Chain plate sizing.

[GILow]

Hi all,

So, I’m going down the path of fitting external chain plates to my Kelly Peterson 44.

I’m not entirely sure what was used originally, but the accepted wisdom (from the kp44 fan site) for replacing the chain plates calls for a pair of 3/8 x 2 1/4 inch flat bars with doubling washers for the main shroud plates and 1/4 x 2 inch for the lowers.

Now my boat has had the plates replaced already (while she was passing through New Zealand twenty years ago) and 10 mm flat bar was used in place of 3/8 (logical enough) while the other dimensions 2 1/4 and 2 inch respectively were honoured, I assume so they’d fit the existing holes in the deck. There are no doubling washers.

Roll things forward, I no longer need to worry about fitting the holes in the deck, but I am running into trouble sourcing 2 1/4 inch flat bar. I can get the 2 inch (well, it’s 50 mm here in Oz) but the next step up is 75 mm (3 inch or thereabouts).

I would REALLY like to do everything out of the same size because the stuff is sold in 4 meter lengths which is pretty much exactly the right length for replacing all the chain plates, including the aft.

Cutting the bar down to 50 mm, or 57 mm in the case of the main plates from 75 mm would be hard work and probably end up looking pretty bad, not to mention expensive and wasteful.

So… thoughts…

Would 12 mm x 50 mm plates solve the problem? I feel like the answer is obviously yes, but sometimes subtleties can reach up from the depths and bite you on the posterior. (Or cause your mast to fall off.)

Any rules of thumb or pearls of wisdom for this stuff?

(And before anyone suggests it, yes, I know titanium would be great but I simply cannot afford it.)

Matt

[Wotname]

12x50 is 600 sq mm
10x50 is 500 sq mm
3/8"x2.25" is ~536 sq mm.

As you say, the tensile strength 12x50 is more than adequate.
The frictional force will be less unless you add more fasteners or perhaps larger diameter fasteners (more torque).

My guess is 2.25" was originally used because 2" was not big enough (frictional surface area wise) so I would be concerned about going even smaller (i.e going to 50mm)

Me - I would go either 114mm wide or add more fasteners or larger diameter fasteners (in that order).

[Searles]

To me the next number is the dia of the rigging screws toggle pin or bolt,the dia if the hole is normally not more than 1/3 the width of the plate ,all in all 2 inch by 1/2 should do it ,if in dought use 2205 polished bar .⚓️⛵️��

[GILow]

Originally Posted by Wotname

12x50 is 600 sq mm
10x50 is 500 sq mm
3/8"x2.25" is ~536 sq mm.

As you say, the tensile strength 12x50 is more than adequate.
The frictional force will be less unless you add more fasteners or perhaps larger diameter fasteners (more torque).

My guess is 2.25" was originally used because 2" was not big enough (frictional surface area wise) so I would be concerned about going even smaller (i.e going to 50mm)

Me - I would go either 114mm wide or add more fasteners or larger diameter fasteners (in that order).

So I’m going from chain plates that were attached to timber knees with a fibreglass shell inside the boat to external plates bolted through the hull to a stainless steel hanging knee in place of the old manky timber knees.

Would that address the issues of friction do you think?

Also, not sure what you mean by 114 mm wide plates? That’s VERY wide indeed.

Finally, would making the chain plates longer than the originals address the friction issue?

[GILow]

Originally Posted by Searles

To me the next number is the dia of the rigging screws toggle pin or bolt,the dia if the hole is normally not more than 1/3 the width of the plate ,all in all 2 inch by 1/2 should do it ,if in dought use 2205 polished bar .⚓️⛵️��

Is 2205 better for the job than 316?

And can I polish it myself?

Edit: a bit of googling suggests that you are correct, 2205 does seem better for the job.

[Searles]

2205 s/s is a better choice overall ,check on web sight of s/s types and uses,hand polishing any stainless to a good finish is a major job without the correct tools .⛵️⚓️

[GILow]

Originally Posted by Searles

2205 s/s is a better choice overall ,check on web sight of s/s types and uses,hand polishing any stainless to a good finish is a major job without the correct tools .⛵️⚓️

Gotcha.

I’ve got a bench grinder with buffing wheel. Would it be adequate with the correct buffing compounds?

(My labour is free. [emoji28])

[Searles]

Lots of fine sanding ,finishing with 600 wet and dry then much polishing compound,use heavy gloves ,watch out for heat buildup ,but doable .⛵️⚓️

[Wotname]

Originally Posted by GILow

So I’m going from chain plates that were attached to timber knees with a fibreglass shell inside the boat to external plates bolted through the hull to a stainless steel hanging knee in place of the old manky timber knees.

Would that address the issues of friction do you think?
Dunno, the frictional aspects would be (I think) somewhat proportional to the maximum torque applied to the fasteners; if the steel hanging knee allows for greater torque on the fasteners then it might be OK.

Also, not sure what you mean by 114 mm wide plates? That’s VERY wide indeed.
Very bad arithmetic on my behalf, I meant 57 mm. Dunno why I multiplied by 2 - sorry.


Finally, would making the chain plates longer than the originals address the friction issue?

Yes. Without doing the maths, I think they would only need to be about 14% longer if using 50mm.

[Manateeman]

We are not sure what you mean by double washers. Just at the top?
We’ve done exterior chainplates that look sort of like leaf springs . So the first layer near the hull is full length. The second layer starts inside the toggle and extends down one or two bolt holes. Kinda a laminated chainplate so it’s thicker at the top and tapers as it extends down.
We’ve only done these in bronze with button head bolts. The holes are broached square and we use full interior backing plates and extra heavy nuts.
We can cut threads so there is no thread in the chainplate area.
The nice thing about bronze is that you can bend the upper portion of the chainplate to match the turnbuckle angle and then the toggle is nice in line.
Mark

[Wotname]

^^ Nice

Lots of work but very nice!

[GILow]

Originally Posted by Wotname

^^ Nice



Lots of work but very nice!

Lots of $$$ too, alas.

[coopec43]

Originally Posted by GILow

Gotcha.

I’ve got a bench grinder with buffing wheel. Would it be adequate with the correct buffing compounds?

(My labour is free. [emoji28])

No trouble at all with a bench grinder!

I used a very fine abrasive pad to start the polishing then two grades of polishing compound(?)

[coopec43]

Originally Posted by Wotname

12x50 is 600 sq mm
10x50 is 500 sq mm
3/8"x2.25" is ~536 sq mm.

As you say, the tensile strength 12x50 is more than adequate.
The frictional force will be less unless you add more fasteners or perhaps larger diameter fasteners (more torque).

My guess is 2.25" was originally used because 2" was not big enough (frictional surface area wise) so I would be concerned about going even smaller (i.e going to 50mm)

Me - I would go either 114mm wide or add more fasteners or larger diameter fasteners (in that order).

I don't know why GiLow doesn't check out the laser cutting services in his State if he must have a specific width?

The bow fitting on my yacht was laser cut and that is about 8mm thick 316 SS. It wasn't cheap but a few 2M strips wouldn't take long to cut.

https://laserworld.com.au/laser-cutting-services/

[Reefmagnet]

Yep, laser cut. That's what I did. Got the bolt holes cut slightly undersized too, so just needed to ream the correct size drill bit through them to finish them off.

Cut from mill finish 10mm plate, I linished and polished to a mirror shine in no time.

Easiest way to do it.