Downeasters, keel variations and following seas

[YankeeBoater]

Hello people - I am concerned with boat manners in a following sea, and in particular, minimizing the tendancy to broach. I have a fondness for the character of Downeast boats, some of which have the reputation for being eager to broach in a following sea.

The traditional boats are "built-down" (deep draft for a semidisplacement boat) with a full keel and a deep, fine forefoot, very low deadrise aft, and inboard power. My understanding is that the deep and sharp bow is great in a headsea for splitting it; but when running downsea it tends to bury into the wave in front of it, and "steer" like a rudder in a random fashion. With the flat run aft there is not a lot of resistance to the stern pivoting if it gets a push from behind, all of which can spell trouble.

So, assuming that people agree with the preceding (feel free to correct, clarify or elaborate), nowadays, on modest sized boats like this (mid 20 foot range), there are powering options, often accompanied by modifications to the keel. My question is regarding the merits of full keel/inboard powering versus a shallow keel that is cutaway about 2/3 of the way back to give clean water to an outboard. Which should have safer handling running downsea? I have heard it argued that the full keel is the best, because it provides resistance to the transom swinging around. But, I have also heard it argued that it's the worst *lol* because if the boat pivots a little, the keel can actually give the seas more to push against from behind.

Can anyone clear this up for me? Is the full keel a help or a hinderance in a following sea? I can see that the outboard (or an I/O) would give greater steering force at the stern than an inboard/rudder. I can also see that a little fuller bow, while it may increase the impacts of waves from a headsea when running at speed (12-18 knots for one of these), it should be less susceptible to burying in a wave when running downsea (basically for the same reason, it offers more resistance to penetrating the water).

What I can't see/sea is what the keel possibilities are going to do running downsea? Is it a function of speed also?

Thanks in advance for sharing your knowledge,
Barry

P.S. Designs from different vendors vary, but I have attached some rough profile figures to illustrate.

[tom28571]

The idea that a following wave is going to push against the keel and make the broach more likely seems faulty to me. Assuming the keel is immersed all the time and the actual motion of the water is not faster in the direction of wave travel than the boat, how could this be true? The water particles are actually moving in a circular path within the wave and the net forward motion is very small. If that is true and I think it is, a keel or skeg is helpful in preventing broaches.

A sharp, low-buoyancy bow and a broad, low-deadrise stern is a recipe for broaches in following waves. A less sharp entry, forward flare for greater buoyancy, steeper aft deadrise, aft keel or skeg, waterline beam that narrows toward the stern and a powerful rudder are all good anti-broach factors, but not guarantees.

[FAST FRED]

The hassle is most lobster boats will have a small rudder , optomising speed , and expecting the prop wash to assist in steering.
When you slow way down the rudder becomes too small to overcome any bow steering.

Simply pay the price in drag for a good sized rudder. Look at any sailboat for the concept.

The wider the immersed transom , the MORE rudder is needed in following seas..

FAST FRED