stern tube

Does anyone know the size of our stern tube? I know we have a 1" prop-shaft, but I haven’t been able to locate the stern tube size in my reference material. Thanks!

Hello - The sterntube is 2" OD, 1-3/8" ID, 15" Long. There is also a larger concentric sleeve which form the tappered knob on the outside of the hull that the cutless bearing retaining screws are tapped through.


Jeremy, thank you for sharing your BCC knowledge and experience, it is very much appreciated. I intend to order a PSS Maintenance Kit and didn’t want to find out I had the wrong size AFTER I was on the hard. On one hand PYI Inc advertises their PSS Shaft Seal to be maintenance-free, however, they also suggest preventive maintenance by replacing the bellows every six years. They also recommend replacing the o-rings & set screws in the stainless steel roter…which are also included as part of the kit.

I lifted those dimmensions from the SLM drawings. So if your boat’s sterntube is fabricated to that standard, it should take a 2" belows. I’m in the process of repowering my boat, and replacing all the existing running gear. I plan on putting in a new traditional packing box - and loading it with “dripless” GFO packing. To be seen how well that works for me. The bellows style units scare me. Bellows failure is one nightmare; the other is shaft mavement after catching a pot or some other obstruction. The nightmare is compounded by the awkward access to the seal on the BCC. So i’ve settled on something that might drip, but shouldn’t ever gush.


Thanks, traveler, for the heads up about the need for preventative maintenance, replacing the bellows of the PSS.

A note of caution on the outside diameter of the stern tube - note well that Jeremy wrote “if your boat’s sterntube is fabricated to that standard”. I’d emphasize the IF. And suggest you measure the OD of the sterntube of your boat.

PYI has one model of PSS for 2" OD sterntubes with a 1" shaft (Product Number 02-100-200) and another for 1 7/8" sterntubes with a 1" shaft (Product Number 02-100-134 - also used for 1.75" sterntubes, suggesting that a bellows for a 1 3/4" stern tube can stretch to 1 7/8" but not that a bellows for a 2" shaft can be adequately compressed to seal on a 1 7/8" sterntube).

The drawing of the BCC sterntube in my copy of the BCC Construction Manual is a trifle ambiguous about the OD of the sterntube, suggesting either that it is 1 7/8" in OD or that the forward part of it could be machined to 1 7/8" if needed.



(edited to make meaning clearer)

Bil, I think I will heed your advice and crawl back in the engine compartment to measure the stern tube before I order the PSS maintenance kit, I also see that our Max-Prop requires servicing every two years, so I plan to order both kits from PYI Inc at the same time.

From reading the PYI website and other forums, I noted that the majority of boats reporting problems with the rubber of the PSS bellows perishing were motor vessels.

And two causative factors - acidic vapor from lead-acid batteries; and grease, oil, diesel fuel, and gasoline contamination - associated with the premature perishing of the rubber bellows.

I’ve another factor to add to the list: Mechanical stress. I noted that when Zygote is in tropical or sub-tropical waters, that two weeks of idleness (ie not rotating the prop shaft by engine or by hand) can lead to stiction between the stainless steel rotor and the carbon stator.

Figuring that the initial turn of the stator against the rotor must cause some mechanical stress to the bellows, I adopted the practice of entering the engine room and gently turning the prop shaft by hand, while holding the stator, before starting the engine. Which leaves unknown whether oils and grease transferred from my hand to the rubber bellows is more deleterious than the mechanical stress.

And I don’t know what causes the stiction (and whether it occurs in cool temperate waters too). I guess that the decay of organic matter in seawater (the same decay that leads to hydrogen sulfide smells in the head, for example) might be involved.



I too read the causative factors regarding the bellow degradation on the PYI website and noted that grease, oil, and diesel fuel were listed. It is certainly possible that the bellow could come in contact with any of these petroleum based products if we’re not careful anytime we perform work in the engine compartment.

I have ordered a one liter bottle of Vappro 850 Lubricant. I intend to put a small sponge soaked with this product in the bottom of a film cannister and place some holes in the lid before I screw it down on the cannister. This lubricant is designed to emit VCI molecules into the surrounding area, within an enclosure, until it is completely saturated. I would place one or two of these cannisters in my tool bags to keep my tools from corroding. These VCI molecules are safe, invisible and odourless. When the VCI molecules come into contact with the air in the enclosure, an ionic reaction takes place between the VCI molecules and the water molecules in the air.

The VCI molecules in the surrounding enclosure are hydrolized into VCI ions, which form a molecular corrosion protective barrier on the metal surface. This occurs?through polarization, and thus passivate the substrate from atmospheric corrosion. It normally takes approximately 24 hours for the VCI ions to be absorbed on the metal surface.

The reason I mention this Bil is I would appreciate hearing what your thoughts are regarding the possibility of this product degrading the bellow?

Most interesting, traveler!

I’d not heard of Vappro before. My reading of the website didn’t reveal exactly what the proprietary Vappro 851 is, but the website suggests that it is largely or wholly silicone.

In the past I’ve used a silicone surface spray on Zygote’s engine, to coat the metal and hopefully prevent rust. Silicone doesn’t perish rubber in the way that a diesel fuel-type spray (eg WD40) might. But only PYI could give an expert opinion about Vappro 851 and the rubber bellows.

