Where to put the batteries?

[MYStargazer]

I have a semi-displacement mobo, which I want to completely re-wire.

She rolls like a pig at anchor in any swell, so it makes sense to place as much as weight possible as low down, and as centrally as possible.

The (four) batteries are currently under a galley seat: high up and to one side of the centreline.

I want to put them under the floor, in a vented but otherwise sealed box.

Where are batteries normally installed? Would I be daft having them so low down?

Thanks...

 

[sailorman]

I have a semi-displacement mobo, which I want to completely re-wire.

She rolls like a pig at anchor in any swell, so it makes sense to place as much as weight possible as low down, and as centrally as possible.

The (four) batteries are currently under a galley seat: high up and to one side of the centreline.

I want to put them under the floor, in a vented but otherwise sealed box.

Where are batteries normally installed? Would I be daft having them so low down?

Thanks...

Flopper stoppers would be a help

 

[sarabande]

in effect you are altering the CofG. That's fine, but it's the CofG, Metacentric height, and CofB that are in a variable relationship to influence the roll period and roll angle. Of primary importance, though, is the hull shape, so unless you are going to spend ages altering and tuning the three params mentioned, I'd consider the addition of bilge plates. If the problem is only at anchor, then the flopper stopper mentioned by Sailorman, is an easy first step.

If you put the batteries way down low, the boat might tend to 'whip' upright a bit more quickly, though much will depend on the displacement and how low the batteries go. That can be quite disturbing to the inner ear and lead to vestibular disorientation, discomfort and seasickness.

Batteries are normally stored out of the bilge for dryness' sake, and ease of access/checking. Under the bunks is fairly common.

 

[alahol2]

I have a semi-displacement mobo, which I want to completely re-wire.

She rolls like a pig at anchor in any swell, so it makes sense to place as much as weight possible as low down, and as centrally as possible.

Not necessarily, if you move the mass closer to the centre of the roll, you could even exacerbate the problem and make the boat roll more sharply. The inertia of the batteries where they are will be slowing the roll marginally. So unless there are other reasons to move them, I wouldn't bother.
As for the roll, a riding sail may be your best bet.

Edit: Must type faster and learn more big words.

 

[Rossynant]

I have a semi-displacement mobo, which I want to completely re-wire.
She rolls like a pig at anchor in any swell, so it makes sense to place as much as weight possible as low down, and as centrally as possible.

Quite the contrary, as others said above.
Placing weight low the GM (metacentric) will be increased - gives you more 'static stability' which is exactly the reason of rolling in waves.
To make rolling less, especially to make it slower, you need to place weights (mass strictly speaking) outside from axis of roll, as far from center as practicable. Not too high, this will be bad for stability, but to the side. Under a seat sounds good.

 

[Kelpie]

A mate of mine has a large displacement fishing boat, he initially placed a large quantity of heavy chain right in the bilge, centrally. He found that when stopped to fish, the boat would roll sickeningly. He was advised to split the ballast and place it under the gunwales. Not as low down, but much further apart laterally. It does seem counter-intuitive but evidently it works.

Oh and on batteries... unless you can hand-crank your engine, I would place at least one battery relatively high up, in case of a bilge flood. When we first got our boat there was a weeping sea cock somewhere and on our first overnight the water level rose to within an inch of the tops of the batteries. The leak has since been fixed and the engine start battery moved to a higher location!

 

[MYStargazer]

Thanks, chaps!

I wasn't expecting those responses at all. It's completely counter-intuitive, isn't it? Perhaps this explains their current locations.

Have just been trying to find something to explain the science behind this, but they all seem to say low and central is good (which in general terms makes sense), e.g. https://docs.google.com/viewer?a=v&...Z-cEs1&sig=AHIEtbTJ2PNwIOu0MACSsuGeY6BvROAjNg

 

[Rossynant]

It's completely counter-intuitive, isn't it?
Have just been trying to find something to explain the science behind this, but they all seem to say low and central is good

Counter-intuitive? Think anoter way:
Figuratively speaking - what a "righting lever" actually does is keeping the boat level with water surface, parallel to it. What happens when water surface is not level?

 

[MYStargazer]

Counter-intuitive? Think anoter way:
Figuratively speaking - what a "righting lever" actually does is keeping the boat level with water surface, parallel to it. What happens when water surface is not level?

Not entirely sure I follow you!

