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Inducing Mast Bend

5K views 24 replies 11 participants last post by  RichH 
#1 ·
Hello,

I have a Islander 37ms, a keel stepped masthead sloop with single spreaders that run straight out from the mast, fore and aft lower stays, roller fuller on the fore stay, and a single back stay. The aluminum mast extends 40' above the deck.

It is very hard to get any mast pre-bend on this rig without cranking down the forward lowers and being a little light on the aft lowers, and then it ain't much bend.

I am going to paint the spreaders and was thinking of having a shop put a 3 or 4 degree angle on the spreader brackets. The thought here is that I would then add some pre-bend to mast by the always snug upper stays pushing the spreaders at the forward 3 to 4 degree angle instead of straight into the mast. Then I could just set the lowers to hold it where I want it to stay.

I am looking for opinions. Do you think this work without causing other issues, or problems with the spreaders/mounts ?

Thank you in advance for your assistance
 
#2 ·
Hello,

I have a Islander 37ms, a keel stepped masthead sloop with single spreaders that run straight out from the mast, fore and aft lower stays, roller fuller on the fore stay, and a single back stay. The aluminum mast extends 40' above the deck.

It is very hard to get any mast pre-bend on this rig without cranking down the forward lowers and being a little light on the aft lowers, and then it ain't much bend.

I am going to paint the spreaders and was thinking of having a shop put a 3 or 4 degree angle on the spreader brackets. The thought here is that I would then add some pre-bend to mast by the always snug upper stays pushing the spreaders at the forward 3 to 4 degree angle instead of straight into the mast. Then I could just set the lowers to hold it where I want it to stay.

I am looking for opinions. Do you think this work without causing other issues, or problems with the spreaders/mounts ?

Thank you in advance for your assistance
I have to ask why you feel you need to redesign your rig.

Maybe you are as qualified as the guys who designed your boat. I don't know. But I generally figure that the Naval Architects who designed the boat are likely to have done a better job than I can do.

Having said that, I would probably start with an adjustable backstay.
 
#3 · (Edited)
The rig does not have much adjustment to it and is set a little too far back on the boat so it develops quite a bit of weather helm as the wind picks up.

I normally sail in 12-20kts in the afternoons, and have been looking for a way to get rid of the weather helm other then reefing the main at 14kts.

I moved a bunch of weight from forward to just aft of mid ship which helped a lot.
I got a new main sail, which helped a little, but with both the old and new main I can not move the center of effort of the main far enough forward to get rid of that much weather helm and heel when pointing.

Thank you for your suggestion but I have a masthead, not a fractional rig, tightening the back stay does not induce mast bend. It just pulls on the fore stay.

I was asking for opinions here because I have seen some very knowledgeable folks on this board.
 
#9 · (Edited)
The rig does not have much adjustment to it and is set a little too far back on the boat so it develops quite a bit of weather helm as the wind picks up.
Then it isn't pre-bend you want. What you want is more less rake. Different thing altogether and used to accomplish a completely different end.

Please read Giulietta's most excellent rig tuning guide at http://www.sailnet.com/forums/gear-maintenance/42542-adjusting-your-rig.html before going any further. (Sorry, you'll have to copy and paste the URL. Sailnet's brain-dead text-munger would destroy it if I tried to do a handy link.) I'd also highly recommend Ivar Dedekam's Sail and Rig Tuning. You might wish to consider obtaining a Loos gauge, tho one can get by w/o it.

Btw: Normally you want aft lowers on a rig like yours to be little more than hand tight.

Jim
 
#4 ·
For such a single spreader rig, the max. of 'pre-bend' should only be about 3/4". I have a monsterously stiff (Seldén) mast and such requires to maintain a 15% cap shroud tension and a 3/4" pre-bend, I need to apply 20% tension to the lower forwards and loosen to about 10% on the lower afts. I also have intermediates and runners, so Im not too concerned about the slack aft lowers.

Changing the spreader angle by sweeping them aft will also require that you also relocate the chainplates ... I wouldnt do that.
 
