SailNet Community - View Single Post - Taipan 28 - How well offshore?
View Single Post
  #16  
Old 11-06-2003
Jeff_H's Avatar
Jeff_H Jeff_H is offline
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,503
Thanks: 3
Thanked 81 Times in 62 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
Taipan 28 - How well offshore?

Keel material density is important to both stability and motion comfort. In any boat, the lower the ballast the more effective it can be in in developing stability and similarly the lower it is the longer the lever arm for developing a large roll moment of intertia which is important to minimizing roll speed, angle, and roll accelleration.

Cast lead is literally twice as dense as scrap iron and concrete ballast used in boats like yours. In a typical cast lead keel installation on a boat the size of yours, the center of gravity is somewhere between 9 and 12 inches above the bottom of the keel. When you talk about a scrap iron and concrete, the center is often 16 to 20 inches above the bottom of the keel. When you consider that the center of buoyancy on a boat like yours is generally quite deep, 16 to 18 inches below the water line the impact of this lower density is profound. From a stabilty standpoint the decreased density and therefore shorter lever arm means that the concrete/iron ballast will only contribute roughly 70% to the righting moment of an equal weight lead ballast. It has much more profound effect on the roll moment of inertia where the concrete/iron ballast will only contribute roughly 35% to the roll moment of inertia of an equal weight lead ballast. One way to offset this is that boats with low density ballast generally have higher ballast ratios (and higher displacements), greater depth (which your boat is comparatively shallow) and larger keel volumes (which means more drag as well).

Good luck,
Jeff
Reply With Quote Share with Facebook