Mooring and anchoring from the bob stay fitting

[Wind River]

I know there are several threads dedicated to this here already and this post is not really to discuss the merits or demerits of this practice but more of what would it take to make the hull structurally sound enough for this to be safe to do if I choose to do it? The most common statement I here when discussing this practice is “ That fitting was not designed for that”. So what would it look like if it had been?

I am no engineer and I have no way to calculate how thick the layup would need to be to withstand the shock loads and side loads this fitting would endure in practice. The current hull thickness at the bow is 3/4" give or take a little.

The pictures I have attached are of the current new bob stay fitting with proposed side plates that would be fully welded to the fitting to give it more lateral stability. I have up-sized the material of the main body of the fitting from 3/8" plate to 1/2" thick 316 stainless and the 4 bolts are 1/2" diameter. The side plate bolts will be the same and the plate thickness is ¼”. The other pictures are of the inside of the hull at the bow after it was opened up for inspection.

The stem of the bow was encased from the inside so the side plate bolts would have had to be through bolted with nuts and tapered washers on the outside. I never liked the look or strength of this so the design was not finalized and installed. After looking into the enclosed stem of the bow with a bore scope I could see there was wood inside and a lot of space around it. It didn’t seem to me this was a good idea. I cut the void open from the inside and found wood wedges just lying in the void. I could easily lift them out and I did. They did not seem to add any support to the bow except in the places the bob stay fitting was bolted through them. The fiberglass that covered them varied in thickness from 1/8” to ¼” and the bob stay bolts went through it with only washers to serve as backing plates.


Now that this area is open I have access to put nuts on the inside for the side plate bolts along with a nice backing plate eliminating this "useless?" wood. My question is one of engineering that I am not qualified to make and was hoping someone here from that field might be able to provide some guidance.

It is a difficult area to access but I could add additional fiberglass layup to the inside of the hull for an area of about 2 square feet on both sides of the hull. The bulkhead of the anchor locker going aft and the Samson post structure going forward would limit how much area I had to add glass to.

How much glass (thickness) would you suppose would be adequate to provide sufficient reinforcement to the bow for this to be a safe attachment point? Common sense would dictate it needs to be stronger than the breaking strength of the chain or snubber. Chain is 10mm and a snubber would likely be ¾” – 1” diameter. The boat weighs 52,000 lbs.

I am looking for educated input here and real world experience.

If anyone knows of an actual failure someone has experienced anchoring from this point and what were the circumstances were I would love to hear about it.

If you know of anyone doing this and know the particulars of their construction I would like to hear about that too.

If you are a retired engineer and have nothing better to do than figure out the math on this, that would be awesome!

If you have a reference to someone I could talk to educated in this type of design, I would appreciate a referral.

This is going to be a lot of work and if it will never be a structurally sound enough to be safe I will reside myself to abandoning this endeavor.

[slug]

It’s possible if you get the engineering correct

Remember , in a gale the boat sheers like crazy

Perhaps 45 degrees

A bale might work

Similar to the bale on a boom that a main sheet attaches to