Port - Starboard ballasting

[blackrose]

My boat has a slight list to starboard of a few inches due to all the stuff along that side of the boat (batteries, small fridge, small freezer, wooden hi-fi cabinet, exercise weights, cassette, washing machine) which isn't fully balanced by what's on the port side. It's a fairly even weight distribution along the starboard side so I intuitively think that the most effective method (moving the smallest amount of ballast) is to remove ballast from around the middle of the boat (from bow to stern) from the starboard side and re-locate it to the port side around the middle too.

 

Is there any basis for my thinking that the further from the middle of the boat (fore to aft) that the ballast is taken/moved the less effect it will have? Or in other words, if removing ballast, or moving ballast to correct a list, does it actually matter where that ballast comes from & goes to (fore/aft)?

Edited January 28, 2021 by blackrose

[David Mack]

Moving ballast from one side to the other will affect the side to side trim. Moving it along the boat will affect the front to back trim. Doing both as you suggest will affect the trim in both directions. But as the boat is longer than it is wide, the effect of a given movement will be less in the fore and aft direction (and a change in fore and aft trim may well matter less).

[blackrose]

Thanks David. So it follows that ballast moved from the middle of one side of the boat and placed on the other side but in a fore or aft position would have less effect on correcting the list than if it was simply placed directly opposite where it was taken from? 

[Murflynn]

it makes absolutely no difference where along the boat you adjust the ballast port and starboard (athwartships).

 

[David Mack]

13 minutes ago, blackrose said:

Thanks David. So it follows that ballast moved from the middle of one side of the boat and placed on the other side but in a fore or aft position would have less effect on correcting the list than if it was simply placed directly opposite where it was taken from? 

How much and how far you move it sideways determines the effect on sideways trim (regardless of any fore and aft movement). Ditto for fore and aft movement.

The effects are additive.

[Tacet]

22 minutes ago, blackrose said:

Thanks David. So it follows that ballast moved from the middle of one side of the boat and placed on the other side but in a fore or aft position would have less effect on correcting the list than if it was simply placed directly opposite where it was taken from? 

Not necessarily, I think.  It must relate to displacement.

 

Assuming Blackrose is symmetrical about its longitudinal axis and forms a rigid box, it ought not to make a great deal of difference.   Where it gets complicated is that in moving ballast fore-and-aft, the displacement per degree post-starboard might change.  In an extreme (and impossible) example, if it became so bow-up that pretty much only the narrower swim area was below the waterline, it would require less weight to change the athwartships trim by one degree.

 

I  see Murflynn disagrees - and he may be correct!  I suggest you check the list whilst standing to one side at the bow and then again at the stern.  You will need a reference point.  Ideally you would leave the spirit level (or automatic level) in the same, say midship, position throughout so you will need a way/someone to read it.   My guess is that there will be no measurable difference.

 

 

 

Edited January 29, 2021 by Tacet

[Murflynn]

29 minutes ago, Tacet said:

 

I  see Murflynn disagrees - and he may be correct! 

 

 

 

it is very simple applied maths (or physics if you prefer)

just draw a force diagram on a cross-section and a long-section of any shaped boat and all will become clear.

[Tacet]

25 minutes ago, Murflynn said:

it is very simple applied maths (or physics if you prefer)

just draw a force diagram on a cross-section and a long-section of any shaped boat and all will become clear.

 

 So here is my boat:

 

It is a bit odd insofar as it has an asymmetric cross section - a little like a confused aircraft carrier.  For the front part, the element below (nearer) the waterline is on the port side and at the stern it is on the starboard side; the other side can be taken as above the waterline at all times.  When I move ballast from fore to aft, the boat goes bow or stern up, of course.

 

It is made even-keeled by ballasting at bow with the stern end elevated - possibly even above the water.  I move ballast aft along its longitudinal centre line (it is symmetric in plan - anything else would be silly!)  to the stern.  It is now bow-up, possibly even out of the water.  Will it remain even keeled?

