It depends on whether you wish to enjoy it or not and on the size of your boat and the strength of your crew. I have a 28 footer and find up to F4 enjoyable. Above that it becomes less so. Also remember that you can have a flat sea at F4 or very lumpy depending on where you are and wind direction. If the wind is from the land, the sea can be less than 0.5m SWH even when 5nm offshore.
We take ours out if its forecast f7 but a few times we have done f8 gusting 9 but not nice.Worst was biscay f10....never againEdit...My avtar pic was f7 i think, head to wind.
If a planing dinghy you'll be looking forward to a wind of F3-4, if a displacement dinghy ie non-planing and stable, say 16ft - 18 ft then F4 would be Ok.
Wind over tide conditions will create steep waves ; wind with tide flatter longer ones and a good fast sail, surfing and planing.
More practice gives more experience and confidence when caught out in stronger winds. Often a dinghy can benefit from a single mainsail reef being taken in on ,say, a Wayfarer,GP14 ,and then the higher windspeeds become less worriesome.
For some real fun try surfing down the face of 2-3ft waves, moving your weight forward as the stern starts to lift then easing the jib; as she starts to increase speed, bring the crew weight aft - and hang on, ready for the next wave to send you off again, 8-10knots often attainable after practice, and enjoyable even in a displacement dinghy. Love it !!
In a dinghy, just ask yourself, what will happen if I capsize? If you are confident of righting her & clearing the water out, then the limit is whatever you can mostly keep her upright-ish in. I sail a Laser & expect to flip from time to time, but after a couple of capsizes it is sensible to come in anyway because it gets harder as you get colder & more tired.
Do you have any ability to reef? I can part roll my sail around the mast, some (Mirrors) can remove the jib & move the mast forward to keep balance. Bigger dinks like Wayfarer, GP 14, etc may have R/R jib & smaller or reefable mains.
Lakes mean that waves are generally much smaller & even if you fail to get her up you will eventually blow ashore somewhere & be able to sort it all out. The sea is more dangerous altogether - especially with an offshore wind.
It's your decision - ALWAYS! So just be sensible, consider the consequences of it all going wrong & extend your envelope a little at a time as you increase in confidence, experience & ability. Happy sailing.
I avoid a forecast above F4, particularly if strong gusts are predicted. However, I've cancelled a few sails when the weather has been calmer than forecast. And what does one do with a Met Office forecast of 10 mph and 34 mph gusts as given for Morecambe Bay this Wed?
I sail a 25 footer with one crew. Both of us are OAPs, not so sprightly as in past years.
This is a matter of experience. You need to have the boat set up for strong winds with good reefing and suitable jib. Or have a roller jib set up for optimum when small. ie correct sheeting position.
You need to remember that sailing down wind is always easy even in very strong winds. It is working to windward that needs to be mastered. This of course is when the wave height will be a problem as well.
My little (light) 21fter is ok to windward in 25 knots At 30 knots it will get us home to windward with a nasty short shallow water wave but it is not competitive at that wind speed and not really that much fun. Of course it is very wet and usually you just want to get home.
I am usually happy to depart at 20 knots provided we reef everything down before departure. Tiny jib and 2 reefs. In fact there is a satisfaction in achieving a pleasure sail success in strong winds if you keep it short.
We have a local wind speed observation internet site. i always check to see what they reckon we were sailing in. (I am always a bit disappointed) but it is a way of getting used to what is achievable.
If however you are not prepared and get caught down wind in a blow with the boat not set up you are asking for trouble from passengers /spouses who may be put off sailing forever. So be careful. olewill