#5 ·
Great response, thanks.

The "move the chain plates" was the over sight on my part that I was looking for. Glad you saved me from that one.

I have been loading the forward lowers to 15-16% and 8-10% on the aft lowers and it moves just off center, but the mast is pretty straight.
3/4 of an inch should do it. I just need to pull the middle of the sail forward a little.

I'll try loading the forward lowers more, and see what happens.

Thanks again
 
#6 ·
After thinking about this for a minute I have a further question.

Would not the masthead, spreader tips, and chain plates to the uppers still align as they do now because the slight rearward rake in the spreaders would push the mast to move forward to create the alignment?

I can see that 3 to 4 degrees is the wrong way to think about it.

If the spreader tips were 3/4 of an inch behind the mast at the spreader junction with the direct line between the masthead and chain plate would that not make the mast pre bend to hold the line?

This is also a stiff Selden mast also, so we are talking apples and apples
 
#8 ·
Two thoughts:

1. Call Selden. I have a Selden mast and was on the phone with them a couple of days ago with questions (including asking about inducing pre-bend in my masthead rig (although I have a baby stay)) and they were quite helpful.

2. How much rake do you have in the mast? If there is a lot of rake, that might account for the weather helm.
 
#11 · (Edited)
A sailmaker INCLUDES proper prebend dimensions in the sail when cutting the luff section. So, If you dont pre-bend the mast the main will be slightly 'powered-up' with a straight mast; powered-up means SLOW (in relatively flat water).

If you have apparent 'weather helm', then the BEST way to correct (only if you have correct BACKSTAY tension) is halyard/cunningham tension: For 15kt windstrength and using a bolt-roped sail you need to stretch-out the luff (to remove boltrope 'pre-load' that the sailmaker put in) by ~1" for every 11ft. of luff length ..... THEN and only then, if you have 'weather helm' you rake the mast.
If the boltrope hasnt shrunken severely from age (needing replacment/adjustment): go on a beat in 12-15 and with all sails close to perfect trim/shape, notice the helm pressure. If Weather helm, then increase halyard/cunningham pressure/tension until you get a 'dead fish' helm .... neither weather nor lee helm, then release the halyard tension, etc. by 1 to 1-1/2".... and then the boat will 'very 's-l-o-w-l-y' 'head up' when the helm is released. A sail with proper halyard tension will have the tack angle (angle that the TOP of the boom makes with the mast) at ~89° . If insufficient halyard tension, the aft end of the boom will be LOWER than the gooseneck ... and the sail will tend to be powered-up, draft aft, and the battens will be somewhat 'hooked-up' to weather - which all increase weather helm.

If the FORESTAY isnt properly tensioned by the backstay ... then the JIB LUFF will sag too far to leeward and the boat will develop a leeward SKID ... and that is erroneously felt in the helm as 'weather helm'. The dreaded 'leeward skid' is noted by the wake coming off the stern at an angle greater than ~5°.

For Seldén mast tuning do websearch for: "Hints and advice
on rigging and tuning of your Seldén mast" (eg.: www.porttownsendrigging.com/resources/reading/), etc.

;-)
 
#12 ·
Hello,

I am going to paint the spreaders and was thinking of having a shop put a 3 or 4 degree angle ....
I am looking for opinions. ..
My opinion is that you are way out in left field head with this idea and should drop it.

As mentioned by SEMIJ you should deal with rake not pre-bend to affect your helm issue.

Further flattening the main can reduce heeling and provide some benefit, but here again trying to bend a non-bendy mast is barking up an empty tree...
 
#13 ·
Thanks for all the great opinions. RichH thank you for the time you took for the detailed answers

I will drop the "sweep the spreaders" idea as it seems that the opinions are all negative.

I agree that a baby stay would be the best way to do what I am seeking. A previous owner had some discussions with Islander about adding a baby stay in 1975 but they recommended not to add one for various reasons.

I'll go through the other suggestions one by one and see if I can resolve this another way.
 
