Cat design, estimating bare shell weight

Hi All,
I'm in the process of drawing up a 10.5 meter spartan cruiser for myself, for some years now, it's a love project. I would appreciate some input from knowledgeable people on some numbers below. I am trying to estimate the bare shell weight at this point in order to add some displacement if necessary and to estimate the payload capacity. The design is layed up as a complete parametric model so adjustments for increased payload or even hull length or anything can be done very easily.

The next stage would be to polish up design details and contact an NA to get a lamination schedule for all the parts.

Before that step I would like to have an estimation of the shell weight based on my own cloth guesswork in order to finalize the displacement and trim of the hulls.

The cat has 3 beams, a partly solid deck between 2 of the beams, a small central pod.

I have worked out the surface area of all the design elements and it ends up about 190sqm. It will be an infusion or vac build with Airex. PIR and Balsa as core. I already have the core for some years now stored (good offer few years back, bought what I could).

Based on the surface area and core weight I can do a very basic weight estimate and I would appreciate some input on it, whether it is on the light side or ok-ish.

For the hulls I'm guesstimating 750gr basalt triax on the outside, 550gr basalt triax inside. That adds up to 1300gr/sqm, adding 40% resin, ending up with around 2.2kg of skins/sqm. The core averages 1.4kg/sqm (12mm, 15mm, 20mm thicknesses). So I am looking at 3.6kg/sqm of hulls.
That adds up to 190sqmX3.6kg=684kg for 2 hulls. Let's add about 50kg of fillets or more, lets say 800kg, some extra reinforcing 850kg.

2 of the beams will be carbon, let's say 150kg for 2 of them (about 6200mm 300x300x5 ?), the 3rd beam is a big box beam that will be used as storage compartment as well, probably 150kg (cored).
So I am looking at a platform weight of approximately 1150kg.

How does this sound ? I am aware of the fact that 750gr on the outside skin is probably a bit thin, for sure the boat will need to be pampered, but it's what I do with everything I have. The geometry of the surface will be designed to maximize monocoque stiffness and double curvature everywhere possible. Hulls are shaped more or less like the GBE shells, simple geometry will little corners, taping etc.

What are your thoughts ?
Cheers,
Florin

 

As a quick first guess, total panel weight plus 20%. Make sure the internal structures are included in your panel areas. From a global structure point of view, 750g/sqm on the outside is plenty (more than I would use), but you will want to look at local point loading, abrasion resistance, etc., in appropriate areas (hence the 20%).

1150kg for hulls and beams is about right for glass. For comparison, my 12m F40 is 1150kg just hulls and beams, 1750kg ready to sail (just hulls, tramps, rig, sails, motor, no crew or stores) but only has 2 small beams. It's all kevlar/carbon.

Richard Woods would have data more appropriate to your boat. Generally, no matter how spartan you estimate your cruising will be, make sure you include enough displacement for a sensible cruising outfit.

 

I'm not sure how strong the setup is that you want to put together, but I for sure don't like the pamper part. While you may have such plans, the environment may have other ideas.

There is no such thing as a too strong boat, and you will need that when you can least afford not to have it.

 

Thanks guys for your comments. I was hoping for a bit more input, maybe I should move to design or homebuilding forum ?

About pampering - this is obviously relative right, you can never build a boat strong enough to resist what ever crazy loads the environment will throw at you. I'd rather have just enough to handle some occasional bump against moorings and be prepared to repair dings every now and then having a heavy boat.

I've seen and touched boats that had from 300gr - 650gr of glass on the outside of the foam. I was assuming around 700gr on a 100kg/cum core should be sufficient for mine. I ended up adding some displacement to the hulls to compensate for more local reinforcements.

I was even wondering about the use of carbon on the foam, supposedly I will need a lighter cloth compared to glass to achieve the same stiffness, but then the skin is too thin for impact loads.

Can anyone comment on the bump-ability of their boats with around 400-700grams of whatever on the outside of the foam ? I was thinking for decks and bottom I'll double the cloth for weight estimation.

 

On race boats we use around 400gr/sqm carbon either side of nomex honeycomb for deck structures, and it is enough to walk on gently in areas that don't have heavy traffic, and fine for global loads. This gives a panel weight of 1.95kg/sqm. Hull below deck we will use typically 550-600 on the outside, 400 on the inside over 40-70kg/cum foam, so up to 2.9kg/sqm. Race boats see high loads, you would need to consider your loads.

Bear in mind that the carbon is about 2/3rds the mass of glass and has about 4 times the stiffness once laminated, so the same weight of carbon is proportionally thicker, and hence the single skin is stiffer by a factor of 13.5 against local pressure (4 x 1.5^3). This benefit allows the use of lower density core, as the skin supports it better, and/or a thinner skin.

