The Chain Plate Project

The chain plates seem to be a project within a project, requiring their own design and special tooling, so I have decided to have a dedicated page for this installation.  beginning on the 27th September 2011, the project is ongoing.

Original Vivacity Chain Plate!

Original Vivacity Chain Plate... frightening!

The Chain plates required their own design.  The  original Vivacity chain plates left a lot to be desired, and so I started thinking about what needed to be done.  I had several requirements for my new chain plates and their method of securing.  Requirements were as follows:

Carries the loading to the hull

The old U Bolt fittings were woefully inadequate in this regard.  They were attached to the deck only, and it was only the hull to deck joint in way of the U bolt that carried any load to the hull.  I have seen several examples of Vivacities with horrible deck uplift in this area.  This arrangement had to go, hence the hull to deck modifications.  Any new chain plate would have to be a calculated and designed item that carries the loadings into the hull in an engineered manner.

Correct Orientation

The chain plates have to orient fore and aft to look right on the existing deck chain plate upstands.  This makes things trickier as I could have just used traditional chain plate "knees" if transverse orientation was possible.  It is, but I feel the chain plate just would not look right on top of the upstand if it were.  Call me fussy!

Stronger than Wire Break Load

As discussed above, they have to be considerably stronger than the rigging wire break load.  The wire in use will be 4mm.  4mm wire typical break load is approx. 12.6kN, equivalent to a 1284kg, or 1.3 tonne pull.  Saving you the maths and naval architectural mind numbing reams of calcs, I have checked this and it is the right wire to specify for the Vivacity.  Peak Righting Moment is approx. 5.6kNm which translates to 4mm stainless 1x19 wire having a factor 3 safety in the Vivacity application.  My chain plates are designed to have a factor 4 safety on wire break load, i.e. they would fail at 5kN or 5.1 tonnes!  This margin of safety more than covers any poor bonding, and distributes the load into the hull over a very wide area, making the proposed arrangement very much superior to the original Vivacity offering.

Visible/Removable for inspection

A lesser requirement, although important.  The key fabrications that make this design work are visible within the cabin for inspection.  If they ever need removing, this can be achieved by unbolting and extracting the chain plate from its pocket.

Invisible on the outside of the hull

Call me vain, but there is no reason good engineering can't be functional and pretty!  Whenever I design something I always try to see the neatest, most elegant solution, and I am reminded of Theo Jansen, the Kinetic Sculptors words:


"The walls between art and engineering exist only in our minds"

So my answer is the following.  Have the chain plates on the inside of the hull, but through bolted to a substantial moulding.  This keeps things off the outside, keeps the right chain plate orientation, has been engineered to be massively strong, and can be dismounted for maintenance should it prove necessary.  It also puts all the load through the hull and takes the strain off the deck moulding.  Here is the plan...

Current Chain Plate Design

You can see the full drawing in the archive section, and here:  Chain Plate Design.pdf

The calculations that went into the design of the chain plates can be seen here: CALC PACK TO BE ADDED

The Build!

So with that, all that was left to do was build them!  My initial thoughts were to build a mould so I could produce up to 6 units quickly and repetitively.  To this end I developed a plug with MDF.  The plug featured a rebated section that takes a slightly oversize piece of steel to get the right size for the chain plate.  I shaped a plug in MDF, then attached it with hotmelt glue to an MDF back board.  I then used resin to seal the surface of the MDF to ensure it would not absorb gelcoat.  I waxed this too, then gelled and laminated the mould up.  I had a lot of trouble releasing this mould as I didn't wax enough, or let things cure enough!  I was rushing, and I know better....

Setting up a plug Setting up a plug Setting up the rebate depth with my circular saw

As I tried to de mould the new tool, it was clear that it had gained a chemical bond with the underlying plug... doh!  Oh well, you live, you learn...  don't rush these things!  I regrouped and resolved to build a better mould the next day...

Rebate Depth Rebate with mocked up chain plate What its going to look like inside the cabin


What its going to look like inside the cabin Resin coated plug... bad idea! Resin coated plug... bad idea!


Plug gelcoated Plug laminated up... Epic fail!!!

The mould had unfortunately bonded to the plug and was not letting go without a fight....  I could have spent an age grinding and refinishing the tool, but I resolved to find a better way... therefore....