 

As I say, you may be correct.  I have seen Blackrose and it is not entirely like my example but it does have varying cross sections and shifting ballast fore-and-aft will change the effective cross section for displacement purposes.  Not much.  Not enough to notice.

 

 

[GUMPY]

What I find interesting about all of this is that @blackrose has had this boat for about 10 years and has only just got round to deciding it has a list ?

 

[blackrose]

? Over 15 years!

 

Well the list has changed a bit over the years of course. Being wide I don't really notice it from inside, but at the moment I can get a really good view of it from head on and can clearly see a slight list.

[blackrose]

1 hour ago, David Mack said:

How much and how far you move it sideways determines the effect on sideways trim (regardless of any fore and aft movement). Ditto for fore and aft movement.

The effects are additive.

 

Yes, I was assuming any ballast from the starboard side would be placed in the same position relative to the port side. My question was about what effect any fore/aft movement of that ballast would have on port/starboard trim irrespective of bow/stern trim. I'm not really sure you've understood my question.

[bill brown]

Out boat lists a few degrees to port but handles well so personally I'd be loathe to change anything.

Edited January 29, 2021 by bill brown
typo correction

[Murflynn]

19 minutes ago, Tacet said:

 

 So here is my boat:

 

It is a bit odd insofar as it has an asymmetric cross section - a little like a confused aircraft carrier.  For the front part, the element below (nearer) the waterline is on the port side and at the stern it is on the starboard side; the other side can be taken as above the waterline at all times.  When I move ballast from fore to aft, the boat goes bow or stern up, of course.

 

It is made even-keeled by ballasting at bow with the stern end elevated - possibly even above the water.  I move ballast aft along its longitudinal centre line (it is symmetric in plan - anything else would be silly!)  to the stern.  It is now bow-up, possibly even out of the water.  Will it remain even keeled?

 

As I say, you may be correct.  I have seen Blackrose and it is not entirely like my example but it does have varying cross sections and shifting ballast fore-and-aft will change the effective cross section for displacement purposes.  Not much.  Not enough to notice.

 

 

 

it makes no difference.

 

if you move a 10kg weight from 1m left of centreline to 1m right of centreline the moment affecting the athwartships list will be the same if you do it anywhere along the boat.

you do not need to get confused with 3-dimensional considerations - that is why i said draw the force diagram (on a piece of paper having only 2 dimensions). 

the shape of the cross-section makes no difference, provided the boat is symmetrical about the fore-and-aft centreline (which is the case for 99.9% of vessels.

 

please don't over-think it, instead apply the KISS principle.

Edited January 29, 2021 by Murflynn

[David Mack]

20 minutes ago, blackrose said:

 

Yes, I was assuming any ballast from the starboard side would be placed in the same position relative to the port side. My question was about what effect any fore/aft movement of that ballast would have on port/starboard trim irrespective of bow/stern trim. I'm not really sure you've understood my question.

I don't know how many times I or others have to say this: If you move ballast sideways across the boat it will have a certain effect on the side-to-side trim. If at the same time you also move it fore/aft, that will make no difference to the change in side-to-side trim.

 

And you aren't limited to moving ballast from one side to the same offset from the centre line on the other side. Any lateral movement of ballast will change the lateral trim, even if it remains on the same side of the centre line. It's just that the effect will be greater the further you move it.

[Tacet]

29 minutes ago, Murflynn said:

that is why i said draw the force diagram (on a piece of paper having only 2 dimensions). 

the shape of the cross-section makes no difference, provided the boat is symmetrical about the fore-and-aft centreline (which is the case for 99.9% of vessels.

 

please don't over-think it, instead apply the KISS principle.

OK.  I am not sure myself, so I will (soon) let it go but it seems to me to be a 3D issue.  My theoretical concern is that by moving the force along the centreline it changes the trim and thus the boat is not effectively symmetrical about the fore-and-aft centre line.  A triangular boat (in waterline plan) would be effectively wider in beam as the force is moved towards the wide end and the narrower end rises.

 

Maybe I will make triangular boat and try it in the pond.

