Form Base Height for stem clearance - Prospector 16

Welcome to the new Bear Mountain Builders Forum - an interactive internet service we provide to encourage communication between canoe and kayak builders
Post Reply
Rehd Brown
Posts: 148
Joined: Sat Sep 08, 2007 8:23 pm
Location: Fresno, California

Form Base Height for stem clearance - Prospector 16

Post by Rehd Brown »

Hi all..

During my Sairy Gamp build I’m drawing up a set of forms for the Chestnut Prospector, shown in CanoeCraft, 97 vintage.

I have the basic forms drawn w/sheerlines marked.

I’m building on my strong-bench which is 16’ long and 10” wide. I’ve noticed some folks having to carve up their bench because the strips at the end won’t clear and still make it to the stems. My bench is a box-beam so I can't trim it down.

My question is: From the base line down( or up ), how far down/up do I make the base of the forms in order to clear the bench with the stems and strips.?


Rehd
User avatar
Jim Dodd
Posts: 1359
Joined: Wed Feb 08, 2006 11:08 pm
Location: Iowa

Re: Form Base Height for stem clearance - Prospector 16

Post by Jim Dodd »

Just enough so your stem forms clear the strongback surface, by a few inches.
More if you want, as you said, you are worried about strips.

Jim
Keep your paddle wet and your seat dry!
User avatar
Cruiser
Posts: 867
Joined: Sat Apr 27, 2013 10:21 am
Location: Bowmanville, Ontario

Re: Form Base Height for stem clearance - Prospector 16

Post by Cruiser »

Not sure if you are drawing from a set of plans (tracing) or actually lofting stations from tables, but I think the process will be the same. (Please note that I think plans already build this in)

I believe it's the shear that you actually want to follow, and in this case if you look at the values for a the mid form and compare that to the last forms at the stern and bow, you should be able to get a sense of how much the boat is sweeping towards the strongback. The difference between the 2 values gives you the actual distance required to clear the strongback at that point .... i.e. add the difference to the mid point form and the last form will just clear the strongback.

It isn't the total distance required, because you still need to project the space between the last form and the tip at each end and add that to your total shear difference. I add an additional 2" because I wouldn't want the ends right on the strongback, a little breathing room is advisable.

Obviously, you need to use the larger difference value, which will usually be at the bow end for obvious reasons.

I did a post on lofting and drawing up forms from tables, you may find something useful there as well:

viewtopic.php?f=9&t=4658


Brian
Rehd Brown
Posts: 148
Joined: Sat Sep 08, 2007 8:23 pm
Location: Fresno, California

Re: Form Base Height for stem clearance - Prospector 16

Post by Rehd Brown »

Hi Brian

I must be missing something both in the Canoecraft and in your post on lofting. I have the forms all drawn according to the numbers on the Charts. That gives me the form shape, but mentions nothing about the height of the forms off the strong-back. I see in your referenced post that you have the bases drawn on your drawings. Where did you get the length of those bases? I'm not seeing it or just read over it somehow. In my copy of Canoecraft ( 97 ) the baseline is at the bottom (toward me) and is drawn upward toward the sheer. The sheer height is given on each form and that's where the numbers stop. I see no reference as to how high up to go for the base of the form. I'm sure I could figure it out, however, I was hoping there were actually numbers to extend the form for that purpose so I didn't have to strain my brain. They will be different lengths for each form, so, to get them correct, they should all be drawn now, from the baseline of your drawings. I am using 24" x 36" graph paper for my drawings, so I would need to add some paper to a couple of them to extend them upward.

Hope this makes some sense.


Rehd
User avatar
Jim Dodd
Posts: 1359
Joined: Wed Feb 08, 2006 11:08 pm
Location: Iowa

Re: Form Base Height for stem clearance - Prospector 16

Post by Jim Dodd »

You are correct .
Ted's plans do not show the extended form base.
In the newest edition(2015), page 92, there is a photo that should illustrate, what you need to know.
If you have draw the plans, just lay them down, and add to them. Again, go by the shearline or top of your stem forms, they should be just above the base line. Then extend the base lines of the rest of the forms, accordingly.

