How Does a Saildrive Propeller Hub Work?

In this post, we’re going to be looking at saildrive hubs belong to Bruntons Propellers, explain why saildrive hubs are needed, how they work and how to take care of them!

Introduction

This post talks about saildrive hubs and saildrive propellers manufactured by Bruntons Propellers. This hub type is found in both sailboat propeller product lines, namely Varifold folding sailboat propellers and Autoprop self-feathering sailboat propellers. However, the content of this post is also equally applicable to saildrive propeller hubs from other manufacturers. The general internal mechanisms of the hubs is something that’s dictated by the saildrive manufacturers (such as Volvo and Yanmar), not the propeller manufacturers – so there’s a little bit of a commonality between them. At King Propulsion, we do get a lot of questions about the size of the sailboat hubs and why they so different from a regular propeller hub. There are a lot of reasons for the differences in the hubs and this post is going to delve into some of them!

Why choose a saildrive over a shaft line?

The purpose of a saildrive hub is very simply – to remove the shaft line. If you think about a traditional engine, the propulsive power comes up from the engine through the transmission along propeller shaft through a stuffing box, and then to the propeller – this has worked well since the invention of the marine propeller. However with the invention of modern materials and methods, Engineers have found more efficient ways of doing things. For example, an outboard engine makes the whole shaft line compact, we have the engine above, with the drive coming down through a shaft through a bevel box and out to the propeller. This is good for high speed craft as it puts the weight further aft to help with planing requirements. While there are a lot of similarities between a saildrive on a sailboat and an outboard engine on a powerboat, the saildrives on sailboats don’t tend to yaw and are fixed in place. This means that you cannot steer a saildrive requiring them to be used in conjunction with a rudder.

Traditional shaft run on a vessel

The great thing about saildrives is that they change how you can layout a boat at the design stage. Naval Architects are very keen on placing the weights at optimum positions in terms of trim and also keeping fixed ballast down. If you have a fixed engine that can only be in one position, that has to have a seven degree shaft angle down to a propeller, that pretty much dictates the layout of the boat. If you can have a saildrive which is compact, you get a lot more flexibility because you don’t have the shaft line to run through. This is one of the benefits of the saildrive. Finally, to consider efficiency changes, there isn’t really that much in terms of efficiency, either from the shaft transmission, or the hydrodynamic flow changes losing the shaft and then going to saildrive. The flow characteristics will be improved by design, however realizing this gain is not always possible.

Modern saildrive “shaft run”

Owning a saildrive propeller

Saildrive propellers are very simple to own, operate and maintain. To take the propellers off on haul out is easy as they are on a splined shaft rather than a tapered shaft. In use there are no immediate disadvantages to the system only benefits such as the reduction in prop-walk. The nature of prop-walk is a function of shaft angle and also hull characteristics to give a side force pushing the boat in a particular direction; typically owners with saildrives say that they don’t experience much prop walk.

The design of a saildrive propeller hub

The design of the saildrive hub is manufacturer led. There is a damping mechanism inside of the hub which is designed to protect the gearing. For a shaft driven vessel, if you hit something in the ocean, the whole shaft system is robust, chances are you will only hurt the engine mounts; everything else is pretty solid in the shaft line. With a saildrive, it’s not quite the same. The saildrive propeller will be mounted through a series of 90 degree gearboxes (bevel boxes) making the shaft run a Z shape. What the manufacturers found quite early with saildrives is that if you hit anything, and it is a possibility in the ocean, as the object strikes the propeller you can strip teeth and damage the transmission system.

The inner components of a Varifold saildrive hub

To overcome this, the manufacturers use a torsional damping system, often made of elastomer or rubber to suppress the shock loads. It’s a little bit of an agricultural system, but it works tremendously well. Bruntons Propeller have evolved their design over many years and three clear designs of the saildrive hub are in service right now, so let’s have a look at it then.

Mounting a saildrive propeller

First lets take a moment to explain the mounting a propeller on a shaft. For a conventional propeller there is no need for the damping mechanism in a saildrive hub. As you can see from the photo below, the shaft is tapered and has a keyway (not shown). For the propeller to be mounted the taper needs to mate extremely well and the friction in taper mechanism is what drives the propeller. The gearing systems for this setup are large and have more ability to absorb shock loads.

Conventional propeller shaft taper

For a saildrive propeller there is no taper. The propeller boss has a spline going through it and this corresponds to the splined shaft of the saildrive shown below. The mating of the splines transfers the drive from the leg to the propeller. In general, the removal of the propeller from a saildrive is significantly easier than for a conventional shaft as there is no taper lock to break.

Saildrive propeller spline

Early saildrive hub – Type 1

For the first type of hub which is actually off an Autoprop, you can see in the image the splined insert that would be placed inside the hub and bonded to a polymer rubber system. This material is actually cast in place at the factory so it is not possible to physically take it out and service it, it was designed to make a permanent bond. This type of damping system worked remarkably well; the hub shown is 20 years old!

