The tricky part of building the trailer was getting the location of the axle right so we could achieve the right balance and tongue weight. We have a 12’ extension and a removable tongue wheel (click here for a larger image of picture at left) that makes launching easier. Another problem with a light tongue - with the extension rigged - backing down a nice gentle, beautifully paved ramp is no problem. The trailer has a removable 'nose' post to help align the boat when she is being pulled out. In the beginning I had built a pair of wood guides for the keel thinking they would help guide the boat on to the trailer. January 23, 2010 The recent interest in trailers has prompted a review of what additional useful information on Marionette's cradle conversion was available. A key piece of information is knowing the exact center of gravity formula for the Dolphin 24. The measurement formula placed the center of gravity inline with the center of Marionette's aft portlight, which is also a point 11' from the end of the transom - why I considered this relevant I can't remember. Early this year I decided to keep Marionette up in Maine for the summer and fall of 2011, perhaps winter her there and have two consecutive sailing seasons up there. After launching Marionette we took the trailer to a local tire shop in Rockland, bought a new tire and put it on the trailer on the port side, and put that tire on the spare rack. Following up on the subject of my trailer tongue extension (see above) this morning I have finally got around to taking a couple of pictures of the setup. March 6, 2016 We have been working on updating Trailers in the Technical section and we needed some better pictures of Marionette's trailer setup - particularly the support system for her 'keel bed".
At left is an underneath photo which gives some idea of the structure under the trailer keel bed.
Wish we had a photo before we put the 2x6 board on top of the keel bed support grid - and the boat's keel on top.
Get your plans immediately after completing secure checkout through PayPal so you can be up and running straight away. Download file is in PDF format for easy viewing and printing from any device. A number of DXF and EXCEL files are available for download after purchase.
Our Single Axle Flatbed Trailer is specially designed for lighter applications hauling smaller vehicles and loads.
The trailer features an optional chariot style nose guard to protect the cargo from potential stone damage from the tow vehicle.
Instant download in PDF format for viewing on any device with a few useful DXF & EXCEL  files available separately (free).
Plans conform to the Australian National Code of Practice – Building Small Trailers (VSB1). I delivered the Scout to the ferry port and Stranrear where I met David and passed it over.  I'm sure he will have as much fun as we have had over the last four years! Barry Palmer ("Mr Sevtec") retired earlier this year - the full range of Sevtec hovercraft plans are now available from Amphibious Marine. Divide the chassis in half and mark the chassis rails, then accurately measure where you want the spring centers to be - ideally between 2" to 6" behind the center of the chassis and a good compromise for general cartage is midway at 3" behind the center. Set up your springs in their front hangers and position the springs and hangers on the chassis rails (hanger towards the front of the trailer) and using a large square resting on the chassis rail at the spring center mark, adjust the spring position until the spring bolt or pin is in line with the spring center marked on your chassis. Make sure that the outside spring edge is flush with the outside edge of the chassis rail and once double checked, tack weld the spring hanger to the chassis rail.
Once this is completed on both sides, position the slippers over the spring tails allowing for clearance between the spring and the slipper for the spring to move in and out when loaded.
Again make sure the outside edge of the spring is flush with the outside edge of the chassis and tack weld the slipper in place.
If the position of your hangers and slippers do not cover a cross member you will need to cut a couple of packers to support the overhang of these parts and weld into position. Once all components are in position, remove the springs and fully weld around the hanger, slipper and packers. Download files are in .zip format and you will need a suitable decompression utility to extract the files. A practical design, ADR compliant with all the necessary information and detail to get your tandem box trailer build project underway. The plans conform to the Australian National Code of Practice – Building Small Trailers (VSB1).
This involved some assistance from the S&S office - fore and aft center of balance (see sketch below). This can be a bit tricky without a winch to hold her on nose as the boat will settle in whatever attitude she has when she no longer floats - this is a variable event by poppet depending on how level and even the ground under the water is.
That 2 x 4 in the foreground was an early temporary brace for the vertical nose support you see in the prior photo.
And I bolted a couple of aluminum tubes to the fenders into which thin flag poles could be inserted.


On the trip back we talked about maybe a better way - like maybe laying the mast on a simple pad system tied to the deck along side the house, or maybe better to a padded pair of cross 2 x 4s, one across the foredeck in front of the house, the other in front of, or in back of, the transom hatch.
