Stability Calculations

[Unstable]

When you are calculating the stability (RM etc) should you include the keel etc in the underbody volume?

[bhnautika]

...Yes

[Unstable]

...and yet you don't include the keel in the displacement calculation (on fin keeled yachts, at least)...?

[Raggi_Thor]

I think you should include the keel in all calculations.

[SailDesign]

If you don't include the keel in all calculations, then you are fooling yourself, and the paying customer, too.
Steve

[Raggi_Thor]

Do we agree again?
I hope no one is watching this :-)

[SailDesign]

As long as you stay in Norway, and I stay in the US, we're probably safe from someone waching BOTH of us at the same time...
That said, "Fools seldom differ", and "Great minds think alike" are both popular sayings, proving once again that most writers know nothing of what they write about.....
Steve

[messabout]

Unstable:
Why would you not include the weight of the fin and bulb in displacement calculation ??? Only two reasons I can think of are 1. Violating the laws of physics. 2. Designing to some convoluted rating rule.

There is a guy on a nearby thread who calculates that his LCG is not in the same location as his LCB. I will bet a chicken dinner that it will be in the same location as soon as he puts the boat in the water. There may be some new age physics that I missed.? Man I gotta go back to school.

[Stephen Ditmore]

I think the issue is whether you include the VOLUME of the keel in the displacement (not the weight). I agree with those who say the answer is yes.

There's a related question, though, which is whether to include the keel + ballast bulb in the midships area when calculating prismatic coefficient. Where the keel is a distinct fin, the most informed designers I've heard on this subject say no.

[Raggi_Thor]

If the fin is short it seems reasonable to leave it out of the area curve and the Cp, and if there is a bulb it's probably so far away from the hull that it doesn't affect the flow under the canoe body(?).
If the fin gets longer, when should it be included in the area curve and Cp?
Remember the jumbojets with their top floor, it's OK because of the wing volume right behind :-)

[Unstable]

As Stephen Ditmore suggests, it is the volume of the keel that I'm thinking about.
Some texts that I have read say you should include the volume of the keel in long, traditional keel types where it makes up a significant part of the underwater volume. But that it isn't included when doing the calculation for a fin-keeled boat. Quite why, I don't know....hence the question.
Also this is for the displacement calculation. None of them say whether it should be included when doing the stability calcs

[SailDesign]

Basic rule would be to always include everything when doing any calcs. Leaving out deep skinny keels is less "incorrect" than leaving out long, fat keels, but it is inaccurate.
For most calculations, the error can be small, but if you are calculating stability, then the buoyancy of everything - keel, rudder, prop, shaft, strut - should be taking into account. The further it is from the VCB, the more effect it has on overall stability (i.e. leave out the prop shaft, etc. by all means), but the bulb IS important, and will decrease stability results, so is VERY important to leave in.
Steve "picky..."