King post

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Novachris

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I posted a while back about mast compression in my 28ft masthead rigged boat,i have decided the best way to go is to support the mast under the deck by means of a king post, at the moment i have rigged up a temp 4x4inch x 6ft length of timber but my question is what dimension of timber would be suitable as a permanent fix? i dont want to use stainless or any other type of steel.
Thanks
Chris.
 
That is a massive post. The one on my 26 footer was 2.5*2.5, but it does depend on how stiff the coachroof is. If you are relying solely on the post then it obviously needs to be more substantial than if it is just extra support.
 
The compressive strength of dry oak, for example, is around 1000 pounds per square inch. Woods such as fir and pine are higher than this, up to 1,500psi, others a bit less, down to 800psi.
Your 4 x 4 will support about 16,000 lbs. The load on the post is likely to be in the 4,500 to 5,000lb range based on 4mm rigging tensioned to 25% of breaking load, plus the weight of the rig.
So, 3 x 3 would be more than adequate with a big safety factor.
 
Tranona said:
That is a massive post. The one on my 26 footer was 2.5*2.5, but it does depend on how stiff the coachroof is. If you are relying solely on the post then it obviously needs to be more substantial than if it is just extra support.
Click to expand...

The main support at the moment is the main and fore cabin bulkhead but i dont realy like this set up so any post would share the load with the bulkheads.

3x3 sounds about right and wouldnt be too intrusive,
thanks.
 
So long as you pay attention to the load bearing regions of the bulkhead and couple it to the bulhead, a 3 x 3 post is likely to suffice, particularly if the wood is a good straight, fine grained hardwood like mahogany. In an ideal world, I would make it from a 4 x 4 to augment the strength of the ends to avoid local crushing, waist the centre section to 3 x 3 and cap it with hardwood pads, again to put any local crushing loads across the grain of the pad rather than along the grain of the post.

Rob.
 
Novachris said:
The main support at the moment is the main and fore cabin bulkhead but i dont realy like this set up so any post would share the load with the bulkheads.

3x3 sounds about right and wouldnt be too intrusive,
thanks.
Click to expand...

Not sure why you need a post if you have two bulkheads. You just need to find a way of sharing the load between the two. The tabernacle that Scotty shows is one way of doing it. You can also consider putting in longitudinal beams connecting the two inside. I did a similar thing on my boat - although it was between two beams when I removed the post to gain access to the forecabin.
 
Tranona said:
Not sure why you need a post if you have two bulkheads. You just need to find a way of sharing the load between the two. The tabernacle that Scotty shows is one way of doing it. You can also consider putting in longitudinal beams connecting the two inside. I did a similar thing on my boat - although it was between two beams when I removed the post to gain access to the forecabin.
Click to expand...

Have you any pics? There is some sort of cage made out of ally angle iron joining the two bulkheads at present neatly done all tidy under the headlining but i dont like it and would rather extend the mast down to keel level via a king post,I prefer a belt and braces set up.
 
Novachris said:
Have you any pics? There is some sort of cage made out of ally angle iron joining the two bulkheads at present neatly done all tidy under the headlining but i dont like it and would rather extend the mast down to keel level via a king post,I prefer a belt and braces set up.
Click to expand...

One has to ask the question - why do you want to do it? The boat is 30 years (at a guess) old and has managed so far without a post. Is the deck compressing by way of the mast? Is the mast moving? Is there any sign of the bulkheads breaking away? If none of this is happening then you don't need a post - the designer did his job right and made that area strong enough.

Why don't you like the current arrangement - sounds perfectly sensible to me. - much better than a big post obstructing access to the forecabin.
 
Tranona said:
One has to ask the question - why do you want to do it? The boat is 30 years (at a guess) old and has managed so far without a post. Is the deck compressing by way of the mast? Is the mast moving? Is there any sign of the bulkheads breaking away? If none of this is happening then you don't need a post - the designer did his job right and made that area strong enough.

Why don't you like the current arrangement - sounds perfectly sensible to me. - much better than a big post obstructing access to the forecabin.
Click to expand...

I have noticed a slight crack in the main bulkhead(two piece) at the top of the arch in the doorway which i have pencil marked, it has "grown" about 1/4"since the start of the season, the doorway is offset so that the mast rests on solid 2" ply,this area has been beefed up with an extra 4x2" timber bolted onto the ply going down to the ballast keel, so in effect a king post.
If i remove the heads door and put up a curtain the post will be a handy extra hold when sitting on the thunderbox in a seaway.
 
Think I would investigate why after all these years the crack has appeared. Further beefing up of the bulkheads to transmit compression into the hull structure might be an alternative, but I guess if it does not restrict access a post is a simple solution.
 
Tranona said:
Think I would investigate why after all these years the crack has appeared. Further beefing up of the bulkheads to transmit compression into the hull structure might be an alternative, but I guess if it does not restrict access a post is a simple solution.
Click to expand...

I put a plumb line on the main bulkhead and it is not vertical it is splayed aft about 10% but it is glassed in strongly to the deckhead and deck and has not moved.

I know you cant get true readings whilst afloat but the other bulkheads are smack on.

Thanks for your advice, on haul out i will remove the mast and put it all right as the designer designed it.

but had a wonderfull sail last night and felt a bit more happy that the mast was not going to come crashing down.
 
I have done a major repair with this sort of problem.
If without a king post the important thing obviously is to transmit the loads from the mast down through the bulkheads to the strength in the bottom of the boat.
To raise the deck up that small amount to its original position, may be possible from inside, using a post with pad at the top and bottom, with a car jack.
I managed to get things to their original position and then rebuild the original, putting in new oak laminated beams and utile uprights down to the bottom of the boat.
The steel maststeps as in my link on previous posting, then spread the load from the outside accross the position of the bulkheads.
For ease of cutting out of the weak timbers I used a Fein Multimaster occilating cutter (Bosch and others now do a similar cheaper tool) which meant I could get into difficult areas with great ease, saving time and enabling a better job to be done than would otherwise have been possible.
Good luck
PM me if I can be of help
S.
 
Last edited:
Thanks for your advice Scotty,

Your pics of the mast foot look good and have given me food for thought.

As an engineer by trade my carpentry skills are basic,i think with the mast down i will jack up the coach roof as you say about 1/2",cut the bonding to the bulhead, get it vertical ,rebond it and go from there,i may make a sort of brace out of stainless thread bar and plates joining the two bulkheads at deckhead level,what do you think?

I still like the idea of a king post as long as its not to obtrusive,it sort of makes sense.

chris.
 
Novachris said:
i will jack up the coach roof as you say about 1/2",cut the bonding to the bulhead, get it vertical ,rebond it and go from there,i may make a sort of brace out of stainless thread bar and plates joining the two bulkheads at deckhead level,what do you think?
Click to expand...

Hi Chris.
without doubt a king post can add tremendous strength.
Not sure I can visualise your other plan.... sketch perhaps?
S.
 
Use aluminium scaffolding pole and disguise it with wood. Be far better and stronger and you would never know done properly
 
Novachris said:
Hi scotty,
I have drawn up a detailed plan that i will share as soon as i can work out how to post it on here.
Chris.
Click to expand...


Given up tyring to figure out how to post plans on here,
anyway it involves beefing up the area under the mast incorporating the two bulheads to form a strong cage like structure.
 
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