Two Stroke Vs Four Stroke Engines – What’s Best?

Not all leisure craft use the same Engine type.

Small/Recreational hovercraft have traditionally used two-stroke engines. Whilst they offer an excellent power-to-weight ratio, four stroke technology has caught up in recent years and offer significant advantages for hovercraft operating in a maritime environment.

Flying Fish were the first manufacturer to use ‘commercial power’ engines in significant quantities and our R&D department has worked tirelessly to develop them to the point where we have the finest engine package available. The Marlin II uses a Vanguard engine from Briggs & Stratton engines that is famed for it’s astonishing reliability. Once into our workshop, the motors are modified and ‘hoverised’ to match them to the fan assembly, producing astonishing performance from just 35bhp and 3800rpm maximum revs. Four-stroke power means fabulous economy of approx 6 litres per hour and low noise – low frequency noise as well, meaning it doesn’t carry far. It comes with electric start as standard and easy, infrequent servicing requirements. They simply purr away all day…endlessly!

Compare this to a two stroke engine – loads of noise, 25 litres of fuel an hour (plus oil!) and an uncanny knack of breaking down in the worst possible place…your call!

Instead of mounting the engine on a ‘pylon’ like you see on many hovercraft, we locate the engine down low for improved centre of gravity, under a bespoke engine cover system that offers excellent protection from the elements, allows unrestricted airflow and cooling and yet only takes a few seconds to remove.

One engine or two?

Many hovercraft use two engines, one for thrust, one for lift.

Flying Fish craft have one engine which provides lift and thrust, sited at the rear of the hovercraft. The reason being that it is easier and far safer to coordinate one set of controls, much more reliable (because ‘lift’ engines are usually at the front of the craft and end up covered in salt water – causing breakdowns.) It’s easier and cheaper to service one engine, more economical and of course, you get less noise from one engine and fan assembly.

That’s not to say that two engines don’t work well – they’re essential on larger craft (in our opinion, craft over 4.5m should be fitted with two engines) but smaller, recreational hovercraft should work perfectly well with just one engine and fan assembly.

Stopping on land or Water?

The strengthened floor of Flying Fish hovercraft is protected by aluminium skids and runners to prevent damage when stopping on rough ground or sliding to a halt. Planing surfaces are also reinforced during construction to ensure that the hull is up to the knocks and bangs of day-to-day usage.

Stopping on water is a nightmare situation for some recreational hovercraft. This is because they have seriously compromised flotation and some aren’t even buoyant – they can actually sink if they take water on. We think that’s a bit of a design flaw! The Marlin II has positive buoyancy due to the closed-cell foam located in the hull. As far back as 2001, the Marlin was tested for ‘flooded buoyancy approval’ and certified by the French Maritime Safety Authority for use throughout the EEC. Another aspect is the placement of buoyancy. We’ve seen hovercraft with nearly all their buoyancy located in the floor – meaning that they risk capsizing if too much load is placed on one side of the hull. In the Marlin II, the hull is shaped like a flat-bottomed boat, so they’ll happily float whilst you fish the day away or wait for a lock to open.

Another issue that many hovercraft don’t like about stopping on water is that of getting back onto the ‘plane’ (called ‘getting over the hump’ in hovercraft parlance!) This is because they don’t have enough lift and thrust to push their way out of the hole in the water and create massive wash – spraying the unfortunate occupants with gallons of water. This potentially dangerous situation means a very wet, slow and embarrassing journey back to the shore and is completely unacceptable in a modern recreational craft – but surprisingly common. The Marlin II, loaded to it’s capacity, will be ‘over hump’ within five seconds and on its way, with no dramas at all. This is due to great skirt design, lightweight construction and lots of ‘push’ from the fan.

Is the skirt easily damaged? How long does it last?

Occasionally, a hovercraft skirt may get damaged and it’s important to understand the differences in design and construction. The skirt must be thought of as the ‘tyres’ on your car, they will wear out and occasionally get damaged – but the choice of material effects how often this happens and how long it lasts.

There are basically two types of skirt used on hovercraft – bag and segmented (or finger) skirts.

The Marlin II uses a segmented skirt – 78 individual segments made from a bespoke material – neoprene coated nylon which is resistant to salt water, UV and tearing and has excellent wear characteristics. Because the skirt is made up of individual segments, should the skirt be damaged you simply replace the damaged one. Easy! It takes just a few seconds. In any case, due to the nature of a segmented skirt, what actually happens is that the segments either side of the damaged one will simply expand more into the available space – so you shouldn’t even see a deterioration in performance until you’ve damaged quite a number.

The reality is that damaging skirt segments is a pretty rare event in any case – the Marlin has superb lift characteristics so there’s no skirt dragging on the floor to snag obstacles. Heavier craft tend to be more prone to skirt damage for the opposite reason! A well designed skirt should generate almost no spray, something well worth checking when you try out a craft!

In normal recreational usage, we find most customers get around 2-3 years from their skirt before it requires complete replacement. In the mean time you’ll replace individual segments as you go.

The alternative to a segmented skirt is a ‘bag’ skirt. This is a single piece ‘loop’ like a giant inflatable ring, around the whole craft. The advantage of these is that they are very stable (which also means doughy/slow steering characteristics) and are relatively good on water. However they are more suitable on larger commercial / passenger craft and in any case, often have small segments fitted to the bottom. The reason for this is that a bag skirt is more like a giant high-pressure balloon and has much more ground contact, so it’s more likely to be damaged. And if you do damage it, it’s pretty much game over – the whole skirt will need repairing, which may involve removing it all from the craft – a colossal job! It’s certainly not repairable on the beach, where you’ll need to mix up glue and stick on patches!

Ploughing In

“Ploughing in” is a term to describe a problem where a hovercraft suddenly decelerates due to the nose of the craft dipping into the water, usually at high speed when running downwind. It can only happen on water. Some hovercraft manufacturers claim to have a craft that does not do this, by design.

Without getting too techy – that’s entirely possible. If you build a hovercraft which is far too heavy for it’s size, it will generate very high skirt pressure. Coupled with the fact it’ll be very slow, it will certainly resist plough-in very well as it never goes fast enough to collapse the high-pressure skirt. Of course, the down side is that it’s slow, wet (all that pressure blows the water past the skirt – and onto you!) and the handling, maneuverability and general performance will be very poor. In other words, avoid any hovercraft that claims to be immune from the plough-in phenomenon!

So! What’s the answer? A well designed craft with a good skirt system will still plough in (every craft can – even the 400 ton SRN4 cross channel craft used to plough in if driven badly – which must have been a sight!) – it’s just the nature of hovercraft construction, like all aircraft can go into a spin. What we’ve done with our craft is to push the plough-in limit as far as possible, make the craft give adequate warning – and a chance to recover – and minimise the deceleration so that when it does plough, the shallow planning surface simply slows the craft up a little, then it recovers and carries on. It really isn’t a drama, and in reality, rarely happens.

Given the extraordinarily good natured handling and performance of the Marlin II – and it’s resistance to unneccessary plough-in’s, it would be very easy for us to match some of our competitors spurious claims that “our craft don’t plough in” but we’re not prepared to do that – it would be a lie.