Technical Information


Other information

This is a guide to typical servicing times and items to be checked on most models:

Sparkplugs - Check, clean, adjust 50 hours
Sparkplugs - Replace 100 hours
Sparkplug cap and leads - Replace 200 hours
Cylinder heads (32** Hirth) - Re tension First 25 hours
Air-cleaner - Check and/or clean Check fittings Every 20 hours*
Carburetor - Adjust idle and tune After first 10 hours then as required
Carburetor cables and choke plungers Lubricate regularly especially if used primarily on salt water
Water pump and fan cooling belt (3701 Hirth) Check 25 hours. Adjust tension when play is more than 5mm. Replace every 100 hours
Cooling fan belt (32** Hirth) Check 25 hours. Adjust tension when play is more than 5mm. Replace every 100 hours
Fuel filter - Replace Every 50 hours*
Oil injection filter - Replace Every 100 hours*
Fuel tank, lines and fittings - Check for leaks and general condition. Pre flight
Engine electrical system - Check for signs of corrosion Every 20 hours
Starter motor – Remove and inspect. Lubricate drive gear shaft Every 50 hours in fresh water. Every 25 hours on salt water or dusty conditions. Replace every 200 hours*
Battery - Check acid level and state of charge First 10 hours, then every 50 hours
Power Module rubber mounts Inspect. Replace worn if worn or damaged
Forward engine frame - Check and re tension bolts Retention first 20 hours. Check every 100 hours
Exhaust - Check for leaks. Check mountings Pre flight – Retention first 20 hours

* Service interval may be shorter if the craft is used mostly over land or salt water. Follow the engine manual as required.

Engine frame - Check all mounting bolts Within first 10 hours, then every 25 hours
Drive Belt - Check condition, tension and alignment Check every 25 hours. Retention every 80 - 100 hours
Belt - Replace Standard belt on standard drive every 100 - 150 hours*. HD belt on flex hub every 250 - 300 hours
Drive hub assembly - Check for bearing play. Adjust if required Every 50 hours
Drive hub flexible coupling - Check / retention retaining bolts Within 15 hours of new or replacement, then check every 50 hours
Drive hub - Re-pack bearings Standard every 150 hours. Flex hub every 250 hours
Drive hub - Replace bearings Standard every 250 hours. Flex hub every 500 hours
Fan Assembly - Check general condition Pre flight
Fan hub Assembly un coated If salt water operation – lube spray after each use
Fan Assembly – Replace (recommended) Every 500 hours or Five Years
Fan duct - Check condition and mountings Pre flight
Fan Guard - Check mountings Pre flight

*Standard belt on flex hub drives replace 200 - 250 hours

Throttle/choke - Check cable condition and operation Pre flight
Oil injection cable – Check cable condition, operation and adjustment Pre flight
Steering and cable - Check condition and travel. Replace if worn. Lubricate Pre flight and every 10 hours
Steering head - Check shaft bearings and free-play. Replace if worn Every 20 hours
Elevator cable - Check actuator mounting. Lubricate Pre flight and every 10 hours
Rudder assembly – Check for bearing play. Replace pivot bearings if worn. Check upper rudder bar mountings Pre flight and every 10 hours

Skirt segments – Check for broken ties. Check for torn or worn out segments Pre flight and as required
Lower hull and planning panels - Check for damage. Repair before use Pre flight
Landing feet - Replace if worn As required
Which marine spray to use

In a filthy marine environment, we use anti corrosion lubricant sprays to protect exposed metal and aluminum surfaces. Its purpose is dissipate water and protect such parts from corrosion.

While many products offer some corrosion protection, many need to be reapplied after each wash down because the coating only leaves a thin layer. Many general lubricant sprays are better designed to assist in removing rusty bolts and will soften rubber and harden PVC parts if constantly applied.

Ideally, the best corrosion protection sprays are those that leave a light greasy, waxy film, which are not petroleum or silicon based.

CRC Marine 66 works very well in high temperatures. It leaves a partly dry film which creates a good barrier which salt water does not remove. It will not wash off from a fresh water wash down.

Other products such as CRC Lanosheild and Inox Lanox MX4 leave a thinner, wetter film. Both of these are still good in resisting salt water spray and will not wash of from a fresh water wash down, however they will attract dust.

For long term protection, storage, severe conditions, or if if you have no access to wash down facilities for a long period, products such as CRC Anti-corrosion Heavy Wet Film is ideal. It will go on wet, stay wet for a while, then harden to a light grease consistency.

Products that contain silicon or petroleum chemicals should not be used. Silicon will soften natural and synthetic rubber. It will harden PVC. Petroleum based sprays are a fire hazard.

Some lubricant sprays will soften rubber parts with continued long term use. This includes rubber fuel lines, rubber radiator hoses, rubber engine mounts, V Belts, cam timing belts and drive belts. Although all these parts are designed to be fuel, oil and chemical resistant, they will soften if soaked in certain types of lubricant sprays.

