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"Tuning" props for salt vs. fresh water.

Discussion in 'Props, Shafts & Seals' started by Capt Bill11, Oct 7, 2014.

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  1. SFS

    SFS Senior Member

    Joined:
    Oct 4, 2013
    Messages:
    125
    Location:
    Tampa
    I think you are all correct (regarding posts 15, 16, 18 and 19). My copy of Chapman's is in a box somewhere, but my copy of Small Boat Seamanship Manual (Copyright 2002, International Marine), which is in turn based heavily on the USCG Boat Crew Seamanship Manual, talks about bank cushion and bank suction. I'll paraphrase, so as to not violate any copyrights:

    If the channel is narrow enough, the forward movement of the vessel causes the "wedge" of water between the bow and the nearer bank to build up higher than on the other side of the bow. This bow cushion or bank cushion tends to move the bow away from that nearer bank. In addition, as the back of the boat moves forward, screw suction and water fill into the space where the boat was just creating stern suction or bank suction. This causes the stern to move toward the bank. The combined effect of bank cushion at the bow and bank suction at the stern may cause a sudden sheer toward the opposite bank. These effects are strongest when the banks of a channel are steep, and are weakest when the bank gets shallow gradually, and extends into large shallow areas. Similar effects are possible when meeting a vessel close aboard.

    And I see I've been ninja'd by Marmot.