Richard Atkin - re underwater-hull form thread

[Alex.A]

2008-jan Thread-Underwater hull form - you say that you - with the help of Rick W -are designing a cat with little or no rocker.... what happened?
Did it come off? Wondering about the design, as it was an interesting thread.

[Fanie]

Hi Alex,

A sail boat has to have rocker or it will be a bugger to tack.

To limit actual rocking however you have to fatten the stern some. The bow of a displacement hull has to be sharp to cut through the water, so you can do little there to stop rocking, hence the only other place is the stern.

Richard's answer here Boxy Fisher Catamaran

[Alex.A]

Yeah - i realise this - but idea's vary on exactly how much - trying to see what minimum is.... by seeing what others have done.
With big enough boards and use of sails it is viable tho... especially for a cruiser, as tacking is/should be less?
Add to this that a faster hull form would mean faster passages?
Obviously it reduces loading but faster would help there too......

Blue bulls suck and the ref was blind!! =) =)
You know where i'm from now.

[Fanie]

If there is zero rocker it could be very unpleasant to sail. A little rocker is kinda like a shock absorber in a car, it just makes the ride slightly more comfy.

Too much would have the same effect as no shocks. You're going to be bobbing all the time.

Who is the blue balls ?

[Alex.A]

What you not a rugby fan? =)

This would be true - if flat bottomed - but if vee'd along the whole length?
Add volume in ends high up and keep ends light!
Load in middle and have a cruising rig.
Besides there are racing ccats (small) that have no rocker and tack just fine - i realise that this is due to boards and speed into tack... so follow this concept?

[Fanie]

No, I'm a boat fan

Even if you have a vee along the whole length, if the hull widens out to restrict bobbing it will be the same. It's about where the hull will resist being pushed into the water. One reason why should know what the draft is going to be. Doesn't have to be flat bottomed to have or have not rocker, although it is a lot more pleasant to walk on a flat hull than on a hollow one and flat does help, but wider helps also..

If you have a hollow hull you may have to add a floor, lots of extra work and of course weight.

[Alex.A]

Draft 450mm (+-) and yes to the floor.
Are you still doing headrm in hulls and top?

[Fanie]

Hi Alex,

Yes, 1.9m in the hulls. Adds a lot to comfort and I want decent bridgedeck clearance. I haven't started on the cabin, but the hulls is next. Lots of changes as well. Should get the rest of the jigs parts this week.

Be patient. Soon.

[Alex.A]

Strength for dealing with the local suppliers!!!!

[Alex.A]

OK.... just read the main thread for Rich's cat.....
Get a tahiti Wayfarer by Wharram.