In general, what ever your preferred corrosion protectant, avoid applying to rubber or PVC parts, or for detailing purposes.

A good product to clean your hovercrafts exterior, engine, engine bay and keep rubber/plastic/PVC parts in good order is Chemtech CT18. This product is biodegradable, pH neutral and non caustic. It also inhibits rust and corrosion. Rubber and plastic parts will look like new. CT18 will remove dirt, grease, oil film and leave a streak free clean fresh finish. Simple spray 10 parts water to one part CT18 with garden sprayer over the engine. Allow 15 minutes to soak in. Loosen and remove and heavy build up of grease or dirt with a soft brush or rag. Rinse off with fresh water (avoid focusing a pressure washer on vital engine parts or electrics). Repeat to remove any ingrained dirt or grease. Start the engine to remove any excess water from the stator/alternator and belts. Once the engine has allowed to fully dry, reapply your corrosion protectant spray to critical areas subjected to possible corrosion, and avoid rubber and PVC parts. If rubber and PVC do become coated, wipe off with the excess with a dry rag.

Torque Values for A2 or A4 Metric Stainless Steel Fasteners

Bolt Dia (mm) Torque (N-m) Dry Torque (N-m) Lubricated Torque (in-lbs) Dry Torque (in-lbs) Lubricated
5 mm 5.1 4.6 45.1 40.6
6 mm 8.7 7.8 77.0 69.3
8 mm 21.2 19.1 188 169
10 mm 42 38 375 335

Torque Values for Metric Steel Fasteners (Maximum Torque, in foot-pounds, for clean, dry threads)

Bolt Dia (mm) Low grade 8.8 grade 10.9 grade 12.9 grade
6 mm 3-5 7 10 12
8 mm 8-12 17 24 29
10 mm 12-22 33 43 57

As part of servicing, it is important to do regular compression checks. This enables you to see how your engine is wearing, when it is due for a overhaul or find faults.

A new Hirth or Rotax two stroke compression is between 125 - 130 psi, and increases slightly after the piston rings have bed in to around 130 - 135 psi. The compression should remain the same for almost half the engine life before slowly declining as the piston ring gap increases. Piston rings, or a top overhaul is due when the compression is less than 20% of the original state.

The compression must only be done on a warm engine as a cold engine will give erroneous results. Allow the engine to get adequate air by holding the throttle open. Allow the engine to crank continuously for at least five to ten revolutions as that will allow an accurate reading on the compression tester.

Piston rings wear naturally wear over time. There is nothing that can be done to stop the wear, but you can maximize the life of the engine by following simple steps.

Engine temperature :
One of the main contributors to premature piston and piston ring wear is loading the engine before the engine has reached normal operating temperature.

If the piston is allowed to warm up quicker than the cylinder wall, the piston will expand and make contact with the cylinder wall. This will prematurely wear the piston skirt, rings and barrel wall. It is very important not to put the engine under full load until the engine has reached operating temperature. It is also important not to over heat the engine for the same reason.

Applying full power to a cold engine also wears the crankshaft main, big end and little end bearings. These parts also need to be at operating temperature to ensure the clearances are correct.

The correct warm up procedure is -
Rotax water cooled engines: Start and run the engine between 2,000 to 2,500 rpm until coolant temperature reached 60 - 65deg (or 110 deg C cylinder head temperature)
Hirth water cooled engines: Start and run the engine between 2,000 to 2,500 rpm until coolant temperature reached 70deg (or 120 deg C cylinder head temperature)
Hirth air cooled engines: Start and run the engine between 2,000 to 2,500 rpm until the cylinder head temperature reached 140 deg C

Oil and fuel :
The correct oil to fuel ratio, and the correct oil and fuel type is important.

The correct oil is designed for light aircraft and motor bike engines, and should have a minimum 2T JASO FC rating. This type of oil is designed to be mixed at 50:1 to ensure it maintains its lubricating quality's and fuel burn without excessive smoke, ash or carbon build up. Never use two stroke oil designed for a outboard marine engines. Outboard two stoke oil is designed to operate at lower engine temperatures.

Mixing a ratio of less than 50:1, e.g. 40:1 should only be used when "running in" a new engine with cast iron bores. Typically, additional oil should only be used for the first take of fuel. Mixing ratios of less than 50:1, e.g. 40:1 over a long period will lead to loss of power and excessive carbon build up.

Mixing higher ratios than 50:1 will reduce the lubricating qualities of the fuel/oil mix and lead to premature engine wear.

High and low fuel to oil ratios also change the viscosity of the fuel, leading to rich or lean mixtures. A low fuel to oil ratio meant to avoid piston hot spotting will increase hot spotting.

