Dowty and the Military nozzled Marine propulsion system 

A water jet propulsion system works by a large volume of water being drawn in at one end, and then forced out of a smaller opening at the rear of the vessel.

Some books claim that the early Stalwarts were fitted with Saro-Gill propulsion systems. I have carried out extensive searches on the  internet, and am unable to find any mention or record of a company called Saro-Gill.  

There was however a company that was set up to make the Gill marine propulsion system under licence.

With thanks to the staff at Tees White Gill, and Michael Gill

www.teesgillthrusters.com/about-us

In 1922, British Royal Engineer, Major Gill developed a range of Axial flow pumps. Major Gill passed away in 1950.

The Gill Pump and Propulsion Company is formed in 1954 to continue sales of the Gill Pump Jet, with manufacture being undertaken under license by the Rowhedge Iron Works in U.K. Rowhedge is near Colchester in Essex.

The Gill units were adapted into jet propulsion units utilizing Major Gill's idea of a rotating thrust deflector.

In 1960 the Stalwart is developing, and Gill propulsion units are fitted to PV2 and PV3. As you can see, these units are basically U shape, with the inlet and outlet being on the same side.

Below is the mechanical workings of the Stalwart.

The blue is one Marine propulsion unit, with the beige 90 degree gearbox attached. This then couples to the Brown Power take off unit sat on top of the gear box. You can see that there are identical set up either side of the Rolls-Royce 8 cylinder engine.

It would appear that the water jet from the Gill marine propulsion units were being affected by the rear wheels. And also because the jet was coming out the side of the vehicle, rather than the back - so the force wasn't as strong as it could be, and there was no way of reversing.

So lets look at where the Dowty system comes in to all this.

The first Hamilton Jet waterjet was a copy of the American Hanley Hydro Jet, but with the outlet directly out the transom (back of the boat) rather than under the hull ahead of the inlet.

My thanks to Hamilton Jets and the Hamilton family member for their help, and images

Early in 1959 or shortly before, Sir George Dowty gained licence from Hamilton of New Zealand, to use Hamilton's Chinook water jet system. 

Dowty was the only licence holder in the UK.

In December 1959 Pathé News filmed Sir Donald Campbell, in Dowty's brand new Chinook Jet powered Turbocraft.

The second film from 1960, at twenty seconds in, nicely shows the Reverse gate being closed.

The system had a Forward/Reverse lever - marked by the yellow line in the image above. In order to reverse, the lever was moved backwards and, I believe, moved both scoops down, forcing the water to go through the cascade grill. There was no navigation when in reverse.

The layout inside the Stalwart's hull. 

• The blue is the Dowty tube

• The bronze is the Marine bevel box and prop shafts

• The green is the Power take off unit

• The red is the main gearbox

• The area with the red outline below the gearbox is the Transfer box, which gives Forward and Reverse drive to the wheels

During 1962 and 1963, during FVRDE trials with PV3 - Dowty and Alvis tested different nozzles on the end of the Dowty propulsion units. Alvis also turned the "Military outlet nozzle" 90 degrees, so that the reverse cascade grill forced water sideways and forward - rather than downwards and forwards. Alvis installed twin Navigation steering levers, so that both sides could be controlled independently of each other, and give navigation in reverse.

On the Mark 1 - The ingress of water into the hull is automatically kept to a safe level by the bilging system connected to the two propulsion units. Any bilge that collects in the hull being pumped out by the right and left hand propulsion units respectively and pipes at the left hand front and rear corners of the load carrying compartment respectively. Should the automatic discharge of bilge effected by the two propulsion units prove inadequate, the bilge can be pumped out by using the hand operated bilge pump, The hand operated bilge pump is fitted and connected through pipes to the rear strainer, it can be used at any time by oscillating the handle behind the driver's seat. 

This was discontinued on the Mark 2. Major Olsson's letter explains that whilst in choppy water, the water inlet can be out of water and sucking in air. During that time the bilge pump is not working, and hand pumping is necessary. The Mark 2 has electrical bilge pumps.

Propulsion Unit Safety Awareness

The lubricator in the rear coupling flange of the propulsion unit drive lubricates the slip ring inside the seal housing, while the lubricator in the bearing cap lubricates the angular contact bearing, and has a vent plug diametrically opposite. 

The rear bearing, between the two impellers in the output end, is fabricated from Tufnol, and is lubricated and cooled by the water passing through the propulsion unit while the vehicle is swimming.

