What performs better SDM or RDM?

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Morts the 6.3 Race was rigging to the numbers...this mast is 14.5%...It is one of the reasons that I suspect once you start to bend the RDMs past the 30kg IMCS test mark they begin to bend more down low than an SDM does at these higher loads. So fully downhauled on a Race sail the 14.5% RDM is probably closer to a 13.5%-14% SDM in bend curve. That is what I think anyway..need to test it.

I am pretty sure that RDMs do not perform the same as SDMs.
They only match when the same load is applied and that being 30kg.

I feel the RDMs flex more when loaded in a gust and therefore don't have the same drive & can get backhanded although on a cambered sail this may not be as noticeable.
Years ago I rigged a 6.6M Koncept on a 430 RDM thinking I would get a deeper sail but I think it was actually flatter.
What Martin says above explains this.

I would like to a comparison based on say 40kg & then 50kg and review the bend curve %.

Ian, I don't think diameter has very much effect. Most modern sails are shaped by difference between sail luff curve and mast bend. A smaller diameter mast for the same bend curve will just mean slightly more sail area, not enough to notice I wouldn't think.

As previously said the main difference will occur when mast loads exceed 30kg, (providing both masts are equally specked), this will be more pronounced on sails with greater luff curve, and at higher loads. So a light weight with a straighter luff, will notice the difference much less than a heavy sailor with a curvy luffed sail.
Well that's my prediction from the theory, can anybody verify?

Ian.....we are taking the difference between RDM and SDM up in the cam length, so the RDM cams are longer so they can be interchangeable, it is approx 11mm from memory, need to check the CAD file. As Decrepit said yes you will effectively get more projected sail area on the RDM due to the shorter circumference.
Re the bend test I am keen to do a comparison at say 50 kgs just to see what is going on. Logically to me that explains why we see the sails setting a bit flatter around boom height on RDMs when compared to an SDM of equal IMCS bend curve however I am not an engineer and incapable of doing the math to explain it. A bend test should demonstrate if my logic holds or is BS. Need to find some time to do it.

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John340 said..
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Martin, For the same Moment of inertia and Modulus of Elasticity, the deflection in a mast is a linear function of the force. So IF the deflection at 30kgs is the same for SDM and RDM, then the deflection should be the same for both SDM and RDM at 50kgs. But I agree with you, there is nothing like empirical evidence to back up theoretical calculation.

John, that's not intuitive, by my earlier reasoning, there is more movement required of the material in a bigger dia mast than a smaller dia mast, for the same amount of bend. And I'm sure the force needed to distort the material is far from linear, so increasing the amount of bend should be easier for the smaller dia mast.

But what seems intuitive to this old head, isn't always so.

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John340 said...

mr love said..
Ian.....we are taking the difference between RDM and SDM up in the cam length, so the RDM cams are longer so they can be interchangeable, it is approx 11mm from memory, need to check the CAD file. As Decrepit said yes you will effectively get more projected sail area on the RDM due to the shorter circumference.
Re the bend test I am keen to do a comparison at say 50 kgs just to see what is going on. Logically to me that explains why we see the sails setting a bit flatter around boom height on RDMs when compared to an SDM of equal IMCS bend curve however I am not an engineer and incapable of doing the math to explain it. A bend test should demonstrate if my logic holds or is BS. Need to find some time to do it.

Martin, For the same Moment of inertia and Modulus of Elasticity, the deflection in a mast is a linear function of the force. So IF the deflection at 30kgs is the same for SDM and RDM, then the deflection should be the same for both SDM and RDM at 50kgs. But I agree with you, there is nothing like empirical evidence to back up theoretical calculation.

Given the weight penalty an rdm mast incurs to achieve the same stiffness as an SDM, could the rdm just be a little less stiff but still within tolerance, this effect would then be magnified when under greater tension than what a 30kg weight provides.

I am referring to stiffness in the bottom section where the diameters vary.

There maybe also a cost from increased complexity to the carbon layout and raw materials that favours such a bias? Ie thicker walls in the bottom section of the rdm

My 2 cents

John I'm still struggling with this, my problem is that I can't see how the amount of deflection, with force applied can be linear.
I guess that's just my concept of how carbon works, that's faulty. Hang on a minute I'll just rewire the old brain, --- Ah that's fixed it! double the force, doubles the deflection, all clear bow.

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Ian K said..

But John, aren't all those Modulus of elasticity calculations already wrapped up in the IMCS number for both SDM and RDM masts?

Oooooh Ian! So often you ask the most interesting questions!

