Hull extension

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Apr 11, 2008
Vessel Name
Vessel Make
DeFever 48
I was pondering the possiblities of a hull extension on my Semi Displacement hull and was wondering if I could smooth out the stern wake and drag with a canoe stern. Keep in the mind the extension would really just be a molded in swim platform so all the glass work would be near or below the waterline. I know another fat ass square stern like I have would be easier but why not take the extra time and build it canoe shaped to handle following seas better and reduce drag?
Iv'e pondered that for years but not as a canoe shape. I would think that would be a very long extention. My idea was to extend the stern straight back on the sides but to increase the curve upwards of the bottom, aft of the existing transom to or near ( or at )*the water line. To get to the water line ( or near ) the steeper the quarter beam buttock line the better. Less extension would be required to produce a full displacement hull. With flatter bottoms* (*like GBs ) much more extension would be required. A steep quarter beam buttock line may be better in following seas than a canoe stern. The QBBL is a line on the bottom of the aft end of a vessel midway between the keel and the chine. One can roughly say a planing hull has about 0 to 4 degrees, a SD hull has about 5 to 10 degrees and a FD hull has 10 or more. To get good quartering*stern sea handling a good canoe stern or a high angle QBBL will deliver, also depending on other design elements. An extreemly fine example is the FD Krogens. My idea was to take a Sawzall to the aft 1/4 to 1/3 of the stern ( ahead of the transom ) and bend the bottom up. Obviously this would be most easily done to a hard chine boat. Wood boats may even be easier. Have I made myself understandable enough?

Eric Henning*
This may not be much information to be very helpful, but there is a fellow in Bellingham, Washington who has put some major hull extensions on the hulls of a few Nordic pilothouse cruisers. These boats have semi-planing hulls. The molds for these boats are now owned by a company in Bellingham (see photo). But Gordon Lavagier (sp?) of Bellingham Marine Repair has modified at least three Nordics with hull extensions that are at least five or six feet long. They have all incorporated a swim platform on top-- they were not enclosed extensions of the cockpit. And they were not canoe-shaped at the waterline. They were flat across the back of the boat although they were cut in at the sides to accomodate large exhaust extensions.

I don't know if the purpose of Gordon's hull extension was to improve performance, economy, or simply provide a massive platform for the storage of a fourteen or sixteen foot RIB, power launching davit, etc. It certainly makes the aft end of the boat user friendly.

You could call Gordon for more information about his hull extensions at Bellingham Marine Repair (360) 734-6326. While his company installed a new Force 10 stove/oven in our old GB36 in 1998 and he knows me by sight, I doubt he remembers my name. The Nordics I have seen that he put the extensions on were named "Y-Ark," "Well-Adjusted," and "Lavitude" (with a "v" not a "t").

The photo shows the stock hull configuration. I don't have a photo showing one of Gordon's hull extensions so you'll have to imagine this boat with a six foot platform extending aft of the transom but extending down into the water and faired into the bottom of the hull.

-- Edited by Marin at 22:11, 2008-12-27


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I was thinking something on the order of 4' of extension, of course for it to have the classic canoe stern the taper would have to be very dramatic. It just seems to me since I only operate my boat at displacement speeds then why not get the benefit of a true displacement stern as well as the extra hull speed and most importantly in my case the added boyancy aft as my boat drags her tail if I fill the water tanks and that is with the lazerette virtually empty. The added benefit of the larger swim platform would be much appreciated by all as well.
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