#22 ·
... I agree that a baby stay would be the best way to do what I am seeking. A previous owner had some discussions with Islander about adding a baby stay in 1975 but they recommended not to add one for various reasons....
Not really, the babystay is a device to control mast pumping, a condition which is again more likely with a bendy rig. Many, most boats have telephone pole masts that are not designed to bend...if you have one like that, give up on the strategies for bending it, or get a new rig. Not that even that would solve your issue...
 
#14 ·
You say you have a 'new' main - presumably it would have been built taking into account the pre bend (or lack of). Agree that your issue (ie excessive weather helm) is a matter of rake adj, but with respect to SemiJim earlier, you'll need less, not more rake (ie move the rig forward, not back, to alleviate the situation.

As a thought, a number of stock designs with similar issues have been 'improved' by the addition of a short bowsprit, moving the headstay forward and the entire rig's C of E forward with it. The Catalina 30 and Rawson 30s are examples of having this done. A bit more involved, obviously, but perhaps a more long term, more effective solution.
 
#15 ·
You say you have a 'new' main - presumably it would have been built taking into account the pre bend (or lack of). Agree that your issue (ie excessive weather helm) is a matter of rake adj, but with respect to SemiJim earlier, you'll need less, not more rake (ie move the rig forward, not back, to alleviate the situation.
Ack! Right you are. Synapse lapse, there. Thanks for the correction. (I've corrected my earlier comments so as not to lead the unwary astray.)

Jim
 
#16 ·
I have the very little mast rake now the mast is about straight up from the deck. The short bow sprint is the way that others owners have tried to correct the weather helm issue on other islander 37's, with ok results.

The new main sail has been made to a spec that called for 60mm (2.36") of mast bend. Sail lofts numbers.

From everything that has been said here maybe it is not reasonable to try and get that much bend in this mast.

Thanks again for your feedback
"Rake The Spreaders" is dead.

I'll go back to tweaking the rig in the more standard ways.
 
#17 ·
"Thank you for your suggestion but I have a masthead, not a fractional rig, tightening the back stay does not induce mast bend. It just pulls on the fore stay."

This may be off in left field a bit, but that's not correct!
I was on a masthead rigged boat a couple of weeks ago and putting backstay tension on does indeed induce mast bend.
The boat in question was a 52' Custom Willie with a 70' mast (like a telephone pole!) But 3,500 lb of backstay tension produced a mast bend I could clearly see from the chainplates. And we used it to depower the sail in heavy weather.

(We were doing the Swiftsure race out of Victoria, Vancouver Island)

sam :)
 
#18 ·
"Thank you for your suggestion but I have a masthead, not a fractional rig, tightening the back stay does not induce mast bend. It just pulls on the fore stay."

This may be off in left field a bit, but that's not correct!
I was on a masthead rigged boat a couple of weeks ago and putting backstay tension on does indeed induce mast bend.
First of all it depends on the rig and the mast. But even then: In relative terms, a backstay tensioner on a masthead rig doesn't induce near the mast bend as on a fractional rig. What a backstay tensioner mainly does for a masthead rig is tighten-up the forestay.

Jim
 
#19 ·
Actually, (and it does depend on the rig, as SJ said) but we were able to induce 8-10 inches of mast bend on our previous masthead boat with in-line spreaders if the check stays were slack.

This was with hydraulics as well, the effect is similar to holding a toothpick by the ends between your thumb and forefinger and squeezing. Another interesting thing about that is that you are actually bringing the masthead a bit closer to the deck, and your shroud tension can drop as a result. Checkstays are there to limit that bend and minimize the 'shortening' of the rig.
 
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#20 ·
I'd point out that there were some boats that had negative rake—where the mast was angled forward rather than aft.

I'd point out that the problem with the mast rake could also be one of boat trim instead. If the boat is aft heavy, the center of effort will move aft as the mast tips aft, and the center of lateral resistance may move slightly forward, depending on the keel and rudder arrangement. This would result in greater weather helm than if the boat was loaded properly.
 
#21 ·
Any comments on Sailingdog's post above.

