magnum 21

OPEN COCKPIT TRIMARANS - Revolutionary family day boats

  Magnum 21
Family day boat -
par excellence

Where is the Magnum 21 manufactured?

All VirusBoats boats are manufactured in Brittany in France. The designer and founder of the VirusBoats company, the late NoŽl Louvet, started this company in 1992 to make rowing boats, having already established his reputation for innovation in the design & manufacture of windsurfboards and then catamarans. Ahoy-Boats is the sole importer to the UK and we also sell VirusBoats around the world.

How is it made unsinkable?

Main hull and floats are unsinkable with at least 300 litres of closed-cell foam in each float and 200 litres in the main hull. In submersiblity tests the Magnum 21 has been completely filled with as much water as would go into it and it will not sink.

What is the construction material?

Construction material is reinforced fibreglass and high quality polyester resin. The outer surface of the main hull and floats is gelcoat isophtalic resin rather than painted so they are very tough and scratch resistant. There is no delamination risk and none of osmosis either.

Why are the bows of each float not level with the bow of the principle hull?

Good question to which there is an important answer. The fact that the floats are set back from the main hull makes the boat handle better and makes it safer. It is becasue they are set back that the boat behaves more like a monohull when turning. It is not necessary to use the jib to turn the boat as it often is with a catamaran. The boat turns around the centreboard really nimbly. It is safer because the positioning and shape of the floats drastically reduces the likelyhood of pitchpoling. Floats are set back from the bow of the main hull.

Can it be capsized or pitchpoled?

Well only three or four have been capsized so far and that is out of 450 boats sold and 7 years of sailing them so that is a pretty good safety record. Most of the people who capsized were not showing enough respect for the elements. One of them was sending a text message to his wife. And they all, without exception, had the main sheet cleated off when they should have been holding it in the hand so that they could let go in a gust if necessary. If you cannot hold the main sheet in the hand then you probably have too much canvas up. Interestingly the factory demonstrator has never felt the need to take a reef. And yet even in weather in which you would not normally go out sailing he felt no risk of capsize or pitchpoling.

Nobody has ever pitch-poled a Magnum 21. There is a challenge for somebody!

Having the floats set back from the bow of the principle hull means that the substantial volume of the main hull will enter a wave first of all and lift the bow up before the leeward float enters the wave. This is quite different to the situation in a catamaran where the leeward hull can dig into a wave sending the catamaran head over heels (pitch-poling).

Can it be righted if is does become capsized?

This is good question. If it is virtually impossible to turn upside down because of the volume and buoyancy of the floats then it must surely be equally difficult to get it back the right way up.

Fortunately there is a solution. All you have to do is to remove one of the floats. This can be done by hand. So first make sure that you put a line through the eye nuts so that you do not lose them when your remove them. Release the main and jib sheets and then once the float is removed you can stand on the wide gunwale on the side of the boat from which you have removed the float and hang your weight off the centreboard and wait until she comes up. It is also possible to use a motor launch to right the boat by towing.

Are the amas for the Magnum 21 the same as for the VirusPlus?

Amas (floats) are completly different from VirusPLUS ones. Much longer and bigger with a volume of 350 litres against 130 litres. The amas for the Magnum 18 are different again, having a volume of 200 litres.

Would the amas (floats) be large enough to form a Kataram for two?

There is new kataram with the floats of the Magnum 18. It is better than the old kataram because they are longer but it is only a single.

Is there access to the inside of the amas?

Yes. There is a bung at the stern only so thay may be aired. I never touch it. The amas are completely filled with flotation. They cannot be used for stowage but you have room enough in the main hullís lockers (more than 1m3). Actually the forward cuddy is 2m long.

How long does it take to rig or derig?

1/2 hour for one person. Less than 20 minutes for two. It has been made ready for the water in 10 minutes by two experts! Obviously it will take you longer the first time but you quickly get the hang of it. If you are going to leave the mast up and put the boat in a boat park then removal of the floats and trampolines only takes a total of ten minutes. Realistically from the water to trail-ready only takes 1/2 hour and vice-versa.


The Magnum 21 can be rigged and sailed single handedly with ease. Launching into and retreival from the water in still conditions can be achieved single-handed also. However, if there is much wind or stream or waves then the help of one other person will almost certainly be required.

The floats can easily be lifted one end at a time from or to the ground if the beams that normally support them are put (left) in place. This is only necessary in pre-2006 Magnum trimarans.

With support arms in place it is easy for one person to lift the floats on and off the boat, one end at a time.

Up to 2005 the floats have been tied on top of the boat for transport. From 2006 there is a new system that halves the time taken to assemble the boat. But for those with boats built before 2006 a Dutch customer living in Finland has kindly shown how he attaches the floats single handedly without the necessity to lift the floats bodily nor even to have them touch the ground.

Piece of rope Start at the rear Hang rear end at fixing point
Fix front end Rear hangs in line Fix rear
No ground touching

How about raising the mast?

