In designing a propeller that is optimal for the range of modern Saildrives now on the market there are a number of issues that must be considered and prioritised.
Propeller's – like life - are full of compromises and invariably involve design trade-offs in developing an optimal solution.
For Saildrives – virtually all of which are cast from Aluminium alloy - corrosion takes on a much bigger perspective than it has done in years past over traditional shaft installations.
For shaft installations the only metals involved were invariably Bronzes and Stainless Steel – both of which have relatively good corrosion resistance from a marine environment.
While one can debate the corrosion potential of various metals and their associated alloys AB2, Stainless Steel 316, 2205, Monel etc in various applications, all metals suitable for propellers will have a significant electro-potential relative to an Aluminium Saildrive leg.
In addition modern current interruption devices in Marinas have a common ground thus offering another conducting path for boats with and even without shore power as stray currents find their way to the Marina grounding.
To eliminate – not just reduce - the corrosion or conducting potential of a propeller on a Saildrive application, composites offer a route which without debate eliminates any corrosion potential on the Saildrive from the propeller itself. There are still restrictions. Carbon fibre is a not an appropriate reinforcing for composites as the carbon has a high electro-potential relative to Aluminium. Dry cell batteries have a carbon anode for good reason. A full electro potential listing of metals in salt water is at: Galvanic Series
Composites and engineering plastics are not as stiff or as strong as metals – but the issue is whether they are adequate for the purpose intended. The larger boss of a Saildrive unit offers an opportunity to increase the size of the propeller body to lower the stress levels without adversely affecting the performance or appearance of the propeller unit.
All propellers must offer optimal motoring performance both in ahead and astern.
The preferred way to achieve that is to use optimally shaped ogival shaped foils with progressive pitch to accommodate the lower velocity at smaller diameters as used in the well proven traditional fixed bladed propeller.
Reverse thrust in all folding propellers is always less than optimal and particularly so at low speed as these types of units depend upon the mass in the tips of the blades to hold the blades open with centrifugal force. As the blades generate reverse thrust however this acts against the centrifugal forces tending to hold the blades open and in reality the blades are held only partially open leading to very poor reverse thrust particularly when manoeuvring at low engine speeds.
The latest Yanmar and Volvo engines have now reverted back to the 3000 max rpm of many years previous which has had the effect of lowering shaft rpm and thus exacerbating the known performance weakness of folding propellers in reverse.
The traditional approach is to add mass to the blade tips to increase the centrifugal force but this has the adverse effect of producing a less than optimal tip shape and impacting on motoring performance. It also increases both the total mass of the propeller and what is more critical – the moments of rotational inertia or the resistance to the propeller being rotated at a different speed. The propeller acts like a flywheel and resists any change in rotational velocity.
Modern Saildrives have quite strict criteria for moments of rotational inertia and the shock impact generated after the Saildrive clutch is engaged. The very popular Yanmar SD20 units have dog clutches rather than a clutch pack which generates very abrupt and noisy engagements and obviously requires low mass in the propeller.
Some designs have rubber bushes between the boss of the propeller and the body of the propeller that carries the blades to minimise this shock loading at start up. The problem this introduces is that the rubber bushes are very poor conductors so in effect isolating a very large proportion of the metal area of the propeller from the zinc anode.
This can only have a very adverse effect by creating an opportunity for corrosion on both the Saildrive leg and the propeller itself due to the dissimilar metals that each is made of.
Some propeller manufacturers will argue that their bushes are conducting – but this is generally achieved by the addition of carbon particles which in many ways exacerbates the electro-potential problems.
The obvious way to minimise the moments of inertia is to first reduce the mass of the entire propeller unit, and then to attempt to focus the bulk of the mass as close to the centre of rotation as is possible. Yanmar design criteria for Moment of Inertia of Saildrive propellers are avilable at: Yanmar MOI's
All folding and feathering metal propellers must also deal with the often harsh fore and aft shock loads imposed when the blades meet their stops upon opening.
