Stem fitting

[mdevour]

Hello folks.

Today we moved the boat back to the school so we can start working on it again! (Whew!)

http://www.eskimo.com/~mdevour/other_pictures/wrappedboat.jpg Ready to haul

This was the first time on the road with the newly rebuilt trailer and a somewhat too-small car. Everything went surprisingly well despite last night's snow storm. :cool:

Before I can ‘glass the outside of the boat I need to mount everything that fastens through the hull so the holes can be filled and faired. That means finalizing the rigging plan enough to acquire and mount the hull fittings.

What I have in mind right now is to buy bronze plate in several thicknesses from a local metal supplier and cut chainplates and a stem fitting out of it, and possibly parts for the gooseneck as well.

I need a stem fitting that can anchor the headstay and headsail tack as well as provide a strong point to hook onto with the trailer winch strap.

Here’s a photo of the bow and a sketch of a simple fitting that I think I can cut from plate, let into a slot in the stem, and stake in place with bronze drifts.

http://www.eskimo.com/~mdevour/other_pictures/stemtop.jpg http://www.eskimo.com/~mdevour/other_pictures/stemfitting.gif Proposed stem fitting

How does it look to you? Gross overkill? Anything else you don't like about it?

Would bedding this part be another good application for 3M 5200?

I went through a bunch of marine hardware supplier web sites last week. I'm sure I missed something I can use, but I didn't see anything that seemed right.

The best I found was a nice casting that screwed to the front of the stem and curved over onto the top. It would handle the strain of the headstay just fine, but the vendor specifically warned you not to expect to drag the boat out of the water by it, as it would pull the screws right out of the stem. :rolleyes:

Any other ideas, or do you think this'll do?

Thanks,

Mike

[ 03-14-2003, 02:21 AM: Message edited by: M. G. Devour ]

[John R Smith]

Mike

I think that really you need two fittings here -

* The stemhead fixing, which could as you say be a bronze casting from a commercial supplier.

* A ring-bolt or ring which should mount much lower down on the stem and bolt right through the stem and apron. This is for hauling the boat out, and could be stainless rather than bronze.

As to your design for the stemhead, it certainly would be strong. However, unless very well bedded there is scope for water to pentrate down into the stem, and it would be very difficult to replace if you needed to later on (fatigue fracture?).

John

[ 03-14-2003, 06:36 AM: Message edited by: John R Smith ]

[TomRobb]

What John said. Two separate items and a better way to keep the water out. 5200 is for permanant bonding. If you ever have to r&r it, you'll wish you'd not used 5200.

[mdevour]

John writes:


I think that really you need two fittings here -

* The stemhead fixing, which could as you say be a bronze casting from a commercial supplier.
This brings me to a real basic question I have about mounting fittings that will be under strain:

Anywhere you can bolt through some structure and provide a backing plate, you've got a good shot at a solid mounting. That I understand.

For the stem, all of the fittings I've seen would basically have to be mounted using wood screws. The one that hooks around the prow and screws to the front of the stem gains strength by applying strain to the screws in shear mode.

In my case, a simple flanged base with a rib with a couple of holes in it would do the job and be the easiest for me to deal with, provided it could be strong enough.

So, if they're big enough, the wood is sound, and the fitting well bedded, can wood screws really handle mounting something like this? Despite the fact the combined strain from the headstay and tack will be doing its best to pull the screws straight out of the wood?


* A ring-bolt or ring which should mount much lower down on the stem and bolt right through the stem and apron. This is for hauling the boat out, and could be stainless rather than bronze.
Already the plans recommend drilling a hole through the stem about 4" up from the bottom for the painter. Is your suggested ring-bolt a stronger version of this?


As to your design for the stemhead, it certainly would be strong. However, unless very well bedded there is scope for water to pentrate down into the stem, and it would be very difficult to replace if you needed to later on (fatigue fracture?).
This is exactly why I'm asking! smile.gif I don't have the experience to judge these things.

A nice flange to provide a gasketing surface would go a long way toward protecting the top of the stem from rot. You're right that my version would not be easily replaced.

I need to revisit some of the vendors' sites. Can anybody give pointers to a decent bronze stemhead fitting for a 16 foot skiff? While I'm asking, how about chain plates?

Thanks,

Mike

[ 03-15-2003, 12:24 AM: Message edited by: M. G. Devour ]

[rbgarr]

Are you sure you want to mount hardware before you cover with fiberglass? What if you want to move/repair hardware at a later time?

Two small eyebolts, one through the breasthook and one through the stem should be strong enough for your purposes, but may not please esthetically.

[mdevour]

Rbgarr wrote:

Are you sure you want to mount hardware before you cover with fiberglass? What if you want to move/repair hardware at a later time?
Only whatever might be mounted on the inside of the hull that would require a hole through the 'glass.

For example, your suggestion below of an eyebolt through the stem. The only way to get a bolt through would involve a hole in the front of the stem that I'd need to fill and sand fair before finishing the outside of the hull.

Depending on how they're installed, chainplates might be another item that'd require this.



Two small eyebolts, one through the breasthook and one through the stem should be strong enough for your purposes, but may not please esthetically.The underside of the breasthook is accessible for a bolt-through fitting. Going through the stem would be kinda awkward.

I've now seen a couple of decent looking stemhead fittings, but they would all seem to involve wood screws for mounting, rather than through bolts. Am I wrong about that?

If you go to this spot on the Tendercraft Boats site (http://www.tendercraftboats.com/fasteners.html#Sailing) and scroll down slightly, you'll see a good candidate fitting. How is this supposed to be mounted? Bolts where possible and screws for the rest? All bolts, no matter what?

Obviously, this fitting would go on after finishing the hull.

Part of my problem is still not knowing how much you can expect to rely on wood screws for this kind of duty. I hope somebody can explain the basics of this to me.

Mike

[ 03-15-2003, 09:46 AM: Message edited by: M. G. Devour ]

[John R Smith]

Mike

it seems to me you are asking some good questions which are not being answered.

On a boat like yours, the forces involved from the forestay are relatively small, but not to be ignored, as you point out. The truth is, lots of commercial small boat fittings are crap. They look pretty, but seem to be intended more for dock-side poseurs than real life. The snag is that the real stuff, like the fishermen use, is far too big and hunky for our boats.

As to the use of woodscrews in a high-stress situation - the short answer is that no, you shouldn't do it (but should always through-bolt with a backing-pad). In practice, however, lots of us do and it works perfectly well. Lulu's stemhead fitting is welded up from plate which wraps around the stem on both sides, and is then screwed into the oak with long bronze screws. Because the forces on the screws are in sheer, and the wood is dense, so far the mast has stayed up ;)

John

[ 03-17-2003, 04:36 AM: Message edited by: John R Smith ]