Chainplates and shrouds...

[mdevour]

We’ve begun to make good progress on our Weekend Skiff project (http://www.eskimo.com/~mdevour/boatproject.html) again. My daughter and I were able to put in a couple of nice long building sessions at the school this week. We’re almost done with the reinforcing blocks for the thwarts and the final fitting of the centerboard trunk.

To refresh your memory, this is a simple to build 16’ plywood skiff with centerboard and solid mast and spars. Despite it being my first boat building effort I am attempting to home-brew a gaff-headed main and headsail rig which is well beyond the designers’ intent or guidance. I’m trying to make reasonable design choices now and, yes, I fully expect to have to rework some things -- or backpedal like crazy -- once we try her out.

Last week I asked for advice on a stemhead fitting to handle the headstay. Now I’d like to address chainplates for the shrouds.

Keep in mind that this is all being put together by guesswork, so please feel free to question my many assumptions and point out anything else I should be thinking about.

The boat will come in at about 200 pounds, and should safely carry a couple of adults. I figure a reasonable max displacement to be around 700 pounds, allowing for a couple of hefty adults or several children and any gear. The mainsail is 75 square feet, the headsail 25. The difference in height from sheer to masthead is about 12 ft. and the half-beam is about 2 feet. I expect the shrouds to attach about 10” to a foot aft of the mast.

My impression is that it will be hard to find any standard rigging wire or turnbuckles that will be too small for this job, and if done right it will be pretty bulletproof. My main concern is seeing that the chainplates are well made and mounted.

Based on my reading so far, a bronze chainplate of about 1/8 x 3/4 x 8, with four 3/16 or 1/4 bolts should be ample -- probably overkill. I need to re-check the tables in Skene’s or Toss’s books, but that’s about what I remember. I would like to make these for myself from bronze sheet or bar stock.

The hull cross section looks something like this:
http://www.eskimo.com/~mdevour/other_pictures/section1.png
Unfortunately there are no frames or other internal structure nearby except the inwale.

The outwales are already firmly in place, so putting the chainplates on the outside of the hull would mean a bit of clever work with a Dremel or Rotozip tool to make a slot for them, but would otherwise make for a straightforward mount. Something like this:
http://www.eskimo.com/~mdevour/other_pictures/section2.png

A plywood pad on the inside would add some meat to the hull skin, with a backing plate and bolts through all. A block fills the gap between the inwale and hull to transfer part of the load to the inwale. A well bedded slotted trim plate over the top of the chainplates would help keep water out. Final installation could wait ‘til after the outside of the hull has been glassed, making it easily serviceable, apart from a couple of bungs that would have to be pulled.

From the standpoint of looks, it would be nice to mount them inside the hull – either glassing and painting over the exterior bolt heads or doing without through-hull fastenings altogether. But I really don’t have a feel for how I’d do that. Of the few designs I’ve been able to look at, there’s always some internal structure to tie into that, again, puts the plate on the outside of the mounting surface with the fasteners under shear rather than tensile strain.

I welcome your ideas, suggestions or comments.

Mike

[ 03-29-2003, 12:10 PM: Message edited by: M. G. Devour ]

[John R Smith]

Mike

seems to me that your chainplate scantlings and design should be well up to the job. Most traditional sailing dinghies I have spent time with had tiny little chainplates held on by a couple of screws :( but they seemed to work just fine. The ones on Lulu, even, are ridiculously small and just screw into the sheerstrake (but have held up 30 feet of mast for the past 40 years!).

The only thing that I would add is that in my humble opinion, it is a lot better to have the chainplates on the OUTSIDE of the hull as you have them drawn, from the point of view that you then can keep a close eye on them for movement or weakness. This, of course, applies more to a larger decked boat.

Now we'll let the real experts have a go ;)

John

[ 03-27-2003, 04:07 AM: Message edited by: John R Smith ]

[Andrew Craig-Bennett]

No, let me add something before the real experts weigh in!

Your chain plates are certainly big enough for the job and the bolts are well in excess of what you need - I would think your safety factor might be in excess of 10x!

Like John, I prefer chainplates on the outside and I even think they look nicer but if you want them inside then I think you will need to fit a false frame to take them. This need not be much - a hardwood member which has its larger dimension across the boat, glued and screwed to the plywood skin and glued and screwed to the chine and inwale will do the job. Mount the chainplate on the aft face, angled to line up with the shroud. For a very nice job, rake this member so that the chainplate is in line with the shroud both fore and aft and athwartships.

I think, from looking at my own 16 footer, that I would put the chainplate a little further aft than 10" or a foot from the mast. Ours are about 16" aft and they don't encumber the gaff rig at all.

Over to the experts...

[NormMessinger]

On the other hand, while we are discussing this before the experts weigh in, are stays even necessary on a boat this size? Our K.T., a Sam Devlin Egret, is 15'LOA with a 75 sq ft sprit sail, unstayed.

[Ian McColgin]

Nice little unit and it should work inside or outside.

Now for a thought a tad outside your box - a idea stolen from the Thistle.

Have the chainplates inside on the bottom half of the hull using a channel shape with holes on the free sides. Have a hole with nice thimble in it right through the gunnel big enough to let the toggle and turnbuckle through.

