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Old 29-04-2012, 15:22   #1
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how to check if my mast is straight?

hi all I just want to know how to check if my mast is straight, once I read that a halyard can be used to see if the distance to the gunwales is the same, today after some talking at the club, some insisted that you should use the halyard and check it against the keel. I found that clumsy since I need to take the boat out of the water. What are your thoughts?
When I use the halyard I found it difficult to apply the same pressure on startboard and then on port!! help will be much appreciated!
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Old 29-04-2012, 16:17   #2
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Re: how to check if my mast is straight?

when i was racing, years ago, we would tie a spanner to a light rope, hoist it up the mast so the spanner was level with the base of the mast, bribe some guys to stand on the rail till she leaned exactly 15 degrees, measure from the base of the mast to the spanner, then repeat the process the other way. if the measurements were the same the mast must be straight, at least thats what they told me!
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Old 29-04-2012, 16:43   #3
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Re: how to check if my mast is straight?

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Originally Posted by JuanCH View Post
hi all I just want to know how to check if my mast is straight, once I read that a halyard can be used to see if the distance to the gunwales is the same,
That's the easiest way, but if you have a multi sheave mast head (offset to one side), it'll be off a little. But close enough. All boats are not perfect. If it's close, it's good. You'll never notice the difference sailing, it's cosmetic.
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Old 29-04-2012, 16:59   #4
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Re: how to check if my mast is straight?

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When I use the halyard I found it difficult to apply the same pressure on startboard and then on port!! help will be much appreciated!
Use the main halyard. Tie it to a bosuns chair / rock climbing harness. Sit in the harness and mark the halyard with some electrical tape at (say) chest height. Swing out to the cap shroud on each side and mark each cap shroud from the mark on the halyard (make sure you are swinging free in the harness - not supporting your weight in any way). Measure from the mark on the cap shroud to the eye of the chain plate each side. Adjust turn buckle until both are the same distance.
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Old 29-04-2012, 17:14   #5
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Re: how to check if my mast is straight?

There is a little bit of guessing by what you mean by "is my mast straight?" The modern mast tube is normally curved a little aft when looked at from the side. But the mast tube should be "in column" that is straight with no bows or bends when looked at from the bow or stern. Too much tension on the shrouds can cause a mast tube to adopt a "S" shape. Not good.

If you are asking if your mast is perpendicular to the boat in the athwartship (beam) direction, that is something else. A steel measuring tape attached to a halyard and hoisted to the very top so that the shackle is up against the masthead sheave will measure the same dimension to the opposite chain plates when the mast is perpendicular to the boat's level position. But this can only be done when the boat hull is floating level in the water as any heel will cause the mast to lean to one side or the other. That is why the measurement is normally done when on the "hard" with the hull leveled in jackstands.

The normal mast is "raked" aft up to about 4° from the vertical in the perpendicular position measured fore and aft.

A little off perpendicular in any direction is not a show stopper. But with the boat sitting level in the water - that is, distance from port and starboard chainplate top hole to water surface equal - you can easily see any port or starboard tilt if your stand back a ways on land. If you are serious with nothing else to do, borrow a "transit" and level the boat in the water. Then sight up the mast with a leveled transit. The cross hairs should align with both the bottom and top of the mast. This can also be done "on the hard."
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Old 29-04-2012, 18:13   #6
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Re: how to check if my mast is straight?

How about looking up the mast?

b.
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Old 29-04-2012, 19:10   #7
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Re: how to check if my mast is straight?

So, why is this important? Trying to learn everything I can.
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Old 29-04-2012, 22:09   #8
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If you are asking if your mast is perpendicular to the boat in the athwartship (beam) direction, that is something else. A steel measuring tape attached to a halyard and hoisted to the very top so that the shackle is up against the masthead sheave will measure the same dimension to the opposite chain plates when the mast is perpendicular to the boat's level position. But this can only be done when the boat hull is floating level in the water as any heel will cause the mast to lean to one side or the other. That is why the measurement is normally done when on the "hard" with the hull leveled in jackstands

Big BS
You are measuring distance to the deck or chainplates . The boat can be heeled as much as you want ,it will not make a difference.
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Old 30-04-2012, 19:35   #9
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Re: how to check if my mast is straight?


Big BS
You are measuring distance to the deck or chainplates . The boat can be heeled as much as you want ,it will not make a difference.


You need do some ocean sailing - masts will bend to leeward as the windward shrouds stretch to take up the load. The leeward shrouds will slacken and in some cases flop around. The weight of the metal in the mast will cause enough tilt/bend in the mast to screw up the measurement if the boat is not "level."

Setting up the "isoceles triangle" to check a vertical center leg requires the same dimension measured from the top of the mast to an equal and level reference point on either side. Normally your chain plate's top hole dimension is the same on both sides of the boat. And it is easier to measure to a pin than to a place on the cabin top/deck. But you could measure from the deck if you find that your chain plates stick up to different distances.
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Old 30-04-2012, 20:09   #10
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Re: how to check if my mast is straight?

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So, why is this important? Trying to learn everything I can.
Structurally it's not too important, but a perpendicular mast (side to side) is important for performance - Typically, you would want the boat to have the same sailing performance upwind on each tack - just makes life easier to have the boat doing the same thing on each tack. This is the case for almost all boats. There are a few exceptions when you would want the mast to cant to one side, but not many.

Fore and aft mast shape and position is a much more involved subject. For the purposes of cruising, most (but not all) will set the mast up with a little bit of prebend, the top of the mast slightly aft of the gooseneck and fairly tight rigging.

For the more performance orientated, they may change the mast shape depending on conditions and point of sail by adjusting backstay tension.

For the racers, they will be adjusting rig settings for each race and changing mast shape with the backstay as often as they use the mainsheet.
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Old 30-04-2012, 20:49   #11
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Re: how to check if my mast is straight?

For the most part, I like it when mine is pointing away from the water.
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Old 01-05-2012, 00:59   #12
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Re: how to check if my mast is straight?

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hi all I just want to know how to check if my mast is straight, once I read that a halyard can be used to see if the distance to the gunwales is the same
This assumes that the gunwales or chain plates are level, often they are not. I use the main halyard very tight between the gooseneck and the top centre sheave, if the halyard bed nicely in the sail track the mast is straight. It must be no wind and no great sun heat on one side of the mast. I string the halyard like a chalk line it must fall all the time in the track grove. To have the mast in line with the hull I do this ashore. First I level the hull, in my case it is easy because I have inbuilt level marks if not I would compare doorjambs, floor, keel sides etc until I am convinced that the hull is level then I will plumb bob the mast from a distance and then I will check if gunwales and chain plates are level using a water tube, accuracy +- 1.5mm. With the shrouds sets I will give a slight bend backward to set the “column”. Later on to keep the mast in column every opposing turnbuckles must be given the exactly same amount of turn. Depending of the light and heat it is often difficult to judge the straightness of a mast by eyes only.
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Old 01-05-2012, 01:05   #13
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Re: how to check if my mast is straight?

I am not sure if you get the magazine " Practical Boating" in your part of the world but the May issue has quite an informative article on this subject
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