Adding sail trim controls

WNYSobstad · Mar 23, 2015

If I might be so bold as to throw my 2 cents (perhaps overvalued) in here re. masthead rigs and backstays. On a boat such as this with spreaders that are not aft swept cranking on the backstay will indeed put a bend in the mast. What happens is the mast is compressed and bends sort of like a recurve bow toward the bow. Initially is has the effect of tightening the headstay but with more tension the headstay actually gets looser as the point where the headstay joins the mast moves closer to the deck.

What also happens is that as the mast is bent it pulls the mid point of the main sail's luff forward flattening it out and de-powering it. This is a handy trick for conditions where the boat is overpowered but not so much that you want to put in a reef.

If you talk to the folks who made your boat they can give you the specs for mast rake and what is known as pre-bend. This is the target you aim for when setting up the rig before you put the sails on. As a sailmaker this is an important data set as the main sail luff should match this pre-bend. (Contrary to what many might think the main sails luff is not a straight line top to bottom.)

Todd Smith · Mar 23, 2015

Jackdaw said:
Can you explain to me how that happens?? To bend the mast the masthead has to move backwards. That is impossible on a boat where the forestay is attached to the same part of the mast as the backstay. Like the masthead.

Fractional boats bend the mast by having the mast bend around the forestay attach point. Anything above that points bends aft, anything below bends forward.

I've owned racy masthead boats; we bent that mast with an adjustable babystay.

Regarding folding the boat.. ask someone that owns a big masthead rigged racing boat. Often when they crank on the backstay hard, you cannot open or close doors downstairs. One guess why.

See the attached image. The mast is bent like stringing a bow.

Having a baby stay helps even more.

Edited, I see that WNYSobstad said all that already.

I have never had doors stick, but I can see how it would happen on lightly built boats.

Todd

Todd

Scott T-Bird · Mar 23, 2015

I don't think I buy that explanation. The reason a bow bends is because there is an opposing force in the center (and the bow is dimensioned and designed to curve in a pre-determined shape). The mast doesn't have this opposing force and most masts for masthead rig have a consistent cross section and built straight. I stand my masthead rig against a similar height Hunter mast with fractional rig (built for bending) and the differences in cross-section are dramatic.

What your exhibit shows is a mast that is buckling under the compressive force that opposes the tension in both the forestay and the backstay. A round mast may do this, if you could possibly develop enough tension in the stays, but it isn't likely to be a smooth curve. It is more likely to be uncontrolled at a point of weakness.

Besides, most masts are oblong, so the buckling would be to the side, not fore and aft.

Todd Smith · Mar 23, 2015

Scott T-Bird said:
I don't think I buy that explanation. The reason a bow bends is because there is an opposing force in the center (and the bow is dimensioned and designed to curve in a pre-determined shape). The mast doesn't have this opposing force and most masts for masthead rig have a consistent cross section and built straight. I stand my masthead right rig against a similar height Hunter mast with fractional rig (built for bending) and the differences in cross-section are dramatic.

What your exhibit shows is a mast that is buckling under the compressive force that opposes the tension in both the forestay and the backstay. A round mast may do this, if you could possibly develop enough tension in the stays, but it isn't likely to be a smooth curve. It is more likely to be uncontrolled at a point of weakness.

Besides, most masts are oblong, so the buckling would be to the side, not fore and aft.

Masts don't bend to the side, because of the shrouds and spreaders.

Bendy mast head rigs, have mast sections that are intended to bend. Just like the bow is.

I only made the comment because people kept repeating the old saw 'masthead rigs can't be bent with an adjustable backstay'. That could mis-inform some people. Many mast head rigs can be bent, and this can be used to shape the sail. On all of the boats that I race, the bend is easily seen looking up the mast, and it is in a smooth curve.

Whether any particular boat's mast could be bend is another question, same as whether it is worth someone's time, effort and money to add a backstay adjuster.

Todd

Scott T-Bird · Mar 23, 2015

No, they don't bend to the side, but when they buckle that is generally how they fail because of the cross-sectional dimensions.

