Overthinking ballast.

[MtB]

A question I've been turning over in my mind for years. 

 

If I remove 1/4 tonne of ballast from the bow of my boat, will the stern sit a shade deeper in the water as a result? 

 

Or will the stern stay the same and just the bow rise a bit? 

 

All opinions welcome! 

[Tonka]

4 minutes ago, MtB said:

A question I've been turning over in my mind for years. 

 

If I remove 1/4 tonne of ballast from the bow of my boat, will the stern sit a shade deeper in the water as a result? 

 

Or will the stern stay the same and just the bow rise a bit? 

 

All opinions welcome! 

Are you going to remove the ballast regardless of what we say, and to find out who was right

[jake_crew]

Mike.

 

When Pintail was first floated by us, the position of the Glenny (dictated by the internal bulkhead) meant she was a bit low at the back end with 7" freeboard.

 

The BSS inspector (who had already done the hull inspection when the boat was out of the water), had withheld granting the cert until he could see the freeboard.

 

We added nearly 1/4T of steel ballast under the floor at the fore-end to bring the back up to 8 1/2 " F/b.

 

At the same time the fore-end dropped by 4".

 

The inspector was happy with that.  We also stored the fuel and water at the (not so) sharp end too.

 

 

 

 

 

[Tracy D'arth]

1 hour ago, MtB said:

A question I've been turning over in my mind for years. 

 

If I remove 1/4 tonne of ballast from the bow of my boat, will the stern sit a shade deeper in the water as a result? 

 

Or will the stern stay the same and just the bow rise a bit? 

 

All opinions welcome! 

Of course the stern will be lower.  Have you noticed ever what happens when you fill the water tank?

[David Mack]

For a boat which is rectangular in plan with vertical sides and ends it is fairly easy to show mathematically that if you add or remove load from the centre the draught changes equally at both ends, if you add or remove load from the very end, the other end changes in draught by half the amount (and in the opposite direction) compared with the change in draught where the load is altered, and if you add or remove load at a point 1/3 of the way along, then there is no change in draught at the further end.

Real boats aren't simple rectangles, but if you imagine an equivalent rectangle, somewhat shorter than the full boat length, then you can estimate the effect. So if the centre of gravity of the ballast removed from the bow is on the end of the hypothetical equivalent boat, then the stern (of the equivalent rectangle) will rise by half the drop at the bow (of the equivalent rectangle). 

 

[Bee]

Its not a proper see-saw, I can fill the water tank at the front which then sits lower but the back doesn't really rise by anything noticable. there is a difference but to work it out needs equations and physics and all those things that I failed at 'O' level. What is interesting is if you load a barge - e.g. a peniche with some massive steel coils right in the middle it is possible to fold the thing like a jacknife, there are pics on You Tube showing just that.

[MtB]

46 minutes ago, Tracy D'arth said:

Of course the stern will be lower.  Have you noticed ever what happens when you fill the water tank?

 

You’re right. My brakes disappear!!