Forestay diameter criticality

[gartonjh]

I have a Cape Dory 27. I found reference to a forestay diameter of 7/32" and ordered a harken mkiv unit 0 ocean furler. Back at the boat, I discovered the previous owner must've bumped up the wire size during the 2016 re-rigging. This means I either need to downsize the forestay to 7/32 or return the unit 0 furtler and order a unit 1 and maintain the 1/4 forestay. The existing stay is in excellent condition and length will not be an issue as I will order extender plates to accommodate width of stemhead and potentially a future bow roller.
The difference in furler cost is not huge, but every dollar counts.
Is there significant strength lost by reducing to 7/32? Any issue with a 7/32 forestay and 1/4 backstay?

[atomvoyager]

7/32" wire and Unit 0 is the most appropriate size and is what I've used on a CD27 and my own 28' Pearson Triton. I would swap the 1/4" wire instead of buying a bigger furler that is heavier and more bulky. But I've only installed the MKIV, not the cheaper Ocean series so I haven't confirmed if the Ocean version has all around bulkier/heavier parts. You will need to confirm the clevis pin diameter for the upper toggle and order the appropriate sta-lok to fit.

Yes, there is less strength of wire between 1/4" and 7/32" but it's not an issue because 7/32 is more than adequate. Rigging wire should be tensioned to around 10% of the wire's breaking strength in order to prevent premature wire failure due to cyclic shock loading of a loose wire. If you use 1/4" wire then with that formula you should tension it to the point where the mast fittings and spar might be under more load than designed for. In practice, yes you can probably get away with 1/4" wire and just keep it set at 5-8% of breaking strength so as not to overload related components but it's not recommended. When the previous owner upsized the wire he was probably not aware of this or felt it was a non-issue. As for keeping the backstay to 1/4 with 7/32 headstay, there will be a mismatch on ideal tension because they tend to equalize each others opposing tension. So you need to tension the turnbuckles for the smaller wire size and let the backstay be less than the recommended tension. Lots of people have done this, including myself in the past, and had no problem but as a rigger I'm not in a position to recommend it.

Also, you need to confirm the lower toggles and link plates available for the unit 0 and 1. You may find one model is easier to fit to your stemhead than the other in this regard so take a look at all the parts before deciding.

[gartonjh]

Many thanks.
I've attached an image of my stemhead. You will see two holes furthest forward. The forestay was in the second one back when I bought the boat. You can probably see the wear from the toggle in the image. The width of the stemhead at these forward two holes (white and blue arrows) is 9/16" which is too wide for the Harken toggle. The width of the stemhead at the aft two holes (red and green arrows) is 7/16". While the Harken toggle will slide over this width, only the hole indicated by the green arrow will work as the hole indicated by the red arrow is too close to the shoulder where the thickness increases and will not allow the pin holes to line up.

I see plenty of evidence on the stemhead that the hole marked with green arrow has been used in the past for the forestay attachment point. It is about 3.5" further aft than the hole marked with blue arrow.

The question. Should I use one of the two forward holes in conjunction with link extender plates? Does achieving that angle strengthen the rig significantly?

Or, is it acceptable to drop back to the hole marked in green and attach the harken toggle directly there?

[atomvoyager]

If you already have the Harken long link plates the simple solution is to move it to the aft hole. The stemhead fitting is strong enough for that. It will slightly raise the clew and sheeting angle but that should be minimal. Or you could grind down the width of the stemhead and grind inside the Harken toggle until it fits. Although not recommended, the Unit 1 is so oversized that grinding the toggle would not make it fail on a 27' boat with 33' mast. And reducing the stemhead to 7/16 or 1/2" thickness still leaves plenty of strength. Or grind just the forward portion of the stemhead to use the harken toggle in the hole marked red.

If the Ocean is the same as the regular MKIV, assuming you are using the Harken long link plates, the Harken toggle goes at the bottom and the link plates attach to the drum. You could instead use standard non-Harken link plates which means the Harken toggle gets attached to the bottom of the drum, the turnbuckle is up inside the drum rather than below it, the link plates attach to the bottom of the Harken toggle, and the bottom of the link plates connect to the stemhead via any toggle you can find that is wide enough. You can see this in the video below at 1:28. A toggle like the one in the video probably won't fit the 9/16" stemhead width so when I recently installed a harken MKIV Unit 0 on a CD27 I found a double jaw toggle with a 9/16" width and used that:

https://www.riggingandhardware.com/c-10 ... e-jaw.aspx

Another option when using non-Harken link plates is not to use a toggle at the stemhead and attach the link plates with a stainless hex head bolt, Nylok nut and red Locktite. You already have the Harken toggle at the upper end so you don't really need two.

https://www.youtube.com/watch?v=fhpOquIADvQ