When I bought the boat it was on the cradle positioned with the front poppit lined up with the running light (which is just forward of the main bulkhead), and in line with the winch.
By viewing the trailer itself (above) and checking against this sketch you can dimension the trailer. It works but you have to watch for sharp turns, and I think a longer tongue might tow better.
And, instead of sailing up - about 7 days - as I had on 2 previous cruises, I decided to trailer the boat up to Rockland, Maine and launch her there.
That trailer had made the trip from Boothbay, Maine home with Marionette 3 years years ago with no problems, but I confess to have more nervousness about long distance trailering than long distance sailing. The shop guy told us that the piece that 'flew' out of the tire was probably due to the heat and age deterioration of the tire, not to any road hazard, and certainly not to wear. They are susceptible to sun damage, some times not visible, and apparently just 'old age' deterioration.
Someday, I'll get a picture on a ramp - hopefully one of me riding the tongue into the water. Your webmaster has not been able to get the Admiral to approve a pickup to tow Marionette so this gets done by a good friend and his truck. Details of this, including a secure site password is given in the PDF file that you will receive. With a max ATM of 2200kg and a TARE of only 650kg you have a generous load capacity of up to 1550kg. This is particularly useful on long-haul trips and on dirt roads. The design also includes handy features like tie-down hooks, double safety chains and chequer plate bed that could be either full width or strip. Over-ride brakes (mechanical or hydraulic) are only permitted up to 2000kg GTM (about 2200kg ATM with a 200kg ball load). Trailers built to comply with this code of practice can be registered for on-road use in Australia.
They sell kits and parts also but all parts to build a Sev (except Props) can be sourced in the UK and Europe (look at the parts supplier pages for metric equivalnets on this site).  Barry's web site is now hosted HERE on the hovercruiser site - it contains a lot of interesting historical and design information on hovercraft (and hang gliding, etc). Depending on the plan you choose, you will get a number of detailed drawings – at least one A4 drawing for each major assembly such as chassis assembly, body assembly, various sub-assemblies (tailgate, ramps…) and all major parts.
Depending on the payment option you choose, you will receive a download link as soon as payment has been finalized so you can get your plans and be up and running straight away. The plan uses a tandem box body panel kit available from most trailer spares suppliers and a strong RHS chassis construction. Not only do you get 13 part & assembly drawings typical to those shown here, but also a 13 page assembly guide, some DXF files and a number of pictures to assist you with the build. I have provided a chart that shows the spring hanger locations for single axle trailers for you to review, see link.There are two considerations for mounting the spring hangers. After sandblasting her, we welded a short tongue to the frame and bought, through the Northern Tool and Equipment catalogue, a single axle, surge brakes, springs, fenders, hitch actuated hydraulic surge brakes, heavy duty tires, a spare tire, a light package and an 800lb capacity tongue jack, the one in the picture immediately below, It was marginal. Some real nice, expensive hitches have load cells that read out the tongue weight on the truck hitch. If I recall correctly, that tongue wheel was a wheel barrow wheel I picked up at Home Depot.
Not a big problem when the boat's stern is nearly in the water, but a big problem if on the steeper, dry part of the ramp, or in shallow water. Anyway, we always manage to get it done and get her home, sometimes with a pad that we slip between the nose post and the bow. We had no problems with this system but if these horses were lower the center of gravity would be lower - a good thing, and easier to load. The boat had been in this position for nearly 10 years without change and there was no visable hull deflection.
It is a large diameter pipe welded to the frame with a smaller diameter pipe inside, and with a few in line holes drilled though both. I don't have a Dolphin yet but I'm planning on picking up one of the free ones (or possibly the one on ebay lately). Its a 300 mile road trip, 250 at turnpike speeds with 6000lbs of boat and single axle trailer pushing you forward. The unloaded trailer made it back to CT without problems - and is now at my local trailer shop scheduled to get another new tire to replace the 'new' 15 year old never used spare tire, and a complete re-inspection. They may be fine for short trips but the high heat developed at high speeds on long trips are another matter. After consultations with my trailer guy we decided to get 2 new, matched tires for the trailer and only use the new tire I purchased up in Maine after the 'incident', as a spare. The trailer has a load area of 4200x1950mm and includes adjustable vehicle ramps with fixed stabilisers. The plans will provide all the necessary drawings, instructions, parts lists and handy tips to enable you to build with confidence, knowing your trailer will conform to legal requirements.
