View Full Version : A few questions from a hovercraft newbie

CrazyKiwi - n/a
3-Feb-09, 03:38 AM
Hi all

I have bought a robin ec25ps engine and are hoping to build my first hovercraft along the lines of the simple cyclone / challenger design.

I am fortunate enough to have in my possession a copy of the HCGB constructors guide and a copy of a technical publication by nigel beale on breeza fans for hovercraft, so I have some solid information available.

I am hoping to use polaris parts to increase the performance of the ec25 from 20 to 30+ Hp to get a powerplant with a good power to weight ratio.

Currently I am planning a 6 by 10 ft craft with an absolute maximum (all inclusive) weight of 600 lbs. It will have a finger skirt and be very similar to the example hovercraft described in the hovercraft constructors guide.

Hopefully the craft will be an integrated design on a htd reduction system running a 800mm either 4z or 5z multiwing fan.

Can anybody give me advice on the use of the EC25 for an integrated design craft? Am I wise to use polaris parts to get it up to over the 30hp mark (performance parts are available to get it up to 38hp) and lastly do my facts and figures seem plausible, forgive me if they are not, this will be my first craft remember http://hovercraft.org.uk/images/icons/smiley_icons/icon_wink.gif

Thanks in advance for your help.


GavinParson - n/a
3-Feb-09, 06:13 AM
Hi Bruce,

The Simple Cyclone is now considered a very old design of craft and the EC25 is not exactly a modern engine.

If you want an efficient craft that is safe, efficient and reliable I would look at more modern alternatives.

You don't want to build something that you won't enjoy using in the end.

I would recommend contacting the Hovercraft club of NZ and speaking to people who have built craft recently and finding out what is available and what works well. Alternatively contact the Australian hovercraft club.

An 800mm duct is by modern standards quite small. Most craft use 800-1200mm ducts and many use propellers.

In the UK and US four stroke industrial engines of 22-40hp are most popular as they are simple to install and very reliable. These are available from Briggs & Stratton, Kohler, Honda and Kawasaki.

CrazyKiwi - n/a
3-Feb-09, 06:32 AM
Hi, Thanks for that response.

Is 800mm quite small even for a small engine (40hp or less)?

Also what were the problems with the Cyclone craft? (you mention safety)

Were there relaibility issues with the EC25?

To my knowledge the Hovercraft Club of New Zealand ceased to exist some years ago due to not enough interest http://hovercraft.org.uk/images/icons/smiley_icons/icon_sad.gif

(As a youth I had a family member in the club so I have seen a number of craft in action)

I Have a set of plans for a superwedge and for some UH designs but I want to stick with something not too big first up and I arent too keen on the UH designs.

Unfortunately I am kinda committed to the engine now, I bought it specifically for this project. (Brand New can you believe it) On the up side after Fuji Robin stopped producing it in the mid 80's it is my understanding that polaris kept using variants of it in there ATV's right up to 97. This means that a number of parts are still available.

I would have prefered to go with the engines mentioned but unfortunately that size arent too common in New Zealand. Sure you can buy a 25hp Briggs, but expect it to cost probably $3000, which is a good portion of my project budget.

Let me put my questions another way then.

If I were in the UK and starting out building my first craft on a modest budget, what would members of the HCGB be reccomending that I build?

Thanks Again


Keith Oakley - n/a
3-Feb-09, 09:34 AM
Hi Bruce

There's nothing wrong in what your proposing - its a question of managing your expectations. Todays integrated cruising craft closely follows the 1974 Simple Cyclone design (indeed it is the original integrated craft) but with more reliable 4 stroke engines, mostly with more bhp, probably a bit bigger lift area to carry more and keep cushion pressure down, a bit more freeboard and buoyancy to float better on water.

As a result todays cruiser would expect to cruise 50 miles in a morning in fairly open conditions over salt water without problems whereas craft of the late 70s expected to break down in a day spent in a field and a pond. Todays craft you would expect to be fibreglass and full of shapely curves compared to the boxy shape of a plywood Simple Cyclone. But much of it is cosmetic and as long as you meet the weight targets and keep it well painted ply is fine.

I'd increase the length (width if poss) and hull height by say 10 -20% put in plenty of bouyancy and crack on. But expect just to use it in sheltered conditions, school field, local lake river etc with reasonably flat water entry - but you could still have a lot of fun. The EC25 is now a pretty old engine and only surfaces on older craft so I'm not sure what expertise there is now on uprating it - but someone might pop up with info.