Jim
Keep your paddle wet and your seat dry!
User avatar
Cruiser
Posts: 867
Joined: Sat Apr 27, 2013 10:21 am
Location: Bowmanville, Ontario

Re: Form Base Height for stem clearance - Prospector 16

Post by Cruiser »

I don't have the older copy of canoecraft, but for the tables to work, they all have to be referenced from the same baseline.

So on each drawing, you draw a centreline and a baseline, the baseline can be marked in at the bottom of the drawing or any positive value above that .... the rule is that all drawings/stations must use the same reference for baseline though, so if you decide you want a 2" extra clearance/space above the strongback (that's what I used) you just draw all the baselines in at 2" up from the bottom of the paper.

There is no formal "stand off" or support piece measurements. Once the baseline and centreline is drawn in, you then measure up from the baseline and mark the sheer line. If the sheer for a given station is 5", then you measure up from the baseline and draw a horizontal line at 5" above the baseline. All of the table values should/will be above this measurement ( I am assuming build orientation, which is actually upside down, for the canoe).

This leaves you with a space between the baseline and the sheer, this is the support piece ... you just decide how wide you want it and draw vertical lines between the sheer and baseline, this will form the part that attaches to the strongback. As you draw each mold you will see that the space between sheer and baseline will vary depending on the station, as you move from the middle to the bow or stern the space will decrease ... this is describing the shape of the top of the boat, which usually sweeps up as you approach either end.

Hope this helps

Brian
Rehd Brown
Posts: 148
Joined: Sat Sep 08, 2007 8:23 pm
Location: Fresno, California

Re: Form Base Height for stem clearance - Prospector 16

Post by Rehd Brown »

Hey Brian

My copy is opposite what you are describing. Baseline / Center line done, forms drawn and keel is nearest the baseline, and you go out and up to the sheer. Since the sheer is different for each form, I'll have to pick an arbitrary line above and make it the same distance from the baseline for all forms. Which means I will have to tape more paper on top of my graph paper. I kind of figured that much, but, was sure hoping there were numbers somewhere for that..!

Thanks for the help.. back to work here..

Rehd
User avatar
Jim Dodd
Posts: 1359
Joined: Wed Feb 08, 2006 11:08 pm
Location: Iowa

Re: Form Base Height for stem clearance - Prospector 16

Post by Jim Dodd »

I dug out my drawings, of Ted's Chestnut Prospector, from years ago ! I should say, you owe me. But It was so much fun walking down memory lane, I Thank You !

OK. Drawing the plans from the book you draw them with the base line from the keel up. I'm thinking that's what you did. I did .

Take the drawings and turn them 180 degrees, draw a new base line 20" down from your original base line.
Add 8" of height to this new base line. Make this base 8" wide. and you should have it.

I have not built with the extended base as Ted does. To me, it's a waste of plywood. But as they say, "There's many ways to skin a cat" I can see a benefit, it's just that I'm cheap ! :sad

Some one can correct me if I'm wrong !

Jim
Keep your paddle wet and your seat dry!
Rehd Brown
Posts: 148
Joined: Sat Sep 08, 2007 8:23 pm
Location: Fresno, California

Re: Form Base Height for stem clearance - Prospector 16

Post by Rehd Brown »

Well Jim, I can't correct you cause I really don't know if you're cheap.!

But, That sounds like a good idea. Kinda what I'd thought I would have to do. I'll have to add 4 or 5" to the top... uh.. 180* bottom of my paper and do what you suggest.!
:tu

Rehd
Rehd Brown
Posts: 148
Joined: Sat Sep 08, 2007 8:23 pm
Location: Fresno, California

Re: Form Base Height for stem clearance - Prospector 16

Post by Rehd Brown »

Well, I sure feel like a dummy..!!

I mentioned earlier that I'd have to tape some graph paper onto my forms to get it tall enough to add the base pieces... My grandson is over for the night and asked what I'm doing. I told him and he said "Why don't you just turn the paper sideways". I coulda smacked him. I'm drawing just half forms, as I can just flip-flop them to mark forms on plywood. So, none are wider than 17". My paper is 24" x 36" with 1" squares and 1/8" marked in all.

That's why I work with wood and not Rockets..!!


Rehd
Post Reply