Designing a fast open deck catamaran

Michlet is a great piece of software - it allows you to estimate the friction and wave resistance of a given hull shape at different speeds and swimming positions.

Even more interesting, however, is its parametric hull generator and optimizer. It creates a (staggering) variety of hull shapes that are determined by a handful of mathematical parameters...

... some of which you specify - e.g. the weight, the length or the width of the waterline, or its position - and you feed these to the optimizer - which varies the remaining parameters until it finds an optimal combination. You can watch live how the hull shape changes.

It is extremely exciting to see what effect changing one parameter has - how it affects other parameters when you want to get an optimum again.

In any case, you get a good feeling how the lines of a fast boat "should" look. Its all about keeping the area of the main station as small as possible - in fact a simple tube has the lowest wave resistance (but massively disturbes the flow and makes a lot of spray) - while keeping the entry- and exit-angels and the curvature soft.

The autor Leo Lazauskas is here in the Forum. I think I read that there is an integration with Freeship

 

If all living space is in the deck pod I wonder how much smaller the hulls can be. There certainly is some minimum size for fore - aft stability. Too small hulls can easily bury the bow and cause beachcat style pitchpole. In trimaran configuration main hull also is providing part of fore - aft stability so it seems pod cat hulls have to be bigger than similar length trimaran amas. Pictures I managed to find of a podcats seem to confirm this.

Since small slim hulls are more wave piercing than wider taller hulls there would be more chance of pod slamming hard. On 15 m performance cats with normal hulls it is generally thought that 0.8 to 1 m bridgedeck clearnce is good. A similar size podcat maybe need more. Pod with all the stuff for living would be quite heavy, weight high up not good. Suppose I need 1.2 m clearance below pod then 2 meters of pod , a 0.2 m clearence between boom and roof results of boom about 3.5 meters above water, while conventional open deck cat boom may be only 2.5 meters above water. While hulls would be smaller with less air drag there would be extra added drag from deck pod. Higher boom cause higher mainsail center of effort, not good for stability. Seems there is fairly delicate balance. Make pod too big and most of the advantages are lost and instead of 2 big main parts there are 3.

Got it, essentially slim hulls would follow waves less and cause less acceleration and twisting on overall structure. Although I'm not sure how simple straight box beams would behave with such wave piercing hulls. Pierce the wave too deep and then BANG box beam slams hard into water. Fast trimarans have fancy streamlined curved crossbeams attaching from above to amas then curving to horizontal position towards main hull. Those are not easy to make.

 

Excellent, that should save a lot of labor. 0.12 m2 is shockingly small.

I'm not sure why the later woods dory designs tend to have that extra chine on the topsides. Maybe it's aesthetics, because it works better with a plumb bow? Or more usable interior space? Or maybe it allows more flare down below? His gypsy, which is older, looks more similar to what you have drawn with the simplicity of a single topside panel.

You might consider the that the angled cabin panel will make boarding difficult on the forward end of the boat, and also consider adding steps in the transom to ease boarding from the rear.

I've also noticed a lot of woods designs have just two cross beams that connect the hulls and a third beam that just takes the load from the mast. Because the mast beam does not need to resist torsion it does not have to pass through the cabin and allows for more space. If you wanted to do that you might need to move the forward beam back a station or two to make the boat stiff enough. Just a guess.

This sounds amazing! I want to check it out.

 

Boarding from standard floating dock would be more from the rear where hull is lower. If I eliminated angled panel it wouldn't make much easier to board because you would still have to climb some 1.3 m up, while stern would be less than meter above dock level. It would also make aerodynamics a bit worse for what already is fairly boxy boat. I'm considering the angled cabin side would be good place for solar panels, it is out of the way of everything else. I have thought about transom steps, but they don't go easily together with beachcat style kick up rudders. Some kind of hinged fold down ladder type steps probably will be simpler to add.

Wouldn't it be structurally better if all beams go through? Space between mast beam bulkheads could hold storage lockers so it is not wasted space. It just require to bend down a bit to pass from galley area to forward cabin space. Maybe this bulkhead could also have waterproof door if that makes sense for lightweight catamaran. If front beam becomes main load bearing beam then it must be heavier and weight in front is bad.

