If it gets up to 3-5 foot seas, running on plane is
NOT a good idea.
Here's why: Wave height is 9/5 of the height of the seas, because seas are measured from sea-
level to crest. So in 3'-5' seas, you're talking about waves that are from 5 1/2 to 9 feet high from trough to crest. You need to listen carefully to the forecast. If NOAA says "seas" are 3'-5', then wave height is 5'-9'. If NOAA says "wave height" is 3'-5', then seas are only 1.6' - 2.7'. Knowing the difference can be important.
If you get just one steep-sider or cresting wave in 3'-5' seas, you can stuff even a SeaCraft while running at 18-20 mph, with potentially tragic consequences. If you run off the front of a wave you can drop the bow right through the back of the next wave. Either of those events at 20 mph will just ruin your day. I know, I've done it.
I stuffed a 20' Seafari back in '84, running in to Lake Worth Inlet in a sudden NorthEast winter squall. Seas went from 1-3' with a wave period of 5-6 seconds, to 4-7' with a wave period of 3-4 seconds, all in about 20 minutes time.
(It eventually grew to 8'-12') As I was running in from the outer reef off Juno, I was running with the seas at about 18-20 mph, occasionally getting nearly airborne, when I flew over the crest of one wave, and stuffed the bow right into the back of the next wave. Up to that point it had been a lot of fun, a real blast.
The impact ripped my bow rail loose from all 4 forward stanchions and bent it back, and cause minor separation of the cap from the hull along a 4' stretch on the port bow. As the forward cabin hatch was open when we hit the back of the next wave, I must have taken 100 gallons or more of water through the hatch. I slammed my head into the windshield frame, and took a gash requiring 4 stitches. My fishing buddy was standing behind the left pedestal seat, and his weight and impetus ripped the 8 #14 base screws right out of the deck, and he broke his thumb somehow. All because I was in too much of a hurry.
The next time just a few weeks later, being older & wiser, I ran in at the same speed of the waves without any problems, even though the mouth of Lake Worth Inlet was breaking 13'-15'. I just rode the back of a wave in, and it was smooth as silk. It just took longer to get there.
When seas get up suddenly, be prudent, don't give in to an attack of "gethomeitus", use good seamanship, and get back safely.