Use quality fuel with a minimum 95 octane. Avoid using 91 octane on new engines. Ethanol based fuels, especially E85 will damage plastic bowl floats. 91 octane fuel may be a benefit on old engines with below standard compression.

Replace the fuel filter regularly to avoid leaning out.

Keep it well serviced :
Ensure that your air cleaner is in good order and adequate to filter out fine dust particles.

Dirty intake air will wear internal engine components quickly and is the most common cause of premature engine wear.

Keep the engine in good tune. Ensure the air filters are clean and the carburetors are synchronized. Replace the sparkplugs at regular 100 hour intervals and replace the sparkplug leads as per engine manufacturer recommendations.

Check for air leaks such as loose intake manifold rubbers and crankshaft seal leaks.

Replace the fuel filter regularly to avoid leaning out.

Avoid full power :
One of the most beneficial things you can do to maximize engine life, is avoid constant full power. By maintaining an average engine load of 75% or less, will ensure the engine lasts as long as the engine manufacturer intended and reduce your fuel consumption considerably. If applying full power, ensure the engine is at operating temperature and do so in short bursts or only when required.

Rotax UL DCDI UL99 and UL17
Maximum RPM 6,600
Minimum idle RPM 1,200
Maximum cylinder head temperature 150C
Normal cylinder head temperature 110-130C
Maximum coolant temperature 80C
Minimum coolant temperature 60C
Recommended Battery type 280CCA 30ah

Hirth 3701
Maximum RPM 6,300
Minimum idle RPM 1,200
Maximum cylinder head temperature 195C
Normal cylinder head temperature 120-130C
Maximum coolant temperature 95C
Minimum coolant temperature 70C
Recommended battery type 330CCA 45ah

Hirth 2706 / 3202 / 3203
Maximum RPM 6,500
Minimum idle RPM 1,200
Maximum cylinder head temperature 280C
Maximum exhaust gas temperature 680C
Recommended Battery type 280CCA 30ah

Hirth F30
Maximum RPM 6,500
Minimum idle RPM 1,200
Maximum cylinder head temperature 280C
Maximum exhaust gas temperature 680C
Recommended battery type 420CCA 55ah

This JavaScript calculator was developed by Alex Olshove based on James Perozzo's book, Hovercrafting as a Hobby. Juergen Schoepf further developed the calculator by incorporating metric units of measure. Michael Nell further developed the calculator for responsive use on mobile devices.

The horse power and fan diameter outcomes are not accurate for modern multi-wing fans. However, all other calculations are useful to work in conjunction with the Multi-wing or Wing fan selection software.

The shape of your hovercraft is assumed to be rectangular. If your hovercraft is of another shape (round, oval, etc.), then enter an estimated length and width dimensions that will approximate the air cushion area of your craft. Always use the measurement from the ground contact point of the hovercraft skirt, not the overall hovercraft hull dimensions.

The air gap you will specify is the distance between the ground and the contact point of the hovercraft skirt, not the distance from the ground to the bottom of the hovercraft hull, and is typically 0.5 inch. If you double the air gap to 1", it will require double the air flow from your fan. If you reduce your desired air gap to 0.25", the air flow requirement will be cut in half. Using a 0.25" air gap is acceptable as well.

Enter the required data in the following fields, use the metric or imperial boxes, not both.  When done, press the "Calculate" button. The answers will appear below.  Press the "Clear Input" button before starting a new calculation
Enter hull length in meters: or feet
Enter hull width in meters: or feet:
Enter Amount of air gap required in mm (i.e. 13) or inches (i.e. 0.5):
Enter max. gross weight of craft in kg: or pounds:


Results computed from the Input Data:
(best viewed in landscape on mobile screens)

Approximate lift perimeter (m) (ft)
Total hover gap area (m^2) (sq. ft.)
Total cushion area (m^2) (sq. ft.)
Cushion pressure (N/m^2 = Pa) (lbs/sq. ft, PSF)
Cushion pressure (mm of water @ 4degC) (inches of water @ 60degF)
Expected actual air velocity (m/sec) (ft/sec)
Lift air volume (m^3/sec) (CFS)
Estimated lift engine power (kW) (HP)
Estimated fan diameter (m) (inches)

The simple calculation method is: Fan/prop RPM x fan diameter (M) x 3.14159. Divide the sum by 60 = M/sec. E.g. A fan rotating at 2,400rpm, with a diameter of 0.8m has a tip speed of 100m meters per second.

The maximum tip speed of a hovercraft fan or propeller is set by the fan or propeller manufacturer. It is important to follow the limitations for recreational use.

In the case of a Multi-wing, or Wing fan assemblies, the maximum tip speed may range from 100 to 130 m/s. The limitations are dependent on the fan blade material, fan blade weight and blade/hub configuration.
  +61 418649350

© 1990-2023 Viper Hovercraft