Curve of area, displacement, c.o.b. ????

[Jet FiSH]

Hi

I don't really know much at all about boat design, and I'm doing a skewl project where I'm designing (to construct) a craft to sit a jetski on, so that it can be towed behind a bayliner V-hull (outboard)
I've spoken to a boat designer about v'hulls and cathedrals, but the major thing he said was to work out the curve of area for my concepts, then work out the displacement and hence the centre of buoyancy.

i sort of understand the Trapezoidal Rule, but I dont understand where all the STNS (stations) come from.

I understand the displacement and cob calculations once you have the STN and the ORD for each stn.

Can you guys help with the curve of area.

I'll be designing a planar hull, not semi displacement.

pretty lost atm so any help would be awesome

[Jet FiSH]

Bump

Can no-one help

[Guest625101138]

If you are designing a planing hull then the cob does not matter once the hull is on the plane. What matters though is the position of the centre of gravity. The hull will need a planing surface that provides lift. It is best to have the hull bow-up at rest so it does not plough during the initial displacement stage.

This site will allow you to play with the dimensions if you know the weight of the transporter and the weight of the jet ski. It begs the question though - why not just tow the jet ski.
http://illustrations.marin.ntnu.no/h...ing/index.html

Only the first 4 parameters relate to your situation and I guess you would want to minimise the towing drag and avoid porpoising. Having a deeper "V" will reduce slamming.

Rick

[Jet FiSH]

So I need the centre of gravity?
To work that out don't I need to have the curve of area for the hull?

haha so many have asked that question It's a poleski, so even though it is a planing hull, its real centre of gravity doesn't come into effect untill there is someone standing on it, so unmanned the ski just dives if its being towed.

I have a few concepts
All have roughly the same curve as the jetski howeer extended at the bow for the initial displacement

my first concept is a single v hull that simply supports the nose hlaft of the ski and leaves the back of the ski trailing in the water.

second concept is the same central hull, however not a v hull, just a rough sort of curviature to flat at the rear, and it is formed into a cathedral hull with two deeper hydroplane style sponsons on the sides.

third concept is a simple v hull that supports the whole ski, thus not mixing a planing hull with another planing hull poking out at an angle.

The designer I spoke to said that the cob is roughly 3/4 from the bow (is the bow the transom?)

cheers for your help rick

[kach22i]

Originally Posted by Rick Willoughby

http://illustrations.marin.ntnu.no/h...ing/index.html

Nice site, thanks for the link.

Marine technology
http://illustrations.marin.ntnu.no//index.html

This site is a self learning system for marine hydrodynamics and marine stuctures. The state of the art is presented as a "click and play and learn" model. Some parts give you the possibility to specify attributes and see the consequences of your choices. It might be nice to complement the surfing on this site with seminars or some kind of traditional teaching, but you can certainly go by yourself if you want..

[Guest625101138]

What happens if you stick a weight on the stern of the jet ski to get it bow up to start with? It is already a planing hull. You have to be able to get it into an attitude to plane. I expect having the tow point low down on the bow would help as well.

Rick

[messabout]

This need not be a design issue. If you want to tow the jetski in planing mode one of many methods is simple and cheap. A sheet of 3/4 inch plywood will do the job just fine. put a tow eye on the bottom of the ply sheet a bit aft of the leading edge, add some timber runners on the after end of the bottom to keep the ply sheet from skidding. Throw the sheet in the water, drive the jetski partially on the sheet. Tie the jetski down. The ply will plane cleanly with a load of 500 pounds more or less. The only detail is to load the jetski in such a way as to have the forward end of the ply above the water surface. Crude but effective. You can build this thing in an hour or less. You can also have fun riding the ply sheet without the jetski. Do so at your own peril.

[Brent Swain]

Divide your hull into ten equal stations longitudinally. Work out the underwater cross sectional area of each station and plot them on a graph, equally spaced. Cut this graph out of a piece of cardboard, and where it balances on a pencil is your longitudinal centre of buoyancy.
Brent Swain

[Jet FiSH]

Haha love the ply idea!
Thanks for all the ideas, but for the standard I'm doing im designing a craft to put the ski onto.

Have I got this right?
take a hull design and split it into 7 stations (red) of the half hull, then take the cross sectional planes STN 1-7 and calculate the areas of each by splitting them into 7 (blue) and using trapezoidal rule?
Then from there you use Simpsons rules to find the displacement and centre of buoyancy/ centre of gravity of said proposed hull?

Cheers again

[kach22i]

This topic has gotten me to thinking about those floating jet ski docks.

http://www.jetskiworld.com.au/used1.htm


http://www.floatingdockshop.com/EZ-P...-Ski-Dock.html

The following chart provides a comparison between the various EZ Port systems.
MODEL SIZE WEIGHT FLOATATION
CAPACITY*
EZ Port I 118" x 60" x 15" 230 lbs 625 lbs.
EZ Port II 127" x 60" x 15" 240 lbs 800 lbs.
EZ Port III 151" x 60" x 15" 280 lbs. 1300 lbs
Extension** 20" x 60" x 15" 50 lbs 100 lbs
EZ Port IV*** 168" x 80" x 15" 500 lbs 1600 lbs

http://www.canadianpwc.com/showthread.php?t=659


http://www.eaglemarinesupply.com/Dock-Lifts.htm


http://www.huronlagoons.com/marina.htm


I just wonder if you can tow it while jet ski and dock are bound together?

[Jet FiSH]

Could be worth looking into...

Are my calculations correct or have I not quite got it?

[Brent Swain]

Plot the area of each of the stations on a cardboard graph, cut it out and balance it on a pencil . Where it balances is your LCB.
Brent

[Jet FiSH]

So I have got the splitting of the hull into 7 stations correct then?

really appreciate all the help guys
im slowly taking it on, its not only a skewl project really, its an interest thats heavily growing on me

[Brent Swain]

I usually use ten stations , but it really doesn't matter.
Brent

[Jet FiSH]

I am reading John Teale's "how to design a boat" atm and he uses 6 stations with 0 inclusive, I guess its personal preference...
He says in the book to use the half sections at midships to calculate the area under Lwl, but I dont see how he does it.
Anyone else read the book?