Powering a vessel to close to hull speed
takes very little HP if the vessel is propped correctly and the bottom is clean
. I'll bet that I could push that boat to 6kts with an inflatable
and a 15hp O/B with relatively no problem (if it has a clean bottom). A properly running 13hp inboard should have no problem pushing a sailboat at 5.5-6kts. in calm conditions.
When you get into choppy waters, the vessel will be slowed every time that she hits a wave but if you have choppy conditions, you also have wind and should be sailing anyway.
Current does not enter the equation because even if you are in a 4kt current, you will achieve 2kts over the ground. If you had a 30hp engine, you may achieve and addition 1 - 1 1/2 kts. Current (by itself) won't lower your speed through the water
Where you may have difficulty is if you are trying to enter a harbor (as mentioned above). Then you may need to put up the main (sheeted in tight and motor sail, tacking up the channel. In severe conditions, you may have to wait outside but I would say that would be pretty rare unless you are in a very wind prone area.
The boat is obviously under powered but I wouldn't consider it unusable by any means. As stated above, it's a heck of a lot better than no engine at all.