REVERSING ISSUES:

On the basis of over 2500 propeller years of service we have received a number of enquiries which we have documented generally involve a common theme of the engine overloading when reverse is engaged.

Assuming the unit is well greased internally and all the blades are free to move about their mounting as when new then this can be caused by a number of possible issues.

FOULING: Anything caught around the blades such as plastic bags, fishing lines, old rope will of course prevent the blades adopting the correct reverse position.

This should always be the first thing to check if this issue arises overnight.

If it is of a more intermittent nature it is more likely to be a lubrication issue – see below:

REVERSE RATIO: One cause of this can be that the reverse ratio of the gearbox is lower ( leading to higher shaft rpm in reverse than ahead ). This is an uncommon situation and is most likely to be found on Lombardini and some Westerbeke models. Sizing an optimum propeller for a reduction ratio of 2.6:1 in ahead will always lead to constraints on engine rpm in reverse when the reverse reduction ratio is say 2.18:1 simply due to the higher shaft speeds.

OEM GEARBOX: Another issue – particularly with older engines is that they may no longer be fitted with the original gearbox supplied with the engine and may now have an OEM box of another brand or reduction ratio.

It is for this reason we always ask for the actual reduction ratio in both Ahead and Astern.

SPRING TENSION: Another occasional problem we have seen particularly for units that have been re-assembled is that the internal torsion spring tension is too high causing reverse engagement ½ way through cycle leaving blades at ~ 45 deg of pitch. This will cause the engine to overload in reverse. In these situations the blades are in effect beginning to rotate and generating reverse thrust before they have adopted the full reverse position.

A series of pin punch marks align when the boss has been re-assembled correctly. Refer to our Assembly Manual on the web site.

For lower powered units a lower spring tension will assist the engagement of reverse, but for a lubricated unit assembled to specification this should not be necessary.

POWER LOSS: We have also seen a number of situations where after much research as to why the engine was overloading in reverse ( and to some lesser degree in Ahead ) it was discovered that the engine had lost compression on one cylinder ( always nearest the water injection bend ) and that the marginal loss of power coupled with small increases in pitch ( all Kiwiprops go to a maximum pitch position in reverse ) was sufficient to create an overload situation in reverse.

As engines get older corrosion frequently reduces the internal diameter and roughens the surface of the exhaust bend. This is subject to high rates of corrosion due to the hot ( often salt ) water meeting the cast iron manifold.

Even reductions of 10 – 15 % can cause dramatic power losses – particularly in smaller engines. Overloading in reverse should initiate checking of the exhaust elbow for corrosion.

It is also essential to ensure manufacturers specifications for exhaust pipe diameter and constraints are followed as back pressure substantially reduce the power available to the shaft.

Refer to our web page: POWER LOSS for a detailed explanation.

IDLE SPEEDS: We have also seen situations where due to a low idle speed ( which may be caused by auxiliary power take off eg compressor ) the Kiwiprop unit was not able to engage fully before overloading the engine. This could be exacerbated by a cold engine and particularly applies to small engines at the lower end of the power range. It is not an issue engines > 30 hp. The operating manual has quite specific shaft idle rpm constraints – it is important to observe these as the unit was tested within the operating environments of the most popular engine types. This is more relevant with higher reduction ratios such as 3:1.

OIL LEVELS: Another issue we have seen on a number of occasions is low levels of oil in the gearbox. Modern gearboxes that are of a clutch type use oil pressure from the engine driven side of the gearbox to engage the gearbox. Yanmar Saildrives ( SD20 – SD30's ) use a dog clutch and are not affected. Low oil levels can cause a slow engagement of the reverse ratio leading to the Kiwiprop unit adopting a 45 deg reverse position for the blades and loading up before flicking to the normal reverse position as would happen if the gearbox was engaging to specification.

LUBRICATION: By far the most common cause of reverse overload is a lack of lubrication inside the unit. This can be in the boss of the unit and or on the individual blades of the unit which must be free to move easily about their mounting pin at all times.

This assumes there is no binding caused by external fouling – eg barnacles or antifouling.

While we would expect the unit to operate without maintenance between haul outs – units in high use or in very dirty or abrasive operating environments such as encountered in shallow sandy rivers or maneuvering within lock systems will require monitoring to ensure they are lubricated.

Units in chalk or coral areas may also see high levels of deposits inside and around the unit.

It is important to ensure these deposits are monitored to ensure they don't effect the operation of the unit which depends upon low levels of friction within all the moving parts.

Later units to # 8500 have O Rings in the blade roots and while not visible our tests have shown these will contribute to ensuring high levels of grease retention within the unit over time.

Current units now have individually turned neoprene V seals which are superior to O rings as they have greater resilience and ability to accommodate assembly tolerances and wear while still retaining sufficient pressure to ensure the seal remains to exclude any dirt.

The Operating Manual supplied with the unit and available off the web details lubrication requirements both for the individual blades and for the boss.

While in a perfect world the unit would not require lubrication – in the real world the extremely harsh operating environment for all propellers requires they be regularly lubricated to maintain their functionality.

LOCKING IN REVERSE: Some customers report that over time their gearboxes have locked in reverse position and are hard to get back out into neutral.

Basically there are two types of clutches in gearboxes: Yanmar use dog clutches in the earlier and smaller Saildrives – and these never seem to give problems. The downside is they can engage quite violently.

Like all clutches however the faces can wear over time and this can cause them to be harder to get out of reverse gear.

All others use cone type clutches. These offer smooth engagement – but of course have conical clutch surfaces that can and will wear from normal usage. Over time after repeated use and perhaps exacerbated from corrosion on the metal cone faces – these tend to lock together and are hard to separate – which translates as difficult to get back into neutral.

Regular oil ( and seal on Saildrives ) changes as per manufacturers spec help to prevent this.

As engines and drive trains get older – these effects can and will arise more frequently.