Good questions, which I can't answer. But I have one of my own, while were at it. I too have heard that lighter sailors should use a softer mast. Does this mean, for example, that if the length works, I could use a 430-21 in place of a 460-25? (I weight less than 140).

Randy

Will it rig? yes. Will it "work"? IMHO: no.

IMCS is a way to "normalize" MCS, to give you a number that you can compare from mast to mast. For those not acquainted with MCS (mast check system) IIRC, it's determined by hanging a 30 kg weight from a mast's center while it's supported at the ends (I want to say something like 15 cm from the end but my mind is going). Simple physics says that two masts of the same stiffness but of different length will have different deflections, the longer one deflecting more.

That's what simple physics says but the problem would be that folks would have a 460/28 and a 490/28 mast (or a 460/25 and 430/25) and think they are the same. So IMCS was calculated- it gives the MCS if the mast were 460 cm in length (which is why longer masts have a higher IMCS- they tend to be stiffer because they must carry the rig forces over a longer length: think how thick/stiff you want a 5ft long bridge to be compared to a 500 ft bridge. The longer bridge needs to be stiffer because of the longer span)

Okay, back to Gene's ?. The sail calls for a 490/26 mast which gives, again, IIRC, about a 29 IMCS (for the same stiffness, you get a 1 IMCS stiffness change per 10 cm length change... roughly). But with the extension, that really reduces the stiffness back to about IMCS 26. The extension adds leverage to my ability to bend the mast- one reason sailmakers don't want you rigging a 9.0 on a 460 mast with a 60 cm extension. The additional leverage from the long extension turns the mast into a noodle.

So, the sailmaker is really calling for an effective IMCS of about 26. Gene has a mast with an IMCS of 34 which will be softened up to about 31 with the extension. 31 vs. 26 is a huge difference that you can see and feel. Most folks can tell the difference of about 1-2 IMCS. I'd think the sail would be a pain to downhaul and it would have great difficulty twisting off properly for Gene's weight. Yeah, he could do it, but the sail would feel stiff and have a pretty lousy range. It would be super stable as that stiffness would really lock in the draft but it would be lifeless at the bottom end and not twist and accelerate at the top end.

Now- why do smaller sailers need softer masts? Sailmakers design the sail figuring the avg. sailer of about 180 lbs or so will be putting a load on the rig (wind vs. body weight via the harness). When the sail is loaded up, the mast bends off, twisting the sail at the top- which is why the leech is floppy on the beach. When the mast bends, that flop is turned into elegant twist. Try it for yourself by putting a rig on the ground, have one person hold the mastfoot to the ground and another push down on the tip as if the wind were loading the sail. You see the leech tighten up, taking up all that loose sail fabric as the tip bends towards the ground.

Lighter sailors just don't weigh enough to get the mast tip to bend the way the sailmaker intended with the normal mast recommendation. By using a softer mast, though, the mast will bend the way the sailmaker anticipated and behave the way it (the sail) was intended. Likewise for a heavier than avg sailor who exerts a greater force, needs a stiffer mast as he/she will exert a greater force on the rig.

I know this stuff sounds like voodoo but it really works and is apparent even to the casual windsurfer. I've had sails that felt like crap on a recommended mast but using one about 2 IMCS less made them light up (I'm smaller than avg.) I also made the mistake of using a too stiff mast and it made the sail feel like plywood- lifeless.

Huh?

Gene,
An extension softens the mast because it effectively makes it longer. Can you bend a 2 x 4 that's one foot long? Probably not. Can you bend one that's 20 ft long? It'd probably flex under it's own weight alone.

If you've ever rigged a 490 in a sail that calls for a 460 (or vice versa), that's what having about a 3 imcs point difference feels like (it's not inconsequential).
MCS/IMCS explained
http://2002.windwing.com/2001/htmlpagefolder/mcsimcs.html

big masts on small sails:
http://www.boards.co.uk/kit/articles/mast_bit.asp#SMALL

Again, can you rig it? Sure. Will it feel/behave right? I really don't think so.
Nothing against giving it a shot but don't expect much.