But I think the IMCS (Indexed Mast Check System) only accounts for the different lengths being compared. There could be something else there though.?

I will try to drag my lazy bum off the couch tomorrow and if I can find some more 10KG bags of lead shot, I will do a comparative MCS test with 20, 30, and 40, 50 KG (if I can find enough bags.) I have heaps of 30KG data, but zero with different weights.

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Ian K said..
It's the old Hooke's Law, you just have to convince yourself that most elastic things are just coil springs in one way or another. It's near enough to linear.

en.wikipedia.org/wiki/Hooke%27s_law
"
Hooke's law is only a first-order linear approximation to the real response of springs and other elastic bodies to applied forces. It must eventually fail once the forces exceed some limit, since no material can be compressed beyond a certain minimum size, or stretched beyond a maximum size, without some permanent deformation or change of state. Many materials will noticeably deviate from Hooke's law well before those elastic limits are reached."


But John, aren't all those Modulus of elasticity calculations already wrapped up in the IMCS number for both SDM and RDM masts?

Just checked out the URL. Don't you just love Wikipedia. I especially like the the following quote:

"Hooke's equation holds (to some extent) in many other situations where an elastic body is deformed, such as wind blowing on a tall building"

A tall building is very similar to a mast in a sail. The cladding of the building is like the sail material and the lift core of the building is like the mast.

The key point is that under Hookes law the deflection is proportional to the load, only under elastic deformation. Hookes law does not apply to plastic deformation. Carbon fibre is a brittle material. It does not plastically deform, once it gets to its elastic deflection limit, it breaks.

Taking your last point, the IMCS number is simply a comparison of different deflections along the length of the mast under a specific load. Hence it is not just a function of the Modulus of Elasticity, but also of the Moment of Inertia of the cross section of the mast.

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mr love said..
Andrew...That would be great. All I am trying to find out is if there is a difference in bend curve characteristic between an RDM and SDM when they are deflected more than when tested with 30 kg. I am suspecting that an RDM starts to bend more at the bottom 1/4 point relative to an SDM the further you bend it. But I may be completely wrong.

Thanks

I'd guess it would be would be close enough to linear that the differences would come in smoothly.

I happen have an SDM and RDM 400. The SDM has outer diameter at the base of 52.0 mm and ID of 48.8 mm. The SDM has OD of 39.8mm and ID of 33.0 at the base end.

The formula for Area Moment of Inertia of a tube which is Ih = Pi( OD^4 - ID^4)/64.

(Why the engineers pinched the term "inertia" from the physicists in this application I don't know? But that's what they call it)

The "Area Moment of Inertia" is the measure of a cross section's resistance to bending.

For the SDM it is 89,536 and for the RDM it is 64,955.

(Sorry to leave out units, but I've calculated in mm, should use metres to get real numbers, treat the figures as just comparative.)

So according to the formulas used by engineers the SDM will bend less at the bottom. That's if the material's the same, the RDM has 90% on it, the SDM 97%, then there's the bias to the wrap, but 39% down on stiffness is a lot for the RDM to catch up by fiddling with the layup.

But that's for a free bending mast. The bend is constrained by the sail cloth. If the bend wasn't constrained no tension would be imparted to the sail, just a thin line of tension between the head cap and downhaul. Like a bow (and arrow).

Here comes a thought experiment. if you rigged a mast with a piece of string between tip and downhaul it would bend in a natural curve. If you sewed sailcloth onto it at this point there'd be no tension in the sail.
Then when you derigged the sail the luff curve would exactly match the natural bend curve of the mast. We don't want that.

So if you rig a sail on a skinny that is comparatively less stiff at the bottom then it would want to bend further than the SDM in the same sail. So the skinny would be trying harder to stretch the lower section of sail because the luff is constraining it to be further from its natural curve. Even though it's softer down there! A rigged sail is a very, very complicated piece of kit!


A question for the engineers. Does a tube being bent distort to being a little out-of-round? The formula suggests no, I always imagined it would?

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AUS 808 said..

sailquik said..

Ian K said..

But John, aren't all those Modulus of elasticity calculations already wrapped up in the IMCS number for both SDM and RDM masts?

Oooooh Ian! So often you ask the most interesting questions! www.seabreeze.com.au/images/forums/icon_smile_cool.gif' />

But I think the IMCS (Indexed Mast Check System) only accounts for the different lengths being compared. There could be something else there though.?

I will try to drag my lazy bum off the couch tomorrow and if I can find some more 10KG bags of lead shot, I will do a comparative MCS test with 20, 30, and 40, 50 KG (if I can find enough bags.) I have heaps of 30KG data, but zero with different weights.

