Data - Marine Driveline Data #2

Propellers

Everything you need to know about props in one paragraph? Hardly – but here are a few points usefully collated in one spot.

Props for inboard engines come in all materials, at all prices; but 90% of them or more are brass. The most common material is manganese 'bronze', which if you have read the other data sheets on this site, you'll know is nothing of the kind: it is a misnamed type of brass (see: /data/d-galvanic.html). Forget the maker's advertising copy, this is the cheapest possible material to make a prop from. Or at least to make a prop that will last more than five minutes – no doubt you could make one from Mazac (aka pot metal), the cheapest casting metal, which children's toy guns and so on are made from.

However, provided that there are sufficient anodes present, and no stray currents, this material is just about acceptable for propellers. But you should of course note that, irrespective of the manufacturer's glowing blurb, this is the cheapest possible solution – you certainly don't want to pay a lot of money for one. It wouldn't make sense to spend a lot of time and effort balancing the blades and making them absolutely symmetrical on a cheap propeller of this type, for instance. These props cost up to (or should cost up to) around £150 for an 18-inch prop. It is pointless to pay more for a manganese 'bronze' prop, since one made from the proper material is not only much less likely to dezincify, in fact of course it can't. These brass propellers are liable to dezincify at any time if things go wrong; your anodes must be up to scratch. If a prop turns pink anywhere, it's a cheap one, and rapidly on the way out. Alternatively, tapping it lightly with a hammer should produce a good ringing sound; a dezincified prop will give a dead 'thunk', and blades have even fallen off when checked like this.

Better propellers are made from real bronze: aluminium bronze (good), phosphor bronze (better), or nickel aluminium bronze (the best of the bronzes), which all, unlike brasses, have no zinc in them, and therefore of course cannot dezincify. Monel (a type of cupro-nickel) is even better, if you can afford it, and Hastelloy even better than that. Silicon bronze is not normally used for props.

DAR

DAR, or Disc Area Ratio, is the surface area that a prop's blades cover, relative to that of a disc of the same diameter. It is expressed as a percentage, so that a DAR of 55 equals a coverage of 55% of the disc. This can also be termed BAR (Blade Area Ratio) or Fa/F. There is some room for manoeuvre in this definition, for manipulation of the statistics, since some makers refer to the figure as a head-on view of the actual prop (the apparent or projected blade area), others as the theoretical blade area with all pitch straightened out (the developed or true blade area); and no one's telling you which.

If you're still not sure what this means, then go stand behind the prop of a yacht hauled out on a cradle, and look forward through the prop. How much of the circle described by the prop is taken up by metal (ignore the shaft and boss here), is the DAR: it will be a bit more than half, for the average prop.

A 2-blade prop has an average DAR of 35. These are used on older sailboats, where sailing performance takes precedence over motoring efficiency. Their drag is obviously less than that of a 3-blader, especially if the prop can be locked in vertical alignment behind the keel. More recent boats tend to utilise a folding prop.

A turbine propeller, which is simply the correct terminology in English for a standard three-blade prop, has an average DAR of 55. By this is meant that a group of basic bog-standard 3-blade props from a bunch of manufacturers would have an average value of 55% DAR.

An equipoise prop (still a 3-blader), the next up in DAR at average 65, is one with slightly more blade area, and is used on motor vessels rather than sailboats. It could be used on a sailing boat where there was insufficient room to swing the prop size necessary, and the high pitch therefore required would produce cavitation. An equipoise prop will in this case bring the boat up to its full speed – at the considerable penalty of more drag under sail.

A 4-blade prop has even more DAR at 75 - 80, and would be used on faster / more powerful vessels.

Size and pitch

A propeller of 16 x 12 R.H. (or 16 x 12 R) is one with a diameter of 16", a pitch of 12", and right-hand sense.

One inch of size (i.e. on the overall diameter) is about equal in effect to two inches of pitch (or 2.5" according to some sources). Therefore a 17 x 14 prop is roughly equivalent to one of 18 x 12. The 18-inch prop will perform slightly better in heavy conditions; the 17-inch prop will have slightly less drag under sail.

