Full Hull Design Help

Hi again,

I've gone away and read a couple books on ship theory i.e principles of yacht design edition 2 and basic ship theory. This is on run on topic with some further questions from the post i started a week ago (http://www.boatdesign.net/forums/boat-design/hull-design-54817.html).

I come with a couple of questions for a design of a small hull.

1) In both books I read it touched on the various sort of forces that cause the boat to trim and heel. As far as stability design what would be the acceptable degree of trim and heel you would feel comfortable allowing in your design? And what loadings would you check (Wind, waves eg...)

2) When calculating the stresses on a hull, basic ship theory provides a method of calculating hull adequacy when it comes to checking stresses for a stationary boat (under hogging and sagging waves). Is there a software which can provide a more accurate way of doing this? Plus checking if any elements are likely to buckle?

3) When checking the designed hull for dynamic forces can the software package provide the forces and run the same checks?

4) Any other considerations I should make whilst going ahead

I'm sure you may have some questions so please ask them and remember I'm new to boat design and very familiar with engineering so I have a mix of education on this topic.

Cheers,
Sam

 

I'm sure any boat you can envisage can be engineered to stay in one piece, but may be totally unsuited to task if it is the wrong shape, e.g. You need a firm idea of what the task is.

 

You don't say what type or size of vessel, construction material, nor give any idea of what we call the SOR. But it sounds like a yacht of some sort ?

Classification society construction rules should be followed, their construction guides are available for free download and they've been refined through a wealth of real world experience, not just engineering.

An alternative is the ISO path. They publish standards which have to be purchased. I'd recommend GL since they still produce rules for smaller yacht construction and those rules will exceed ISO construction requirements although they are all very similar nowadays.

Another book I recommend you read is "Seaworthiness the forgotten factor" by Marchaj

 

On his other thread........


" I'm looking to design a little boat at first, hopefully out of wood or metal and attach an old motor to it. "

 

o yes should of pointed that out. As Mr Efficiency pointed out above I glance over that in the last thread. It'll be a little boat mainly to test out the design principles and put them into practice. Any other information you'd like me to elaborate on please ask

 

Select a speed range. Select type of waterway ( sheltered, open). Number of passengers, etc. Not to forget, the budget !

 

speed range is something I still don't have a definitive answer for at the moment. Lets say enough to get it to plane. number of passengers 2 to 3 including me. Budget I'd consider before building and if I see it viable/worth the time so lets say no bounds on budget. Waterway would be sheltered definitely not open water.

 

A plank on edge cutter will probably heel 25 degrees on a breeze, while a skimming dish type will only heel 10 degrees or less. Most importantly, software won't do the thinking or learning for you. Unless you understand boat design, what comes out will be meaningless. You won't be able to tell if it makes sense or not. The only thing software is good at, is doing the tedious calculations and saving time. However, you need to know what to input. Basically, it is at least four years of college, or a longer time on the water and shipyard.

 

Sam, what you first do is establish the parameters and goals you'd like the design to achieve. This is called the SOR. With this in hand, many of the basic dimensions, hull shapes, rig, propulsion and "general arrangement" will be roughly defined, which eliminates much of the guessing and considering stuff.

For example, let's say you want a fun boat for the family. Okay, what kind of fun? Powering the kids around, dragging a tube, fishing, sailing, weekend getaways, modest camp cruising, etc. Yep, it starts as basic as this, but what did you learn? Well, how about a powerboat, probably outboard powered (all hypothetical examples of course), which means some shapes are tossed out the window right away. Plane mode performance, maybe at the modest end to keep costs down and your dental work intact, so 30 MPH top speed with 24 MPH cruise to save fuel. This is just fast enough to tow a skier or drag a tube, so you can see how long the kids can hang on in the turns. This kisses off round bilge hull forms, for the most part, though still a possibility, if you're just dying to have one. Crew of 3, well let's call it four, just because as soon as you finish it, you'll have friends looking to ride in this puppy, so a front seat and two jump seats or seat boxes aft. Sounds like it's going to be a small runabout, maybe in the 16' to 19' range. You can go smaller, but you'll enjoy the elbow room of a 17'er more than a 14'er, trust me.

