Does carbon matter? Are long RDM extensions (48 cm) safe to use?

I have an NP alloy RDM extension (US cup). But it only goes to 34 cm. I'm surprised they do a long one in SDM and alloy only. Yet they have sails with over 40 cm extension.

Metals are isotropic, carbon composites aren't. Also Carbon has low compressive strength, wrt to metals.

What this means is:
- your mast being bent, is stretching the outer-wall fibres making it strong
- your extension is being compressed by the mast
- your extension has holes in it, where the bolt goes through... making a compressive point-load.

Long Alu extensions do break, but more often they just bend a little (or a lot !)

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mathew said..
Metals are isotropic, carbon composites aren't. Also Carbon has low compressive strength, wrt to metals.

What this means is:
- your mast being bent, is stretching the outer-wall fibres making it strong
- your extension is being compressed by the mast
- your extension has holes in it, where the bolt goes through... making a compressive point-load.

Long Alu extensions do break, but more often they just bend a little (or a lot !)

Thanks. I'm considering buying a longer extension (instead of a new mast). From what I understand, there is little to be gained from having a carbon extension, because the weight saving at the mastfoot has little effect. But I'm worried that a long alloy extension may damage the mast. Maybe it's one reason why NP does not make a long extension, except for SDM alloy (I believe they are a conservative company).

Weight saving is weight saving. A 46cm Ali extension is 2 to 3 times heavier than a carbon imo & make awesome intruder dissuaders. Maybe NP only make that length mast extension so you need to buy another mast....? Just sayin'. Their sails do rig sweet when you follow suggested mast sizes.
I don't particularly like using extensions near or at max, similar to boom arm extension - just personal preference of course. I'd suggest matching sail to mast suggestion & look for a mast that suits.
Wouldn't mind if mast extensions actually started at 0 or 2 - having a sail that is 402 luff with a 400 mast & extension that starts at 8am sucks.

AB....

40cm something alu ext too heavy, i have a 48cm chinook carbon without any issues. on this extension you can skip a mast size

I have a 30' alloy. And a 46' carbon..., the alloy is way heavier than the 46 ..but both do the job.much prefer the carbon one ,nicer fittings and you don't have to wash it...the alloy one does crusty up if not washed .. .both mine are naish RDM extensions .
i personally would pay the extra and get the carbon ...you have them a long time .

I've been using the Neil Pryde 48 EXT ( Alum ) for years with no problems at all. I run my 7.5 Ryde with a 430 mast and the extension is set somewhere close to 40cm. I can't remember exactly.

Prior to that I had Fusion 7.4 and two different 7.2's in it. I still have the same 430 x6 mast. Never any drama what-so-ever. MXT base however. In fact I swapped the 48 ext from another sailor who wanted one of my 34cm extentions. He used race sails with terrible downhaul on. Still going strong.

I would not worry one bit.

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ULF said..
I've been using the Neil Pryde 48 EXT ( Alum ) for years with no problems at all. I run my 7.5 Ryde with a 430 mast and the extension is set somewhere close to 40cm. I can't remember exactly.

Prior to that I had Fusion 7.4 and two different 7.2's in it. I still have the same 430 x6 mast. Never any drama what-so-ever. MXT base however. In fact I swapped the 48 ext from another sailor who wanted one of my 34cm extentions. He used race sails with terrible downhaul on. Still going strong.

I would not worry one bit.

RDM or SDM?

Long extensions are the bane of RDMs.

I'd consider a tip extension. 150 mm aluminium or the tip of an old mast or something like that.
An alternative is a long SDM extension with an internal stopper plug to receive the RDM. That only works if the extension has pins for adjustment and they project a fair way through to support the plug. You also have to be sure the extension doesn't interfere with the bottom cam if there is one.

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808 said..

ULF said..
I've been using the Neil Pryde 48 EXT ( Alum ) for years with no problems at all. I run my 7.5 Ryde with a 430 mast and the extension is set somewhere close to 40cm. I can't remember exactly.

Prior to that I had Fusion 7.4 and two different 7.2's in it. I still have the same 430 x6 mast. Never any drama what-so-ever. MXT base however. In fact I swapped the 48 ext from another sailor who wanted one of my 34cm extentions. He used race sails with terrible downhaul on. Still going strong.

I would not worry one bit.

RDM or SDM?

SDM

All I know is I have read of a lot of people swearing off carbon extensions after a very long swim. As for weight, I don't personally see this as important, the one place you could afford something a tiny bit heavier would be the location of the extension.

With this discussion we realise that an over extended shorter mast won't have the same curve characteristics as a short extension in the "right" size mast? The sail will behave differently, generally worse.

Totally agree with having the right mast length ...they just work better ,some sails are sensitive to this problem
some not so ...if they recommend a mast on the sail or on the sail specifications ...you will Get a better performing sail
as the top of the sail exhausts correctly ...due do the correct area of bending ...I find my ezzy sails work fine with a 430 with a large extension as well as a 460 with a small .
Where my naish sails ..like the right mast ..with a minimal extension ..( fussy buggers)
Going back to the original question .. Nothing wrong with alloy mr cranky.if they make them they must test them ..
sometimes your extensions are almost expensive as a good second hand mast .( having a second mast is a bonus)
Ive broken a few mast ,but never a extension .

Thanks for the replies.

What motivated this was the fact that NP has a bunch of sails that can all go on a 430 mast and cover the range from 4.5 to 7.5.

Fusions as the smaller sails and Rydes as the bigger sails. The biggest Ryde would need a 46 extension on a 430 mast.

Now whether it's a good idea to rig smaller sails, that specify 400/430, and bigger sails that specify 430/460, all on a 430, I have no idea. For example I've been told that as a lightweight (<70 kg), small sails on long masts are a bad idea.