The mast can be hauled into position by a single person using the purchase of the main sheet blocks provided that lateral movement is prevented by the use of temporary short shrouds attached to the rings that hold the floats on (see below). But even this is unecessary unless you have strong winds. I now lift the mast up in one movement on my own. I have the spinnaker halyard already attached to the bow and once the shrouds are tight I pull on the mast-foot-end of the halyard until it is tight. Then the mast is secure and I can attach the forestay via the jib furler at my leisure.
Mast-detail Stepping the mast Easy peasy lemon squeezy!
During mast erection or lowering a couple of short temporary shrouds will help stabilise the mast in windy conditions or when you are doing the job alone.
Temporary shrouds help stabilise mast when erecting it single handed. Temporary shrouds are best fixed to float clamps. Temporary shrouds remain taught when mast is lowered.
Note the use of the main sheet attached to the ring at the bow intended for the gennaker. This gives plenty of mechanical advantage and makes the job of raising or lowering the mast really easy.


The modern trailer, licensed for Europe, is made of galvanized steel. The capacity is 350kgs with a launching system that is extremely easy to operate. It prevents the rear wheels of your towing vehicle from getting wet. In fact even the trailer wheels do not need to be immersed. Although the wheels are water proof and immersing them will not invalidate the trailer warrantee it is always best to avoid wetting the hubs if you can.

The hull is supported by two dozen rollers so you can leave your pride and joy (MAGNUM 21) on it without worrying about the hull becoming deformed.

The all up towing weight is 499kg.
Trailer Launching on a slipway
Launching Winching
On it's trailer the MAGNUM is exactly 6.5m long and 1.65m wide (less than the distance between the wing mirrors of a Ford Focus). It is actually only necessary to tie it with straps if the mast is in place (vertical).
Magnum 21 at St. Cado's island Less than the width of a Ford Focus!
transport is easy The Magnum 21 can even be towed behind a small car such as the Toyota Yaris but a braked trailer is required for vehicles under 1300kg. Trailer sailing has never been so easy.

Performance with outboard motor?

The motor mount at the stern is simple and works perfectly. It is strong enough to support the engine during road transport and of course for storage. Engine mount now fitted as standard
We have achieved 10 to 12 knots with a 9.8 hp engine with three people on board! You could expect 7.5knots knots with 5 hp four stroke engine. Short shaft engines are best. Motoring with a 9.8HP outboard
you can do 10 knots!

Performance under sail?

The MAGNUM 21 is not a sport catamaran but is, for sure, much faster than a Windrider 17. You will remain completely dry with wind of force 4. We've been sailing at more than 15 knots without any problem during a recent test in just a breeze and speeds of 22 knots have been achieved in a force 7 without the need for reefing the main sail. The boat sails well close to the wind and turns easily. The tiller is particularly light in the hand making the boat easy to handle. Any weather helm in strong winds can be minimised by using the traveller on the transome. This enables you to tighten the main sheet in a downward direction thereby putting more tension in the leach of the main sail. Provision for a cunningham is also provided so that you can add tension to the luff of the main.

Details of the sail plan?

All the MAGNUM 21 sails are made in France by a French sailmaker but the MAGNUM 21S sails are made in Greece or France. The main sails are fully battened, 14 m2, and the jib is 6 m2. The furling jib is standard. The standard mainsail has 1 reef point and is a fast system. You can order an optional 2nd reef point.

The material is the same dacron as sport catamarans. No winches are required.

Light winds?

The boat has been designed with a good safety margin so for light winds we offer a 24m2 asymetric spinnaker, for which you will require the retractable bowsprit (included in the spinnaker kit), and a 16m2 furling gennaker.
gennaker Light winds rig Spinaker

Heavy weather performance?

Thereís absolutely no risk of pitchpoling in heavy weather. Weíve tested this boat up to 30 knots of wind (without reef) with 3 people on board, without any kind of problem.

The cockpit remains completely dry up to wind force 5 and 1 metre wave height.
22 knots in a force 7

In fact we have a customer who has sailed a classic Magnum 21 in a force 8, gusting 9, and achieved speeds over 25 knots. He has regularly achieved more than 20 knots with 6 people on board. But this is an experienced multihull sailor who knows exactly where on his lake to get the best conditions for achieving these sort of speeds. It is not for the feint hearted.

How high is the boom?

The bottom of the boom is 32 inches above the cockpit seats so there is plenty of room and you will not have to duck too much during tacking.

What about a boom tent?

There was a cockpit tent designed that covered the cockpit and some of the trampolines area (more than 8m2). This tent was high enough under the boom to allow 6 people to sit and eat in the cockpit. As both the cockpit floor and the trampolines would remain dry this would also make a very comfortable sleeping arrangement. However, it was expensive and very heavy and bulky and a decision had to be made whether to take it along or not when going sailing. A better alternative is to buy small, lightweight, geodesic tents for the trampolines.

What colours are available?

The regular colour for hulls and floats is white, and the sail colour is white. This used to be with a blue or red or yellow flash near the top opf the main sail. Now though the sails have a more modern cut. They have no flash and a squarer head with more roach and a diagonal top batten.

Other hull colours are available as an option at extra cost. Standard alternatives are Yellow (very visible at sea), Ferrari Red (smart racey colour), and Navy Blue (classic up market colour). From 2004 there has been a new brighter Mediterranean Blue instead of the navy. If you choose a colour other than white then you can still order the deck to be in white or grey, which is good for reducing glare. In fact you can have any colour scheme you like within reason. However, bear in mind that white is an easy colour to maintain. If you choose an odd colour then it might prove difficult to sell your boat later and it might be difficult to match the gelcoat in the repair of a damaged section.
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