I need help for design of the compression post

[Djubaca]

Hello,

A little history :
I have a fiberglass Carter 32 with keel stepped mast wich was in very bad condition and it was retired. I bought a new mast which is deck stepped.
Now i have to build a compression post .

I will make a compression post from SS tube with diameter 89x4.

Is it necessary to do some strengthening to the top of the compression post to transfer a moving force to the hull of the boat.

Attached some drawings.

 

[sarabande]

I would certainly consider some re-inforcement immediately under the base plate of the new mast to help spread any lateral loadings (e.g. when heeled) across the deck to the sides of the hull. The webs you have drawn at the top of the post - are they held steady by the red box section ? That seems a bit inelegant, and a glassed-in tube under the deck running from P to S to act as a deck beam might leave you more head room.

 

[boatmike]

Your design appears sound enough. There should be very little sideways load on the point where the mast joins the post. Consider your keel stepped mast. This passed I assume through the deck with some form of gasket at the join which took no load. The sideways load should be taken entirely by the rigging in both cases and this does not change when the boat is heeled. It is common to have a small amount of lateral movement in the mast. With a keel step the point where the mast meets the keel provides a "hinge point" and the deck gasket allows clearance. With a deck mounted mast this hinge point is transferred to the bottom of the new mast where it meets your post and this is normally stepped to allow a small amount of movement. The post and the mast therefore are not rigidly braced vertically where they join and angular movement of the mast either sideways or fore and aft takes place at this point so that no lateral loads are transmitted to the post. All that is necessary is to maintain the post in a vertical position by cross bracing and usually the monocoque structure of the deck is quite sufficient to avoid any buckling forces provided that your post is positioned accurately in line with the angle of the mast. I would consider however incorporating a square plywood plate into the design. This should be large enough to spread the load across the deck and glassed in to it between the post top and the deck underside. The new tabernacle to accept the mast should be bolted through this and the deck to the plate at the top of the post. Hope this is clear.....

Mike

 

[30boat]

As boatmike says a plywood plate could be glassed over the coachroof under the mast sep.IMO the post doesn't have to bee stainless.Wood is perfectly adequate if properly sized.That's what my Fulmar 32 and many other Westerlys have without any known failiures.Mine is mahogany later ones are probably teak.It is important hovever that the base of the post is kept dry.I should add that being fractional my rig is more highly loaded than an equivalent mast top rig like yours.

 

[Djubaca]

sarabande - the deck is not horizontal and straight and easies way to do bending beam with rectangular section is by welding ( there is no easy way for me to bend a tube ). Post is in the toilet so I don’t care much about elegancy ( btw all materials are SS 316 L )

Mike, I don’t inconvenience about lateral forces. Main question is about the moving force ( I think it is the biggest horizontal force in steps of mast which is pushing the hull forward ) . In keel stepped mast it is transfer directly to the steps . In deck stepped it is in tabernacle which is in deck level.

So if I understand clear, I have to do something like attached file. And it is no need to do anything else ? ( Of course this elements will be strong enough )
And another question. Is the post have to be rake in line with rake of mast ?

Thank you
Regards
George

 

[boatmike]

I think we have a language problem here so I will try to make it simple.

1. The post should be in line with the mast. If the rake of the mast is one way or the other so should the post be to be in line with it.

2. The post should be straight, not bent. If you draw a line through the centre of the mast from top to bottom this line should run right through the centre line of the post and finish directly below it at the keel.

3. The mounting for the tabernacle which is the component that holds the mast above deck level needs to be at 90 degrees to the post not at an angle. If the deck is angled then you need to build it up top and bottom with glassed over tapered plywood to achieve a 90 degree mating face both below and above the deck.

4. The deck itself should ideally be solid and the bolts should go through the flange at the top of the post, through the inner plywood, through the deck and through the tabernacle base. If the deck is not solid but foam or balsa cored I would be inclined to remove the core from below and glass the inner plywood directly to the outer skin. If there are any upstands or flanges that the original mast gasket fitted around they should be removed. From the post top to the tabernacle bottom should be solid.

Hope this helps

Mike

 

[LittleSister]

There should be very little sideways load on the point where the mast joins the post. Consider your keel stepped mast. This passed I assume through the deck with some form of gasket at the join which took no load.

I am not convinced there is 'very little sideways load' at the point where the mast joins the post, and I suspect this is what the OP is questioning.

