The
deck stringers are pieces of 18 mm. x 60 mm. Oregon. There would be
five of them if it were not for the hatches, but where the hatches go
the continuity of the midline one is broken. The two immediately
lateral to the midline are spaced as widely apart as is necessary for
the hatch, which depends on motor and access considerations. The
plan
allows for 600 mm.
The
foredeck stringers laid out on top of the bulkheads
where they will be housed.
They sit vertically in grooves cut in the bulkhead top frames, except
where the width of the stringer is too great for the width of the top
frame, such as occurs at the transom and at the outer end of bulkhead
B. Here, half lap joints are used instead.
As with the floors in the cockpit, there are things I would do
differently on the deck stringers. At least the floors were horizontal,
but the top frames of the bulkheads are cambered, so even scribing a
set of perpendicular lines 18 mm. apart is not easy. It would have been
preferable to have cut these housings in the frames before they were
glued to the bulkheads and shaped. Too late now.
The stringers are let into the inner lamination of the sheer planks
forward of bulkheads A and B, and in view of the changed arrangement I
am making for the windscreen, they will not terminate at bulkhead C,
but will continue backwards into the cockpit for a variable distance
which is determined by the angle between the two halves of the
windscreen and the location of the front end of the coaming. The bottom
windscreen frame will be attached to them, so they need to have angles
cut across both their width and thickness to accommodate the rake of
the screen as well.
Because they are vertically upright, their upper edges have to be
planed to the camber and slope of the deck.
Before the days of epoxy these stringers had to be fixed to the hull by
screws or nails into the sheer, or, preferably, into a clamp attached
to a frame attached to the hull, so that there was no danger of the
stringer splitting around a screw placed too close to its end. Now, all
that is really necessary is to fix them in place temporarily while the
epoxy sets. Later, when the deck is laid, there will be a line of glue
running around sheer, stringers and bulkhead top frames, which will
bind the entire deck structure together, aided by numerous blocks which
will be located beneath such deck hardware as cleats, fairleads,
bollards, etc.
The first step is to mark the positions of the lap joints on the top
frames. They start with the inner stringers, 600 mm.
apart, and the outer ones are about 250 mm. lateral to those. At
bulkhead C the depth of the top frame at 300 mm. off centre is 140 mm.,
which is enough to house the entire width of the stringer. At 550 mm.
off centre it is 120 mm., which is still enough. At bulkhead B those
figures become 120 mm and 90 mm., so the outer joints here will need to
be halved.
Positions of the deck
stringers marked on the dash bulkhead (left), and the centre line on
bulkheads A,B and C (right).
At bulkhead A the figure for the inner stringer is 90 mm. (The outer
stringers
do not cross bulkhead A.) So you would imagine that a halving joint
might be necessary here too. However, bearing in mind the slope of the
deck that may not be the case: one way of producing the necessary curve
in
the stringer would be to plane down its width at the forward end, which
would leave a narrower board for joining to the bulkhead A top frame.
(The
only other method would be to bend the stringers into a curve, just as
the carlings were, but that would have the same problem associated with
it, namely that between the bulkheads the stringers might tend to take
the shortest route, a straight line instead of a fair curve. I imagine
that there would also be a fair chance of them snapping).
The flat white batten is nailed to
the bulkhead top frames. The dark wood, which is a proper springy
batten, is clamped to it at the frames,
but the difference in curves is plain to see. The flat
batten takes a straighter course between frames, while the dark one
makes a fair curve.
Using the batten
like
this has also shown up an unfair deck slope. With the batten fixed at
the bow and at bulkhead C it sat happily on bulkhead A, but rose a few
millimetres above bulkhead B. The original plan for this boat was for a
20' launch, but the alterations for the 21'6" version merely stated,
rather poorly in my opinion, that the measurements for the camber of
the top frames of the bulkheads was approximately the same as that for
the shorter boat. The "approximately" has now come back to bite me.
The batten rises
above bulkhead B.
To overcome this,
in
an area of the boat which will be highly visible, I decided to add a
shim to the top of the bulkhead so that the batten touched it in a fair
curve. The shim, a left-over piece of spotted gum from the stem, is 3
mm. thick, which is a tiny bit thicker than is required. After the glue
has dried I have to plane it back to fit precisely, and feather it onto
the sheer.