Awesome discusion. I've learned a lot -- even if it did make my hair hurt. (Where are you, Mr. WindLord Herderich, when we need you? Aren't you going to clarify all this for us?)
When I got into windsurfing in the mid-to-late 90's, I wondered, What's all this fuss about MCS and IMCS when I can't find ANY 460 masts on the market that have a MCS/IMCS even 1 point different from one another. Same within the 430, 400, and 490 lengths. Later, when I became a dealer, people would call me all worked about the issue: "It's critical that I have a 460 mast of 25 IMCS for this new sail!!" I'd say, "That's good, because that's the only way they come." (In carbon, anyway.)
It's a very important obsevation for all you newer windsurfers: When comparing masts of a given length, virtually EVERY carbon mast (up to a 490cm), regardless of brand or CARBON % CONTENT, has the exact same MCS/IMCS rating.* So don't worry about the imcs rating # of any new or used masts from recent years (unless you're getting into the big sails, above around 8.5 these days.) Just worry about the right length -- the exact length suggested by the sail's designer -- and how much carbon you can afford to buy. Stiffness is not necessarily the same as reflex-ability. Higher carbon content generally means lighter weight, which is a good thing, and greater reflexability -- another good thing.

*I just checked the '03 and '04 mast spec charts of Powerex, North Sails, Fiberspar, and to be fair, an "off" brand, Sailwork's Joystick, Lipstick, etc. The charts are identical on mcs/imcs within a given length up to the 490cm size. The 30% carbon 460 has the same mcs/imcs as does the 75% 460 and the 100% 460 in all these brands. One exception: North Sails and Fiberspar each have one 490 model at 28 imcs and all others are 29 imcs. You do begin to find a minor variance of 1 or 2 points in the 520 and 550 lengths with in carbon-content models or brands.

Also, it seems obvious to me the length of the extension will not affect the masts stiffness, i.e., I think a 490/26mcs/29imcs on 40cm of a good, quality extension is still effectively 26mcs/29imcs on 5cm of extension. The extension does not really add FLEXABLE length to the mast. The extension does not flex like mast materials flex. I don't think a quality extension flexs at all. I.e., hanging the IMCS test weight from the center of a 490 mast attached to an unbending, rigidly-mounted 40cm extension or 5cm extension will deflect exactly the same. The mast doesn't know it's mounting point (on the extension end) is beginning 40cm, or 5cm, or no cm at all, from the usual vise grip or whatever they hold it with. This doesn't mean where you're hanging your weight from doesn't vary along the length of the mast,and thus, in some way affect the behavior of the mast;, but I don't think the essential flexing properties of the mast changes.

Chuck wrote:
"Also, it seems obvious to me the length of the extension will not affect the masts stiffness, i.e., I think a 490/26mcs/29imcs on 40cm of a good, quality extension is still effectively 26mcs/29imcs on 5cm of extension."

It doesn't change the mast's stiffness but... it does increase our ability to bend the mast. The mast is effectively longer. Leverage 101. If the sail designer wants you to rig a 520 luff on a 490 mast, he's already figured out how much the mast is going to "soften" up with the 30 cm extension. If you put a 520 with the same imcs, you aren't going to end up with what he (any female sail designers we know of?) intended.

Maybe we are saying the same thing here but if we aren't, think of it this way. Say you have a 10 ft 2 x 4. If I add a 2 ft piece of steel (much stiffer than the wood) to the end, I'd have an easier time bending the 2 x 4 when supported only at the ends. Doesn't matter how stiff the "extension" is, I have a longer effective 2 x 4 and it will bend easier.

I have a 465/28 (so about a 28 imcs) that when you rig it with 30 cm of extension is significantly "softer" than a 490/25/28 imcs rigged with 5 cm of extension. Why? You are right that the mast isn't magically any softer but I get an extra 30 cm of leverage on that mast. (Think of the extension as what it is - a pry bar). That's why when the sail maker calls for a 490, they really want a 490 in there.

North- Fiberspar... afaik, both are made by Excel of Finland which would explain the similarity.

If I ever saw a case of wind withdrawal then this must be it. Let’s all hope for wind so that we can start talking about all the triple backward loops we were able to complete at Van Pugh instead of the finer points of MCS/IMCS.

