paul-essex
Well-Known Member
a oak 4x4 fence post would do a nice job and cheapish
Mast Support Post
- Thread starter richardbrennan
- Start date
charles_reed
Well-Known Member
Almost makes me glad to have a keel-mounted mast - even if the original did pant itself into fatigue fracture just above the coachroof.
If you use wood, a thoroughly seasoned softwood 4x4" over 60" has a max load of 5500 -6500kg: a 4x4" structural hollow section x 10mm thick walls about 35000 kg. A very rough comparison as the wood is from US standards (and measurement) and the RHS from TUV SiS figures converted.
Converting the post to round 304 stainless would reduce the loading a bit, but I suspect you know which way you'll be going.
You're rather stuck with making use of a typical brewery pipework section and going over the top.
Do you know the mast compression numbers? I suggest that 3" schedule 5 having proved inadequate, you'd be safer going to a much larger section and a thicker wall.
Working by guess and gosh is very amateur.
If you use wood, a thoroughly seasoned softwood 4x4" over 60" has a max load of 5500 -6500kg: a 4x4" structural hollow section x 10mm thick walls about 35000 kg. A very rough comparison as the wood is from US standards (and measurement) and the RHS from TUV SiS figures converted.
Converting the post to round 304 stainless would reduce the loading a bit, but I suspect you know which way you'll be going.
You're rather stuck with making use of a typical brewery pipework section and going over the top.
Do you know the mast compression numbers? I suggest that 3" schedule 5 having proved inadequate, you'd be safer going to a much larger section and a thicker wall.
Working by guess and gosh is very amateur.
Last edited:
SHUG
Well-Known Member
This used to be a problem with the Sigma33 when they had Dacron sails.
As the sails stretched it needed more and more mast pre-bend to retain a nice sail shape. This was achieved by making the top shroud tension higher and higher until it becomes noticeable that the deck was "dished" as the wooden mast-post crushed up, and the bulkheads pulled away from the hull. This then meant a winter of remedial fiberglassing.
With the advent of Genesis sails , the problem was less acute as the sail held its shape without these extreme tensions.
The Sigma was generally a strong boat but for racing, the normal design limits are sometimes exceeded.
As the sails stretched it needed more and more mast pre-bend to retain a nice sail shape. This was achieved by making the top shroud tension higher and higher until it becomes noticeable that the deck was "dished" as the wooden mast-post crushed up, and the bulkheads pulled away from the hull. This then meant a winter of remedial fiberglassing.
With the advent of Genesis sails , the problem was less acute as the sail held its shape without these extreme tensions.
The Sigma was generally a strong boat but for racing, the normal design limits are sometimes exceeded.
richardbrennan
Well-Known Member
Update
The table has been dismantled, which was fairly easy, the mast is now down and the original post has been removed. I have spoken to Solent Welding who expressed surprise that the post bowed and think that it must be fairly thin walled to have done this; I am having the post delivered to them tomorrow and they will determine what the wall thickness is. They have suggested that rather than going for a wider diameter tube, I go for a thicker wall, which would not require any surgery on the table, so is an attractive option. However, I am not sure how the strength varies with thickness; would doubling the thickness double the strength or is this too simplistic, perhaps a question for Vyv? Also are there any other reasons why it would be unwise to go down this path?
The table has been dismantled, which was fairly easy, the mast is now down and the original post has been removed. I have spoken to Solent Welding who expressed surprise that the post bowed and think that it must be fairly thin walled to have done this; I am having the post delivered to them tomorrow and they will determine what the wall thickness is. They have suggested that rather than going for a wider diameter tube, I go for a thicker wall, which would not require any surgery on the table, so is an attractive option. However, I am not sure how the strength varies with thickness; would doubling the thickness double the strength or is this too simplistic, perhaps a question for Vyv? Also are there any other reasons why it would be unwise to go down this path?
Pagetslady
Well-Known Member
My Corsair has a mast support approximately 2 inch dia like yours it has the saloon table mounted around it, and it is solid mild steel bar covered with the same foam backed vinyl as the rest of the boat a much cheaper option and your table will remain original, surely that should be strong enough.
Mike
Mike
richardbrennan
Well-Known Member
Thank you Pagetslady; that's an option I might consider, especially as Solent Welding tell me a suitable tube is about £30 a metre and unless it's a stock size they will have to purchase 6 metres!
Chris_Robb
Well-Known Member
Hi, the one in the Oceanlord is definitely not solid - about 3 inches in diameter, I would be surprised if the Corsairs was solid. There may be some non stainless metals which could be stronger and cheaper than stainless - Hope one of the experts comes along soon.
West Coast
Well-Known Member
Not a structural expert here, but if the tube is bowing as per your original post, this is buckling and would only be effectively countered by a larger diameter tube (assuming similar steel material). Thicker wall tube of same overall dia would make minimal difference. If you are ashore, with old post removed etc, do the job properly and go for a larger dia steel tube!
vyv_cox
Well-Known Member
No, doubling the wall thickness will not double the buckling strength but it will help a lot. Increasing the tube diameter is a better solution from the strength point of view but could well involve more major modifications to associated equipment, e.g. the table. Be aware that in piping sizes up to 12" an increase in wall thickness goes on the outside, not reducing the bore, so there still might be a necessity to modify the table etc.
richardbrennan
Well-Known Member
Thanks Vyv
I will see how much I can increase the diameter and perhaps combine that with a thicker tube. I would like the advice of a surveyor, but the one I have spoken to so far was helpful but was not able to give formal advice on what is basically a structural engineering problem as it would not be covered by his Professional Indemnity Insurance.
I will see how much I can increase the diameter and perhaps combine that with a thicker tube. I would like the advice of a surveyor, but the one I have spoken to so far was helpful but was not able to give formal advice on what is basically a structural engineering problem as it would not be covered by his Professional Indemnity Insurance.
Pagetslady
Well-Known Member
Mine is mild steel and 2inch dia I once tried to drill a hole in the base to thread a cable up to the head lining. Having drilled a 6mm dia hole half an inch deep I am assuming it is solid,
theoldsalt
Well-Known Member
The mast support on my Moody425 appears to be a stainless tube 4"" diameter. I have been advised that inside is a mild steel pillar that actually holds up the mast and that the S/S tube is mainly for show.
30boat
N/A
Why don't the builders just use a lentgh of the same extrusion as the mast?It wouldn't cost much and would look neat .
