Installing and Fiberglassing the Sterntube

The Final Fiberglass Layout
The Final Fiberglass Layout

Recall that Jean-du-Sud had no (inboard) power train prior to the engine install. In particular, it had no propeller and thus, no sterntube. The first practical task in the engine installation was to find the aperture and the former sterntube hole.

When Cortez arrived to the « New World » (America), he ordered that his ships be burned so that there would be no turning back (Smith, 2022). I began the install with grinding the aft part of Jean-du-Sud’s keel so as to find the aperture and the sterntube hole. It was of course a necessary step, but it was also a commitment. I burned my ships!

The Aperture, 40 Years Later.
The Aperture, 40 Years Later.
Aperture (Grinded)
Aperture (Grinded)
A Newly Found Sterntube Hole
A Newly Found Sterntube Hole

In more practical terms, I had to install a new sterntube. I chose to use a fiberglass model over bronze because of its superior adhesion to the hull. Furthermore, fiberglass does not corrode, reducing the list of future potential problems by one. The sterntube is 18 centimeters long (7 inches), accomodating a one inch diameter shaft. I started with a longer sterntube and cut it to the right dimension once the engine was installed. Only 34 milimeters stem out of the aperture (picture below), meaning that the remaining 14.6 centimeters are inside the hull.

Maybe this is too long?
Maybe this is too long?
Shaft and Sterntube at the Proper Length.
The Shaft and the Sterntube at the Proper Length.

As a part of the whole, I fiberglassed the sterntube after installing the engine on its bearer. It is easier to fiberglass the sterntube with the proper alignment than aligning the engine to the sterntube. From a design standpoint, fiberglassing the sterntube after the engine is installed gives an additional degree of freedom during the alignment. In practice, the shaft just aligns itself on its pivot point and the sterntube follows, greatly reducing the needs for adjustments. So if anyone can do it, I highly recommend the approach.

For fiberglassing the sterntube, the three most useful online sources are from Timothey C. Lackey (2009), from the Practical Sea School (2021; video below) and from Far Reach Voyages (2020). Lackey’s web page was my starting point to understand how to do it and remained my main reference during that part of the install.

Replacing the Sterntube by Practical Sea School (2021).

There is also a video made by James Frederick (2020) that uses a bronze sterntube. It is not useful from a fiberglass standpoint, but since it is performed on an Alberg 30, it can help get a sense of the dimensions (Frederick, 2020). Finally, Costa Blanca Yacht Services made a video showing how they did a structural repair of a sterntube (Costa Blanca Yacht Services, 2020). The video is pretty instructive, especially for how to think about the finishing coats. That being said, I find that their filling of the sterntube’s surroundings with polyester resin putty to be a risky choice.

For the exterior fiberflass, I used five pieces of 16 oz cloth as the structural layers to fix the sterntube. The design of the largest piece is shown in the drawing below (first picture of three below). Each other piece has its dimensions reduced by one inch on each side. Each piece was layered in ascending order of size, so that each new layer would overlap the previous one.

Between each layer, I added 2 centimeters strips of 6 oz fiberglass around the sterntube, so that the ends of each strip would “latch” onto the hull (see the second picture below). Each strip was latched in a different orientation, so as to increase strength in all directions. I finished the layering with two 6 oz pieces that were one inch larger than the largest 16 oz piece. The final layering is shown in the third picture below.

Sterntube patches (largest shown).
Sterntube patches (largest shown).
Latching strips (?).
Latching strips (?).
The Final Fiberglass Layout
The Final Fiberglass Layout (Epoxy fairing to be done).

If for some reason the shaft were to jam in the sterntube, there would be a lot of load on these exterior pieces of fiberglass. To reinforce the bond, I decided to pour thickened epoxy on the other side of the hull. I used a cardboard piece reinforced with chicken cage to create a mold where the epoxy would thicken. The length was chosen to guarantee that the seal shaft would have a long enough lip on the sterntube to be firmly attached (pictures below).

Dry-fitting the Shaft Seal Priot to Pouring Epoxy.
Dry-fitting the Shaft Seal Prior to Pouring Epoxy.
The Cardboard Mold Installed.
The Cardboard Mold Installed.
Epoxy Poured in the Mold
Epoxy Poured in the Mold

Want to Know More?

Have a look at the « boatwork » section of this website for more practical tutorials and techniques.

References

J. Frederick (2020). Installing a New Bronze Stern Tube on an Alberg 30 at The Ventura Harbor Boat Yard, YouTube Video, retrieved online in October 2023 at this url.

T. C. Lackey (2009). Systems: Sterntube and Shafting, The Triton Daysailor Project, retrieved online in October 2023 at this url.

Practical Sea School (2021). Replacing the Stern Tube – Part 1, YouTube video, retrieved online in October 2023 at this url.

D. Smith (2022). Sometimes It’s Best to Burn Your Ships, retrieved online in October 2023 at this url.