As you work through what you need, other things come up, such as length, beam, general hull sections for the performance envelop you're targeting, etc. and you start "mowing" around the design spiral, until you fine tuned things sufficiently enough, to not worry much come launch day. As a rule, on your first design(s), you'll stick pretty close to known examples of other boats, in this size and performance range. This is the safe way to learn about stuff, because if you jump way out on a design limb, you're not going to have a "comp" to make assessments against, so you're back to the guessing stuff.

What's all this mean for you, well make up a comprehensive SOR, listing everything you think is relevant, right down to the cup holders in the portable head (I'd recommend they mount below the tissue paper dispenser or you can learn the hard way). Prioritize this list and break it into sections, such as convenience, propulsion, accommodation, etc, so the separate items can fall into a category and this category or column of items, can receive a rank in the priority list. The more anal you are about this, the easier it will be to design up the boat you have in mind.

Once all this is done, you still haven't cracked open the notebook and you're not ready yet. Sit down at the kitchen table, with a plain piece of paper and hand draw a boat, roughly to a handy scale, about the width and length you want and see where you'll put stuff, like seats, tanks, the built in cooler full of beer and Fedel the wonder dog. It doesn't have to be particularly precise (yet) or pretty (yet) and once you got this basic thumb (literally) sketch, you've just done the rough GA. It's at this point you'll want to open up the notebook and work with a spread sheet, so you can start playing with how much things might weigh and where you might need to shift them, when things don't trim as you'd hoped (they never do). Now with the rough GA and a rough weight estimate, you can think about drawing a boat with software, because you now know how big to make the boat, for all the stuff it'll have in it and met the performance expectations you've envisioned (pretty cool right). Be grateful, as there was a time where there weren't any notebooks and the weight calculations were done by hand (extremely tedious) and the drawing stuff was also with a pencil, triangle and Tee square. I learned this way, using a slide rule for the center of masses calculations. Be very grateful and good luck.

 

Would you be happy to design a boat, that, on the balance of probabilities, would at best be no better than, and likely inferior to established designs, just to be able to say, "I designed this" ? I can't really see a sound reason to be getting involved in designing a boat unless you have a close familiarity with what already exists, and have a definite idea on how to improve the breed. I suppose it is boatdesign.net, but it is remarkable how many want to design boats from a base of little or no boat knowledge or experience.

 

To Mr Efficiency. Yes. I'm looking to get the basic theories of design under my belt. Building the boat is a second concern to me. I want to be able to have a set of numbers and dimensions sorted so that I can play around with them until I'm satisfied. I personally don't see the point in just replicating another design, wheres the fun in that.

To PAR, thanks for the reply. I've got vague dimensions sorted and as expressed in my reply to Mr Efficiency, I'm looking to start the design iterations to learn how things interact and come together. The hull I design I may not even build, I'm just at this stage trying to sort out how the hydrodynamics and buoyancy principles funnel into a safe, steady hull design. This is why I'm concerned with things like finding a software package to get max stresses on a planing hull or the waves types I should be checking the heel degrees for or how to even do that for a matter of fact. This process I'm venturing on is one of learning not of efficiency or practicality.

Cheers,
Sam

 

If you have a passion for boats and boating, it is much more likely you will soak up useful information like a sponge, but coming into it more or less "cold", I doubt a satisfying result is likely. Gonzo said you need to either go through the formal education, or a much longer less formal period of involvement with boats, seems about right, you gradually get a feel for the similarity to the juggler's art of keeping a whole bunch of things in the air at once, which is needed to get a good result.

 

Agreed, a casual approach will be quite time consuming. There's no software package that will tell you if a particular set of lines or weights or volumes, will be well suited to your needs. This is governed through understanding the various engineering disciplines involved. This doesn't mean you need to understand how to design a supertanker, before you can draw up a 17' bowrider, but it does mean you'll know why the supertanker is done the way it is, when you have enough knowledge to do a good 17' bowrider.

 

Thanks again for the replies. How about I change my question slightly. for any select ship, how do you validate the stresses the boat will endure can be sustained by the ship. Be it by hand, by software or other method I just want to know how an established designer goes about this.

 

I'd say the stresses likely to be encountered are reflected in formalised scantling rules, which vary according to the type of boat, hull material etc. Just a lot of accumulated experience.