Mast questions.

[mystymike]

Hello.
1. What would the implications be in moving a mast 100mm forward of it's present position.? (to bring it squarely over the bulkhead strongpoint)

2. Would there be any detrimental effect in leaving a mast in the "pre-bend" state for the whole season or longer . Would this induce fatigue or ''memory'' in the mast material over time.?

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[GrahamSC]

Hi
You will almost certainly affect helm balance. What boat ?
GrahamSC

<hr width=100% size=1>Never recovered from loss of Tot, 1974

 

[Trevor_swfyc]

I think the effect of moving the centre of effort forward of the centre of balance would be to reduce weather helm ie the tendency to head up. Clearly heading up in a gust is a good thing so if you went to far and the boat headed off in a gust it could get a bit hairy.
The position of the mast is a design critical factor so I would leave well alone.

Trevor


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[boatless]

Mike, Hi

If the existing position is causing problems (cracks etc.), and as long as you leave the masthead where it is, moving the c of e approx 70mm isn't going to make a lot of difference. If the boat is a bit neutral at the moment you may notice a difference. Most adjustable mast heel fittings have at least 100mm travel.

As you'll be well aware, as long as you don't go past the elastic limit, there will be no appreciable permanent deformation. 99% of fractional rig boats leave pre bend in all year. Even with extreme pre bend, none of mine ever had more than 10 -20mm in 10 metres permanent set.

John

<hr width=100% size=1>the above is complete rubbish, probably.

 

[sofka]

could you reinforce the bulkhead/under the mast heel with a knee? effectively like a shelf bracket. a nice piece of hard wood or laminate strips together to a nice shape. This would transmit much of the down force into the bulkhead, thus you can leave the mast where it is, unless you want to move it of course. Presumably the bulkhead is designed to take the pressure of the mast.

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[mystymike]

Thank you for the replies.
The reason for the question. The centre of the mast foot is about 50 mm from the bulkhead. Last season I found that the bulkhead was distorting under mast pressure when tightening the rig. I have removed the mast and built in twin 50mm by 75mm compression posts, jointed into a new 36mm marine ply door arch with the required footing onto the hull, shaped and glassed in. the bulkhead is now very sturdy. I would like to centre the mast over the newly stiffened bulkhead as "belt and braces". I am wondering about the effect on the upper shrouds which are straight. I've considered the "knee" option also. I will do some checking Monday when I visit the marina. Mast is Kemp 8mtr and still on trailer in my driveway.

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[penfold]

That flexing sounds a bit scary; are you sure you're not putting too much tension on? Cruising rigs usually don't need to be that tight, and macwesters are fairly conservatively rigged.

flexure aside, would it be possible to fit a strut or post immediately below the mast, or would that obsruct the cabin? Also, now you have braced the bulkhead, I'd have thought the flexing will be unlikely to recur.

cheers,
david

<hr width=100% size=1>OMG, Schrodinger's cat is dead!

 

[mystymike]

Hello. The bulkhead flexing occurred whenever I put the bend at the top of the mast. The bulkhead is 12mm as to the spec. but the posts I have fitted back and front of the doorway, like door posts, are what I believe should have been fitted in the first place as a single post directly under the mast would block the doorway. I am satisfied with the structure now and it blends well with the cabin.
My Engineering brain won't retire with the rest of me so these little problems have to be resolved...................so there could be more of them. Thanks again. Mike.

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[VicS]

It sounds to me as well that you are overtightening things. This is an old fashioned design and would surely have not have had any bend in the mast, unless the original has been replaced with a modern bendy one more at home on a racer.

The correct position for the mast is surely in line with the cap shroud chain plates!

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[mystymike]

Hello.
Yes, you are right, as the mast is parallel section for the whole height. My boat has twin backstays which always feel "floopy" and it was in tensioning these that the bend took place and the bulkhead flexing was found.. It was a good way to find out little weak points.

The second question has more to do with a larger boat I have my eye on.

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[kds]

Prebend was really developed for racing to windard and, having done a few miles in them, I would say this is hardly the forte of the Macwester. You are asking too much of the bulkhead and overtightening the backstay. Twin backstays exert a lot of force as they are at much of a less acute angle to the mast and will feel floppy. A good guide is to never use more than handtightening on a bottle screw on a cruising boat.
ken

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[mystymike]

Hello.
I understand what you are saying about tension in the rig. I had the rig set to a little over handtight to take the shake out of the forestay when going to windward. The boat had really to be on the mark to prevent the shaking but once tensioned that little bit extra it was o.k.
I am not very experienced in these things but I read quite a bit and I have had a year of sailing her with enjoyment. I still believe that the ballast is light as my boat does not have the weight of bilge keels, just the standard 650kg lump of iron. Do you think that adding some more ballast will help her ( to compensate for the loss of the bilge keel weight) Thanks for your reply. Mike.

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