what about hard chine aft

[jelfiser]

i'm in truble with aft design of my hard chine plywood 6 meter sailboat design -You can red more at (http://boatdesign.net/forums/showthread.php?t=12496)
I mean i absolutely don't want problem with following sea but I prefer a large aft for having more volume on back and planing easier , my question is why I have never seen on a one chine hull an aft like this ? usually they are more rectangular
Someone can tell me ?

[CORMERAN]

To jelfiser,

check out the very successfull one design sail boat, the THUNDERBIRD.
Sailed for many years in the Pacific North West.
Made of ply. With hard chines.

It was faster than many more " modern " boats.

Dont feel that the aft beam needs to be super wide. Getting on plane
will happen without excessive width - especialy if you keep the WEIGHT
down.
Being pushed sideways - out of control - is not fun. So this is one area
of design I prefer to be conservative.

[Ike]

The question should be, will it rise to a following sea? This depends partly on volume but is more dependent on shape and weight. The shape you have is good, and as someone said well proven. Thunderbirds are famous in the Pacific Northwest. The other is to make sure you keep the weight out of the ends. The weight should be centered. Don't be storing anchors and chain and placing engines in the stern.

[frosh]

Hi Jelfiser, the stern shape in your picture to me resembles a semi-planing hull that should behave well in big seas in dispacement mode, which is how it will likely sail most of the time. For better planing you need a flatter V and more width at the stern at the waterline. Then other factors such as weight and sail area are also critical.

[Billymac]

Ike, interested in why the weight can't be put at each end rather than centre. I thought this would make a more stable boat.

[CORMERAN]

Hi Billymac,

weight increased at the ends might be of use in a race car. As it effects
the Polar Unit of Inertia.
In simple terms - the car wont want to spin as quickly - making it more
forgiving to drive fast. Like a long wheel base, front engine Ferrari compared to short wheel base Porsche.

However: cars live, somewhat, in a 2 dimentional world. Whereas, boats have to function in 3 dimentions.

Weight placement, in small planing boats is a whole 'nother debate.

Specificaly, to the vessel shown by jelfisher - said vessel will most likely
be operating - in the most part at DISPLACEMENT speed. With moments
of excitement - when surfing down waves - at " planing " speeds.
Therefore - FIRST it must function as a displacement vessel.

Somewhere, close to Herreshoff's birth place ( his designs won the
America's Cup at least 5 times) - there is a large rock.

Cut deeply into it, is this law, defined by himself:

" With displacement craft, weight needs to be kept out of the ends.
Placing as much of the weight as possible, close to the centre of
the vessel."

[jelfiser]

Just seen thunderbird , it seem really interesting, i'm going to experiment and evaluate some modification to see how does it work .. does anyone heve some information more on this boat(cp, angle of entrance.. etc.)?

For the weight ...as you can see in an approssimate weight extimate in previous forum ancors and motor are keeped at prow, i have to balace the weight on the bow, two people:150kg is a lot but i cannot governate the boat in the middle !!!
yes, it have to be a semi planing hull and i was thincking to a criteria to evaluate how flat should be plywod panels over and down the chine, also the eight of the chine at the aft i'm just making some consideration on waterline with the boat helled of 25 deg .
tanks you all for interesting considerations

[im412]

the japanese and chinese use to use an open transom for water balast in a following sea on their wide buoyant transoms eg junks even down to little skiffs
something to consider

http://www.amateurboatbuilding.com/a...se_boats3.html

[Ike]

Comeran, thanks. You explained it better than I would have. The main thing to remember is that the more weight you have in the ends the slower the are to accelerate upwards. So you get more water over the bow and more on the stern. If they are light they rise quickly. Of course this is a balancing act (bad pun). You don't want the boat to "hobby horse" either and you need enough structure there for strength. But heavy objects hould be as close to the center as possible.

By the way Australian 18's also use an open stern.