A Fog Horn.
The sound signal that you should hear when a power boat is in the fog is one prolonged blast at intervals of no more than 2 minutes.
A boat make bbbbbbbbbbmev sound.
According to the Navigation Rules, a rapidly ringing bell every minute signals a boat at anchor in the fog.
Fog?
What's fog? Fog is caused by the condensation of water molecules in air. This leads to the increase in the density in air. So, the layman answer can be: Velocity of sound is greater in Denser materials, so during fog velocity of sound is greater. Mathematicaly: v2=Bulk modulus/density =stress/strain*density we can say stress due to sound wave normally and in fog is constant. Therefore, v2 is inversely proportional to strain*density. during fog this value is less than normal, that's why velocity of sound wave in fog is greater.
The vowel sound of the OA in "boat" will be a long O (oh)sound, as in coat and float.
You can find the recipe for homemade witches brew at http://www.associatedcontent.com/article/1058983/childrens_halloween_party_treats_homemade.html. You can purchase a sound effects CD and to make a ghost costume, use a sheet. To make fog, you will need to buy a fog machine though.
Slow to minimum speed
It is a misspelled mast. Or it could be some kind of sea fog.
water+air=fog
The adjectives "thick" and "all encompassing" can describe fog. Example sentence: The thick and all encompassing fog prevented the boat from safely entering the harbor.