PYI Dripless Shaft Seal

[sailingdog]

Unfortunately, you're blaming the PSS for something that was caused by faulty craftsmanship on the part of your boat yard. The failing of the PSS was an effect of them not properly re-installing the prop shaft. As Maine Sail said, the coupling should have been replaced—it wasn't and when the prop shaft moved, due to the bad coupling, that is what caused the PSS to fail. Properly installed, they're pretty damn reliable.

 

[Ross]

At the very least the shaft should be dimpled for the set screws. To expect a couple of set screws to take the thrust of a 20 horse power engine with no mechanical advantage is not good workmanship.

 

[Nereussailor]

I bought one for my boat 2 years ago and wouldn't have it any other way!

Dave

 

[Maine Sail]

This should be a given

At the very least the shaft should be dimpled for the set screws. To expect a couple of set screws to take the thrust of a 20 horse power engine with no mechanical advantage is not good workmanship.

Ross,

This is part of the ABYC P-06 standard and SAE J756 any good marine mechanic should be well familiar with this standard but of course many are not..

From ABYC P-06

NOTE: In order to accommodate the requirement of a
light press fit, TABLE V may be used as a guide.

6.8.1.1 A minimum of two radial set screws shall be
used.

6.8.1.1.2 Spot drills having the same tip angle as the set
screws shall be used to increase the grip of the set screws.

 

[Jerry Clark H356 SV Persi]

Just had one installed prior to launch 3 weeks ago. Spent one hour breaking it in with slight spray mist off the seal. Last weekend - first time in 6 years my boat is DRY! I wish I had put it in when I commissioned Persistence. Looks like exactly what I wanted.

 

[tsmwebb]

What you describe is actually very typical of poor quality boat yard mechanics. ...

You should always make it a habit replace a solid one piece coupling once it has been removed and do a full fit and face. Couplings are cheap compared to a sunken boat.

... Hamilton Marine will do mail order fit and facing through their shafting source at very reasonable rates.

Unfortunately this problem is not the fault of the PSS but it gets a bad name regardless...

Hamilton Marine will mail it to Vanuatu?

Folks who cruise to remote places will find the need to have repairs done there from time to time. They will have to make do with the resources at hand. They just aren't going to be able to do everything as it should be done. So, products for cruising boats should be somewhat fault tolerant. This is not a one size fits all kind of thing -- folks who are going to make passages against the trades in the remote SW Pacific have different needs than folks have access to first rate yards and on the water support. Still, it seems to me, that because of its design the PSS seal is not a good choice for the voyager who will spend a good deal of time away from the developed world. If that is a "bad name" then it is deserved.

--Tom.

 

[Ross]

Any shaft installation should include a drivesaver or similar flexible coupling. This will go a long way to compensating for slightly off face/bore tolerances. I certainly wouldn't go to far away places with strange sounding names without one.

 

[henkmeuzelaar]

Ouch, this board moves so fast that one can hardly wait a day or two with answering before having to dig the thread out of the garbage dump again........
In case anyone is still listening:

MaineSail, thanks for the trademark, technically superb response! You are absolutely right about the importance of accurately fitting the shaft flange, as described by you. I barely dare to admit, however, that the last key was probably hand-filed in pretty much the same way that Afghan gun-smiths build their fearsome rifles. This is why I recommend that every long-distance cruiser wanting to use PYI dripless couplings while unable to access high-tech machineshop facilities on a more or less regular basis should make sure that the shaft cannot travel forward in case the set screws might be slipping.... Losing auxiliary engine power on a sailboat is not the end of the story but having to battle a potentially massive leak could well be.

Ross has a point: in fact, we tried to order a drivesaver flange several years ago but once it arrived in Vanuatu (quite a project in and by itself) we were unable to get a perfect match on the hole patterns. Moreover, after pondering the fact that one of the possible modes of action by which it is supposed to help protect the transmission is a sacrificial failure mode I chickened out on trying to install it in combination with a mechanically vulnerable PYI dripless shift seal mechanism.

As Tom Webb rightfully pointed out; if one decides to cruise in these places (or run a 9-yr humanitarian assistance project, as in our case) technical vessel maintenance and repair becomes a whole new ball-game. To illustrate this point a bit further: three seasons ago I had to hand-file the aluminum flange of our 15-year old Maxwell windlass motor down by approx. 1/8 inch in order to re-engage the 3/16" drill bit (taking the place of the original break-pin over the past 7 years) with the clutch of the 8:1 (?) transmission. Before putting Rivendel II on the market in Brisbane, Australia last November I did put a new windlass motor in to save the buyer potential further agonies. However, the whole thing was working just fine, as far as I was concerned.

Have fun!

Flying Dutchman

 

[John Nantz]

Re: PSS Seals

In lieu of a collar I used a spare zinc which works well. Might not be perfectly balanced, and I can't tell if it is or isn't, but it works.

 

[Maine Sail]

In lieu of a collar I used a spare zinc which works well. Might not be perfectly balanced, and I can't tell if it is or isn't, but it works.

The speeds the shafts turn at would not really affect any changes in shaft balance that close to the coupling. The only problem with zincs is they tend to gunk up the shaft so if you needed to pull it off, and slide the stainless rotor off the shaft, you'd need to sand it with some wet sand paper for passage of the rotor over the shaft smoothly.

 

[henkmeuzelaar]

Thanks for the informative, detailed answer, Maine Sail!

I just read it for the first time today as I have been traveling a lot.

Since there is a new PYI/PSS thread running right now, I thought it might be helpful to bring this thread back to life since it contains much of the valuable information you have contributed to this topic on the HOW-forums.

I completely agree with your advice to always install a new shaft coupling after the shaft has been pulled in the yard. On the other hand, I have to presume that you have not yet been in Vanuatu, or similar third world countries, yourself.... Let me say it this way: ANY replacement part that can be found there within 1 - 2 months (including parts possibly cannibalized of sunk or stranded vessels) is a godgiven blessing!

Also, I have a question to ask about one aspect of your answer that was not quite clear to me, namely whether the brunt of the forward force imparted by the propellor (including the frequent shock forces caused by slamming into waves) should be borne entirely by a properly fit shaft coupling plus set screws or that it is better/safer to make sure that the shaft bottoms out entirely into the engine output flange. In other words, is the coupling primarily meant to transfer engine torque to the shaft or should it also be able to handle all longitudinal propulsion and shock forces encountered at sea??

Flying Dutchman