VCG on 40.7 and effect of 100kg+ keelboot

[Anticipation407]

I am a boat owner but not a naval architect.
I need to establish the approximate theoretical effect of adding 100kg or more below the bottom of a 2.4m deep 40.7 keel as some of our fleet have done this and most have not.
I have access to Maxsurf and SPAN
I can approximate the 40.7 hull in Maxsurf but need a reasonable estimate for VCG to run it in SPAN with normal keel and the booted keel.
Looking through this forum it would seem to range from deck height for motor vessels to around DWL to 0.25m below DWL with or without crew weight at deck level.
And do I need to know the weight of the keel and bulb?
Can anyone suggest what to do?

[MikeJohns]

I take it this is a Benetau 40.7 ?

Have you tried contacting the designer? (Bruce Farr) he should be able to tell you whether the keel supporting structure is up to the added weight too.

LOA 39'3"
LWL 34'10"
Beam 12'4"
Draft 7'9" (lead standard keel), 6'2" (iron shoal-draft keel)
Displacement 15,211 lbs.
Ballast 5,952 lbs.
Sail Area 799.83 sq. ft.
SA/D 20.85
D/L 162
L/B 3.18

[Guillermo]

To estimate VCG (or KG) you may try this rough approximation:

BM = Bwl^2 / 12Tc
GM = (0.44*Bwl/T)^2
BG = BM-GM
KB = Tc/3
KG = KB + BG

being:
Bwl = waterline beam
Tc = body draught
T = natural rolling period (from rolling test)

K located at root of keel

Cheers.

[Guillermo]

Another (equally rough but more complicated) formula:

KB = (0,843 - Cb/3*Cwp)*Tc
BM =(0,07032*Cp+0,01039*Cp^2*Cwp^2)*Bwl^3*Lwl/Vol
GM = (0.44*Bwl/T)^2
BG = BM-GM
KG = KB + BG


Cp = Prismatic coefficient
Cwp = Waterplane area coefficient
Vol = displacement volume.

Cheers.

[Powerboatman]

This is an interesting one. From the manufacturers data I see the 40.7 has two keel options (one iron, one lead, each with different draft/weight). I also see Beneteau upgraded the model with a different mast/rigging option so are you looking at the new or old model? Depending on which option the key dimensions (displacement/draft/DWL) are likely to vary. If you want to do an accurate theoretical exercise for both types you will have 4 different VCGs. Option 1: lead keel/old mast, option 2: iron keel/old mast, option 3: lead keel/new mast, option 4: iron keel/new mast and then 8 options when you add lead to the keel. With a published light displacement of 6970kg the addition of 100kg equates to a 1.4% increase in overall displacement. Despite this being theoretical you are likely to find that real world conditions and loading (eg crew, fittings, freshwater load, diesel load, battery fit etc) will have a far greater effect on actual VCG than the extra 100kg on the keel. Good luck.

[DuncanG]

IMS certificates contain useful info on righting moments and VCG.

You can find one for a 40.7 here - http://www.avomeripurjehtijat.fi/certlist/index.php