SB 18 centre of gravity/centre of flotation

Sail Boats 15' and up. Please include the boat type in your question.
fragi
Frequent Poster
Frequent Poster
Posts: 159
Joined: Sun Jul 19, 2009 5:56 am

SB 18 centre of gravity/centre of flotation

Post by fragi » Tue Jun 22, 2010 1:25 pm

Where are centre of flotation and centre of gravity located in SB 18 CB version. Where should ballast be installed according to these theoritical points?



User avatar
jacquesmm
Site Admin
Site Admin
Posts: 27315
Joined: Wed Aug 14, 2002 1:00 am
Location: Florida USA
Contact:

Re: SB 18 centre of gravity/centre of flotation

Post by jacquesmm » Tue Jun 22, 2010 1:51 pm

The LCG is 9.62" aft of the tip of the bow, the LCB at 9.21.
The plans shows where to put the ballast.
Note that our calculations are based on our own assumptions of weight distribution. In such a small boat, small differences in weight can produce large differences in trim.
I always like to install the small inside ballast after launching the boat or I keep some as trim ballast.
Jacques Mertens - Designer
http://bateau.com

fragi
Frequent Poster
Frequent Poster
Posts: 159
Joined: Sun Jul 19, 2009 5:56 am

Re: SB 18 centre of gravity/centre of flotation

Post by fragi » Tue Jun 22, 2010 2:18 pm

In the plans is reffered 114kg of ballast installed at the sides of the trunk, nothing more. Should ballast be installed at the front part of trunk, near the high load blocks, or aft to frame C. Is it possible to put it forward near the mast base? Declinations in weight trimming, in relationship with ballast position, is to possible to drive me in dangerous conditions or simply trims the boat aft or forward? Does it affect planning. Should I worry about a missed construction and lower performance?

User avatar
jacquesmm
Site Admin
Site Admin
Posts: 27315
Joined: Wed Aug 14, 2002 1:00 am
Location: Florida USA
Contact:

Re: SB 18 centre of gravity/centre of flotation

Post by jacquesmm » Tue Jun 22, 2010 3:04 pm

Too early to worry.

That's what I call trim ballast.
The main ballast is in the keel bulb, the inside ballast is trim ballast.
Put the ballast somewhere in the middle.
Launch the boat, rig the boat, get your gear onboard as if sailing, put the normal crew in the cockpit and move the ballast so that the trim is correct. That is the best way to do it since small variations in weight are impossible to avoid.
Jacques Mertens - Designer
http://bateau.com

fragi
Frequent Poster
Frequent Poster
Posts: 159
Joined: Sun Jul 19, 2009 5:56 am

Re: SB 18 centre of gravity/centre of flotation

Post by fragi » Tue Jun 22, 2010 4:58 pm

Can I add ballast on the sole near the trunk?
Can I put it near the sides?
Can I install less ballast? (what about the vertical position of CG and CB?)
I have built the CB version.

fragi
Frequent Poster
Frequent Poster
Posts: 159
Joined: Sun Jul 19, 2009 5:56 am

SB-18:stringers

Post by fragi » Tue Jun 22, 2010 5:09 pm

Do I need to add additional stringers below deck, cockpit sole, at the foredeck and at the sides of cockpit for additional support to decking?

User avatar
lncc63
Very Active Poster
Very Active Poster
Posts: 527
Joined: Thu Feb 02, 2006 8:46 pm
Location: Philippines

Re: SB 18 centre of gravity/centre of flotation

Post by lncc63 » Wed Jun 23, 2010 1:01 am

Hi guys,

You must be really busy Jacques. I believe you have the two version mixed up. Isn't the 114kg of internal ballast for the CB version? The bulb keel version does not carry any internal ballast.

Thanks for pointing out the need to be able to move some of the internal ballast around if one wants to adjust the trim. I pretty much fixed my mind on trimming with live ballast.

Fragi the plans do not call for any additional support for the deck, seats, and soles. Quite a few builders, including myself, did add supports though. I think it would really depend on the plywood you have. In retrospect, a better option might be to glass the underside of those surfaces if you have plywood like mine. Best to wait for what the designers say.

Louis

User avatar
jacquesmm
Site Admin
Site Admin
Posts: 27315
Joined: Wed Aug 14, 2002 1:00 am
Location: Florida USA
Contact:

Re: SB 18 centre of gravity/centre of flotation

Post by jacquesmm » Wed Jun 23, 2010 8:03 am

You are correct Louis, I mixed the CB version with the bulb keel one.
The LCG and LCB are correct. For the ballast, see the plans.
Jacques Mertens - Designer
http://bateau.com

fragi
Frequent Poster
Frequent Poster
Posts: 159
Joined: Sun Jul 19, 2009 5:56 am

Re: SB 18 centre of gravity/centre of flotation

Post by fragi » Wed Jun 23, 2010 8:07 am

this is the exact position for LCB and LCG for CB or Bulb. I am confused. What about my other question about additional stringers?
And when I will trim the boat should it have an inclination forward, aft or no inclination?

User avatar
jacquesmm
Site Admin
Site Admin
Posts: 27315
Joined: Wed Aug 14, 2002 1:00 am
Location: Florida USA
Contact:

Re: SB 18 centre of gravity/centre of flotation

Post by jacquesmm » Wed Jun 23, 2010 10:18 am

The LCG and LCB are the same for each version but what are you trying to do? Which version are you building?
Jacques Mertens - Designer
http://bateau.com

Post Reply

Who is online

Users browsing this forum: No registered users and 3 guests