For the record, I agree with William; longer extension = softer mast. Although I plan to stick to carbon masts, no matter how similar the 2x4 bending characteristics may be. (just can’t get that 2x4 into the mast sleeve no matter how hard I try)

Gene,
I guess I should have read the link more closely... because I don't agree with that bit. Look at their chart at the bottom- DIN / IMCS / MCS CONVERSION TABLE. The columns are masts of the same nominal stiffness. An MCS 25 mast goes from an IMCS of 25.5 at 460 cm to 22.4 at 490 cm. 30 cm = 3 imcs points. It may feel like 2 points but the math says otherwise.

Googling around a bit, I think I've divined what they really meant. When using too long of a mast that pokes out the top, North Sails once estimated that the extra was worth about 1mcs/20 cm (probably because you are rigging the top closer to the middle of the mast where it's stiffer). To say extension doesn't affect things really is saying that the sailmaker, as we alluded earlier, takes that into account when designing the sail. So by that measure, it doesn't matter.

Marek's right. I want to go someplace warm and sail.

http://www.bluefinz.com/technique/masts/

scroll down to "using masts as a tuning tool"

I guess my main concern regarding the above ( except for the fact that I now have a lot of material to place in my new procedural manual on windsurfing ) is what exactly are you referring to when you use the words Heavy Sailors. Is that the same thing as calling a person FAT :?: And don't you think the mast stiffness formula could also use a component from the South Beach diet or the Akins diet to address what may happen if that FAT, I mean, Heavy Sailor loses weight suddenly?

This is some really tuff stuff to think about. Excuse me while I get some asprin.

Peelskid,
Not to worry, I think you safely can use the stiffer masts.

Way to hang in there William - the work he does for us all!
Gene - I also believe in the "just try it approach" - please let us know.

"Einstein" thought experiment:

Imagine some tether-ball poles - remember those?

1) 1 foot tall 2x4 wood - not flexy if we push on the top to the side.

2) 8 foot tall 2x4 wood - flexy, we are getting good leverage ( force x length).

3) 7 foot steel with 1 foot 2x4 at the top - not flexy. Steel is not bending at a distance by us and the soft wood part is close to where we are pushing.

4) 1 foot 2x4 wood with 7 foot steel at the top - pretty flexy. The 2x4 will flex over that one foot length like a hinge, we will have good leverage on it. But not as flexy as #2.

Note: flexy is a technical term for Someting-MCS.

So...hmm...don't put the stiff mast extension at the top of the mast...I guess. Which you weren't going to do anyways. Good.

Masts bend along their length, not just at the base, and actually not at the base at all since this is on a universal. So this example...sucks...but it was fun. No, seriously...there will not be much/any flex in the extention, everyone agrees, but from there on: there will be this bigger sail load, but also, the boom will be closer to the less flexible base...

Even Einstein had to do many thought experiments - not every one was successful.

Tetherpole #5. 8 ft 2 x 4 on top of 1 ft of steel (9ft total). Easier or more difficult to bend than pole #2?

Now, I've got one for you Marek. Suppose you were on a street trolley on your way to your job in the patent office in Bern carrying your 2 x 4. It's 8 ft long and pretty flexy. Now, as the street car accelerates and approaches the speed of light, it appears to shorten. Does it flex just as much?

I started a new thread instead of posting a reply, so to make sure that we have an accurate record for this topic, here is my comment from my 2x4 post.

OK, I finally got that 2x4 into the mast sleeve with some effort (it worked, but I will not be able to resell the sail in "as new condition" anymore).

There are some benefits though (even with the few rips in the mast sleeve). After nailing two 2x4s together and jamming them into the sail, I can get that perfect length mast buy just cutting of the excess with skill saw, no extension required. I guess with that perfect mast length, I don't need to worry about the DIN / IMCS / MCS number any more.

Now to answer William's question about the speed of light and my 2x4 mast stiffness:
The answer is "I don't know"....But I do know that if I'm on the lake and I spill on a jibe, and there is no one there to see me fall - did I really fall in? No way, I carved that turn like never before...at least that's the story I'm sticking to.

Marek

Don't worry Marek. We may not see it but it'll register on our home seismographs.

Sorry the "Next page" technology escaped Einstein and myself.

To answer William:
> Tetherpole #5. 8 ft 2 x 4 on top of 1 ft of steel (9ft total). Easier or more > difficult to bend than pole #2?

Same as #2.

Marek - just remember the tetnus shot before sailing with the nail filled 2x4 mast.

Richard,

Use small nails...so they don't stick through.
Now I wonder if a plywood fin would work…