Calculating ballast

[LUCE1923]

HI you wonderful people, I have a 36ft springer (so has a keel) boat  which I have just had replated. I am looking to put the floor down but am wondering about ballast. Currently there is nothing on the boat but obviously its been replated so heavier. It now weighs 7 tonnes. I intend putting a wood burner, cooker, shower etc and also have to line it out. Its currently in the water any idea how I work out what if any ballast I need?

[BEngo]

How much is the current draft in metres?  Fore and aft.  How much do you want the final draft to be, in metres?  Fore and aft.

Add fore and aft together and divide by 2, for each case.

Take the difference between the now figure and the final figure. Multiply by the beam  (2.1m) and the length at full beam, (about 10m I would guess) then by 1000.  That is the extra weight you have to add,  in kg.  Approximately.

Deduct the weight  of your  fit out and what is left is the ballast you need.

 

Easiest to fit out first and leave a way to add ballast.  The method above will still tell you how much ballast you need.  You need to be able to add/move/extract ballast to get the trim right in any case so access is important however you go at it.

N

[LUCE1923]

Thank you makes sense aside from draft?

[BEngo]

How deep is the boat in the water? Distance from waterline to the bottom of the V, ideally.  That is draft, or draught.

 

N

 

[Tracy D'arth]

Possible very little ballast needed on a reskinned Springer of 36' but you will need to trim the weight side to side to level the boat after you have added everything inside.

[Murflynn]

4 hours ago, BEngo said:

How deep is the boat in the water? Distance from waterline to the bottom of the V, ideally.  That is draft, or draught.

 

N

 

not quite.   for a Springer you need to measure the draft at the side and at the centre ('keel'), add together and divide by 2 to get the mean draft at any location.

[LUCE1923]

its 2.23 metres currently

[BEngo]

Just now, LUCE1923 said:

its 2.23 metres currently

I am afraid you are measuring incorrectly.  2.33 m is over 7ft and

A.  There a few canals that deep

B.  A Springer drawing that much will be under water at the gunwales.

 

You need the distance from the bottom of the V hull to the waterline, as it is now and as you want it to be,  at the bow and at the stern.  It is likely to be about 200 -300 mm now at a very rough guess and about 500 mm when finished (the counter plate should be just in the water and there should be a slope up along the gunwales of about 25-50 mm between front and back).

 

Murflynn above is technically correct, if you want to estimate actual displacements,  but all that detail falls out in the calculations if you just want to work out the CHANGE  in displacement, which is what you want.

 

N

[tehmarks]

12 minutes ago, LUCE1923 said:

its 2.23 metres currently

Has it sunk?

[LUCE1923]

oh this was the measurement all the way down, I measured in 3 places from water level to water level, so the draft should be halved so 1.15mtrs is in the water its has a V shaped hull

[Iain_S]

That's still very seep draughted *; 3ft 9in approximately for those who thimk imperial.

 

edited to add " *for a narrowboat."

Edited June 12, 2020 by Iain_S

[BEngo]

Inside the boat, measure the height from the bottom of the V to the gunwale.  Outside the boat, while standing on the bank, measure from the gunwale to the waterline.  Subtract the second measurement from the first.  That is the draught at the place you took the outside measurement.

N

 

 

[WotEver]

2 minutes ago, BEngo said:

Inside the boat, measure the height from the bottom of the V to the gunwale.

Vertically, not sloping up the side...

[LUCE1923]

ok will do that tomorrow

[Murflynn]

11 hours ago, LUCE1923 said:

ok will do that tomorrow

...............  after the salvage company has re-floated the boat ??  

 

 

seriously - good luck with your measurements, we all have to start somewhere.