Topic : Prebend - or where has it gone

"set the spreaders for deflection fore and aft, and side to side, measuring from the shroud. About 40mm outwards, and 15mm forward deflection, which would be for powering up a fairly soft mast section." As per GGGGGGG

my mast bends 10mm backwards?? What measurements are people using for not so soft mast?? Forwards backwards mine stays the same unless 3 inch aft deflection.

Posted: 16/09/2005 18:08:35
By: puzzled
What mast section have you got?  Deck or hog stepped mast?

Posted: 16/09/2005 20:32:46
By: Dave Lee
deck stepped proctor carbon

Posted: 17/09/2005 00:15:15
By: puzzled
My proctor carbon (ex Tales) and my superspars ally (ex WAPB) are both approx
length mast to shroud 36 cm
width shroud to shroud 70 cm
depth from shroud to shroud projection perpendicular to mast track 15 cm

both masts bend the right way and worked fine, although the ally on the WAPB had just a touch too much prebend, but not enough if moved to the next hole.

Posted: 17/09/2005 09:37:16
By: Alan F
I looked up prebend on Google and found it is in the "Dictionary of difficult words"!

It's some ecclesiastical term so I guess you just pray you get it right.

Going back to university days, its also the name of a bridge in Durham.

Posted: 17/09/2005 16:28:17
By: Researcher
Proctor carbon masts can be a bit tricky to set up - allegedly the bend characteristics of the Mark 1 version (internal sail track) varied considerably from mast to mast, so copying the set up of another boat may not work that well.  However, as a starting point, the spreaders on the Mk1 Proctor carbon I had with 3546 were set at about 695mm between the tips, and 135mm from the sail track to a line between the spreader tips.

The Mark 2 Proctor carbon (external plastic sail track)was very, very stiff. Don't know about settings for this section, but GGGGGGG may be able to help?

Best bet is to get along to a MROA training event and get one of the top guns to look at your rig, or speak to the sailmaker who cut your mainsail.

Posted: 17/09/2005 18:42:25
By: Dave Lee
thanks for your replies.

Spoke to the sailmaker
and have managed to get a starting point prebend.

Next question -

When and when not, do you use prebend?? This has always confused me. Does prebend not flatten the sail somewhat and reduce power?

Posted: 18/09/2005 11:07:30
By: puzzled
CORD LENGTH. Ask yer sailmaker and set within that.

Posted: 26/09/2005 07:58:28
By: Admiral
In a nutshell, prebend for light weather, straighten up for full power (when you can get crew to windward) prebend when overpowered.


Posted: 26/09/2005 08:08:13
By: Chairman GGGGGG
But have we not kind of moved away from prebend with the new Chipstow rigs?

My mast with the standard Winder spreader settings is very straight (Questionably inverted) with 6" of rake. I have struggled to get the boat to go well in light winds (Could be me) but have no such trouble when it's windy and large amounts of rake are used.

Since pretty much everyone using a Winder boat and a chipstow mast are using the same spreader settings (128mm from shrouds to luff track) I am self concious about trying anything too differant, but a feeling in my water leads me to suspect more bend is required in the light stuff.

So how do the top guys acheive this? What is so good about this 128mm spreader setting?

Posted: 26/09/2005 18:10:49
By: Chris M
Prebend still required when a flat sail is wanted. Trade off is straightening when the power is needed.  Spreader settings are always a compromise, and 128 seems to be about optimum.


Posted: 27/09/2005 08:39:08
By: Chairman GGGGGG
I understand that, but what i'm struggling to understand is that with the mast at around 6" of rake the spreaders do not want to bend the mast.

We never had this problem with the superpsar becasue it needed more prebend as it's a stiffer spar.

So do you use more rake in lighter winds to acheive your prebend?

Posted: 27/09/2005 08:44:01
By: Chris
I Know this is granny and eggs Chris but is your mast foot in the same place as everyone elses?

Posted: 27/09/2005 11:21:33
By: Jon
To clarify, I know the step will be in the same place but i mean the foot within it........

Posted: 27/09/2005 11:22:25
By: Jon
Yup the same as other winders.

Posted: 27/09/2005 12:26:59
By: Chris M
Sailmaker said, set prebend to suit sail there is therefore no set aft deflection of spreaders - set to get the prebend even if you go as far aft as 160mm for a stiff mast.

Then just use the lowers to adjust the prebend as and when to get the required power.

If that doesn't work, phone him back!

Posted: 28/09/2005 14:06:28
By: puzzled
It is not one design!

You can come up with your own solution - heaven forbid. If you want to do the same as everyone else then buy a 400 - or maybe two 400s!

Bending the mast removes fullness from a main sail. How much fullness does your main sail have? How much do you want? At the same time the amount that the mast bends will then have an impact on the slot that the air flows between the main sail and jib. What do you want this to look like?

Posted: 28/09/2005 14:37:56
By: bender
Well, yes i thought this myself but pretty much everyone is using this 128mm measurement and not just those using p&b sails.

If it's literally just the compromise position i feel more confidant about experimenting.

Posted: 28/09/2005 15:30:41
By: Chris M


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