I’d guess that Vappro is expensive. To keep corrosion away from my tools on Zygote (and a few of those tools were my father’s, so they’ve been around!), I have in the past sprayed them with WD40, Boeshield, and silicone spray.

But for what it’s worth, my favourite treatment for tools is to coat them with stick lubricant - the white wax stuff that is sold in a cardboard tube, especially at auto accessory shops (and without the premium price it might bear if it were called “marine xxx” or sold exclusively at West Marine or any other retailer for boat bits).

Stick lubricant is a wax (so is the stuff left from Boeshield). It doesn’t smear or leave oily or greasy marks. Applying it to a favourite wrench or other tool is a meditation. And the result feels pleasant, with a non-slip finish. Jolly inexpensive too.

I store my hand tools in canvas rolls (wrenches in one, screwdrivers in another, and so on). The rolls keep them quiet. And I take the opportunity of a sunny day to sun the tool rolls, so they get hot and dry.



Hi Bil,

I finally got a set of micrometers on my stern tube, the reading was 1.8" which equals 1 7/8" if I’m not mistaken. I’m going to order PSS Shaft Seal Maintenance Kit (02-100-134) from PYI this week. Thanks for the sound advice!

Great news traveler!

I’ll be interested in hearing how your work goes. And particularly whether the job can be done by just withdrawing the shaft from the shaft coupling or whether the shaft coupling has to be removed from the transmission flange.

Here’s my take on (A) routine maintenance necessary for a PSS; and (B) the steps that might be necessary for changing the bellows:

A. Routine Maintenance

  1. Daily operations:
    ? visual check for splits or cracks in the bellows;
    ? visual and olfactory check of engine room to ensure that it is free of battery acid fumes and splattered oil and grease;
    ? before starting engine, rotate shaft by hand, while holding the stator steady to break stiction.

  2. Preventative Maintenance
    ? at 6 year intervals, replace the bellows

B. Bellows Replacement

  1. Purchase replacement bellows (PYI PSS Maintenance Kit)
  2. Check maintenance kit and read instructions

With the boat on the hard (I’ve read of people changing bellows with the boat in the water, fixing a shaft anode on the shaft so it does not slide out and wrapping an old t-shirt tightly around the shaft to control water ingress):

  1. Using a marker pen, mark on the propeller shaft the location of the forward face of the stainless steel rotor;
  2. Undo the set bolts holding the shaft to the shaft coupling and withdraw the shaft from the coupling;
  3. Unfasten and remove the shaft coupling from the transmission flange;
  4. Move shaft aft far enough for removal of the rotor and the bellows (3? should be enough? so no need to remove the rudder??);
  5. Check and when necessary use 400 grit paper to polish rough edges and corrosion deposits from the forward face of the shaft and the shaft coupling set screw;
  6. Use Allen key to remove the double-stacked set screws from the stainless steel rotor;
  7. Lubricate shaft forward of the rotor with dishwashing detergent + water;
  8. Slide the rotor forward and remove;
  9. Use wrench/screwdriver to loosen hose clamps fastening aft end of bellows to shaft log;
  10. Slide old bellows assembly forward and remove from shaft;
  11. Clean exposed shaft to remove any oil, grease, or silicone;
  12. Check and if necessary use 400 grit paper to remove rough edges from where the rotor set screws cut into the shaft;
  13. Inspect inside forward end of shaft log;
  14. Slide new bellows + carbon stator on shaft and position so forward end of shaft log does not protrude into convolutions of bellows;
  15. Check that the mark noting location of forward face of rotor is still visible;
  16. Chase one family of set screws into rotor, then back them out so they are recessed;
  17. Lubricate internal O-rings of rotor and forward end of shaft with detergent + water;
  18. Slide rotor onto shaft until it contacts stator;
  19. Attach shaft coupling to shaft;
  20. Attach shaft coupling to transmission flange and align with less than 0.1 mm difference around the circumference of the flange;
  21. Orient and position bellows on shaft log such that the forward end of shaft log does not protrude into convolutions of bellows, orient the hose clamp screws at 180? and then fasten the hose clamps;
  22. Locate rotor in the neutral position, just touching the carbon stator;
  23. Using the marker pen, mark on the shaft the location of the forward face of the rotor in the neutral position;
  24. Using that new mark as the reference, slide the rotor aft to compress the bellows by 0.75? or 19 mm ? that should be at the old reference mark;
  25. Use the Allen key to drive the first family of set screws down until they cup the shaft;
  26. Use the Alley key to drive the second family of set screws into the rotor so the set screws are double-stacked, and;
  27. Check tension of all hose clamps and fit jackets to protruding tails of the hose clamps.

After re-launch:

  1. Check for drips from bellows;
  2. Hand compress the bellows to purge air and ensure a flow of clean water through the shaft log;
  3. Sea trial: drive propeller for one hour minimum while checking for leaks;
  4. Check/readjust alignment of transmission-to-shaft coupling.



Ahoy Mike , You might like to check your “Private Messages” on this forum !

FWIW , I too did the reasearch that you are repeating , Good Luck , as I said once before on another thread , you may have to live with a poor choice for a long time !