Mast Prebend

Started by Thomas Leith, June 22, 2009, 01:59:32 AM

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Thomas Leith

I see that the minimum reccommended prebend on the mast is now 4-1/2" or 114 mm. My understanding is that the Hyde sails on #80 are cut for 4" of prebend, so this means we're a little flattened at the "minimum" (which is fine because we weigh in at around 520#). 

Will the sailmakers now start cutting for a 114 mm MINIMUM prebend? And what does this really mean? I have heard tell of sails cut for about 2" of prebend. It seems to me this would be quite flat if we crank in 4-1/2" or 5" prebend in a breeze-on condition. You might want a sail like that in a windy locale. But in a light air locale the skipper would either have no power, or he'd have much less prebend that the "minimum", maybe even pulling some bend out of the mast with the lowers. And what is a reasonable maximum prebend?

Now that I think about it,  I presume this "minimum" is not particularly important in light airs -- say below 10 knots. If I wanted to tune the rig with 4" or even 3" of prebend in order to power up,  this would be fine. Yes? No? Have I missed something?


Dan Tucker

I assume you're reading the 4 1/2" of prebend from the Brian Bennnett tuning guide on the class site? That's a good starting point for the Hydes in 10-12 knots. Other sailmakers may cut a main with less luff curve, so they start out with less pre-bend for the same conditions. More prebend, does equal a flatter, depowered mainsail. More turns on the uppers gives more mast bend, which depowers the main.

The key thing to remember is that guides are just guides. If you're overpowered, put a few more turns on the rig. If you're not fully hiked, take a few turns off.

Two key things to remember when sailing in breeze with the kite up - keep plenty of vang tension on and don't ease the main out past the spin turning block. You need the leech tension pulling aft to act like a backstay, to keep the mast in column.
Race it like you stole it.

Thomas Leith

Quote from: Dan-T on June 22, 2009, 10:20:55 PM

Other sailmakers may cut a main with less luff curve, so they start out with less pre-bend for the same conditions.

I understand the basic mechanics of mast bend and chord depth. Trouble is we're allowed only one sail per year, and in practice we'll probably never have two in fabulous condition. So how do I choose?

My understanding of the Bennett Tuning Guide is that the 4-1/2" of prebend is MEANT TO PRESERVE THE STICK, and that this is sort of independent of the main sail. What I mean is "your main sail might not work very well for you unless this constraint is well accounted-for by the sailmaker".

If this "minimum prebend" is only for heavy air (say 16+) then there's no worries for most people -- it is what we all expect anyway. But especially if it is meant even for medium air (say 12 - 15) then some of the really flat sails I've heard of can't work well at all, and might not work well even in heavy air. (i.e. 2" luff curve sail on a 4-1/2" prebent mast).

Also I had heard comments last summer that a 4" luff curve might be a tad too full, maybe a 3" luff curve would be better for the "average team".  But if the "minimum prebend" is 4-1/2" what would this mean?

I asked about the maximum reasonable bend on the mast because that might choose the sails we want to use for our team. If 6-1/2" is unreasonable, maybe we want to start out with a flatter sail.

I do like Bennet's remarks about "what to measure" and his inclination to talk about turns on the turnbuckles versus Loos gage readings. And I appreciate your remarks about guides being guides -- we will of course adjust up & down, but I must know where to start and what the limits are.


Dan Tucker

I was in Miami when masts came down in the burst. My personal opinion is that vang tension and not over easing the main will do more to prevent those sort  inversion failures than the amount of prebend.  There was a LOT of discussion about it that weekend. In Charleston. We had flatter luff curve sailis sailing in big breeze, with no ill effect. With more prebend the mast has to work harder to invert, it's the builder's recommendation to reduce the liklihood, but it's not an absolute guarantee against inversion.
Race it like you stole it.