I only ask because I have found it to be the other way around on my boat.
More weight forward, more weather helm.

Hi Sailingdog, hope all things are well with you
 
#23 ·
As I said, a lot depends on the specific boat. :D

I'm good... just back from a delivery trip from the Bahamas... getting used to life back ashore is taking a bit longer than I thought it would. :D
 
#24 · (Edited)
I can agree somewhat with dogs observations but such are not the 'mainstream' cause of most 'weather helm' problems. Certainly there are boats with manufacturing error and designer problems - where the mast was placed too far aft which require either 'straight up' or slightly forward raked masts BUT these issues are/were usually solved by 'sail shaping recommendations' that reposition the "position of maximum draft- POMD" in the mainsail, etc. (Ie. radical sail re-SHAPING by hard mainsail halyard/cunningham tension) OR simply having sails cut purposely with the POMD further forward than the 'typicial 40-50% of cord length. As an example, some of Bob Perry's designs need this correction - because of 'yard execution and design errors'.

Weight distribution, especially in light weight boats (especially in the bow), can affect the relationship between the overall CG and the CLR ... but on most boats of over 30-35 ft. lengths the 'water immersion value' (the amount of how far a boat's waterline 'sinks' or gets deeper when overloaded) is in the range of 1000 - 1500 pounds per inch of deeper immersion of the waterline .... something that becomes mathematically trivial as the boat becomes 'larger'.

Most designers, nowadays, have good view of the relative or redundant 'buoyancy' of the bow and stern ends, have better control of the manufacturing yard's 'execution' to insure proper mast positioning, etc.

I make the claim, that probably most 'weather helm' is probably caused not by errors in the CE-CLR relationships but principally rather by RIGGING TENSION ERRORS, especially of the relationship backstay - forestay tensions !!!! ..... and this is where probably MOST sailors confuse actual 'weather helm' for the boat 'skidding off to leeward'. Without proper forestay (via backstay) tension the 'luff hollow' that the sailmaker cut into the jib to compensate for normal 'sag to leeward' of the forestay ... if this forestay SAG doesnt exactly MATCH the 'luff hollow' shape that the sailmaker cut into the luff of the jib the boat WILL skid off to leeward and the developed side forces from the skid is ROUTINELY and erroneously felt/thought to be weather helm ... when its NOT 'weather helm' causing the 'helm pressure'. At least this has been my observations when setting up a LOT of racing boats.
Weather Helm in my experience is PRINCIPALLY from POOR sail *SHAPE* and POOR *rig TENSION*. ... as I can 98% of the time bring this relationship back into 'perfect balance' by sail SHAPING and RIGGING (backstay)adjustment; ~2% of the time a "mast rake correction" is needed.

The simple fact is that MOST sailors dont know how to raise and tension the luff of a boltrope luff sail!!!! Dont beleive me, next time out sailing simply note HOW MANY boats when beating or high reaching HAVE THE AFT end of their booms LOWER on the horizontal than the Gooseneck ... the mainsails are NOT raised enough so that the mainsail does NOT take the shape that the sailmaker intended/designed (for when in windstrength of ~15 kts) ... and THATS the principal main cause of 'weather helm'. With a boltroped luff mainsail ****you MUST stretch out the luff 'preload' of the boltrope (~1" for every 11ft. of luff length) ... or you get 'weather helm'*****.
(When racing, if I see an 'ahead' competitor with the boom aft end 'drooping' ... I'll immediately get into a tacking duel because I now KNOW that that boat has 'weather helm' problems ... and I WILL win.)

Thats my story, and Im sticking to it.
 
#25 ·
To better explain the 'forestay - backstay' tension relationship (forestay sag), here is a copyrighted articled that I did several years ago. This will infer that a LOT of weather helm issues are really SKID to LEEWARD issues.
This is copyrighted and may not be copied, etc.
(Sorry the graphics on this article cant be uploaded)

Matching “Luff Hollow” to Forestay Sag.
Why doesn’t my boat ‘point’, it just heels over and skids off to leeward? .... LOTS of 'weather helm'.

