Paper and pencil

It is interesting that for about a million years, a designer visualized a boat in a tree trunk. Carved it out and paddled away. Then some smart guy thought, If I just cut a narrow slit then carved it out and used heat to spread out the slit, I would have a wider boat. Next some guy thought if, I tied a plank to that top of the spread portion, I would have a wider and deeper hull. Then some other guy thought, why do I need a tree trunk, why can't I just build it buy fastening planks together at the edges. Then someone figured, why not put some framing in there so I could build a bigger boat.*

Then some one finally figured, if I just drew that on some paper, someone else could build it...

I guess the point I am trying to make here is that whether you design it on the computer, draw it on paper, or just knock it together it all starts with being able to see it in your mind and having some idea how boats work.

I suspect that Laukejas's desire to do it on paper comes from a lack of a technical background.

Certainly, he has ignored the implication we have given that he could buy a really nice computer for what it would cost to set up to do it on paper.

My recollection is that it was almost impossible (OK, merely difficult) to accurately calculate displacement when you did it on paper, at least that was why I made those models way back then. The funny thing is, I joined this list mostly to look into what boat design software was available these days, and this thread was the first one I came across.


*Pretty much the history of the development of the Viking Long Ships.

 

Calculating displacement on paper is not really hard; it only takes time. I would say it is one of the tedious jobs the computer takes off your hands.

 

There are some rough formulas to get close to a displacement (and other stuff), but computers as a rule, make these tedious calculations easy. The only other real advantages are changes and repeatability. With a hand drawing, if you need to move something, you've got hours of erasing and redrawing, but with a computer, you make the change and print out a new drawing.

 

All right, I'll get this one. Thanks, that should help a lot.

Beautiful. Yet terrifying. I tend to forget how much time, timber, money and effort goes into building such a ship. And it isn't the biggest sailing vessel either. And often it seems that they tend to over-engineer it. For example, the number and thickness of ribs... It reminds me of a documentary where a scientific explorer group built a sailing vessel that was meant to sail during summer to North pole (as close as possible), wait until winter, get encapsulated in forming ice, and as ice closes up, it would pop out on top of it. The idea was to see where the ice (along with the ship) would drift. I remember that the ship was made exceptionally strong to survive the crushing force of ice. This video reminded me of that. On the other hand, maybe it's just my inexperience talking, maybe this is normal on ships of such size.

Don't tempt me

Thanks, I'll save up for that.

With due respect, I must disagree. The analogy you provided is misplaced. Using inferior materials in music as you suggested would have no educational value, nor would it teach any skills you couldn't learn with modern made instruments. Modern instruments (classical guitar) made of modern materials don't replace any of musicians abilities.
While computers do replace. It does the tedious work for you, and often it doesn't require you to understand how it does that (for example, I've calculated hydrostatic properties of various hulls with Delftship hundreds of times without understanding how software does it). The first part is an advantage, the second is a drawback.

You may have mistaken me for a Luddite. I'm not planning on ditching the advantages software computing offers. I just want to develop enough skills so that I can design a boat without it, should the need arise.

Just like a sailor would want to learn to navigate by stars, even though he has GPS. There are skills you must know, no matter how much technology surrounds you. You learn the skill, and keep it sharp. But you use GPS daily, because it's more convenient.

Same here. I will use software for these tedious tasks to save time and effort, but only when I'll know that I'm not dependent on them.



Yesterday, I manually calculated displacement for a 5m yacht using trapezoid formula from Skene's Elements of Boat Design with 98.2% accuracy. That doesn't mean I'll do that every time. Just now and then, so that I wouldn't forget it.

I hope I made this subtle difference clear.

Calculating wetted surface area seemed even more tedious for me... Haven't understood that part yet.

 

Wetted surface by hand means measuring the girth at each station and then simpson ruling it. No harder than counting squares if you don't have a roller wheel that measures lengths

Because redrawing and fairing a hand drawn lines plan is so tedious you actually draw in a different way than you do on a computer

You would estimate your basic dimensions, length, disp, Cp, Am etc and then draw a rough Sectional Area Curve. The draw maybe three stations, 3,5,8 say, based on that SAC. So the hand drawing method won't have helped quite as much as you think when you go back to a computer

Keep refining and draw then build lots of boats so you know what works and what doesn't

The hard one to do manually is a stability curve

BTW use double sided mylar film and draw your (very precise) grid on the back, your boat on the front. And get a good erasing shield. Don't use tracing paper as you then need ink not a pencil to see the lines and anyway it distorts too much

Richard Woods

 

That method under-estimates wetted surface area if the spacing between stations is used as the interval in Simpson's rule. The difference from true wetted surface area will be small for a narrow boat, and larger for a wide boat with bluff bow and stern. The differences may or may not be significant.

 

If you need a quick estimate of displacement, the modified version of Tchebycheff's Rule is very good. A lot of books don't mention it, but it's usually accurate to a few percent (can vary between 1 and 10, depending on weirdness of hull).

It works much like trapezoidal, except that you only add the areas of stations 1, 4, 6 and 9, then multiply by one quarter of the waterline length instead of by the station spacing.

Trapezoidal is accurate enough for most purposes. The English used to insist on using Simpson's, but for most work you don't really need to. Trapezoidal has the other advantage that its estimate of volume will always be slightly low, and boats tend to end up a bit heavier than planned anyway.

Wetted surface is easy enough in principle. Instead of taking station areas you take perimeter lengths. These are then multiplied by station spacing, as usual, then by a fudge factor to account for hull surface curvature. Fudge factors of 2 to 4% are usually about right.

ETA: Cross posts.

 

Thanks, Richard, I haven't thought about that. Simple method. Skene suggested much more complex and laborious calculation.

What would you suggest as a more accurate calculation?

 

Maybe fudge factor could be used for displacement calculation with trapezoid method as well? I like it far more than Simpson's. Simpler and easier to remember.

 

Yes, use Tchebycheff's, like I said. It'll get you in the ballpark quickly. If you need something more precise for fine tuning, use trapezoidal or Simpson's.

 

If you calculate the volume or the surface of half a sphere (values which mathematically can be calculated with accuracy) with the method of trapezoids and Simpson, you will see that mistakes can become very large.

 

Accuracy depends on how many stations do you use, but basically that's correct.

What more accurate method would you suggest?

 

Use the distances taken off the bilge diagonal. But as David says it depends on the boat

Of course in England we use Simpson - wasn't he English? but otherwise I agree, Tchebycheff works

If you are bored and you want a little maths exercise, work out how Simpson/Tchebycheff rules actually work, and while you're about it, how a planimeter actually measures area

RW

 

Ok, I dug out Sailing Yacht Design by Douglas Phillips-Birt. This is another old one you don't really need, but is worth looking at if you get a chance. It contains a table of common error values for Simpson's and trapezoidal rules when used on normal yacht hulls of the period.

According to this table, when using the standard ten stations at equal spacings, Simpson's will overestimate by 0.12% and trapezoidal will underestimate by 0.68%.

For eight stations the figures are +0.3% and -1%. For 12 stations it's +0.06% and -0.48%.

He also says that the error for Tchebycheff's, again for common yacht hulls of the period, is between 0.1% and 1%, not the 1% and 10% that I mentioned earlier (I hadn't looked at the page for literally years, and my memory was out by a factor of ten ).

Short version: for what you want to do, any of these will be good enough.

 

I actually red an article on how to make a planimeter myself. It explained how it works. Simple mechanism, complex maths. I understood some of it. First time I learned about integrals, anyway. As I said, my profession is far from maths