Designer's Notebook: Drilling the Shaft Hole for
an Inboard (Cont'd)
The first item on the
agenda is to determine where the hole is to be
drilled. Some, like David V. Lott2, use
mathematics to ascertain the shaft centerline
location. Some do a layout on the hull from
information given on the plans, or as Rich
Coey3 did, use a full size expansion as
shown in this excerpt from the book " Inboard Motor
Installations":
The shaft angularity, and drop of the strut when
known, can be used to determine the point of the
shaft centerline using this table from "Inboard Motor
Installations":
A Method to Determine the
Shaft Angle for a Straight Shaft
Installation
It's not practical to drill a hole at an acute
angle; a cleft must be chiseled in the keel or a
pre-drilled angled block used so the drill will not
creep.
Many respondents used the strut as a guide with a
custom built drill made from tubing attached to a shaft
commensurate with that of the strut to drill the hole.
This was a surprise; we made one but only because we
would be using it repeatedly in our shop.
The strut must be positioned and anchored securely in
place when this method is used. The area on the keel
must be flattened because the boat bottom is usually
vee'd and the strut base is flat. Although struts
are made in various angles it is possible to alter the
angularity to a minor degree by recessing the fore or
aft part of the strut base in the keel. If the boat has
considerable vee in the bottom severe flattening of the
keel for the strut base may be impractical. In such
cases partial flattening and using epoxy with fillers
can be used to fill in the void. Use plastic wrap over
the strut base that will contact the hull, coat the
area the strut will contact the hull with thickened
epoxy (fiber fillers for larger voids) and set the
strut in place making sure it is vertical. Flare the
excess epoxy filler to the hull surface. After cure the
strut can be removed and the area fiberglassed.
Drilling the shaft hole with a simple jig to determine
angularity doesn't use the strut/boring bar method.
An auger bit coupled to an electrician's extension
is used as the drill. A spade bit, Forstner bit or hole
saw can be substituted, however the latter will require
frequent cleaning. Although a brace with bit is shown,
a slow turning power drill could be substituted. Or, as
one correspondent stated "Why not use a standard
ship's auger with brace to drill the shaft hole?
That's what they are made for".
A method we initially used required little but a brace
and bit. A generous cleft was made in the boat bottom
so the drill point could start at the correct position.
A plywood angularity guide was blocked to the hull
bottom and with one person drilling and the other
sighting for the correct angle while the hole was
progressively drilled.
Several brought up the problem of splintering on the
inside of the keel as the drill came through. A surplus
block of wood temporarily fastened to the inside of the
keel over the exit area eliminates this problem. And
make sure all screws are removed in way of the shaft
hole; they can be replaced after drilling.
The "lazy man's" method off making the
shaft hole was also suggested. The shaft hole is not
cut at an angle. Rather, a slot is cut in the keel,
usually about 6" long or less for a 14° -
16° shaft angle using a 1 ½" thick
keel. The ends of the shaft hole are hand tapered with
a wood rasp or similar tool. Be sure the shaft log used
will cover the slot. Make a sketch to ascertain the
slot length required for the keel thickness and
angularity of the shaft of your installation when using
this method.
Pre-drilling the shaft hole with a pilot drill was
also suggested but will be of little value if the
boring tool does not have a centered lead bit. However,
this does eliminate a "goof"; if required,
the small lead hole can be filled and then tried
again.
The shaft hole may simply be epoxy coated,
particularly if the boat is trailered; others prefer
fiberglassing the hole's interior. The latter can
be somewhat difficult. One method is to use a dowel,
rod, or pipe wrapped with wax paper or clear plastic
followed by a wrap of fiberglass cloth. The shaft hole
is coated with catalyzed epoxy and the fiberglass dowel
assembly inserted in the hole forcing the fiberglass
against the inner surface. Rotate the dowel covered
with plastic while forcing it against the hole's
inner surface. With a little patience the cloth should
adhere to the wooden keel. If necessary wedge the rod
against the fiberglass at the top of the shaft hole to
insure adhesion. Read Smith4 inserted an
elongated balloon on a dowel, and then blew up the
balloon forcing the fiberglass against the keel shaft
hole.
In either case, remove the assembly after the
fiberglass is securely held to the surface and add more
epoxy as required. Trim the excess cloth flush with the
top and bottom of the keel. Radius the outer and inner
exits of the hole and lap the exterior fiberglass
covering onto it. The inside of the keel, around the
shaft hole where the shaft log will be, is optionally
fiberglass covered; epoxy alone is a viable
option.
A fiberglass, metal or plastic tube is an excellent
method of lining the shaft hole. Fiberglass tubes can
be purchased while some builders have used stainless
steel or copper metal pipe, PVC plastic pipe or brass
plumbing tubing. A fiberglass tube is easily made over
a mandrel that may be a prop shaft, a pipe, cardboard
or (as Bruce Dow5 did) a roll of newspapers.
Wrap with wax paper or plastic and several layers of
fiberglass cloth or tape progressively saturated with
epoxy resin. After cure pull out the mandrel and you
have a custom made shaft-hole liner.
The liner is inserted into the shaft hole, projecting
several inches on each end. Create a dam with masking
or duct tape between the keel and tube on the
underside, then inject catalyzed epoxy in the cavity
between the tube and the keel. Rap the area with a
mallet to be sure the keel/tube cavity is filled with
epoxy. If the space between the tube and keel is rather
large the epoxy should be thickened. In lieu of epoxy
some prefer, particularly when a metal tube is used, to
pre-coat the liner with a polysulfide sealant before
inserting into the shaft hole.
This covers drilling the shaft hole where a strut is
used.
In the next WebLetter we will cover drilling
through a keel or deadwood.
The following list links to builders' comments and photos on how they drilled the shaft hole in their boats (click on Builder's Name to link to his comments/photos):
BUILDER CONTRIBUTORS TO THIS ARTICLE IN ORDER OF MENTION
Paul Kane - Email:
Subjects Covered: A, B, C, D, E
David V. Lott - Email:
Subjects Covered: A, B, C
Rich Coey - Email:
Subjects Covered: A
Read Smith - Email:
Subjects Covered: G
Bruce Dow - Email:
Subjects Covered: A, B, C, E
SUBJECTS COVERED KEY
- Shaft hole tube saw drill
- Shaft hole location
- Using strut as a guide
- Rectangular slot shaft hole
- Shaft hole liner tubes
- Fiberglassing shaft hole
- Skegs
ALTHOUGH NOT SPECIFICALLY MENTIONED THE FOLLOWING BUILDERS ALSO CONTRIBUTED TO THIS ARTICLE
Dwain Colton
Subjects Covered: A, C
Bill Edmundson
Subjects Covered: D
Doug Harrison
Subjects Covered: G
John Johnston
Subjects Covered: C
Graham Mackay
Subjects Covered: G
Paul Miller
Subjects Covered: A, C, F
Wayne Robertson
Subjects Covered: A, C, E
Ken Schott
Subjects Covered: A, C
Garry Stout
Subjects Covered: G
Brian Walters
Subjects Covered: C
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