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Technical Q&A

PUBLISHED: 18:03 05 August 2013 | UPDATED: 18:03 05 August 2013

'Mystery noise' diagram

'Mystery noise' diagram

Archant

A selection of reader questions.

'Rudder judder' diagram'Rudder judder' diagram

WHAT’S THE MYSTERY NOISE?

Q: When in Drive at about 1400 to 1500 rpm, I am getting a noise that I can only describe as a ‘metallic grinding’ which can be eliminated by throttling back to around 1250-1300 rpm although this can vary. It is, however, not always present, when leaving a lock recently, I noticed that the noise was not present at 1500 rpm and did not manifest itself until I had to change engine speed for a reason. Could this be cavitation? If so, what causes it, how is it cured and why is it a new phenomenon to me? The noise can only be heard from outside the boat – when driving it or when leaning over the side but not when you stick your head in the engine room! I have just upgraded the propeller as the old one was under-powered, but this noise was present with the old one albeit at higher revolutions. The boat is now six years old and this noise has only been around for the last six months or so.

A: I do not associate “metallic” sounds with cavitation. Also a narrowboat is likely to have a low power engine driving via a reduction gear so cavitation is less likely.

One possible explanation is the weed hatch – its cover should have a horizontal plate hanging from it that very nearly fills the weed-hatch hole level with the hull (just above the prop). It is an anti-cavitation plate and these have been known to vibrate against the hull at certain engine revs.

Inspect both the plate and the area around where the plate sits to see if there is any sign of bright or polished metal or loss of paint/blacking. If so the cause has probably been found.

If not make up a plywood cover for the top of the weed-hatch and test the boat (taking care not to allow too much water to leak past wood into the boat).

If the sound is different or has gone, again the problem has been located. Have a set of heavy metal plates welded between the hatch cover and anti-cavitation plate so it cannot vibrate. You may have success taking an angle grinder to the plate and reducing it to clear the problem spots, but this might give rise to cavitation.

If it is not vibration I would next suspect propeller ventilation. This is where the prop draws air down from the surface around itself. It is more of a problem on narrowboats where the restricted water flow past the boat can allow the prop to remove the water from under the swim and effectively bring the water level down to the return at the top of the swim. It is then easy to draw air into the prop.

This is all to do with basic design and may be difficult to stop. When the noise is apparent have a good look at the water level all around the stern of the boat. I would want to see it well over an inch above the return to the swim plate – the higher the better.

If it is not vibration or ventilation then there is every chance it is cavitation and unfortunately curing it can be difficult. Things to try include using an angle grinder to smooth and round the edges of propeller, moving the anti-cavitation plate up or down, ensuring at least one or two inches clearance between prop blades and hull/skeg or moving the rudder away from the prop.

Fitting a “large surface area” propeller is another solution – talk to an inland prop specialist like Crowthers.

Also talk to people with a similar hull to yours and see what they have had to do.

TO BOW OR NOT TO BOW

Q: After almost 30 years hiring and sharing, I have decided to take the plunge and buy my own boat. I have criteria of what I want and require. One of those is a bow-thruster. I have ignored many first class boats because I have been led to believe that the retro-fitting of a bow-thruster is either too expensive or too impractical. However, at a recent boat show I was advised by the representative of a well-known bow thruster manufacturer that it should be no problem to retro-fit one in most boats, it was more an unwillingness of brokers to recommend such action for pre-owned boats. Can you please advise.

A: Well a salesman would say that wouldn’t he! Narrowboats vary so much that this question is virtually impossible to answer. You can do almost anything if you throw enough money at it! It all depends upon the design at the front of the boat. If the water tank is below the front cockpit step you would have to move the thruster back into the cabin and find a way of accommodating the transverse tube and keep the motor above the water level. It can be done, but not aesthetically in my opinion.

Look at the cross section diagram of a typical modern narrowboat. There is nowhere to accommodate the bow-thruster because of the gangway – but all boats are different. If there is a void under the cockpit floor then it is probably possible.

Now we come to accommodating either two very thick cables running the length of the boat or a bow-thruster battery close to the actual device. This has BSS ramifications over fume venting. Again it is not impossible but may not be aesthetically pleasing.

A large hole needs to be cut in each side of the hull – and they are not likely to use a jigsaw! Unless good access can be obtained to the inside of the hull and/or all flammable material (including paint films etc) can be removed, the flame or plasma cutter could set the boat on fire.

If I had to advise without inspecting each boat in question I would side with the brokers. If you look at the full tube and thruster kit I am sure you could see the problems associated with fitting it to any particular boat.

PROP SHAFT SIZE

Q: I am having a 50ft narrowboat shell built for me. I have bought a 2.5 BMC engine (secondhand to recondition myself) with a PRM gearbox. What size prop shaft is best suited to this engine, 1.25 or 1.5in diameter? My original plan was for a 1.8 BMC engine but the 2.5 came available so I went with that. Can you see it causing me any problems, because I have since been told by an ‘expert’ it is too big for the length of boat?

