Is it worth increasing keel weigth on particular boat?

Discussion in 'Stability' started by cuorefocoso, Dec 22, 2007.

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cuorefocosoJunior Member

terhohalme,
I think the same, I would like to have it some heavier.
But would i gain something from it? Eternal question ? And again, what more should I do not to damage the boat?
From my point of view I should reinforce the bottom a bit, specially in places of keel fixing and a bit around. Just for info, here are some pictures of the boat and how the keel is attached http://cuorefocoso.blogspot.com/

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cuorefocosoJunior Member

What a hell and where did I go wrong? :!:
I did rough calculation, and got finally stuck; according it, my new keel should be even better working against heeling than original one. Evenmore, I think the guy who changed the keel was thinking something similar.
What I did: I tried to estimate COG of both keels. Assuming the underwater part of the boat is approx 0.3m, the original keel height would be D1.5-0.3=1.2m and new keel D1.8-0.3=1.5. As per original lead keel, I would say COG=height/2=1.2/2=0.6m from point of attachement to the hull; if we count it from WL or better form COB as Guillermo advised the arm for an old keel would be ~0.6+0.3-0.1=0.8m. For new keel it is a bit more complicated - I have estimated COG as following: lower lead part should have its own cog in 1/3 part from down part of it (see the picture), I think it is something as 0.3m height, so ~0.1m; upper GRP part could be estimated as (height of keel)-(height of bulb)=1.5-0.3=1.2. As it is not very complicately shaped, i would assume cog of fin should be approx in the middle of it, so 0.6m. By method of approximation, I get COG of new keel something as 1.26m from point of attachement to the hull; so the arm from COB should be ~1.26+0.3-0.1=1.46m.
Now, if we would like to get kind moment, we should multiply it by weight (shouldn't? ), then old keel = 800kg x 0.8m=640kgm, for new keel = 450kg x 1.46m=657kgm. As I said, I am confused looking at this.
(first and second pictures are same boats as mine with a bit different but mostly "conservative" keels; third and fourth pictures are my keel bulb views)

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3. Joined: Mar 2005
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GuillermoIngeniero Naval

Rimas,
there is an additional consideration to take into account: by taking off 350 kg from the keel (and thus from displacement of the boat) draught is less and so waterplane area -and its inertia- will be smaller. Let's asume KG remains more or less the same: depending on hull forms, the metacentric radius (I/V which varies with Bwl^2) may significantly diminish, thus reducing GM.

As I told you, you need a full and proper stab analysis to be performed.

Cheers.

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JoakimSenior Member

The problem is that you are refering to COB, which does not work at all when you are mainly interested in sailing performance at normal sailing heels (0-30 degrees).

Refering moments to COB suggest that adding weight at COB doesn't increase stability and adding weight above COB decreases stability. Both of them are not true!

There are even several boat types, which have a floating or at least very light keel and still have a good stability due to water (non-movable) or other ballast inside the boat.

The situation is more compex when you consider safety and heel angles of ~90. You don't have problems with that, since your boat has a limit of positive stability already around 115 degrees, which is a good value for a boat of this type.

http://www.atm.ox.ac.uk/rowing/physics/basics.html#section6
http://www.owlnet.rice.edu/~nava102/presentations/lesson23.ppt
http://www.globalsecurity.org/military/library/policy/navy/nrtc/14057_ppr_ch12.pdf

5. Joined: Mar 2005
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GuillermoIngeniero Naval

Rimas,
Also your new keel is deeper in draft, which shifts the center of lateral resistance (CLR) downwards. Assuming the rig and sails have not changed, the heeling arm, or the distance from the sail plan’s center of effort to the keel’s CLR, has increased. This causes the heeling moments being bigger than they were, which counteracts the equal (let's asume that) righting moments, so the boat heels more.

Cheers.

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cuorefocosoJunior Member

Not sure I told it, sometimes it happens - you have it in your head, and think that everyone knows it...
For power compensating when the boat will gain weight, I have a plan to use genoa 1 instead of genoa 3 now, or 22 sq.m compared to 15. In terms of handicap it should make no sense, I mean bigger weight will bring the rating down more than increased sail area should keep it up. So, the only concern might be downwind in light air, but I hope to live with it somehow.

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