What performs better SDM or RDM?

Comes down to flex numbers and where the softest and stiffest parts of the mast are, compared to the flex needed in the cut of you sails, to the body weight and load you put on the sail, depending if you rig big, small, or in the middle.
Nothing is set in stone.

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. This helps big, fat slalom sailors keep the power on. Lighter, handsomer sailors can use rdms on race sails to good effect.

For loft sails I preferred RDM as a lighter sailor due to the greater flex in the head of the sail meaning that I was less likely to be stood up (lifted) in the gusts and the easier rotation of the cams.
In bigger sizes the sail definitely rigged flatter and had less bottom end but I used a 460 rdm in the 7.8.

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decrepit said..
Lao, that indicates a different bend curve to me, the op is asking about identical bend curves.

I got the impression that larger diameter masts work better in terms of reflex all things being equal.

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decrepit said..
Lao, that indicates a different bend curve to me, the op is asking about identical bend curves.

Here is the info from Monty (Loft) from poehaaa's link. Talking about loft rdm vs sdm (they are designed for both and supply both)
RDM
Feel more forgiving generally compared to SDMs. Their more elastic dynamic character is better suited to lighter windsurfers.
Their dynamic softness making the rig more forgiving for the stronger winds associated with smaller sizes.Are generally better suited to rough conditions, especially severe chop, where their softer character gives a ‘dampened’ feeling to the rig.
Cam-equipped designs rotate more easily on RDM masts than on SDM masts.
Tend to set the sail flatter through the lower body on RDMs compared to SDMs.
Generally allow the rig to set lower down onto the board compared to SDMs because their softer character results in slightly more mast bend through the middle, which in turn allows more twist through the lower leech that lets the sail twist off more just above the boom, so that the boom settles in, ‘closing the gap slightly more than SDMs do. This can make upper end sailing more efficient and more comfortable.

This is exactly what I found sailing the Loft sails on RDM for many sessions.

For all I know this could be due to a different curve in the SDM vs RDM but I think it is more to do with the diameter and wall thickness affecting the way the mast flexes under load. Rdm will flex more under the same load but on the same curve characteristic (I could be talking rubbish here! )

SDM masts will give a better gust response in transferring the gusts load to the sail but as a lighter salior sailing overpowered that is not what you want. The rdm allows the sail to exhaust and then gives the power. Less of a shock load that unsettles the board and allows you to transfer the power in to speed/ forward motion rather than lift.

That all makes lots of sense, but when they say SDMs are stiffer than RDMs, and that affects how the sails set, it sounds like they are talking about static conditions not dynamic, in which case they are comparing 2 different specked masts.
Dynamic response is a different kettle of fish, and I guess it's possible that same specked masts have different dynamic response, due to wall thickness and diameter.
Without having the two identically specked masts, (and hoping the specs are accurate), it's impossible to tell if this is the case.

So unless evidence turns up to the contrary it's safest to go with manufacturers recommendations, I guess not all of it is hype and marketing.

As mentioned, identical flex RDM and SDM behave DIFFERENTLY when loaded, flexed, and sailed in the water.
Reflex is quicker on SDM. Softer feel is inherant on RDM.
But what exactly are we looking for?

I have been messing around with RDMs on some KA Race sails and doing some testing. We recommend RDM for the smaller Koncepts...5.8 down which are actually designed and tested on RDM specifically and the 5.5 Race which rigs on 400. Larger sails though I have always felt perform better on SDM.
My experience rigging larger sails...6.6 up is they rig flatter on RDM. I have been trying to get an understanding of why that is the case as I have rigged on SDM and RDM masts..both very close in bend curve...the RDM 13.2 and the SDM 13.5 yet the sails rig quite differently.
I do have a theory but I am yet to do a mast measuring session to prove it out.
The IMCS test is done with a 30kg weight and the actual deflection at 30kgs is not huge. On modern Race sails with high luff curves we are bending the masts a lot and I suspect the 30kg test is not telling the real story. I would like to test some masts at 50kg and see how differently the RDM and SDM behave...I suspect quite differently at higher loads. I have sort of proved this out a little by testing a 6.3 Race and 7.5 Koncept on a couple of sample RDM masts we had made a few years back, trying some more flex top masts for speed sailing. From memory they were 14.5% bend curve. The sails rig really nicely on these masts..yet also rig almost exactly the same on 13.5% KA SDM masts. When I rig them on 13.2% RDM they are noticeably flatter around the boom. So something is going on with the RDMs at high loads...they are bending more around boom height.

