When to consider displacement hull over planning hull?

[TitoSoto]

Lets say you have a really skinny 6:1 B/L 3 ton powerboat.

Obviously it would easily surpass its hull speed.

Would it be more efficient to have a displacement hull or a planning hull for such a boat? Let say, 15 knot target speed...

[CaptaindeJong]

Originally Posted by TitoSoto

Lets say you have a really skinny 6:1 B/L 3 ton powerboat.

Obviously it would easily surpass its hull speed.

Would it be more efficient to have a displacement hull or a planning hull for such a boat? Let say, 15 knot target speed...

Are you asking about FUEL efficiency?

Planing if on calm water, displacement if on rough water.

[SantaFeTrailer]

Purely anecdotal, but maybe a data point. Two weekends ago we towed our Sunfish up the lake with our 18' Chris Craft outboard. Displacement mode towing the Sunfish and planing coming back without it. Now the Sunfish added a bit of weight and drag, but I wouldn't think all that much, and we used about twice as much fuel towing it.

[a64pilot]

Your numbers mean not a very big boat, so I would say planing in smooth water, and you don't plane a smaller boat in rough water, it beats it to death if your kidneys can handle it, the boat can't over time.

I assume with your numbers a well less than 30' boat and to get that to 15 kts even with thin hulls would take lots of power?

[Lepke]

Speed is why you'd pick. In the days of rum runners, a narrow, semi displacement hull was built. Fast, but would carry cargo. I owned one. Usually 10-12 feet wide and 40 plus feet long. CG patrol boats of the time were also built this way in the hopes of catching the bootleggers. Mine was owned by the mayor of Sausalito. Politicians were corrupt then, too.

[TitoSoto]

Originally Posted by Lepke

Speed is why you'd pick. In the days of rum runners, a narrow, semi displacement hull was built. Fast, but would carry cargo. I owned one. Usually 10-12 feet wide and 40 plus feet long. CG patrol boats of the time were also built this way in the hopes of catching the bootleggers. Mine was owned by the mayor of Sausalito. Politicians were corrupt then, too.

So would this boat of yours beat you to death when "semi-planning" on rough water? How much did this boat weight?

[Lepke]

Originally Posted by TitoSoto

So would this boat of yours beat you to death when "semi-planning" on rough water? How much did this boat weight?

Actually I had 2. One I completely rebuilt for profit. 2 Chrysler Royal straight 8s. It would plane at about 19 knots and used a lot of gas. I only operated a couple times. Never figured the mileage. Only in a river. The 2nd boat I didn't plane it. It was a commercial fishing boat when I got it. About 20-25 tons and heavier and deeper in the water than built. Bottom rub rail was submerged when loaded. Would do 14 knots with the engines screaming, but propped for trolling.
Current boat is also a semi. Built for the USCG in WWII and converted to a Monk design in the 70s. It did 22 knots with 2 850hp gas engines burning about 75 gallons/hour. Now I cruise at 10 knots with 2 200hp DDs using about 8.5 gallon an hour. 83', 80 ton.
At the end of WWII most PT boats were scrapped. Almost all had broken keels from pounding in rough water. A friend bought one as built for tuna fishing. Before converting, he took friends and family for a joy ride. Ran outa gas and had to be towed in. Unknown to him, it used 50 gallons an hour/engine at speed (3x50).
I ran several planing boats in the military. Fine in small waves or smooth water. Pounding is miserable in big waves. One type had a tendency to flip in 3'+ when put hard over at speed when lightly loaded.
There is no solution to speed in a bigger boat without a high fuel price. And running flat out leads to frequent engine overhauls.

[Lepke]

One more thought. Just because you have a high hull speed doesn't make a economic running vessel. Speeds costs. I rode some WWII built destroyers. 375' x 35' x 9'. We could do about 15 knots on 1 boiler, 27-28 knots on 2 boilers. 35 knots on 4 boilers. So at full speed for 7 more knots, double the fuel. Even from 15 knots to 27, we doubled the fuel. No idea on the gallons an hour, but was measured in tons of bunker oil per hour.

[Hasbeen]

I ran a 110 Ft displacement passenger launch, with a couple of 8L3B Gardiners in her. 280 passenger capacity, she cruised at about 12 knots, using 20 gallons of fuel an hour.

I ran a 40Ft fishing charter boat with a 4LX Gardiner. Slept 8 on reef trips, cruised at about 9 knots, using 1.6 gallons an hour.

I ran a couple of alloy international cats, 100 Ft with a couple of 16V92 1200 HP GMs in them. 325 passengers, they used 55 gallons per hour at 24 knots, & 400 gallons per hour at 30 knots.

Yep, speed costs.

[David M]

Speed costs bigtime. I know of one powerboat that gets 6 MPG at 5 knots and 0.8 MPG at 17 knots.