The shim on top of bulkhead B (held
down by brads) allows
the batten to ride true over all the foredeck structures.
The shim overriding the sheer (left) and needing trimming,
and over the
top frame (right).
Now, to house the
stringer into a full width trench in bulkhead A, and another in
bulkhead B for example, would require the stringer to take a straight
line from the
top of A to the top of B. Clearly, they have to be left a few
millimetres proud of the top frames to allow for the rise between the
bulkheads. The line of the true batten can then be scribed onto the
stringer, and the shape achieved by planing.
The first trench cut then is in bulkheads B and A, and I will start
with the
midline one, cutting down about 50 mm. to start. Since this short
stringer does not have to cross the full thickness of the bulkheads it
seems neater to house them in a less than full thickness trench. That
means using the router, so a guide is set up on the bulkhead, aligned
to the vertical, and a groove is cut with it. I happen to have a 20 mm.
router bit, so I leave the stringer 20 mm. thick, rather than the 18
mm. specified in the plan. Its under side is given a short shoulder to
fit tightly up against the mortise, and the fair line of the batten is
scribed onto its top. The shape is planed, and the first stringer is
finished. It is not glued in yet, for ease of access to the fuel
compartment later.
The short stringer between bulkheads
A and B.
The
router is also useful for cutting the full thickness trenches, and much
easier than doing it by hand, even allowing for setting up a jig for
each cut. 300 mm. to the port of the midline the second stringer is put
in, but only after a struggle with the extremely difficult
stringer-to-sheer joint, which is basically trial and error and
whittling. Again
the batten is used to scribe a fair line onto the stringer, and it is
planed to shape.
A line of housing trenches is
ready for the stringer (above), and the batten is used to fair the
curve (below).
The
stringer is left hanging into the cockpit, as it will be needed for the
windscreen frame later, and, again, it is not glued in because the
compound angle which will be needed on its cockpit end has not yet been
determined.
The stringer after trimming to a
fair curve.
Another
day, another stringer. Because the outer ones are taking a direct route
to the sheer they are initially high on all bulkheads except the middle
one, bulkhead B, and are planed down to the fair curve line. This,
however, means that they are less than 60 mm. wide at bulkhead C (55
mm. in fact), 60 mm. at bulkhead B, and less than that again at
bulkhead A. It does not affect their strength, but the hatch is
supposed to lie on drip channels which are attached to the underneath
surface of these stringers. Either the hatch will have to be built
thicker at the front than the back, or the channels will have to be
constructed thicker at the back than the front, or a wedge shaped shim
could be added to the bottom of the stringers to equalise their widths
at C and B.
Only the two outermost
stringers are left to be fitted here. Then a short half beam between
the inner stringers will
provide one support for the hatch close to the dash bulkhead, and a
similar one attached to the rear of bulkhead B
will provide the other one.
The third and fourth stringers
are fitted.
In the end I
decided
to add shims to the underneath surfaces of the stringers. They were cut
wedge shaped and glued on, but in order to make them both exactly the
same width I planed them down together when the glue was dry.
Shims on the stringers stretch from
bulkhead B back to a point 600 mm. to the stern of it.
The overhang of the stringers into the cockpit is trimmed to
approximate length and angle.
The spotlight which I
have acquired is one which would have been
mounted on the ceiling of the wheel house of a cabin boat. It can be
directed by turning the knurled wheel which projects through the
ceiling into the cabin.
Now, if I am to be
able to use this mechanism I need to have the wheel come through the
deck and into the cockpit, where it can be reached from the driver's
seat. That means that a centrally placed rearward extension of the
deck stringer will be in the way. It will have to stop at the level of
the dash bulkhead, and two short supporting struts will come back
in its place to straddle the spotlight mechanism and anchor the central
post of the windscreen behind it. Alternatively, I could dispense with
the central one altogether, and replace it with two spaced ones. This
was my solution:
That does not leave much room for the wheel to rise if the light is to
be pointed upwards, but plenty for swinging from side to side, although
it will lose another 10 mm. when the deck is added. If necessary I can
cut some scoops into the short central stringers. Meanwhile, on to the
hatch.
