Deck Deflection around mast base

[boatingworld2004]

Hello

I wonder if anyone has exprienced this issue. I have a steadfast 24 motor sailer with a masthead rig and when we came to tension the rig we found the deck deflecting down in a forward aft direction. The deflection was about 5mm in the centre of the mast foot. Does this present a problem? I have taken some tension off the rig for the moment and things got a bit better but now I'm unhappy with the rig tension!
Maybe some more support form under the mast or a large deck plate? The deck has a piece of ply glassed on the back which goes full width to the hull deck join but this seams a bit of a design weakness?

any thoughts greatfully recieved

 

[wongaero]

Don't you have a pole in the cabin then?

 

[boatingworld2004]

no afraid not! Thinking of installing one !

 

[wongaero]

Right! so the mast is held up by, um...

My dad's old Samphire had a 2" stainless pole and he even managed to bend that - it was slightly offset though I think...

 

[Tranona]

This is quite common in boats that do not have enough stiffness in that part of the deck. A mast pillar is one option, but they often restrict access forward, and you need to be sure that the bottom goes onto something solid like a floor.

Alternatives are to laminate a stiffer beam under the deckhead, perhaps supported by two posts, one either side of the passageway and picking up on the berth or seat fronts. I installed a similar arrangement when I removed an obstructive mast compression post

Hope this helps

 

[saltwater_gypsy]

If you can't open the heads door. your rigging is too tight!
A pole, or two with a beam between as suggested above will solve it. Other essentials are a car jack to push the deckheadt back up and some extra lamination to strengthen the stuff thats been weakened by bending.
Will only take a few minutes (NOT)!

 

[vyv_cox]

<<< If you can't open the heads door. your rigging is too tight! >>>

Au contraire, I am told that a Sigma 33 will not win races unless the rig is tight enough that the heads door won't open/close. Another good reason for not buying a heavily-raced boat!

 

[boatingworld2004]

I thinking of tackling this job later in the year ( when it's warmer for epoxy to cure etc!) what does everyone think about using the rig in the current state. With the slack rigging the mast seams to move a lot in the centre section. I guess bashing into a head on sea with the mast shaking away is less than ideal?

 

[dulcibella]

Sounds like a bad idea to me. If it were my boat, I'd bite on the bullet and do the upgrade of the mast support as soon as possible, then enjoy the season with no fear of any nasty structural failure.

 

[Tranona]

You could try a temporary compression post using an Acrow if there is something solid at the bottom end to take the load.

 

[Sans Bateau]

Is there any reason why you cant fit a compresion post (mast post)?

If fitting one will not restrict movement etc, then that will be the best solution. But it must be stepped on the keel, or something that will transmit the loads to the keel, you not want to bend a hole in the hull, do you?

 

[boatingworld2004]

thanks for the comments

A compresion post seams a sensible option. Without the pole being in the centre of a small walkway into the forward berth, i guess i would fit it at the side of the walkway with the pole contacting the underdeck about 10cm away from the centre of the mast. Having a plate on top of the pole to spread the load may also feature in the plan. I've just looked at a boat that has had a similar problem and the solution was a large stainless plate on deck under the mast foot?

Might use both approaches

any thoughts on this as a solution?

 

[Sans Bateau]

Whatever you do, remember the downward pressures on the deck from mast under load, increased with transmitted sail loads will be enormous. Just make sure the loads are transmitted to very strong parts of the boat to avoid further damage/failure.

You might do well to get a rigger or boatbuilder to take a look and provide confirmation of your plan.

 

[boatingworld2004]

I've been doing some reading about mast compression! What sort of load is applied to deck with this sort of rig when the rig is fully loaded? Is there a way of calculating this ? Had the boatyard owner round for a look and chat and he suggested the pole solution as well. I need to know the loads involved to sort out the pole size?

has anyone done this sort of mod before?
cheers

 

[Tranona]

I expect there is a way of calculating the load, but by observation a 50mm diameter stainless tube is commonly used. The post I removed from my boat was actually wood just under 50mm square and my rig is much bigger than yours. The important thing is to spread the load both ends. Under deck you could either laminate in an extra beam or a pad that is about 100mm larger all round than the mast shoe. At the bottom end you will also need a pad, fore and aft step or transverse floor depending on the shape of the keel area. The important points are to locate the ends of the tube securely and spread the load over the structure.

 

[ex-Gladys]

A few years back I looked at a Robber that had a deck compression, albeit due to rotting of the plywood core. It was going to be a big job (£3000 for a boat priced at £6000). PErsonally this is such a fundamental element of a boat I wouldn't put it off.