[GUMPY]

1 hour ago, blackrose said:

? Over 15 years!

 

Well the list has changed a bit over the years of course. Being wide I don't really notice it from inside, but at the moment I can get a really good view of it from head on and can clearly see a slight list.

I couldn't remember exactly how long just knew it was longer than a lot of people have owned their boats ?

[Bobbybass]

I buy those cylinder....plastic covered exercise weights...whenever I see them in charity shops ( 5 kilo...10 kilo etc)

They are quite clean and neat.

I put them under my lounge seats....and bottom of cupboards.

I must have over 200 kilos now.

As my loo tank fills up....I can shift some to the other side to keep us flying level.

 

Works for me.

[Murflynn]

2 hours ago, Tacet said:

OK.  I am not sure myself, so I will (soon) let it go but it seems to me to be a 3D issue.  My theoretical concern is that by moving the force along the centreline it changes the trim and thus the boat is not effectively symmetrical about the fore-and-aft centre line.  A triangular boat (in waterline plan) would be effectively wider in beam as the force is moved towards the wide end and the narrower end rises.

 

Maybe I will make triangular boat and try it in the pond.

in theory you could have a very odd-shaped boat will offer less or more resistance to listing to one side if the waterline plan is unconventional (e.g. triangular) and the ballast is moved fore or aft, but that excludes all narrowboats and widebeams.

 

perhaps an Americas Cup racing yacht or a Vendee Globe round the World wedge-shaped boat, but irrelevant for canal boats.  basically if the waterline plan of the boat is very roughly symmetrical fore and aft (like all conventional canal boats) then you can ignore this imaginary complication.

 

as I said - it doesn't pay to over-think it.

[Tacet]

3 hours ago, Murflynn said:

in theory you could have a very odd-shaped boat will offer less or more resistance to listing to one side if the waterline plan is unconventional (e.g. triangular) and the ballast is moved fore or aft, but that excludes all narrowboats and widebeams.

 

perhaps an Americas Cup racing yacht or a Vendee Globe round the World wedge-shaped boat, but irrelevant for canal boats.  basically if the waterline plan of the boat is very roughly symmetrical fore and aft (like all conventional canal boats) then you can ignore this imaginary complication.

 

as I said - it doesn't pay to over-think it.

You did say not to over think it.  But you also said it makes absolutely no difference (your bold) where along the boat you adjust the ballast port and starboard (athwartships) and it applied to any shape boat.  Then it was subject to being symmetrical along its longitudinal axis - which I accept is very common.  Now it is subject to (I think) the boat being symmetrical across it mid section - which is uncommon.

 

Most narrowboats and widebeams boats are not much different front and back at the waterline - but even by design, there is usually a difference particularly if a counter stern comes into play.  So, in practice no appreciable change to list consequent upon where, fore and aft, the ballast is positioned.  But in theory,  there is a difference unless the boat is symmetrical in plan about its central point.

 

I shall devote the time saved in not building a triangular boat towards thinking whether being trimmed by the head (or stern) has any effect.

[manxmike]

My boat developed a list for no apparent reason. One day level, two weeks later a distinct list to port. Fridge, cooker, couch, bed all to port side of boat. I managed to acquire a number of old sash weights from a company that installed double glazing and put them under cupboards on the starboard side.

I carefully checked the bilges - dry as a bone, nothing new added to port side. I can only assume that as the galvanic action eroded the hull, more went on the starboard side (where the people using most electricity from the mains were moored) than on the port. This is pure speculation, I have absolutely no scientific back up and am probably well wide of the mark.

I no longer have the boat, when the hull was surveyed at 2mm in places I was relieved to sell it (at a huge loss) as it would have cost more than I had available to over plate the hull.

One day I will hopefully be afloat again, preferably with an aluminium hull and an electric motor (ooh, that's put the cat amongst the pigeons!)

[Drayke]

Hi Blackrose.

 

Why not try taking some ballast out of the starboard side and take it off your boat then check how much of a difference it makes to you trim, port/starboard and bow/stern, then adjust accordingly.