Chain Plates take 2!

This time I resolved to use a superior technique.  I used parcel tape for a mould release this time, and I waxed the hell out of it!  The results were much better and I produced a useable tool this time.  After a little sanding to remove rough edges and an aceone wipe got rid of the wax.  I used parcel tape on the tool face this time too just to be sure, then waxed that as well.

Setting up the new plug Setting up the new plug Setting up the new plug - release tape added

I used hot melt glue to hold things together again, and things were looking better with a bit mote attention to detail this time.

Gelled up and ready for laminating Laminated with CSM & Surface Tissue, cut at the green stage of cure Excess cut away at green cure

The mould released cleanly and easily this time.  There was a fair bit of heat output from the mould as it was a fair mass and the exotherm was noticeable.

Mould demounted and looking not too bad First product!

First product!

So I set up the mould for producing the chain plates.  I applied the mould release tape to the mould surface and then waxed the hell out of it.  I laminated the chain plates with epoxy and biaxial cloth.  There was a fair bit of heat build up as the cure progressed again as it was a heavy lay up, but things went well.  So far 2 units have been produced and they are excellent.

Prepped for production Unmoulded product Product offered in for a look

To come: more production of chain plates, 6 in total, and then installation to the hull!


23rd March 2012

So with the vee berth now complete and access much improved, I have decided to drive on the chain plate installation somewhat.  To this end I have produced another 4 chain plates.  I opted to do these 4 in polyester and chopped strand as a cheaper option.  They are heavy enough that this will be fine.  I will use the two I have already manufactured in epoxy biax for the main shrouds, and use the remaining for for the fore and aft lowers.  The mould tool, low quality to begin with, has really deteriorated with repeated mouldings and I struggled to release the final two mouldings, but I have them out now!  Sanding them up now, and will be ready to install soon.  I will get some stainless chain plate stock ordered up and begin the fabrication soon.

Chain plates made up in polyester and chopped strand Chain plates made up in polyester and chopped strand



25th June 2012

So I managed lately to get one of the slots cut out for the chainplates using a drill.  Since I wanted to paint out the vee berth and take that area to a finish, I decided to get a move on with the installations.  I got a length of 25mm x 6mm flat bar in 316L stainless steel from Grampian Steel, who were very nice and delivered it specially.  Today, I decided I would have a go at getting the chainplate backing plates installed, so I got 8 chainplates cut out - 6 for use and 2 spares, and had a go at drilling and tapping them.  Needless to say I struggled with the drilling, so I drove down to Stonehaven and used the big drill press at the sailing club.  This went much better than my high speed drill!  I tried tapping one hole and that went fine, I will just have to take my time and use lots of coolant.  So thats where we are at as of today.  The next steps are to pair up a chain plate with a backing pad and drill out the backing pad to matched holes.  Then get the holes in th stainless tapped out to M8.  The chainplates are all cut to 250mm which is over long, but I will trim to suit when each is fitted.

Chain plates all drilled now



26th June 2012

So this evening I got 4 of the chainplates made up with holes in the backing pads and the stainless bar tapped.  Tapping the stainless was slow and careful work,even so the tap wanted to stick, "galling" itself in with the work going into the steel.  I was aware of the torque stress placed upon the tap and felt that it was only a matter of time before I broke the tap, even with the copious lubricant I was using.  I managed to get 4 chainplates made up before I broke the brittle tap leaving one end well and truly stuck in the chainplate.  Thats why I made spares!  I ordered a nice new high quality M8 x 1.25 tap set that cuts the thread in two stages, which should require less torque for a good cut.  Now that I knew the exact length required for bolts after an allowance for washers and over laminating of the backing pads, I ordered up 30 M8 x 1.25 x 25mm socket head button screws in A4 stainless, with the corresponding M8 Form C washers (20mm dia x 1.4mm thk).  I will probably install the backing pads one at a time with the make up bolts I have just now, then swap these out for the correct hardware once everything is bonded up and well cured.  Initial impressions are good.  Once can clearly tell that you could lift the whole boat with this set up if one so desired!  Indeed, the calcs show that this is more than possible.  Would I ever do it?!!  hmmm....  Might get the structural guys at work to check my numbers first!!!

Last Updated (Saturday, 07 July 2012 11:56)