Any composite propeller that mimics a traditional folding unit design will have insufficient mass in it's blades to generate any reverse thrust from centrifugal force. Some form of mechanical motion will be required to ensure that any composite propeller always opens fully and stays open in reverse. At this stage another design issue emerges. In a traditional folder there is very little resistance to rotation of the propeller unit once the blades are fully folded. Any design for a composite unit then has to have the blades residing in a folded or low drag position such that sufficient resistance is generated from the water to enable any mechanical action to take place when the propeller is engaged into either ahead or reverse rotation.
This functionality is likely to conflict with the obvious requirement for a low drag configuration of the unit when sailing.
Including some form of spring into the mechanical action involved in opening the blades will perform three important function:
* Remove any shock loading from moments of rotational inertia to meet and exceed all Saildrive
manufacturers criteria for start up forces at clutch engagement in both the ahead and reverse directions.
* Ensure that the blades are returned to their low drag position under the positive mechanical force of the springs
thus ensuring that a simple reliable and effective mechanical action always forces the blades out of the engaged position when sailing.
*Remove any mechanical motion of the blades about their mountings when sailing thus eliminating wear
potential and any auto rotation caused by partial feathering or folding.
Variable pitch is a critical design feature for any propeller in that it allows for fine tuning of optimal settings for each and every vessel, and even for tuning personal preferences for engine cruise rpm on identical vessels to deliver optimal cruise speeds.
The nature of all Saildrive gear trains is such that they all have the same reduction ratio in both ahead and astern. This is very different from shaft drive installations where the reverse reduction ratio is often much higher than in ahead and consequently the pitch setting in reverse should be higher than in ahead.
Thus variable pitch for a Saildrive unit requires that when adjusted the pitch is modified equally in both ahead and astern to the same setting automatically.
Variable pitch is also important from an economic perspective in that it reduces inventory in the supply chain dramatically.
Traditional geared folders require inventory by Diameter x Pitch x Blade #.
[ Being geared each blade requires the teeth in a different position so is a unique part # -
It also places constraints on reassembly as each blade location is unique]
With typically 7 diameters from 14.50" to 19.50 ", 5 pitch ranges from 10" to 14" and 3 blades for a 3 bladed folding unit - this translates to 7 x 5 x 3 = 105 line items for blades required in inventory, versus just 7 for an optimal composite design – with the blade root common and only the tip shape varying to obtain the different diameters.
Thus the injection dies can be constructed with just one common die and a set of inserts that go to make up the die set for each of the required diameters. This lowers the die costs substantially as the one main die block can then service the entire range of propellers.
A composite unit with common rotation and variable pitch will thus only require inventory by blade diameter thus reducing both die costs and inventory dramatically and at the same time offering customers what is optimal as regards diameter and pitch as distinct from what's in stock.
Symmetric blades ( ie the same identical blade being used for each blade position ) thus emerges as a design issue in lowering the overall cost of the unit and therefore it's price and value proposition to the end use customer.
Corrosion will always be less in any marine environment where the internal parts of any propeller remain fully lubricated throughout the sailing season. In addition to ensuring smooth operation and minimising any potential wear from the operation of the unit – the coating of marine grease will assist in minimising any corrosion potential that exists with the metal components that will still be required for the highly stressed attachment bolts.
Traditional folding propellers universally have exposed gears with the potential that creates for marine deposits to introduce significant wear in the gear tooth mechanism of the unit.
The ability to simply grease the whole of the unit without removal from the Saildrive is thus an important design feature. Ensuring the unit is internally greased with no exit pathways will primarily ensure smooth operation without wear but also ensure a minimum of scale deposits from hard water which in certain locations, typically water with high lime content, can cause scale deposits which may interfere with the smooth operation of the feathering or folding function over time.
