Math problem, estimating disp., SA

[DCockey]

Originally Posted by Tim Hall

Ok, so I got my curve and my displacement is pretty darn close to what I had guestimated when starting the project.

So I'm looking at this curve, and it occurs to this is possibly a nice analytical tool (I really appreciate DCockey for pointing this out, BTW).

Is it possible to estimate my LCB from this? If I, by trial and error, find the x-coordinate (distance on the WL) that divides the displacement in half?

The location which divides the displacement in half may NOT be the LCB.

Longitudinal postion of the centroid of the area curve should be the LCB.

[rwatson]

Originally Posted by DCockey

The location which divides the displacement in half may NOT be the LCB.

Longitudinal postion of the centroid of the area curve should be the LCB.

This is what Freeship/Delftship does so easily - its really worth the trouble unless you need the brain strain.

[sorenfdk]

Using FreeShip, DelftShip or any other software doesn't make you a yacht designer (or a naval architect!) They are just advanced calculators that make life easier for the user.
It seems to me that what you need is to read a book about yacht design. Principles of Yacht Design by Larsson & Eliasson would be a good choice.
BTW: "SA" is normally used for Sail Area. Wetted Surface Area is shortened "WSA".

[Kestrel]

Hi,
if You want to enter the matter, and understand things too, even if less known, Gauss-Legendre integration method, with 4 integration points, is very fast and precise, once you can make a diagram or a tab. of the integrating function (example, cross sections area to calculate the volume, wet beams to calculate hull wetted plane, wetted contours to get wetted surface, and so on...) and it fits well also with automated software procedures (sample: http://www.idet.it/idra.html).
Bye
K.

[Tim Hall]

Using FreeShip, DelftShip or any other software doesn't make you a yacht designer (or a naval architect!) They are just advanced calculators that make life easier for the user.

Yes, this is why I'm spending a lot of time drafting in 2D. My background is in architecture (not naval), and my experience is doing a lot of drawing and re-drawing teaches you a lot. After several iterations of midsection profiles I can intuitively figure how a subtle change will effect the entire hull form. And I've already come up with a few of my own drafting conventions to make sure I'm drawing a fair surface.

My only interest in Freeship is to do a stability calculation and find the swamping angle, which would take forever if I tried drafting the design displacement at so many angles of heel. I think pushing points around in 3D software is very misleading...especially when you're looking at the cool model which may or may not actually be a good design.

BTW: "SA" is normally used for Sail Area. Wetted Surface Area is shortened "WSA".

Ha ha, yes I just realized this when I saw my own heading before replying this time. WSA is what I'm concerned with.

[Tim Hall]

Originally Posted by rwatson

This is what Freeship/Delftship does so easily - its really worth the trouble unless you need the brain strain.

Yeah, at the moment I want to learn about my design. Between drafting the line in Illustrator and using AutoCAD's number crunching capacity, I can easily get the centroid of area under a curve. These two programs I know. I'll get into Freeship or Delftship a little later on in the process.

But yes, I look forward to seeing if my numbers match to what this kind of program says.

[Tim Hall]

Thanks everyone for the replies.

[Tim Hall]

Originally Posted by sorenfdk

It seems to me that what you need is to read a book about yacht design.

I have been reading about canoe/kayak design. But it seems to me there's a lot missing here. I think you're right...I used to have that book some years ago, and would like to revisit it.

[Wynand N]

Originally Posted by Doug Lord

====================
Tim, here is a table direct from Skenes for the Simpsons rule. It will give you, displ., CB, and CP. The table works with any even number of evenly spaced stations starting at zero(intersection of bow with wl). If you make up a table like this so you can use it in the future it will be a quick and easy way to accurately estimate these parameters. Might even be able to do an Excel version. I've used it for 40 years and it is accurate.

Way back in 1990 when I started my small boat design diploma course, boat design software was scarce and expensive and with no internet at the time and stuck in SA, I made my own spread sheets in Excel that proved very effective and helpful, very primitive though looking back on it.

Tim, if you are interested, I can send you copies of my Excel spreadsheets (10 & 20 stations) based on Simpsons multipliers, basically in the same photo copy format as shown by Doug.
It calculates Displacement, LCB, VCB, CP, CWP, immersion and moment to trim all on one page.
Another spreadsheet calculates water-plane area and center of flotation.

If interested Tim, just send your email by PM and its off to you.

[Wynand N]

Check your email Tim, the postman has delivered

[Tim Hall]

Wynand, those are very nice spreadsheets! Thanks again.

[ancient kayaker]

FreeShip gets discussed a lot in these pages. If you do a forum search - or use Google's site search which is more powerful - you will find lots of threads with information. Programs like FreeShip are specifically for hull shape design. If you can find a free download for Carene2008, that is a very easy and intuitive program for a beginner, although less flexible. It's exported offset tables will import directly into FreeShip: this is a route I sometimes take to get into FreeShip's tools quickly. If you're likely to design more than one hull the learning is worth the effort. Last time I looked the Freeship manual was a pain several inches above my kayak's seat but there was a tutorial on the net which was good. Search and ye shall find . . .