Early Autoprop saildrive hub

Once they system degrades and starts to fail the bond breaks due to the torque load. Once failed the insert will spin because there is no physical restrictions because it’s a cylindrical shape then you lose drive. Once the unit slows down the rubber will set again in the cold water and some level of drive returns – that is how it would fail. This saildrive hub worked well from the beginning, but it was superseded by a better system where you start to get some mechanical connection.

Modern saildrive hub – Type 2

The figure below shows the updated saildrive hub. It is identifiable by the lack of fasteners on the back of the hub. The hub is assembled using an internal circlip to secure the splined shaft and the outer propeller body together. This part of the assembly would be done on a large press to push it all together until the circlip clicks in place forming a permanent connection. It works amazingly well and there are a lot of these in service right now. You may be watching this video and you may have a set of these on your sailboat.

Modern circlip style saildrive hub

The disadvantage of this design is the marine environment. As you can see, there’s some shell fouling on the hubs shown in the images. This is a rubber dowel system used as the damper and it’s not sealed, so the water can get inside. Obviously being rubber, over about 10-15 years in the ocean they and the circlip start to degrade. So when you try and service them you have to destroy the spline insert to get them apart. For saildrive hubs intended for use below 40 hp they were made with a plastic called Zytel which is simple to machine, above 40 hp the hub inserts were aluminum and hard to machine.

Zytel (left) and Aluminum (right) saildrive hubs

To machine the hub apart requires a drill press or milling machine to drill out the hub and find where the circlip was inside. Then the circlip has to be spun to find the holes to fit circlip pliers, squeeze it, and then using the pressure on the hub, push it out. So it’s a little bit complicated to get this apart. From the picture below it is clear that the internal part has to be destroyed to replace the rubber dowels. To reassemble you would replace the spline insert, new dowels and get the new clip and squeeze it back together. Unfortunately Bruntons Propeller have not manufactured these parts for a very long time. So these are now obsolete and only available from used parts dealers.

Pre-assembly of a saildrive hub

Despite the lack of serviceability the hub works remarkably well. You can see from the interior that there are flutes on the hub and insert which is where the dowels sit. As the torsional load comes in, it’s absorbed by the dowels between both sets of flutes. The insert is not going to spin like the first one could – a big step in the right direction!

Current saildrive hub – Type 3

The current design, which is used on both Autoprop and Varifold product lines has a refined internal mechanism and also the ability to service the dampers. The spline insert has 3 fins on it where a series of smaller elastomer dampers are held inside the hub. These correspond to 3 scallops in the hub, which once pressed together form the torsional damper. The system also is waterproof and can be disassembled for service.

Current saildrive showing dowels and the bearings (gray)inside the hub

This saildrive is not like the first one where it can spin once it fails, it has mechanical fin restraints which, as the torque load comes in, absorbs the shock load. Another benefit of the current system is that there is a bearing in the bottom of the hub which is actually watertight. This helps protect the elastomer by extending its life and also allows the gaming mechanism to effectively float. Again this hub needs a hefty press to put it all back together but there is now three bolts to it all locks together.

The fin / scallop system used on the current saildrive damper

Servicing saildrive propeller hubs

King Propulsion is a factory certified repair center in the US and repairs Varifold and Autoprop. The company is able to service saildrive hubs, some more than others. For Type 2 (circlip) we have sold parts for customer repairs in the past when they were available but but it was a very involved job to perform. The hubs were quite expensive to service, but customers did service them. The old saildrive parts are now out of stock and Bruntons Propellers are not making any more – it is an obsolete part. So one of the purposes for doing this article was to give some advice on how to look after saildrives, because if the circlip style saildrive fails, the propeller nut will be firmly in place with only the spline insert left on the saildrive leg and there’ll be no propeller!

This type of failure is a rare occurrence, but it’s something to think about as it can happen. Fortunately there are some telltale signs to watch (or listen for). There will be an increase in vibration as everything loosens a little, it’s not something that just happens. One of the things we encourage owners to do is when the sailboat is on the hard, or if you dive the boat, grab a hold of the propeller blades with the engine in gear. There should be no rotational or axial play in the hub.

A badly degraded damping system – this propeller was returned to service after a clean up and new parts

We have had a recent propeller in for service where the dowels had failed. You can see that if there’s no rubber system in here, it’ll move and it’ll also move actually and you’ve got some rotation. Left unchecked this could have resulted in the unit breaking apart and loosing the propeller. So when you’re on the hard, if you can physically move it or move it this way, it feels like the nut is loose, then it’s time to have a look at the saildrive hub. And I would strongly encourage you to get in touch and get a quote on how we can replace these for you. It’s a very straightforward process. Takes about 20 minutes. It’s not expensive. You could just send us the hub and we can replace the interiors and get it serviced for you. If the hub is part of an Autoprop, we can include this as part of the Autoprop service when we replace the blade bearings.

Final thoughts

So that’s a quick overview of saildrive hubs. I think in years to come, the dominance of saildrive is going to increase because they are easy, flexible, and allow the manufacturers flexibility with the Naval Architecture. The power absorption by the saildrives is already starting to increase, so we’re going to see larger and larger saildrives and larger and larger saildrive hubs.

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