Pitch is measured at 70% out from the prop centre.

Sailing drag

If possible, the propeller should be locked while sailing, since this provides several advantages: there is less drag, and wear on the driveline is reduced. Mechanical-shift gearboxes can be locked while sailing by putting them into gear; reverse is normally used. In very heavy weather, forward may be a better choice, since the engine can sometimes be started by the tremendous drag and kick of running down a wave at well over hull speed, and there is a risk of it being started in reverse rotation if the shaft is locked in reverse gear. If this does happen, don't touch the ignition key, but pull the diesel stop to kill the engine. However, with the alternative arrangement of an ignition key off / fuel solenoid engine stop, it shouldn't be able to start.

A hydraulic-shift box cannot be locked. Even if you stop the engine in gear, the gearbox cannot lock the shaft when the engine stops, as the clutches only lock due to oil pressure generated with the engine running. These boxes are always referred to it seems for short as hydraulic gearboxes, though of course they aren't, since that refers to a different type of equipment. Your prop will spin under sail; in the past this was a worry because some gearboxes could not maintain sufficient lubrication without the engine running, and consequently were damaged. However, as far as I am aware, all gearbox manufacturers have now heard of sailing boats, and this problem has been resolved. You can allow the prop to spin while sailing.

Unfortunately, the drag from a spinning propeller is higher than that from a locked one; the noise of the rumbling shaft can be annoying; the sterngland must also be greased exactly as if you were motoring; and wear is taking place on all components, just as if you were motoring. A shaft lock is the answer here; but these are hard to obtain, expensive (around £600), and extremely difficult to fit on the average sailboat, whose engine has generally been installed with a shoehorn and a 'fit & forget' philosophy. Many sailboat gearboxes are completely inaccessible due to appalling installation practices.

Incidentally, some feel that drag with a spinning prop is less than that from a locked one; but consider the case of a helicopter. If the engine fails, the pilot performs an 'autorotation': the gearbox is taken out of gear, and as the helicopter falls, air resistance causes the rotor to spin, and the machine achieves a stable descent attitude and a much-reduced rate of fall – the high drag from the spinning rotor allows it to land with a paraclutic descent, albeit heavily, whereas with the rotor locked it would simply fall out of the sky. You can check this if you have a mechanical-shift gearbox (or a shaft brake) – test your speed with or without the prop locked; you'll sail faster with it locked.

On a larger sailboat, a smaller secondary shaft can be installed and fitted with a small propeller, to drive an alternator for additional 12-volt power generation. When the shaft is locked, there is less drag than when the propeller is spinning and generating power.

A folding or feathering propeller may be fitted, to reduce drag, and to stop the shaft spinning under sail. These are an extremely popular choice now; and there are an endless number of models to choose from, all of which cost at least three times more than a standard prop. One or two of the feathering props will still spin under sail, so you should beware of this especially if your main reason for obtaining one is to stop shaft spin. Among those that still spin the shaft under sail, in some circumstances, is the Brunton Autoprop, a popular choice due to its fine powering reputation in both ahead and astern.

Hand of rotation

A prop can be right or left-handed. Though most are right-handed, the direction of rotation in forward drive is dependent on the output of the gearbox.

The hand can be determined by standing aft of the prop, looking forward. The forward edge of any blade is further from you than the after edge; if the forward edge is to the right (i.e. starboard) of the blade at the top pointing upward, it is a right-handed prop. The blades will screw the boat through the water by rotating to the right, or clockwise.

A right-handed prop kicks the stern to port initially, when reversing. If your stern swings first to starboard when reversing, you have a left-handed prop.

