Stability of small trawlers

[tranmkp]

Please help me understand: a small trawler, with full keel, mostly there for tracking and to house the shaft I suppose.


Would would be the effect of adding ballast inside the keel area? Would it modify the boats roll characteristics? Or would it just make is slower?

[TollyWally]

I would imagine that the speed issue would revolve around whether or not it is a displacement hull if the weight is kept within reason. The roll charactoristic will probably speed up with more of a snap. Possibly less comfortable but able to stand up to harsher conditions. My .02, others will chime in with perhaps more qualified opinions

[Ike]

Well, it depends. On some it is just there to help the boat track straight and protect the hull and prop. On others they actually do put ballast in the keel to iincrease stability. Of course as Tolly said that shortens the roll period and gives it more of a snap. It all depends on the intended use of the trawler and the area where it will be used. If the area has bad weather and seas much of the time then stability becomes really crucial. The maybe a little ballast may help.

[Guillermo]

Adding ballast in the keel will certainly increase initial stability but may be detrimental to stability at big angles of heel (ultimate stability), if it significantly diminish freeboard. Every boat is a case.

Cheers.

[MikeJohns]

Often people ballast fishing boats thinking that by adding stability they will make it more comfortable.
The resultant increase in GM usually makes the roll far more violent and the boat feels even more unstable.
It is ironic that as the roll becomes more gentle and even 'lazy' in its response to waves the vessel is approaching a dangerous level of instability, this is counter intuitive to many people who confuse comfortable motion with stability.

Cheers

[tranmkp]

This is all good info...

[Highlanderwj]

My experience- in the gulf of Maine 8-10 ft seas, the boat was rolling too slowly or hanging there at the end of the roll. Very scary. So - the water tanks are above the water line and I pumped them out as fast as possible.
Much better! A little snap roll is way better than a terminal swim.

Carl

[jehardiman]

As Guillermo and Mike Johns said, it really is dependant on the current stability and hull shape and without some data is almost impossible to know the true answer.

FWIW, here is the problem:

GM is the distance between the Vertical CG (called KG for "Keel to CG") and the apparent point of rotation of the vessel, the Metacenter (KM for "Keel to Metacenter"), so GM = KM - KG. The righting moment just as the vessel starts to roll is the sine of the roll angle * GM * displacement. So you can see that the larger the GM, the more the vessel wants to return upright. If GM is too large, the vessel is jerky in roll, if GM is too small, it doesn't want to roll back upright.

The real issue with adding weight to manipulate GM is in how KM relates to KG. KM is actually the sum of two factors: KB, the vertical distance from the keel to the center of buoyancy and BM which is the distance from the center of buoyancy to the Metacenter. BM is calculated by Ixx/V where Ixx is the inertia of the waterplane and V is the displacement of the hull in volume. (for a full discussion of all the math in this go here (http://www.boatdesign.net/forums/boat-building/calculation-gm-8852.html#post60405) and ignore my snide comments in that thread). When we add ballast low a number of things happen. KG decreases, KB increases, V increases, and Ixx can either increase or not depending on hull shape. Depending on the interplay between those factors, GM could increase, could decrease, or could remain the same.

So in answer to your original question, tranmkp, adding weight will change your roll period, but right now there is not enough information to say wether it will make it "better" or worse.