Twin Keel Righting Moment

[Willallison]

Here's a "how long's a piece of string' question for you!
What you you regard as a typical RM30 for a twin (bilge) keeled, 50ft sailboat?

[SailDesign]

Never seen a 50' bilge-keeled sailboat...

[bhnautika]

Why would twin bilge keels have anything to do with RM30? Displacement X GZ

[MikeJohns]

Originally Posted by Willallison

Here's a "how long's a piece of string' question for you!
What you you regard as a typical RM30 for a twin (bilge) keeled, 50ft sailboat?

Will

Exactly the same as its typical sistership with a single keel.

[Willallison]

yes, yes - sorry - too much, yet not enough info! I know it will be the same as a single keeled job...
let me rephrase... does 14700 kg.m seem a sensible RM30 for a 50ft sailboat?

[MikeJohns]

Will
Yes that is a perfectly sensible figure.
Still a lot more to consider ....intended use....also consider whether she will be jerky ie, what will the roll inertia be , how much damping from her keel.......

Is this a theoretical exercise or a commission ?

[RHough]

Originally Posted by Willallison

yes, yes - sorry - too much, yet not enough info! I know it will be the same as a single keeled job...
let me rephrase... does 14700 kg.m seem a sensible RM30 for a 50ft sailboat?

Um ... close, what LWL?

14700 kg.m = 106,000 ft lb

From Skene's I'd expect that RM30 on a 41-43 ft LWL

[bhnautika]

What displacement are you estimating?

[Willallison]

Mike - No, no commission.... a little bird tells me you live in these parts.... I'm sure you'll hear about it the day I manage to land my 1st 'job'! ... just a lesson submission.

bhnautika - Off the top of my head, displacement is about 16 tons...

[MikeJohns]

Will , Rhough

Intended use infers consideration of all the design ratios before you start worrying too much about conformity with norms, so if the figure looks sensible its good enough for the design spiral, very easy to lower RM with out any major changes but much harder to increase it. So often best to shoot high then see how your rollover curves look, remember that stability drops drastically with all the parephenalia that gets afixed these days, so you should always account for all sorts of extra gear in your W & M calcs.


I have a 7 acre bush block in the hills behind Margate with 'glimpses' of the channel and storm bay. Getting time to sail North for winter.

[bhnautika]

Will, Do you think your design can get .9 of a metre GZ

[RHough]

While we're on the topic of RM ... What would be a good target for RM when the masthead is in the water (RM90?)?

for reference, the Mini-Transat rule requires that the boat be self righting with (IIRC) 45kg at the masthead, the ballast in any position and the masthead in the water.

Is there a guideline or rule of thumb that gives a number to shoot for?

[Willallison]

Mike - funny how small the world really is isn't it.... I've got 5 acres (and a shed almost big enough to build a boat....) just out of Kingston!

bhnatutika - I'll take a look tonight when I get home, not sure what the GZ was off the top of my head...

[bhnautika]

Rdough the mini-T’s have a max mast height of 12 metres above the waterline, assuming about 11 metres from the mast head to the new centre of buoyancy at 90 degrees of heel that’s around 495 kg\m. Displacement for these boats is around 900 Kg. A GZ of .6 of a metre would be the minimum at 90 degree.

[Willallison]

bhnautika - GZ is 0.76m

Incidentally, I've been thinking about the comments you guys made about RM being the same regardless of keel type. At the time this made sense, but on further thought I'm not sure I understand why....
The keel(s) are included in the 'model' used for the RM calculations and as there is so much variation between the two forms I would expect variation in the RM as well.