The
plan allows for hatches in both fore- and rear decks, which are
nominated to be about 600 mm. x 600 mm. but whose size can be
determined by engine size. In Ariadne's case, the
motor is much smaller than 600 mm., but I am not, so in order to gain
access to the motor compartment I shall leave the dimensions as they
are. The foredeck hatch beams and stringers are supported on combined
drip channels and hatch supports, which, in turn, are supported by the
bulkhead B top frame, deck stringers, and, at the aft end of the hatch,
by a single manufactured deck
half beam running between the deck stringers only. The hatch will have
to have a camber continuous with the deck, which means that its beams
will have to be curved. That is easy enough with the beam which lies
against the bulkhead B top frame, as its curve can be taken from that
member, but the other hatch beam will have to correspond with the deck
half beam. I favour forming the athwartships drip
channel/hatch supports from laminated stock rather than solid, but both
hatch and deck stringers can be made from solid lumber carved to the
required shape, and that is how I intend to shape the camber on the
half beam.
Here, the half beam between
the inner stringers is having a piece added to it above for shaping to
the deck camber.
Rather than try to make a curved drip channel athwartships, to match a
curved deck half beam and curved hatch beams, it seems simpler to have
them straight, with the hatch shaped more like a chest top. And,
instead of making the hatch in one piece and hinging it to one side, I
think it looks better to have it made up in two halves, opening from
the midline.
I had intended to
join the half beam to the deck stringers with sliding
dovetail joints, but they are not very strong in soft wood like the
Oregon I am using. Instead, I used dados which I will reinforce with
corner blocks.
The original drip channel/hatch supports were made up of three
thicknesses of 9
mm. marine ply. The first two are shown attached to the underneath of
the lateral stringers. But I was a bit dubious about the 8 mm. x 8 mm.
dimensions for the channel. That might be all very well in the Thames
drizzle, but I reckon that a Sydney storm or some hoon of a water skier
could dump a whole lot more water than that on the hatch, right on top
of the electrics! So I replaced the supports with larger ones made up
of two pieces of solid lumber, which allowed a greater cross
section of flow for any water.
A ply drip channel/hatch support is
attached to a lateral stringer.
The new drip channels/hatch supports being made of Oregon...
...and presenting a slightly wider channel (12 mm. X 20 mm.)
This still leaves
unresolved the question of how to shed the water which may enter the
central gap between the two halves of the hatch, but there appears to
be no mechanism visible in the few pictures I have seen of these boats
with their hatches open. I believe that they rely, instead, on the
chrome trim diverting the water onto the sloping body of the hatch
cover, and directing it towards the peripheral channel.
The hatch frame
in the plan is made up of 25 mm. x 50 mm. pieces of Oregon, which will
be joined by
finger joints. The tops of the frames will have to be planed down to
the camber and slope of the deck, and there will have to be a clearance
gap of about 2 mm. from the hatch surrounds to allow for expansion. The
two halves will be hinged on their sides, and, after the decking is
laid on them there will be a cowl vent on each one which will act as a
handle for lifting. The cowls will need some device for excluding water
from the engine compartment, but that can be constructed later. Here,
the frame pieces have been laid over the hatch opening to check for
dimension, prior to jointing.
But, because I am
using a flat bottom on the hatch, instead of one which follows the deck
camber, there are some members of its frame which have to be wider than
50 mm.. To accommodate that I have added extra width to the central two
longitudinal pieces, and all four athwartships members. The outer two
longitudinal pieces can remain at 50 mm.
After joins are cut the trim lines are marked onto the hatch frames
from the adjoining bulkheads or beams and stringers, and they are cut
to approximate shape. The final shaping is done after the frames are
glued up.
The hatches prior to shaping...
...and after.
The last step at
this stage is to apply the first skin of 6 mm. ply to the hatches.
Because of the number of clamps required only one can be done at a
time. But by nightfall there were two hatches completed, and the first
indication of the eventual look of the sub-decking.
52.Fitting the Shaft Seal
Once again, it is so much cheaper to
order these sorts of things by mail from the USA than to buy them here
in Australia. The PSS Shaft Seal came to A$260, as opposed to A$500-600
which I was quoted here.