The ongoing secure attachment of both the blades to the propeller body as well as the propeller unit itself to the Saildrive are a critical design issue. Not a season goes by where a number of users report the loss of propeller blades or the loss of the entire propeller from their Saildrive unit.
Being mounted on a Spline as distinct from taper where the propeller can pull up tight on the shaft – Saildrive propellers by their very nature will move slightly on the spline whenever they are engaged into either ahead or astern. This is also a common cause of propeller loss as the locking mechanism fails and over time the attachment nut comes loose.
A composite propeller unit must incorporate fail safe mountings for both the blades and the propeller as a whole while at the same time ensuring the unit is simple to mount with a minimum of tools. The ability to mount and dismount the unit without having to remove the blades from the body of the unit offers a very useful feature from a customers perspective as Saildrives require the frequent changing of the zinc anodes. On many models this entails the removal of the whole propeller from the Saildrive unit. Newer units have a split zinc which can be replaced without removing the propeller.
Low drag when sailing is an obvious design objective for any propeller, but because of the nature of Saildrives where the leg and boss at the bottom of the unit present a substantial frontal area and thus drag, the design of the propeller can focus on just the incremental drag from the design of the blades. The boss of the propeller will have no incremental drag effect as it is streamlined behind the larger lower gear case of the Saildrive itself.
While two bladed folders are often seen as having the lowest drag configuration – that is only strictly true when off the wind. On the wind and reaching conditions will generate streamlines, due to the leeway made by the yacht, that generate a significant projected area relative to the blades if they are in a vertical position.
Projected area basically equates to drag – so that more correctly the drag of any two bladed folder is greater on the wind and reaching when the blades are aligned vertically than off the wind when leeway is virtually nil or when the blades are aligned horizontally.
Drag from the blades will be less when the blades are folded in a horizontal position.
An optimal blade shape will such that it will tend to shed any weed or flotsam encountered when sailing. In the case of a Saildrive this requirement is lessened due to the large gear case at the foot of the leg which tends to act as a first point of deflection for any impact.
In addition the lower sections of the blade will be of sufficient radius to prevent any damage from a rope being caught around the propeller under power.
Cumulatively the above criteria and objectives make for a very challenging design requirement for any new composite propeller that is targeting Saildrive units to primarily eliminate – not just reduce – the corrosion potential from the bronze propeller components acting upon the Aluminium Saildrive legs in use today.
Couple this to compelling economics of the delivered unit which are taken as a given makes for an even greater challenge.
After 5 years of design iterations and over 30 versions of the propeller constructed and tested in all operating modes onboard a sailing yacht we are confident that the approach we have taken on the SDC propeller design makes the optimal trade-offs.
In addition to normal motoring and reversing these included multiple engagements ahead while sailing, engaging reverse while sailing, engaging ahead whilst motoring astern, engaging astern whilst motoring ahead, feathering from a sailing mode, static thrust and full throttle tests.
The next stage of the test program involved a documented usage program on a Bukh 20 Saildrive rated 20 hp @ 3000 rpm on a 2.25:1 reduction delivering 1333 shaft rpm @ max for over 100 hours over a six month period. This was designed to simulate normal daily usage with multiple astern and ahead engagements followed by motoring periods.
Our experience with the Bukh engine range is that it is very conservatively rated and that this can reasonably be compared as very similar to a Yanmar 3GM30 rated 24 hp continuous @ 3400 on 2.64:1 delivering 1287 shaft rpm @ max.
After examination at the end of this period no discernable wear or structural issues were detected in the unit which was in as installed condition.
The next stage of the program will extend testing to the very popular Volvo 2003 2030 and newer D1-30 range plus the Yanmar 3GM30 and 3YM30 range – all rated 28 – 29 hp with various shaft speeds between 1360 and 1460.
We believe the unit addresses the very complex design issues facing such a product in a manner that by providing equal emphasis to the functionality and life cycle economics will deliver on an ongoing basis a superior life cycle value package to all Saildrive users within the defined power range of the unit.