Brian Nystrom wrote:

How many boats have you seen that have the exact same hull profile
forward and aft of the centerline? Not many are designed that way.

"Exact same profile" not required, just at a single point. How many do
you want? These are 20 year old craft. The model names have been posted
so you are welcome to look them up and let us know. Not many are
designed symetrical fore-and-aft today but at one time all canoes and
kayaks were symetrical. The many ways designers use to keep selling
more boats is a topic for another discussion.

Say they are not symetrical. Then the front section can be inserted
into the rear section and locating the cutting lines is not as
critical. Asymetrical would be easier.


That wouldn't be anywhere near strong enough. To do it right, you need
the inner glass, but you also need to feather the joint back a couple of
inches on the outside, lay in new glass to build the joint up to the
original thickness, then apply new gelcoat.

It may or may not be strong enough in a patch or a scarphed panel but
this is a tube. Any tension on one side is compensated by equal tension
on the other. Just sand off the inside surface, position the two
halves, cellotape around the outside to keep the resin from oozing out,
and glasstape the inside. No need to do any fancy finish work to the
outside. One does need, however to cut the hull carefully so the two
pieces fit.
People have reported using single sided taped butt joins on plywood
boats to avoid the problem of finishing the outside surface. Apparently
it does actually work there.



The bottom line is that this project simply isn't practical.

Where does practicallity enter into the hobby of paddling or
boatbuilding?
Giving up a day's pay to drive 200 miles to paddle down a river is
practical?
My own boatbulding is motivated as much by curiosity as practicality.
Can it be done?
Some don't like to spend money but time is no problem.

Wm Watt wrote:

at one time all canoes and kayaks were symetrical.

Care to back that up with something resembling a fact.

It may or may not be strong enough in a patch or a scarphed panel but
this is a tube. Any tension on one side is compensated by equal tension
on the other.

You really don't understand what a hull does in the real world. Tension has to
be balanced by compression. Local loads have to be accommodated, not just
overall bending moments.

Mike

Michael Daly wrote:
Wm Watt wrote:

at one time all canoes and kayaks were symetrical.

Care to back that up with something resembling a fact.

Moreover, at one time all river kayaks were not only symetrical but
were a standard 13 feet long. It wasn't until computer programs became
available that all the different shapes appeared.


It may or may not be strong enough in a patch or a scarphed panel but
this is a tube. Any tension on one side is compensated by equal tension
on the other.

You really don't understand what a hull does in the real world. Tension has to
be balanced by compression. Local loads have to be accommodated, not just
overall bending moments.

Any tension on one side is compensated by compression on the other.


Mike

Wm Watt wrote:
Michael Daly wrote:
Wm Watt wrote:

at one time all canoes and kayaks were symetrical.
Care to back that up with something resembling a fact.

Moreover, at one time all river kayaks were not only symetrical but
were a standard 13 feet long. It wasn't until computer programs became
available that all the different shapes appeared.

A look at Charlie Walbridge's old book on whitewater kayak making proves this to
be nothing but bull****. It also doesn't address sea kayaks and recreational
kayaks.

To this day, there are people designing kayaks without computer programs. It
ain't rocket science.

Any tension on one side is compensated by compression on the other.

Glad you sorted that out.

If you only reinforce on one side, as Brian pointed out, you'll get a weak
joint. Hull-to-deck seams on some cheaper sea kayaks are only taped on the
inside and are known to have split. No one I know would trust a one-sided seam
for strength.

Mike

Michael Daly wrote:

If you only reinforce on one side, as Brian pointed out, you'll get a weak
joint. Hull-to-deck seams on some cheaper sea kayaks are only taped on the
inside and are known to have split. No one I know would trust a one-sided seam
for strength.

Hull to deck seams are not only problems on fibreglass kayaks but also
on fibregalss sailboats of all sizes, even very large, very costly
ones. It's not because they are only taped on the inside. It's design
and workmanship.

I have a couple of open plywood sailboats which get quite a bit more
stress than kayaks do. Sails can pull 75% dieways and only 25%
forwards when going upwind. That putsa lot of twisting stress on the
hull and pulls it over in the water, all too often to the point of
capsize if you don't stay alert. The plywood panels on my boats are
butt blocked with adhesives that are 1/20ht as strong as epoxy (urha
formaldahyse and polyurethane). I've n to had any problems with teh
butt joins even when the plywood has been delainating.



