Ballasting and trim

[Brent Swain]

If you have made some changes to a design, and are not sure where to put your ballast so she will float level, the solution is simple.
Jack her up and put a piece of shaft under the keel at the designed Longitudinal centre of gravity. Move your ballast inside, fore and aft, until she rocks easily on the shaft. That is where your ballast should go.
As most boats tend to squat , and go down by the stern when you load them up, slightly heavier in the bow is a good thing.
Trim ballast?
On a cruising boat, that is your tools, spare anchors , scuba gear, and any other heavy stuff

 

[sarabande]

How do you then resolve issues with the centre of buoyancy and the centre of flotation ? What about moments of polar inertia in relation to adding ballast ?

 

[lw395]

I tried to do this once.
Fairly small errors in the hull lines move the centre of buoyancy a surprising distance.
The easiest way to find the centre of buoyancy turned out to be floating the boat, ballasting it level, then drying it out and seeing where it balanced.
Two examples of a 'one design' class balanced differently.

 

[Beelzebub]

I tried to do this once.
Fairly small errors in the hull lines move the centre of buoyancy a surprising distance.
The easiest way to find the centre of buoyancy turned out to be floating the boat, ballasting it level, then drying it out and seeing where it balanced.
Two examples of a 'one design' class balanced differently.[/QUOTE

I was once shown around one of the Beneteau factories by Mme Roux, the chairman. She explained that Beneteau only used female laminators as they followed the design spec exactly whilst males tended to be too generous and this could result in trim differences, not to mention extra material costs!

 

[Brent Swain]

How do you then resolve issues with the centre of buoyancy and the centre of flotation ? What about moments of polar inertia in relation to adding ballast ?

If she balances on that point, she will float level when she hits the water, period!
I weld lifting eyes in the bulwark at that point. Several of my boats have been lifted out and launched with cranes that way. If one end hangs down a bit ,a comealong to the hook, lightly loaded, straightens her out easily.

 

[Brent Swain]

I tried to do this once.
Fairly small errors in the hull lines move the centre of buoyancy a surprising distance.
The easiest way to find the centre of buoyancy turned out to be floating the boat, ballasting it level, then drying it out and seeing where it balanced.
Two examples of a 'one design' class balanced differently.[/QUOTE

I was once shown around one of the Beneteau factories by Mme Roux, the chairman. She explained that Beneteau only used female laminators as they followed the design spec exactly whilst males tended to be too generous and this could result in trim differences, not to mention extra material costs!

This is why some of my clients prefer welding students. They will listen to you. One hired some long term professional welders who worked mainly with thick steel. Knew it all . Couldn't tell them anything. They sure warped hell out of his deck!

 

[Brent Swain]

I tried to do this once.
Fairly small errors in the hull lines move the centre of buoyancy a surprising distance.
The easiest way to find the centre of buoyancy turned out to be floating the boat, ballasting it level, then drying it out and seeing where it balanced.
Two examples of a 'one design' class balanced differently.

This completely avoids the risk of a small math error screwing things up , making everything from that point on wrong. Practical solutions are far more reliable.

 

[lw395]

This completely avoids the risk of a small math error screwing things up , making everything from that point on wrong. Practical solutions are far more reliable.

You miss my point entirely.
The point is that the actual centre of buoyancy is very often not exactly the design centre of buoyancy.
Unless the hull is some weird double ender, it will also move according to the displacement.

 

[DownWest]

This completely avoids the risk of a small math error screwing things up , making everything from that point on wrong. Practical solutions are far more reliable.

Brent, you are now on the racing design section of the forum. Such 'suck it and see' methods of resolving buoyancy and weight problems don't work. Boats are designed to close tolerances to conform to rating rules, by maths. It is not difficult if you spend some time learning how.
Racing boats are not corrected by chucking cruising kit in either end to get it balanced. Expect to be at the back of the fleet if you do.

Also, let's say you increase the fullness of the aft sections a bit. The actual extra material in the hull will not be much, so will not affect the balance on a bit of bar. But, the extra volume will be significant and raise the stern.
Area of the hull increases by a square law, volume by a cube law. Google Simpson's rule. It might help.

 

[Brent Swain]

You miss my point entirely.
The point is that the actual centre of buoyancy is very often not exactly the design centre of buoyancy.
Unless the hull is some weird double ender, it will also move according to the displacement.

Where she floats is her real centre of buoyancy.

 

[Brent Swain]

Brent, you are now on the racing design section of the forum. Such 'suck it and see' methods of resolving buoyancy and weight problems don't work. Boats are designed to close tolerances to conform to rating rules, by maths. It is not difficult if you spend some time learning how.
Racing boats are not corrected by chucking cruising kit in either end to get it balanced. Expect to be at the back of the fleet if you do.

Also, let's say you increase the fullness of the aft sections a bit. The actual extra material in the hull will not be much, so will not affect the balance on a bit of bar. But, the extra volume will be significant and raise the stern.
Area of the hull increases by a square law, volume by a cube law. Google Simpson's rule. It might help.

Yes, volume is far more affected than surface area. An inch more material means a huge increase in volume. I have juggled those numbers and changes on all my designs, in the initial calculations.
Simpsons rule, or law of mechanical similitude?
Herreschoff has some interesting points on that in his book, "The Common Sense of Yacht Design ".