Daffy, I knew you would cave in eventually & do the test for me I have put it out there a couple of times but no bites.
I figured you would be the only person with enough different masts, weights & the time to do it

The propeller heads are going strong on this but a basic test will sort it pretty quickly

It will have to wait till at least next week. Got a bit distracted by chores today and there is a snow dump forecast to end the season so I am off the the snow for a last fling.

And now we are entering the Sandy Point Storm season, I could be distracted next week by a 40-50 knots speed day.

Now I think about it, I may have already done a test like that with 20KG. I will have to check my data sheets. But I was more interested in what difference it made to the bend curve.

Ian, The MCS tests don't take into consideration the tip and base of the masts, but I don't think that would make much difference anyhow. I think it might be the layup orientation and the carbon modulus that makes the difference. All the production RDM masts I have measured (340 to 460) have been well within the bend stiffness spec. A few were actually stiffer which I have put to good use for testing. They typically vary by less than 0.3 IMCS which is getting close to the resolution of the testing anyhow. Same with the SDM's.

I have a couple of test sample RDM masts that are as light as the lightest SDM's that were made with a higher modulus (stiffer) Carbon. The problem is, they came out one mast size too stiff as well. (IMCS 19 for 370 and IMCS 21 for 400) To make them the desired standard stiffness, the walls would have to be thinner which would make them more prone to breakage. There are always tradeoffs. My neighbour is 'testing' the 4m test mast in the surf with a 45cm extension as a 430 replacement in his wave sail (comes out very close to 430 bend and stiffness spec) and has not broken it yet! I think it would be beneficial to use even smaller diameter with this material, but that can't be done on existing mandrels.

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Ian K said..

mr love said..
Andrew...That would be great. All I am trying to find out is if there is a difference in bend curve characteristic between an RDM and SDM when they are deflected more than when tested with 30 kg. I am suspecting that an RDM starts to bend more at the bottom 1/4 point relative to an SDM the further you bend it. But I may be completely wrong.

Thanks

I'd guess it would be would be close enough to linear that the differences would come in smoothly.

I happen have an SDM and RDM 400. The SDM has outer diameter at the base of 52.0 mm and ID of 48.8 mm. The SDM has OD of 39.8mm and ID of 33.0 at the base end.

The formula for Area Moment of Inertia of a tube which is Ih = Pi( OD^4 - ID^4)/64.

(Why the engineers pinched the term "inertia" from the physicists in this application I don't know? But that's what they call it)

The "Area Moment of Inertia" is the measure of a cross section's resistance to bending.

For the SDM it is 89,536 and for the RDM it is 64,955.

(Sorry to leave out units, but I've calculated in mm, should use metres to get real numbers, treat the figures as just comparative.)

So according to the formulas used by engineers the SDM will bend less at the bottom. That's if the material's the same, the RDM has 90% on it, the SDM 97%, then there's the bias to the wrap, but 39% down on stiffness is a lot for the RDM to catch up by fiddling with the layup.

But that's for a free bending mast. The bend is constrained by the sail cloth. If the bend wasn't constrained no tension would be imparted to the sail, just a thin line of tension between the head cap and downhaul. Like a bow (and arrow).

Here comes a thought experiment. if you rigged a mast with a piece of string between tip and downhaul it would bend in a natural curve. If you sewed sailcloth onto it at this point there'd be no tension in the sail.
Then when you derigged the sail the luff curve would exactly match the natural bend curve of the mast. We don't want that.

So if you rig a sail on a skinny that is comparatively less stiff at the bottom then it would want to bend further than the SDM in the same sail. So the skinny would be trying harder to stretch the lower section of sail because the luff is constraining it to be further from its natural curve. Even though it's softer down there! A rigged sail is a very, very complicated piece of kit!


A question for the engineers. Does a tube being bent distort to being a little out-of-round? The formula suggests no, I always imagined it would?

Beautifully explained. I came to the same conclusion, about the difference in area moment of inertia, after I measured my 430 RDM and SDM masts later this morning. This is not what I expected. Therefore we can conclude one or more of the following
1. They don't have the same IMCS - Daffy's measurements will confirm or deny this
2. They have different modulus of elasticity - as you have explained this is unlikely
3. Something else is at play
I suspect its the first option. If that's the case then the same sail rigged on a RDM and SDM will behave differently under load.

