Mine is a Chinook and a Baja mast extention, and both RDM and SDM go up to 48 and 46cm.
Generally speaking if you need more than 30 cm then you should be using one size up in length and importantly stiffness of mast.
Lot of sails these days (severne at least) are speccd with a shorter mast with lots of extension. In my opinion the more extension in the mast the better.
It's not that silly a question.
Good point Sparky, Even though my extensions go to 46 and 48, I seem to only go up to 20. Most of the time I'm at 10-15. I guess I just like having that cushion of extra extention available and also knowing I will never be short of extention.
Long extensions are sweet if you need that length, but they sux if you don't need any extension or just a couple of cm, as most bottom out in the mast around the 8cm mark
Shelly,
That's interesting to hear. I have never had my 46 and 48 extension bottom out on me. I have used them on Fiberspar, PowerX and Ezzy Hookipa masts and never a problem. What masts gave you trouble with the extension bottoming out?
The minimum extension one can put is 5 - 8 cm
depending on the extension
if the luff on a sail is say 462 - one cannot put 460 mast with 2 cm extension
sail makers s/b aware of this and avoid such "odd numbers"
i have seen less and less of this happening with the sails I have been encountering
but since Shelly mentions it ....
If you get some sand in the bottom of your mast then the 'fused' extension will extend out to something over 4m 
!!
Select to expand quoteMagic Ride said...
Shelly,
That's interesting to hear. I have never had my 46 and 48 extension bottom out on me. I have used them on Fiberspar, PowerX and Ezzy Hookipa masts and never a problem. What masts gave you trouble with the extension bottoming out?
Thats a good pt
I use current ka 100% carbon masts and i use ten yr old ( still going from when i started) np extension, 34cm and that bottoms out around 4, i dont even bother with the 48cm ext anymore, but it bottoms out about 12cm, i know this because i tried to rig my 7.1, 465 luff with it, no chance with a 460
Select to expand quoteSparky said..
Generally speaking if you need more than 30 cm then you should be using one size up in length and importantly stiffness of mast.
very generally, I have a couple of NP at 34cm (and they are very different age sails so its not just one model / year)
I think 32-34cm ext is nice as it gives you two mast choices, important for wave gear where u might break one and just not happen to have $1000 in your wallet 
I'm yet to see or hear of a mast failing because of a contact point from the extension.
I was more trying to say, some sails are speccd to have a shorter mast with lots of extension. Better to have an extension that goes to 46cm than an extension that goes to 30cm if you need 30cm of extension.
Select to expand quotedarcyn150 said..
Hi there guys what is the longest mast extension
I've never felt comfortable using the top holes on an extension, I've bent too many aluminum extensions doing that (in waves only).
I've since gone to carbon, but I still feel the same way. I use medium length ones only, and I wouldn't trust, so I've never owned a tall extension.
But I've also seen carbon extensions break using the top holes, hence my caution.
Some people I know have even added more holes at the top of the extension, crazy, which will result in the base of the mast splitting, seen that also.
If in the waves, you should be using the extensions conservatively.
There are Euro pin extensions that get to zero I believe. Also the tallest extensions are carbon and more expensive, aluminum extensions only come in lengths that are shorter than the tallest carbon ones. If you use a really long extension such as over 30 cm the mast will end up being one size smaller and of course softer, not as stiff which could be an issue for a heavy weight person.
If a long extension on low setting jams up in the mast just grind the end bung down a little ( simples )
Also know that a long extension will flex more than you may think. I use 32 cm on a 46 carbon ext with a cammed sail and placed a straight edge on the inside to see a gap of 1 to 2 mm , that's a scary looking bend. So keep in mind that a long extension will bend a lot more than the shorter mast , it's bend on bend , a shorter mast with longer ext will be WAY flexier than the next size mast. 
Select to expand quoteImax1 said..
If a long extension on low setting jams up in the mast just grind the end bung down a little ( simples )
...
NP extensions, unlike most, have a snug fit in the mast so you can't do that.
Select to expand quoteNotWal said..Imax1 said..
If a long extension on low setting jams up in the mast just grind the end bung down a little ( simples )
...
NP extensions, unlike most, have a snug fit in the mast so you can't do that.
On NP extensions the end bung is a bigger diameter than the extension.
This diameter can be ground down a little to let it slide up the mast that last bit.
I can't see why you can't do this.
I do this on my extensions to fit all the way in on all masts.
If there is slight clearance when using on a lower setting it can't be a problem , if you can imagine when rigged up all the pressure is on one side and there will be clearance on the other no matter how tight it is.( unless it is smacked in with a hammer tight )
Mabee I'm missing something
Select to expand quoteBeaglebuddy said..
... If you use a really long extension such as over 30 cm the mast will end up being one size smaller and of course softer, not as stiff which could be an issue for a heavy weight person.
I'm pretty light and on wave sails I prefer to use a shorter softer mast and a lot of extension.
The rig is more compliant and less tuggy in gusts.
I sold my aluminium chinook 48 ext to a mate.
I used to fit it in my Point7 masts with no issues for over a year.
He used it in a North mast and the bung stayed stuck in the mast when he removed the extension.
Next time he went out, the mast snapped just where the bung was stuck.
Select to expand quoteNotWal said..Beaglebuddy said..
... If you use a really long extension such as over 30 cm the mast will end up being one size smaller and of course softer, not as stiff which could be an issue for a heavy weight person.
I'm pretty light and on wave sails I prefer to use a shorter softer mast and a lot of extension.
The rig is more compliant and less tuggy in gusts.
I also prefer it as a heavyweight
Everyone needs 2 extensions as 48cm up a mast stiffens the bottom a lot.
I'm still using a Tyronsea extension from 1986, it goes out to an awesome 56 cms.
Select to expand quoteMark _australia said..
I also prefer it as a heavyweight
Everyone needs 2 extensions as 48cm up a mast stiffens the bottom a lot.
this is a question I asked Barry Spanier - previously of MauiSails and now S2Maui Sails
he stated that ALL their sails were tested with standard length i.e. longer SDM (and RDM where applicable)
there seemed to be NO data to show/prove that the longer extension inside the bottom part of the mast causes it to stiffen up
the bottom of the mast seems to bend the least anyway
would be happy to hear if there is truth in what you say Mark ...
^ A snug fitting NP extension will stiffen the bottom of a mast.
Most SDM extensions are smaller in diameter and only touch the mast at the collar and at the bung in the top. They allow the mast to bend without affecting its stiffness.
RDM extensions are usually a snug fit because they need all the diameter they can get. They would stiffen the mast bottom too.
It might make some difference to the tension in the sail foot and fullness in the bottom but it doesn't seem to be an issue.
