Need help interpreting my lines plans (pic)

[dmede]

I’m lofting my plans for the skiff Maud & Emeline and have a question about some of the measurements in the plans. These plans do not use a table of offsets but rather give you the offsets as measurements on the lines themselves. Everything is pretty straight forward except for the stern where some of the measurements don’t add up.

In the scan below you can see that for the stern in both the profile and section view there are two sets of measurements for the same lines and they don’t match each other. On the section lines the height is shown as 16 ¾” and right next to it is 15” plus 1 3/8” (distance between LWL and bottom of stern plus total face of transom), they should be the same but there is a 3/8” difference.

In the profile there is a 16 ¼” measurement for the height of sheer at the stern and a 16 1/8” measurement for the same line. I had a hunch that the smaller measurement (also the inner one) was the height of sheer at the inside face of the transom while the bigger measurement was the height of sheer at the outside face of the transom and after checking in my lofting that seems correct. But it could just be a coincidence.

Can anybody tell me what’s going on with these stern offsets?

Dave

http://static.flickr.com/68/153691665_8f091e2465_o.jpg

[Jay Greer]

A quick glance tells me that the developed stern has a height of 15" as it takes in for the added height due to the rake of the transom. Projecting this line to the LWL
adds the extra 1/2" inch which is the projected distance as opposed to the plumb line height of the transom below the crown line. The heights of the inner and outer faces of the transom are affected by the bevels in relation to the projected sheer line. The drawing appears to be correct.
JG

[dmede]

I think I get the profile measurements (they seem to be for the inner and outer face of the transom at the sheer line).

But in the section lines the two measurements on the left of the transom (16" and 15" plus 1 3/8") I don't understand. They seem to be referring to the same height above LWL (if you add the 15" and 1 3/8" together), but they are different measurements.

I'm also confused about the measurements of the space between the transom bottom and the LWL. In profile it's 1 5/8" and in section it's 1 3/8". But to me it seems like they should be the same regardless of whether or not the transom is being shown as developed or not (the bottom should always be that fixed amount above the LWL).

[dmede]

ok wait, I think I understand what your saying now. The 16 3/4" measurment is in line with the 15" line (the angled face of the stern) but extends past the bottom to the LWL at that same angle? I think I'm confused by the addition of these numbers becasue to me it seems like information I could get from the lofting and not really need to be given in the plan.

For instance I can't see any use for giving me up front the 16 3/4" line in the section. I lofted the stern properly using the other info and can get that measurement from the lofting (If I needed it).

[Doug Hamilton]

Here is a submission for the true shape of the transom derived from the given three dimensions for the profile, viz. the 14-1/2" and the 1-5/8" at the transom and the distance of 33" between the base line and the construction datum.
--------http://i73.photobucket.com/albums/i207/Dougha/s2.gif
I'm so full of myself for having learnt how to post an image onto the forum that I've made this drawing just to marvel at seeing the image appear.
Thanks to Braam Berrub for giving me a clue in his post in the thread titled "Posting pictures" in the bilge on 04-08-2006.

[Jay Greer]

On another note; Don't be too concerned if you find a few measurements or offsets that are a bit out of whack. That is what the lofting floor is intended to be for, a proof of the design. Often designers make mistakes. L.Francis Herreshoff often made off set notes and measurements that I have had to correct during lofting. L.Francis also had a unique way off spelling things as well. And, he would sometimes trace a part in reverese! I make mistakes too and, sometimes, find that what I have drawn doesn't work in the construction phases and I have to compensate or just wing it.
Us boat builders ain't infallible!
JG

[dmede]

Nice work Doug! Of course I still have to expand the transom myself incase some of my lines arent exactly per plan but your image may still help.

I worked on the lofting again last night and today and I think I have everything in hand. The duplicate numbers were confusing me but I think I get what they are referring to now.

[John E Hardiman]

Just a note for both dmede and Doug.

Remember when developing the transom that the sheer and bottom are rolling in. It is therefor necessary to develop the expanded transom to the INNER face of the transom stock, not the outer face/lines in order to have the necessary dimension to cut the bevels back (i.e. the inner face is larger than the outer face). I can't tell because the image is small, but that may be why the dimensions in Doug's development doesn't match the "developed stern" of the lines plan.

[Doug Hamilton]

John H:
Thanks for showing interest.
I could do no better with the limited data given in the posting.. I assumed none of them pertained to the inboard face of the transom.
dmede:
One aspect that is puzzling: the transom is shown to extend to the level of the bottom of the boat, as if the bottom planks do not cover the bottom of the transom.. Could the outboard stern-post provide a sufficient longitudinal clamping force?. No way!. Look elsewhere in the drawings for an answer.. Atkin progeny are not available for questioning, I take it.

[dmede]

John H:
Thanks for showing interest.
I could do no better with the limited data given in the posting.. I assumed none of them pertained to the inboard face of the transom.
dmede:
One aspect that is puzzling: the transom is shown to extend to the level of the bottom of the boat, as if the bottom planks do not cover the bottom of the transom.. Could the outboard stern-post provide a sufficient longitudinal clamping force?. No way!. Look elsewhere in the drawings for an answer.. Atkin progeny are not available for questioning, I take it.

The lofting is actually of the "stern" not the transom. So it's drawn to show the lines to the outside of plank (side and bottom). Same for the sections. In the getting out process I deduct the plank and bottom thickness to get the molds and transom (the transom has some other stuff going on such as deducting more than the bottom thickness to account for the beveled face of the bottom etc.)

When I get out the transom I use the bevels off the lofting to determine the inside face dimensions. I've been working on it again today so it's all fresh in my head. I'm just about ready to start building molds and get a pattern for the stem (once I nail down the rabbet and bearding lines).

[Doug Hamilton]

I don't have much experience at building boats and I know I'm about to belabour this topic of transoms beyond my limitations. However, its fun.

One way to build a transom is to form a panel of a set, uniform thickness and mark both the outboard and inboard shapes on its outboard face, saw-cut it to the inboard shape, bevel it to the outboard shape while on the bench, then mount it, as a finished piece, into place on the stocks..
Another way is to make a panel of no set, uniform thickness and mark on it the outboard shape, saw-cut it wide of the marks , mount it, as is, into place on the stocks and there pare the edge down to suit each individual plank as it is being offered..

John Brooks promotes the first way.. I tried it, failed and resorted to the second way successfully.. I expect to fare better on the next transom.