Good luck


Superwedge - n/a
3-Feb-09, 11:44 AM
Hi Bruce I have seen a 3/4 size Turbo Superwedge powered by a EC25pm engine and it works great for a single Occupant..



GavinParson - n/a
3-Feb-09, 03:22 PM
Hi Bruce,

Many of us are using 35hp engines and we all use 900mm ducts minimum. 6 years ago I had a racing craft with 40hp and a 750mm duct. I fitted a 900mm duct and it transformed the performance.

I can't comment on the EC25s reliability. I used to have one but never used it because there was always something better to use.

My concerns on safety with a Simple Cyclone would be the low freeboard and low buoyancy if you were to use it on open water.

Look at some of the Ausie craft and see what you think.

CrazyKiwi - n/a
3-Feb-09, 04:24 PM
With Regards to buoyancy and freeboard;

I will use either polystyrene or urethane foam inside the hull to give a safe level of floatation. If I were to make the depth of the hull 300mm would this be sufficient freeboard, If not what would you reccomend?



GavinParson - n/a
3-Feb-09, 08:45 PM
300mm absolute minimum to the bottom of the duct in an integrated craft. Go for a deck height of at least 500mm.

Put extra buoyancy at the rear and under the duct.

Jamie Lewendon - n/a
4-Feb-09, 11:47 AM
Hi, Thanks for that response.

Unfortunately I am kinda committed to the engine now, I bought it specifically for this project. (Brand New can you believe it) On the up side after Fuji Robin stopped producing it in the mid 80's it is my understanding that polaris kept using variants of it in there ATV's right up to 97. This means that a number of parts are still available.


I don't want to dishearten you but you appear to be confusing 2 complettly different engines. The EC25PS was produced up until about 1978, and was mainly fitted to small snowmobiles and microlights. It produces approx 22HP, and if left standard is pretty bullet proof.

The EC25P<u>F</u> was produced upuntil 1998 and was fitted to ATV's, it can be tuned fairly reliably to give approx 38HP, however it is a completly different engine, and the only common parts are the conrod and sparkplug.

lastly, how did you manage to acquire a 30 year old EC25PS brand new?

If you want parts for either versions, let me know, I've got a shed full of bits and bobs, new and used


CrazyKiwi - n/a
4-Feb-09, 04:30 PM
Ahhhh so the penny drops at last and I see where I have gone wrong.

The pistons are even different? Because it looks like the pistons are all the same for all the polaris 250 models.

As an update, (I know everbody is going to roll their eyes at this one) I have aquired a secomd ps engine and have a crazy idea that it mightn't be too difficult to convert it into an inline twin.

Is there an way I can get in touch with you jamie regarding these engines, their parts you have and their differences.

I cant believe the brand new thing either. The second engine I have bought is near new also. I just come across them when browsing a New Zealand online auction site.



CrazyKiwi - n/a
8-Feb-09, 05:28 AM
Hi Gavin, can I just confirm something you said in an earlier post on this topic.

A deck height of 500mm; does this mean 200mm or so of hoverheight on top of that ie deck 700mm off the water?

Did I understand you correctly, that seems quite high?

Also the Constructors guide reccomends 1/4 of the deck area as lift, can a portion of this be a bulkhead above deckheight?

Thanks Again


PS; I run up one of the Robins on the weekend, all I gotta do now is do something as far as expansion chambers/ silencers for them then i'll be right. (Well... the neighbours will be happier anyway)

GavinParson - n/a
8-Feb-09, 06:42 AM

If your duct in an integrated craft sits lower than the top deck of the craft (as is usual in order to duct the lift air into the hull)then the bottom of the duct should be at least 300mm above the floor of the craft.

From the floor of the craft to the to the top of the hull measured inside the craft should be at least 500mm. A craft longer than 12ft should have more than this.

Hover height is a different matter. That is the height from the underside of the hull to the ground below measured at full lift.

BTW I don't think joining two engines will work successfully. If you try re-inventing the wheel you'll just get frustrated with your project and give up hovering like so many before you.

I'd always recommend buying something that works first, learn how it works and enjoy using it. If you eventually go on to build something then you'll be armed with the experience and knowledge your first craft has given you.