They are available in a number of sizes, depending on the diameters of
the stern tube and the shaft itself. Mine is the smallest they make for
the 3/4" shaft and a 1-1/4" tube. (The actual measurement of the tube
is 1-5/16", but the recommendation is that you use the smaller size,
rather than go up to the next size, which would be for the 1-1/2" tube).
The fitting instructions are simple. The shaft has to be withdrawn far
enough to allow the seal to be fitted over the tube, which may mean
removing the rudder if it is already in place, so it is best to do this
before it is. The shaft itself has to be sanded down to absolute
smoothness with 600 grit paper where the nitrile 0-rings of the seal
are
to be slid over it, otherwise they can be damaged by small burrs. This
is especially important in the area of the keyway. Then the back end of
the bellows is
tightened over the tube, and the separate steel rotor over the
shaft.
The shaft tube has been cleared of epoxy where the seal is to
fit, and the shaft is withdrawn into the tube.
The seal is slid over the stern tube back to a point marked
on the tube
which is the length of the rubber neck before
it reaches the bellows, in this case 35 mm.
Because there is not enough height in the shaft compartment
to allow
the vent tube to exit the seal upwards, the
mechanism has to be rotated so that the barbed anchor is pointing
sideways. The vent tubing will pass under the
longitudinal floor timber which marks the compartment boundary, and
into the motor compartment through the
dash bulkhead. The shaft can now be slid forward and have the rotor
attached to it.
With the shaft now re-extended and locked onto its motor
fitting, the bellows is retracted (by 20 mm. in this case, and the
rotor is slid back to make contact with the carbon flange at the front
end of the bellows. The rotor is tightened onto the shaft in that
position, and that is that. No oils or greases are to be used to aid
fitting, but
dishwashing detergent can be used. Before tightening the seal over the
shaft, the thrust bearing should be fitted and locked onto the motor
mounting baffle such that it is in its correct operating position.
The rotor is slid back to the carbon flange.
There used to be a slow speed version of these seals for boats whose
speed is under 12 knots, which did not have vents to allow for water
cooling of the seal, but not any more. They are all vented now, and
allowance has to be made for the vent tubing to be routed up above
water level without allowing pooling in low spots which could cause a
water lock and prevent effective cooling. There have been reports of
small injections of water coming up the vent tubes when motors are
thrown into reverse, so it would be reasonable to discharge the vent
into a reservoir or overboard, although I think that it is unlikely
that the electric motor will cause a sufficiently violent thrust to
inject much in the way of water at all. In fact, considering that the
top speed of Ariadne will be about 6 knots, I don't think that
overheating will be a problem either. Nevertheless, the vent tubing has
to end somewhere, as does the tubing which drains the drip channels
around the foredeck hatch. I am not inclined to empty them into the
bilge for the bilge pump to take care of, so they might all be directed
overboard via a through-hull fitting or into an onboard reservoir.
Provided that they can be
appropriately segregated with one way valves there is no reason that a
single through-hull or collection vessel could not be a common
discharge point for all of
them.
The vent tubing is attached to the bellows, and fed under the
longitudinal floors to the cavity which will be behind the cockpit
lagging. From there it will pass through the dash bulkhead into the
motor compartment at a raised level,
so as to avoid the batteries which will be sitting immediately in
front of the bulkhead at floor level.
The fitting instructions for the seal warn against "running a loop near
the top end of the vent tubing" as that could cause a siphon effect and
draw water into the boat. I am not sure exactly what is meant by that.
Unless such a loop ended below the waterline it could not siphon water
in. In a heeling vessel such as a yacht that may be a consideration,
but not in a slipper launch. As long as the vent terminates well above
the waterline there cannot be any real danger of flooding.
Mind you, the instructions do warn of a fine water mist being released
at the seal during its initial running in period (of 1 hour), but other
than that the bilge should be bone dry.
Boats of 12 knot speed capability need to have water plumbed into the
seal via the vent, because of the venturi effect drawing cooling water
out of the stern tube at this speed. Again, Ariadne, with her slow
revolutions and low speed will not need such refinements.
