Here is a link to wind conditions for the lake at Marblehead but it is usually accurate for the entire western basin. If the forcast for Sunday is correct it may be to choppy for your 18 ft boat. It has been my experience that a steady 15mph wind from the NE will produce steady 3Ft to 4ft waves and that is not including the wind gust into the equation or the occasional rouge wave. This could change by then so use good judgement and be safe. Another thing about the wave forcast is the NOAA sites do not take into consideration the valleys in the waves but measure from the low water datum or the current average water depth, so the wave can and will be 1/3 larger than stated when actually on the water and in the valley's of the waves.
If you rely solely on the NOAA Weather station reports you will be disapointed more often than not. It will end up being calm on days that they have forcast as windy and windy on days that they forecast as being calm, so be aware of this.
http://www.iwindsurf.com/windandwher...orecast+Graphs