Bobstay fitting?

[oldbilbo]

I'm looking to fit a bobstay to a robust GRP 4T boat, and seek b/builders' suggestions regarding whether to fit a s/s bow-eye.... or this arrangement?

Access inside is difficult, for internal reinforcement. The boat is out of the water.

 

[PlankWalker]

Anything but that arrangement! Surly that's for pulling onto a trailer?
A bobstay needs to be bomb proof, and I fear that arrangement could chafe through when the going gets tough.

Plank

 

[William_H]

I think you will find the GRP very thick in drilling into the bow. (fore and aft). You may find it thick enough to tap a thread into the GRP so 2 bolts to fit a saddle to shackle the bob stay to. Or if you don't trust a tapped thread then you need bolts long enough to get far enough in to a backing plate. Or perhaps a threaded rod going right through to where it is accessible. All sorts of options to bolt on a saddle, but that hole through the hull looks horrible and would need to be glassed in on the inside anyway. good luck olewill

 

[Javelin]

I'd be inclined to fit a long threaded m10 to m12 u-bolt using 6" drilled ss bar on the inside in place of washers (unless there's a nice flat area inside) and nylock nuts.

I've done one from the outside before as it was impossible to get any access inside.
I ground out the glass about 15mm deep and then re glassed a ss T bracket on and then re-gel coated an area of about 4" around the fitting.
As usual, getting a good colour match was the hardest part.

 

[john_morris_uk]

Something similar to this would be better surely?

https://www.s3i.co.uk/commercial_eye_bolt.php?pid=2816&gclid=CLX3vJX97MwCFZUW0wodIbQE5g

Otherwise a fabricated plate that fitted like a saddle round the bow with a 'U' or 'eye' welded on and extending far enough back along and round the hull to get a couple of bolts through either side.

I understand that access inside might be difficult but surely it's not impossible or non existent?

If it really is incredibly thick layup and impossible to access, then fix the saddle with studs/bolts set into epoxy. The force will be a shearing force and unlikely to pull out.

A surveyor said he'd seen winches mounted in this way when I was dubious about putting studs in cast epoxy for a engine mounts. The studs were very strongly set when completed.

 

[Simondjuk]

An appropriately sized eyebolt with a decent backing plate ought to do it.

 

[PaulJ]

The Bobstay fitting comes under very considerable strain so I don't like the idea of an eye-bolt as all the strain is in one place. At least a U-bolt spreads the load over a slightly larger area and provided it has good backing plate and the bow is well reinforced inside, it should not pull through. I like John's suggestion of "a fabricated plate that fitted like a saddle round the bow with a 'U' or 'eye' welded on and extending far enough back along and round the hull to get a couple of bolts through either side"......... this would spread the load over a wider area though it may be more difficult/costly to do.

Paul

 

[Simondjuk]

The bobstay sees the tackline load. In fact, many bobstays are effectively an extension of the tackline running free through the forward end of the bowsprit such that it acts only as a spreader. In such setups there is often a fixed line between bowsprit and bobstay tang as well, but this acts solely as a backup should a sheet or the tackline get around the sprit and impart an upwards load and is generally slack in normal circumstances. Open 60s are one example where such setups are sometimes used. Once either the tackline or tack snap shackle breaks, there is no longer any load. Make the bobstay and its fittings somewhat stronger than those and you're good.

I can say with certainty that M24 eye nuts and studs (eye bolts are not available with sufficiently long studs to reach through the moulding at the bow) are quite capable of taking the bobstay loads on 68 foot, 35 ton boats flying big kites.

What sort of boat, bowsprit and sails this question relates to has some bearing of course. I'm assuming a relatively modern mono flying A sails from a modest length bowsprit, and not a massive long spar such as a classic yacht may carry on her bow, which along with a bumpkin may all but double her LOD.

 

[oldbilbo]

Open 60s are one example where such setups are sometimes used. Once either the tackline or tack snap shackle breaks, there is no longer any load. Make the bobstay and its fittings somewhat stronger than those and you're good.....

What sort of boat, bowsprit and sails this question relates to has some bearing of course. I'm assuming a relatively modern mono flying A sails from a modest length bowsprit....

Yup! I patrol the pontoons, on the lookout for ideas that reflect Colin Chapman's dictum "Build in simplicity and add lightness." My platform is a venerable 70s monomaran, and I'd simply like to hoist and use an A-sail for my convenience and 'grin factor', instead of all the faff of a last-century SymSpi, pole and lines. Here's where the above pic came from...

 

[rogerthebodger]

The point about that arrangement is that the bobstay would break before the hull was damaged. In the same way as road bridge parapets are designed to break away before the bridge structure is damaged.

 

[Simondjuk]

Yup! I patrol the pontoons, on the lookout for ideas that reflect Colin Chapman's dictum "Build in simplicity and add lightness." My platform is a venerable 70s monomaran, and I'd simply like to hoist and use an A-sail for my convenience and 'grin factor', instead of all the faff of a last-century SymSpi, pole and lines. Here's where the above pic came from...

In that case I'd keep the bowsprit short and the bobstay attachment to the bow relatively high up, as this will still give an effective working angle onto a short bowsprit with the advantages that if the bobstay is all above pontoon level, a pontoon bump won't yoink the bowsprit downwards and you won't have so many issues with water ingress via the hole through the bow that you use to locate whatever you end up using as the tang if it's well above the waterline.

An M12 s/s eyebolt has a break load of about 3500kg, so almost as much as your boat weighs. I'd be confident that would do it, but go for an M14 or M16 if you want even more peace of mind and something handy to hang the boat up by!

 

[PaulJ]

My comments about the load on a bobstay were based on a rescue in which I was involved in the middle of the Pacific in which a bobstay fitting had pulled out of the hull leaving a hole in the bow.........

The failure caused other complications, not least injury to the skipper. The crew were taken off by a passing ship and the boat was lost. I guess it depends on the setup but a bobstay CAN come under very severe strain.

Paul

 

[oldbilbo]

Thanks for all input.... so far.

 

[rogerthebodger]

Thanks for all input.... so far.

The point about that kind of attachment is that if the bobstay did break through the hull it would not compromise the strength or integrity of the hull where an eye bolt bolted through would if pulled out.

 

[Simondjuk]

Assuming the layout permits, which is likely to be the case on the majority of cruising boats, I'd locate the bobstay fitting in the part of the stem that falls within the boundary of the anchor locker, such that damage or failure of the fitting is unlikely to lead to flooding of the interior of the boat. I'd hope that the bow of most boats would be of robust enough construction that any damage to or failure of the fitting wouldn't propagate downwards far enough to compromise the hull below the waterline or upwards far enough to compromise the forestay fitting.

 

[bbg]

The arrangement you show in the first post is the arrangement that is, as far as I know, the preferred one with high-performance / high-load applications. Here is a photo of the bow of my ex-boat, showing the same arrangement.

I would be reluctant to use something bolted onto the structure, for the reasons others have given.

Edit - it was an articulating bobstay, and in this shot shown in its normal upwind / not-in-use position, stowed up against the lifelines.

 

[Simondjuk]

Jeepers, that's a monster!

Perhaps I'm wrong, but I had in mind that we're talking about something much smaller here to keep the tack of an asymmetric kite or cruising chute just clear of the bow/pulpit/etc. Either a tubular deck mounted type, that would be all but self-supporting but would be made more robust and kinder to its mountings by way of adding a bobstay, or a small A-frame type, such as the below:

 

[oldbilbo]

...but I had in mind that we're talking about something much smaller here to keep the tack of an asymmetric kite or cruising chute just clear of the bow/pulpit/etc.

Yes.