Outboard
profile
The
out board profile is what anyone looking at your vessel from the side
will see. One of the perils of drawing outboard profiles by hand is
the perspective effect caused by you looking at the profile with its
top edge away from you. What appears to be a neat boat on the drawing
board suddenly doesn't look all that swell when pasted up on the
wall. The advantage of computer drafting is that you are already
looking at the boat as it would appear on the wall.
When
you are drawing the outboard profile you should include everything
that you, as the designer, planned to have fitted on the boat, the
builder/owner may have other ideas. So be it.
So
we start with the outline of the hull as we drew it in the
construction drawing. And remove any extraneous lines.
So
we need to add a house. If you recall the original design called for
a place to get out of the wet and an open cockpit. The design was
also for up to three adults so two can be out of the wet and the
other can steer. So the house should be at least half of the
waterline length. And a way for those inside to see outside, a port
perhaps and or a hatch. Now a comfortable seat needs to be about 15
or 16 inches high and with the same depth and minimum head room is 38
inches so the cuddy top should be some 4'6“ above the midship
frame. A hatch needs to be a minimum of 2 feet square. The port can
be any size or shape but should fit with the overall design. I
always like a small foredeck, not that you would want to stand on it,
but it gives a place for a substantial samson post forward and a
small storage locker below.
So
all that would look like this.
The
cuddy would be open at the back for air in the cuddy and also to
provide air for the engine which is air cooled. We could also decide
upon a rudder at this stage.
The
rudder could be transom hung or be under the boat on a rudder shaft.
Because this is a small slow boat I think that a transom hung rudder
would be better and fits with the type of boat.
OK you say but what about the propeller and engine. Well in this profile you
can't see the engine but you're right about the propeller. The
problem is you can't show the propeller until you decide the angle of
the shaft and to do that you need to decide where the engine is going
to go. Which brings us to the inside profile.
I
lied when I said in the last post that we would do the deck plan, we
really need to establish where the engine will go and what the shaft
angle will be because then we can finish the outboard profile
and the construction drawings for the keel.
Inboard
Profile
We
start with the almost completed construction profile, see why this is
easier on a computer.
Then
we add the cuddy that we drew on the outboard profile.The
port, samson post, fore deck and some seating at the transom and in
the cuddy.
Now
for the engine. We have the specifications for our engine. It is 15.4
inches long, 12.6 inches wide and 13.6 inches high, the shaft height
above base is 4.17 inches to the centre of the shaft. The propeller
that we have is 8 inches in diameter so once we draw that in with the
shaft exiting from the back edge of the keel and the propeller tip 2
inches away from the bottom of the boat, to stop thumping, we can establish a shaft
angle of 5 degrees which will not interfere with the lubrication of
the engine.
There
is a little bit of a problem, the prop spins just a little below the
keel which, if the boat takes ground, will cause damage to the prop
so we'll put a skeg on the keel to protect the prop.
Now
that we have established the shaft angle we can extend that back into
the boat and determine where the engine will sit and draw in the
engine bearers. But we have a problem, the optimal RPM for the prop is
600 RPM the engine runs optimally at 3600 RPM so we need reduction
gearing. There are two ways to accomplish this, an actual reduction
gear set up or v-pulleys. Given that this project is driven by cost,
V-pulleys it is. A 2” steel pulley on the engine output shaft and
an 8” pulley on the propellor shaft.
All
of which means that the engine must be several inches above the shaft
centreline probably, about a foot. There are calculators for this at
http://www.gizmology.net/pulleysbelts.htm.
For the purposes of this exercise we're going to place the two shafts
12” apart. Given that distance we get this,
The
square box is the engine, the area under it is the engine mount, the
beds should cover at least three frames to reduce vibration and
strain on the hull. Covering the shaft coming into the boat is the
shaft log. The concentric circles on the engine are the exhaust. We
can extend the exhaust to show on the outboard profile and draw it
in.
And
there is the almost complete outboard profile and the almost complete
inboard profile.
Outboard
Profile
Inboard
profile
Next
time we'll finish both these drawings and move on.
