0 Members and 1 Guest are viewing this topic.


Ian Brooks

Some hovercraft terminology
« on: Feb 18, 2012, 6:27 pm »
Some hovercraft terminology?



Freeboard

In simple terms, freeboard is the height of the lowest part of the hull above the water when you are floating with the engine stopped. Put simply, its how big a wave would have to be to come over the side.



A cruising craft should have at least 12 inches. If it hasn’t then it won’t be much use as a cruiser! Look for high sides and front bodywork.

This really is a key safety issue. When the weather turns against you, or you cross the wake of a large ship, waves over the front or back can turn a Sunday outing into a life-threatening Mayday situation. Larger is better for Cruising.

Plough-in

A plough-in occurs when the front (usually, but it can happen to the side) of the craft suddenly dips down into the water, causing the craft to violently decelerate. All control will be lost, and sometimes passengers can be thrown out of the craft – in any event, it is not good.

All hovercraft are susceptible to plough-in unless fitted with effective anti-plough devices. It is caused by hydro-dynamic forces acting on the skirt, causing the skirt to be dragged under the craft. Unless fitted with effective anti-plough devices, most light hovercraft are very susceptible to ploughing in, and the driver will need to become skilled at predicting and preventing it. However, there are well known solutions to plough-in, so look out for craft fitted with such a device. I personally wouldn’t buy a craft without.

In addition to effective anti-plough devices, good craft have a hull designed to reduce the effect of plough-in, so that if it ever happens you are not ejected from the craft – although there is still an uncomfortable deceleration period before control can be re-gained.

Any craft that ploughs in so badly that the Pilot is ejected from the seat is dangerous and should be scrapped.

The best craft have active control of plough-in using a compartmented cushion and responsive skirts. In this case, the plough-in phenomenon can be brought under control and used to effect a kind of emergency brake.

Hump performance

When a hovercraft is at rest on water, it floats like a boat – that is, it is in ‘displacement mode’ or ‘boating’. When the craft goes to move off, it must transition from displacement mode to non-displacement (hovering) mode. The transition occurs, for a small craft, at about 8mph, and is known as ‘going over the hump’, because the craft must ride over a wave that forms in front of the craft just before ‘hump speed’.

Many otherwise excellent craft will not go over the hump – you could liken this to driving a car on the motorway with no clutch – at the first traffic jam you are stuck with no way of getting restarted. This is a key safety issue. It means that you cannot stop on water, because if you do you cannot get back into hovering mode. You will have to ‘boat’ the craft back to land at about 4mph, enclosed in your own torrential rainstorm. Often the spray is so bad that vision is impaired and secondary engine failure occurs, whereupon the situation can take a turn for the worse.

You should have the hump performance demonstrated to you before you buy. If you buy a craft that will not go over hump, it may not be possible to improve it.

Back
 
Ian Brooks
Gloucester, UK