Drew Harper

We're setting 5-6 inches of prebend, but we're sailing in 20-25, puffing 30. At that there are times when the rig is completely straight, particularly with acceleration loads stuffing into 4 foot chop. We keep hard cunningham tension, a minimum of 3/4 vang load and never ease the main past the mainsheet mark, as per the guide. The problem is, of course, there are times when we crash HARD as we see a monster puff with a wind shift and there's just no ease left and down comes to late without a solid read coming from the crew.

My personal observation is that prebend is pretty critical here. The positive loading on pre-bend enables the mast to better stay in column.

We are maintaining a 2" block forward of the mast partner. We only see that block load with 6 inches of prebend. I would measure your luff curve. 2" curve seems pretty skimpy unless the panels are broadseamed...probably not though.

We carry make rake and are at the rearmost pin on the mastbase to counter the leehelm issue in the chop. This has helped greatly.

Hydes, I believe, are 7-8" of luff curve. The Norths have been cutting to a 4" curve but I do believe they've seen the light and will be amending that.

#189 UK Built Mark IV Viper "DILLIGAF"

Thomas Leith

So it sounds like 6" is not too much for the mast or the hardware. Do I hear 7??

The 2" sail is a west-coast rumor I've heard. We have the standard Hyde sail which I'm TOLD has a 4" luff curve. I suppose I should measure it, but I can't imagine it is anything like 8" or the boat would be too powered up all the time -- the mast would be very straight compared to luff. It would be slow in light air too, with all that depth and drag.

We've had that "ooops! we didn't bear off early enough" feeling ourselves. We can generally see a puff coming but we're not good yet at figuring out whether it is also a shift. Brian tells us "by the time you feel it, it is too late." This seems right, based on recent experience, but we don't get to sail enough in fresh winds to learn quickly.

Jay Harrell

Quote from: Fat Chick on June 23, 2009, 03:11:37 PM
So it sounds like 6" is not too much for the mast or the hardware. Do I hear 7??

The carbon mast can bend A LOT with no ill effect, you just don't want the load going the wrong way like it does in an inversion.

Quote from: Fat Chick on June 23, 2009, 03:11:37 PM
The 2" sail is a west-coast rumor I've heard. We have the standard Hyde sail which I'm TOLD has a 4" luff curve.

I suspect the 2 inch and 4 inch numbers you hear are not sail luff curves, but rather are the mast pre-bend numbers recommended by the sail maker.  In other words, you start with 2 or 4 or whatever, and bend goes up as the rig loads up in the wind.  I know the standard Hyde has more than 4 inches of luff curve built in to the sail, because that's the minimum recommended pre-bend for it.


Justin Kelleher

Quick tip,

The amount of mast bend your main will tolerate is a function of luff round and broadseam.

If your Hyde is built with 4 inches (hate that word) then your main will probably cope with 7-8 without too much trouble.

Thomas Leith

Oh, I see. I have confused "has a X" luff curve" with "wants/likes a X" mast prebend". I have to learn about what "broadseaming" means. In any case, I will make some measurements this weekend. The first think I want to know is how much mast prebend we have cranked-in right now. I doubt my ability to eyeball it within 1/2". Or maybe even within 1".

Thanks for the comments,

    t

Thomas Leith

Here's an example of what I'm concerned about:

North says:

http://www.northsailsod.com/class/viper/viper.html

"The OEM supplied [Hyde] sails require an extreme amount of prebend to get near their designed shape our main requires only 1?-3? of prebend."

But if something like 4-1/2" is the minimum for rig safety, how can this sail work properly?

Something is not adding up, but I don't know what (yet).

     t

David Guggenheim

I own Black Mamba and we sail with Hydes and North sails . The Hyde's we set up with 4- 5 inches of prebend and pull the mast ford  at the deck. The Norths which i used at Charleston we set up with 1-1 1/2 pre bend in 0-10 and chock the mast, in 10-15 less chock and about 2-2 1/2 pre bend . The caps shrouds  start at 18 on the loos and build to 25-30 as the breeze builds to 18-20 knots. I broke my mast in Miami because i was set up for 10-12 knots and put my spinnaker up in 28-30 knots, also had my lowers on way to tight so the mast couldn't bend