A: As this is a canal boat that has to get intimate with whatever is sitting on the bottom of the canal I would tend to favour the larger shaft. This will be better at resisting the forces when the prop jams a rock/trolley etc. against the skeg/hull and stops dead.

The only thing to watch is the space required to fit it. I cannot see that being a problem on a new build if you are specifying the stern gland, otherwise the size of the gland will dictate the shaft size.

Also check the easy availability of stock spares. I would ask a few chandlers what sized prop boss hole they normally stock.

It is true that diesels like to work hard but it has been calculated that a 60ft narrowboat only draws 1 or 2hp from its engine at canal speed so almost any diesel is too large on that score. On modern systems I suspect that the alternator(s) take more power than the prop. Theoretically your engine might be more prone to bore glazing than a smaller one but care in using low spec oil (say API CC or CD) and occasionally giving it a good thrash should solve that possibility. In any case BMCs are not known for bore glazing.

Anyway you appear to be well equipped to deal with glazing if it arises. When you rebuild the engine make sure you glaze bust (hone) the bores if you are not getting them re-bored.

Depending upon the reduction ratio in the gearbox it is possible that you may not have space to swing the optimum diameter prop and will thus run at higher engine revs than the optimum. However, this is not such a problem on canals because the engine speed is governed by boat speed and the alternator(s) often require higher revs for maximum charging. A number of hire boats are deliberately under propped to allow for this.

A larger engine with a good prop match would give better ‘braking’ than a smaller one and would also be better for use on rivers when there is a danger of high current flows.

Practically the only downsides that I can see are less space for working around it in the engine room and, being longer, it may limit you in choice of flexible shaft coupling (if it is to be flexibly mounted I would urge serious thoughts about an Aquadrive or Pythondrive).

Expert’s theory and practice do not always agree!

RUDDER JUDDER

Q: I have a 1994, 36ft narrowboat. The rudder has become increasingly loose in the top bearing so that the slightest attachment of weed causes alarming judder in the tiller. The bearing clearly needs replacing and I would be grateful for your advice. Is this a DIY job? I suspect not as the old one will probably be very reluctant to part company with the shaft. If not what sort of cost should I expect? Should I stick with the current design (steel ball-race I think) or is there a cheaper/easier option?

A: Is it a DIY job? That depends on what experience you have in this type of process. As you will see, it can be tricky. I doubt any boatyard will give you a firm price because no one knows how easy it will be to get the old one off or what they will find.

There are as many variations in rudder stock designs as there are builders, so without looking I can only guess at what you have. If you have ball bearings you would change the whole top housing. I expect four nuts or bolts on the top of the fuel tank hold it down. (Tip: if bolts, put some gasket sealer around their threads when replacing to render them rainproof).

Unless you have looked when the boat has been out for blacking you will not know how the rudder is located in the skeg, so fit a length of rope through the hole on the rudder where the blade breaks the surface of the water. Thus if it falls out of the skeg when you lift it later in the process, you can retrieve it without getting too wet!

The first thing to do is to remove the swan neck tiller arm from the fixing at the top of the rudder stock. With a bit of luck this will have a big nut on top, holding the boss of the swan neck onto a square or round taper. It may or may not use a key to locate it. Otherwise the boss could be split with a pinch bolt clamping it to the shaft or a taper pin like a cotter pin on a bicycle crank.

If it is a taper get someone to hold a sledgehammer against one SIDE of the boss while you attack the opposite side with the mooring hammer. Hit as hard and as fast as you can which should cause the boss to jump free from the taper. If it is not a taper you may require a very strong gear puller to pull the boss off the stock.

Apply coarse emery paper to the shaft that is left sticking up from the bearing, and clean it to bright metal. Do not stint on this and the later stages of the operation will be a bit easier.

In the sides of the raised collar just above the actual bearing you will see two holes. In each is an Allen screw (socket head screw); remove these – drilling them out if you have to. Undo the four nuts/bolts that hold the bearing housing onto the stern deck. Refit the big nut that held the swan neck on and tighten until it is just about proud of the top of the thread to protect the thread.

Now either apply the puller to lift the bearing assembly up the shaft or pack the underside of the bearing housing, close to the shaft so it is raised no more than half an inch (otherwise you might lift the stock out of the skeg). Use metal as packing because wood has too much ‘give’. Apply the mooring hammer or sledge to the end of the shaft, ideally with a short length of brass or aluminium round bar as a drift. As the shaft drops through the bearing, re-pack and repeat the blows until the bearing assembly comes off the rudder stock.

I think you can now get this type of housing with a nylon bearing. This may be better than a ball type because it should not rust away. If you have to refit a ball type, do remember to re-grease it every month or so. I have also seen the ball bearing type with an oil seal at the top face. This would prevent the spurts of water when turning under power. Your local bearing supplier should be able to match your assembly and suggest alternatives. They may also be able to supply a new ball bearing assembly to fit the original housing.

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