Originally Posted by Nostrodamus
I believe it has something to do with swell height and the time period between swells but i'm not sure.
You are correct. Wave height is meaningless unless you know the interval. A swell of 12' every 17 seconds is far more benign than a swell of 8' every 8 seconds, the latter of which will entail steeper waves and will thus affect the boat more.
Realize that swells tend to spread out the further they get from the source of energy that generates them. A long swell interval indicates that they may have traveled for days from away from a low pressure system. This can create frustrating conditions for sailing because you may not have the necessary pressure to deal with the swells. Upwind this means you won't be able to point as high, downwind this means that your sails
will not be able to hold their shape much of the time, even if using a preventer.
A following swell will have the greatest effect on a boat that is able to surf. That same boat will have greater difficulty carrying its momentum into the swell, especially when the waves get square. In other words, swell will have more effect on a sailboat that accelerates well than a sailboat that doesn't.
I used to race
for a syndicate that actively campaigned two boats. One was light displacement
, the other was an ultralight. We could look at the wind forecast
and wave report and know which boat was more likely to win on any given day. Unfortunately, what this means in terms of the OP's original question is that how swell affects boat speed is highly dependent upon the boat. It's ultimately a question of VMG, and this not only becomes highly complex, but it becomes highly variable. The only real answer is: it depends not only on the swell, but upon the boat.
Regardless, there's no way someone could answer, "Add or subtract 10% of boat speed for any swell over 6'." It just doesn't work that way.