As I see it, under sail the whole boat is being dragged through the water by the chainplates, backstay fittings, stem fitting, the main and foresail sheet blocks, and the base of the mast. When the mast was keel stepped, the stiffness of the mast took the lateral loads from the lower part of the mast down to the keel, and the boat was being dragged from there. (The rubber gasket at deck level took no load in that partucular case.) With a deck mounted mast that's not the case - there's no such stiffness (even you called it a 'hinge point'), and the propulsion loads on the lower part of the mast (lower part of sail, forward part of boom, etc.) need to be transmitted to the boat here instead of at the keel.

The compression post holds the weight of the mast and counters the downward force of the stays but, unlike the section of keel stepped mast passing through the cabin top, does nothing to handle the lateral, propulsion loads, which must be greater than either those through the mainsheet or that at the masthead. Quite how much of a structure is needed to handle them i wouldn't know, but they are not 'very little'.

 

[30boat]

I am not convinced there is 'very little sideways load' at the point where the mast joins the post, and I suspect this is what the OP is questioning.

As I see it, under sail the whole boat is being dragged through the water by the chainplates, backstay fittings, stem fitting, the main and foresail sheet blocks, and the base of the mast. When the mast was keel stepped, the stiffness of the mast took the lateral loads from the lower part of the mast down to the keel, and the boat was being dragged from there. (The rubber gasket at deck level took no load in that partucular case.) With a deck mounted mast that's not the case - there's no such stiffness (even you called it a 'hinge point'), and the propulsion loads on the lower part of the mast (lower part of sail, forward part of boom, etc.) need to be transmitted to the boat here instead of at the keel.

The compression post holds the weight of the mast and counters the downward force of the stays but, unlike the section of keel stepped mast passing through the cabin top, does nothing to handle the lateral, propulsion loads, which must be greater than either those through the mainsheet or that at the masthead. Quite how much of a structure is needed to handle them i wouldn't know, but they are not 'very little'.

I would think that some doubling of the coachroof with glassed in plywood would take those loads well enough providing the area covered is large enough.The coachroof on boats with keel stepped masts is usually as strongly built as those with deck stepped masts.Most likely no reinforcement is needed but for peace of mind it could be done.

 

[William_H]

I would have thought the structure of the deck around the original keel stepped mast would be quite strong. It has to take the weight of people standing on it plus possibly halyard turning blocks. This question is a bit subjective however and OP must feel confident in the end result. I might suggest a few layers of carbon fibre layed to the inside of the deck out to a fair distance from the mast. Easier to lay on top of course but then has to be painted or similar to make it look good. Even then much easier than plywood.
The sideways loads will be very low while mast stays straight in compression but huge if it starts to buckle. I don't believe there is much drive for the boat transmitted via the mast. Only some drive from the mainsail when running.
As a matter of interest most boats around here have to lower the mast so hinge at deck level is needed. Boats built with keel stepped mast (usually fractioanl rig) have a hinge put in the mast about 30 to 40 cms above the deck. This consists of 2 flanges welded to the mast either side of the cut in the mast but at 45 degrees. Back side has the hinge front has 2 bolts clamping the flanges together. This means the mast has integrity and bend characteristics remain as original. ie bend comes from keel to the top with movement at the deck level. good luck olewill

 

[dur]

Is not the principal advantage of a keel stepped mast that it is supported laterally at the deck (by the partners)? This is what lends additional stiffness to the section of the mast between the deck and the lower rigging. A deck stepped mast can effectively pivot at the deck which conversely requires extra mast strength for that lower section. You would expect a deck stepped mast to have to be a stiffer (perhaps greater diameter) pole than for a keel stepped version.

 

[savageseadog]

Make sure that your deck stepped mast is bolted or pinned to the mast step as the foot of the mast can jump off the step in bad conditions.

 

[30boat]

Make sure that your deck stepped mast is bolted or pinned to the mast step as the foot of the mast can jump off the step in bad conditions.

For that to happen you'd need a very slack rig or a very bendy hull.

 

[Debenair]

Or hit a Dutch bascule bridge with your main shrouds as happened to a boat we were accompanying in the IJsselmeer some 20 years ago.
The mast on the 45 footer was stepped in an SS cup about 50mm deep and supported by a post some 150mm diameter.
The mast "jumped" the cup and the post was permanently deflected some 20mm out of vertical.

