Paul,
You are not giving us enough information to work with. Tell us more about your boat. 
I'm a shoal keel with a three blade prop kept in reverse when sailing and vintage Dec.1990.
Using a 110% jib, full battened loose footed main sail, not reefed and 18 kts. of wind and a very clean bottom, we can normally do 7.7 kts. close hauled.
If we do any planing, we figure for 5 kts. max.
Hope that helps and good luck on your trip.
Bill

On a beam reach, a clean hull and 20 knots of wind, not surfing, 8.3 knots. But normally with that kind of wind 7+ knots. 
130% genoa reefed a few turns and one reef in the main. 
Standard rig.
Enjoy your trip!

Henk.

If the boat is fully powered up (sufficient sail and wind) it will max out at the "hull speed" as it tries to climb the bow wave in front of her.  In more than 15 kts of wind speed, we normally have more sail power than we can use, so added friction such as props or bottom fouling just are overcome. You cannot overcome the bow wave.  With a 30 ft waterline, theoretical max speed is 7.1 kts using the formula for a displacement boat of 1.3 x Sq Rt of LWL (​https://sailboatdata.com/sailboat/catalina-36-mk-ii) .  I have seen above 8 kts when surfing on waves, but that doesn't last more than a few seconds.  No matter how good your sail trim, how smooth her bottom, or how much wind, you will not sustain more than about 7.1 kts.  

For passage planning, we normally pick either 5 or 6 kts, depending on how agressive and optimistic we are feeling.  For under a day or on deliveries, I will pick 6 kts, and figure that I will motor sail if there's not enough wind.  For leisurely sailing or battling upwind, I'll drop it to 5 kts which basically adds two hours to a 60 mile passage (12 vs. 10 hours).   Maximum speed never figures into cruise planning - completely impractical to sustain max racing speed for days on-end, unless you have a race crew to watch her around the clock.