Mast Prebend

[hornblower]

We have a 43ft modern cruising sloop with a fully battened mainsail. A friend of mine said that I had a problem with the mast because there is no prebend.

The mainsail sets well and I thought that the cut of the sail together with it being fully battened would mean that a straight mast was probably specified.

Are there any thoughts on this?

 

[Allan]

Not an answer to your question but I think I have read somewhere that fractional rigs have more prebend or it is more important than on a masthead rig. If that is the case, which is yours? If I have time I will have a look for a suitable book.
Allan

 

[savageseadog]

It depends on the cut of the sail. Race boat mains tend to be cut fuller and require more mast prebend. I guess your fully battened main is flat so a straight mast will do. If you find the sail is too full put some bend in with the cap shrouds assuming it's a fractional swept back rig.

 

[gardenshed]

I have a 41ft boat, masthead rig, fully battened mainsail.
You need to have some prebend. Never ever set up your mast to be dead straight as the danger of it inverting (mid section bending aft) is very high, usually quickly leading to collaspe.
If you have decent mast sliders (re-circulating ball bearing type) then the amount of bend will have almost no impact on how easy or hard it is to hoist or drop the main.
My rig has two sets of lower shrouds, keel stepped, double spreaders. To set up the rig my sequence is:
forestay / back stay and deck chocks set to give me a few degrees of aft rake
Centre the mast head using the cap shrouds (measure to the chain plates using the main halyard (dyneema)
Evenly tension the Cap shrouds, continuously checking to see that the mast head remains centred
Next tension the intermediates checking that the mast remains straight by sighting up the luff groove
finally, tension the forward lowers to
a) get pull the mid section forward (approx 8-10 cm)
b) keep the mast straight athwartships (looking up the luff groove)
Get similar tension as caps and intermediates
finally tension the aft lowers to match the tensio of the forward lowers.
finally, lock off all bottle screws, spray with WD40 and then protect with tape /plastic tube.

I like to have lots of tension. As my rigger friends have said, you can never over tension shrouds at the dock with hand tools.

What do I want:
Stiff stable mast that evenly bends fore and aft and minimises any athwartship bend.
Ability to put in more pre bend by tensioning the backstay
no pumping (mid section moving back and forwards) when going up wind in a chop
Selden's website has some good tips if you don't want to follow my advice.

 

[nickf]

Selden do a useful booklet "Hints and Advice". Try seldenmast.com and type in "Hints" in search window should bring up link to the document.

 

[30boat]

You should be ok without prebend if you have a decent babystay.

 

[matelot]

Just been going through the setting up exercise myself, and investigating whats involved. The sequence seem to be to get the required degree of aft rake first of all using spacers / adjusters at the bottom of the forestay. The aft rake governs the weather helm so you adjust until you have enough weather helm under balanced sail plans to ensure you dont get lee helm. Usually somewhere in the 1 to 5 degree range - measure aft rake at the gooseneck using a halyard and do a bit of trig.

Next step is to set the curve in the mast which should be about half the mast width for non roller furling masts and evenly spread over the whole mast height. This is to ensure that the mast doesnt invert under sail pressure. Might be less of a worry if you are cutter rigged but in my case the forward lowers go maybe one third up the mast at most so they alone arent enough.

Then its an issue of tension. Basic rule seems to be 20% of breaking load which you can get by measuring and marking off a 2 m length of each stay when untensioned. Then for normal 19 wire, 20% corresponds to 4mm stretch in that 2 m length. Thereafter adjust so that when under full sail hard on the wind in 10 to 15 kn the lee shrouds are only just losing their tension.

Try this site http://pdf.nauticexpo.com/pdf/selden/hints-and-advice/21696-6227.html

 

[tabernacleman]

Hi, in my humble opinion some prebend ensures that in a blow the pressure in the mainsail will take out the pre-bend thereby straightening the mast, a straight mast is 'longer' than a bent mast therefor the forstay is tightened preventing the genoa from sagging just when you want a tight luff, so the answer is yes, pre bend is necessary, and most of previous posts appear to have good advice, final tuning re cap shroud and lower shrouds tension is normally done under sail, force three to four sailing on the wind, how slack is the leeward shroud? By the way, backswept spreaders will obviously invoke pre bend when tensioned on a fractional rig, the adjustable backstay then alllows you to de-power the main by bending the top section of the mast, de-powering the head of the main and this is the bit in most wind, on a masthead rig the top of the mast doesn't bend but the backstay helps with the tension in the forstay, easy to rig a back stay tensioner and can make a big difference. Again with a masthead rig prebend is invoked with the forward baby stays, when the mast is bent to your satisfaction tension up the aft babystays. Try to get a mate in his boat to sail alongside and make your adjustments while he keeps the same settings, known as 'brushing' certainly indicates if your 'tweeking' has helped or hindered.

 

[savageseadog]

Re mast inversion, swept back rigs

With the main sheeted and kicker on bend is induced into the mast, the mast is very unlikely to invert even if there is no prebend. Some prebend is always desirable and pretty well inevitable unless your rigging is set up very badly.

 

[lw395]

Mast inversion is usually only an issue when off the wind, as the kicker/gooseneck ceases to help. It can be a real issue when the spinnaker pole puts a lot of rearward force on the mast. Another possible cause of failure though inversion is letting the boom out so far it levers against the shrouds.
There seem to be some subtle variations on rigs and how they should be set up, I found the Selden Website and Ivar Dedekam's (sp?) book helpful. But basically, you either want controlled backward bend or keep it straight. You don't want uncontrolled 'pumping' in big seas, that's when you need to get the babystay on. The acceleration aloft when the boat pitches can be considerable, particularly if the boat slams into a wave or two.

 

[hornblower]

Thanks all - I always intended to contact the designer's office and the builders' agent (Premier Yachts) and see what their take on it is but I wanted to see if there were opinions on here so that I could fully understand what they were saying to me.

 

[[32511]]

Where is this Mast, and what diocese is it's prebend in?
-sorry!

 

[30boat]

On my Fulmar prebend actually increases under sail,even when going to windward.I fitted checkstays to deal with it and it works fine.