 

I'm entertaining the idea of a podcat and it seems that while hulls can be made smaller with less surface area they still have to be fairly substantial to have enough freeboard and reserve buoyancy. In my case with slimmed down hulls I loose about 25 m2 of surface area per hull, but I gain about the same area from deck pod. Not sure if it is worth it. Boom is higher reducing mainsail size. Overall windage possibly even worse because crossbeams are more exposed and pod is sitting high up. Lowering the pod would encourage slamming especially with slimmer hulls that would cut more into waves. My original design allows to have large work table in non living hull. Living hull is house which can be kept clean, other is storage and workshop where stuff can be taken apart, fixed and don't have to worry too much if something smelly spills. Podcat arrangement will loose that.

It seems that to really gain from podcat arrangement I would have to have even more slimmed down hulls and smaller pod which would again make living area cramped.

 

Every basic concept has its own strengths and weaknesses and challenges that need to be solved. ;-)

Otherwise - assuming the same bridge height and the same speed - it doesn't matter which concept you use to tackle a 2m wave. The height that the hull has to climb so that the wave doesn't crash into the bridge - and the time to reach it - remain the same. The bow volume must of course be "large enough" for this.

But where will the movements be more comfortable to bear? And the curved beams of racing cats - are they really that high - or aren't the hulls simply lower? (OK, they are high too, it's all a question of dimensions ;-)

You are of course right about the windage of the pod, as well as the higher sail pressure point, and of course about personal preferences.

Here is the download, attached to the first post in the thread:
Michlet 9.33 Released https://www.boatdesign.net/threads/michlet-9-33-released.52865/

 

Take a look at the CW Atlantic 42 Cat design and check out how the “pod” is aft not centrally positioned.

Your lounging area is just in front of the pod instead of behind, and then you have a big net out to the bows.

I imagine keeping all that weight aft not only helps to keep the bows up, but also creates less slapping perhaps?

There was one moored next to me most of last Summer. At first I found the layout odd, but then saw the advantages, including while at rest on anchor.

 

"I imagine keeping all that weight aft not only helps to keep the bows up"

---

Bow Up

For me this is the big big Big question because the bad behavior and even the capsizing of monohulls and catamarans begins with the bow sinking, so

1) hydrostatic resistance of the bow to sinking

2) good longitudinal balance: reconciled centers (CF, CB, CG) and back/aft (60% LWL)

3) hydrodynamic hull trim: High (H) pressure in the bow (hydroDynamic Lift) and Low (L) pressure in the stern, suction.

That is: do not pay any attention to models obsessed with resistance because the big question is not winning a regatta or a rowing competition or whether a large ship uses less fuel, no, the big question is a good Attitude of the Hull for Comfort and Safety

(bow to the left):

 

...

Optimising Hull Lines for Performance https://www.graingerdesigns.net/the-lab-library-02/optimising-hull-lines-for-performance/

The funny thing is that I agree with the drawing of this catamaran and, however, I don't understand the text

 

I completely agree with this for example:

"Pitching is slow. It destroys the airflow in your sails and the flow around the hulls, and your performance is suffering from slamming loads.

The single most effective way to counter pitching is with asymmetry in the water planes. You can achieve that in the with a fine bow and broad transom. Or you can achieve it with V sections forward and a flattened U shape aft. Or you can achieve it in the profile view with a very straight run forward and a bump in the aft sections. A flatter rocker line is better for resisting pitching than an evenly curved one with deeper draft in the middle.

The final result is a combination of all three of these factors.

On a cat like Design 256 the weight is concentrated well aft so we need to get buoyancy well aft."

But the curious thing is on the one hand, as always, everything must be justified in terms of greater speed and, on the other hand, the four key pieces of Dynamic behavior in water/Hydro at speeds greater than 0.4-0.5 Froude are absent in these reflections: the lateral and vertical component of the High pressure at the bow, and the lateral and vertical component of the Low pressure at the stern

 

Bridgedeck-slamming: motor cats often use a third mini hull - shorter than the main hulls, above the water in normal conditions, sharp v-shaped bow with steps like yuo see them in fast speedboats. They argue the wave must be broken, bevore she reaches the bridgedeck.

Derek Kelsall used a similar concept in his latest sailing cats - for the racers as well as for the cruisers - the forestay attached to the bow of this mini hull, without needing a front beam. He said this arrangement can better hold the strong forestay tension, but i think this is also about wave breaking. In fact they are tris with a degenerated main hull - shorter han the others and well above the water

 

Yeah, it's kinda a bit of a conundrum balancing all aspects. What I like the most about podcat is 360 degree visibility from cabin and you can keep watch while inside in a heated space if weather is poor. However I loose full headroom space in the hulls. No big work table with full headroom and all the tools within reach. Smaller mainsail too. Question arise if I have to have fairly substantial hulls for seaworthiness anyway then why not make them a bit bigger so I can use them for living and don't have to build extra part - deck pod.