I'm sure there are some mast head rigs which have a mast section with a designed pre-bend and a cross-section that allows it. Possibly some tapered masts are masthead rigged. But I contend that the great majority of masthead rigs will have a non-tapered section that also has no pre-bend. Mine is a MORC section which is an extruded aluminum cross-section which is constant from top to bottom. Sure, it would bend a little if configured with shrouds and a baby stay to guide it. But it is not going to bend if it is simply put into compression by tensioning the forestay and the backstay as the exhibit suggests. All that does is put the mast in linear compression. The ultimate end result is buckling, and it will be perpendicular (or across) to the long axis, somewhere between the shrouds, probably just above the spreaders

Luckily, it probably isn't feasible to cause the mast to buckle by over-tensioning the backstay only, so the end game is to rake the mast aft and reduce sag in the forestay. The "old saw" is basically correct about the reason for tensioning the backstay on a masthead rig. I think it is pointless to try to argue that it is not correct. Bending a masthead rig is generally the exception, not the rule. And bending a masthead rig also depends on rig assemblies that are designed to induce bend ... generally more detailed than simply tensioning the backstay.

Although, I would agree that a keel-stepped mast is likely to act more like a cantilevered beam or a fixed end than a deck-stepped mast. A deck-stepped mast will tend to pivot, keeping the mast straight. A keel-stepped mast fixes the mast at the deck penetration, where the aft tension creates bending.

bigpapaporsche · Mar 23, 2015

Sail Trim Adjustment

Larry, everyone has an opinion, here mine. I've seen a lot of sailors make all kinds of modi cations to rigging, in the hope of improving performance by changing the sail's shape etc. I do like the addition of a Traveller. However, IMO I think you have to first find a way to graphically be able to see/measure what the sails are doing, i.e. see the air-flow going over the sails. Only then can you determine what affect the Sail Controls are having on the sails! By the way these items are cheap. I like Tell Tails on both sails, as well as on the ends of the Battens. I also added Black Trim Stripes on both of my sail this season, so i can see what is going on better. Remember the KISS principle. Don't forget your Reefing System.

Scott T-Bird · Mar 23, 2015

Au contraire ...

Todd Smith said:
Masts don't bend to the side, because of the shrouds and spreaders.

Todd

Actually, masts are MOST easily bent to the sides ... because of the shrouds and spreaders. You can come up with all kinds of crazy S shapes and sweeping curves, all depending on which combination of loosening and tensioning you come up with for the upper and lower shrouds.

Admittedly, that's not the point, because the shrouds are intended to be set specifically to keep the mast straight in the sideways dimension.

But the orientation of the long axis front to back and the variety of controls at the midpoints are what makes bending to the sides more readily accomplished.

All U Get · Mar 24, 2015

Hope you had your fenders in.

All U Get

Todd Smith · Mar 24, 2015

Scott T-Bird said:
No, they don't bend to the side, but when they buckle that is generally how they fail because of the cross-sectional dimensions.

I'm sure there are some mast head rigs which have a mast section with a designed pre-bend and a cross-section that allows it. Possibly some tapered masts are masthead rigged. But I contend that the great majority of masthead rigs will have a non-tapered section that also has no pre-bend. Mine is a MORC section which is an extruded aluminum cross-section which is constant from top to bottom. Sure, it would bend a little if configured with shrouds and a baby stay to guide it. But it is not going to bend if it is simply put into compression by tensioning the forestay and the backstay as the exhibit suggests. All that does is put the mast in linear compression. The ultimate end result is buckling, and it will be perpendicular (or across) to the long axis, somewhere between the shrouds, probably just above the spreaders

Luckily, it probably isn't feasible to cause the mast to buckle by over-tensioning the backstay only, so the end game is to rake the mast aft and reduce sag in the forestay. The "old saw" is basically correct about the reason for tensioning the backstay on a masthead rig. I think it is pointless to try to argue that it is not correct. Bending a masthead rig is generally the exception, not the rule. And bending a masthead rig also depends on rig assemblies that are designed to induce bend ... generally more detailed than simply tensioning the backstay.