To hook up, feed the stay through the hole in the gunnel to a good hook up hole, pin it, and adjust.

The big advantage here is that you do not have the chainplate tang sticking up and in you way when simply rowing or handling the boat on shore.

Thistles have this feature and that's a far more highly loaded rig than yours. It will work and be treselegante . . .

G'luck

[htom]

I'd put the chainplate on the outside of the hull and gunwale, with a spacer between the hullplank and the chainplate the thickness of the outwale. You can then put the attachment point below the gunwale where it won't be in the way of anything inside the boat (but it can still hang up on a dock, I suppose.)

[George Roberts]

NormMessinger brings up an interesting point:

"Are stays even necessary on a boat this size?"

I don't know how the bottom of the mast is held in place. I believe you said you increased the sail area. I don't know the design wind conditions ....

It is hard to say anything. That will not stop me ...

instead of plywood doublers perhaps you can make false frames (1"x3/4"x8") to run the screws into.

[mdevour]

Well, I have to say you guys are makin’ me think! I like your suggestions. :D

While we were at school today for the kids’ classes I spent some time staring at the boat with your various comments in mind. I took some measurements and photos of the area we’re talking about.

First off, the dimensions are actually more like this:
http://www.eskimo.com/~mdevour/other_pictures/dimensions.jpg

As you can see, if the shrouds attach a little further back, the nearest frame member is almost in reach, particularly toward the bottom of the hull. It’s reasonable to lead them to a point closer to Andrew’s 16 inches aft. (The dimensions in my earlier post were the best I could do from memory.)

Do we need shrouds at all? For the purposes of a simple lug or sprit rig? No. For my own despicably clever reasons I’m trying to put a headsail on her, however. So the shrouds would be needed to balance that strain, wouldn't they?

It’s good to know my initial mounting suggestion would be okay if it was lightened up a bit. I even like how it would look.

That said, we have two very good suggestions for inside mounting. The first, adding another short frame piece, is simple and makes perfect sense, angling it to line up with the stay, at least fore and aft. It turns out to be pretty close to the existing frame, but I really don’t think I should run them back far enough to tie into it instead.

Next is Ian’s clever suggestion of feeding the shroud through a thimble in the wale and anchoring it below. In fact, the existing frame is located almost perfectly for this. I can see attaching it to something as simple as a flanged eyebolt through the frame:

http://www.eskimo.com/~mdevour/other_pictures/shroudend.jpg

One thing both of these mountings do, still, is change the direction of the pull in the lateral plane at the sheer:

http://www.eskimo.com/~mdevour/other_pictures/shroudpath.jpg

I see no way around this if I’m going to use the full width of the hull. Is it an important problem?

Ian, does the shroud wire actually bend at the thimble? Is chafing a problem? Or is the lead straight on your Thistle? I agree it sounds like an elegant solution. :cool:

George, I sure don’t know what the design wind conditions ought to be! tongue.gif

To answer your other questions, the mast steps into a block on the keelson and passes through the partners about 9 inches higher up. The suggested sail plan is a tiny, loose footed lugsail of about 40 sq. ft. that you can turn unsupervised kids loose with without worrying about them getting blown over.

The book also shows photos of a larger lugsail with boom that looks to be about 60 sq. ft. The errata sheets that came with my copy also gives a plan for a gunter main of 75 square feet, with the mast moved about a foot forward. None of these rigs use a stayed mast.

You’ve certainly given me some ideas to work with, folks!

Thank you,

Mike

[Ian McColgin]

If you locate the chain plate on the after side of that frame and put a radiused protector against the inwale and frame, it all should follow nicely with no strain on the planking.

The protector could be as simple (making both sides at once, as an inch or so of bronze pipe where you warm and flare one end, then cut it in half the long way. Set the half-round in nestled such that the center of the semi-circlular section is aimed about at the corner of the frame to inwale.

Then it will be very easy to feed the stay through as it's an open space rather than a small hole. The little bend will not hurt the wire any, but if you really want to get perfect, file a half'round depression where the stay lays such that it will hold the wire from flattening - like the groove in a proper wire pully.

G'luck

[Carlsboats]

Ian's points are well taken. The rig used on the Thistle, Highlander (and some other boats) is
very simple, strong, adjustable, and low cost. It works fine on boats this small. Save the standard chainplates and turnbuckles for bigger, heavier boats.
The multi-hole U-channel is a stock item in the catalogues, and is long enough to
distribute the strain on the topsides. If yours is a thin plywood hull, you could glue on a doubler -- a piece of 3/8" ply would do fine, and then through bolt the whole thing. You would tear the sides out of the boat, or rip the rig off the mast, before that lower end fitting would let go.

[Mad McGee]

Just an alternative...

On my marisol skiff (12 6"), gunter rigged cat, the chainplates are just glued pieces of mahogany and the rigging is rope with deadeyes and lacing. Looks great, is more then strong enough and suits the wooden hull better. From you pictures it strike me that it would be a minimum of fuss to do the same. Also, I don't think that a gaff rig will particularly reward you by being strung up tight with metal rigging.

