Cat design, estimating bare shell weight

John Perry, you're right, I read "block coefficient" when it was "prismatic coefficient". My mistake.
I'm not saying that the numbers are not consistent, I have no items to judge that. A part of the error that I made, I keep saying the same thing before (I repeat that I have not done any calculations) a depth of 0.32 m (submerged body drawn in green in the picture), with these forms, there is little to achieve a displacement of 2.5 tons. I also still think that the freeboard is high. But I repeat again, this is my ideas to the drawing view shown.
Regarding the distribution of volumes of the hull, we can not say anything without knowing the weights of the parts of the boat and its distribution. I will venture, however, to say that I see little volume aft but we all know that these initial forms can be very different from the final forms.

 

If the draft is .32, then WL beam will be .64 for a semicircular hull, but 14:1 ratio makes it a bit wider than that. So Am is say .18. So displ per hull is approx 1.23cum

To John. Maybe its better to say monohull and multihull designers have different problems to solve. For example, few 10m monohulls will have cruising speeds over 10 knots, this boat should be double that. So there will be potential pounding and nosediving problems to design out. No monohull has the torsional strains that an open deck catamaran has.

Weight and CofG estimates are more important on a multihull than a monohull,There is no ballast that can be used as a "fiddle factor" It has to float on its lines with no help. Australia 11, for example, a famous state of the art monohull that had to have its keel moved significantly after building to trim properly.

And that is one reason why I, and others, say you cannot design the scantlings without knowing much more about the boat. For example a hull with a "bird cage" interior might have lighter skins than a monocoque. A boat that is designed to be beached (which few monohulls are of course) might need a thicker keel layup than theory says.

Then sailing in flat water needs a different hull shape than one that sails offshore. The rig size will be determined by the prevailing conditions. A tradewind boat is very different from a course racing boat (look at those scow minitransat boats, not good in a short chop to windward, ideal for power reaching/running)

Everything is related if one truly wants to optimise a hull

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Richard you are right on the money of course. Hulls displace 1.225 of fresh water and around 1.25 tonnes of salt water, for the same waterline. There is no need to doubt the numbers.

For Tansl - don't know why you say the freeboard is very high ? It is a cruising cat, not a racer, indeed it will have higher windage compared to racers, but it is far sleeker than most cruising cats on the market, far far sleeker. The "sheer" sits around 1200mm above the waterline. Adding the draft and a discrete "cabin" top provides a max of 1800mm standing headroom in a limited area.

I agree with all the points about weight estimation. The design is a progressive evolution of ideas, lots of things changed over time. My initial question was regarding a very rough weight estimation, based on that I added some displacement, and displacement might still change, but you need to start somewhere. With a programmed parametric model, these type of changes take seconds, I doubt most of the NA's out there can set this up quickly for freeform shapes.

I don't fully agree about the weight being far aft. I think boats end up like that due to relatively poor design and planning. I'm talking about the extreme cases. I don't think it is good practice to have VERY high volume aft to accommodate heavy weights. I believe a more balanced approached is better. I will not have twin diesels, like most condomarans.

High volume aft also has a tendency to bury the bows quicker. All that weight has inertia, when its inertia points upwards, it will stuff the bows deeper. I know this discussion can go on forever, but I'm planning for a more balanced weight distribution, coupled with high Cp hulls.

Thank you all for your input, highly appreciated and critique leads to progress, if well founded. I assume more comments will arise over time when there are more platform details.

 

isvflorin, you have done the calculations and therefore you know how is your hull and its displacement. I thought that 2.5 tonnes was not a real figure, but I was wrong, that's all.
As the freeboard, I'd estimate a value of 1 m and it seemed high. It still seems rather high, but surely you have some reason unknown to me now, to give this value.
As for weight distribution and therefore volume of submerged hulls, is not about following a trend but to see how it will navigate the boat and get a suitable areas curve trying, among many other things, minimize shear on the hulls. If you have not significant weight aft, certainly will not need to add thrust in this area.
Cheers

 

The acid test I use when designing a boat is "could my mother get on board"?

