Reversing Issues
Sep 2023:
On the basis of well over 85,000 propeller years of service going back over 25 years - we have received a number of enquiries from earlier units with higher times in sevice which we have documented that generally involve the engine overloading when reverse is engaged.
NB: Many of the issues identified below - other than the lower reverse redn ratio - have been addressed through our ongoing continuous improvement program which has seen a switch to triangular shaped Tri Rollers that reduce force per unit area on the blade root, a sealing cap on the top of the reverse rollers which both retains grease and prevents fouling on the bearing surfaces plus the addition of what we term Impact Screws at the point of maximum force into the blade root thus eliminating blade root wear whilst ensuring the ongoing free movement of the Tri rollers as they are rotated by the forces of reverse engagement.
Referenced as follows: Tri Rollers & Impact Screws:
Assuming the engine idle speeds are in manufacturers specification, the unit is well greased internally and all the blades are free to move about their mounting as when new, the reverse rollers are free to rotate - then this can be caused by a number of possible issues.
Our experience is that ~ 90 % of these issues are due to lack of lubrication of the blades and the boss - forward and aft, and / or fouling and binding of the reverse rollers. If the problem has become slowly more frequent over time, this will almost certainly point to lubrication as the cause.
We caution in our OPERATING MANUAL not to paint the reverse rollers on the rear of the boss with antifouling paint - yet frequently see units where the rollers have been painted over and are frozen to their M8 central mounting screw. They are then unable to turn freely about their bearing when engaging the blade root during the reversing funtion. This additional drag - which forces the rollers to generate a sliding motion vs a rolling motion will make reverse engagement more difficult and can lead to partial engagement as described in B below. Over time the inevitable build up of carbonate type marine deposits can also freeze the rollers. It is important at haul time to rotate these rollers with a set of pliers in a clockwise direction to ensure there is no tendency to loosen the screw. Applying CRC in an aerosol can will assist in blowing lubricant into the bearing area and assist in freeing up the rollers so they turn easily with finger pressure.
It is essential that the reverse rollers are always free to rotate easily about their mounting screws as a pre-requisite to any analysis of reverse engagement issues. New Tri Roller design will address this with a sliding and forced rotation through 120º with every reverse engagement maintaining ongoing mobility of the Tri rollers about their M8 axis.
There are two distinct situations that can arise:
A: Occurs when the shaft rpm in reverse is such that at the maximum pitch setting of the Kiwiprop in reverse ~ 24º - coupled with a low reverse reduction ratio leads to high reverse shaft speeds which prevents the motor achieving close to max rpm in reverse. For a given reverse ratio - this is addressed with smaller blades set at higher pitch to maintain Ahead thrust while reducing the power absorbed in reverse. You would expect this situation to be repeated unchanged on every occasion. Vessel reverse thrust will be significant - and the vessel will quickly build speed in reverse.
Where there is a large difference between a lower Ahead reduction and higher Reverse reduction ratio eg 2.60:1 in Ahead and 1.86:1 in Astern which leads to much higher shaft speeds in reverse - in some situations sufficient to overload the engine in reverse at idle - one approach is to reverse the rotation by swapping the control cable lever thru 180º so you now have 1.86:1 in Ahead and 2.60:1 in reverse - but does require the reduction box to now run in what was effectively reverse which may be an issue. Higher idle speeds will generally address this - but vessel speed in reverse will be significant. Very few gearboxes have these signficantly different ratios.
B: Occurs when due to any one or combination of the issues described below - the unit instead of flicking through 100º on the internal clutch inside the boss when reverse is engaged - the blades stick on the reverse rollers half way through this motion leaving the blades at ~ 45º which will overload the motor and generate little reverse thrust. You would only expect this situation to occur when a specific problem arises. The key characteristic of this situation is manifest as little or no reverse thrust with consequently little rearward motion of the vessel.
It is important to try and identify which of these issues is relevant in each situation.
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.
REVERSE ROLLERS FREE: It is critical that these are free to rotate about the M8 screw they are mounted upon. Any antifouling that has been applied must not lock these up so they cannot rotate freely.
SPRING TENSION: Another 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.
CAUTION: If excess mastic is squeezed forward into the spirng recess during re-assembly this can set and despite initially testing free - cause binding after it hardens and prevents the spring condensing under torsion. This forces rotation of the unit before reverse has engaged the full 100º movement on the boss leaving the blades at ~ 45º excess pitch. An overloading situation will thn exist with little reverse thrust and motor overloading.
ROPE WRAP AROUND PROPELLER: If this has occured it is highly likely the rope will have wrapped around the nose cone and in stalling the motor or breaking the rope the nose cone will have moved on the boss. As these tail the internal torsion spring any movement of the nose cone will alter the spring tension and cause reverse engagement problems.
Every unit has a set of pin punch marks to indicate initial alignment on each component:
See page 10 of the OPERATING MANUAL .. Amy movement from the initail settings will require repositioning the nose cone.
DAMAGE: Always check that the 3 / 4 reversing rollers are present and free to rotate. Scenarios exist where these can be sheared off or bent generally in an impact situation during reverse. The absence of a roller(s) will allow a blade to adopt an Ahead position in reverse and lead to severe vibration.
These should always be the first things 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: If the condition was to progressively occur more frequently over time, that can also be a likely indication of lubricant loss.
As a temporary workaround - give the unit a very brief kick ahead prior to immediately engaging reverse. This pushes the blades into the Ahead position which is the same as the Reverse position. Then you will have the blades fully engaged in the reverse position and will have full normal reverse function.
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 2.13:1 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 of the current installation.
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.
We have also had situations where for smooth running - idle was set very high at ~ 1100 which on a 2.00:1 redn leads to very high shaft idle speeds. This was causing partial engagement leaving the blades at ~ 45º when they loaded up prematurely and overloaded the motor with poor reverse thrust. This was a relativly small high speed motor installation. The problem was cured simply by reducing idle rpm to ~ 850. Similary we have seen issues when the idle speed was very low. We have a web page on IDLE SPEEDS that expands further on this issue.
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 Saildrive model SD20'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. This is also available in current format on our web site.
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. Blades that are partially engaged at ~ 45º as described above will generate increased reverse loads and exacerbate this issue.
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.
Others use cone type clutches. These offer smooth engagement – but of course have conical metallic 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.
Today all Saildrives - with the excepetion of the SD20 described above - and most shaft gear boxes have gone back to traditional clutch packs with smooth engagement and easier dis-engagement forces.
Regular oil ( and seal on Saildrives ) changes as per manufacturers spec help to prevent this. Over long time frames cone type clutches will eventually need their surfaces reground back to original specification. Yanmar specify 250 engine hours between re-lapping of these surfaces on their older Saildrives.
As engines and drive trains get older – these effects can and will arise more frequently.
Higher than expected idle speeds of course provide additional loads on the clutch which can make dis-engaging from reverse more difficult.
EXCESS BLADE ROOT WEAR: If over time the blade root has become severely scored - often as a result of lack of free rotation of the reverse rollers mentioned above, the friction forces of the reverse rollers acting on the blade root will increase and restrict blade movement to ~ 45º pitch thus overloading the engine.
Current models have for some years incorporated Impact screws in the bade root to offer a metal on metal contact at the point of maximum compressive force during reverse engagement which has virtually eliminated this issue.
SUMMARY: There are a number of variables to be consider when reverse engagement and dis-engagement is an issue.