Roll reduction solutions

I have a 50’ powerboat, a displacement hull with a fat keel that ends about 6’ ahead of the propeller.
Most of the boats voyages are in open ocean, often in much less than ideal conditions, where she exhibits an uncomfortable amount of roll.
Would extending/weighting the keel have any significant effect towards reducing the roll?
I think the added keel could hold a ton of lead, which might help bring her down nearer the DWL.
The waterline is quite a bit lower than the paint line in the picture, transom is only about 2” submerged at rest, and I’m thinking that the vessel is likely chronically under loaded, but I have no idea where the DWL should actually be.
Any thoughts or suggestions appreciated!

 

Ballast always helps a lot, but it can make the rolling period too rapid. Bilge keels don't raise or lower the boat's center of gravity (CoG), but they do cause some of the energy that causes rolling to be used to move more water, thus reducing the amplitude and speed of the roll. So I would do the following:
- Place bilge keels in the middle two-thirds of the length of the hull.
- Calculate the appropriate amount of ballast.
Since you don't have the boat's body lines, I would eliminate the second option.

 

That should be a warning to you that you are taking the boat out into sea states that she is not designed for, i.e. you should not be going into open ocean with such a boat.
I would change to a bigger and more suited boat.

However, if changing to a different boat is not possible and you insist on going out into seas that you should not...all you can do, as, as you note, add some weight to the keel, how much, trial and error - try incremental amounts and gauge the effect. Each time adding more. Does it do what you want...who knows given the hull form.?
Since, as noted just adding weight low down, does not always solve the issue - but it helps. Since it provides more restoring lever and may - or may not - change the roll period by an amount that satisfies you.

Additional, adding bilge keels, roughly as shown, will reduce the roll amplitude, but not the period.

 

Adding weight at the keel will reduce the roll amplitude, but also reduce the roll period. The movement will be faster and "snappy" which is more uncomfortable. Adding weight to the bottom of the boat at the inside, spread out would make a more comfortable roll. Bilge keels are also good because they dampen the roll.

 

Frankly, I don't know why "inside" could be better than "outside." Is there something that justifies that claim, some physical law that I'm unable to guess?
"Comfortable" is a completely subjective concept. Engineers, they say, don't work with impressions but with facts.

 

Yes it is called "moment of inertia". It is a physical law that was recently discovered; sometime in the 17th Century. Motion induced comfort assessment has been quantified and is a branch of engineering. It is closely related to ergonomics; another branch of engineering. You need to go back to school

 

I consulted with my school, and they very kindly confirmed what I already knew: the rolling period does indeed depend on the moment of inertia, but on the water line shape. And that value, for the same amount of ballast, is the same whether the ballast is outside or inside the boat.
The term "ergonomics" sounds familiar, and I know it has nothing to do with the sensation experienced by one person or another when the boat heels.
My school is amazing, isn't it? It always agrees with me.
I'm very sorry that you, judging by what you've shown, don't have a school to go to.

 

Pretty presumptuous of you to determine from thousands of miles away that I operate my vessel in an unsafe manner, but I guess I did ask for comments, so thanks I guess. I did say “uncomfortable “ roll, and did not infer that it was unsafe.
The question I intended to pose was more concerning the effect of lengthening of the keel on rolling.
I see the keel as a large fin extending below the hull, that sweeps from side to side when waves tip the boat.
Wouldn’t making the fin larger offer more resistance to rolling?

 

Yes, more keel will dampen the rolling.

 

How about putting less added weight in a gyro stabiliser instead ? Possibly less increase of fuel consumption, probably more comfort, somewhat tunable roll behaviour ... but you'd also forgo the additional protection of shaftline and prop, and certainly spend a lot more $$$

 

Flopper stoppers?

Or

A short mast with a stabilizing sail, as one does on a motor sailer. The effect on roll behavior of a stabilizing sail is well worth the effort.

Edit: stabilizing sails don't work when there is no wind. But you will always have wind.

 

Also, that is a remarkably round section, so it would be astonishing if she didn't roll like an expletive..

Bilge keels, and a short mast and sail.

 

Steady sails were traditionally used on ballasted round bilge hulls to dampen violent motion. I have been aboard one in a heavy beam sea and it was hard work as we had little warning and objects were flying across the deck.

 

That hull looks like a typical lobster boat. Down East they traditionally say that a rolly boat is a good sea boat. The reason being that the boat will roll with the waves instead of being stiff and shipping water.