Hi Eric,

Even the best “stuffing box” will be defeated by a leak in the hull 🤣

…but I am pleased that you have found the cause of the leak…..and that you now seal the prop and rudder shafts to prevent any water ingress - always the best policy.

Bob.

So.. fellow sailors and builders, I’m extremely grateful for all your input and I’m really pleased it’s been helpful to others too, a great discussion! I have a confession however.. after all that, it turns out that there was a crack in the hull of the tug!! I can’t flippin believe it. I followed the advice re the nylon washers with the grub-screw collar and got the propshaft all greased up, then tightened everything up, I even greased the rudder post just in case.. did a bath test - and water started coming in straight away! I checked round underneath and immediately saw the crack 😳 it’s now sealed up and waiting for the sealant to dry before testing again. Will report back concerning the prop shaft but I’m pretty sure it’s watertight 😜😜 Thankd again, Eric

Thanks, Toradog, for explaining this to me.
I think I'm exaggerating with the precautions; even less precautions are enough to keep water out.

I know I'm asking a lot, but if you'd like, feel free to do so. Do you have photos detailing how you built yours?
Please only post them if you have them. Don't go crazy looking for them. You've already been too patient.

Alesandro,
To be honest with you, I would only use a cup seal or an o-ring seal if the model's stuffing box were really deep. Maybe 6 inches or so. All other time, I simply use grease in the stuffing box and a nylon washer on the inboard end of the stuffing box, held in place with a wheel collar.
KISS principal.😊

Thanks, Toradog, for your replies.

You write: "
In your drawing of the shaft seal, if you were to mount an O-ring between two of the bearings, your water leakage would be zero."

But in reality, I don't have any leaks.
Furthermore, I would never disassemble the propeller shaft because, even if possible, it's too complex. I wouldn't do it even for small leaks.
Luckily, not even a drop gets in.

However, if I wanted to build another system myself, would you recommend I also install the O-ring? Where exactly?
I ask because I can never be too careful, but perhaps I'm exaggerating.
You say to place it between the two bearings, but which ones exactly, since there are three.
Wouldn't it be better to place it (where possible) between the last bearing and the plastic cap?
This protects the bearing furthest aft.

Allesandro,
In your drawing of the shaft seal, if you were to mount an 0-ring between two of the bearins, your water leakage would be nothing.

Alessandro,
Here a a couple of links:
You should be able to find comparable items on your side of the Pond.

https://www.mcmaster.com/products/u-cup-seals/

https://www.dubro.com/products/nickel-plated-dura-collars

I'm posting this message with a significant delay due to translation.

The different perspectives you two, ToraDog and GrantH, have had are very interesting to me.

I find ToraDog's advice very useful and GrantH's observations very stimulating.
Actually, I'll add a side note: ToraDog, could you give me some specific commercial advice (even direct links if you'd like) on the sealing systems, such as the thrust bearings, you mentioned?
I'm asking because so far I've used homemade methods that have worked, but I'd like to have other ready-made alternatives. I'd also like to be sure we're talking about the same thing.

Returning to the substance of your discussion, I have to say that in the past I've encountered a major problem with the method recommended by Grant.
One of my first approaches to building the propeller shaft drive was precisely the one Grant suggested.
Today I would have simply bought a ready-made propeller shaft online, but back then I racked my brains.
I found that placing one end of the propeller shaft tube above the waterline causes the shaft to tilt excessively with respect to the keel (or the surface of the water).
To prevent the angle between the propeller shaft and the keel from being too great (which would cause the propeller to point too low), you have to move the engine forward and lengthen the shaft.
This is what I tried (if I can find photos, I'll attach them to better explain what I mean and what I did).
This method proved unsuccessful, and I immediately abandoned it because such a long shaft created excessive noise, an unbearable clang, and a very high current draw (despite the propeller spinning freely and not in the water).
To prevent water from entering the hold, I use a homemade method (see attached photos).
The propeller shaft rotates inside the tube, supported by three bearings; the tube is filled with grease.
In this specific case, the shaft is a 4 mm diameter brass rod. The bearings have a 4 mm internal diameter and an 8 mm external diameter. The tube containing the bearings, spacers, grease, and shaft is made of aluminum and has an 8 mm internal diameter and a 10 mm external diameter.
At the point where the brass propeller shaft exits the hull, there is a small perforated plastic protective cap with an additional feature.
As you can see in the drawing, water can hardly penetrate three bearings. This has never happened before, but anything is possible. However, even if I don't have any problems with water entering the hold, I run the risk of water coming into contact with the bearings and damaging them, which is why it's important to add and top up with grease.

Thanks Tora! The drawing however, is not mine (internet). My drawings are on my PC. Anyone looking for ideas for stopping boxes you can search the internet. They're even drawings to help make a short stuffing box to retrofit a boat that doesn't have one.
Lew

LewZ is on the money with his description and drawing.
In almost all cases his system will work beautifully. To help keep grease in the tube, one can add a washer and set collar( or stop collar) to the prop shaft just snug enough to block grease from coming out.
A couple of other methods, hat's off to bubbleheads(submarine builders): both use o-rings. What is important is to fit an o-ring that just barely touches the shaft, ie, it o-ring ID is just a few, may .010 of an inch smaller than the OD of the shaft. The same applies to metric. one system uses compression fittings that allow the adjustment of pressure of the o-ring onto the shaft. the fitting can be soldered or epoxied to the stuffing box. Then the o-ring placed on the shaft and the compression nut added over it.
Another variation uses brass tubing. Here, using various sizes of tubing, a housing is made that retains an 0-ring between smaller diameter tubing, placed inside an other brass tube "case". The case is then placed over the stuffing box and shaft. In both cases, grease can also be used, but if done correctly, it should not be needed. Beware that it is the water in the stuffing box that lubricates the o-rings and therefore the shafts should not be run "out of water" for any length of time or at any speed.
Another method one can use is to use actual cup seals, just like the crankshaft seal on your car/truck motors. Very small diameter ones can be challenging to find, but they exist. Again, a brass tube house is made(cut and the seal placed into the housing) with the open pressure side facing the water pressure. The water again lubricates the seal. On subs that use the Engel system and pressurize their hulls during ballasting, back to back seals will keep the water out and the pressure within the hull.
I'lll work on some diagrams.
Remember to KISS. It does not take much to keep water out of our boats.