Terrapin Mk I amphibious truck

Terrapin amphibious truck

The Terrapin was a British eight-wheeled amphibious truck. It was designed by Thornycraft and constructed by Morris over concerns the Americans might not be able to supply enough of the the DUKW amphibious truck (p.W:DF87). Aside from the centrally mounted engines, the most distinctive feature was the arrangement of the wheels. There were four equally-spaced wheels per side, but only the middle four were in constant contact with the ground. The front and rear pairs were raised (the front more than the rear) to aid in transitioning from water to land (such as coming out of a river).

The 500 Terrapins built gave useful service, but not without problems. The main flaws of the design could be connected to the two engines being mounted near the center of the body. The first problem was that there were two engines, each driving the wheels on that side (or the screw propellor, in the water). While this arrangement might make sense on a boat (which was Thornycroft's main business), it was a poor choice for a land vehicle; if for some reason power was lost to one engine, the Terrapin would suddenly and violently spin about. Also, mounting the engines centrally split the cargo hold into two sections. While total load was actually greater than that of a DUKW, neither hold had sufficient space to carry a vehicle or field gun. The driver was located between the engines, where is had a very poor view ahead of him, despite being in an open seat. A canvas spray screen over the forward cargo hold only made visibility worse. To solve the visibility problem, one or two extra crewmen were carried to aid the driver as lookouts.

The Terrapin Mk I has a crew of one, the driver. It uses 6.4 gallons per hour at routine usage.

Subassemblies: Medium Wheeled chassis +3, eight off-road wheels +2.
Powertrain: 2x71-kW gas engines w/2x71-kW all-wheel drive wheeled drive train and 2x10-kW screw propellors, and 50 gallons fuel in standard fuel tank [Body]; 4,000-kWs batteries.
Occupancy: 1 CS Cargo: 100 Body

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Armor
Body: 4/22
Wheels: 3/5

Equipment

Body: 45 VSP open forward hold, 55 VSP open rear hold, 2.5 VSP equipment locker, bilge pump.

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Statistics

Size: 23’¥9’¥9’	Payload: 5.25 tons	Lwt: 13.2 tons
Volume: 175	Maint.: 132 hours	Price: $2,300

HT: 9
HP: 330 [Body], 28 [Each wheel]

gSpeed: 15

gAccel: 3

gDecel: 20

gMR: 0.25

gSR: 5

GP: Very high (1/6)

wSpeed: 5

wAccel: 0.3

wDecel: 10

wMR: 0.1

wSR: 2

Draft: 5

Floatation rating 13.5 tons

Design Notes
Design gSpeed was 52 mph and wSpeed was 4.8. Both numbers were adjusted to the historical figures. Calcualted draft was 2' but was increased due to the wheels. As well, the floatation rating has been reduced to better match this reputation. Design weight was increased by 9% to match historical.

One source listed the vehicle as having 8mm of armor, while others made no mention of this. As the design was badly underweight (empty weight without the high DR was some 4.5 tons short of listed unloaded weight), it was decided to go with this armor figure.

Because of the wheel arrngemnt, only half of the wheeled surface area was used to calculate ground pressure.

Variants
The Mk II )1945) was a single prototype seeking to solve most of the problems of the Mk I. The driver was seated near the front and the cargo was in a single bay. Length was increased to over 31 feet. The war ended before it could enter production .