waterweight below the waterline
Think of a small "experiment" to make this sink into your mind in an easy way to remember.
Say you have a stick with a weight on one end like a ol grandfather clock pendulum.
depending on where you grab the stick between thumb and forefinger, as a pivot point, will affect the way the pendulum swings (and hence the how the stick above your grip point also swings).
If you grab somewhere near the "balance point" the pendulum will swing back n forth about the same distance & at the same rate for the stick above your grip and for the stick and weight below your grip.
Think of the fulcrum point you grip the stick as the metacentric height of a vessel now.
As you move your grip down toward the weight - the top of the stick, will swing further and further over, - the shorter bottom part of the stick with pendulum weight will travell less and less, the closer your grip gets to the pendulum weight at the bottom.
If you get close enough to the pendulum weight where the top part of the stick actually weighs more than the short part left and it's pendulum weight combined - the whole thing will tip over...
This is basically what happens to any vessel, as it's metacentre is lowered...below it's point of bouyancy.
The balance point of the vessel is where it's downward gravitational weight, meets and balances it's upward bouyancy point from the water it displaces.
Adding weight below waterline, is like adding weight to the pendulum on your stick, it affects both the balance point, and the way the pendulum swings (the boats ability to right itself).
We need to understand, that adding weight above the blance point (usually above decks in our small boats excercise) lowers the vessel further into the water as it now displaces more water...ie it lowers the balance point, at the same time as it adds weight to the top of the imaginary stick, ie it counterbalances the weight on the bottom of the stick...making the stick easier to tip over and slower to come back to straight up and down (it's ability to right itself).
Pretty simple really - adding weight below the waterline, similarly lowers the balance point as it also displaces the same weight, it's just that it doesn't add weight above the metacentre, it actually has the effect of making your pendulum heavier thus righting the vessel easier (quicker)...even tho it's sitting lower in the water.
Getting the picture?
Whether mono or cat - they are both just a hull displacing water, in one case a single hull displacing water or in the case of a cat two hulls fixed toegther with a plank across the middle, so the effect on metacentric height/hence roll moment is the same (assuming the water below waterline is equal in both hulls - i.e. you've flooded both sponsons equally, you haven't just holed ONE sponson - which is a different kettle of fish entirely).
Vessels that self right quickly, and only roll over a little, coz the metacentre (pivot point) is a long way up from the pendulum weight, are said to be very "stable" - they self right quickly - they almost snap back to straight up and down..these can at times be quite uncomfortable to ride long distances in some sea conditions.
Vessles that roll a long way over slowly and a long way back slowly are said to be tender (and people get sea sick in em easy).
The best riding deep V boats (24degree) tend to be "tender" - they roll back n forth a long way slowly....adding weight up high like fly bridges makes em roll further. More passengers up there makes em roll even further again!
All the passengers up there at once, when fuel and water below decks is almost used up, can sometimes with a little wind assistance and a good beam swell - tip em over!
Flat bottom boats like a john boat are said to be stable but they ride bad enough to knock your fillings out and make your kidneys pee blood.
Somewhere in between is nice...
Single centre aft Rudder steered "tender" boats, tend to be hard to steer for the novice. Thats because as they roll a long way back n forth - the rudder angle varies with the roll, and the novice tends to steer against the roll - basically building forearm and bisceps muscles like Popeye, rather than holding the wheel steady thru the roll and only making course corrections to the helm at verticle, then waiting for them to take effect thru the next two roll moments..
It takes some doing to train yourself NOT to "oversteer" on such vessels.
For those of us with OB powered boats the above steering comments have little application.
clear as mud?
Cheers!
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