Richard Woods

 

How tall is your mom?. I ask it thinking on the freeboard as, I suppose, is the reason for your comment.

 

Another test. Could my crew run to the bows and fend off if I mess up coming alongside a stone wall when under power?

There are lots of design compromises to make that don't involve sailing performance. Only on full crewed race boats can you ignore every day practicalities, so it isn't necessarily a good idea to slavishly copy what you see on the top racing boats

Richard Woods

 

Beware the fashion

Gday Florin

I was wondering around the boatyard today (Kankama is getting her bottom done) and looked at the cats here - big, fat and heavy. I did think that reasonable performance cruisers would be the future of cat design 10 years ago but cheap charter cats have skewed the market.

Designing your own boat you have to do some predicting as to what the future holds. I think your design has two and maybe more features that will be looked back on as abberations in cat development.

The reverse bows I cringe at. I like them on AC 45s and A class cats but with a forebeam, nets and paraphenalia up the bows they really do not naturally occur as a result of the design process in cruising hulls. A Schionning cat here has them and they are added on by squashing the topsides above half the freeboard creating a bulge where there should be a smooth curve. I think reverse bows will be along with racy cabins with highly sloped front windows as affectations that looked good until you had to use the boat. Few modern cats have slopey cabins now although they were the rage 10 years ago. One design here has fabric shades over the windows creating great windage.

I would be worried about the rather flat hull sections aft. A few Grainger designs of about 8 years age have added little pods under the stern to stop the flat sterns from slapping in chop. Grainger talks about putting the stern sections underwater to stop this but this reduces light wind performance.

I also hope you will draw in some stern steps. Stern steps make a boat and my second folder had no stern steps for various reasons but I took extra time and retro fitted them. They truly make a cat a great cruising boat.

I will again state that I don't think the boat will sit on its lines when cruised. It is a cruiser so it needs to have volume and the worst thing a designer can do is to miscalculate the volume. It is so hard to remedy. You could draw up a spread sheet and add things up but that is what all designers already do and almost all of the cats I see are sitting below DWL. We have a terrible desire to fill our boats up with stuff and stuff will weigh you down.

But weight is not a bad thing - when designed for weight is fabulous. It makes out boats stable and reduces acceleration and this is where I really get annoyed at performance cat designers.

It is interesting to draw fast looking boats and say "Won't that go quick". If it knocks the crew around it will not. In fact is is rare that you can put the foot down and let a boat rip. Most of the time when the wind blows the seas come up and so you slow down. A lightweight and spindly craft with too little volume will give its crew a rougher ride then a moderate cat. In more than 15 knots you will be slowing the boat most of the time. Every now and then you will get some great water and let her go but for the VAST majority of the time you will be throttling back because the autopilot can't handle it, the waves are confused, you can't be bothered, you want to cook bread or read a book and so the fastest boat is the smooth boat that quietly looks after you and patiently clicks the miles over and gets you where you want refreshed and calm. (I am a racer in my youth - won state titles in Lasers. Tasars and 420s so I love to sail fast and do when its good to do so.)

This is the bee in my bonnet I have with racey cruisers. I cruised in one and it was much less enjoyable than now in a moderate 4000kg 38 footer. Who cares if you get to there you are going 30 minutes earlier if you get flustered or if your partner gets scared? You wouldn't go touring in a Porsche as it would be too hard on the nerves - you go in a van. Add a bit of volume to the hulls so they can carry the kayaks and the dinghy outboard. Make somewhere to carry 400 watts of solar panels so you can make bread every day with an electric breadmaker. Carry 300 litres of water so you can shower after you swim every time and go away from civilization for a month. Get rid of the reverse bows so you can walk up the front to take a mooring and so the bridle doesn't rub on them in wind against tide conditions. Remember accelerations will reduce comfort. That is why Newicks are so loved by their owners with the seakindly lines rather than power shapes.

I blame the antifoul fumes for the rant.