When a sailmaker designs a headsail thats either attached to a foil or is attached by hanks, the sailmaker ‘assumes’ that the rigging is at normal tension (usually about 12-15% of the breaking strength of the wire). Unless specified otherwise a ‘cruising’ sail will be designed for ~15kts. of wind and the sailmaker will expect that headstay to ‘stretch’ a wee little bit when the sail is wind-loaded at the design windstrength. When beating, the stay will also begin to ‘sag’ off to the lee-side of the boat ... usually quite predictably. This expected sag is ‘compensated for’ by the sailmaker cutting a smooth
curved amount of sail material from the luff section of the sail - called “luff hollow” (fig 1). If the hollow isn’t cut to this shape and when the headstay does sag all that material at the midsection of the luff would be ‘pushed’ towards the center of the sail and the designed point of maximum draft -wide grey line D1 in fig 1.- would move aft to D2 in fig.2 (increasing weather helm) and the amount of draft would get much deeper causing the boat to heel more ... sometimes causing even more apparent weather
helm. Also, since the center of effort of the sail is now moved/sagged off to leeward ... its like someone moved that headstay several feet to leeward or radically somehow changed the sail’s angle of attack: the boat ... heels over, slows down, can’t point, and starts to ‘skid’ off to leeward ..... and all because the headstay sag no longer *matches* the luff hollow that the sailmaker designed into the sail !!!! Its all action and reaction.

How to increase headstay sag (and make pointing ‘worse’): loosen the backstay; apply too much winch tension to a sheet (all that applied load to the jibsheet eventually ‘distributes’ to the *headstay* which increases the sag); sail in wind well beyond what was the target design wind-loading for the sail.

How to know when the luff hollow shape is matching the normal sag in the
headstay
1. Ask the sailmaker how much luff hollow was cut into the sail, and where on the luff
up from the tack is that maximum amount. When on a hard beat and with a Mark 1 eyeball near the tack, look up along the luff and simply estimate that the ‘sag’ you see is approximating that value that the sailmaker gave. If too much sag, tighten the backstay or release some sheet tension; if too little sag, loosen the backstay or increase sheet tension.

If this is too approximate, there is a more precise way ... .
2. Take the jib/genoa and lay it FLAT on FLAT clean ground or floor and make an ‘acordian fold’ about 2 ft. back from the luff (Fig. 3). The acordian fold will allow the ‘curved’ 3D shape of the luff of the sail to lay FLAT on the ground. Work out ALL wrinkles from the luff section - you want the luff to be absolutely FLAT. Then take a string and pull tight along the curved luff shape .... what is ‘missing’ between that tight string and the sail is the ‘luff hollow’ that the sailmaker cut from the luff edge of the sail. Measure and record or REMEMBER that ‘hollow’ shape if you want that sail to take the ‘shape’ as was designed.
For easy bombproof ‘precision’, take that taught string and move it a few inches across the luff and so that its parallel to the ‘first’ string position but now laying over sail material. Then get some 3/8" wide adhesive backed ‘draft stripe’ material and apply ’straight as an arrow’ just behind the leading edge of the FLAT sail luff on the ground. When sailing and having ‘pointing problems with increased weather helm’ ... just take a walk forward and ‘see’ if that vertical stripe along the luff is deadstraight (headstay sag now *exactly* matching the ‘luff hollow in the sail’). Do any adjustment necessary (backstay tension, jibsheet tension, running backstay, etc.) to
keep that stripe ‘straight’ .... and your boat will now ‘point like a banshee’, with less heel, and wont be skidding off to leeward (and with the helmsman erroneously blaming ‘weather-helm’, etc.). Its very easy to overload a jibsheet on a large genoa ... and totally destroy the critical forward leading edge shape of the luff.

Want to race, quickly move to the side of a squall, point higher,
or get to your far destination faster?
... **match** the headstay sag to the “luff hollow”
RichH Ty37 #423 “Aquila” 12/07 ©2007 - R.Hampel
 
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