Intend to build chapelle's 17 hampden

[Boreas]

Hi there guys. I just made up my mind and intend to build chapelle's 17 hampden (from his book Boatbuilding). As it is my first build, I will be very careful. The very first is to loft the boat full size but I want to make sure I'm gonna do it right. So, for practising my lofting -and avoid mistakes while lofting full size- I'm lofting the same Hampden 17 but at a 1:5 scale using millimetric paper. Now my problem is that I found the offset for the sheer profile at station A to be wrong. As many (maybe most of us) know and may have the book I wonder if you could help me check it.

Thanks to you all.

[johnw]

It's pretty common to find a mistake in the offsets when you loft. Some designers are more careful than others, but at the time this design was done, it was all done by hand, and it was pretty easy to make a mistake. A lot of boats have been built to these plans, so perhaps someone will come along who found the same mistake.

[Jim Ledger]

Those plans in Boatbuilding were usually measurements taken off existing boats, some probably very old. Finding discrepancies should be expected due to the difficulty of taking accurate measurements under less-than-ideal circumstances.

[Bill Perkins]

You can get that plate of the lines at scale ( 3/4 in.= 1 ft. ) from the Smithsonian for $10 . You may be able to spot the error with a scale rule applied to that drawing . I would think you'd want that and the sail plan at their true scale for building anyway . There are probably some undimensioned parts it would save time to measure with a scale ruler .

You could enlarge the books illustration to a scalable size at least . A crisp clear copy of the original drawing would be more inspiring and legible though . It might help you see something .

[TerryLL]

My 1941 edition of Boatbuilding shows the sheer height at station A to be 3-10-2. Scaling off the body plan with a set of dividers I find that offset to be pretty darn close.

I've found that doing a scale loft at the following scales works well:
One sixteenth inch equals an inch, 3/4" to the foot, or
one eighth inch equals an inch, 1.5" to the foot, or
one quarter inch equals an inch, 3" to the foot.

[Boreas]

My 1994 edition also shows the sheer height at station A to be
3-10-2. As I'm working with milimetric paper, I have converted the whole offset table to metric system.

1' = 30.48 cm
1''= 2.54 cm
1 eighth = 0.3175 cm

Then:

= 3-10-2 (imperial)
= 3(30.48)+10(2.54)+2(.3175)
=91.44+25.4+.635=117.475 cm

I'm gona check everything again and then I'll tell you.

[boatbear]

Gidday Boreas. I played around with a ‘virtual’ build of the 17’ Hampden last year, with lots of good help from WBF members. It was a lot of fun and I learnt a lot. If you are interested here are the links to two threads including pictures of some lovely existing boats to whet your appetite. Good luck.

http://www.woodenboat.com/forum/showthread.php?t=62205
http://www.woodenboat.com/forum/showthread.php?t=68846

[TerryLL]

I get 117.475cm also. This would be the full-size loft offset.

The scale offset (1:5) would be 23.495 cm.

Are you doing the scale loft first, and fairing all the lines as you go, and then using the scale offsets (multiplied by five) to establish the full-size loft? If this is your method, then any slight adjustments to obtain a fair scale loft will be multiplied by 5 when you transfer your new offsets from the scale to the full-size loft. A .1 cm adjustment in the scale loft will become a .5 cm difference at full size.

Chapelle took many of his lines off old existing hulls, and the offset table should only be considered an approximate guide in establishing a full-size fair loft for a new hull. The full-size loft will smooth out the minor irregularities found in most offset tables. That's one of the main reasons for doing a full-size loft.

[Boreas]

Everything was Ok. It' was only that at the scale offset (1:5) instead of measuring 23.49 cm I measured 22.49

[johnw]

All's well then...