Phil

 

Phil,
thanks, I really appreciate the input and the tone. As I said previously, only the underwater part is relevant for now, the profile of the hull (one version) was shown to have an idea of freeboard height. Again, it is a collection of ideas and it is an optimization problem. I do insist on lightness though, as I said, I already live on a boat and I have a solid idea of what I will have on the cat in terms of weight. If some extra displacement will be needed I can adjust a few parameters and hatch a new set of hulls in no time, this will be after engineering the structure and the laminates. There will be a feedback loop from/to who ever will engineer the structure and laminates, her/his input will require changes and everything will be kept in check.

There is one thing far worse than reverse "wave piercing bows" in cruising cats. And that is, the fake reverse bows, meaning a V section strangled at the top to give a negative profile rake, absolutely disgusting, an aberration as you said.

The hulls will have almost vertical profiled bows, a sharp-ish upper tip filleting down to a radiused water entry. There is much to the design which is not shown, therefore I guess it is normal that people rushed into conclusions of "copying" (everything is a copy of something else after all).

To be a bit clear of what I'm doing now, I'm not designing the shapes of the hulls or platforms. I'm writing software that allows me to design the shapes of the hulls and change them instantly to adapt to various constraints. I'm happy with what I've written so far and I can spit out hundreds of variations of bow profiles, deck camber, rocker, displacement etc.

Yes Richard, it is a very good point, and one that is so obvious that I don't need to read it on a forum in order to incorporate it into my design, but it is fair, as I think you were mislead by those early hull mockups. So yeah, my crew can run to the bows on a relatively flat forward section to fend off. I wish Irens caught on that too (Gunboat 55). But hey, cool sells right ?!

I read some thread around here asking for innovations in cat design. Almost ridiculous. I believe the major innovation is HOW the design process works for a client. As I have noticed, most of the designers use antiquated ways to convey their concept to a customer, then spit out paper plans laboriously so they can go to production, if a small change is needed, almost everything needs to be re-drawn. Let's envision this scenario. A designer writes a software that allows the client to customize a series of parameters, giving a certain family of shapes for the hulls, cabins, platform etc. A web interface allows the client to input the displacement and other parameters, producing an instant 3d model, that gets delivered to a composite engineer in real time. No waiting. The engineer does it's job, or maybe it is already done for a broad range of weights and small adjustments, than the client can receive the design information in either printed matter or even manufacturing information (file to factory). How many naval architects do this ? This is what I do for a living in architecture, programming geometry.

Regarding the slamming, this needs to be looked at carefully together with a broader image of the design. Lots of cats need lots of volumes aft due to their weight distributions, I still call that not optimum design. To accommodate that volume, you either have a deep rocker until the exit, or you widen the waterline giving you flatter bottom. I dare to assume most of the cruiser having this slamming issue with flat sterns have lots of weight aft. We need to talk numbers, like surface area and weight, acceleration etc. Taken to the extreme, it is obviously something to avoid and figure out how to combat the possible slamming. But I am far from the extreme, I have little weight aft and the flat surface area is rather small, there is little weight to accelerate down and slam, I believe the motion will not be that dramatic.

Imagine a a wooden cube falling on the floor next to you dead flat on one face. You can feel all that slamming through your bones. Now imagine the same cube being made out of foam and falling dead flat next to you, you will feel the slamming but it will be less dramatic. Surface area being constant, the weight will be the major factor affecting slamming.

I leave you with a beautiful shot of suicide bows. Go run there Even the tramp stops a few meters short of the bow tip.

http://sailinganarchy.com/wp-content/uploads/2014/01/gb-5501-2.jpg

 

I like the idea of having a modifiable shape in a design. It would be interesting working out what parameters could be fiddled with to change the shape but keep the basic idea the same.

I would think that sometimes prospective clients can be a little bit unaware of the processes involved. One guy came out with me on my first folder and told me the boat was okay but that it really should have two 1.2 metre wide double beds in a cat that folds down to 2.1 metres.

