many moons ago, I asked this question on the MauiSails forum
Barry was interested, but both he and the forum have disappeared ...
from MauiSails that is
.
if 10 cm of extension is suggested, is it best to use the minimum mast extension - like 12 cm ??
if 20 cm is called for, use a 28 cm extension ??
the plastic sleeve seems to be the piece holding extension to the mast
what's the point of having more inside - no sexual connotation intended 
True Joe ...you don't need any longer .
i cut one of my 40's down ,now I have all the correct size mast for my sails ..just got in the way .hack saw 
it is the strongest part of your mast ,
Every Mast has it's perfect extension size.
Most masts assume they prefer the longest extension set point but the long term performance of the mast can suffer.
I wouldn't use an SDM extension on an RDM mast and Vice Versa as you're not going to get the desired performance from either.
There are also Mast and Cup arrangements and also Extensions with levers on the outside. Both until recent times haven't been well accepted in mainstream society but are becoming more common since the introduction of Kitesurfing
I don't think it makes much difference, the last bottom part of the mast does not really bend. I have a pretty long one that sometimes have used at minimum extension and did not notice any problems or difference. Of course you need to make sure to have at least 12-15 cm of extension inside the mast, usually that section has no holes.
Slightly off the subject, but only slightly.
I've never understood why the super long extensions are even on the market, they make no sense if used at or near the max extension, everything about it is wrong. Your mast is too short, you don't get the desired bend characteristics, and if it's an aluminum extension, it can easily bend. A waste of money for the cheap desire of convenience, which just hurts the rig's performance. 
Select to expand quoteMastbender said..
Slightly off the subject, but only slightly.
I've never understood why the super long extensions are even on the market, they make no sense if used at or near the max extension, everything about it is wrong. Your mast is too short, you don't get the desired bend characteristics, and if it's an aluminum extension, it can easily bend. A waste of money for the cheap desire of convenience, which just hurts the rig's performance.
Sometimes manufacturers (North, Avanti that I know of) recommend two mast sizes for their sails. North had their Type-S three sizes with one 460 cm mast. Avanti 8.4 Machine 3-4 can be rigged with 460 cm for overpowered conditions with softer feel and 490 cm for heavier sailors and/or light breezes. It works well as advertised, and I think it is a benefit- not a detriment.
agree with SL55
more extension just makes it softer
on the other hand ....
does 30+ cm extension inside the mast change the mast bend ?? really ??
Select to expand quotejoe windsurf said..
agree with SL55
more extension just makes it softer
on the other hand ....
does 30+ cm extension inside the mast change the mast bend ?? really ??
You know how a mast bend curve is tested right..? Without typing for 10'. A weight is hung from the center, the difference in deflection is measured at 1/4 1/2 and 3/4, the difference is a measure in a percentage, from 1/2 to the others. This forms the basis for mast bend curve soft or hard, soft or hard on both bottom and top.
if you measure the same mast, with a extension installed will it change the curve? It will most assuredly change, how much?
may also be a factor issue
if the mast is a 520 or 550 versus a 400
30 cm may be a bigger factor in bottom mast bend on the shorter mast ??
Select to expand quotejoe windsurf said..
agree with SL55
more extension just makes it softer
on the other hand ....
does 30+ cm extension inside the mast change the mast bend ?? really ??
It will make the top softer only, while making the bottom stiffer, which answers your question, yes, really.
Sound like you're considering buying only tall extensions, or have only tall extensions.
And as for sl55, the only advantage is to those sail makers, "buy our sails, you won't need so many masts".
I know,,,,,, I'm sounding like a purists, but the questions are purist in nature.
I was told that it wasn't good to use a really long extension in the mast as it put strain at the top where the ext and mast meet...This came up because I tried to use a long extension and it wouldn't go all the way in as the diameter of the extension was too wide for the mast at the full height and it wedged in.
I now have a short extension for those that need only 10cm or less and the longer ones for those that need 30cm etc.. I use severne gear..older
I wouldn't think a long inserted extension would make much or any noticeable difference to mast bend.
Most SDM bases have a collar on the bottom and a end cap that is larger than the ext shaft ,so the points of contact are only at the base of the mast and the end of the extension letting the mast flex between both points .
I use a carbon SDM extension with 32cm extension on a race sail with heaps of downhaul and the thing flexes a lot. Put a ruler on it and it seemed like a 2mm gap ( scary ) !
Select to expand quoteImax1 said..
I wouldn't think a long inserted extension would make much or any noticeable difference to mast bend.
Most SDM bases have a collar on the bottom and a end cap that is larger than the ext shaft ,so the points of contact are only at the base of the mast and the end of the extension letting the mast flex between both points .
I use a carbon SDM extension with 32cm extension on a race sail with heaps of downhaul and the thing flexes a lot. Put a ruler on it and it seemed like a 2mm gap ( scary ) !
Exactly. In a well designed extension system the mast will still bend somewhat.
But in any case, sails designed for a shorter or longer extension will normally have the slight change in the rigged bend curve accounted for on the sail shape/design.
In my experience, it will make a difference in theory, but is so little that in practice it is very hard to tell.