The propulsion units MUST NOT be permitted to run any longer than necessary after the vehicle has left the water. Overheating and wear will result in the Tufnol bearing failing. The result of which can be catastrophic, and result in loss of vehicle.

As soon as possible after leaving the water, disengage the propulsion unit drive by depressing the crutch pedal, moving the outer lever on the right of the drivers seat forward and sliding the stop down the shank of the lever.

DO NOT

Run propulsion unit drive when out of the water for more than

• Mk I 30 seconds

• Mk II 2 minutes

• 
  

Note: 

To minimize corrosion it is recommend  that the propulsion units are flushed through with fresh water as soon as possible following operation in sea or esturine water.

Below are some accounts of Stalwarts that sunk

This is an extract by Roland Ramp, a German collector of British military vehicles

Taken from https://www.livesteammodels.co.uk/dhmg/rola001.html

You know that I have restored some Stollys to swimming condition. It is no secret that in 1985 (1983) the British Army issued an instruction ordering the removal of the Dowty Hydrojets units and later on the swim-gears. For the restorer - to get all the missing parts at the swim-gear/steering is one side of the story, but the main problem is to make them pretty watertight. So I take care to fit the rubber gasket at the rear bottom hull engine door and the dropsides – that’s easy. Also to take a closer look at the exhaust pipes is not wrong. Because 2/3 it is under the waterline, every crack or little hole can be disastrous if the driver stop the engine. You don’t believe it, but two years ago a Stolly from customer must recovered out of the eastern sea. He used his stalwart as a swimming island for diving activities and shut the engine. The next hour was time enough, the seawater filled some rust-holes of the exhaust, through the pipes/valves/combustion chambers into the engine. And it didn’t start again. Take care of the exhaust if you want to swim!

But my most horrible experience was a FV 623 Stolly sunk in only four Minutes ! What’s happened. After doing all the work I transported it with my low loader to a nice event at Pocking/Bavaria. Next step was going into the lake with a friend of mine on the loading compartment - looking into the hull to watch incoming water. Because I was an "amphibious expert” I put the Dowtys in and going to sail. Joe, my friend behind me, told me : "Roland - water comes in !" Why I don’t stop at hurry and goes backwards out of the lake - I don’t know that. Next cry from Joe : "Roland - The hull is nearly filled! I was able to get my Stolly near to the shore. Engine quit and water around my seat. In seconds I was out of the cabin. The heaven takes care of sailors - the bottom of the lake at that place was only two and a half meters deep and Stalwart’s cabins roof stays above the waterline. It needed some work to recover it from the lake with friendly help of a Hotchkiss Tank driver. The rest of the time at the event I use to make Stolly’s engine dry and running again!

After that I found in two other Stollys also these sleeves missing. The army mechanics don’t fit them because Stalwarts don’t need to swim anymore! Beware of it.

The following is the Marine Accident Brief from the National Transportation Safety Board  (NTSB), when an Alvis Stalwart, the MV Minnow, sank in Milwaukee harbour in the USA, on 8th September 2000.

Post accident On-Scene Examination

After the Minnow was salvaged, Safety Board investigators found no evidence of blockage from debris in the intake and discharges for the port and starboard waterjets and no abnormalities in the starboard waterjet tunnel. They observed that the blade of the port waterjet impeller had cut through the waterjet tunnel housing the impeller, leaving a 1/8-inch-wide circumferential gap through which water from the port waterjet tunnel could enter the amphibious vehicle.

When the port and starboard waterjets were disassembled, investigators observed that the port impeller shaft inside the stator bearing was severely damaged, and that the bearings inside the port stator were broken into fragments and discolored. Neither the stator bearing nor the conical fairwater at the end of the port stator had any lubricant. Investigators found that the lubricating fitting (spigot) for the port stator had broken off. The bearing’s retaining nut on the port impeller shaft was severely corroded.

The underwater area of the hull was in good condition and showed no evidence of leaks. The three drain plugs on the hull were secured in place. No other sources that potentially could have resulted in a significant flooding of the hull were identified. Investigators tested the Stalwart’s two 12-volt positive-displacement bilge pumps and found that one of the pumps was inoperative. Two of the six impeller vanes were broken and were blocking the suction and discharge openings inside the pump casing. 

Probable Cause

The National Transportation Safety Board determines that the probable cause of the flooding and sinking of the Minnow was the inadequate prepurchase mechanical evaluation and subsequent inadequate maintenance by Minnow Tours before placing the vessel in passenger service.