[Steve Holland Posts: 1580 Status: Offline]

Hi,
 
I too looked at bike engines, I currently have 750 Suzuki with the 4 cyl 16v engine. They are reliable but I wouldn't use one for cruising as they are relatively heavy and produce their power top far up the rev range for a cruising craft. There are increasing numbers of all aluminum engines out there that would work well, get the electrics right and you should have no problems.
    As for diesels, they will be used in the larger cruising craft more often but are still heavy and have almost as much electrical components to fail. The only one I would consider just now is the three cyl VW 1.4ltr. It was fitted to the Polo or Fox. You would be best to get the whole car so as to get all the little important bits and you could use some of the other parts off your car for the hovercraft too.
 
Steve

[Ian Brooks Posts: 3550 Status: Offline]

Hi

No-ones fed up with you for asking questions - keep asking!

Modifying hovercraft to get better performance is really, really hard. The designer of the Sevtec, Barry Palmer took 30 years to perfect the Sevtec designs, and from a skeptical beginning, I've learned the hard way he is almost always right! I lead a design and development team - so improving designs is my job - but it took me 3 attempts and hundreds of hours to make my "improved" Surveyor equal to the standard design!

When I looked at bike engines, I found that most rev'd too high and were surprisingly heavy. The exceptions are the BM's. The truth is there are no ideal hovercraft engines out there - although for many people, the larger Briggs V twins are as near idal as you can get - I use the populer 35hp Briggs. Diesel in considered by many to be a good idea, mostly for reliability reasons, but up to now weight has been a major problem (and I would not put a turbo onto a hovercraft) as there are still (to my knowledge) any truly lightweight diesels available. Maybe that will change, but for now you would in my view need a large craft to carry a diesel.

Cheers
Ian

[Ian Brooks Posts: 3550 Status: Offline]

If the chain comes off, you have a bad day. If a belt comes off, you stop, put another on, and go home!

Atters will fill in the details!!!!!

Ian

[John Robertson Posts: 5121 Status: Offline WWW]

5K should build you a nice craft with care.
 
Check the max power RPM for the engines (there are many different Bandit engines up to 12500rpm). Also check the weight with the gearbox - some bike engines are surprisingly heavy! If the rpm is high you could leave it stuck in 3rd or 4th and use the gearbox as part of the reduction drive. I have to ask the obvious question - how come these engines are so easily available as there appears to be a reasonable s/hand market for them?
 
It is hard to get engine weights - the only real way is a visit to your local scrapyard - if you can't get it off the ground then it's too heavy
 

the other thing i have always wondered is way auto gear boxes arnt use on boats and hovercraft?

Because you don't need a gearbox at all (although outboards have a fixed reduction/reverse gearbox). In general, the load curve more or less matches the power curve of the engine because you are driving a fan or prop - unlike when you try to start off up a 25 degree hill with your car.

[John Robertson Posts: 5121 Status: Offline WWW]

I don't think you've p***ed anyone off either Fastflow
 
My responses may be a bit long but that's only because I try to explain the why's rather than just saying something is good or bad.
 
Don't let me put you off using a diesel engine - just make sure you know all the details first (another one - diesel fuel is 15% heavier than petrol - less volume of diesel fuel is needed than petrol but it weighs more - weight is everything for a hovercraft). The only two advantages I can see of a diesel are the lack of an HT system and their low-revving nature (and the fact that diesel is cheaper than petrol per unit of energy) . I'm not at all sure that there are any others but feel free to convince me otherwise?
 
Hovercraft are almost unique in that almost every part of the craft has an effect on every other part. It is very easy to end up in a spiral of "enhancing" that results in something that doesn't work well (or even not at all!). The Donar craft certainly looks really nice but I have to ask the obvious question - [cynical mode ON] if it works so well why are there not hundreds of them out there?? [cynical mode off] In my view, the part of a hovercraft that could do with further development is the skirt system - it affects drag, hump performance, stability and efficiency - almost everything!
 
Bike engines can be a good choice but you need to look at where they develop their power. For a cruisng craft you really want peak power as low down the rev range as possible - this will help reduce noise and make the transmission system simpler (it's a bit difficult to get a belt reduction drive to operate at 200000rpm for any length of time . The BMW engines are a good choice as they are relatively low revving units. For cruising, reliabilty has to be top of the list when choosing an engine - you need one that will happily run day after day, ALL day at 2/3rds power. As Jon says, the craft must cruise at 1/2 power under ideal conditions - the other half is needed when things get hairy!
 
The chances of picking up an old Subaru engine must be almost nil nowadays. There are, however, tons of suitable small aluminium car engines out there. Almost anything will do (it's hard to find much under 100HP nowadays), but look at the peak torque RPM and the max power RPM that will give you some idea of it's most efficient operating speed - the lower the better.
 
If you are really serious about this stuff then I would recommend joining the club - there is a wealth of information available on this site to members, including a 40 year magazine archive that has technical articles and examples of almost every type of hovercraft design ever made.

