Keeping up Appearances - Part Three

If there are any joys of boating to be experienced when your boat is on the hardstand, then I confess that all of them, without exception, have escaped me. The once a year haul-out is something I look forward to with as much enthusiasm as a trip to the dentist, but it has to be done and moaning doesn't help.

At least it is a lot easier than in years gone past, with modern travelifts and good cradles replacing the old-fashioned slipways or ? even worse ? the method of tying the ship to two pilings, scrubbing as the tide went down and painting as it rose. I used to use that method a lot in North Queensland because it was free, but that was its only recommendation.

Well, let's have a look at the work that may possibly confront you when the boat is sitting in the cradle, having been placed there by the yard crew. Most of us will elect to have the hull blasted with a high pressure water blaster before proceeding any further. If you haven't and are going to rub the bottom clean by hand, then all I can say is let me send you the address of the Masochists Society of Australia, as I believe they are always looking for new members.

To continue, a careful check of the hull reveals that the dreaded pox (osmosis) that we discussed in last month's article is not in evidence, so we can turn our attention to the running gear and make sure that is up to scratch.

The underwater running gear of a boat consists of the propellers, skegs, shafts and rudders at the stern. This is the business end of the vessel and needs careful attention if the boat is to perform at its best.

PROP TALK
Firstly, we will have a look at the propeller, or propellers if your boat has twin engines. A propeller generally has its size stamped on the hub, usually with two numbers, the first designating diameter, and the second pitch.

The diameter is the diameter of the circle the blades make as they rotate, while the pitch is the distance the propeller would advance through the water in one full turn. Thus a four-bladed 24 x 6 propeller would be one that has a diameter of 24in and would move 6in ahead in the water for one rotation.

In fact the second figure is theoretical only and is never achieved in reality. The difference between the theoretical and actual is termed 'slippage'.

The number of blades generally affects the smoothness of the propeller, but more blades decrease the efficiency of the unit as a whole, so there is a limit. The most efficient propeller is a single-bladed unit, but because it would be impossible to balance it is only a theoretical curiosity.

The most efficient, practical propeller that can be built is a two-bladed one, but these can produce hull vibrations. The reason is that every time the blade passes under the bottom of the boat, a pressure wave from the blade will strike the hull and can in some instances sound like someone hammering on the bottom of the boat. Adding more blades tends to smooth this out as the pressure waves are closer together for each revolution and the noise becomes much smoother, but adding blades means that they start interfering with each other and thus the efficiency of the propeller falls away.

The designer of the boat will determine the correct propeller based on engines, displacement and various other data. It is seldom an exact science and it is not uncommon for a few attempts to be required before the right combination is achieved.

Remember, some owners will want a propeller that gives them the best possible performance, while others will want top acceleration and some may opt for economy. It is inevitable in balancing all these desires that areas will have to be compromised; my only comment is that if you are not satisfied with the performance of your boat, then see a specialist who knows and understands propellers, because it is possible to apply quite a range of modifications to your existing prop without having to go to the expense of a new one.

TAPERS AND KEYWAYS
After all that theory on propellers, let's turn our attention to how they are attached to the propeller shaft. An understanding of this will help when it becomes necessary to remove one.

The end of the shaft has been turned in a lathe to produce a taper and at the end of the taper is a thread, onto which will screw a large retaining nut. An inspection of the taper will reveal a keyway cut into the shaft. A keyway is simply a square groove, generally 50mm or so long. Into this square groove will fit a square bar of metal approximately the same length as the groove, but twice as thick as the groove is deep.

If we now have a look at a propeller that has been removed from the shaft, we notice that the inside of the propeller hub has also been machined to a taper, which in fact exactly matches the taper on the shaft, and that there has also been a keyway cut, identical in dimensions to the shaft's. The purpose of using a taper fit for a propeller, rather than a parallel fit, is to facilitate installation and removal.

When a propeller is installed, the key is first placed into the keyway of the shaft and then the propeller is offered onto the shaft, making sure that the keyway in the hub is aligned with the shaft key. As it slides up, the two tapers mate together, at which point the entire surfaces of both are in full contact.

The key prevents the shaft from turning inside the hub of the propeller and the retaining nut at the rear ensures that the taper is fully home and prevents the propeller from coming off when the boat is put astern.

The advantage of the taper fit is obvious when it comes to removal. The propeller has only to be moved back the smallest amount and it is released from the shaft, whereas with a parallel fit, release is only achieved when the entire hub is dragged from the shaft.

Many years ago I assisted in the removal of a parallel-fitted propeller and it took us two hours to get the thing off, succeeding in the end only by heating the hub to almost red heat whilst chilling the shaft and then pounding away with a large sledgehammer. That was the most convincing demonstration I've encountered of the necessity of taper-fitted propellers.

Before leaving the subject, have another quick look at the retaining nut, and you'll notice that the end of it has little sections cut away in such a fashion that it looks like a little castle. This is why it is called, wait for it, a 'castellated nut'. The purpose of the cutaway sections of the nut is to allow it to align with a hole drilled at 90º to the axis of the shaft at its threaded end. When the nut is done up dead tight, a split pin can be inserted through the hole and the two halves bent around the nut, thus preventing it from coming undone.

Although this looks foolproof, it can fail. A small launch I once owned shed its propeller at Port Stephens when I went astern, which ruined the entire day! Always fit a new pin if it has been removed.