The assembly guide included in the plans will assist you in the construction and give you lots of guidance on brakes, axles, suspension and other components.


If you’re having any trouble opening the .zip folder, download a FREE evaluation version of WinZip. Of course you can also manufacture the body panels if you wish – all the drawings and detailed dimensions are provided.
After positioning on the ramp, the trailer wheels are blocked, the trailer disconnected, the tongue wheel moved to the down position and the jack swung up. This owner has, many times, put his 200 lbs to good use as insurance by standing on the tongue over the extension wheel for the ride to deeper water.
I learned one lesson about trailering a heavy load at 65mph on the highway - do not look at the trailer - its scary. And thinking about all the large dents you see in steel bridge trusses on the highway would no longer fog the mind.
There did not seem to be any good reason to change this so the next step was to position the single axle under the cradle to get the desired tongue weight. An accident up ahead closed Route #1 and we were detoured off on to a side road where we sat for nearly an hour.
The boat ramp set up is as follows: the trailer wheels are blocked, the nose wheel installed beneath the tongue and the bolts tightened.
The assembly of all the parts is fun as long as you have a mechanic friend who knows about things like hydraulic surge brakes. I thought you would never ask (click here) Total cost was less than $2000 plus lots of mental stimulation vs the Triad no brainer but expensive $3300, at that time (1997), now maybe close to $6000. The jack wheel is raised to get the hitch off the truck, then lowered to get the weight on the nose wheel, then the jack is tilted back.
With this one axle system you have to be sure you have your weight distribution right, not only for towing, but when using an extension, especially on a steep ramp.
By adjusting the jack up forward you can get one set of holes to line up and re-insert the pin to block it. Knowing we dodged a bullet (the single axle bullet) we changed the tire with the brand new, never used, spare (15 years old).
This puts the spring shackle at an angle from the hanger back toward the back of the trailer. The tires on the trailer probably did not have 1000 miles on them - but they too were 15 years old. This angle should not exceed 45 degrees if installing a spring with an unknown spring hanger spacing distance, from the spring manufacturer.
In the heat of the moment I forgot to take pictures, and I know this will get me in trouble with the webmaster. The spacer blocks are steel and are used to vertically position the plates on the tongue at a spacing equal to the axel length. Once you have the spacing figured out you need to place the two hangers so that proper tongue weight is achieved. A Triad with boat would be about 8’ longer, at least several thousand in additional, larger barn, construction costs on land that I really could not spare. The fender already had a dent in it, (look closely) from a too tight turn I took out of my driveway on my way to get the trailer serviced a couple of weeks earlier. A general rule of thumb is that 60 percent of the total trailer weight should be forward of the center line of the axle or axles on the trailer. Ideally your trailer should have 10 to 14 percent of the gross trailer weight on the hitch ball coupler.Installing a trailer suspension requires precise layout so the trailer will be safe and handle properly without trailer sway. If you are unsure of the process a local trailer shop or welding fabrication shop can help you with installation of the critical parts. You should be able to easily make the trailer and craft under 750Kg (non-braked suspension units and hitches are much cheaper!).
The cushion will collapse when the nose is on the trailer and the back end on the ground and a gap opens up in the middle. Was strong enough but there was no way of lifting the deck from the horizontal position with the 118Kg Scout on it (I had intended using the trailer winch to pull it up).
Second attempt involved removing the Scout air drive and fitting it onto a table type structure over the UH18 lift fan (with the Scout hull still sitting on the top deck above the UH18). Last attempt was to remove the rudders from the UH18 and fit the Scout air drive partially into the UH18 thrust duct. The air drive is mounted on a two legged table - the legs sit on the back edge of the trailer and the front of the table has two horizontal extended arms which fit into the end of two square tubes of the UH18 engine mount frame (and are locked in by simple cross pins). I re-made the top deck so the Scout hull was nearer to the top of the UH18 roof - it is now lifted onto the UH18 thrust duct and the Scout skids fit onto a roller on the front of the top deck - it's then lifted and rolled up onto the deck. The roller trailer was a bit of a disappointment - see the prospector build diary for the result.



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