Mike

Wm Watt wrote:
Michael Daly wrote:

If you only reinforce on one side, as Brian pointed out, you'll get a weak
joint. Hull-to-deck seams on some cheaper sea kayaks are only taped on the
inside and are known to have split. No one I know would trust a one-sided seam
for strength.

Hull to deck seams are not only problems on fibreglass kayaks but also
on fibregalss sailboats of all sizes, even very large, very costly
ones. It's not because they are only taped on the inside. It's design
and workmanship.

I have a couple of open plywood sailboats which get quite a bit more
stress than kayaks do. Sails can pull 75% dieways and only 25%
forwards when going upwind. That putsa lot of twisting stress on the
hull and pulls it over in the water, all too often to the point of
capsize if you don't stay alert. The plywood panels on my boats are
butt blocked with adhesives that are 1/20ht as strong as epoxy (urha
formaldahyse and polyurethane). I've n to had any problems with teh
butt joins even when the plywood has been delainating.

I got kicked off the computer at the public library at the end of the
time slot. So to continue ...

The hulls on these two plywood boats are less rigid than the hulls on
the kayaks I've paddled (about 20 models so far). I've done fibreglass
repairs after reading one or two books on the subject. In spite of what
you read from promoters of epoxy, polyester repairs can be made stonger
than the origninal hull. The same repairs with epoxy are many times
stronger than necessary. That's why I do not see any strength problem
in cutting through a kayak hull and joining with interior taped butt
joins. For the other reasons we have mentioned in this discussion
(cockpit, reduced stability and bouyancy) it's not a good idea to take
the extra lengthout of the centre of the hull so this part of the
discussion is moot.

Wm Watt wrote:
Wm Watt wrote:

Hull to deck seams are not only problems on fibreglass kayaks but also
on fibregalss sailboats of all sizes, even very large, very costly
ones. It's not because they are only taped on the inside. It's design
and workmanship.

You don't get it, you never will. I'm a structural engineer and I know bad
designs when I see them. Asymmetric joint construction is not good design.

The hulls on these two plywood boats are less rigid than the hulls on
[...]
stronger than necessary. That's why I do not see any strength problem
in cutting through a kayak hull and joining with interior taped butt
joins.

You do what you want. But don't advise others to do what is widely regarded as
bad design and construction.

Mike

Wm Watt wrote:
Brian Nystrom wrote:


How many boats have you seen that have the exact same hull profile
forward and aft of the centerline? Not many are designed that way.


"Exact same profile" not required, just at a single point. How many do
you want? These are 20 year old craft. The model names have been posted
so you are welcome to look them up and let us know. Not many are
designed symetrical fore-and-aft today but at one time all canoes and
kayaks were symetrical. The many ways designers use to keep selling
more boats is a topic for another discussion.

What some people have done as far as design is irrelevent. More than
likely, these boats are not symmetrical.

Say they are not symetrical. Then the front section can be inserted
into the rear section and locating the cutting lines is not as
critical. Asymetrical would be easier.

Are you kidding me? It's also quite likely that the ends are different
enough that one will not fit into the other. For that to work, the fore
and aft decks would have to be the same height, which is not common at all.

That wouldn't be anywhere near strong enough. To do it right, you need
the inner glass, but you also need to feather the joint back a couple of
inches on the outside, lay in new glass to build the joint up to the
original thickness, then apply new gelcoat.


It may or may not be strong enough in a patch or a scarphed panel but
this is a tube. Any tension on one side is compensated by equal tension
on the other. Just sand off the inside surface, position the two
halves, cellotape around the outside to keep the resin from oozing out,
and glasstape the inside. No need to do any fancy finish work to the
outside. One does need, however to cut the hull carefully so the two
pieces fit.
People have reported using single sided taped butt joins on plywood
boats to avoid the problem of finishing the outside surface. Apparently
it does actually work there.

It only works because the boats are fully glassed inside and out after
the panels are stitched. Perhaps some designs substitute fillets and
tape on the seams for inside glass, but either way, the boat is
substantially reinforced. I've never heard of a kayak kit or plan set
that uses/recommends but joints without exterior glass and some interior
reinforcement.

The bottom line is that this project simply isn't practical.

Where does practicallity enter into the hobby of paddling or
boatbuilding?

There's got to be a line somewhere. Perhaps others think this is a good
idea, but I don't.

Giving up a day's pay to drive 200 miles to paddle down a river is
practical?

I wouldn't do it.

My own boatbulding is motivated as much by curiosity as practicality.
Can it be done?

Maybe you should offer to shorten these boats for the OP, just to see if
it can be done.

Some don't like to spend money but time is no problem.

Knock yourself out, then!