efficient 10m displacement powercat

This boat has a claimed lightship displacement around 3 tonnes, max displacement is 5 tonnes, and its fitted with a pair of 100hp yamahas... gets along quite well too;

 

But surely that's an open deck boat with no interior??

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

What is the Supercat 38's WOT speed with the 100 obs?

 

They claim WOT speed is 27kts...

Richard, yep theres bugger all accom in the hulls, couple single bunks and the like.. Again, its a fishing boat, not a liveaboard cruiser... I plan to keep my boat along the same lines -very minimalistic, nothing but bunks and a very small bathroom (which will never get used) to keep the admiral happy. The Admiral is a land lubber and always will be... My design does not have an enclosed saloon either, its open to the back deck... i considered the clear vinyl curtains idea with no real cabin structure, but the spray is always present therefore the side/front curtains always remain down so might aswell have proper windows that can be opened all done in lightweight composite. Id hate to think what all that stainless flybridge structure weighs, not to mention the cost...

 

Here's a pretty impressive vessel, bigger than the subject of this thread, but it handles chop like it's not there. There is a lines drawing of it on the website, and it has a deeply immersed transom. It is running a fair bit of power, presumably that could be scaled back somewhat.
http://www.youtube.com/watch?v=wGh2rML-41Q&feature=related

 

Groper - not sure if you missed this last post from Willallison.... Like you I suspect, I'm here as an interested amateur. My experience is that even though you might not like what you are hearing from a number of these guys, pretty much everything they tell you will prove to be correct (in the case of the pro's that is....)
Oh - and I rather like glass cubes too!

 

I agree those weights imply careful design and build

But 6mm lexan would be strong enough for the windows, so say 7kg/sqm

If you use 10mm foam you are probably OK with 40kg/m3 density and you'd end up with roughly a 13mm laminate thickness total (approx 1.5mm skin each side sounds right). I'd use more on the bridgedeck bottom though and you'll find the decks will flex (disconcerting to some, even if strong enough)

But there is no margin for error and most people would only use a boat built to those scantlings inshore in flat water.

Remember, scantlings are usually higher than theoretical to allow for building errors, operator errors and to extend structural life

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

From calculations to ISO12216 6mm is not enough for such big windows. Actually it gives thickness few percent higher than PMMA, because also stiffness is considered.

 

I've not run numbers to check the validity of any of the stated "scantlings"... I was just pointing out that in at least one area (the glazing) the given weights are out by 50%! It's not meant as a cheap shot at groper, I'm merely trying to reinforce the need for using realistic numbers.
But Alik is quite right... 6mm would certainly not be thick enough for the fwd facing windows.

As to the foam, again, I've not run numbers... but 10mm 40kg/m^3 foam with 2 x 1100 gsm skins (approx 3mm total thickness) certainly sounds a bit light on to me, unless you plan on having a great deal of closely spaced interior support structure and even then you face problems with the potential for puncture. This is intended to be a fishing boat. That implies offshore use and a fairly tough life to me.... I would prefer to be a little more conservative with the scantlings and be confident that that boat I was heading out in is likley to bring me home again....

 

1. We use 40kg/m3 foam for only for furniture. Likely it will not comply for sides and bottom.

2. 2x1100g/m2 skins? There is min laminate weight criteria for outside skin, it will not comply. Just for reference - for Lloys SSC rules min bottom outside skin is 4.5mm thick; similar numbers for DNV HSC. For ISO12215-5 it would be lighter but not just 1100g/m2.

 

Ok guys i need to clarify some things... firstly about the panel weights in the table i posted earlier and regarding window thicknesses etc;

The way the software calculates the weights is by the thickness, density and area of the surfaces so drawn... however, when dealing with composite panels which can have different laminates applied to the core, and different core thicknesses, the density of the panel is always changing... therefore i cannot simply use the true panel thickness and density without recalculating the density everytime im using a different laminate. So, what i do in some instances, like with the windows etc, is simply change the density until i acheive the desired weight (which i calculate longhand by adding the resin/glass/foam weights per m2 of laminate) and leave the thickness alone or visa versa... So the thicknesses you see in the table are not correct and neither are some of the densities, but the panel weights should be close - provided i havnt made a mistake in my longhand calculations somewhere which is possible and is something i need to check.

There is no way im using 10mm lexan for the windows btw, way too heavy...im going to try 6mm... and there is no way the window lexan will be the size of the surfaces as shown, the window surface as shown is actually a composite panel including the windows within them, which has to hold the roof up etc... so imagine a frame around the windows dividing them up into smaller peices (which will also make them stiffer)

I dont have the tools nor expertise to model the boat 100% accurately, and i do realize the boat will be heavier than whats shown in the table i posted earlier, much heavier in fact, but i feel there is enough headroom with the designed displacement of 3500kgs. It will not be built to survey standard - its for private use only. ill get to the scantlings later in the thread...

This boat shown earlier in the thread, was just launched at around 2500kgs; it doesnt have as much beam as mine, length is 10m, built on balsa core not foam i beleive...

 

Theres no way im using 40kg/m3 foam for the hull... if the original post was read correctly, its to be used in internal furniture construction and glass laminated stiffeners/stringers....

 

Not the software we use. Those ones are using weight of glass fabric, resin content, actual sore, etc. After all, we add some overweight margin to cover resin penetrated in core, some reinforcements, etc.

Starting from l/b>2 only the width b effects the thickness of plate. So would You split the windows vertically also?

It might be substandard design or build, or launched without some equipment, or marketing numbers, or wrong measurement... Just make real table of weights for Your boat, no need to guess.

FYI: mass of structure ms for powercats, kg:

ms=e*L*B*D

where L, B, D are hull measurements, m;
e=28…34 kg/m3 for planing catamarans in FRP; e=20…26 kg/m3 for displacement catamarans in FRP

 

My initial scantlings are as follows;

Hull, cabin walls and top deck = 15mm 80kg/m3 PVC foam with 900gsm triax outside and 750gsm triax inside. Additional 600GSM Kevlar on hull bottom/bilge.

Bridgedeck = 20mm 80kg/m3 PVC foam with 1500gsm triax both sides

Beam bulkheads/webs (2 per beam) = 20mm 150kg/m3 EG.balsa with 3000gsm uni-directional both sides. Additional uni added to top and bottom beam flanges (bridgedeck and topdeck) to close the beam box.

Using these scantlings, the rough weight of the shell including 250kgs for the outboard motors comes to 1386kgs... i already added extra density in the chart i posted earlier to allow for things like tape joins and epoxy fillets etc... which is why it came out around 1500kgs.... ill post a new panel weight chart soon, but i gotta run to work now... the original chart had different thicknesses as it was originally going to be built from 12mm balsa core instead of 15mm PVC foam...

Do these sound like realistic scantlings, or am i still dreaming?

 

What is the basis for Your scantlings?