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Appropriate Mast Bend?

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Ben
Master Marine Consultant

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USA
1234 Posts

Initially Posted - 05/05/2014 : 08:03:17

Hi All, I tuned my mast recently and am wondering if this is an appropriate amount of mast bend? the boat is a Beneteau 361 with a furler. Aft swept spreaders, aft lowers (no forward lowers), and a baby stay. They're all tensioned correctly (according to a variety of reference material I found), but I thought I'd post the picture here and see what the brain trust thought.

Thanks so much!

:)

Ben
Beneteau 361
Viking Kitty
Columbus, Ohio

Edited by - on

shnool
Former Capri-25 Tech Editor

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1032 Posts

Response Posted - 05/05/2014 : 08:47:14

it's pretty hard to tell from that picture, and of course harder still to get perspective, but looks like only a couple inches of prebend 2-3 maybe? Easy way to tell how much you have is to run the main halyard against the mast down to the gooseneck (hold it against the mast at the goose)... and eyeball how far it "diverts" and where (which should be nearly halfway between)... all this should be with adjustable backstay at "loose."

I'm only working with my Capri 25 knowledge here.. rigging is an art. Rig tuning requires a doctorate.

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Steve Milby
Past Commodore

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Response Posted - 05/05/2014 : 09:58:56

Ben, I don't know anything about the Beneteau 361 rig tuning in particular, but look at the line drawing of the Beneteau 361 at this website. http://sailboatdata.com/viewrecord.asp?class_id=6333 It shows the mast raked aft, but it doesn't show any discernible mast bend. To the best of my knowledge, whether prebend is needed will depend on how the mainsail is cut by the sailmaker. If the sailmaker intended it to have some prebend to give it the best shape, then you should do it. But personally, I wouldn't worry about it. If you have established the correct amount of rake and the correct amount of tension on all the stays, then your rig is probably going to be awfully close to optimal. To get it closer, I think you'd probably have to consult the sailmaker as to whether they designed the sail for prebend. In any event, if the sail has a few years of age on it, the original shape has probably changed enough so that it doesn't matter that much. If it's an older sail, I wouldn't bother trying to establish prebend. If you buy a new sail sometime, then you should discuss that with your sailmaker, and follow his recommendations for tuning the rig for the new sail.

From the photo, it looks like you have given the mast a modest amount of prebend. Without knowing more, I'd say you made at least a very good guesstimate at what it should be.

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Ben
Master Marine Consultant

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1234 Posts

Response Posted - 05/05/2014 : 10:07:07

Thanks John and Steve. I'll post this question on the Beneteau forum too to get some input there. I didn't intentionally go and put pre-bend in the mast, but that's just how it ended up after I tensioned everything. I could probably ease up on the baby stay, which should reduce the bend a little, right?

I do not have a backstay adjuster, so this is what it will always be, unless re-tensioned.

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shnool
Former Capri-25 Tech Editor

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Response Posted - 05/05/2014 : 11:06:40

I'm trying to wrap my head around "baby stay," are we talking about a stay-sail stay? Otherwise called an inner (fore stay shorter and lower and inside the actual fore stay)... Or are we talking aft lowers?

Verus

Edited by - shnool on 05/05/2014 11:08:53

Ben
Master Marine Consultant

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1234 Posts

Response Posted - 05/05/2014 : 12:08:26

John, when I say "baby stay" I'm referring to what you're referring to as the staysail stay. I think the Beneteau documentation refers to it as a baby stay.

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shnool
Former Capri-25 Tech Editor

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Response Posted - 05/05/2014 : 12:36:27

No problem... I am not questioning your terminology... just trying to understand... YES that'd certainly pull the mast forward from the middle then, induce pre-bend.

If it's any consolation... all else being equal, that looks pretty good... assuming (and we can't tell) there is proper rake, and tensions are all to spec... that amount of bend is pretty good.

NOW the big question is, HOW does it sail? Do you have slight weather helm in Force 3-4? start to get severe by force 5 (does cranking the backstay take it out?)... if so you are pretty darned close to perfect.

Watch for floppy mast... it'll dip in the middle to leeward on a beat if the lowers aren't right... the top will dip if the caps aren't right... Tuning is not by numbers, or rather should only be used to get you started... the rest should be done under sail.

While beating if you arent' getting much dip in the middle to upper mast, then check the lowers, look for sloppy... if you have them sloppy, tighten JUST to get the slop out... tack, check the other side... repeat with uppers/caps.

The way I do it is (backstay off):
check for upright first (even port/starboard)... then tension up evenly...
Check fore/aft, set maximum forestay...
Measure rake... adjust backstay as needed for tension, forestay only if the rake is too much.
Now lowers, snug them up only...
Measure bend (should be zero)...
Create pre-bend now, by adjusting forward lowers...
ease aft lowers to help balance against forward lowers... Check tensions...
If I did it all right, when I go from zero backstay to full on backstay, the tension moves from the forward lowers to the aft lowers without either getting sloppy...
Then I go sailing and adjust leeward to remove slop... tack adjust same amount...

From there I either tighten up for heavier air, or loosen up for lighter air... Your baby-stay will act like my forward lowers.

Again, if your numbers are all trued up, and you're in column, your rake is correct, then that bend is likely correct.

Edited by - on

Ben
Master Marine Consultant

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1234 Posts

Response Posted - 05/06/2014 : 04:27:18

Thanks again, John. My tuning process would be a bit different, since I don't have forward lowers or a backstay adjuster, but I do understand the "fine" tuning is done while sailing. Hopefully the first sail will be the weekend after Mother's Day. I just got the sails on this past Sunday (had been too windy to put sails on), and I'm not going up on Mother's Day because of family commitments. So that's the plan. :)

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awetmore
Master Marine Consultant

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1144 Posts

Response Posted - 05/10/2014 : 08:37:51

Baby stays are very common on boats. Many C&Cs have them and so do my friend's Yankee 30s. They usually don't go as far forward as a staysail stay and are there to prevent mast pumping and to induce some prebend, but aren't for hanking a sail onto. shnool, if you walk your docks a little bit you'll probably find a boat with one. Sometimes they are attached to a track so that the tension can be adjusted while sailing.

Mast prebend on my Pearson (which has a very stout mast) is about 1.5" and it goes to 2.5" when I put on a lot of backstay tension. The new main is being cut to match these dimensions so that I can flatten it a little bit for high winds, and have a big more depth in low winds.

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