Although, I would agree that a keel-stepped mast is likely to act more like a cantilevered beam or a fixed end than a deck-stepped mast. A deck-stepped mast will tend to pivot, keeping the mast straight. A keel-stepped mast fixes the mast at the deck penetration, where the aft tension creates bending.

I don't think we have any disagreements about facts. Just about percentages

The old saw that I was referring to was "You can't bend a masthead rig, you can bend a fractional rig". It incorrectly states that you can't bend a masthead rig. That is only true for some percentage of boats.

Todd

Dave Stone · Mar 29, 2015

Mast bend on mast head rigs

can be induced by cutting the bottom of the mast at a small angle so that the heel of the mast does not touch the mast step under "normal" conditions. this makes the forward side of the mast longer than the back side. Haul in a bunch of backstay and pretty soon the heel of the mast is on the mast step and the mast is bowed forward somewhere in the center helping to flatten the main.
Catalina 22's do this. The center of the mast is held in place by the four lower shrouds (pre-bend) to help induce forward bend & to limit how far forward the middle goes. The mast is compressed and the masthead moved aft by the backstay adjuster. The forstay gets pretty tight. So do the aft lowers. I pulled the deck up where the lowers were attached. I also ripped out one of the interior bulkheads that the uppers attach to. It was held to the liner by a couple sheet metal screws. Check everything and reinforce any weakspots.

Also, old sails may or may not respond to sail controls as you operate them. Mine didn't. New ones did. The old ones were soooo baggy that the controls just could'nt move enough material to change the shape enough for me to see.

Joe · Mar 30, 2015

Dave Stone said:
r. The forstay gets pretty tight. So do the aft lowers. I pulled the deck up where the lowers were attached. I also ripped out one of the interior bulkheads that the uppers attach to. It was held to the liner by a couple sheet metal screws. Check everything and reinforce any weakspots.

This is a perfect explanation of why every sail trim guide I've read does not recommend trying to induce mast bend, on a mast head rig, with the backstay.

SailorTom · Mar 30, 2015

My 2cnts which is a deal at half the price

1. new sails. They are your motor! Can't flatten or adjust draft etc..in old baggy sails!
2. cunningham. It's cheap and functional
2. You didn't mention but a good speed display. It's real hard to tell if an adjustment helped 1/2kts or hurt 1/2kts. A Speedpuck is nice but crazy expensive for what it is

Be careful if you put in backstay adjuster, good for tensioning the headstay, but that old boat might fold up if you put too much stay tension in it!

shnool · Mar 30, 2015

To continue the off-topic argument... mast bend on a masthead rig. The Capri 25 I sailed for 3 years most assuredly bent the mast with a backstay adjuster. The sailmakers guide recommended a pre-bend in the rig, under static conditions and baskstay off... Jack is very much correct in that increase in backstay tension on a masthead takes forestay sag out, but with every action there is an equal, you get the point... the masthead actually moves aft in that arrangement. If lowers are essentially static, and prevent the center of the mast from moving, the effect is bending the mast.

Its NOT MUCH... with the same tension a fractional would turn to a pretzel, but the masthead rig WILL move. I also started with a significant amount of rake... the Capri 25 is a dual lowers, single spreader rig, with a floating SINGLE pin mast step. This allows the heel of the mast to pivot all it wants. Anyway, startoff on the Capri 25 was with 8-10 inches of rake, mast straight up port/starboard, with about 1 inch pre-bend (this is induced by tightening forward lowers)... Aft lowers are set to hand tight (because again Jack is correct, as you tighten backstay, forestay sag goes down, mast wants to bow forward in the middle, so you must let it!)... So measuring rake with FULL ON backstay I was at about 12-14 inches. Looking up the mast from the tabernacle, you'd see a nice bow forward to max forward at the spreaders, then curving ever so nicely back to the top of the mast.

