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A Dumb Question About Rabbets


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For some time I have been wondering about the profile of rabbets. My understanding is that in its simplest form the rabbet is a housing used to hold the garboard strake so the outer face merges smoothly with the keel. In almost all the documentation I can find I have found the rabbet carved as a v-shaped notch thus.

 

post-12145-0-42910800-1422873948_thumb.jpg[
 

This means the edge of the garboard strake would need to be shaped to match the angle at A, whatever that angle may be.

In fact I believe the rabbet would always be cut with the angle at A being a right angle (ie 90 degrees)  to accommodate the square edge of the garboard strake. This would occur regardless of the angle at which the strake meets the keel as shown in versions 2 and 3 below.

 

 post-12145-0-56414300-1422873957_thumb.jpgpost-12145-0-45037800-1422873973_thumb.jpg

 

This approach would also mean only one timber needs shaping (the keel) rather than two (the keel and strake) which I imagine would have been preferred by the shipwrights.

Any thoughts?

Dave

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Greetings Dave,

 

Good topic. I can give an opinion. I expect there are as many ways to shape the rabbet as there are shipwrights. I expect that the angle of the rabbet would change multiple times along the hull length as the garboard strake was installed. In some cases the shipwright may have chosen to bevel the plank edge rather than hold a specific rabbet angle. Further, caulking is always a concern, and I am not sure holding the lower outside corner of the plank at the outer face of the keel would facilitate a good caulk job. I would want to bevel the plank and let it in to the keel to insure a good caulk joint. You may be right as rain - let's see what other opinions may be.

 

wq3296

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I concur with druxey.  The plank is not re-shaped.  All the cutting is on the keel.

I wrote a short piece with diagrams for my club's newsletter.  Go to:

http://uvsmgshipmodelguild.wikispaces.com/

Click on 'Now Hear This' on the left of the page, then 'What is the Rabbet?'

I concur with both druxey and dgbot.  There are no dumb questions.  There's too much to learn in this business not to ask when you get stumped.

 

Joel Sanborn

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Greetings,

 

As usual, there are few absolutes, no matter what you are building. If you are talking about models, then maybe only the keel is cut as an expedient, and leakage is not a problem. However, on real ships, at least on the new Mayflower, the garboard strake is beveled, more like Version 1 above. Further, I think Dave is referring to real ship building since he uses the term shipwright.

 

wq3296

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I would expect that the rabbit for any full size ship would be cut to a mininum depth into the Keel that provides the needs of the joint. A V cut into the keel with a 90° bottom angle would be one with equel sides and only deep enough for a good connection to be made. Would requile trimming the Garboard Strake to fit, which could be fitted as it is laid along the keel and ribs. I would not want any keel of mine to be cut any deeper than the mininum needed. The builders, building and planking those ships had skill levels that would not be tested by  fitting a garboard strake into such a rabbit.

jud

Edited by jud
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I've just checked two respected sources and, where the construction is as in the original presentation, the plank edge is square and the rabbet is cut to accommodate that square corner.

This is a tough joint to keep watertight so you want as much wood as you can get to fasten the plank edge to the keel.  If you taper the plank edge you will have a weaker joint.

 

Joel Sanborn

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Many thanks for the learned replies. I am now happy to shape the rabbet only on the keel with a 90 degree inner angle.

 

This then begs a question when it comes to the rabbet on stem and stern timbers particularly high on the stem with a bluff bowed ship (such as HMB Endeavour). In such a ship the ends of the planks are almost perpendicular to the stem so is the rabbet very shallow or is it let into the stem timber much like a dado housing and if so, how deep?

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I'm sorry if this make you less happy, but the whole is more complicate, as far as I know... :D The garboard plank was beveled in mostly cases to match the rabbet. As clearly shown in those drawings:

 

http://collections.rmg.co.uk/collections/objects/85449.html

http://collections.rmg.co.uk/collections/objects/80965.html

http://collections.rmg.co.uk/collections/objects/80745.html

 

here the later practice:

http://collections.rmg.co.uk/collections/objects/80704.html

 

and here a couple of frame drawings that show tre rabbet and garboard planks:

http://collections.rmg.co.uk/collections.html#!csearch;searchTerm=Telavera_1818;start=10

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Dear Alex,

 

Those are good examples of cross-sectional drawings you have cited.  I absolutely agree that the earlier ones show more of a 'birdsbeak' joint amidships. However, was that actually how the rabbet/garboard joint was carried out in the shipyard?

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Greetings Dave,

 

It looks like Alex M and Jud agree that of course you shape the garboard to fit into the smallest rabbet (Version 1). Based on your last question, you can see why the garboard has to be beveled (fitted) to the rabbet. If the rabbet was to deep, you would think that the keel could be weakened in those areas. I expect it was a judgement call, based on experience, that would dictate how much the shipwrights would shape the garboard and/or the keel for best fit and watertightness. I like what Jud said, that shaping the garboard would have no challenge to real ship builders.

 

wq3296 

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Greetings Dave,

 

It looks like Alex M and Jud agree that of course you shape the garboard to fit into the smallest rabbet (Version 1). Based on your last question, you can see why the garboard has to be beveled (fitted) to the rabbet. If the rabbet was to deep, you would think that the keel could be weakened in those areas. I expect it was a judgement call, based on experience, that would dictate how much the shipwrights would shape the garboard and/or the keel for best fit and watertightness. I like what Jud said, that shaping the garboard would have no challenge to real ship builders.

 

wq3296 

Actually, amidships the birdsmouth shape removes more wood from the keel than a square corner rabbet would.  It does give more plank inside the rabbet for fastening.

 

Joel Sanborn

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Don't believe the Garboard Strake should be fastened to the keel, nor do I believe it should be inside or far into the Keel. Reason, the forces working on the planks and ribs are different than the forces working on the Keel. A perpendicular 90° bottomed rabbit, only deep enough to make the joint, will provide enough surface to seal with Oakum and tar, you want the rabbit only deep enough to make a flexible seal joint between the Planking and Keel, its not there for structural reasons. That joint needs to move, were it rigid there would be failure of the plank close to the joint. Attaching the Garboard Strake is just like attaching any other plank to the ribs. That attachment makes the rib and plank components into one unit that moves and flexes together as a unit. The Keel controls other forces, which vary because of wave action creating a live load, shifting along the Keel as the ship works and results in an up and down flexing of the Keel, any sideways motion should be controlled by above the Keel structures within limits.  Without that mix of motion, the joint could be a solid joint, kind of like the connection between the foundation and the structure of a house, but even that connection is intended to allow for some flexing.

jud

Edited by jud
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