how is draft, waterline, displacement determined

I built a sailing dinghy (D4 by Mertens-Goossens) last year. She sails well for my purpose. However my wife and I are a bit cramped in it. So I decided to build a new boat that can carry 2 adults and a kid.

I started drawing plans until I realized that I didn't know how to determine stuff such as draft, waterline, displacement and other technical stuff.

So my question is "How are these things determined? Is there a theory or mathematical formula for this?" Can someone please point me to the right direction?

Thanks in advance,

eric

 

Hi,

You are kind of asking the same as if I asked about a car: how do you determine the lenght, width, height, wheel size and engine size... it all depends on what you want to do with the boat.

Your first criteria seems to be more room. Room for what - sitting, sunning, sleeping, cooking, etc... and room for what 'stuff'?

But keep in mind the eventual size - if you are trailering, etc.

The is this a sailing for powerboat.

Once you know these, the best thing if for you to browse the internet for boats that come close to what you want, and note not only the displacement, etc. but also the shape of the hull... as this is key to all aspects of your boat - its performance, sea-keeping ability, etc.

In fact, unless you like designing a lot, you might want to consider buying some plans for around 100$. Will save you a lot of time, and you will get something you know works and looks like you like. Maybe some others can suggest designs, or sites of designs?

Cheers,
Grant

 

One nice book is "How to design a boat" by John Teale(?).

If you draw your boat seen from above with the accommodation you want, then you have established length and width. If you estimate the total weight with crew and gear you have the displacement. If you divide the displacement in tons by the waterline length and then multiply with 2 (typically) then you have the underwater area of your midship section. Then you can sketch this section and you have a nice start

Correcting myself... Depl = Cp * Am * Lwl

Example:
Your boat is 5 meter Lwl and the estimated total displacement is 500kg or 0.5 tons.
2 * 0.5m3 / 5m = 0.2m2.
If you have a simple v bottom that is for example 0.2m draft and 2m waterline width.

 

John Teale:
http://books.boatdesign.net/boat/books.cgi?Operation=ItemLookup&ItemId=1574091573&locale=us

 

Understanding Boat Design by Ted Brewer...exellent for the basics and more.

Steve

 

there is a very good thread on this forum that goes deeply into the various methods calculating these. sorry i cant find it quikly

 

Thanks to your replies.

I want to build a boat like the mirror since its large enough for me and my wife but still small enough to carry around. But I can't get hold of plans. Mirror Kits are being sold but they're too expensive for me. So I started drawing based on pictures and some known measurements such as LOA =10'10" and beam 55" and hull displacement = 50kg.
I think my question should be how would my boat sit on the water? My drawings may look nice on paper but what if the bow goes too low (or too high) in the water once it's built.

thanks again,
eric

 

thanks to all, I found the two similar threads that treat the subject of waterline.


eric

 

not in the process calculating but when you still got these links post them again please
looked the mirror up: 3000 bucks for the plans while i saw a poly hull for 3500

 

Eric, I would be interested in those threads you found on the subject of waterline. Would you mind posing the links? Thanks!

CET

 

found some links but that crazy 3d camera with software that draws and calculates foto's taken from a few angles of an object / ship (think posted by Steven Ditmore) i still havent found back...

http://www.boatdesign.net/forums/showthread.php?t=951&highlight=Simpson
http://www.usna.edu/NAOE/courses/en200/ch02.pdf
http://www.boatdesign.net/forums/attachment.php?attachmentid=1161&d=1077732498

 

Yipster thanks for the links, heavy on the theory but I guess I need those too.

the thread was started by Baldur entitled "formula for finding waterline?" its on page 25 under boat design. It had 13 replies and Yago's replies explain a lot.

http://boatdesign.net/forums/showthread.php?t=7818
http://boatdesign.net/forums/showthread.php?t=3884

eric

 

Intended use

Henry Morschladt once owned Pacific Seacraft sailboat company. While not a naval architect, his designs were very succesful. When asked how he came up with one of his most popular designs, Henry said that he designed an efficiency apartment then put a boat around it. The boat was a highly functional place to be for a weekend and didn't sail bad either.

While working out all those waterlines and butt lines and lcg and lcb and metacentric height, keep in mind the efficiency apartment.

djs