Good luck and nice boat.

ARM

[mdevour]

Ian, Carlsboats, and Mad McGee,

You're all bringing up good ideas...

McGee's talk about rope rigging brings me to a question I was going to ask eventually...

I've already been thinking about using rope stays and lashings instead of wire and turnbuckles. I also wonder if I really need the rig "strung up tight with metal rigging" as McGee puts it.

How about some of the high tensile low stretch polyester or fancy polymer braided line? They could be less than 1/4" diameter and still have several thousand pounds breaking strength. It's similar in cost to wire plus fittings, given that I can splice or serve the soft-eyes and thimbles into the ends myself (...with practice!).

McGee's simple wooden "chainplate" with a faired hole could take the lashings directly. I'd just tie or splice a tail onto the thimble in the shroud-end, give it a few turns through chainplate and thimble, a half hitch or two, and be done.

The drawbacks I can see would be the extra time and bother over a simple shackle or pin, and having no control over the rig tension from one set-up to the next, which would leave us at the mercy of the "helpful" soul who would inevitably string things way too tight.

So a simple shackle hooked into a chain plate, mounted any of the ways discussed above, with the lashing already made up and tied off, would be quick and a little more foolproof. It could attach at the sheer, or still be routed below per Ian's suggestion.

Serviceable? Too "creative?" :rolleyes:

Ultimately, I'm thinking that installing a false frame canted in the general direction of the masthead will at least let me build the thing and test 'most all of these different ideas later with little fuss.

Thank you for your consideration, folks.

Mike

[Carlsboats]

Now that you are talking about rope, I can help you with some experience drawn from Alden's X-dinghy. This is a 12 footer designed in the 1930s, undecked, with an outer rail and an inwale. I have an "X", and the shrouds are a combination of wire and rope.
Thusly: Wire comes off mast, just like a regular shroud. However, the shroud stops about a foot above the rail, ednding in a thimble big enough to take a piece of 5/16 line, sloppy fit. The 5/16 line begins at the rail just forward of the mast (it goes down through a hole in the inwale, and is held by a stopper knot on the bottom). It then goes up through the thimble, and leads back down to a small block attched to the rail aft of the mast. This gives a two-to-one purchase on the shroud. From the block, the line leads aft to a cleat. I have found that this two-to-one purchase gives all the power needed to set up the shrouds.
The X-dinghy also has a forestay, which is on a turnbuckle held in place with a quick release pin.
To set up the rig, attach the headstay, then set up the shrouds. The turnbuckle lets you crank in more or less rake in the mast. Never had any problem with rope chafing in the thimble. Taking down the rig takes less than a minute: pop the quick release on the bow, unreeve the rope, and you're done.
My wife and I once circumnavigated Shelter Island in winds of 18-20 with our "X." We got pretty wet with spray and did a lot of bailing, but nothing broke. A rope and wire setup like this would certainly be strong enough for your boat.

[Wooden Boat Fittings]

Lots of good ideas here by talented people (despite their modest disclaimers to the contrary.)

Here's something else I think would be worth considering -- use your two halyards as shrouds. This is rigging on the KISS principle, and for a rig this size should be quite satisfactory. No chainplates, no wire, no thimbles or rigging screws, just a belaying pin at each end of the mast thwart.

And you could also do away with a forestay by setting your jib flying, too.

Mike

[mdevour]

More interesting suggestions... Thanks gents.

Carlsboats: I looked for something on the X-dinghy on the web and couldn't find anything. Do you know of anywhere I might I find a few pictures or drawings? Sounds like a fun boat.

The rig you describe is startlingly like something I'd already thought up, but figured was too complicated! :rolleyes: Just goes to show ya! smile.gif

Mike: I've played a little with that notion, too, and it is tempting. And, hey, I could actually start calling it a jib again! ;)

I'd think about it more if I felt the mast and hull structure was a bit stronger -- which is probably just me being timid again. :eek: I've seen 2 or 3 different single sail rigs up to about 75 square feet recommended for this boat, all with unstayed masts.

If we consider the loads involved, my 75 sq. ft. gaff main won't behave much different from a lug sail or spritsail when flown alone, and the halyards will add lateral stiffness the other rigs don't have.

There'd be no drawback to leaving the jib halyard rove and acting as a stay when running without. The main would normally be up first and down last, so the jib wouldn't have much chance to pull on the mast without the rest of the rig to balance it...

I wonder how it'd set up with the wind astern... The jib halyard would want to slack off as the mast moved forward. But the jib wouldn't tend to draw much, either, so again we're back to the same size rig they say the hull can handle.

Maybe if I contrive a way to belay the halyards just a teensy bit further aft... :D Maybe a regular cleat on the aft side of a false frame?

Yes, it's tempting!

Alegra and I are at the school working on the boat tonight. I'm in the process of prototyping the masthead layout to see if I can get all the lines to run fair -- while she's finishing the last of the mounting blocks for the thwarts.

I need another package of clothesline to rig the gaff halyards, topping lift, and sheets, but the headstay, shrouds, and headsail halyard are all looking good.

I'll try it before I'm done, Mike. Thanks for all the suggestions people!

Mike