The
Jet Boat Project

About the project

Welcome to the Jet Boat Project Website. This is a documented project about the restoration and conversion of an old propeller driven speed boat into a water-jet propulsion system.

The boat was originally a 1971 15.5ft Scorpio Craft fitted with a 90HP OMC (Outboard Marine Company) Stringer Electromatic Stern Drive. The original OMC power unit was a marinised version of the popular Ford Pinto type 1600 OHC 4 cylinder engine. These have now been replaced with a Rover 3.5L V8 engine driving a Hamilton 771 Series Jet Drive.

The project was meant to be a usual rebuild and restoration, but trying to keep everything as original as possible. Having previously owned speed boats with outboard motors, I wanted to rebuild one with an inboard motor. Inboard speed boats are usually more expensive than the outboard versions so my intension was to rebuild an old one over a period of time to spread the cost.

As with most projects like this, the aim is to be cost effective but not to skimp on quality or safety. A very good way to get new parts at a very reasonable price is to search the Internet auctions. Here you can get new parts cheaper than some dealers are asking for used or second hand ones. The majority of parts were bought through the Internet and some from local boat jumbles.

The above boat was purchased and stripped down to the shell, with all of the interior and fittings being discarded as they were rotten, broken or worn.

One of the first stages of the project was to make sure the OMC engine and Stern drive was serviceable. Although the engine was old, obtaining parts wasn't a problem as this was a common engine and the marinised parts were still serviceable. The Stern Drive was going to be a little trickier as the OMC company had gone bankrupt in December 2000 but parts were still available. Both were stripped down and a list of parts gained for the rebuild.

The OMC Stern Drive needed new bearings throughout and a complete new water pump. Whilst searching the Internet using the word pump, I came across a new Hamilton 771 Jet Drive for sale, these are specially designed propulsion systems used on Jet Boats which are commonly referred to as pumps. These devices suck water in from a hole (intake) under the boat and is then pumped at high velocity through the rear (stern) of the boat to push it forward.

Purchasing this new jet drive to replace the old OMC Stern drive added the word conversion to the project taking it to an entirely new level, so the idea of keeping things original went out of the window. This was just as well as I could now do my own thing. Now that the drive system was going to be changed, a more powerful V8 motor seemed appropriate and well within the scope of the project. The old OMC engine was scrapped and the Stern drive was sold.

All that remained of the original boat was the fibreglass hull, the Triplex glass windscreen and part of the main wiring harness. The project has taken over 7 years so far. Most of the work has been completed by myself with the appreciated help from friends and family.

If you have any pictures or information you would like to share regarding the make or model of this boat, please mail me, I'd be very interested to know and perhaps I can add a viewer's page later on.

Here is a video of the boat in action during a family day out at Cardiff Bay.

GALLERY

Here is a small selection of photos with information taken periodically throughout the project. For more photos and other information please feel free to browse the numerous pages within the site.

Any questions or comments, please contact me. Happy browsing.
Last update-07/08/2010

• This is the Scorpio Craft hull just after purchase and in need of some TLC. There are many hours of work ahead with numerous modifications including the installation of a Rover V8 engine coupled to a Hamilton jet drive, a complete overhaul of the hull, plus the fabrication of a new trailer.
• The old fibreglass gelcoat was beyond polishing so the complete hull had to be prepared for spraying. A 2 pack (2k) polyurethane paint system was used as it is tough, durable and bonds well to a fibreglass gelcoat. As the boat is trailered and not in water constantly, this paint job should give many years service.
• The jet drive water intake is cut out of the bottom of the boat. Note how the hole tapers into the keel of the boat. This is to promote a smooth flow of water into the jet drive intake. As the keel is lower than the intake, it would otherwise be stepped and create turbulence which can damage the jet drive impellor and mar performance.
• Because the aft section of the keel had to be removed, the area around the hole has to be reinforced to regain the structural integrity of the hull. Here the area around the jet drive has been reinforced with new braces and engine bearers.
• The Rover 3.5 V8 ready for strip down and rebuild. This is from a 1985 SD1 saloon with a Range Rover front timing cover and water pump fitted. Note the very long nose on the water pump, this will be cut off to save a few inches seating space. The two Stromberg carburettors are unsuitable for marine use so will be changed for a Rochester 2bbl marine carburettor.
• Rebuild nearly complete. The previous owner claimed he had just rebuilt it. The crankshaft, pistons and cylinder liners were in good condition but it needed new piston rings, main bearings, big end bearings, camshaft chain and gears, camshaft, camshaft followers, camshaft bearings, valve springs, gaskets, seals and a good clean.
• While the engine was being rebuilt, an old scrap engine block was used as a template to line-up the prop shaft and take measurements for the new front and rear engine mounts. The front mounts will go onto the newly fibreglasses engine bearers as per norm, but the rear mounts will bolt onto the jet drive base directly under the prop shaft.
• The Rover V8 engine is significantly longer than the original OMC 4 cylinder engine, intruding into the rear seating area. This had to be kept to a minimum by shortening the water pump and using the shortest drive coupling possible. This prop shaft was assembled by a local specialist using two Hardy Spicer 1310 universal joints and a sliding spline.
• Most of the mechanical, electrical and body work is done with just a few refinements needed here and there. The 2 pack paint is well hardened by now so some stripes and lettering will be painted up the sides to brake it up a little, though it did look quite nice just as it was.
• Initial test saw 34knots @ 4200rpm, should be 4800rpm. This was possibly due to the old marine carburettor and excessive cooling water injected into the exhaust system causing too much back pressure. Future modifications may be a redesigned exhaust system, a new carburettor or a fuel injection system. UPDATE 09/04/2009: Carbutettor rejetted and ignition timing adjusted, now 40knots @ 4500rpm. Looking to squeeze another 300-400rpm.

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