Query: Lengthening a design and its effect on rocker

[Holmes375]

How is the designed degree of rocker affected when lengthening the design by moving the stations farther apart (equal distance), and lengthening the stem mold by the same amount?

What if you move all the stations farther apart by a small amount and leave the stem forms as originally designed?

I stretched a NWC Cottager a few years ago, a friend's build. It came out nice but I did it differently. Spaced the two forms fore and aft of center by 1", the next two by 3/4", next two by 1/2" and the last by 1/4". Stems molds were left as per design plans. The hull finished out at about 15 foot, 10 inches. I've only paddled it a few times but it paddles quite nice for a recreational style tandem. I have no baseline though to compare it to, of course. My friend and his family like it very much.

I'd like to stretch a NWC Portage Solo 6-8" but having little design knowledge I'm concerned about altering the liveliness of the boat. I understand the additional length in itself will have an effect but I don't want to lose the amount of rocker in the original design. Altering the length of the stem molds makes me nervous as it would seem small changes there might have a dramatic effect.

The Portage Solo is a 15 foot design, max beam 29", waterline beam 26.5" , 2" rocker at the bow and 1" at the stern.

Most of you know more than I about canoe design and I'd be interested in hearing how you would stretch this design without drastically altering its original characteristics.

Thank you.

 

[Jim Dodd]

Great question Holmes !

I've altered several designs. This to me adds a lot of excitement to the build !
Just looked at the" Passage" solo, the design, I think you are referring to ? The stem profile isn't shown in Dennis's description.

I think you would benefit by stretching it, I'd shoot for 15' 9" or so. That's about all you can get from a 16' plank. 16' would be even better, if you have longer strips, or don't mind splicing ?

I'd add a little more towards the middle, and gradually reduce the spacings as you go. I have also tried to add a little A-symetry to a design, (my Pearl) . Trying to give it a sharper, sleeker entry. I really couldn't tell much difference !

Rocker ? I really don't think you will notice much difference, but rounding the stem forms a little should help

Years ago, a good friend built two Bruce Kunz Merlins. The Merlin as Bruce designed it, had a near Plumb stem, making it track straight. It was a little difficult to turn, creek paddling. We added rocker to the stems, and this made quite an acceptable difference !

The most important thing to remember when altering a design is FAIRING it !
When you play with form spacings, lay a strip against the forms, both the sides and bottom, and make sure the forms follow the strip ! You don't want any funny Lumps in the hull. Make them as gradual as possible.

I'm excited ! Enjoy the adventure of designing !

Jim

 

[Canotrouge]

Lots of white water boats were modified in length w/o to much lost in rocker or design specification by lengthening(or shortening) the middle section only of the boat. some time as mush as 12-14 inches. John Kazimierczyk did that a few times on different boats, his latest was on the Blink to create the 20/20, The boat handles the same way, but have more speed and track better!!

 

[Sven]

I built the design which I believe you are talking about. I did no stretch it so I have no idea how that affects the handling. Anyhow here are my two pennies. I built with forms spaced at 12" and ended up with a boat that was only 14'7" long, not the 15' stated on the specs...I wish I had stretched the design a little bit (about 12" total) but really only for the increased volume for tripping.
I can tell you for certain that the rocker in the bow is almost completely attained in the first 2-3' of the boat. Therefore I would imagine stretching it would not affect the turning much.

Here are a couple photos for reference

Untitled by Steven Fike, on Flickr
overlay of bow and stern forms

Untitled by Steven Fike, on Flickr
profile shot of the bow

 

[Holmes375]

Hey Jim.

15'-8" or 9" is the tentative target. I build with short strips - 7' - thus butt joints are my friend I'd like to stay under 16' just to avoid the extra gunwale scarfs. And my storage slot for this boat is 16'1" long.

I'd like to make it a pooch friendly paddler for my 45 lb kibble cruncher. I have a Magic set up for tripping and its a superb boat but I'd like this one to be a little more playful in the lumpy water and a bit wider amidships for the dog.

For fairing my forms I cut several full length strips from a scarfed 1x. I cut these at 3/16" so they're nice and flexible, easy to tack in and out. I have to build a new strong back and I'll make it user friendly for moving the forms around with clamps.

The traditional and symmetrical hull of the Cottager made the stretch quite easy. I built an 18' x 20" wide sea kayak and stretched it 4" to accommodate my size 12s and 35.5" inseam. Took measurements until I was blind trying to figure that out. Got lucky and it fit perfectly. Amazing since it was my first build.

 

[Holmes375]

Lots of white water boats were modified in length w/o to much lost in rocker or design specification by lengthening(or shortening) the middle section only of the boat. some time as mush as 12-14 inches. John Kazimierczyk did that a few times on different boats, his latest was on the Blink to create the 20/20, The boat handles the same way, but have more speed and track better!!

Very cool! Thank you.

 

[Holmes375]

Hey Sven.

I followed your build - and others' here - quite closely. Immensely educational.

Thanks for posting the stem form pics - that's helpful. They must be shorter than I expected to finish at 14'7".

 

[Jim Dodd]

Hey Jim.

15'-8" or 9" is the tentative target. I build with short strips - 7' - thus butt joints are my friend I'd like to stay under 16' just to avoid the extra gunwale scarfs. And my storage slot for this boat is 16'1" long.

I'd like to make it a pooch friendly paddler for my 45 lb kibble cruncher. I have a Magic set up for tripping and its a superb boat but I'd like this one to be a little more playful in the lumpy water and a bit wider amidships for the dog.

For fairing my forms I cut several full length strips from a scarfed 1x. I cut these at 3/16" so they're nice and flexible, easy to tack in and out. I have to build a new strong back and I'll make it user friendly for moving the forms around with clamps.

The traditional and symmetrical hull of the Cottager made the stretch quite easy. I built an 18' x 20" wide sea kayak and stretched it 4" to accommodate my size 12s and 35.5" inseam. Took measurements until I was blind trying to figure that out. Got lucky and it fit perfectly. Amazing since it was my first build.

The Yak looks Great !
Stretching will not only give you more volume, but you should notice an increase in speed.
With the doggie payload, i'd give the mid section more of the stretch.
I'm with canotrouge on the handling issue !

Jim