Bend goes from 1 inch pre, to what 3 inches post? Something like that. You also note that backstay full-on puts the aft lowers under significant tension, where they were hand tight to start (no backstay)... This allows you to depower the mainsail by flattening it... it also adds halyard tension and effectively moves draft forward. Of course this also tensions the forestay, which flattens and depowers the entry of the jib/genoa.

I am told there are masthead rigs with bendy masts, and those without... perhaps THAT is the real key to your arguments over whether you bend masthead rigs or not. The Capri 25 was most assuredly a bendy mast.

My prior boat was a Capri 22, which is a bendy fractional rig... I didn't race that boat, and now I wish I had, to learn more about what made it tick.

My new boat, an S2 7.9 (26 foot) is a well known factional rig, with what many call a telephone pole for a mast. That being said, I can't wait to learn this rig, and more about mast bend. This is an odd boat with highly swept spreaders and single lowers, with a single spreader. The mast extrusion is about 1 foot taller than the Capri 25s, but is significantly heavier than the Capris... and larger diameter. I'm told though that mast bend is a huge part of how to make the boat go well to weather. I'll let you know. I can already tell you that this thing can out-point most J 24s, especially with a 110 jib.

Don Guillette · Mar 30, 2015

I wonder if we're getting confused with mast bend and mast rake. As has been discussed at length on this forum in previous topics -- there's a difference between the two items. Guys like RichH, Joe from San Diego, Stu J, Scott T bird, Jackdaw and some others explained both elements in such detail that I got a brain freeze reading all the details!! There is no way I could "bend" the mast on my C25 or C30. If I raked the mast on those two boats they thought I had picked up the mast and moved it aft and weather helm was the result.

On the C30 I had a backstay adjuster that Garhauer gave me. I never used it. I only kept it attached was because it looked trick!!

Scott T-Bird · Mar 31, 2015

Dang Larry, you're racing already?

Joe said:
This is a perfect explanation of why every sail trim guide I've read does not recommend trying to induce mast bend, on a mast head rig, with the backstay.

We're still waiting for the ice to melt. Our lake is still iced completely over - I've never seen this much ice this late! I guess this just means I have more time to argue about the cause and effect of backstay adjustment on a masthead rig.

Keep in mind that the engine for your masthead rig is the head sail. Talk about bending the rig to improve the shape of the main sail is just talk. The purpose of backstay adjustment on your rig is to reduce sag in the forestay and improve the performance of your genoa ... the real engine of your boat. If you are using a larger genoa, the backdraft from your genoa when you are tacking up wind has more effect on the leading edge of the main sail than any bend that might be induced. You also appear to have one set of lowers (similar to my boat), which are slightly swept back. When you tighten your backstay with an adjuster, your lowers will go slightly slack - which means your mast isn't bending.

I'd also point out that the opposing force from the forestay means that adjustment of the backstay can only induce bending by attempting to force the mast into extreme compression. By contrast, a fractional rig that is set up for bending needs far less stress on the rig because the force at the top of the mast is un-opposed and the force is truly in bending, not compression. For that reason, the mast is also tapered to encourage bending. Your mast has a constant shape with the long axis fore and aft which really prevents bending. Any attempt to bend it puts the mast in extreme compression, requires far more stress on the rig and acts to fold the boat as others have described.

Rig stress is a topic I was reading about for your boat. You might want to consider an upgrade in rig diameter as discussed here ... http://www.sealsspars.com/cal2-27.html I tend to agree, as my boat, with slightly less sail area has 7/32" rigging. Does yours have 3/16"? Or has your rigging already been improved?

I think a backstay adjuster would be a significant improvement for your boat because it improves the shape of the head sail when apparent wind increases on those upwind legs. A Cunningham is also worthwhile because it can be done on the cheap ... but improving the shape of the leading edge of your mainsail is less important than improving the shape of your head sail if you are talking about performance.

Scott T-Bird · Mar 31, 2015

We looked at a Cal 2-27 (1976) before we bought our boat. If we weren't a day late with the offer, that's what we would be sailing right now. I might have something to say about an A-4 engine in that event!

Adding sail trim controls

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