CNC Plans not Included

This is nice photo of my friends bridgedeck floor showing the stiffeners.

 

Ok, that sounds good.

I will figure on a 4" core for a clearspan deck for now.

That is going to make a drastic difference in a lot of things but mostly positive I think especially on the construction side.

 

photo isn't showing?

 

The norm would be 1" on a cat thia size jorge 4" h80 pvc foam core will cost you a bomb!

 

1" and no beams, wow, hard to believe! I think the span with the skinny hulls is something like 16 feet! Anyway I played with lowering the deck and it has it's own caveats.

The profile def gets sleek but I think the sheer should be lowered too. I might have a few inches of play there but there is no play in the bridgedeck clearance or headroom of the bunks. But it's something to think about because if you do away with the aft bath and leave the bunk in the hull that would clean things up topside.







Here is the old version with the modified salon. Not sure which I like best lol.

 

I agree that your design is looking much better now - the deep void you had below the bridgdeck was an obvious waste of space and resulted in unnecessarily high superstructure. I agree with your own sugestion that you should now consider some reduction in freeboard, if you can manage that you would save some weight and windage and I think improve the aesthetics, although that is of course subjective. But dont compromise the clearance under the bridge deck. I think there is at least one thread on this forum discussing bridge deck clearance.

Looking at your latest sketches I think you are in danger of not having sufficient torsional strength and torsional stiffness in your structure. The structure of a multihull needs torsional strength and stiffness to cater for situations such as the lee bow 'digging in' causing much of the total weight of the craft to be briefly supported at one corner of what is basically a rectangulr structure.

There seem to be two basic approaches to providing torsional strength and stiffness in a mulithull structure. The first approach, normal in cruising boats, is to have a single torsionally strong box like cabin structure connecting the hulls. The second approach, normal in lightweight racing craft, is to connect the hulls with fore and aft cross beams that are have relatively little torsional stiffness but which are stiff and strong enough in bending to transfer torsional loading onto the hulls which are capable of carrying that loading since they are much larger in section than the cross beams. Many multihulls do of course have more than two cross beams but in such cases you will usually find that only two of the cross beams provide the torsional strength and stiffness of the craft as a whole.

The cabin structure that you have sketched does not seem to offer much torsional strength or stiffness - being open at the rear and having almost continuous windows it is like a shoe box without a lid as opposed to a shoe box that has had the lid glued in place!

Since you have chosen not to have a torsionally strong and stiff cabin structure I think you need fore and aft cross beams. The front end of your bridge deck structure will act as a forward cross beam, but where is the aft one?! I suspect you have left that out because you fancy having a flat unobstructed deck right back aft. I suggest you need to make some compromise there by including an aft cross beam, but you may not like to have to step over it to access your tender which I suspect you will want to hang in davits aft. Actually, the bending moment on a multihull aft cross beam is minimal at the centre of the beam so there is an option to have a beam that tapers down in height towards the craft centreline, giving fairly minimal obstuction to strolling aft along the bridge deck - just a thought.

 

Yep i also noticed the lack of a rear beam and figured Jorge simply had not got around to drawing it yet

Indeed you will need some structure back there - which also needs to accomodate the mainsail sheeting arrangement and davits etc - this is why so many catamarans have such a similar rear box beam arrangement that you see on most designs.

Also - as to the bridgedeck floor - the stiffness of the floor is important as a flexible deck feels completely wrong to walk on even if the floor itself is strong enough. Thus the design is deflection driven. For a floor in most buildings/homes designers typically design to a deflection of 3% to keep the floor from feeling too flexible. Id say continue this approach to a floor in boats. This is why i added the triangle beam under my bridgedeck - its was never part of the original design. Once i walked on the floor it simply didnt feel stiff enough, so then i designed the beam which i added later to correct the problem. I designed the beam to a 3% deflection limit based on a 100kg load (person). You can work this out by taking the span/360 = deflection limit. For mine IIRC it was approx 6mm deflection in the center of the unsupported span with 100kg load. It worked very nicely and the floor felt great afterwards - still a touch springy but not feeling wrong or cheap...

