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Old 04-16-2015, 10:42 AM   #7
Nomad Willy
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City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 16,147
Ranger wrote;
"How much improvement could there be? A half-gallon/per hour/per engine? One? Or...?

Opinions vary on that but roughly I'd say 1.5 on a SD hull typical of psneeld's Albin or an IG and 2 times as much as a planing hull ... like the CC in this post. A narrow boat would be better than a wide one. With the CC one would half the fuel by turning up the stern. That has nothing to do w the efficiency of power .. just the drag of the hull. FF mentioned 3hp per ton for FD .. that's for a really slick FD hull or just slower speed. My FD hull is at 5hp per ton and won't make hull speed. At least I don't think. The lowest hp per ton I've seen for a mono hull is 2hp per ton. Even after a converted stern to FD the CC's hull won't be as efficient as a typical FD. And if the turned up stern turns up abruptly there will be less gain at 1 knot below hull speed as it would have if the stern curved up gradually over a longer section of the hull. Barges (the typical double ended variety) have a very abruptly turned up stern but the advantages in lower drag must be very substantial indeed as I've never seen one w a stern typical of a basically right angled bottom to transom angle.

But it would be a lot of work justifiable only if most all of the work could be done by the owner. I've thought through the years that it would be easier to convert a wood boat than a FG but the shape of the hull may more of a deciding factor.

Opinions vary probably more than reality on this. TAD (our resident NA) I'm quite sure thinks the drag of a SD hull is much closer to a FD at cruising speed (1 knot below hull speed) than I do and a conversation about this on boatdesign.net had a lot of varying opinions. But if one were to row a flat sterned skiff and then a propper row boat (like what is called a "pulling boat" your mussels in your body and the very sluggish speed (or lack thereof) would be very convincing. I've seen people ballast the bow of a skiff so the stern is out of the water and row it backwards to basically turn it into a FD hull. Must be a bugger to keep going straight.

But the conversion of the CC in question may make it steer better especially in following seas and there would be other advantages and disadvantages. But because it would be a big job I may never see one done. And the end result would never be as good as a proper FD hull. Just like converting a sailboat to a powerboat wouldn't be as good as a powerboat ... ha ha but would probably be more efficient one knot below hull speed.

But if one had the skills a conversion could be an economical way to obtain a large trawler-like boat for very little money w almost the same fuel efficiency of a FD hull. For understandable reasons big gas powered cruisers are hardly worth anything now and w the right surrounding circumstances one could buy a nice boat for really cheap. But leaving the square sterned CC above as is would make the conversion probably not worth doing. But w a really light boat like one of plywood the conversion would become much more beneficial w the hull left as is. HP per ton - half the weight - half the power required.
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Eric

North Western Washington State USA
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