Our process for designing the electric motor compartment has had to be somewhat adaptable. We started from knowing nothing and so it has been a constant process of learning and then changing our plans.
The work over the last two weeks (Stuffing box flange is off, Yanmar Diesel 3JH5E for sale and Friday progress #21) has been a catalyst for some more changes. We now have a much clearer understanding of how all the parts between the motor and the propeller fit together. We also have a much better idea of what we need to fit (and many thanks to Tristan for his comments on our post Staycation Electric Motor Progress which got us rethinking our drivetrain).
So we are now close to deciding upon a PSS Pro Shaft Seal to keep the water from coming into the boat through the stern tube. We like this dripless seal as we shouldn’t need to provide a raw water supply to lubricate it. That is good because as our motor is air cooled we don’t have any raw (salt) water to divert into the seal. Some brands require 4 litres per minute which would mean installing both an extra seacock and a pump.
The challenge of providing water lubrication to the dripless seal isn’t just that we would need to provide it when the motor is running but that we would also need to provide it when sailing and using the spinning propeller to generate electricity using the regen feature.
So the PSS Pro shaft seal allows you to provide an air vented hose if your speed will be less than 12 knots (if we ever reach 12 knots it will be a short lived and no doubt terrifying moment!). However, if we find that water lubrication is required to reduce wear when running in regen mode for days at a time, then we can add a seacock salt water inlet and connect it directly to the seal without needing a pump.
The PSS Pro shaft seal is also available with a wider range of support for propeller shaft and stern tube flange sizes. We hope/plan to reuse the flange that used to hold the stuffing box, it is a larger diameter than would otherwise be the case and most dripless seals can’t cope with that.
So that is all good 🙂 The only downside is that the PSS Pro Shaft Seal is a bit longer than many of the solutions.
That brings us to then next piece of the puzzle which is where Tristan was so helpful. Our initial plans used thrust bearings within the motor frame to absorb the push and pull from the propeller. These have two grub screws that pass the thrust from the propeller shaft onto the motor frame and then that gets passed through the motor mounts to actually move the boat.
It turned out that as a very basic and cheap solution it was flawed. Two grub screws are not very much when it comes to transmitting the thrust generated by a 40hp motor spinning a propeller ar 1600 rpm to move 9 tons of boat. Also if the motor mounts need to transmit all the thrust to the boat they can’t be very flexible and so they won’t absorb much vibration.
So we are adding an Aquadrive to the drivetrain. This helps us in several ways. The propeller shaft ends at the Aquadrive which is fixed in perfect alignment with the cutlass bearing. So vibration and wear is minimised. The Aquadrive then passes all the thrust directly to the boat, so no thrust is acting on the motor which can therefore be mounted on much softer mounts so less vibration is passed onto the boat. Plus the connection from the Aquadrive allows for a lot of freedom in alignment for the motor requiring a less accurate installation.
Apart from the cost of the Aquadrive (nearly £1,000) this is all good. However, the impact on our layout is that the Aquadrive is over 250mm long.
With the PSS Pro Shaft Seal and the Aquadrive our motor needs to be moved forward so much that instead fitting the motor batteries (2 rows of 2) in front of the motor there is barely space for one.
So, we think we are switching things around. We will move the motor to the forward end of the motor compartment, a longer shaft will connect the motor frame to the Aquadrive. The motor batteries will then go aft of the motor above the shaft and Aquadrive (it raises them by about 200mm).
We will probably move the house bank batteries to above the motor to keep the weight distribution approximately the same fore and aft. The centre of gravity will be a bit higher although we think still lower than with the diesel and full tanks.
We have not tried to fully plan where all the electrical items will go (motor controller, and inverters are the biggest) yet.
The plan is to build from the bottom up. So
- Remove the old bulkhead to the corridor to ease access to the motor space. Finish the cleaning, then sand the whole area.
- Fill the old seacock holes from the cockpit drains and the diesel water cooling (these will end up somewhat hidden by the battery box).
- Paint the whole of the motor compartment and the cockpit locker.
- Fit new Cutlass bearing (need to sort out grub screws to hold it in place), then the propeller shaft with the dripless seal. Add the (still to be fully cleaned) propeller and an extra zinc for galvanic corrosion protection.
- Next will be the Aquadrive, which includes building the frame that will transmit the thrust. We should be able to fix this to the moulded in engine bearers.
- That will allow us to mount the motor and fit the new shaft connecting it to the Aquadrive.
- Now we will be able to fit the new cockpit drain seacocks where we can get easy access and route the hoses efficiently.
- Then the battery box for the 48 volt motor battery bank of 4 x 300AH can be built above the shaft/Aquadrive.
- Then the battery box for the 12 volt house battery bank of 4 x 120AH (position a little uncertain at the moment)
- This will allow us to build the full bulkheads separating the motor compartment from the cockpit locker and from the corridor (and also steal a bit more space into the aft cabin).
- Then we can fit all the electrical items and wire everything up (big job).
Fortunately, while the list is long the uncertainty is getting less. The biggest unknown is now how well the default program settings of the motor controller will work. Will we need to hire or buy the tool to reprogram it? Within that the biggest questions are about the regen and we won’t be able to know much about that until we are actually sailing.
So quite happy with all this :-).