Cutting Hole for Propshaft

[KelseyB]

Here’s an interesting one. We are replacing an external compartment on our 30’ steel sailboat due to a corrosion issue. This compartment descends from the hull and surrounds the propshaft, connecting to the aft end of the keel on its forward end and a strut supporting the rudder on its aft end.

This compartment, which I’ll just call “the box,” is accessible from inside the hull through a small cutout on the bottom of the hull. The opening of the cutout is not as big as the box and is only about 6 inches wide, leading to restricted access, especially of the aft end of the box which can’t be reached with a hand from inside the boat (a design flaw I could gripe about for ages, but I’ll digress). Theoretically the cutout could be expanded for better access, but it’s unclear whether we could expand the cutout enough to grant full access.

Here’s the issue. The old box has been cut out and the new box (should be nearly identical) has been fabricated and is ready to be welded on. However, we asked the fabricator not to cut out the hole for the cutless bearing and shaft on the assumption that the new box will inevitably be slightly different from the old one and, for alignment purposes, that it would be better to get it welded on first and then cut out the hole for the shaft. But, now we’re not sure how we’re going to cut out that hole in the right place.

We’re thinking of using a laser sight to pinpoint where it should be in reference to the transmission coupling, but that would only tell us where the hole should be on the inside of the box, and it will be extremely difficult or even impossible to cut the hole (or even drill a pilot) from the inside. We’ll almost certainly have to do the actual cutting from the outside. But from the outside, there won’t be a reference with the transmission and where the hole should be.

I’ve attached some photos and diagrams. Of course we know we’ll have to do some engine realignment regardless once it’s in, we just need to be able to get the hole in approximately the right spot. Any ideas?

(The attached photos are of the old box, which has since been cut out - never mind the horrifying corrosion there. From the inside, you can see how inaccessible it is - you can’t even see where the shaft exits the boat in that picture. That’s why there’s the third photo, taken with the phone shoved into the old compartment, where you can see the stern tube attached to the aft end of the compartment. The prop shaft runs inside of the stern tube).

[markxengineerin]

Interesting problem, I don't know, but it seems every new boat being manufactured would have the same challenge. I think one or two of these and some homemade "tooling" would get you in the right ballpark, but maybe there's a better way.

The one time I saw this being done, in a yard, they had made a template out of plywood that was clamped to the hull/rudder, and fit tightly on the old shaft, before cutting out the original "box". I didn't see the drilling of the new hole, but I assume they re-installed the template as a guide for where to center the hole (drilling from outside), and perhaps a couple angle gauges on the drill to get the angle roughly correct based on measurements of the old shaft.

[wholybee]

Do an initial sight before the box is installed. Sight from the rudder through the center of the strut to the coupling. Leave the laser in place, and then it will be properly sighted when the box is welded in place.

[Bowdrie]

Originally Posted by KelseyB

the assumption that the new box will inevitably be slightly different from the old one

we just need to be able to get the hole in approximately the right spot. Any ideas?

Going by key words; "slightly different" and "approximately".
Drill a small hole, (1/8th",) in the box from the outside in the "approximate" position, shine a light on it from the outside.
Now go back inside and see what the difference is between where the laser is hitting the inside and your 1/8th" hole.
Adjust difference and proceed with larger hole.

[Oldwheels]

Say you were putting a new engine in. You would move the engine around to achieve proper alignment.

Install new box best as can. Your welder should have some skills. You then may have to adjust engine placement a little bit to achieve perfection.
Yes, bore the box before installation. There are portable boring machines but those folks are very pricey and slow.

[kayakerChuck]

I run into similar dilemmas fairly frequently.

If I can, I usually do some variation of this:

Cut the hole in the box big enough that it will not interfere with anything.

Cut another piece with the correct size hole for your shaft/bearing. Slide this "bushing" onto your shaft before aligning the shaft/motor.

Once you are happy with the alignment, weld the "bushing" piece in place.

[Bruce K]

Originally Posted by kayakerChuck

I run into similar dilemmas fairly frequently.

If I can, I usually do some variation of this:

Cut the hole in the box big enough that it will not interfere with anything.

Cut another piece with the correct size hole for your shaft/bearing. Slide this "bushing" onto your shaft before aligning the shaft/motor.

Once you are happy with the alignment, weld the "bushing" piece in place.

This!

[kenbo]

Originally Posted by Bowdrie

Going by key words; "slightly different" and "approximately".
Drill a small hole, (1/8th",) in the box from the outside in the "approximate" position, shine a light on it from the outside.
Now go back inside and see what the difference is between where the laser is hitting the inside and your 1/8th" hole.
Adjust difference and proceed with larger hole.

That's the way I would do it too.

[skipperpete]

Originally Posted by kayakerChuck

I run into similar dilemmas fairly frequently.

If I can, I usually do some variation of this:

Cut the hole in the box big enough that it will not interfere with anything.

Cut another piece with the correct size hole for your shaft/bearing. Slide this "bushing" onto your shaft before aligning the shaft/motor.

Once you are happy with the alignment, weld the "bushing" piece in place.

Exactly this, kayaker chuck knows his stuff…..big loose fit between stern tube and box cutouts but a tight-ish fit of the 2, 3 or 4 annular rings on the stern tube. It’s critical to do a dummy run and alignment with the whole thing tacked in place before doing any serious welding.

[NorthCoastJoe]

Tack weld the box in place, then using a long 1/8" bit, I have seen them 18" long drill where your laser mark is as a centermark. You can also get bit extensions if you need to. Then use a hole saw, slow and lots of cutting oil to make your hole. If you can cut the tack welds and move the box to a drill press for the hole cutting that will make life easier. If you have room I would look into putting an Aqua Drive in there. No more engine alignments and all the thrust forces on the hull not the transmission.