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Mast prebend, fixed spreaders?


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 I have a CL 16 with a Dwyer DM 2 mast. I want to put some pre-bend in it. I’m thinking if I make the ends of the spreaders adjustable, so I can lengthen them and use forestay tension I should be able to induce prebend? Yes?

The jib is set aft of the forestay and is on a Holt Allen highfield lever. I don’t want to prebend the mast with jib luff tension. 

Does this sound workable so far?

What about spreader angle? Would there be an ideal spreader angle for any given amount of prebend. Selden make a nice adjustable spreader bracket  but I’m not sure I want to go there yet.           Should I let the spreaders piviot freely or fix them at some average angle?

Yours, in analysis paralysis, J

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39 minutes ago, JulianB said:

Can I ask why you want to pre-bend the mast??

What are yo trying to achieve?

              jB

That is a good question. Unless you are balls to the walls racing in a competitive CL 16 fleet, you won't notice any difference. However, if you just like to tinker I would get some Wayfarer info - pretty much the identical boat. Although the CL mast will probably be different with that swivel. 

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Firstly I like to tinker, The whole boat is a big tinker

Secondly, I have a Wayfarer mainsail which doesn’t have the right luff curve for the mast’s characteristics. I can’t get the top two telltails flying until the breeze is up. If I can bend the mast somewhat like a Wayfarer’s then the sail will hopefully work and I will have the tuning options as per Wayfarer tuning guides.

My question was really about the spreaders about the necessity or not, to have them fixed fore and aft 

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Being pragmatic, go get a sailmaker to re-cut the luff (do a luff curve) will improve the boat 100 times more and cost 1/10 of getting into pre-bend.

It will also work across the range, of wind-strengths, so it wont be notchy.

 Spreaders require a lot of rig tension to make them "pre-bend" a mast.

I doubt that a CL16's spreaders have been designed for that sort of load/work.

My guess is they are simply there to maintain the wire is a safe working distance from the mast.

And to up-spec that is another level of head-ache!

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If you want to play around and tinker I suggest you make a model mast. You can learn all sorts of cool stuff that way. You'll also want to make a model sail too. Also cheaper than the real thing and save s a lot of trouble later when you go to mess with the full sized mast.

But yes, luff curve is really best handled by sailmaker. I suspect you have a bagged out old sail that is not worth spending on...is that correct?

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One big problem of trying to get pre bend into a deck stepped telegraph pole is you need too much spreader rake and too much rig tension to achieve this easily, if your boat is not properly engineered you could have problems with the shroud bases, or the mast step. You will also have a forestay that is way too tight in light air, and if the breeze does come up the mast will have a tendency to want to bend too much as soon as you start using vang because of the high shroud compression load and spreader rake. Pre bend works best with keel stepped masts going through a gate, with or without a strut or mast ram , or on a rig with more than one set of shrouds. So, as suggested above, your best option by far is to have the luff curve on your sail modified to suit your mast as it is.

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3 hours ago, Major Tom said:

One big problem of trying to get pre bend into a deck stepped telegraph pole is you need too much spreader rake and too much rig tension to achieve this easily, if your boat is not properly engineered you could have problems with the shroud bases, or the mast step. You will also have a forestay that is way too tight in light air, and if the breeze does come up the mast will have a tendency to want to bend too much as soon as you start using vang because of the high shroud compression load and spreader rake. Pre bend works best with keel stepped masts going through a gate, with or without a strut or mast ram , or on a rig with more than one set of shrouds. So, as suggested above, your best option by far is to have the luff curve on your sail modified to suit your mast as it is.

It is not a deck stepped mast. It is a dead knock off of a Wayfarer, so extremely sturdy and seaworthy. (Note I mistakenly said above the Wayfarer does not have a swivel. Wrong word - pivot would have been better - and the Wayfarer does have one). The story goes that CL didn't want to pay fees to an association (sounds familiar), so they rebranded it, as it is a great daysailer as well which CL was targeting.  Anyway, the tuning is exactly the same. North, McNamara and the Wayfarer Assoc website all contain info. I sailed in the Wayfarer Worlds (admittedly ages ago), and the fast guys have adjustable spreaders which they adjust between races in changing conditions.

  • Image result for cl 16 sailboat
     
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39 minutes ago, Bill5 said:

It is not a deck stepped mast. It is a dead knock off of a Wayfarer, so extremely sturdy and seaworthy. (Note I mistakenly said above the Wayfarer does not have a swivel. Wrong word - pivot would have been better - and the Wayfarer does have one). The story goes that CL didn't want to pay fees to an association (sounds familiar), so they rebranded it, as it is a great daysailer as well which CL was targeting.  Anyway, the tuning is exactly the same. North, McNamara and the Wayfarer Assoc website all contain info. I sailed in the Wayfarer Worlds (admittedly ages ago), and the fast guys have adjustable spreaders which they adjust between races in changing conditions.

