Affect of tucking rails on boat geometry

[sbroam]

A while back the subject of tucking the rails, or shortening the thwarts, of an open boat came up. I've done it before and subjectively observed that the boat felt "faster" and "drier" with an indeterminate effect on turning. Tonight I had an opportunity to take some measurements on an empty hull with no thwarts yet in place.

The boat is a Mad River ME, about 15' long, shallow arch cross section. I took the width measurements at the location of the center thwart, slightly behind dead center. The widths are outside to outside of the gunwales. Depth is taken with a spirit level on the thwart ; measured vertically from the floor to the top of the gunwale with a carpenters square. Bow height similar.

*tucked in
width : 31"
depth at center : 14"
height of bow : 22.5"

* no thwarts, relaxed
width : 32.75"
depth at center : 13.5"
height of bow : 23"

* factory thwart width
width: 35.5 "
depth at center : 12.75"
height of bow: 24"

* spread out
width : 37.5"
depth at center : 12.25"
height of bow : 24.75"

There is a pretty clear direct relationship between with and bow height (presumably also rocker, however that is measured) and an inverse relationship between width and depth. No surprise really.

I'm curious what is happening to the cross section - that's harder to see - when tucked in, do I get a bulge amidships in the sides? I'd think that might help turning?

Any how, I'm inclined to pull it in from the factory width to the "relaxed width" of 32.75"

Scott

[Mike W.]

Hey Scott, We pulled in a 17' Old Town Penobscot. I did not measure 4" waterline width. It will look like a bulge at midship on the sides at or above the waterline. I think it will actually be a little narrower than stock. Pulling in the gunwales has more effect at the gunwale than at the waterline since the bottom tries to maintain it's width. You may also notice a slight arch in the hull after you pull it in. That's where the increased center depth came from.

Pulled in, it felt like the boat was faster, tippier, tracked better & was harder to turn. Everything is a compromise. Play with it & see what you like.

[yarnellboat]

Scott,

I pinched my tandem Prospector from 37.5" to 35". I find it has less initial stability and has more "edges," it's more fun to turn for sure. It's probably faster too, as it's narrower and probably more arched. I also moved the seats in towards centre to compensate for any loss in rocker/bow height.

It's describebd better on the Canadian Canoe Routes forum:
http://www.myccr.com/SectionForums/viewtopic.php?t=9138

But I also think differences are pretty minor tweaks, it's not like you can pull in the gunwales and make it into an entirley new type of boat.

Pat.