Stretching a 7'7'' Nutshell Pram by 5%

[Songololo]

In order to make maximum use of the 2.5m length limit (i.e. the limit above which boats need to be registered here), I'm considering stretching the JW 7'7'' Nutshell Pram by 5%. The 7'7'' is the sheer length to inside of planking and outside of bow and stern transom. I have factored in the additional length as a result of the transom sections outside of this.

Having had a look at the WB #60 article "Building the Nutshell Pram" (p101-113), this would be my approach:

• convert all Feet and Inches to cm/mm (sorry, used to working in Metric )
• increase the bow, stern, molds 3 and 4 spacing proportionally by 5%
• leave the bow and stern transom angles unchanged
• leave daggerboard up against mold 2
• move mast (step and thwart) back (the Wee Rob balanced lug will be used here)
• redraw inner fore keel to take account of increased length and shallower angles
• plank from patterns taken off the molds (battens along laps)

Note: I don't plan on altering the mold or transom sizes i.e. the sections.

Planks will need to be scarfed as they will no longer (except for the gardboards) fit on single sheets ...

Will this work? Suggestions?

Anything else that needs to be considered?

http://books.google.ch/books?id=Maf4...page&q&f=false

Lance

[Ian McColgin]

That small a change should not make a performance change, either adverse or positive, noticable to most folk. Some designers make designs, like LFH's tender to the MarcoPolo, that are deceptivly simple but that would not tolerate tampering. Something about the porportions of the bottom rocker maybe. But the Nutshell, lovely as it is, does not appear to be in that class, so . . .

G'luck

[James McMullen]

I stretched the 8' Eastport Pram to 9' purely by spreading the stations further apart and leaving all of the sections absolutely the same in order to take advantage of some 4x10 okoume panels Edensaw had in stock. Worked great!

[Songololo]

Did you do that with a stitch-and-glue CLC Eastport Pram?

[Gib Etheridge]

An easy way to do that is to multiply all of the lengths, widths and heights by 1.05 using a carpenters calculator. As you increase each one you can select to show each measurement in metrics as well, which will save a lot of work and possibly avoid mistakes. A carpenters calculator is a wonderful tool, well worth having.

That's not enough of a change to affect a noticable difference, even though you'll actually be increasing the capacity of the boat by almost 16 percent (1.05 X 1.05 X 1.05, lengths, widths and heights).

I would use the same scantlings and increase the sail plan by the same amount.

Here's a site where you can enter imperial feet, inches and fractions and get the metric equivalent. You'll need to then multiply time 1.05.

http://www.blocklayer.com/English-Metric.aspx

Have fun.

[David G]

I'm not looking at the plans, so Ian may be correct. I will caution, however, that small boats are sometimes less amenable to such tweaking than larger ones. I'll always ask the designer before undertaking such a modification.

[James McMullen]

I worked from the Eastport Pram plans, Songololo. It's actually lapstitch, not stitch and glue, but these techniques are broadly similar. A pretty easy build, actually.

Another shot taken from the inside.

[Songololo]

That's rather interesting! I assume that the EP planks on the plans are laid out so that the station lines are perpendicular to the edge of the plan i.e. also the plywood sheet. Increasing the offset spacing along the edge to fit 10ft sheet length would be a relatively straight forward exercise.

I was wondering if the same could be done with the Nutshell plank offsets. The only thing is, I'm not sure if they are laid out to maximize the usage of a plywood sheet or laid out so that the station lines are perpendicular to the sheet edge ...


Lance

[JimConlin]

If you're finicky, the rake of the transoms will increase, too.
Do the same 1.05 scaling on the longitudinal positions of the tops and bottoms of the transoms.

[rbgarr]

I'll always ask the designer before undertaking such a modification.

That will be tough, but let us know if you get an answer.

[Songololo]

Jim, yes, after having given it a bit more thought, that makes sense. I think that a 1/4 scale lofting exercise is probably a good idea to give me a feel for how different the transoms would be, compared to just moving the current 'ends' out by a factor of 1.05.


Has anyone had any Nutshell experience with:

• making the plank patterns directly from the molds?
• stretching the plank offsets shown on the plan?