Shortening a kayak?
 

Hanta-Yo-Yo wrote:

You might just clip the ends back to the desired length, and then glass
over the opening to be water proof. That way, the load handling central
structure will not be compromised, as well as the rocker that is needed
to turn easily. Also you will not have to mess with the combing if you
plan to use a spray skirt.

The sharp pointy ends are not needed really as demostrated in the newer
generation of playboats and creekers. You are not looking to achive
great speed characteristics, and depending on how good you are at the
glass work, will tell how the finished product will be.

You could always mount some sort of maiden form on the bow to scare off
the swamp gods, or maybe attract a mermaid! HYY

This is the best suggestion so far to take off a foot or so. I'd leave
the bow alone unless more has to be taken off than can com eoff the
stern. Cutting a foot off the back won't noticably effect bouyancy or
stability. I'd put on a plywood transom. It would be easy enough to
hold a piece of plywood against the cuttoff end and trace around with a
pencil to get the shape. Then, if using an electric jig saw, set the
base plate at an angle equal to the taper of the boat when cutting it
out. A square cut transom won't fit the boat as well. The transom can
be glued into the hull reinforced by bocks of wood glued in before or
nails through the hull or both. The inside sruface of the plywood will
have to be painted before it's put on the boat. My grandfather did this
with a cedar strip canoe that had one end damaged. It worked well.

Shortening a kayak?
 

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

Shortening a kayak?
 

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

Shortening a kayak?
 

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.

Shortening a kayak?
 

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

Shortening a kayak?
 

Michael Daly wrote:
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.

Gee, I got an "A" in engineering math at the UofT but note of that is
relevant to the discussion. You've never tried splicing a kayak or any
other boat, have you? What we see here is the triumph of theory over
practice and knowledge over experience.

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

That's a piece of advice I'm not about to take.
I've read over 50 boatbulding books, including books on scantlings and
strengths of materials and on boat repair, done a lot of calculations,
used and wrote boatbulidng computer programs, and tried a lot of
different things, some of which did not work out.
Butt joins are nowher condsidered bad desing or construction. Testing
shows teh opposite. People can do their own test. Butt join two panels
and then bend the result until it breaks (deforms in technical lingo).
On plywood panels with epoxy adhesive the plywood breaks before the
butt join. It's stronger than the rest of the hull. People can decide
for themselves who they want to listen to.

I've seen Michael's website which I like. (It's not about boat repair.)
We've disagreed before, because he's been wrong before.

Mike

Shortening a kayak?
 

Wm Watt wrote:
Michael Daly wrote:
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.

Gee, I got an "A" in engineering math at the UofT but note of that is
relevant to the discussion.

There's a lot more to structural engineering than a bit of math. The essence of
structural engineering is stress analysis and proper design within safety limits.

On plywood panels with epoxy adhesive the plywood breaks before the
butt join. It's stronger than the rest of the hull.

Butt joins on fiberglass and butt joins on plywood are not the same thing. The
difference in strength between the base material and the adhesive is
significant. Plywood is thick and weak, fiberglass is thin and strong. Lapping
a tape over one side of a plywood joint is not going to have the same effect as
doing the same with fiberglass.

As I've already pointed out - if single taped butt joints in kayak seams were
sufficiently strong, we wouldn't see so many failures and so many paddlers
condemn the manufacturing technique. If the joint was sufficiently strong, it
wouldn't be so sensitive to labour and quality of construction as you have claimed.

Mike

Shortening a kayak?
 

Michael Daly wrote:

Butt joins on fiberglass and butt joins on plywood are not the same thing. The
difference in strength between the base material and the adhesive is
significant. Plywood is thick and weak, fiberglass is thin and strong. Lapping
a tape over one side of a plywood joint is not going to have the same effect as
doing the same with fiberglass.

Wood is as strong, pound for pound, as steel. Fibreglass versions of
plywood boats weigh more. Plywood is as strong as fibreglass used on
the same boat design. I don't get your point here.


As I've already pointed out - if single taped butt joints in kayak seams were
sufficiently strong, we wouldn't see so many failures and so many paddlers
condemn the manufacturing technique. If the joint was sufficiently strong, it
wouldn't be so sensitive to labour and quality of construction as you have claimed.

I assume you mean single-sided taped butt joins. (I don't know if it's
proper to call them "scarfs".) Single layer slingle-sided taped butt
joins would problably be okay but if I were doing it I'd use
single-sided double-layer joins, 4" glass over 2" glass. It adds so
little in wieght, cost, and effort.

A lot of experiments have been done in single- and double-sided taped
butt joins in plywood with different adhesives and fibres. People have
built with polyester, epoxy, and polyurethane resins, and glass and
polyester fibre, to my knowledge, with satisfactory results. On the
19-year-old fibreglass kayak in question I'd stick with polyester
(epoxy for those who want to spend more money) and glass fibre.


Mike

Shortening a kayak?
 

Wm Watt wrote:

A lot of experiments have been done in single- and double-sided taped
butt joins in plywood with different adhesives and fibres.

And other people have done the same with different results. However, we are
talking about fiberglass boats here, not plywood. When you load a butt joint in
tension, the asymmetric, single-sided joint will fail with a significant flexure
in the joint. If it is symmetric, double sided, it will be in pure tension.
The difference in strength is considerable. The difference in fatigue
performance is considerable.

Single-sided seams along a sheer line will _buckle_ if the hull is subjected to
serious load - I've seen this happen. It buckles because the joint is not
symmetric. Under those conditions, the single sided joint can split - I've seen
this happen too.

Why do you assume that _your_ experiments count for more than the experience
that many others have had with single-taped joints? Good kayaks use a lapped
and double-taped seam. That is much stronger than a butted joint and much, much
stronger than a single sided seam.

Mike

Shortening a kayak?
 

Michael Daly wrote:

And other people have done the same with different results. However, we are
talking about fiberglass boats here, not plywood. When you load a butt joint in
tension, the asymmetric, single-sided joint will fail with a significant flexure
in the joint. If it is symmetric, double sided, it will be in pure tension.
The difference in strength is considerable. The difference in fatigue
performance is considerable.

Single-sided seams along a sheer line will _buckle_ if the hull is subjected to
serious load - I've seen this happen. It buckles because the joint is not
symmetric. Under those conditions, the single sided joint can split - I've seen
this happen too.

Why do you assume that _your_ experiments count for more than the experience
that many others have had with single-taped joints? Good kayaks use a lapped
and double-taped seam. That is much stronger than a butted joint and much, much
stronger than a single sided seam.

A sheet of paper is also thin enough to fold. Once a tear is repaired
with tape that part is stronger than the rest of the sheet. That's what
you see in a butt join. It's thicker and more rigid than the rest of
the hull. It won't flex. It won't fold. It can take more stress than
the original hull before failing. If the hull's going to fold and fail
it won't be at the butt join.

Longitudinals and butts aren't comparable. Before there was
firbreglass or plywood plank-on-frame hulls would work open along the
seams but not at the butts where the planks were joined end-to-end.
Longitudinals, being long and thin, are weaker.

Butt joins are strong. The reason people scarf plywood and feather
fibreglass is to get sufficient surface for adhesive strength. With
butt joins the whole butt is the adhesive surface. A 2" butt is
stronger than a 1/2" feather. As I wrote, I would probalyhy use a 4"
over 2" butt.