  • Image result for cl 16 sailboat
     

 

39 minutes ago, Bill5 said:

It is not a deck stepped mast. It is a dead knock off of a Wayfarer, so extremely sturdy and seaworthy. (Note I mistakenly said above the Wayfarer does not have a swivel. Wrong word - pivot would have been better - and the Wayfarer does have one). The story goes that CL didn't want to pay fees to an association (sounds familiar), so they rebranded it, as it is a great daysailer as well which CL was targeting.  Anyway, the tuning is exactly the same. North, McNamara and the Wayfarer Assoc website all contain info. I sailed in the Wayfarer Worlds (admittedly ages ago), and the fast guys have adjustable spreaders which they adjust between races in changing conditions.

  • Image result for cl 16 sailboat
     

Sorry, the photo I looked at was from the bow and it looked like the mast was deck stepped. This being the case it might be possible to pull the mast forward at deck level, that should induce pretend with low rig tension.

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If you have to, try fitting Morrison wires, crimped to the shrouds just above the spreader tips, down to bottlescrews an front side of mast just above mast heel, you can then preset the prebend.

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 Thank you for the replies.  To your points,  The mainsail is a Wayfarer main, two seasons old  and yes cut for a wayfarer mast. The Dwyer DM 2 mast is not really a tree trunk. Its about 2 pounds heavier than the un tapered proctor section that came with the boat and almost the same dimensions (not quite as wide, thicker wall) You can certainly bend it with the vang but at the expense of leach tension. It does have a pivot just above the step, exactly the same as a Wayfarer.

The Wayfarers are limited in the length of spreader, class rules,  but I’m not so a longer spreader should push the mast forward better than asking it to act as a lever. The forestay which is a piece of spectra runs around a sheave at the stem and back to the center of the boat. This was done so I could lower the most from inside the boat but it also makes it easy to put a purchase on it so as to adjust tension.  I uploaded an image of the spreader tip. Remove the piece of aluminum that secures the shroud and is retained by the clevis pin and replace with an aluminum bar with a series of holes and some arrangement to hold the shroud. Slide in and out to desired length and secure with pin.  I am now thinking that a spreader that is free to move for and aft will better direct the load forward regardless of mast position due to its bend (shroud load or vang load)

 I could just pay a sailmaker to re-cut the luff but what’s the fun in that?

image.jpg

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3 hours ago, duncan (the other one) said:

repeat after me.

Spreader angle controls prebend, length controls lateral stiffness.

 

jesus... its even in the UK wayfarer rig setup instructions

The last thing to check is the mast bend.

Pull the main halyard tight against the mast at the gooseneck and measure from the back of the mast to the halyard at spreader height.

Too little bend - Adjust the spreaders backwards a small amount

Too much bend - Adjust the spreaders forward a small amount

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4 hours ago, duncan (the other one) said:

jesus... its even in the UK wayfarer rig setup instructions

The last thing to check is the mast bend.

Pull the main halyard tight against the mast at the gooseneck and measure from the back of the mast to the halyard at spreader height.

Too little bend - Adjust the spreaders backwards a small amount

Too much bend - Adjust the spreaders forward a small amount

This is mostly it. The other part is that without rig tension there will be no bend. As you put the rig tension on with the jib halyard you will have to crank that on. Spreader angle has the biggest impact on pre bend, but more tension will equal more bend. Little tension will equal no bend.

 

FFS don't go adding loads of weird wires and shit ..

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 I made a little model of the rig. I could adjust the spreader angle and spreader length. Although the wooden dowel for the mast was a little thick, it was clear that spreader angle and not spreader length is the determining factor in Mast bend. My apologies to you gentlemen, you were correct. 

 I tried to upload photo  (2.2 mb) unsuccessfully 

Edited by jasonsansfleece
Angle and length the wrong way around
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39 minutes ago, jasonsansfleece said:

 I made a little model of the rig. I could adjust the spreader angle and spreader length. Although the wooden dowel for the mast was a little thick, it was clear that spreader angle and not spreader length is the determining factor in Mast bend. My apologies to you gentlemen, you were correct. 

 I tried to upload photo  (2.2 mb) unsuccessfully 

it makes sense if you do the trig. Bending force is Fc x sin(alpha). Fc = spreader compression (proportional to rig tension), alpha = spreader angle.

Although you will get more bend for the same rig tension with longer spreaders, when you slack off the rig (reduce spreader compression) for the same lateral mast stiffness (lateral spreader force on mast = Fc x cos(alpha)), the bend will be the same as you had with the shorter spreaders and more tension.

 

This assuming that the stays are always in tension, which they may not be (eg: some dinghies tune for the leeward shroud to be slack upwind).

 

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41 minutes ago, duncan (the other one) said:

This assuming that the stays are always in tension, which they may not be (eg: some dinghies tune for the leeward shroud to be slack upwind).

 

And other dinghies make it so (slack), no matter how hard you tune them...

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DavidYacht, thanks for the name "Morrison wires" I wondered what they were called. Given the OP's original request and the simplicity of this solution which means being able to adjust mast bend independently of rig tension and the minimal cost involved it seems ideal. 

 

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