But at this stage of your design, simply know that unsupported spans need to be kept reasonable. You can use those furniture box items shown just after of the galley, also the fore/aft aligned bench in your galley. Basically you wont have a problem in the forward part of your floor, however as youve drawn it now - you have large unsupported spans in the aft part of the floor. For my boat, I added another transverse bulkhead which delineated the "inside" from the "outside" of the saloon area. It was mostly all window and door openings but the ring structure gave me support for the roof and also for the floor. It also acts as a water barrier from rain runoff and heaven forbid "green water" that finds its way onto the aft deck and stops it running inside the main saloon/ galley area. You have to step over it to get into the saloon but it has kept lots of water which would have otherwise flooded my interior on occasion!

This brings up another point - your roof structure. Its huge and will be difficult to get stiffness without using excessively thick core material which is very expensive. You need to get more support somehow or make it compound curved - which adds stiffness but also adds alot of work. You can provide stringers on the top of it which also double as solar panel mounting points and things like that, but even so simply supporting it from 4 corners is gonna be a big ask...

 

Gropers comments around the roof are spot on too, stringers can be added externally especially coinciding in way of genoa tracks as these take some load that needs to be delt with also- that these can add stiffness & not cut into headroom a bonus, window mullions can extend into the roof structure a little as well, sometime a structural "Pelmet" can break the span and act for conduiting wiring for lights and light fittings in a neat way, you see this on flybridge cruisers/gameboats as well. Wiring runs need to be planned in advance, it gives me the heebie geebies to see nice laid composite fibers cut for rough access later.
The typical bridgedeck connective structure of 3 to 4 full width bulkheads has evolved over decades, the work Lock Crowther drew into his designs has been much copied and evolved to simplify the use of uni fiber to upper and lower tabbing of connective bulkheads. It's worth looking back over Gropers build history to check how he detailed it as well.
Jeff.

 

Thanks for the observations...

Yes on the aft beam. I drew it in a pic in the previous page, just not on these new pics as I wasn't liking the deep cockpit and stopped.

I can drop the sheer by 12" by dropping the sole a bit and cutting headroom. That will get rid of the deep cockpit effect and get me at least 48" of BD clearance but ...

That throws off the interior making beds way high and still leaves me with the really unsightly raised areas in the cockpit. What I would like to do is recess BD, just around the mattresses, by about 12". I can taper it so it's not a hard edge.

I heard slamming is mostly caused by waves meeting in the middle of both hulls. If that is the case it seems I would be fine as the mattresses only infringe under BD by about 3.5 feet and at that point BD clearance would be around 36". If I can do that then those raised areas are back to seat height and I think this boat is really close to being perfect for my intended use and aesthetic.

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Yes the roof is another story and will require engineering, I am just going for the look right now and if I have to use 1.5 inch core on that fine too.

My pipe dream is to have something like this - https://youtu.be/u0AH_MDgBNc ... Not the overlapping shingles but the smooth glass tiles. They are becoming lighter and cheaper so I have my fingers crossed.

PV glass is another thing I have my eye on. It's heavy because it's structural but I am going to ask if they can substitute lightweight acrylic since my roof will be the supporting structure ... I don't want the traditional rack install system, very ugly and heavy when you have 40-50 panels.

 

Here is what I am talking about dropping the BD in aft cabin for mattress. I would taper it forward.

Then that area can be used for seating instead of being mostly wasted. It also gets me easier entry into the bed.

I think my sheer will be reduced by about a foot in the middle of boat.

As for the BD ... I think I would just do a double sandwhich with stringers between. The bottom surface and stringers or stiffners infused with both hulls in one shot and the top surface infused on a table and glued to the stringers or stiffners later -no fairing either side ))

That lets me drop roof and boom 12-18" ... I like that!! and should give me a very rigid surface and if I place stringers cross wise to length, it makes chase channels all the way down the boat.

 

Yep your BD clearnace will be fine like that jorge. The main area you dont want to compromise the BD clearance is up forward. You see what you have drawn on many other cats also- again for the aft berth arrangement.

Your BD detail looks fine and neat also, for all the reasons you mention. The only downside is its more expensive compared to to the standard method of using a single panel with bonded on stiffeners... the floor area is quite large and thats alot of foam core sheet - doing it twice adds thousands of dollars...

 

sneak preview Tons of improvement from many perspectives I think.

 

With your bunk steps it's typical to run the steps for the full length of the underwing with them transitioning/tapering out forward, down aft it seems to ease the transition from the connective bulkhead into the hulls, the longtitudinal aspect of the step adds to stiffening the span also.

Jeff

 

As long as it doesn't slam that would be ideal, I could lower the fore bunk too. I am totally loving that I don't need a step on that aft bunk!