Spreaders: replace or repair?

[savageseadog]

I am not saying it is wrong but --where does "accepted minimum" come from?

It's an angle that gives a reasonable amount of support to the mast without having too much compression in the spreader. At 0 degrees the spreader would perform no function, at 20 degrees the compression would be massive.

 

[TimBennet]

It's an angle that gives a reasonable amount of support to the mast without having too much compression in the spreader.

The designated angle is to provide a reasonable amount of support to the mast without too much compression in the mast itself. Most of the load on a shroud is downwards. If the angle the shrouds arrive at the mast is less than 20 degrees, then you either need a wider shroud base or an additional set of spreaders.

The spreaders can't be longer than half the shroud base, but can easily be made strong enough for an compression they may experience. Reducing spreader length increases mast compression.

 

[mjcoon]

Spreaders should bisect the rigging at the tip which also means that they should point up. Shortening them will reduce the angle, the accepted minimum is 10 degrees (or 170) between the upper and lower spans, preferably 12 degrees. I would have no hesitation in shortening if the geometry is right.

How much can you shorten the spreaders until the bottle-screws run out of thread and the shrouds have to be re-terminated?

Mike.

 

[Daydream believer]

It's an angle that gives a reasonable amount of support to the mast without having too much compression in the spreader. At 0 degrees the spreader would perform no function, at 20 degrees the compression would be massive.

So you made it up then & have no actual design figures to prove 10 degrees

I am quite aware of the theory but have you considered that many large yachts have horizontal spreaders

they are able to do this because they have discontinuous rigging
the idea of balancing the angle- which is what you are saying ( & is quite right except the quote of 10 degrees)- is to avoid/reduce vertical stresses at the spreader tip because the angle of the shrouds on a horizontal spreader is trying to push the tip downwards. this is prevented on discontinuous rigging set ups

your comment re horizontal compression is ( only in my opinion) is wrong although I am not sure how you are measuring the angle. The wider the rig base the better the mast support as the forces are more square to the force applied on the rigging & therefore reduces rig load.

Not sure if i can explain properly but i cannot post sketches & calculations on the forum

 

[savageseadog]

I have to say that it's wise not to take too much notice of the nonsense that's put on here at times

 

[30boat]

From the manufacturesrs themselves:

 

[Daydream believer]

I have to say that it's wise not to take too much notice of the nonsense that's put on here at times

Absolutely- that is why it is important to challenge false claims asap to avoid others believing it to be true

 

[savageseadog]

From the manufacturesrs themselves:

Exactly

The tip of the spreader bisects the angle of the shroud and the angle deflected is 12 degrees

 

[DJE]

I did the trigonometry on this some time ago and found that for the spreader to bisect the deflection angle at it's tip then the triangle formed by the spreader the mast and the top section of shroud is isoceles - assuming that the part of the shroud below the spreader tip is parallel to the mast. Therefore the distance from the top shroud termination to the spreader root is the same as the distance from the same termination to the spreader tip.

The OP should also note that some heat-treated aluminium extrusions loose about half their strength in the heat-affected zones close to welds.

 

[Daydream believer]

Most people are focusing on the vertical angle of the spreader.
However, If the spreader sits in a root that restricts fore & aft movement that may still put pressure on the spreader fixing bolt & the shoulder of the spreader
If the mast is fractional then adjustment of the backstay may well bend the mast forward. If the rigging is taught then the angle ( on plan) may change

If the mast is allowed to pump a little the constant change of force ( albeit slight) may work the joint over time

If the mast is masthead rigged it may be that the stay goes to a point on deck aft of the mast but the spreader does not have quite enough, or too much, aft rake
This can also stress the bolt point
Whilst some have posted drawings of the spreader bisecting the vertical angle I could never acheive this on my boat as there are 2 wires passing through the spreader tip.
One going up to the shorter upper spreader thus a near horizontal angle would be correct
The other going to the root of the upper spreader & a bisect of the angle thus formed would be ( I guess) much more than indicated
in fact on my fractional rigged 14 m high mast the spreaders are horizontal-- by design

On my squib & my Phantom dinghy the spreaders are constructed to allow them to pivot fore & aft at the mast

 

[sailorman]

I did the trigonometry on this some time ago and found that for the spreader to bisect the deflection angle at it's tip then the triangle formed by the spreader the mast and the top section of shroud is isoceles - assuming that the part of the shroud below the spreader tip is parallel to the mast. Therefore the distance from the top shroud termination to the spreader root is the same as the distance from the same termination to the spreader tip.

The OP should also note that some heat-treated aluminium extrusions loose about half their strength in the heat-affected zones close to welds.

Make off a halliard end / shackle off at the spreader root then swing the halliard out the the tip & lock-off ( Selden ) @ that length