With some gearboxes it doesn't matter what hand of rotation your prop is. You can pick up a cheap prop at a boat jumble and just fit it anyway, it'll still be fine, whether left or right-handed (as long as the shaft size, taper, boss length, and keyway are right, that is). This is also a good way of trying any prop size out. You can do this for instance with some Newage PRM boxes; both the reduction ratio and the gearboxes' ability to supply continuous drive under full load are identical in both forward and astern. One might say that these gearboxes have no forward or astern at all, just whichever way to suit your prop... Well, except for the fact that PRMs have an emergency 'get-you-home' feature that allows you to bolt the gears together solidly by removing a small top cover plate and locking the clutch, if it should by some chance fail. It only works in one direction, so actually you'll still need a right-handed prop; unless you don't mind reversing home.

Propeller size

When choosing a prop, you should be aware that many boats are under-propped, and as a result either cannot achieve their full speed potential with that particular engine, or cannot motor efficiently to weather in stiff conditions. If, when putting your gearbox in forward drive with the engine at low revs, there is no discernible movement ahead initially, then this is almost certainly true in your case. A boat should be kicked forward immediately as soon as forward drive is engaged, and driven forward at a knot or so even at tickover engine speed. Provided that you have enough engine power (many boats don't), and enough prop (again, uncommon), then you should achieve a comfortable fast cruising speed at an easy three-quarters of max engine revs. If you have to rev the engine hard to get a decent speed from your boat, allowing of course for waterline length since smaller boats are slower, then either you don't have enough engine or enough prop.

All the prop sizing charts, calculator programs, and computer programs that I have seen give a propeller size one to two inches small, or two to four inches underpitched, in several size ranges that I have checked out in practice.

Therefore, when buying a propeller, it is best to buy one oversized, since it can easily be reduced in size or pitch if necessary; but the reverse is probably not true. Propeller repair shops have queues of people getting their props repitched upward, trying to achieve the speed they know their boats are capable of.

If, on the other hand, your engine is 'lugging', that is, it cannot reach full revs and produces black smoke at full throttle, then you may perhaps be overpropped. Since the chances of this are rather slim, you should first try opening up the engine compartment and allowing the engine to breathe properly. This often fixes the problem, since they are frequently 'strangled' and do not receive sufficient air at full throttle. Removing the air filter can also help, since this is sometimes plugged up, or simply undersized. One of these measures often fixes the problem, pointing the direction you must then work toward.

Formula for minimum prop size:
D = 4.07 x [square root (BWL x Hd) ] – all dimensions in feet
...where:
D = min prop dia. in inches
BWL = waterline beam
Hd = draught excluding foils

Formula for ideal prop size:
D= [ 632.7 x ( shaft HP exp 0.2 ) ] / ( RPM exp 0.6 )

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A Popular Prop Myth: Cavitation

When a propeller is spun too fast for its pitch, it develops cavitation: it does not operate efficiently, and develops air bubbles on the blades that collapse and implode, making a noise audible on the boat, reducing efficiency, and eventually causing prop damage. Pitting occurs on the blade surfaces.

At least, so we are told repeatedly and interminably. You might scratch your head for a long time, trying to work out from where the air bubbles are supposed to appear. However, cavitation is of course caused by the formation of vacuum bubbles, not air bubbles. A prop with less pitch, and a greater diameter or a higher DAR is needed.

Gearboxes with a 1:1 reduction are sometimes responsible for this, as they spin the prop much too fast for a sailboat, and are generally best avoided for all displacement-speed boats.

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Gearboxes

Many people will say that the true test of a genuine marine gearbox is that it runs on engine oil, and not ATF (automatic transmission fluid). At least in the small and midrange areas that the majority of sailing boat gearboxes inhabit, this opinion is often voiced. Therefore one would expect that hire fleets would use PRMs and suchlike, which run on ordinary oil and have a good reputation in the leisure market for reliability (the cost and/or weight of something like a Twin Disc or a Hundested being out of the question).

However, I now hear that at least one hire fleet operator and gearbox reconditioning agent does not (in part) concur. In their opinion the most reliable small & mid size boat gearboxes are Borg-Warner. My surprise was reinforced by the information that, in any case, a Borg-Warner gearbox should not be filled with ATF, but with engine oil. Apparently ATF is specifically required only when the gearbox is subject to very high revolutions, a situation not encountered in sailboat diesel installations.

