Questions
from
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We have been fielding builder's questions for years, and we've done our best to answer their questions and keep them headed in the right direction. Of course things now are digitized which makes it possible to ask questions and receive answers much quicker than ever before.

Our books were actually written to address problems that everyone faces sooner or later, but these questions that arrive by email these days call for much more specific responses. We've been saving the more interesting emails for quite some time, and may eventually put them out on CD. In the meantime, we thought we would include some right here on the site. If you would like to see a CD, please let us know.

All emails you'll see are just like this one, in conversational format. The original query appears in dark red...answers in black.

From: Walter Simmons <ducktrap@midcoast.com>
Date: Sun Nov 10, 2002 9:22:27 AM US/Eastern

Subject: Re: Ribs in Lapstrake Hull
[This discussion concerns the yacht tender, Sunshine]

Douglas,

You're over thinking the job. This isn't a 90' dragger, she's a yacht tender. Answers inside your letter...

Dear Walter:
When placed into a hull, the flat face of a rib must make line contact with each strake at or near its upper, inboard edge and make some surface contact with the keelson. It seems to follow that the placement of each rib would be in a plane at right angles to the keelson.

Amidships, yes, towards the ends of the keelson, not necessarily (though it's certainly easier to get them to lay tight against the keelson if the run is perpendicular. To my way of thinking, contact with the planking at the laps is the more critical of the two.

It seems also to follow 1) that ribs that are placed along a straight portion of a keelson would be in parallel transverse planes and would not slope toward amidships as they rise from the keelson to the sheer and

No it doesn't...your initial premise is a bit off.

2) that ribs other than those that are placed along a straight portion of a keelson would be in transverse planes that above the keelson would slope toward amidships and that the slope of the transverse plane of any rib would be greater the farther that rib is situated from amidships as a consequence of the change in the slope of the keelson which is upward if it changes from being straight at all.
Your drawing of "Sunshine" shows no ribs, therefore it falls to the builder to lay out the ribs.

Apparently you decided not to order the book, which does explain rib spacing.

The stated spacing is ten inches. I expect that spacing must hold good only along the keelson.

No, no, no. Put your ruler away and use your eyes and sense of proportion. The rib spacing keys off the stations. When I build any of these small craft, I first put a rib at each station...and they do look best if they cross the keelson at 90-degrees and stand vertically (or very nearly). The fly in the ointment is that the tops of the forward ribs tend to lay aft and the tops of the after ribs tend to lay forward, owing to the configuration of the hull. Do the best you can, but keep them as close to those station marks as possible. The end ones will want a little twisting to sit tight against those laps. If the temperature of the oak is right, they'll do that just fine.

since any slope in the transverse plane of any rib would result in a closer spacing at the sheer.

Yes, but for aesthetic reasons, you don't want to let that happen. If the station ribs are in place first, it's no problem to space the intermediate ones evenly along the sheer. Of course that means they will be farther apart at the sheer than at the keelson, which isn't a problem unless you happen to be trying to hold to a specific measurement.

I expect that marks must be made on the keelson at that spacing. Each mark would indicate one point in each of the transverse planes in which a rib must be situated. The line of intersection of each such transverse plane with the inboard surface of the planking is the position for a rib.

Long way around the barn, and there is an easier way...

The question is if at each mark on the keelson a builder is to bend a thin batten of the same width as the rib into contact with the inboard surface of the planked hull will the position taken by the batten be at a slope suitable for the corresponding rib?

Certainly you can do that. In fact I do just that in order to mark the locations of the tops. I then wrap the lining batten around the sheer, tick off those rib locations, and then transfer them to the opposite side of the hull.

Having said all of that, rib spacing should always be settled before planking even begins in order to get the lap fastenings right. I use a pair of lap rivets between every pair of ribs. They are all #12 rivets, so when the rib rivets are installed, the overall array of fastenings shows even, proportional placement. To make sure that happens, I position my lap clamps where the ribs will be, and then it's a small matter to space those fastenings by eye as I'm planking.

Hope that helps,
Walter Simmons