 

[William_H]

I don't agree at all. The keel stepped mast is that much longer so more susceptible to bend. The chocking or "partners" at the deck would mean that part of the mast is located laterally so no different to a keel stepped mast. But many keel stepped masts on fractional rigged boats are not chocked at the deck so that the mast can bend keel to top.
the only thing you cnaa say about a keel stepped mast that is chocked at the deck is that the stiffness of the mast will tend to hold the bottom section stiffer. This might seem good but the lower shrouds should hold this point located laterally anyway. But any movement of the mast above the deck will result ina tendency to bend (break) the mast at the deck. So i have never seen any point in a keel stepped mast except perhaps cheaper for the builder. good luck olewill

 

[boatmike]

I don't wish to be rude but there is a lot of unqualified opinion expressed in this thread which the OP needs to discount.

Firstly, with a keel stepped mast there should normally be NO mechanical connection between the deck and the mast on a fully stayed mast. The only exception to this is an unstayed mast like the freedom rig or one where the deck is deliberately designed to take sideways loads and is as suggested chocked to the mountings. This is not such a case and such considerations are not relevant.

Secondly, with a deck stepped mast the only load taken on the deck itself is a COMPRESSION load. All side loads are taken by the rigging. Remove the rigging and the mast falls down!

IN THIS CASE the only consideration the OP has to take into account is that the mast step must be securely located to the deckhead by any means be it a conventional pinned socket or tabernacle and that this in turn is securely mounted on the top of the deck and fully supported underneath by a post transmitting the load through to the keel.

All other considerations are simply confusing the issue and not at all relevant.

And before some pedant points out that when sailing the sails impart a sideways loading component on the bottom of the mast, these loads too should, in a well designed rig, be taken by the lower stays and any resultant component on the base of the mast will be too small to worry about compared with the monocoque stiffness of the deck.

 

[William_H]

Well yes and no. Obviously the deck itself must provide lateral location of the step. In the extreme of no deck then the mast step might allow the mast compression post to get out of column so colapse. But the deck itself in sideways load will be both in compression on one side and tension on the other if there is any tendency to move sideways or fore and aft and get out of column. So my suggestion to OP's concerns that this strength of the deck in lateral movement is not so critical but must be considered. olewill

 

[boatmike]

This IS now getting pedantic, but of course any small side loads on the deck will cause a horizontal component which will result in relatively small compression and tension loads in the deck horizontally. These loads are however tiny compared to the VERTICAL compression load which is frankly all that the OP needs to consider in this design. The other HORIZONTAL load and probably greater than the loads across the beam of the boat are fore and aft loads. These however are also relatively small as much of this load is transmitted through the stays and boom downhaul. Also most decks are considerably stiffer in the fore and aft direction than they are athwartships so this load, being relatively small is not a consideration in this case.

Let's keep to the OPs question and not get academic about detail. The primary and only consideration the OP needs to concern himself with is VERTICAL COMPRESSION load. This, with consideration of the weight of the mast itself, and more relevant still the downward force of the rigging in tension, is massively greater than any horizontal loads on the deck.

 

[LittleSister]

These [horizontal] loads are however tiny compared to the VERTICAL compression load which is frankly all that the OP needs to consider in this design. .... The primary and only consideration the OP needs to concern himself with is VERTICAL COMPRESSION load. This, with consideration of the weight of the mast itself, and more relevant still the downward force of the rigging in tension, is massively greater than any horizontal loads on the deck.

Yes, the vertical loads are greater than the horizontal loads, but that doesn't mean that horizontal loads can be ignored. Imagine the mast end of the boom and the lower mainsail not being attached to the mast, and trying to hold it in place by hand when you're beating (or even reaching or running). It's unlikely you could on any but the smallest of boats. Where is that force normally going? It is distributed by the mast: partly to the lowers, and mainly to the deck (in a deck stepped mast, or a keel stepped mast chocked at the deck) and to the keel in a keel stepped mast which passes freely (save for a weather-proofing gaiter) through the deck.

The OP's compression post will handle the vertical, compression loads. His very reasonable question was 'Is it necessary to [also] do some strengthening to .... transfer a moving force to the hull of the boat'. I don't have the knowledge to say whether the existing deck structure will be adequate to transfer this force, but I do know that the answer is not that there is no such force.