If you get a handle on your loads, and set up a spreadsheet that calculates your structural mass on the basis of both stiffness and strength locally and globally, you'll see that the use of carbon can actually be little more expensive than glass in a boat with negligible payload, but once you start to add cruising kit, then the structural weight becomes a smaller proportion of the total, and hence the benefits diminish. Until you've got numbers for your payload requirements, then you can't determine where you sit on the curve.

 

Don't get optimistic

Gday Florin

All designers have trouble working out what their boats will weigh. Have a good look at old tris and cats and almost all are below their lines. A sistership to my 38ft cat sits 7cm lower in the water. Both boats have the same structural laminate but the sistership has heaps more inside.

In my first cat design I was a little too optimistic about what I could build it for. It is a good boat but can't take as much weight as it could for cruising. The laminate schedule has less bearing on total weight than you think. Grainger wrote about how his early boats came out heavier than he thought. He surmised that all the taping and filling inside increased the weight. One of nice early cats was the Mystery Cove 38 (I thought about building one). A friend has one with a volume on waterline of 3800 litres. After people built a few the Mk 2 version had about 5200 litres volume.

If the boat is going to be used for racing then I would be fining the hulls down and reducing immersion rate. Still I would go along to a boatyard and look at some similar boats. By looking at the interior fitout and the waterline you can see if the boat is heavy/ light and whether it sits high or low. Then look up the design plans and see its stated displacement.

My 38footer was 22cm above DWL when launched as a shell. That is a good thing as she sits with her stern just kissing when fully loaded and fitted out. I always shake my head when I hear of boats that sit on their lines when launched - a cat will need to sit high when launched. How high depends on you. I don't have twin diesels, lavish interior, permanents gensets etc. The sistership even had a stained glass drinks cabinet and lost much performance and payload.

Considering that almost all cats sit too low when cruising - look at their sterns - I would urge great caution about setting a weight based on a shell. On Kankama we store about 400 litres of water, 80 litres of fuel, food for two for up to two months, my tools, a single 25hp outboard, a dinghy, two kayaks, a light generator, sewing machine, 40 metres of chain, anchor winch, three anchors, lots of assorted rope, boom tent, solar panels and arch, 200 AH battery. There is more too.

Thankfully Kankama is pretty simple inside so she sits and sails well. If you have a look at similar boats to your own and then work out what your type of boat truly weighs on the water you will start far ahead.

As to the laminate. I would recommend having a walk on some foam boats. Foam more than any other core is susceptible to microcracking of the laminate and high shear loads in the core. This can cause the glass to lose the ability to handle loads easily and the core to let go leading to squishy decks. A friend who had a lightly built foam cat did not like watching the deflections of anyone walking on the deck and as his boat was getting on the decks were getting more flexy as the glass fatigued. I would think that 1100 triax would be good on the deck top laminate with 750 underneath. Don't used balsa laminates as a base as they have lighter skins

cheers

Phil

 

Those last two posts tell you all you need to know. No one will commit themselves without seeing drawings

About 35 years ago I worked on a 38ft cat design where the bonding glass/resin weighed more than the shell laminate. So it is not a new problem. You really have to reduce all the overlaps and joints as much as possible

700g/sqm glass on the outside doesn't sound excessive to me

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks guys, much appreciated. I will definitely keep this thread alive, or make a new one for the design and details, as I would love to hear some comments on this cat. Unfortunately there are no recent 3d models as I am now programming the design in Rhino.Script. There should be a draft 3d model ready in a month or so. I can only indulge with a model from a few years back, the design changed quite a bit over time. I appreciate all the input regarding weight estimation and i am aware of the traps that one can fall into, this is why I'm trying to keep most of the variables under control.

Now, an interesting question would be, once I end up working with an NA, how do I know he/she is doing a good job ? I assume if nothing breaks, then the job is either great or overbuilt. Surely if something breaks within calculated efforts than the job was crappy. This is not my area of expertise so I am paranoid about having not so necessary heavy details. But, sure i don't want things breaking in "normal" conditions.



why doesn't the pic display ?

 

That's clearly in part down to you. You will decide on which NA to use, and by implication you will look at how successful their past work is and decide on that basis.

A NA will not just design the structure, they will be modifying the lines, sail plan etc as well. For a boat cannot be broken down into its constituent parts, it has to be considered as a whole.

More of a problem is suppose the NA says "you need to change such and such" and you say "No, it is my design I want it that way". Who will win in that scenario? If it's you why are you employing anyone? If not you, why not go to the NA with a blank sheet and just say "I want a 35ft open deck catamaran that sails fast"

Maybe better to involve the NA now, just to give you ideas and point you in the right direction, then you go away and you design the boat

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Richard,
I'm an architect, I'm used to working with engineers and change stuff around. I believe I am capable of designing the platform for my own uses and don't need an NA for that part, plus it is the design part that is very rewarding for me. Without hesitation I can say I more capable and skilled in geometry and modeling than the average NA out there.

