Getting the rabbet profile correct

[HarryH]

I received some great help here recently sorting out profile/keel rabbet lines on my lofting project, and I'm hoping for some similar help with this latest (and I hope, last) stumper. So, Bruce, John, mmd, et. al., your help is very much welcome.

I have finished just about all the lofting of my Downeast designed 23' pocket cruiser powerboat. I've learned alot; had to paint over my work here and there and redo a line or two more than I wanted to, but for the most part things reconcile pretty darn well. The diagonals, buttocks and waterlines fair well, with only a couple of minor corrections to the body plan needed.

But the stem.. I drew its profile from measurements given on the lines plan, and faired it up, with one point evidently a mistake by the designer, off perhaps 3/8". No way the batten would pick that point up, but it did not hinder his intent. I used the stem rabbet offsets from the table, one for each of three forward stations. (The stem rabbet was not delineated on the lines plan as was the stem profile).

Now here's the part that throws me a tad. In all the drawings I find in my pile of boat building books, the rabbet point on the half-breadth plan is given by the intersection (along a waterline) of the outside of the planking with the half-siding of the stem. If I follow this convention on my lofting, the back rabbet gets less than 1" from the boat centerline in places. Further, if I project the plank and stem half-siding intersection up to profile, the rabbet line then shifts significantly aft. In other words, the faired rabbet line found this way shifts away from the stem profile much further than it was when drawn from the offsets.

Shucks. Why is mine different? I wonder if the designer's desire to keep stem construction simpler for the amateur accounts for some of the drawing discrepancy? He calls for the stem to be laminated from 1/2"x4" wide laminars; when assembled the stem is molded 5". When gluing up the stem, he suggests leaving off the last three laminars until later, and beveling up to the back rabbet at this point. Then he suggests gluing up the rest and working them over with a rabbeting plane: "This will be a very close approximation to the rabbet as shown on the plans". It will also save a lot of hammer & chisel work.

And the topper is that his drawing of rabbet and waterlines in plan look similar to the stuff in the books I described. And his rabbet line on the profile he drews looks a bit different than mine, too. I am fairly certain my waterlines are drawn in correctly, and I have gone over my work several times.

Has anyone been confronted with this? Is there something I need to doublecheck?

Much Thanks,

_Harry

[Jon Etheredge]

I will take a stab at helping you out here but there are several things in your query that aren't clear to me. Maybe you can clarify. BTW, I am assuming that you are drawing lines to the outside of plank.


In all the drawings I find in my pile of boat building books, the rabbet point on the half-breadth plan is given by the intersection (along a waterline) of the outside of the planking with the half-siding of the stem.
You are correct that the rabbet line is the intersection of the outside of plank (which is represented by the waterlines in the half-breadth view) and the stem half-siding.


If I follow this convention on my lofting, the back rabbet gets less than 1" from the boat centerline in places.
You are saying that the half-siding of the apex line is approx. 1 inch? That sounds okay. Remember that it is a half-siding so there will be 2 inches of wood left between the apex lines after you chop out the rabbet.

If you feel that there should be more wood left between the apex lines you can always increase the sided dimension of the stem. But this will change the location of the rabbet line so you need to get it lofted as the designer intended first and then you can draw your modifications in the lofting.


Further, if I project the plank and stem half-siding intersection up to profile, the rabbet line then shifts significantly aft. In other words, the faired rabbet line found this way shifts away from the stem profile much further than it was when drawn from the offsets.
This part has me confused.

You say that the designer gives offsets for the rabbet (in the profile view). But you say that you are carrying the intersection of the waterlines and the rabbet from the half-breadth view to the profile view and the points aren't in agreement? I think you should do this the other way around.

Start in the profile view. Use the rabbet offsets given by the designer to lay out the shape of the rabbet in the profile view. Now, draw the half-siding of the stem in the half-breadth view. Then find the intersection of the waterlines and the rabbet line in the profile view and transfer those intersections to the half-breadth view by drawing vertical lines up (or down as the case may be) to the half-breadth view. The point where these vertical lines intersect the half-siding of the stem will be the ending point of the waterlines in the half-breadth view. And both views will be in agreement.


I wonder if the designer's desire to keep stem construction simpler for the amateur accounts for some of the drawing discrepancy?
This is a little confusing too.

Are you saying that the designer has you bevel the inside edges of the outer stem piece before gluing it in place? This would be unusual in my experience. The inside face of the outer stem is usually flat. The rabbet line will remain the same as with a tradtionally rabbeted stem. But the apex line will shift so it coincides with the rabbet line in the profile view. In other words, the apex line is exactly behind the rabbet line in the profile view in thsi type of construction. The ends of the planks are not cut at a right angle to plank face as is usual when the stem is rabbeted but will instead be planed off perpendicular to the boat centerline.

But, it really doesn't matter. The construction of the stem is really a seperate issue until you have the rabbet line and waterlines layed down correctly.


And the topper is that his drawing of rabbet and waterlines in plan look similar to the stuff in the books I described. And his rabbet line on the profile he drews looks a bit different than mine, too. I am fairly certain my waterlines are drawn in correctly, and I have gone over my work several times.

Has anyone been confronted with this? Is there something I need to doublecheck?
I think you need to recheck your waterline endings in the half-breadth view. It sounds like you may not have found these waterline end points correctly.

Or maybe I have completely misunderstood what you are describing smile.gif

Maybe you can provide more info to clarify things.