Checking this rather surprising information revealed it to be correct, and I am now wiser. The gearbox manuals clearly state this. I would expect many of you to be equally doubtful – feel free to verify it yourself.

Reduction ratios

Cogboxes generally come in a choice of ratios: you can order your new box with a 1:1, 2:1, or 3:1 ratio in forward drive, and probably with a slightly reduced choice in astern. The most popular choice for the ahead ratio, in small to medium-size sailboats, is a 2:1 ratio (pronounced two-to-one). This is something of an 'average' yacht gearbox ratio, since it is almost always satisfactory.

In case you aren't clear what this figure means, it's the ratio of the engine speed to the prop shaft speed: the gearbox reduces the engine revs to a more practical speed for a propeller in water. So, an engine spinning at 3,000 revs, with a 2:1 box, has a prop turning at 1,500 revs (per minute) – a nice top speed for a propeller on a sailing boat, and at 25 revs per second it's the most you'd want. The type of props that can be fitted to a sailing yacht become extremely inefficient at over 2,000 revolutions per minute.

In fact, for maximum motoring efficiency, you would fit a 3:1 ratio gearbox, together with a larger propeller of less pitch. This bigger, slow-turning screw would give you the optimum transfer of power from engine to water, at the very slow speeds (usually below 9 knots) that sailboats are capable of. However, your sailing performance would suffer, due to the drag from the larger prop.

This being the case, a compromise is sought, and a 2:1 pitch in ahead, with the smaller prop that the faster shaft speed indicates, gives the best match between sailing and powering performance. Going further, to a 1:1 ratio, is a poor choice for a sailboat. Although this works for a higher-speed vessel, at displacement speeds the propeller will be very inefficient, and there will sometimes be cavitation. All the engine power will not be able to be transferred to the water.

Reverse

A ratio of between 2:1 and 3:1 is the popular choice for astern. Speeds are slower in reverse due to several factors, and in any case neither optimum speed nor optimum power transfer are sought.

Controllable-pitch props such as the Sabb* coaxial propshaft system for small boats, and the Hundested CP prop for large ones, rule the roost here; the Brunton Autoprop has good reviews, and would be the first choice for a medium-size boat with a tricky marina berth requiring reversing under difficult conditions.

Driveline height

The majority of gearboxes provide an output coupling directly in line with the engine's crankshaft. The propeller shaft will be in a line with the engine output, straight through the gearbox. This is not always the case, though: a drop-centre gearbox has its output typically four inches below the crankshaft centreline. These gearboxes, of which Newage PRM are among the best known, can be used to solve several types of installation problems.

Be careful when re-engining, since replacing an engine that has a standard in-line gearbox with an engine mated to a drop-centre gearbox, will require the engine beds to be raised by around four inches; the converse, requiring a drop of four inches, will hardly be possible.

* Saab and Sabb:
Saab is a Swedish car firm that does not make marine engines or gearboxes. Sabb is a Norwegian marine engine and gearbox firm that does not make cars.

Sabb marine diesels are heavy, simple, basic, have a good hand start, and are very reliable – which is why they are often fitted to ship's lifeboats. Their gearboxes are in much the same mould. They do not operate on the usual principle, though: instead of forward and reverse gears, they have an axially-bored propshaft with another shaft inside – a co-axial propshaft. The propeller pitch is variable from full ahead to full astern, with the inner shaft controlling this. The engine / gearbox always runs in ahead direction.

Well, of course it would, you say: how can the engine go backwards?

Aha – you'd be wrong if you thought this had never been used. The old Dolphin marine petrol engines used to have to be stopped, and then restarted in reverse rotation, in order to get reverse gear; and boy, was that fun on a windy day in a tight harbour...

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Thanks to the contributors to this page who have made corrections:

Bob Mutter of .ca [yeah, my Scandinavian geography ain't too good, Bob...]

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