Goran Marstrom and Kare Ljung the designers behind the M5
Please open the page to get the inside story and ideas behind the Marstrom M5 A-Cat design from the Marstrom design team, enjoy the read and some nice wallpapers.
Marstrom Catamarans better performance, longer life
The M5 design from the designer’s point of view
Goran Marstrom and Kare Ljung are the designers behind the latest Marstrom A-cat platform the M5 and with their huge experience to design and build superfast sailing catamarans it’s very interesting to get the designers point view behind the M5 design. Goran is as well still doing excellent results in the A-cat racing scene, in the world championships his always in top of the master class and also with very good overall results.
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The M5 has no wave piercing bow, what is the reason for that?
Our own full scale tests shows that "wave piercing" bows gives more drag than our normal bow for a certain tip moment and the lack of reserve displacement at the bow makes "wave piercing" boats much easier to tip over. Another disadvantage with "wave piercing" bows is that the more narrow deck makes the hulls much less stiff and contributes to a less rigid platform. The advantage of C bows is less surface in the hull so you can save some weight but at the cost of a less stiff hull. We at Marstrom think that a Catamaran platform shall be as rigid as possible and does not believe in the old school idea that a soft platform can be good because the hulls can move with the waves.
The stiffer the platform the easier it is to control the boat and sails equals better speed. One thing people pro "wave piercing" bows claims is that the wind drag is lower. The relative wind angle close to the water is very small so the so the difference in wind drag between a normal and "wave piercing" bow is extremely small and should not affect the performance We have notice that several of our competitors new designs are not really "wave piercing" but in essentially normal bows that has been reshaped close to the deck to look like a "wave piercing" bow. We at Marstrom think "wave piercing" bows are just a fashion and we can already see tendencies that is on its way out.
The bottom shape as well as the scoop and rocker line is quite flat on the M5, what kind of effect will it give and what is the idea behind that?
Our philosophy is to design semi planing hulls with rather fully end ships that float more above the water than actually trying to cut it. The trick is to find the right amount of rocker line. Too deep and the hull will be slow in stronger winds and too flat the boat will be slow in lighter winds. The traditional catamaran hulls are designed on the idea of "cutting" the water and the simplified theory that a prismatic coefficient of 0,5 gives the lowest wave drag. Must traditional hulls have a Cp of 0,53 to 0,57.
Prismatic coefficient or Cp = A number to describe volume distribution of a hull. Higher numbers equals more volume in bow and stern, a square box or a cylinder has Cp of 1,0 and hull made out of two cylindrical cones has a Cp of 0,33
Full size tests and tests in ship tanks for the last 70 years has shown that the prismatic coefficient must be much higher than the normal 0,53 to 0,57 to get the minimum drag. This is mostly due to drag contributions from skin friction. Other things like spray, three dimensional and surface effects also come into play.
You say that the M5 is semi planing, why not planing hulls and what does semi planing mean.
Planing boats creates enough lift at high speed to get on top of the water. Displacement boats have hulls that create negative lift with speed and actually float deeper with speed. Semi planing is a term used for boats that creates none or just a small amount of lift. Creating enough lift to plane from a narrow hull cost a lot of drag and will slow down the boat in most sailing conditions but the top speed should be higher. That's why we design semi planing catamarans with just a small amount of lift.
The first edition of the M5 had the straight daggerboards but then changed to the Marstrom designed asymmetrical curved foils. How does the curved foils works compared with the straight boards?
The first obvious effect of the curved boards is the created lift they give. Creating lift with the daggerboards cost much less drag than creating lift from a planing hull. Less obvious effects are that you can get up on one hull earlier and that it's easier to keep the boat sailing on one hull.
It seems that the asymmetrical curved foils shape is very sensitive and it’s easy to fail with the design. How sensitive do you think the shape is for the curved foils compared with straight daggerboards?
Yes the shape of the foil is very critical. You can totally destroy the performance of the daggerboard with a bad paint job or a small damage at the leading edge or even some dirt on the foil. The shape of the foils is so sensitive that it's very hard make a copy of a daggerboard with the same performance. If you have enough knowledge to make a good copy then you have enough knowledge to make your own design.
There are some problems to consider when it comes to designing curved foils. Sometimes the boards will create side forces working against to each other and in other conditions you will have a negative lift at the root and positive lift at the tip of the foil. Both these examples cost drag and one has to design the curved foils to minimize the drag from these two cases. Our solution has been to develop a slightly asymmetrical profile and then experiment with the angles of the foils. We load the tip of our curved board more than on a straight one and we load the leeward board more than the upwind board, all this to find a good balance between lift and drag.
There is a lot discussion where to place the daggerboards and the mast, some designs move everything further back. How far do you think it’s possible to go and still get a good balance in the platform?
The most important thing is to find a good balance in the boat. If you have a boat that have a tendency to dive then you have to move back the mast and daggerboards. If you move the daggerboards backwards then you will unload the rudders, but if you move them to far back then it will become difficult to tack with the boat, control it at the starting line and sometimes it can even become hard to steer the boat in a straight line. Our hulls has another volume contribution than most of our competitors so our boards are placed a little bit further fwd than most other Acats. Actually the curved boards are placed a few cm further fwd than our straight boards.
The M5 is built with epoxy prepreg carbonfibre and autoclaved baked with overpressure and heat, i.e. state of the art technology similar as used to Formula 1 cars and the aircraft industry. Carbonfibre is a fantastic material but how important is it to use the right production technology in order to bring the outmost from the carbon fiber properties?
The production technology is defiantly important for the result. Producing parts in with prepreg materials placed in vacuum bags, a pressurized oven and cured in high temperature gives normally a huge improvement in quality compared to cure the laminate without he pressurized oven. Both the temperature and the pressure helps in minimize the number and the size of imperfections in the laminate.
How does this effect the life time value?
This makes the product last much longer. It's no secret that the Marstrom boats have a long life expectancy.
The beams on the M5 are bolt on to manage easy assembling and transportation. How is this possible without affecting the overall stiffness?
If it's done correctly then a bolted beam/hull connection can be very stiff and the weight penalty compare to glued joint is small.