Bend in a wooden mast

[nigelhudson]

I've been looking at a GRP trailer sailer that has a gaff rig with wooden mast and spars. The manufacturers current spec for the mast and spars is "Sika Spruce treated with Sikkens" and I guess that this would also have been the case in 1984 when it was built. My concern is the mast which is deck-stepped and original I think.

The mast is about 15' long and has a visible bend to port in the top third such that the top is probably 40-50mm out of true with the bottom half. There is no evidence of cracking so I think that it is entirely down to the warping of the timber. My previous experience of wooden spars is confined to the boom and gaff on a Mirror dinghy so I'm not sure whether this bending is material. Obviously there is no such thing as a completely straight wooden spar and they will all distort slightly with age. My question is - what is an acceptable amount of bend in a 15' spruce mast?

Thanks

 

[sarabande]

it would be handy to have a pic if poss.

Much depends on whether the bend is gradual along the length of the mast, or just in the last couple of feet or so. Which is why a pic would be useful.

Have you been able to look for local damage e.g. crushing or rot ?

 

[DownWest]

That will be siTka spruce. Best way to correct it will be run it down on a bandsaw in the fore and aft plane and reglue it with epoxy while clamped to a straight bench/beam. It is not important to get a dead straight cut or plane the faces as it will fit anyway. This is advice from Herreshoff, who knew a bit.
A

To add: If the shrouds go to the top ( I assume they do on such a short stick) then the curve could cause the mast to fail due to the compression under high loads. But I doubt it, so I would do it anyway for cosmetic reasons as I would hate to spend my time checking the flow of the wind with that in view.

 

[oldfrank]

... or you could try hanging the odd weight on it to see if it'll straighten. It could just be the result of unsupported storage?

 

[Bunchats]

Bend in mast

We too have a mast that has a curve in it all along its length (22").
We have decided it will be fine when the mast is up and shrouds are tightened to make the mast straight. This is due to the fact that when leant against an object its elasticity is such that it bows.

We bought the dead straight timber and brought it home on the roof-rack. Three hours travel in the wind is what bent the timber and we worked around that.

Moral: cover timber up when you want it to remain straight on a roof rack journey - we hope!

Any thoughts anyone.

 

[tonijon]

I agree with old frank or the other way of doing it is hang the mast with the curve of the bend facing upper most and leave it for a few weeks keeping a regular eye on it, and you may find that gravity will have straighten out the majority if not all of the bend I have managed to do it on my 37ft mast although the bend wasnt as extreme.

 

[Bunchats]

On the bend

Thanks for the reply Tonijohn - I did do that after discovering the bend in the orignal, untrimmed length. even hung /stood five and one, gallon water containers at televant places but after two months nothing.

this made me decide to make the mast with stern bend and pull that forward (2-3") to straighten it when it is on the boat. You could call it pre-bend!

That's what I'm going to work with now and think it will be fine.

Again, thanks.

 

[KAIROS]

Straighten a mast......

I had a similar problem with a mast. Here's what I did to fix it......relatively simple......

A Possible Solution:

On my boat, the starboard upper shroud had been twanging taught while the port one was slack. The starboard turnbuckle was wound in to it's limit, while the port was out. Still the mast was curved above the spreaders. The lower part of the mast was in the right position (straight and vertical and centered).

Ashore, with it well supported from heel to masthead, it had a bend of about 8-inches to port, but only above the spreaders. That's 8-inches in about 17 feet.

We rolled it on it's side well supported for it's whole length EXCEPT the upper part where the now upward bend was. For months we hung 75 lbs on the masthead hoping the upper mast curve would be straightened by the relentless force. We had a yardstick for reference affixed next to the masthead so we could see and record where it bent to initially. We expected over time that the solid (actually 4 glued up quarters in section) spruce mast would slowly give in and relax toward straight.

After months of weight on the unsupported masthead, it hadn't budged. The reading on the yardstick hadn't changed one little bit. I was thinking of various complicated schemes - some involving steam.

My friend (a complete lubber) says to me....."how about using an electric blanket wrapped around it covered with a space blanket, where you want it to bend." He'd bent some wood for some lubberly application, and had the impertinence to tell me to skip all that steaming and moisture nonsense....heat alone will relax the wood.

It worked. It was amazing. Sure you can see it work on a stick heated all around over a campfire, but I never held a mast over a campfire.

Imagine the mast laying on sawhorses, the lower mast well supported, weight hanging from masthead, a sawhorse under the upper mast where we wanted to focus the bend. The yardstick was still standing there telling us nothing had changed in months, and when we removed the weight the upper mast would spring back into it's curved shape.

Now wrap a 7-ft section of the mast with the electric blacket, straddling where you want to focus the straightening force (drawing, below). We put a digital thermometer inside the bundle against the mast to keep track of the temperature.

For the first bend we used sawhorse #1 (only, no #2) under the upper part of the mast, so that we were trying to straighten the lower part of the bend. We set the blanket on heat setting 8 of 12, and watched. The temperature inside hovered at about 150-degrees (F). Within 2 hours the weight had brought the stubborn masthead down 3 inches according to our yardstick.

Next we added saw horse #2 to focus on the upper most part of the bend. The blanket was moved so it was nearer to the masthead than shown below.

Tips:
1. Be careful that the electric blanket kindling temperature is not reached....the space blanket apparently really holds the heat in.
2. Allow the mast to cool before removing the weight. It will spring back more if it cools unfettered.
3. The mast will spring back SOMEWHAT after days, in the direction of the original curve. So, you need to 'overbend' it. How much? Good question. Our first 'overbend' concerned me since I thought we had gone too far (how much can you torture a mast?). We just bent it and let it cool then bent it again.

When it is ashore laying on saw horses, it still has a slight bend. By rigging the upper shrouds to the mast as it lay there, we could see that by tightening the turnbuckle on one side 1/2-inch more than on the other side, the upper mast would be straightened. On the boat, this worked as expected.

Now the mast stands straight with only slightly more tension on the upper shroud on the one side. We'll see if it continues to behave.

[IMPORTANT: THE MAST DISCUSSED HERE WAS GLUED UP USING RESORCINOL GLUE, WHICH IS NOT AFFECTED BY THE HEAT INVOLVED HERE. IF YOUR MAST WAS BUILT USING EPOXY, THE METHOD DISCUSSED HERE MAY MAKE IT COME APART]

 

[Bunchats]

Going straight

Thanks for that Kairos - I will try it over the winter and am certain it's going to work.

 

[electrosys]

So much depends on the setup - that's why a photo would be useful.

If it's possible, I'd be inclined to leave the bend alone, and simply rotate the mast through 90 degrees, so that the weight of the gaff and sail tend to straighten the mast when rigged.