16" Prospector Station 7 & Stem Issue

[bullitt168]

Hello everybody,

I started on my Prospector 16" and lofted the plans from Canoecraft.
I set up the mold, built the stems and up until now everything worked very well.

When shaping the Stem, i realized that there is a big gap between the contact surface of the inner stem and the contact surface of station 7.
(See first picture).

The contours of the stem contact surface and of the station 7 don't align well, so there must be something wrong.
In order to align the stem contour with the station contour, I'd either have to move the station up by approx 1/2" or the station had to be wider by approx 1".
What I did so far to in terms of error checking:
- I checked for errors when reading the table from the book
- I checked all measures of the station & stem
- I checked whether i deducted the hull thickness from the station correctly
- I checked whether the station is too low (too close to the strongback)
- I checked whether i deducted too much/too little from the stem mold when cutting station 7 in halfes
- I checked the distance from station 7 to station 6

Obviously, i didn't find any error or significant inprecision yet (all measures are within +- 1mm tolerance)

So... Do you have any hints what may be wrong? Is this normal and i should just carry on living that some strips don't touch station 7? (I read somewhere in this forum that some people just ignore station 7 at all.)
Should i change station 7 to follow the "logical" curve of the hull?

I attached some more images to give you a better overview of the situation.
Thank you for your support and happy easter from the black forest in Germany,
Bastian

[Cruiser]

When I lofted my Freedom 17, I took off stem thickness from the stem forms asince I was using stems.

However, my understanding was that there was no need to account for the strip thickness on the rest of the forms, that was already assumed to 1/4" ... that may just be my version of CC, but I was reasonably certain that applies to the tabular plans.

In that last pic of where the stem meets the form ... I have always carried the flat area to the end of the stem, it doesn;t look like you have and also may not have the stem high enough to do so, I think there is an issue, maybe having to do with your choice of deducting the hull thickness ( if I understood you correctly).

Brian

[Stephen]

Bastian

I'm not completely sure what caused your problem, but I did spot something in your method that may have contributed.

- I checked whether i deducted the hull thickness from the station correctly

The following is from the FAQ on the Bear Mountain site. If you're using the German edition of the book I don't know if the page numbers will match. https://www.bearmountainboats.ca/pages/faq

The offsets in Canoecraft are for the station molds. There is no need to reduce the offsets for the thickness of the planking - that has already been done. Unfortunately, this is stated incorrectly in the text on page 42, second column, fourth paragraph of the 2000 edition.

When you plotted your stem mold, how much did you remove from the curve of the stem mold to accommodate the inner stem? Did you just remove the thickness of the inner stem, or did you also include an allowance here for the hull thickness. If you removed the hull thickness from the stations but not the stems that would cause a mis-alignment.

It seems that what you have done will make a canoe 1/2" narrower than designed. If you can tolerate a narrower boat my best guess is that you would have to rework the stem molds to match the smaller hull, but I'm unsure what the best approach is going forward.

Happy Easter
Stephen

[bullitt168]

Dang!

I am using the 2000 edition of CC :-(

So i ran exactly into the error described in the FAQ.
Thank you very much @Stephen for the hint.
At least now I know what's the root cause of my situation. I paused the project already for several months when i found the issue, so I am actually kind of relieved that I now know what's the matter.

I am also not sure yet on how to proceed. I see the following options:

1. Tolerate the 1/2 narrower Boat. in this case, I will have to rework station 7 and find a "middle ground" between stem and station 6
2. Add thin strips of the hull thickness to the molds to compensate the previosly deducted hull thickness. While this would bring me "back on track", it's kind of "hacked". I'd also have to raise the stem mold by a little bit to be flush with station 6 again.
3. Scrap all Molds and re-do them. While of course this would be the clean solution, it's frustrating and a waste of time & ressources to do everything again.

Thank you again @Stephen for finding my root cause.
In case anybody has any other clever solutions on how to proceed I am curious to learn about them.

Bastian

[Cruiser]

Personally, I would work on option 1 .... you know now what you did, so adjusting the stem, 6 and 7 molds should be straight forward. You will never notice a 1/2" trim job on the canoe IMO.

When you do your "adjustments" just be sure to take strip and check the fair carefully, the front needs to fit the rest perfectly.


