Andrei, the speed of a sailboat is limited by its length, as is intimated above. If your boat is 21', that is its Length On Deck
(LOD). But there's not 21' of boat in the water
: the Length at WaterLine (LWL) is going to be a bit less than the LOD. Let's give you 18 feet (with a little heel).
To calculate your boat's hull speed, take the square of the LWL (the square root of 18 is 4.24) and multiply by the constant 1.34, for a theoretical hull speed of about 5.7kts., or right on 6.5mph. That assumes adequate wind, a clean underside, and efficiently trimmed sails
. With 10kts. of wind or so you might reach your hull speed, but additional wind will not translate into additional speed as you seem to imagine: excess wind (maybe above 15kts. in a boat your size) will have to be compensated for by adjusting your sails
to "spill" the excess through without harnessing its power. That is a topic for a whole treatise, but in general, you ease the sheets
(the lines that control the sails' trim) to spill wind through them and keep the boat from heeling over excessively. You will learn some of this as you go.
You're not likely to see that max. hull speed on a regular basis. You will likely avg. around 4kts. Hey, let's be realistic: winds shift and die, the bottom of the boat is of unknown cleanness, and you are a novice
sailor who should not expect to get the most out of the boat.
To plan your trip, I suggest you use 3kts. of boat speed to do your figuring. That makes a 7-hr. trip out of 20 miles. If the wind is coming from, or from near, the direction you want to go, you will have to tack back and forth (sail a zig-zag pattern) that will put your miles travelled into the high 20s, because adding up all the zigs and zags will equal much more than the distance of a straight line from start to destination
(a line called a "rhumb line" on a chart).
That's very conservative, and you may do the trip in significantly less time, but figuring conservatively is prudent. If you have to sail into the wind, it's conceivable that this trip could be longer than daylight hours will give you in one day. If you sail after dusk, you will need to display navigation
lights, or find some place to pull up near shore to spend the night, so an anchor
is a necessity.
So after I'd anticipated and allowed for those difficulties, I'd make sure I had enough food
for two days and enough water for four days; a functioning electrical system
, including navigation
lights, and a working anchor
and plenty of anchor rode
, and some kind of chart for the area. Now I could choose either to sail through the night or take a break and get some sleep.
In addition, there is a whole long list of other equipment
that would be nice to have: a couple of good flashlights might top my list. Take some time to think out how you will eat and what appliances/supplies that will require, what additional clothing
might come in handy (think hours and hours of unrelenting sun, and also more and colder wind than you expect, and dry clothes to replace the ones that will get wet), if you over-night, what you will use for bedding, what might be useful to tie things together (a coil of line or a few shock (Bungee) cords), a handful of basic hand tools, etc. Going through your camping gear
might yield several good choices.
If you're adamant about doing this without an engine as a novice
, it could be a great little sail, but you should be prepared in case you have a few challenges along the way.