Lofting the Bobs special

[FHB02]

Hi Bear mountain builders

I am new to this forum and I have begun building my first cedar strip canoe.
I am slowly getting familiar with boat lofting and I have loftet plans to bobs special from the table of offsets presented in the 2000 version of canoecraft. However my first problems have started to appear. Something seems to be off with my stem mold. I found a revised table of offsets in the FAQ presented on the website of bear mountain boats. The value: sheer - 1-02-7@1 5/8" is presented and I am confused how to understand this value (is this a coordinate? (Butt x WL)). Furthermore I feel like I need a profile value like with the other molds (I figured that this this value might be the same as the profile of station 6 minus 3/4" (thickness of inner stem))?

Lastly I was wondering if I could use a semi waterproof PVA wood glue for glueing the strips together? or if its a must using a fully waterproof wood glue? (since i figured that the canoe would be waterproof anyway covering it in epoxy and varnish)

Kind regards
Frederik from Denmark

[Cruiser]

Hi Frederik,

I did a post on lofting canoes from the CanoeCraft tables, hopefully the answer is there, you can find the link here: viewtopic.php?p=31740&hilit=lofting+canoecraft#p31740
The stem sheer values was confusing, but there is a section with pics and explanation, ot should help some.


In my last build thread I did a small section on glue selection, it answered what you are asking, it can be found here (post #64): https://www.canoetripping.net/threads/l ... ost-105920


Brian

[Stephen]

Hello Frederik

The value: sheer - 1-02-7@1 5/8" is presented and I am confused how to understand this value (is this a coordinate? (Butt x WL)

Your understanding of this is essentially correct. Make an extra waterline at 1 5/8" and measure 1-02-7 along it to get the sheer point. I'm pretty sure that Butt is a term that applies to station molds rather than stem molds, but this distinction isn't too important for your purposes.

Furthermore I feel like I need a profile value like with the other molds (I figured that this this value might be the same as the profile of station 6 minus 3/4" (thickness of inner stem))?

This is partially correct but needs clarification. Per the Bear Mountain FAQ:

The stem offsets are horizontal measurements from the station listed in the table to the outside edge of the inside stem. Therefore the stem mold that you make should be cut back to allow for the thickness of the inside stem (normally 3/4" on the canoes)

First you will plot the points from the FAQ for the stem and use the profile value for Station 6 WITHOUT deducting 3/4". You will then connect the points in a fair curve using a flexible batten to get the forward edge of the inside stem. (This step is optional but may help you see what comes next. ) Then you will deduct the 3/4" from this curve, always measuring perpendicular to a tangent of the curve. A coin or machine flat washer of 3/4" diameter is one way to measure it. Finally connect these new points in another fair curve to get the pattern for your plywood stem mold.

Good luck with your build.

Stephen

p.s. a note about Brian's excellent post on lofting. Because Bear Mountain does not supply the bow and stern sheer values of the (asymmetric) Freedom 17 he is lofting, he has had to extrapolate the sheer values from Stations 15 to 16 to stern and from Stations 1 to 0 to bow. Since the FAQ page has already supplied these values in the case of the (symmetric) Bob's Special, you can skip that particular step. Additionally, you can make one stem mold pattern from which you will make two identical stem molds.

[FHB02]

Hello Brian and Stephen

Your replies were indeed very helpful. I appreciate it a lot. I have now completed setting up the mold :)

kind regards
Frederik

[Stephen]

Frederik

Glad that I could help you.

Stephen