Impact of design characteristics on powerboat handling in following seas

[njbob]

I'm interested in the design characteristics that make for either good or bad performance and handling of inboard and outboard powered boats in following seas. Boats in the size range of say 20' to 50'. For example, how does the wide, shallow, flat stern sections of a typical downeast boat perform in a following sea compared to a typical deep vee sportfishing hull? How does the long deep keel of an inboard powered craft compared to the shallow or no keel outboard powered boat? Does more power and speed capability translate into better ability to handle following seas? Displacement hulls vs. planing hulls? Light vs. heavy? Etc.

I'm in the early stages of thinking about a boat for coastal cruising and would like to be aware of things to consider and not be too surprised by the handling characteristics of any boat I might end up choosing.

Thanks for any input to this conversation.

 

[gonzo]

Considering only operation in following seas, I think an inboard with a shallow keel and significant skeg works the best. Outboards often get their propeller ventilating because the stern lifts too much. Also, out boards and sterndrives depend on thrust for steering, Unlike inboards with rudders, they need more speed to maintain steering. Planing boats, on the other hand, can stay on top of wave but it requires expert handling.

 

[comfisherman]

Broad flat wide stern with no taper makes for poor performance on stern seas. We joke that our boats are perfectly designed to be terrible in a stern sea. Shallow draft doesn't help either with a wide beam.

Power helps, a little more throttle can really help tracking with a heavier stern swell.

My first boat had a sister ship that was identical except in the keel shape. Mine was several inches deeper and dog legged up and did not go fully to the bow, the sister ship had a full length keel that didn't go as deep. In every way identical except the keel, small gear ratio and prop size. Mine was deeper getting over sand bars but had a distinct advantage in free running situations. One of my haunts back then had a prevailing wind that ended up on your nose or stern depending on the day. Between the better bite of a bigger wheel and deeper ratio and the arrow fletching like nature of the keel always seemed to help keep me from spinning. Noticeably better than the sister ship.

My new boat is slightly wider, slightly shallower with a big fat flat stern, following sea isn't much fun but the extra couple hundred hp helps. Was a trade off as this one is better on the shallows but it does make me work harder in a following sea.

 

[lovesthesea]

The biggest threat with following seas is broaching. A flat bottom with no V entry is the best way to avoid broaching for the same reason you never see a surfboard with a V entry. A flat entry cuts into the wave instead of sliding down it.