrap it in a sail, in air, the effective percentage mass reduction drops even further.
I'm surprised folks claim to tell the difference. All I can put the perception of faster response down to is that high carbon masts generally have a smoother finish, less sticktion from the luff sleeve.
There is a definite difference in feel between a stiffer mast and a more flexible one, I think the difference in feel comes more from lack of deflection in the first place, than reflex response though.
The big advantages come (for me at least) when i have to pump back onto the plane after my quite regularly botched gybes. There's no mistaking the feel of the top of the rig flopping backwards and forwards hindering my efforts to get back on the plane. Stick a stiffer mast in and the problem is all but gone.
Just getting along too, the rig deflects as you bounce over chop. stiffer mast= less deflection.
