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Old 04-13-2013, 01:57 PM   #1
Nomad Willy's Avatar
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 11,907
QBBL and High AR propeller.

Since there's no design section I'll put it here.

QBBL is the Quarter Beam Buttock Line and is the imaginary line along the bottom of the aft section of the boat in-between the keel line running fore and aft and the outboard part of the hull's bottom more often than not the chine. There are quite a number of other things that often classify a boat as being full displacement (FD) or semi-displacement (SD). Submersed transom being one of the others. But the QBBL is the most dependable and defining feature that separates one type of hull from the other.

In the pic we see a seine boat hull w a very steep QBBL. Notice the keel is parallel to the ground. A QBBL angle half as much would firmly establish this boat as a FD craft even w some submerged transom. Unless the QBBL is steep submerged transoms usually identify a hull as being of a SD type. Notice the blue boat that has considerable submerged transom and a QBBL that is much much flatter than the seine boat but steeper than most trawlers. I would submit this blue boat as about the break-off point as a bit steeper QBBL and less submerged transom would make it a FD craft and bit more submerged transom and a flatter QBBL would make it a SD hull.

This may not be true as there is no bullet proof definition but this is at this time as close as I can come w/o further input from an idea that I like better. I invite you to be that person or analyze your take on it or my concept presented above.

Mostly because of restrictive clearance or other considerations our trawler props are of a lower aspect ratio than this fish boat. That is they have wider and shorter blades. The longer narrower blades are more efficient but in our sizes are very hard to find. Like w aircraft longer more slender wings are more efficient.

Last pic is of our April snow.
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North Western Washington State USA
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