[ 08-19-2004, 12:01 PM: Message edited by: Jon Etheredge ]

[HarryH]

Thanks for the reply, John.

Surprisingly, you did very well understanding what I was trying to say. Thanks for hanging in there.

The paragraph in your reply where you say are confused about my statements is at the nub of my hang-up, I think.

I ended the waterlines on the half-breadth plan, where the PROFILE OF THE STEM (from the profile plan) projects down to the half siding of the stem, not the where the rabbet projects down. I got this from Steward (Boatbuilding Manual) where he talks about long line endings: "Considering the bow...the profile of the stem has been faired and drawn permanently. Each intersection of the stem profile with one of the waterlines is a definite point in the profile plan, and the corresponding point in the half-breadth plan is found simply by projecting the intersection in the profile down to the line representing the half-siding of the stem face in the half breadth plan..." His drawings seem to illustrate this.

Jon--that's how I ended my waterlines...the waterline endings I find in other books seem ended this way too. Do I misunderstand something?

When I started the lofting job, was I not supposed to fair the rabbet profile with the points given in the offset table when I faired the stem? I picked this faired rabbet off the profile along with the sheer profile, top of deck at side, etc. to give the rabbet height in my body plan, too.

The stem is given in the plan and notes as sided 3", and I thought that was a given to be observed. I drew that in on the half breadth plan before I faired in the long line endings...I guess I was thinking where this waterline ending crossed the half-siding of the stem would match up to the rabbet point in my profile drawing.

Incidentally, if this is too laborious because of my inability to describe my difficulty, perhaps I could give you my email address or phone you or such...but this is certainly a big plus right now.

Thanks,

Harry

[HarryH]

OOPS! Minor error in above description...the half siding of the stem is 2", since it is molded and sided 4x5...the 3" wide keel was stuck in my noggin

[Jon Etheredge]

Thanks for the reply, John.
You are welcome. My apologies if I have muddied the waters smile.gif


I ended the waterlines on the half-breadth plan, where the PROFILE OF THE STEM (from the profile plan) projects down to the half siding of the stem, not the where the rabbet projects down.

Jon--that's how I ended my waterlines...the waterline endings I find in other books seem ended this way too. Do I misunderstand something?
Okay, the statement that I made about waterline endings is wrong for many drawings and not very clear for the rest of the drawings :(

I'll try to straighten out my error by considering two different cases:

1) Steward is showing a stem that is faired off so the sides of the stem are in a plane with the outside of the planking. Your interpretation of his statements is correct and the waterlines will end at the stem profile (face of the stem). Be sure to notice that the waterline endings in the half-breadth view are not at the half siding of the stem though, they are instead at the half siding of the <U>face</U> of the stem. The stem face will be narrower than the rest of the stem in this case. Maybe this is the source of the error in your lofting?

2) The second case is less common. In this case the stem sides are faired off but at a shallower angle than the outside of the planking. In this case my original statement is sort of correct but it is misleading since the waterlines do continue on to the face of the stem. It would have been better to say that there is a discontinuity in the waterline curve at the point where the waterline crosses the half-siding of the stem. I think of this discontinuity as "the waterline ends at the half-siding of the stem". At any rate, this is probably an isolated case that is only seen in some small boats. Again my apologies if I caused more confusion.


The stem is given in the plan and notes as sided 3", and I thought that was a given to be observed. I drew that in on the half breadth plan before I faired in the long line endings...I guess I was thinking where this waterline ending crossed the half-siding of the stem would match up to the rabbet point in my profile drawing.
Yes, the point where the waterline crosses the half-siding of the stem in the half-breadth view should agree with the point where the waterline crosses the rabbet line in the profile view.

A discrepency could be caused by one of these things:
- The stem half siding is layed out wrong in the half-breadth view.
- The waterline endings in the half-breadth view are not correct.
- The layout of the rabbet line in the profile is incorrect.
- The offsets given by the designer for the rabbet line are not correct (this one is the least likely).

Hmmm, I bet there are other possible causes but that is all I can think of right now.


Incidentally, if this is too laborious because of my inability to describe my difficulty, perhaps I could give you my email address or phone you or such...but this is certainly a big plus right now.
It is challenging to try and debug lofting problems without sketches and hand waving smile.gif

If you would like, you can email me (click the envelope icon near my username to get my email address) and I'll send you my phone number and you could try to reach me by phone over the weekend.

[HarryH]

Well I'm thinking I stumbled onto the source of my mayhem. The error I mentioned earlier was in fact, much greater than I thought. The rabbet given in the offset table was awry on a station right in the middle of the stem. I think it is off by about 3/4" in height. Because the stem is rakish at that spot, it translates into much more measured horizontally. When I project up from the half-breadth to get points, and allow the batten to find itself around the wayward point, everything looks pretty good. I am kinda surprised that this apparently glaring error was still around on the second edition of the plan, redrawn about 8 years after the first. When I bought the plan in 1973, the designer told me I had set #138...seems somebody would have barked before then..?

Another "side bar" while I am enjoying your fine help: the keel is sided 3" wide (or thick if you prefer). The stem is milled to 4" wide. The rabbet line is drawn straight from stern to stem on the plans, so it looks like the area the plank lands on at the stem, i.e. from apex to bearding line is pretty fat, and the rabbet apex will be cut a little deeper than if keel and stem were of same thickness, no? Hope I stated that in a plausible way...

Again, thanks for your help Jon... I am going to take the liberty of emailing for small (but userful!) talk, as you suggested.

_H