If your serious about working the San Juan currents to your advantage you will want to purchase this current atlas, produced by the Canadian Hydrographic Service. The Atlas is available at West Marine as well as many retailers for about $30
When cruising, having this information handy can save you time and fuel and make the difference whether you get to your planned destination. For instance a boat heading north up Haro Strait near San Juan Island will get a big boost, however by checking the next several hours one would see that the free ride is going to end soon and the train will come roaring back. As a rule of thumb the currents flow north on a flood tide and south on ebb tides but as you can see, rules have exceptions. This forecast is just before the flood tide reverses and becomes ebb, an apt description would be "confused" The confusion will soon end, get your ticket to ride, its free.
This is what the currents looked like on July 13th at 7:49 am, also July 14th at 8:39 am and July15th at 9:26 am. As you can see, your vessel will get a free ride south just about everywhere, but each day at a slightly different time. When planning your passages and your cruise it would be very helpful to consult the Current Atlas first, a simple free ride one week will be the wrong direction the next week. You may want to choose a different departure date.
While were on subject of currents, picture in your mind these big arrows meeting opposing winds and you should then visualize big, steep (dangerous) waves slamming your boat. Then after a few hours the current changes, and all is well in Camelot once again.
this post should be read if big waves bother you >>>wind-versus-current-and-predicting waves
Lately, I have made it part of my cruising to use as little fuel as possible, one cruise we covered about 120 miles over 8 days and used 6 gallons of gas, and most of the fuel was used battling an opposing current between Jones Island and James Island. Unfortunately my schedule dictated the time.