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]

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]

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'!:p ... 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.

[SailDesign]

Will,
Not so much that the RM _will_ be the same, but that you would _want_ the same to provide the same power to carry sail for a given displacement. You'd look awfully silly doing 2 knots uphill because yuou couldn't carry enough sail......
Steve "my 2-cents"

[Willallison]

Ah - now that makes sense:)

[bhnautika]

Will sorry I mis-understood the intent of the original question. There are obviously two parts. CG and volume distribution in RM’s. Yes there would be a small difference in the distribution of volume of the two styles of boats as they heeled but it would be small difference in the GZ (mm), I was assuming a set CG. If we are talking CG then bilge keelers tend to have a higher centre of gravity and a lower RM than deep keelers.
Not seeing the sectional shape I think .76m might be a bit optimistic for a bilge keeler. As it is, you have about 12000Kg\m RM30

[MikeJohns]

There are many hulls that come in two forms Single inline or twin keel. The hull is often identical in both only the keels vary. The COG could easily be made identical and hence your heeled RM.

As Steve pointed RM is your power to carry sail and you do not see reduced rigs in the twin keel versions.

A problem some people have is a perception that one keel of the twin will have more influence when heeled, but of course the COG remains firmly fixed.

[bhnautika]

Mike I am fully aware that the CG is fixed. My point is the centre of gravity of the keels is higher on the bilge-keeled boat than a deep keel boat so it influences the total CG of the boat and so the righting arm. On two identical boats with the two types of keels, yes you could make the cg’s the same but which one would be easier to adjust and why?

[Willallison]

In theory I'd agree with you Mike - though in practice, I found that in order to obtain the appropriate CLP / lead etc the total underwater volume of the twin keels needed to be much greater than would have been the case with a single CL keel.
Which in turn may answer bhnautika's rather cryptic post. The increase volume would allow simpler movement of ballast - and hence the CG. Then again this is really only in the LCG. If you were trying to alter VCG I'd expect a CL keel to make things easier

BTW bh (I hope you don't mind me calling you that...;) which bit of Oz do you hail from?

[bhnautika]

will sunny vic

[Willallison]

isn't that an oxymoron...:p ;)

[bhnautika]

Ok sometimes sunny