Ya gotta go read this! Stability issues eh?

[Boston]

ya I been following that thread over at WB for a while
those guys are classic
I like the comment at the beginning just as they are about to launch/sink
"now we'll see why it is you really dont need an NA"
and then
ooppss

Originally Posted by Alik

Richard, Do You remember the photo of 60' cat I posted here? Designed by amateur, sitting bowdown?
Yes I do!
Do You know what they do now? The have cut FRP hulls at WL, removed the old bottoms and adding new bottoms with increased and re-destributed displacement! So if someone needs two 60' kanoes - welcome, CIF from Thailand... Maybe I should picture that!

Oh yes, please post a picture. Though I do´nt understand, what´s wrong with the method?

[MikeJohns]

Originally Posted by Alik

...............The have cut FRP hulls at WL, removed the old bottoms and adding new bottoms with increased and re-destributed displacement! ................

I've seen this done with a 20 million 45m alloy vessel. A sudden red faced discovery by the chief NA, the design team forgot to import one of the major modules for the weights and moments calc.

To err is human but to really stuff up don't question the numbers from the computer Always tricky the first vessel of a new class, after than it's easier to spot the errors.

[Alik]

Originally Posted by MikeJohns

Always tricky the first vessel of a new class, after than it's easier to spot the errors.

For that boat no calculations have been performed... Building such a big cat by trial and error - gamble for customer's money.

[powerabout]

I didnt realise that building a scale model costs more than building the real thing so thats why they dont do it???

[aranda1984]

It seems like they couldn't make up their mind what to build.

As far as I know, we do not have a computer program that would give you a perfect design just by asking for the type of the boat and the length!
What you put in is what you will get out!

By no means am I an expert, but the old two thirds rule seems to be right in everything. (From art to engineering!)

The first deck is two thirds of the hull, the upper deck is two thirds of the first deck...

This is clearly just gestimating, but if you make your boat using this principle and if you place the Cg about 56% aft, your boat will float safely!

Imagine being out on rough water in that top heavy contraption with 40-50 Knt winds...
For that "thing"to be safe, with all those structures built on deck, the hull would have be at least 50' longer.

I had a teacher friend once who was teaching machine shop practices in a small town High School. He built a 75' sail boat in his "spare time" out of steel.
The craft had that "starving horse" look as all the welds at the ribs were showing.

Once finished, he had to arrange transportation for an extra-wide load for a 500 miile trip to the sea!
Well, at least he had the smarts not to put the lead ballast into the keel before shipping!

The right use of the computer is to do an FEA on the design before even buying one screw for the boat!

Chears,

Stephen I. M.