I think you can do this type of scaling already in many CAD packages.

cheers

Phil

 

Your impression of how NA's work is not representative of my experience for the last 20 years. We use software packages to develop parametric series of forms, not just to develop simple geometric parameters, but also resistance and dynamic characteristics as well as FE models and material cutting files in a fully integrated process. These aren't perfect by any means, but are constantly developed and for the preliminary design iterations are very quick indeed. Getting decisions from the client is usually a much longer process than actually generating the designs. You may be reinventing the wheel with your software (not a reason not to do it, though!), and you need to be careful that the algorithms you choose to include don't overly constrain your shape options.

Bear in mind that the custom-designed private small craft segment of the NA field is not typically representative of NA practices as a whole. It is a tiny segment with relatively little revenue, equivalent to designing domestic house extensions in the civil field, also something that people frequently do without recourse to a civil architect.

I'm sure there are still designers in this segment using pen and paper, and personally I don't think that should be a deal breaker. You are paying him for his design skills, and I would select an NA (or civil architect, for that matter) on this basis rather than his methodology. The duration of the design phase, even with an indecisive client, is usually small relative to the total project duration.

Also, when considering other designs you should bear in mind not only the aesthetic design brief from the client, but also the requirements of the boat. The Gunboat 55 will be driven very hard on race courses by some of its owners, and the bows must be capable of being stuffed into the backs of waves without tripping the boat. You may not have this requirement, so may not need such a bow profile.

 

I don't get what all the fuss is about? You create all the structural panels in the boat, and each panel is assigned a density. The weight is totalled automatically when all the surface areas are multiplied by the assigned panel densities. Shell weight done. This already happens in all the paid naval architect software and all the freeware I've seen also. What I don't understand, is what your going to do differently to make it all worthwhile?

 

apologies

Hey guys and gals,
I'll start this post with an apology. Posts are getting rather aggressive, I don't want to be stepping on anyone's toes or insult your design skills. Maybe I already did that and therefore I truly apologize, it was not my intention. I am grateful for all the knowledge shared on these forums and hopefully I can share some myself. I am humble before you, I appreciate sharing your knowledge and I might even work with you or for you in the future.

Hump (sorry don't know your real names), I was specifically referring to the small boat segment, homebuilder or smallbuilder one off segment. As you said yourself, parametric design is not that ubiquitous there and sure there are reasons for it as it is indeed, I assume, a small revenue segment. Having the ability to customize an already predesigned model, is something I don't see on any website right now. I'm not re-inventing the wheel, just putting in a different wrapping at best.

I constantly seek out designs and check out the work of other people, some times I can say - I'd build that but too bad it has this and that, if I could only change it. I'd like to have a web interface with fields for various parameters that a layman can understand and a 3d model with various estimations displayed next to it. A client would not need to use any software to adjust her/his design. Of course, the design is constrained by how much you allow to change, but I still think it is quite significant.

What about adjusting the displacement to your cruising/racing/type of construction needs ? Standing headroom ? Solid deck/tramp, freeboard, rig height/type etc ? If there is anything like this already somewhere please let me know as I want to have a look.

One problem is pretty severe though. I can see this robbing NA's of work, and if NA's end up doing this in the future, the work will get highly competitive. Right now, if you want to homebuild, the options in the 10-12m range are quite limited if you have a solid idea of what you want from your future boat. A parametric approach would broaden those options.

Interestingly, there are a lot of older design Gunboats, with traditional flat decks forward. I'm wondering if the hull changes are something really necessary for the owners or just style trend. I'm pretty sure the older Gunboats are also raced hard around the course. Of course there are pros and cons for everything, in my line of work, you can justify a giant plastic pink rabbit as artwork or architecture, if you have the right discourse.

 

In short, you simply can't design a boat like that.... unless you understand naval design, you probably cant see why this approach is not workable. ..

 

Isvflorin, with all due respect, I fear that you want to discover America. What you say does not sound like new. Many years ago, naval architecture programs allow we to modify an existing model, either parametric or not, in a few minutes and that's what a NA makes, on multiple occasions, during what is called the "spiral" in design process.
With all due respect, if you look at what already exists in the world with your creative ability, maybe you can develop some really new tool.
Maybe what you're telling us is a new tool, but I am not able to see.