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-   -   Long mast extension (http://www.star-board-windsurfing.com/forum/showthread.php?t=3589)

Unregistered 25th March 2008 04:09 PM

Long mast extension
 
Hi,

I'd like to hear your comments regarding two concerns I have when using a rather long mast extension (40-45 cm):
- situation A: Major part of the extension inside the mast. Let's assume that te mast has a constant curve characteristic. Will the extension inside the bottom of the mast spoil the performance? We' re talking about 30-40 cm alu piece that probably makes the lower part of the mast stiffer than should be.
- situation B: Using the extension at ful length, only 5-10 cm of it is inside the mast. Due to rather small conntact between the mast and extension, I think there's quite a pressure acting at the mast. Is there any risk to crack the mast?
Thanks in advance for your inputs,

G

Unregistered 25th March 2008 04:25 PM

I have often contemplated your questions and at one point asked the same questions on the SB forums many years ago. My opinions ...

a) Yes. It makes the mast stiffer but it does not concern me too much. I think the effect is relatively small. If you have the cash just buy a couple of extensions, otherwise don't worry. It really won't cause you to lose the PWA championship.

b) No. All manufacturers basically have a similar minimum length with their extensions into the mast. They wouldn't put holes so high up on the extension otherwise. I am sure there are true engineers working on the design & spec of carbon masts - perhaps not with the brand names like Neil Pryde, North, etc but at least at the mast manufacturers like Italica, Maclean etc. Also the connection between the top & bottom section of mast has a similar length of insertion compared to the minimum length of mast extension into the mast.

steveC 26th March 2008 12:47 AM

G,

Regarding your situation B, you would be running a very high risk of mast failure unless you have a least 15cms (6") of the extension inserted in the mast. Any less than that doesn't provide a sufficient bearing surface and unduly point loads the mast. However, I think the minimum could be argued a bit contrasting RDMs and SDMs, since RDMs have a thicker wall structure. Still though, poster 2 offers a very good rule of thumb to follow by highlighting the length of the ferrule that joins the top and bottom sections of the mast.

Roger 26th March 2008 08:38 AM

Hi Unreg,
I'd have to disagree with the 2nd poster here.
I've seen some pretty bad results when too much extension was used up inside the mast.
This can definitely "stiffen" the bottom of the mast and can make the lower camber or even RAF battens very hard to rotate. (Your situation A).
The reason they make several length extensions is so you can use an extension that supports the mast well, but does not stiffen the bend characteristics apprecialbly.
In your situation B, even a 45 cm (long/tall extension has an additional 15-20 cm of tube at the top to support the base of your mast.
So, you can go out to the full adjustment (45 cm) and feel fairly confident that you are not unduly stressing your mast.
Certain size rigs (7.5-8.0 mostly) use 460 (IMSC 24-26) masts that are extended out to near 500 cm.
The bend characteristics of a 490 (IMCS 28-30) are just too stiff for these sails to work well on a longer mast, so the luff length specification is 460 cm + 35-40 cm of extension.
You could use a 490 mast here, but the sail would not twist of as designed and it might not rig well at all.
Also, if you are a big guy, then maybe (only maybe) you'd like the feel of a longer stiffer mast in this type of sail, but most sailors < 100 Kg. seem to prefer the "best" mast in these cases.
So, your situation A is almost worse than situation B.
I have seen several masts "point load" at the top of a long extension inserted 35 cm or more up into the mast.
The "point load" simply "snaps" the mast right at the top of the extension because the mast cannot bend with the extension inside it down lower and consequently breaks off at the first point were it can bend.
Hope this helps,

_RUS-12 30th March 2008 12:10 AM

I have some experiments of this kind last year.
I suggest you use not recommended mast if you have to use very long extension. ;)
In my MauiSails MS-2 9.5 I had to make custom extension. Otherwise it did not work properly.

sit.B If you use very small part of extension in mast(and too long extension), a backlash of extension in mast cause critically wrong shape of the sail.

wsurfn 30th March 2008 01:45 AM

So what is the ideal amount of extension inside a mast ( i.e. how much is too much)? Is it more or less critical in RDM vs. SDM?

I have Chinook Carbon Tall extensions (both RDM/SDM) I use for my entire quiver.
(Severne Blades 5.3 -6.2 with PWX RDM 430, a 7.5 Gator with PWX SDM 460, and I also have a MS2 8.5 and Nitro4 9.3 on a FS7000 SDM 490).

Unregistered 30th March 2008 09:01 AM

QUOTE - RUS_12 ---- "sit.B If you use very small part of extension in mast(and too long extension), a backlash of extension in mast cause critically wrong shape of the sail."

Huh !?!? That is crap. Why guess? .... Just follow the manufacturers recommendations, then it is easy.

If you need flexibility & you are on a budget, then you could use North gear to make things cheaper with their Carbon Xtenders. The recommended mast in numerous sails, is actually using the Carbon Xtender often with 30+cm of extension. I use mine as per their ideal mast recommendations and they rig perfectly ... and I also am complying with their specifications in case there is any warranty issues.

Roger 30th March 2008 09:37 AM

Hi wsurfn,
Think the extension designers have pretty much answered this question.
Take any short, medium or long extension, slip the adjustment collar up to the highest
setting, and measure the amount of extension from the seat on the collar to the top of the extension.
My guess would be about 5.0" (12.7 cm) based on a NP 26 cm max (medium size) extension I just found under my chair.
As far as how much is too much, measure the front of the lowest batten in your sail from the foot, then see how far up inside the mast your extension goes.
If it extends beyond the height of the lowest batten (the foot batten) then you need a little shorter extension.
This is not critical on RDM masts and probably not on small wave and freeride sails, but the higher end sails, with lots of shape in the foot of the sail, can have some real problems if the mast base extends beyond the front of the lowest batten and "stiffens"
the mast in an area where the designer intends it to start bending to allow the foot batten to rotate freely.
Yes, you can often "fix" the poorly rotating foot batten by backing off the tension, but then you get wrinkles and less than the design amount of draft in the draftiest and most power making area of your sail.
Hope this helps,

Unregistered 30th March 2008 04:34 PM

QUOTE ... "As far as how much is too much, measure the front of the lowest batten in your sail from the foot, then see how far up inside the mast your extension goes.
If it extends beyond the height of the lowest batten (the foot batten) then you need a little shorter extension."

Yes, this is a rough guide but if in doubt dont waste your time trying to be a windsurfing engineer .. just read the designer's recommended specs.

Plenty of my North sails have the Xtender connection much higher than the foot batten, and that is as per the OEM spec!

So make it easy .. just read the specs.

mike 30th March 2008 09:49 PM

The north mast extenders are a different issue. As I understand it, the problem is a long extention INSIDE the mast. The extention collar is the main contact point with the mast and is kept tight with the downhaul. If there is a long extention deep in the mast, when downhaul is applied, the mast flexes and the end of the extension makes contact with the mast, stopping the flex in this area (possible damage??). This would to have most impact on sails that require a lot of flex in the mast in the initial rigging (my retros flex alot, my north ices don't).


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