We know this:

- The KA sails (in the specified sizes) work well on both RDM and SDM masts. We know this from testing and comparison.
- Some sail models and sizes work better on either RDM's or SDM's for certain sized sailors and for certain uses. Often it comes down the a certain feeling the sailor finds most comfortable for them.
- We have made changes to the shaping of the small Koncepts to make them work better on RDM for speed sailing for most sailors.
- The KA masts, both RDM and SDM are carefully tested before delivery to conform to specification, which includes IMCS stiffness and bend curve. The specs are the same for both types.
- RDM masts are a lot more durable in 100% Carbon than SDM masts so they are great for any activity where masts are stressed a lot like wave-sailing, Freestyle and Speed-sailing.
- SDM masts can usually be made lighter and are therefore often preferred for high performance racing, especially with large sails.
- The development of a sail does not happen in a vacuum. We don't start with a specific mast and a clean sheet and build a sail around it from Scratch. The process builds on what we have learned from previous models and prototypes tested on various mast combinations and specifications. It is a process of refinement guided by a philosophy on desired outcomes. I am sure every other designer does the same.
- I have personally tested dozens of different masts in production and prototype sails. One soon gets a picture of how they react to different spec masts, but there are always new things to learn. As certain aspects of the board and sail change, so do the design goals to complement those things.
- Most of us have noticed that RDM masts impart a sightly different feeling and subtle differences in shape to a sail than SDM masts. The quest if to explain why. It's arguable that we don't need to be able to explain exactly why if we know it happens, but enquiring minds do want to know.
- We don't know everything about everything.....yet.

Rob.....what I am talking about is what happens to the masts...RDM vs SDM,s when measured beyond the 30 kg IMCS test. All the KA masts are tested in the factory, extensively measured and conform to the specified bend curve within tolerance. This is the universal IMCS test with the deflection tested with a 30 kg load, (we actually take more data points than the standard test )
The sails are designed and tested on these masts, KA masts that confirm to the IMCS stiffness and bend curve that we specify, so no not a random bunch of masts.
This bend curve was arrived at a number of years ago by testing lots and lots of masts. We eventually settled on a bend curve of 13.5% as it a) worked great and b) was not so far from standard CC that you could not use one in a KA sail. There were pros and cons for different bend curves and some extreme ones have been tested, we are very happy with the curve we settled on which works both on the wave sails and the big race gear.

This debate is about the performance of RDM vs SDM. Remember that RDM masts were originally developed for Wave sails due to the obvious durability advantages. More recently customers have requested the ability for flat water performance sails to work on RDM masts and I guess this thread has been centered more around that. Our sails work on both yet I do observe subtle differences. Some riders do prefer one over the other. The original question is "which performs better?" I would answer that it can be subjective. For example I am a heavier rider so prefer SDM on the bigger Race sails. My experience is the sails have better low end and feel more stable at the top end on SDM, however some of the lighter riders like Andrew use and really like RDM. I have observed in the design/ testing process that the sails do rig slightly leaner at boom height on the our RDM's, more noticeable on the larger Race sails (I have tested RDM on as large as 7.5, no bigger as we do not offer a 490 RDM) I am speculating that this small difference could be that RDM masts may vary from SDM masts in bend curve when tested at a higher load than the standard 30kg IMCS test. No this not something I have tested as it is not part of the normal stiffness and bend curve test, I will do it now if for nothing else than fueling my curiosity. I never stop learning.

Anyway I did not intend this to turn into a KA fest, I just wanted to share my experience having rigged lots and lots of sails on both RDM and SDM. My personal answer to the question is that I would use nothing else but RDM on Wave sails and freeride sails up to 6.6. I would use RDM on the speed sails up to and including 5.8 and SDM on the bigger sizes. I am 90 kgs and this is what works best for me.

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Ian K said....Although my quick measure of a mast under moderate flex showed no distortion of diameter, any tube bent to destruction from Bunnings seems to show a distortion just before before kinking. If some tubes can get into that distortion zone but recover without damage then that would show up as a non linearity in flex. If the diameter in the flex direction reduces even slightly it will be falling behind to the 4th power in stiffness. That could be quite different between SDMs and RDMs.

Well I'm a bit hesitant about commenting after my last stuff up, but wouldn't that be in the wrong direction?
It seems to me that the thinner walled larger dia tube is more likely to go elliptical than a smaller dia thicker walled tube. Which is going to make the SDM softer under bigger loads.

Didn't mean to open a can of worms. I just have an observation from rigging sails and attempting to explain it...If you guys who are way smarter than me think my theory is BS I am totally willing to accept that. Will still do an "excessive" bend test at some point though to satisfy my curiosity...not this weekend though...footy and wind!!!