I'm thinking it may work because it's easy to swing a dangling weight from one side to the other (as with low-down, central ballast), but hard to lift a weight (as with ballast spread laterally out towards the sides of the hull). Is this what you mean?

 

[Rossynant]

No, what I mean is boat stability tries to keep the boat lying parallel to water. So when wave comes from the side boat will be forced to conform to the slope of water surface The more 'stability' she has, especially at small angles of heel, the more she will heel - and more quickly. Stiff boat responds more quickly as it attempts to assume the slope of the wave. With less stability she will not be so quick and wave may pass under before she gets to the slope angle.
Another thing is period of roll - seasickness is more related to period (quickness of movement, accelerations) then to angle of rolling.
Actually there is a formula for natural rolling period of boat: it's dependent on stability and inertia. The higher GM (static stability) the quicker motion is; higher inertia moment - slower motion.
Inertia here is not about lifting the mass on the side, but similar - it's hard to turn boat when masses are spread far from axis of turn, outside.

I'm not so good at English language, but look for explanation in here http://en.wikipedia.org/wiki/Metacentric_height

 

[macd]

Several suggestions of flopper-stoppers up there. The OP has a mobo, which is much less amenable to deploying the simpler kind of flopper-stoppers since it has no boom. (Also trickier to fit a riding sail ). Flopper-stoppers are also substantially less effective against local waves than against swell, since their recovery period is much reduced.

 

[alant]

A mate of mine has a large displacement fishing boat, he initially placed a large quantity of heavy chain right in the bilge, centrally. He found that when stopped to fish, the boat would roll sickeningly. He was advised to split the ballast and place it under the gunwales. Not as low down, but much further apart laterally. It does seem counter-intuitive but evidently it works.

Oh and on batteries... unless you can hand-crank your engine, I would place at least one battery relatively high up, in case of a bilge flood. When we first got our boat there was a weeping sea cock somewhere and on our first overnight the water level rose to within an inch of the tops of the batteries. The leak has since been fixed and the engine start battery moved to a higher location!

Your mates fishing boat, has probably got a battery at a high level, to serve VHF etc, where it will still function when rest of boat is swamped - think its in their regs.

 

[alant]

I have a semi-displacement mobo, which I want to completely re-wire.

She rolls like a pig at anchor in any swell, so it makes sense to place as much as weight possible as low down, and as centrally as possible.

The (four) batteries are currently under a galley seat: high up and to one side of the centreline.

I want to put them under the floor, in a vented but otherwise sealed box.

Where are batteries normally installed? Would I be daft having them so low down?

Thanks...

You could always fit a mast & when affordable, hang some saily things on it!

 

[MYStargazer]

No, what I mean is boat stability tries to keep the boat lying parallel to water. So when wave comes from the side boat will be forced to conform to the slope of water surface The more 'stability' she has, especially at small angles of heel, the more she will heel - and more quickly. Stiff boat responds more quickly as it attempts to assume the slope of the wave. With less stability she will not be so quick and wave may pass under before she gets to the slope angle.
Another thing is period of roll - seasickness is more related to period (quickness of movement, accelerations) then to angle of rolling.
Actually there is a formula for natural rolling period of boat: it's dependent on stability and inertia. The higher GM (static stability) the quicker motion is; higher inertia moment - slower motion.
Inertia here is not about lifting the mass on the side, but similar - it's hard to turn boat when masses are spread far from axis of turn, outside.

I'm not so good at English language, but look for explanation in here http://en.wikipedia.org/wiki/Metacentric_height

Nooowwww I geddit! Thanks!

 

[Kelpie]

Does the same effect give bilge keelers a more comfortable rolling motion?

 

[jwilson]

As others have said, moving batteries low and central will almost certainly make rolling worse, though it will improve the ultimate stability (abilty to self-right from being VERY heeled over).

 

[Rossynant]

Does the same effect give bilge keelers a more comfortable rolling motion?

Not really - but bilgekeels provide more damping against roll, they resist sideways movement. In fact bilge keels were used on displacement motor boats or passenger ships for this reason.
Usually - considering the same sailboat in two keel versions - stability and inertia will be similar for fin keel and twin keels, at least this is what designer wants to achieve. But bilge keels will have more lateral area, so will require more energy to push them aside. There is also more to this, for instance shape of keel - bilgekeels tend to be shallow but longer then finkeel, this shape generate more sideway force; if they are really located at turn of bilge then it's farther from roll axis; etc.