Offsets for 15 Footer

[Robert (Alerion Sailor)]

Does anyone know  to what point the Herreshoff Buzzards Bay 15  offset are measured, I think they are measured to the inside of the planking but I want to know for sure before I start. The only reason I ask is because I read other article on Wooden boat ( Building the Maid ) that spoke of the offset measurements to the outside of the planking.

I think the offsets from MIT are the originals from HMC and would therefore be no need for these offsets to be to the outside of the planking.

Robert

[Jon Brooks]

I'd get a hold of Alec at Artisan.  He's built quite a few now (working on another 18 right now).  He should have the answer you seek!

Jon 

[Adam]

Our great leader Steve Nagy can probably tell you as he restored "Elf" some years ago....It's only a matter of time until he wakes from his retirement nap and reply's... 

[Steve]

Actually, I can't.  I contracted the re-framing and planking work out.  Assuming you have a copy of the drawings from MIT, study the hand-written notes ... the answer to your question may be in there.

[Robert (Alerion Sailor)]

I did study several times so far I have found any indication. I suppose I can loft the first couple and reference them from the the bow, I may be able to get a safe reading from that.
Robert

[Erick Singleman]

NGH made most, if not  all of his designs, from hand carved half models.  Once the models were completed he used his ingenious machine to take the offsets from the half models.  These offsets would be representative of the outside of the hull.  I know it was this way for the 12 1/2s.  I believe they lofted to these outside hull measurements and then built the molds by subtracting the planking thickness and frame thickness from the half breaths. 

Now you may think as I did that this works at the beam of the boat where the hull is parallel to the centerline, but what about  as the hull tapers fore and aft from the beam? There is a cosine of the angle to deal with, however this is a relatively small distance to compensate for over the frame and plank thickness (up to approx. 1/8" on a Herreshoff 12 1/2 footer depending on angle of the hull to the centerline).  I think they probably ignored it, because I don't imagine that someone took the angle measurements of the hull taper at each waterline level and paged through trig tables and calculated that they had to add back .089" at this frame WL level and .054 at the next one, etc. etc.  And, I don't believe HMC used lines drawings that would allow them to easily take the angle measurements.  I believe NGH had a book with the offsets and that was what they worked from to loft the boats, and that's it).

Additionally, this 1/8" would be insignificant when you consider that they would be making minor adjustments to the molds anyway after they set them up to make sure a batten lays fair across the length of the hull.

That's my take on it.

[Robert (Alerion Sailor)]

Interesting theory, yes the offsets were taken form the half model which represented the outside of the hull.


I saw a video of Halsey on you tube using the measuring devise and while he did it he said that you needed to subtract the thickness of the planking.

So my question still remains to be answered is weather or not the offsets from MIT are direct off the half model with or without the planking consideration.

[Erick Singleman]

Alright, here's my perspective fro 30,000 feet (or leagues, which ever applies).  And I am not saying this is correct, just logical to me based on how I would do it if it were me way back then.  Play along, this could be a wild ride...

As I see it NGH would have written down the offsets in his notebook as they were displayed off of his machine.  Which I believe was in feet, inches, and eighths, if I am correct.  Now if it were me, i would loft the boat to these outside dimensions in full scale on the shop floor or wall, or my mother-in-law's backside, you know, something big, because there would be some corrections to do because taking the measurements at the scale he took them and translating them to the larger scale would have some error no matter how careful he was or how good his machine was. (sorry for the long sentence, good at math, not English).

So after they have the full size corrected lofting you have two choices...  1) you can set up a grid on your wall or your mother-in-law's... (yeah, pass on that one) and measure a new set of offsets to a line that is the frame and planking thickness away from the lofted lines for each water-level and diagonal line and re-record these measurements in the same or another book, then make a pattern from the new offsets.   OR, 2) you can set a large sheet of paper over your lofted half breaths and make a pattern that is the plank and frame thickness short of the original lofted half breaths.

If I were the production manager at HMC I would save all of the measuring and re-measuring and go with method 2.  And If I were doing it myself I would go with method 2 because I am a engineer, and engineers are lazy people because they invent stuff to make life easier (like elevators, and shoe mirrors).  Also I believe it would be more accurate to make new half breaths using method two because you are doing only one measuring step instead of two.

Now after all that nonsense, I still can't tell you what they actually did.  So I apologize for wasting your time.

[Erick Singleman]

Your dilemma is an easy one to solve, I just realized, duh.   If you get the offsets from MIT and you want to confirm whether they are inside or outside the hull measurements or the mold offsets.  Just compare them to measurements taken off one of the boats in the museum.  You'll have your answer in five minutes.

[Adam]

1. I can never look at my mother in law the same again.
2. will someone just call Adam Langerman or Halsey....
3. what the @#$% is a shoe Mirror?
4. why do engineers give me a headache?
 

[Charles Barclay]

LFH indicated in one of his books that as a teenager he and his brothers would transcribe the measurements into the offset book.  Since the H 15 was 1899--that would be Sidney. 

In further support of Erick's on the outside measurement comes from Barry Thomas's "Building the Herreshoff Dinghy" pp-6-7 which includes a diagram 2-2 for a simple tool to correct for the planking width. 

 

[Robert (Alerion Sailor)]

I will make some phone calls

[Erick Singleman]

"3. what the @#$% is a shoe Mirror?"

A reference from Pee Wee's Playhouse, where Pee Wee would have mirrors on the tops of his shoes to look up the skirts of unsuspecting females.  I figured that was probably invented by an engineer given our stereotypical awkwardness with women.   

[Adam]

DOH Erick! 

[kdrolt]

My first post but I can answer this, however dated the thread might be, the boats are even older.

I have the offset booklets from MIT on #486 and #514. These also apply respectively to #536 and #515.  These four were built from the same half model (which I have seen). My grandfather owned #486 at one time, so I am reducing the offsets into decimal so I can make a half model for my dad.

The offsets have a note on the first page denoting what the timbers and planking sizes are, so NGH wrote a note to the shop to 'make deductions' from the offset measurements so that the molds would be correct.  The offsets represent the exterior surface of the hull both below and above the WL.  The same page also lists the frame spacing (in inches) on a "common measure."  So all the frames can be measured from the fwd face, or the center, or the aft face so long as they are all done the same way.  I can't answer how frame 1 is measured from the fore peak of the stem, but I don't think this would matter much to the shop because they have to make everything fit even if there is a half inch missing somewhere.

The offsets themselves have errors.  I can see that from plotting the frame profiles on a common graph.  The errors could have been on the B & S offset machine, or transcription, or both.  I also know that #486 and #514 had some common frames, per notes in the #514 booklet.  Well that's not exactly true because the offset measurements would have had to been identical, two years apart (1897,1899) --- and they aren't, but they are close.  Again, the shop gets the job of fixing any errors made prior.  FYI, HTH.