REMOVING THE SHAFT
So why is it necessary to take a propeller off in the first place? Well, generally it isn't, provided that everything is in good condition. We will need to check a few things.

Firstly, notice that the propeller shaft is supported just ahead of the propeller by a bearing. Because this is generally in a skeg, it is consequently called a skeg bearing. The skeg bearing is made of rubber or other synthetic material.

Grasp the propeller and see if the shaft has any movement between it and the skeg bearing. A small amount is permissible, but more than a millimetre indicates that it has worn beyond acceptable limits and must be replaced. This is a tiresome task that is sometimes easy and often difficult, depending on the original engineering of the running gear.

The first item to be removed is the propeller itself. Straighten out the split pin around the castellated nut and remove. Undo the nut, sometimes a bugger of a job in itself; cleaning the exposed threads at the end of the shaft first will assist this.

A good smack with a large hammer and a block of wood on the front of the propeller hub will generally release the tapers. However, don't overdo this method. If it refuses to budge, borrow or rent a large puller and use this to apply rearwards pressure on the hub by tightening the screw of the puller.

If the propeller still won't release when a lot of pressure has been applied, the application of another good whack will generally do the trick. If it doesn't, then the next recourse is to apply heat to the hub using oxy acetylene. It's best to engage an experienced person for this, as setting fire to the boat will not assist in propeller removal.

Using heat is the worst-case scenario and is not often necessary due to the ingenuity of taper fits.

With the propeller off, the fun now begins. Go into the boat's engineroom and check where the shaft joins the gearbox at the back of the engine. This is called the shaft coupling and this coupling consists of two flanges bolted together, sometimes with a vibration damper in between.

It should not be necessary to undo these flanges, but we will need to release the shaft from the rear end of the coupling. Generally this is a keyed parallel fit into a split hub. Undoing the bolts that clamp the split hub to the shaft will normally allow the shaft to be withdrawn easily, but often the coupling is covered in corrosion due to its proximity to the stern gland, which likes to flick saltwater over everything when it leaks. Sometimes the only way to remove those frozen bolts is to saw through them with a hacksaw blade, a job that will get you a peerage in the Masochists Society, believe me.

Another thing to brighten your day is the accessibility of the coupling, or the fact that it was designed by a demented person with a degree in cooking. Over the years I have gazed upon some baffling arrangements that defy belief and thanked my lucky stars that I haven't been the one to dismantle them.

OOPS, MIND THE RUDDER
Let's look on the bright side and assume that all has gone smoothly, the shaft has slid out of the coupling and we are back at the stern of the boat. It is now with horror that we realise the shaft still cannot be withdrawn because the rudder is in the way. Bursting into tears will not help, but it may make you feel better.

Some boats have the rudder slightly offset to the line of the shaft, in which case there is no problem as it will neatly slide past; this is called good engineering.

Often the rudder is relatively easy to remove because the designer realised the necessity of such things and accommodated for it; this is called effective engineering.

Occasionally it seems as if the rudder was the first thing placed on the assembly platform and the rest of the boat was constructed around it, so that it will now require a least a degree in rocket science to figure out how to get it out of the way to allow for the shaft's removal. I don't know what sort of engineering this is called, but a few choice words spring to mind.

So in the fullness of time, often days, we finally get to withdraw the shaft from the skeg bearing, and for all those of you with two engines, remember this will have to be repeated for the other side. I said there wasn't much joy in being on the hardstand at the start!

A NEW SKEG BEARING
Okay, lets press on and remove the offending bearing. There are so many ways of holding this in, including gluing, that its removal will have to be left to individual inspection. Due to the soft nature of the bearing, it can generally be removed fairly easily by destroying it with a tool such as a chisel or saw blade.

When success has been achieved, measure the diameter of the bearing aperture and the diameter of the shaft and head off to your local chandlery to purchase a new one. If the shaft and skeg are of relatively standard sizes, the right bearing will be obtained easily; otherwise it will have to be machined to correct dimensions in a lathe.

If you are operating your boat in a lot of shallow sandy water and find that this is causing you to wear out the bearing very quickly (the sand gets drawn between the shaft and the bearing), then you may want to consider fitting a bearing with grooves machined longitudinally. This allows the sand to flush through and reduces wear.

Standard-sized bearings are commonly sold in lengths of around 300mm or so; just cut this down to fit your skeg.

Whilst on the subject of skegs, you will often see a small pipe set at right angles at the front. Its purpose is to introduce water between the bearing and shaft to assist in cooling and flushing.

While the shaft is out of the boat, it is a good idea to renew the packing in the stern gland, as its removal is easy with the shaft out. If your boat has a lip seal gland rather than the old-fashioned packing, then don't disturb it if it is not leaking.

Dig the old packing out of the gland but do not renew it until the shaft has been re-installed. Use at least three individual turns of packing around the shaft. Tighten down the gland with gentle pressure but this will have to be checked when the boat goes back in the water.

At times on the hardstand you begin to wonder if you will ever get the boat back in the water, as the work list seems endless. Well, it only seems endless because it is. Remember that a boat is not finished until it is sunk, and only then in deep water!

Hopefully next month we can round off these out-of-the-water chores and get on with more of the fun side of boating. Meanwhile, I think I'll just check those rudder bearings, then fit the new anodes, then... See you next month.