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Old 02-10-2015, 10:23 AM   #1
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Hull Speed, DeFever 48

Saw a DeFever 48 on Yachtworld with twin 375HP Cats. Most of these models I see with sub-200HP Lehmans or Perkins.

1986 Defever 48TRAWLER Power Boat For Sale - www.yachtworld.com

How fast can a DeFever 48 really go? A full-displacement hull of that size probably would never exceed 13/14 knots (really pushing it), I should think.

Curious if anyone on here is the owner or why one would put that much HP in a full-displacement hull? Where's the benefit? And more so, what's the top end and fuel consumption?
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Old 02-10-2015, 10:53 AM   #2
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Well I am sure that a DeFever expert will probably chime in here, but since I understand that there are both full-displacement and semi-displacement DeFever designs, I am guessing that this would be one of the semis. No point in having 750hp to push a boat along at displacement speed! In fact, even if the '48' in the name referred to waterline length (more likely the waterline length is significantly less), FD hull speed would be less than 9 1/2 knots.
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Old 02-10-2015, 11:11 AM   #3
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Originally Posted by kraftee View Post
Well I am sure that a DeFever expert will probably chime in here, but since I understand that there are both full-displacement and semi-displacement DeFever designs, I am guessing that this would be one of the semis. No point in having 750hp to push a boat along at displacement speed! In fact, even if the '48' in the name refers to waterline length, hull speed would still be less than 9 1/2 knots.
Our boar is similar (52' yachtfisher, semi-displacement hull) with the same powerplants.

Hull speed is 9.6 knots, and that is at 1100 RPM +/-. Fuel burn is about 6-7 gal per hour combined. Top speed we've seen is 24 knots. This was with no fresh or black water onboard, 1/4 tank of fuel, and the vessel stripped of all gear adrift. Fuel burn is 50+ GPH combined.
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Old 02-10-2015, 12:12 PM   #4
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Originally Posted by Pau Hana View Post
Our boar is similar (52' yachtfisher, semi-displacement hull) with the same powerplants.

Hull speed is 9.6 knots, and that is at 1100 RPM +/-. Fuel burn is about 6-7 gal per hour combined. Top speed we've seen is 24 knots. This was with no fresh or black water onboard, 1/4 tank of fuel, and the vessel stripped of all gear adrift. Fuel burn is 50+ GPH combined.
Ouch 50GPH that hurts to think about
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Old 02-10-2015, 12:57 PM   #5
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We have the power- but we only exercise it when needed....
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Old 02-10-2015, 02:35 PM   #6
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Quite comfortable cruising at 7.6 to 8.1 knots with fuel burn 4 to 6 gph for both engines. Yes, the Cats are more HP than needed. But the vessel is 27 years old and who knows what kind of shape any engine brand is at that age without some real investigation.
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Old 02-10-2015, 06:21 PM   #7
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You can definitely outrun some storms with that setup.
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Old 02-10-2015, 08:57 PM   #8
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Hull speed has little meaning for any boat but a full displacement boat and as far as I know De Fever made no FD boats. Displacement yes but not FD. SD craft and planing boats don't benefit from the stern wave like a FD boat. The stern of a FD boat surfs to an extent helping the boat along like the Bart subway trains do turning their motors into generators going down grades. The up side is that the SD boat like a De Fever can run at hull speed and usually (I think) burn less fuel doing so at that speed than a FD boat. The drag of a FD boat running at hull speed is so high that anyone wishing reasonably good economy will slow down 1/2 to one knot and usually their FD craft is more fuel efficient at that reduced speed. So hull speed is something to do on a SD boat but to almost always avoid on the FD boat. IMO
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Old 02-11-2015, 08:56 PM   #9
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My Defever 48 has the 3208 Cats. I rarely run fast but with a clean bottom and 1/2 filled tanks, I can go 19kts. I don't find it very comfortable to run fast, mostly run around 7.5kts at obviously much better fuel mileage.

I've had this boat since 1997 so know it well. It's a great boat.
PM me if you have other specific questions.
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Old 02-11-2015, 11:21 PM   #10
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[QUOTE=manyboats;306760]Hull speed has little meaning for any boat but a full displacement boat and as far as I know De Fever made no FD boats.

There is a practical meaning of the common term hull speed in a SD boat. It is usually a slightly foggy point beyond which the fuel burn curve gets steep and the higher above that Hull speed point the steeper the curve gets. So somewhere in that foggy point the boat changes its mode of traveling through the water as a functional FD boat to a SD mode and must use more fuel to do that.
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Old 02-12-2015, 06:41 AM   #11
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A boat with lower power to cruse cheaply will run out of power as the bow attempts to climb its own wave and the stern falls in the trough.

They call it hull speed and it is very very inefficient to climb that water hill , perhaps 3X the fuel burn compared to 1 or 1.5K slower.

With boodles of power the semi plaining can actually climb on top of the bow wave and at great expense ride there .

A plaining boat hull and operation is quire different , it flys on the after surface of the hull on top of the water, some boats that are light enough actually reduce the Gal per NM at higher speeds due to the lower drag,

Light construction is expensive , big engines are expensive, so those that can afford 20K or 30K at 50 to 100GPH seldom want boats that look like trawlers.
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