I think curved beams are the key here, they get good clearence while hulls remain fairly low. However they are hard to make. I follow S/V Lynx build and they struggled a lot with making a curved front beam, probably hundreds of hours of work for what in other comparable designs are straight aluminum pipe.

Interesting arrangement although I would prefer a deck behind the pod and pod being easy to walk around. Open that big front door at wrong moment and receive a big splash straight in the cabin. If some kind of jam develops at the rear end of the boom then sorting it would be quite precarious while sitting on the rear edge of the pod. Integrating tiller steering probably not possible with this layout.

I wonder how much lower it would allow the bridgedeck bottom to be. It seems with this design slap would be more often but less severe although probably still quite loud inside. One of the advantages of deck pod high up is it would be fairly quiet there being isolated from the hulls and noise they make allowing better sleep. Fast boats are loud no matter sail or motor driven.

 

If your building a boat to eat miles then you might want to look at factors like sea-kindliness and short handed management, high speed cruising will beat you down with fatigue without a large crew, big boats need big rigs and big rigs hurt people, not just by accident but by fatigue and attrition. I'm thinking your wish list needs a reality check because you might end up building an unwieldy monster.

 

I'm kinda thinking about it. If going just by dimensions 15 x 8.5 meter cat is fairly big. However since I plan to essentially have a 15 m boat, but with weight and cabin space of your average 11 - 12 m mass production cat I should be able to get away with smaller and less loaded rig than typical production 15 m cat that may weigh 20 tons. Plenty of boats both mono and multi in this size range sailed by couples and solo so if rig is set up right it shouldn't be a big problem. Wendy Globe sailors manage to race their far more demanding boats around the world non stop. I will probably have around 1/3 sail area of IMOCA.
I definitely want a fast boat like my existing 9,5 m catamaran. Since I also want reasonable cruising payload in 2 - 3 ton range easiest way to achieve it is having long hulls that give desired performance and load capability. As a big bonus I get enough room for everything living related in one hull which simplify internal fitout. If I cared only about cabin space I could get all the space I need in a 10 - 11 m monohull.
Sailing fast is noisy and more fatigue causing on a long trip, but it is always possible to slow down for the night and put pedal to the metal again next day.

 

Sure, but if you think about it the forward crossbeam will still be taking most of the load. When you design the crossbeams you calculate for stiffness, not strength. With 3 evenly spaced beams and a torsional load (eg opposite corners of the cat on blocks, and the other two corners floating) the outer beams are taking the bending load and the middle beam is in torsion. So the middle beam isn't contributing much to the strength of the boat, especially on a pound for pound basis.

Also, when the beams are calculated it is done as a cantilever, so attaching to the wider point of the hull reduces the load and allows for a lighter beam.

You can see if the beams are too far from the ends the forces are higher and the stress is higher. And if the beams are too close to the ends the hull is narrower at the attachment point and the stress is higher. So there is an optimal placement that minimizes stress on the beams: close to the ends, but not too close. Again, you can have the mast beam double as a crossbeam, but it's not needed.

Regarding Rolf's conjecture that skinny trimaran hulls are better for avoiding capsize, I'm not convinced. In your case the length of the boat, the displacement and the Cp are essentially fixed (you do want a high Cp like the 0.65 you designed into the first hull), so the volume in the ends below the waterline is also fixed. Above the waterline you can have more or less flare and more or less freeboard. I don't see how less freeboard helps because once the hull submarines you're going to have high drag from the beams and the added wetted surface of the deck. If you watch videos of hobie cats pitchpoling, see here at 1:20:



you can see as soon as the hull submarines it catches and the boat starts decelerating and rotating. Also, from a pure physics standpoint having more volume in the corners of the boat means it will float higher as it goes over diagonally (as multihulls typically do) and will require more work/energy to flip. A floating beach ball rotates easily because the buoyancy is concentrated in the center, but a donut does not because the buoyancy is distributed about the perimeter. I believe a catamaran is more like the donut - so more stable, where a trimaran is more like the ball with more buoyancy in the center, so less stable.

These are just ideas I have, I'm certainly not an authority on buoyancy/stability or structures so take them with a grain of salt.