Based on more than 2500 cumulative years of propeller service, we have received a large number of questions very often centered around the theme of engine overload in reverse.
The causes of these problems are diverse, as long as the internal lubrication of the propeller is correct and all the blades rotate freely on their axis as when they were new.
Anything that can be taken or stick to the propeller, plastic bag, floating ropes, fishing lines, etc., prevent the blades from orienting properly in reverse. This is the first check to be made when a problem of this type occurs unexpectedly. If the problem is more intermittent, its origin and probably to look for in the lubrication (see below).
A reduction ratio that is too low in reverse (producing a higher shaft rotation speed in reverse than in forward) may be the cause of the problem. This situation is not common and is mainly found with Lombardini engines and some Westerbeke models.
Determining the optimal size of a propeller with a reduction ratio of 2.6:1 in forward always leads to stresses on the engine speed in reverse when the reduction ratio in reverse is for example 2.18:1 simply because of the higher rotational speeds of the shaft.
Another problem, especially with older engines, is that they have not always kept the original mounted inverter and can now be equipped with an inverter of another brand and/or with a different reduction ratio.
That is why we always ask for the actual reduction ratio in forward AND reverse to be indicated to us.
The internal torsion spring surge is another occasional problem that appears especially when the propellers have been completely disassembled. In this case the blades are only half oriented in reverse, limiting themselves to an angle of about 45 °. The overload on the engine is then significant in reverse. In these situations, the blades begin to rotate and generate thrust in reverse, before reaching the correct orientation.
LOSS OF POWER
We have also seen a number of situations where after a lot of research on the origin of the engine overload in reverse (and to a lesser extent in forward motion) it appeared that the compression ratio had decreased on a cylinder (always closest to the water injection elbow) and that the marginal loss of power coupled with the small increase in pitch (all Kiwiprops propellers™ orient themselves at the maximum angle in operation). ) was sufficient to create an overload situation in reverse.
As engines age, corrosion frequently reduces the internal diameter of the exhaust elbow and deforms its internal surface. This part is subject to a high corrosion rate due to the coming into contact of hot (and often salty) water with the cast steel manifold.
A diameter reduction of 10 to 15% can cause a dramatic loss of power – especially in small engines. Any overload found in reverse shall cause an examination of the exhaust elbow for corrosion.
Refer to our power loss webpage for a detailed explanation.
We also encountered situations where due to low idle speed (caused, for example, by the power absorbed by an external device such as a compressor) the Kiwiprop™ propeller could not fully engage before overloading the engine. This problem can be amplified when the engine is cold but only affects the least powerful engines.
It does not appear with engines with a power greater than 30 hp. The user manual indicates specific constraints on the speed of rotation of the shaft at idle. It is important to respect these recommendations, as the propeller has been developed to match the operating characteristics of the most common types of engines. This problem most often appears with high reduction ratios, 3:1 for example.
Too low an oil level in the inverter is another problem. Modern clutch inverters use the oil pressure created on the engine side of the inverter to engage the gearbox. Saildrive Yanmar (SD20 – SD30) uses a claw clutch and is not affected by this issue.
Too low an oil level can slow down the engagement of the reversing ratio, which causes the Blades of the Kiwiprop™ propeller to orient themselves at 45° in reverse and to load the engine before reaching the normal orientation in reverse as is the case when the inverter engages normally.
The lack of internal lubrication of the propeller is by far the most common cause of overload in reverse. This insufficiency can be located in the hub of the propeller or on the individual blades which must constantly rotate freely on their fixing pin.
No seizure should therefore be caused by the accumulation of dirt, anative for example, or antifouling.
Our propellers normally operate without maintenance between two annual fairings. However, the lubrication of those frequently used in waters very loaded with dirt or abrasive particles must be regularly checked, as found in the sandy bed of shallow rivers or on canals with many locks.
Those in waters rich in limestone or in coral regions may be subject to significant deposits.
It is important to regularly monitor these deposits to ensure that they do not affect the proper functioning of the propeller which depends largely on the low level of friction inside all moving parts.
Recent propellers are equipped with O-rings at the blade emplanture and although these seals are not visible, our tests have shown that they contribute greatly to retaining grease inside the blades.
The User Manual supplied with the propeller, also available via the Internet, describes in detail the operations and lubrication standards of each blade and hub.
In a perfect world, the propeller would not require any lubrication – but in reality the extremely aggressive environment in which all propellers are used, requires regular lubrication to maintain proper operation.
Some customers report that in the long run their inverter locks in reverse and that it is then difficult to bring it back to neutral.
There are two main types of clutch in inverters: Yanmar originally used a claw clutch, still used today in small Saildrive headers, and this system seems to have never been a problem. The disadvantage that it sometimes presents, is to engage quite violently.
As in all clutches, however, the contact surfaces can wear out over time making it more difficult to engage in reverse.
All other engines use cone clutches. The interlocking is softer – but of course, the conical surfaces eventually wear out over time. Over time, after many uses, and sometimes reinforced by corrosion on the metal surfaces of the cones, this wear tends to lock the connection between the cones which are then difficult to separate. It then also becomes difficult to return to the standstill.
Regular replacement of the oil (and the seal on the saildrives) in accordance with the manufacturer’s specifications facilitates the prevention of this problem.
These problems become more common as engines and transmissions age.