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):


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

  1. Shaft hole tube saw drill
  2. Shaft hole location
  3. Using strut as a guide
  4. Rectangular slot shaft hole
  5. Shaft hole liner tubes
  6. Fiberglassing shaft hole
  7. Skegs


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