Dowty – Restoration

My Dowty as purchased. Looks OK, but on break down not so good , neither was the trailer

Before you embark on a restoration of a boat, do not think it is as easy as a classic car. Many items are often unique to the craft and can be unobtainable within about 10 years. Nearly all parts will fight you to remove them or break. Nuts and bolt are always difficult, but often ‘automatic gear oil’ will help. You will also require a lot of skills or people to assist in the re manufacture of parts.

This documentary of my Dowty was possibly one of the most difficult and complex to do. It required skills from Toolmakers to a Casting Foundry. From a windscreen maker to an upholsterer. From a carpenter to a F1 fibreglass specialist and many more. To all the people who assisted I extend my gratitude and thanks.

My Dowty Turbocraft

I purchased my Dowty Turbocraft in 2010 a 1958 MK1 (14’ 6” test boat apparently).  It looked OK on the outside, but revealed a series of monster problems on strip down, a full rebuild was required, plus a donor boat for the obsolete parts and a new trailer.

The hull was solid, but damaged with general use, however all the mechanicals were in a real mess, with corrosion and broken bearings, seized up controls and the jet unit was rubbished. All this can be seen in the Dowty restoration section.

As in many marinised car engines it was not good, with a cracked block repaired with fibreglass filler. The engine cooling was clearly in a big mess. Oil cooling was a primitive 15mm copper pipe in the sump which had burst, which of course diluted the oil and blew it out into the water.

The engine was sea water cooled via a take-off from the jet outlet and flow could only be regulated by a gate valve. Flow is dependent on engine revs and water from the pressure inside the jet casing. There was no water pump or thermostat, so cooling water flow rate was erratic and engine temperature impossible to manage. Also, the iron castings in the engine block and cylinder head were subject to corrosion, as was everything else on the engine.  These poor engine cooling arrangements caused many engine overheats in Dowty’s, so the temperature gauge is vital. A full rebuild was required and new engine block was required. The exhaust manifold was OK.

The through transom exhaust is silent at slow speeds when submersed, then rather pleasant when the engine note can be heard when on the plane. The sealed double row ball thrust bearing and the water lubricated cutlass bearing supporting the impeller shaft were both completely trashed, possibly due to sand and flint being drawn up by the jet at the cutlass bearing end. The large bearing even had the sides knocked out (can be seen in Dowty restoration section). The original stainless steel impellers were fine. 

The hull and deck were structurally sound, but cosmetically poor. Marks and surface damage on the hull were made good, by a professional. The interior trim panels were replaced because they were smelly and rotten. A new curved and bowed windscreen was made and fitted at a cost, plus a new dashboard.

Driving a jet boat is an acquired skill, so the foot throttle had to be repaired correctly. The Dowty Turbocraft is different for sure, but fun especially when a 360 turn can be done at top speed, let alone the shallow water running, but the Dowty Turbocraft is not without its quirks for sure.

The stern and its antiquated steering nozzle arrangement. Steering cables had no guides through the hull so acted like a saw through the stern

Dowty data – 1958 UK built Turbocraft

Length: 14’6″ (4.42 metres)
Beam: 5’9″ (1.52 metres)
Draught: 9″ (0.27 metre)
Power Unit: Marinised Ford Zephyr MK 11
Propulsion Unit: Dowty-Hamilton Chinook Marine Jet (2 stage)
Maximum Speed (1958): 35 M.P.H. (56 KPH)
Dry Weight: 1 ,350 lbs (612.35 KG)
Fuel Capacity: 10 Imperial Gallons (37.85 Litres)
Seating Capacity: Up to 5

Interior looked OK and was serviceable but not solid and I wanted to use the boat to its maximum

Dowty Engine and Water Jet Restoration

The original engine had a cracked block, so I purchased another and rebuilt it. Right is the result a lovely Ford Zephyr straight 6 – 2500cc engine. The replacement block came from an RAF compressor for starting jet engines

The sequence of the break-down and rebuild at the moment is likely to change as it is a large piece of work.

Engine repair was simple. The water jet dismantling was difficult. Help was found with an excellent Toolmaker friend who manufactured some of the obsolete parts, mostly in stainless steel and aluminium. Below is a large, pictorial file on the strip down and rebuild of the boat with basic details:

Ensure when replacing the cutlass bearing, the correct end is set up for the ‘Leading Edge’, which is marked on the edge.

Not a good idea to stop a leak. This was someones idea to stop water ingress. It was not even at this point, but at the stern. The pictures above also show the main leak area.

Dowty jet unit rebuild

A word about the main bearing. This displays the importance of setting up the ‘thrust’ spacers, which is the most difficult part of the jet unit assembly. You can see clearly that the bearing has pushed out the side of the inner bearing holder. The outer semicircle is a bearing filling point, which may indicate a wrong replacement bearing. Information pointed out by Ian on our Forum, thank you Ian. Bearing tolerances are critical

Assembled drive shaft after full refurbish. Note new sleeve behind 2nd (inner) impeller. The shaft is 1 metre long. There are 9 seals and 1 massive bearing in it, plus a cutlass bearing between the impellers.

Dowty jet unit base plate re-manufacture

Possibly the worst job of all. It required the valuable help of a foundry, toolmaker and a fibreglass expert, large milling machine, specialist alloy welder and of course me.

IMPORTANT NOTE: The base plate has an inherited weakness. The weakest point for leaks and rarely found is at the rear of the intake housing, take a look at ‘Inlet casing’ picture, there is a dark stain below the water exit‘. Pressure of the water being forced out of the exit nozzle causes the seal and even the fibreglass structure to break down. These water-jets produce a considerable amount of pressure.

Dowty Forward/Reverse shift mechanism

Dowty trailer replacement

Dowty Dashboard