keel bolts fully threaded or part threaded bolts?

[Niander]

My bilge keels are held on by nuts and bolts 12mm about 12 on each bilge keel that are made of steel.
i want to replace them and am wondering what to use fully threaded or part threaded bolts ?

Also would you recomend stainless or zinc plated steel bolts?

 

[Niander]

Do i need high tensile?

 

[guyd]

Pedant mode

Bolts are partially threaded, screws (machine screws) are fully threaded. Both work with nuts.

 

[Niander]

do they ?your smart?

 

[Niander]

Spose i can't go wrong with high tensile.

 

[Tranona]

Definitely not zinc plated, but either galvanised or stainless. Sounds like a lot of heavy duty fastenings so doubt there is much load on them individually. Presumably they go through from the outside and that most of the length is in GRP or wood. So bolts are the way to go, not machine screws. Make sure the complete length and under the head is covered in sealant so no water can get to the metal and set the nuts down on a steel plate washer.

 

[rob2]

How funny, without full detail my take on the description was almost the exact opposite! I doubt that many bilge keels are through bolted like a full keel. Those bolts are manufactured from mild steel rod with the lower end upset to form a stopper wedge as they are pushed up into the keel. The top end is threaded to receive the nuts. The bolts are usually then parcelled in cutton and tar before insertion and more molten tar poured down the hole to seal it before the nuts are tightened.

Most bilge keelers I've had a look at have a flange of some type against the hull and this is bolted up to a (hopefully) strengthened area of the hull. Some have countersunk heads to lie flush with the flange surface and some areactually of coachbolt design so they are locked against tuining and present a shallow domed head to the outside. As Tranona has said, bolts used underwater need to be properly hot-dip galvanized or stainless (316). In all cases, a correctly sized (length) bolt is better than a screw as it presents an unbroken bearing surface to the components being aligned and will not bite into the sides of the hole as would a screwthread To achieve a really good fit, it is better to have the unthreaded portion minutely over long if neccessary and allow a washer to take up the difference, but only one washer. If that cannot be achieved with standard "off the shelf" sizes, a stainless bolt could have the thread cut a little longer to suit with a die. With mild steel, any customisation would mean having it regalvanized!

Whilst the keels are off, assuming they are steel, it might be worth getting them hot-dip galvanized too!

Rob.

P.S. Do consider the possible glavanic reactions of dissimilar metals. If you use stainless bolts on steel keels the keels will suffer locally to the bolts - plus there is a risk of crevice corrosion in the bolts. Galvanised bolts are a better match to the keel material, I'd be inclined toward the galvanised bolts but would still consider using tar to seal them and to some extent isolate them from the keel. Once back on the hull, another choice is how you finish the assemble to prevent water contact and then antifouling - it never ends...