On the other hand, I don't have experience with calculating lamination schedules for intricate parts, beams seats, loaded areas etc, this is why I want to employ an NA only for the engineering. There is no "win", it's just about stating the problem accurately and finding the optimum within a certain set of constraints.

 

This is always an issue when employing specialists, whatever the field. Standard industry practice in the commercial shipping field is that if the owner cannot judge the quality of the work (and few can or are willing to) then they employ an owners representative who is able to make this judgement to control the specialists and ensure the project moves in the direction the owner wants.
The small craft field is fraught because it is full of companies that are run marginally, producing products for owners who are not adequately represented. Hence all the horror stories of owners losing boats and companies going under.
My advice, as one of the specialists, would be to choose an NA based on their track record and reputation in the area you want. A good NA won't risk their reputation by doing a sub-par job.

Edit: Just seen your last post, sounds like you are happy to represent yourself from a design standpoint but need help with the engineering: If I was in your shoes I would employ a composite structural engineer, and also learn enough about structures to perform simple calcs so you can check the ball park of his answers. Again, if you select a reputable engineer, he can help you with this process, but you will have some degree of independent validation.

 

I always cringe slightly when I hear the term 'spartan' cruiser. It's not that I'm against the idea it's just there are a lot of cats and tris out there sitting way below their DWL and brimming over with cruising junk.

I think the problem mainly boils down to owners not keeping a good account of what is onboard. Racing boats also suffer the malaise for example when they cleaned out the main hull and floats on the Formula40 Running with Scissors they found about 300lbs of extraneous junk just lying around.

 

Corley,
I hear you, and I'm preparing for accounting everything if possible. I believe I'm sitting farther ahead when it comes to planning. I am living on a 28 footer for more than 1 year just to see what crap I need on my boat.

Anyway, been programming the new hulls, just the underwater part is relevant now. Shallow 320mm rocker, 0.64 prismatic, 2.5 tonnes in salt water. Length to beam ratio (waterplane) is 14.

 

After viewing the 3D model you have shown us, it seems very shallow draft for a displacement of 2.5 tons. The block coefficient 0.64 seems a little high. The freeboard seems very high, unless you need a lot of space inside the hulls.
Again, this is only an opinion because I have not done any calculations.
Another suggestion: one of the most delicate points in catamarans, is the calculation of the beams connecting the hulls and how to join them.
Good luck with your project and one more advice, let yourself be guided by those who know more than you in naval architecture. Here I include the weight distribution and, therefore, the overall distribution of hulls and deck. By the way, you can not do serious weight calculation and distribution if you have not calculated the scantlings of the ship.

 

Why do you say that? A very quick sum with a pocket calculator suggests that the numbers given are pretty consistent. The waterline beam is effectively defined by a length (overhangs look negligible) and a stated length to waterline beam ratio. The rocker and the shading on the picture gives an idea of the midships underwater transverse section, combine that with the stated prismatic (not block) coefficient and you are pretty close to 2.5 tonnes displacement.

I would guess that this is the easy part of catamaran design. Once you have an idea of the displacement, as determined by the proposed build method together with your stress analysis and the required payload, and you obviously know the length, as limited by class rules, the cost of marina berths or the space available to build it in, or whatever, then you really dont have all that much choice over the general underwater hull shape.

Conventional wisdom rightly or wrongly suggests that it should have a sharp point at the front, and your drawing certainly has that. There is also a trend towards sterns that are relatively wide and flat underneath, also as shown on your drawing. This stern form gives a small increase in hydrostatic pitch stability compared with canoe style sterns of some older designs and that is (probably?) to the good. It also allows spade rudders that stay close to the hull as they turn and broad steps down the stern to the swimming ladder. Generally weights tend to end up well aft in modern cruising catamarans, that may be because accommodation aft is drier and a bit more comfortable, also weight aft inherently resists pitch poling. Your drawing shows the centre of bouyancy fairly well aft, which all fits in with this. Ad Hoc has repeatedly told us on this forum that the fine details of underwater hull shape are almost irrelevant to multihull design, he pointed out that a crude model roughly sawn from a rectangular block was almost as good in tank tests as a nice smooth shaped hull. Mind you, I suspect that even if the drag difference is small, it may well make a difference to race results between otherwise equal boats! So how hard can it be? Much harder for monohull designers who have to cope with righting moment, drag, steering moment, pitch and leeway all changing as as the hull heels and changing in ways that depend on hull shape!