Brian

[bullitt168]

Yeah, After some more time considering I will proceed with Option 1 and re-do station 7 (and maybe 6 as well)
Thanks!

[Captain Barnacles]

Hi Everyone,

I am just starting to build my first canoe, a Prospector 16, and have found a similiar issue to that posted by the OP except that in my case it is station 6 that is causing me concern. When I clamp the first strip to the molds and look along the length of it I have a compound curve. From station #0 to #5 I get a lovely fair curve but then at #6 it comes in more sharply and begins to curve outwards again towards #7 then it changes again to curve back in towards the stem so looking from above I have a very loose "S" shape.

If I release the clamp from #6 the strip follows a fair curve but I am left with about a ¼" (~6.5mm) gap between the strip and mold #6. Checking my set of plans it looks like the curve straightens out towards the bow/stern but doesn't appear to change direction. The effect is the same at both ends and on both sides of the canoe, at least I know I have some symmetry I suppose!

I have checked the accuracy of the molds and everything is very-near-perfect (I was very meticulous when tracing and cutting the molds etc). I have checked and re-checked everything that I can think to check and it all seems to be as it should so I am left wondering whether what I have is correct or not.

I'd be really grateful if anyone can advise me how the curvature of the first plank should look please. I remember reading a boatbuilding book that said something along the lines of "if it looks right it probably is right". To me, what I have just doesn't look right.

Paul.

[Cruiser]

This is where a few pics would help "us" help you.

If the forms are producing anything other than fair curves there is a problem ... regardless of the cause.

If you look at the plans (assuming you have a half set), look at the spacing between forms, if they all look regular, then you need to take a hard look at your form 6 and the one preceding and following ( any of them can cause this issue). If the forms seem to have regular spacing, you likely have an execution problem and need to find that before proceeding.


Brian

[Traveler]

From your description it sounds like station 5 is wider than 6, which should not be the case given that I believe 7 is the middle and widest station. Are you sure the numbering didn’t get reversed somehow in the setup, such that what you have assumed is station 6 is in fact station 5 (and vice versa)? Worth a check if you have not done so already. I have the forms for this canoe cut but unused in my garage and can look at what I came up with tomorrow if that might be helpful.

[Traveler]

Hmmm - as I read the first parts of this post I think I have a different station numbering system in mind. My station 7 is the mid station for the whole canoe but your station 7 seems to be the stem side pieces? If so, then ignore my post above as I am working from a different set of plans than those for your prospector. Thx

[Captain Barnacles]

This is where a few pics would help "us" help you.

If the forms are producing anything other than fair curves there is a problem ... regardless of the cause.

If you look at the plans (assuming you have a half set), look at the spacing between forms, if they all look regular, then you need to take a hard look at your form 6 and the one preceding and following ( any of them can cause this issue). If the forms seem to have regular spacing, you likely have an execution problem and need to find that before proceeding.


Brian

Many thanks for your helpful replies and apologies for my delay in getting back to you sooner.

I had a very careful look at the spacings and alignment of my molds and the only thing I could find that I wasn't sure about was how I had measured station layout lines from the centre mold. I contacted a Barry Biddlecombe of cedar-strip.co.uk (from whom I bought my plans and cedar strips etc) and he diagnosed that I had indeed measured wrongly. I was initially reluctant to remove all of my molds and reposition them, after all it was only a matter of about ¼-½" or so.

After getting over the realisation that I was going to have to remove all of the molds and start over repositioning them I got stuck in and finished the job in about 3 hours. I laid a strip along the molds expecting that I'd still have the same unsightly curves but lo and behold the strip sat beautifully and looked very "right". I could not believe that such a small change in the position of the molds made such a profound difference in the lines.

Hmmm - as I read the first parts of this post I think I have a different station numbering system in mind. My station 7 is the mid station for the whole canoe but your station 7 seems to be the stem side pieces? If so, then ignore my post above as I am working from a different set of plans than those for your prospector. Thx

:) Yep, I have my molds numbered from 0 in the centre to 7 on the side of the stem mold. This is just how they were numbered on the BMB plans.

And so I have finally begun to build my canoe. I am about 18 strips up on each side, I am working staple-less (for now! - it is getting harder as the stems are still almost vertical but towards the centre the strips are now almost horizontal) and managing to do two or three sets of three strips per day so it's taking shape pretty quickly.

Thanks again,
Paul.