How to measure "prebend"

[isaksp00]

Finally got my 1991 H23 in the water, for first time as its new owner.

Now comes the mast tuning. I have seen the various comments about using approx 2% rake, and another about 6 inches of prebend (at least for a 21).

I can see how to use the outer stays and the swept spreaders to control bend of the upper mast (more or less, at any rate), and have seen how to use the lowers (inner stays) to pull the center of the mast side to side to straighten it. When people say "X inches prebend" what does that mean? Is the 6 inches the measure from a straight line from masthead to mast base to the mast, at the longest distance from that imaginary line to the mast? Is it how far back the masthead is from vertical? How on earth do you measure it?

Visually, mine has a noticeable aftward bend from the spreaders on up - if I had to guess (well, I guess I do have to guess) the masthead is maybe 6 inches behind the totally straight-mast position.

Maybe it is easier just to sail it, and observe the sail shape and handling? That begs the question - what do you observe in the sail shape to tell you what to adjust for the bend?

 

[Alan]

All good questions.

When the mast up but not yet tensioned the first thing you do is set the mast rake. This is done by adjusting the length of the headstay. It is measured by adding a weight to the main halyard as a plumb bob at boom height. Measure back along the boom from the aft surface of the mast to the plumb bob for the rake measurement. One to two percent is the usual amount. This is calculated in inches of mast height. Example: mast height of H23 is 33' or 396". 1% = 4", 2% = 8"

Next the mast is tensioned at the upper shrouds and measured for 'in column'. The amount of tension put on the shrouds is a percent of their SWL. This will induce an amount of bend in the mast. Not to worry, as the lower shrouds are tensioned this bend will be reduced. Reduce the amount of bend with the lower shrouds till the correct prebend is found. A general rule for prebend is one mast diameter along the long axis. It is measured at the widest point of separation between the main halyard pulled tight at the gooseneck and the aft surface of the mast.

Last, by sighting up the luff grove of the mast verify that the luff grove is dead straight all the way up (NOT front to back) side to side. Minor adjustment in the shrouds are used here for fine tuning this. (Note: any adjustment of a cap shroud after the initial 'in column' has been set must be matched by an equal adjustment of the other cap shroud)

The backstay can now be added. It is HIGHLY recommend that you have a backstay adjuster to control the power in the main. In light air the backstay can be nearly loose and as the breeze builds the adjuster is increased to bend the mast aft which flattens the main de-powering it.

 

[isaksp00]

Alan, thanks very much! Very clear.

BTW, where on LI? I grew up in Huntington, and we had a Thistle in Huntington Harbor.

I just did more adjusting this morning. I used a 100 ft plastic tape measure up the halyard to get the mast in column side-to-side (at least, I'd say within about a quarter to half an inch of each other, measured to the chainplate pin). I then fiddled with the inner stays to get the mast as straight sideways as I could. I have no idea if the shroud tension is correct - they seem pretty tight, we'll see under sail.

I was not able to truly measure mast rake - a bit too windy to have a halyard hang straight (OK, if I could use a cinder block, it would have been vertical). However, the Harken furler is adjusted well in - to the tight end of the turnbuckles screws - so unless the prev owner really had the basics screwed up, it is about as far forward as it can go. On a calm day I'll try to measure the rake percentage. I will also (next time I'm out) try to eyeball the mast bend to one mast diameter using the halyard or wire topping lift as a "straightedge".

I have a split backstay, where the single vertical wire starts about chest high. What is a recommended adjuster for this kind of rig?

 

[Alan]

I sail out of Oyster Bay. I'm moored off West Harbor Beach in Bayville.

After you start to tension the rig you cannot get a reading on mast rake anymore. It must be done with an untensioned rig. When you head out sailing just be sure that the boat exhibits a small amount of weather helm. If you have lee helm the mast is too far forward and must be moved back. Lee helm is dangerous to have and must be avoided.

Here's a pretty good explanation in video format of how a backstay works.
http://www.youtube.com/watch?v=9GDro9i1ye0

I would highly recommend Don Guillette's book 'Sail Trim' available from the Store on this site. It puts everything into plain English with easy to understand explanations.