[John Robertson Posts: 5121 Status: Offline WWW]

Sevtec have a great design and its that way for a reason. I have tried to improve on craft before and it was just like throwing money away.

Something else to consider is that, if you change something, how will ever you know whether the change is an improvement over the original design if you don't know how the original design would have worked because you didn't build it and test it?
 
I'm the same as everyone else in thinking I can do it better - the fact is I've learned the hard way that 9 times out of ten I can't :blush:. The reason isn't always down to expertise or qualification - it comes down to being in possession of all of the design criteria and performance parameters (and a big lump of real-world experience of hovercraft) and then being able to make the best compromise decision (EVERY design decision is a compromise).

[John Robertson Posts: 5121 Status: Offline WWW]

.. but at 2.5 gallons per hour =16mpg in a nut shell that is a bit naff.

I don't think you will find any other water craft that can operate at that kind of speed (40mph) and return that kind of consumption. Reduce the speed and the consumption falls - just as it does with any other vehicle. In my experience, hovercraft are around 30-50% more efficient than similar size boats (at the same size/payload/speed). The reason is mainly reduced drag (less energy used to overcome drag versus energy used in the lift system - the faster you go, the more efficient they become)
 
The amount of energy you get from a litre of fuel is the same no matter what 4 stroke engine you burn it in (two strokes ARE less efficient, diesels appear more efficient but diesel fuel has a 20% higher energy density than petrol). If you need 40kW to operate a hovercraft it makes almost no difference to fuel consumption what engine you choose to burn your fuel in (you will actually need to consume more than 100kW of "fuel energy" to get 40kW from an internal combustion engine :eek:).
 

air movment.
u are right in that i dose go in any gap but airflow is affected in a simular way to water ...... there is also an inbalance on the prop because free flowing air on the top half and restricted air flow on the bottem half which in turn causes friction and ware on breaings etc.

The intake airflow pattern is a 3D cone shape (the prop inflow shape extends far out the the side and above the craft). The prop pulls in several hundred times more air than the lift fan does (the lift fan only has to replace cushion air lost though the skirt gap - the prop has to throw vast quantities of air backward to generate thrust). The lift fans are "stealing" a very small amount of the air being pulled in by the prop making any efficiency loss almost insignificant (do the calcs and you will see what I mean - or do the fluff-on-the-end-of-a-stick test on a real craft.
 
You may have noticed that there are all kinds of lift fan locations and orientations out there - everything from horizontal to vertical. They all have different advantages and disadvantages.
 
It is very important from the noise point of view to reduce intake turbulence but, as with all designs, there are other factors involved and compromises always have to be made.

[Ian Brooks Posts: 3550 Status: Offline]

The subaru is light, cheap and reliable... it works and I would be inclined to build to plan. John Robertson's Prospector uses one and has impressive performance.

Likewise the lift fans - they work... It turns out that air can get around surprisingly tight bends!

Cheers
Ian

[Ian Brooks Posts: 3550 Status: Offline]

Hi

Have you seen the Hoverclub of America site?

http://www.hoverclubofamerica.org/

Ian

[Ian Brooks Posts: 3550 Status: Offline]

Hi

I built a surveyor - which I am very pleased with...

It took 14 months to build in it's original trim (it had another engine since then). You should budget anywhere between £3k and £5k, depending on what sort of a nose you have for a bargain.

Ian

[Ian Brooks Posts: 3550 Status: Offline]

Hi

Curves are possible... it just takes time! There are limits to what can be achieved with the foam/fibreglass construction, if you look at the sevtec site theres a section on other builders craft,that should give an idea what is possible

http://www.sevteckits.com/sevteckits/html/builders.htm

Ian

[Ian Brooks Posts: 3550 Status: Offline]

Yes - you can fish off them, they are stable and float happily when "off cushion". If you surf the Sevtec site, there will be details there somewhere.

The design is customisable to some extent - everything below the upper skirt attachment line should be done strictly to plan, also the prop, lift fans and engine frame kept to plan. But the upper decks can be customised, provided that weight is kept under control. You could add a small, lightweight cabin if desired.

Cheers
Ian

[Ian Brooks Posts: 3550 Status: Offline]

You would need something like this:

http://www.sevteckits.com/sevteckits/html/prospector.htm

Also check out:

http://www.hovercruiser.org.uk/building-a-hovercraft/prospector.html

However, not much that could be used in force 6. The Prospector will operate in force 4 - 5, but at force 6 it would be getting a bit hairy.

Ian

[Ian Brooks Posts: 3550 Status: Offline]

I have never flown a hovercraft before so I will be cruising to start out but eventually would like to get into racing.

Hi


Don't be too hasty to dismiss homebuilt hovercraft - some of the most capable and successful cruising craft are homebuilt - the key is obtaining good plans and sticking to them. There will be plenty of advice regarding which plans to choose should you go that way - but you will need to buy them! And without storage, I fear that a homebuild would not be feasible (although it has been done in an attic with a crane used to lift it down when it was finished!)

This is my homebuilt:

[ATTACH]4003[/ATTACH]


Ian