Straightening a fin box?

[tjhc*]

I've bought a second hand scull (an Aylings AX2) and have found that
the fin box is not mounted in line with the rest of the shell. I
hadn't spotted the problem when I tried the boat out...

The fin sits square in the box - it doesn't appear to have been
knocked. It's the mounting itself which is not straight. But this
leads to the boat pulling towards the middle of the river and my
getting a rather tired right arm.

I'd like to get the box straightened, but wouldn't know where to start
myself. There is no access hatch to the rear canvas at the moment -
only a drainage plug hole.

Any idea where to make a start?

[Jim Dwyer]

An easier fix would be to mount the fin so it is straight when it protrudes
from the box.
You may have to shim opposite edges of the fin or even bend the fin so it
protrudes straight out of the boat when installed.

Jim


"tjhc*" wrote in message
news:c36666ea-16ea-4e66...@fv28g2000vbb.googlegroups.com...

[Jim Dwyer]

I remember Alberto Demetri's boat was not going straight so the boatman cut
bottom of the back of the fin along the boat and then bent it straight.
That was in St. Catharines at the worlds way back in the 70's.

I am guessing it was not an aerofin!

Jim


"tjhc*" wrote in message
news:c36666ea-16ea-4e66...@fv28g2000vbb.googlegroups.com...

[Carl]

Tom -
It used to be incomprehensible to me that a finbox could or would be
incorrectly aligned as, when it happens, the boat's run is seriously
compromised. Unfortunately some builders have been less than careful in
this critical department.

The turning force exerted by a fin, until stall occurs, is directly
proportional to the angle it bears to the boat's axis and proportional
to the square of the boat's velocity. This means it also matters
whether any part of the finblade is itself bent. In particular, it
matters whether the tip is twisted, since the tip is out in the fastest,
least-disturbed flow.

I agree with Jim: if you can do anything to re-align to fin within its
box, then you should do that. However, your scope for re-alignment is
somewhat limited. If the fin root is anything up to 150mm long but only
2mm wide, & you file up to 1mm from one side ahead of the mid-point & a
similar amount from the other side to the trailing edge, then you can
only rotate the fin by ~3/4 degree (& then you have to wedge it in the
box like that). It's all a bit tricky.

As a guide to what that means in real life: a fin which is misaligned
by 1 degree will appear to sight up with a point lying about 10cm to one
side of the bow. I've seen fins which are that badly aligned so I hope
yours is not.

Getting the existing box out of the boat may not be easy, but on that
particular boat I'd guess it's bedded into an epoxy or polyester dough,
& with skill, patience, old hacksaw blades & various other instruments
it may be doable for you.

Failing that, a bodge would be to twist the tip (& as much towards the
root as you can) towards the required alignment.

How to check alignment? Have the boat inverted in or on rigid trestles
with the ball touching a wall. Mark where the ball is touching with a
vertical line (or use a handy brick course joint). Stand well to the
stern & with 1 eye sight along the fin to see where that line of sight
hits the wall. You don't need lasers or anything els, just your eye.

Cheers -
Carl

--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Email: ca...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)

[sully]

On Apr 16, 4:37 am, Carl <s...@sss.jjj> wrote:
> On 15/04/2012 23:24, tjhc* wrote:
>
> > I've bought a second hand scull (an Aylings AX2) and have found that
> > the fin box is not mounted in line with the rest of the shell. I
> > hadn't spotted the problem when I tried the boat out...
>
> > The fin sits square in the box - it doesn't appear to have been
> > knocked. It's the mounting itself which is not straight. But this
> > leads to the boat pulling towards the middle of the river and my
> > getting a rather tired right arm.
>
> > I'd like to get the box straightened, but wouldn't know where to start
> > myself. There is no access hatch to the rear canvas at the moment -
> > only a drainage plug hole.
>
> > Any idea where to make a start?
>
> Tom -
> It used to be incomprehensible to me that a finbox could or would be
> incorrectly aligned as, when it happens, the boat's run is seriously
> compromised.  Unfortunately some builders have been less than careful in
> this critical department.

As someone who works constantly with old
forlorn or rejected equipment, it's 99% certain
the finbox was mounted incorrectly during a
repair, or it might simply be unrepaired damage.

push on corners of the box, make sure it isn't
moving around, else follow previous advice given!