Time to measure some masts at 50KG to prove out my theory or find out that I am full of s..t.

Here is a 6.3 Race on a 14.5% RDM....It is great on this mast.

So my take at this point is RDM OK up to around 6.0-6.6 but I would use SDM on larger sails but......if you can get an RDM mast about 1% more flex top than the recommended bend curve it will work fine on larger sails.

Yes it makes sense, for me it's rdm up to Koncept 6.2 and SDM for 6.6 and above.
Martin, I have noticed that latest models larger Koncepts are a bit flat in the lower section, and that all sizes in the latest years seem to have the leach a bit looser. Is it my feeling/wrong rigging or is it by design evolution?

I apologise if this is slightly off topic. I have a KA 100% carbon 430 SDM mast and Naish 100% carbon 430 RDM mast. I've rigged my 2016 KARace 6.3 on both. For me, there is no discernible difference in performance except in one area. On the SDM the cams do not rotate without pushing them into place with my fist and foot. On the RDM they rotate beautifully. Even if the SDM was faster, I still prefer having a sail that rotates properly.

I've got it set up now with RDM cams, but from memory there were no spacers for the SDM cams. If I ever change it back I'll pay attention to the spacers.

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decrepit said..
I'm wondering if there is a definitive answer to this question, has anybody done wind tunnel tests, side by side racing of two similar sailors, swapping rigs several times, to compare speeds.

Just taking a guess here, but I doubt you'd notice much difference. If there's a weight difference that could have more effect than aerodynamic.
(Are RDMs lighter than SDMs? for the same specs).

I think you are correct Decrepit. It comes down the the characteristics you are looking for. Aero difference seems insignificant.

Speaking of 100% carbon masts: RDM's for a given stiffness are lighter than RDM's. But RDM's are far more durable.

RDM's also have a different dynamic feel. For me,as a lightweight, that seem to have a more forgiving flex characteristic in strong winds with smaller sails for speed sailing. I use RDM's for 6.2m and down. Generally SDM's for 7m and up for lighter weight and crisper power feel in cambered sails. For non cambered sails, I think the same applies. RDM's over 430cm seem to get very heavy in comparison with SDM's.

In short, The 'performance' differences are not so much about pure power or speed, as about handling and durability.

Yes, that definitely makes a difference. But the smaller KA Koncept sails and Race sails are designed for RDM to compensate for that. But this allows you to 'custom' tune your sails by changing masts. If you rig a 5m or 5.4m Koncept on an SDM it will have a slightly deeper lower profile but will probably feel more rigid. It could be useful for bigger sailors.

For some years I have been fiddling around with mixing top and bottom sections of different sized masts to see the effect on sail shape and feel.

For instance, in one sail designed for a 4m mast, I tried a 430 bottom and 370 top section. It can be very interesting, and in some conditions, and with some sails, advantages can be had. It can also turn out quite horrible on other circumstances, so experimenters beware!

I have also experimented quite a lot with using a 15cm mast tip extension on some RDM's with a corresponding decrease in mast base extension. This effectively makes the mast slightly more flex top and softer. It can really make a difference if the sail feels a bit too stiff and unresponsive in gusts.

Great topic .I reckon Sdm s are faster ..due to personal test .in flat water and gusts.
the Sdm doesnt warp the sail out of shape as much and delivers stiff drive .
How every if its choppy and a little more forgiveness and control is required ,RDM s have there place .
I have 100% RDM and Sdm ..so its a fair test ..used on the same 4 cam sail.
i like using the RDM and do get more belly in the sail but the SDM ...it's faster ..highly noticeable in the top end .no loss in speed .
As the RDM is still flicking around and exhausting that air you need for top end speed .

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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.