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Author Topic:   Fast displacement hulls of the future?

cooper1958nc posted 10-16-2008 04:14 PM ET (US)
With fuel and carbon concerns increasing, powering boats as efficiently as possible is always an interesting topic. Recently I operated a pontoon boat and was pretty surprised at the speed this 22' boat would make on 50hp. Even loaded to the gills with 10 people or so, cruise speed on the 50 resulted in 12-13 mph. This is way beyond hull speed yet there was no bow rise and the pontoon hull is not designed to lift. Sailing cats also seem to sail way above hull speed, yet it is hard to say they are planing in any realistic sense. There is not much literature on this, but I would not be surprise to see major manufacturers going to fast displacement cats if they can be driven a moderate to low speeds with very little power.

20dauntless posted 10-16-2008 05:13 PM ET (US)
Glacier Bay Cats are all displacement hulls...you should be able to find more information on them by searching google or checking their site.
Unlike displacement monohulls which are limited to 1.34 x the square root of waterline length, displacement cats can achieve much higher speeds.
boatdryver posted 10-16-2008 05:55 PM ET (US)
I tested a Glacier Bay 26 four years ago on choppy San Francisco Bay with a sales rep. I was very very impressed with the ride. But then the rep spied a container ship entering the Harbor at 10 knots and decided to cross the ship's wake at 22 knots. Well, the wake was much higher than bargained for and my buddy did a face plant on the deck-he looked like a road kill.
The thing that turned me off, though, other than the sticker shock, was the fuel flow meter which showed 17 gallons per hour at about 22 knots.
So even though displacement cats can exceed traditional (1.34xsquare root of waterline length) hull speed it appears they require a lot of energy to do so. Does anyone have personal experience with one of these?
JimL
pglein posted 10-16-2008 06:08 PM ET (US)
Sailing catamarans are not displacement hulls. They don't have to hop up on top of the water like a flat-bottom or deep-vee to be considered "planing". If they're producing lift, they're planing.
TransAm posted 10-16-2008 07:32 PM ET (US)
Won't happen. Consumers will flat out reject it. Technology will overtake any need for such a thing.
cooper1958nc posted 10-16-2008 07:37 PM ET (US)
"Sailing catamarans are not displacement hulls. They don't have to hop up on top of the water like a flat-bottom or deep-vee to be considered "planing". If they're producing lift, they're planing."
How do you know if they are producing lift?
Round-pontoon boats will not produce much if any lift, and they run flat at like zero angle of attack.
But they go far beyond hull speed on small amounts of power.
"Won't happen. Consumers will flat out reject it. Technology will overtake any need for such a thing."
Well, this is a type of technology, and further, technology can't change physical fundamentals.
pglein posted 10-17-2008 07:50 PM ET (US)
Lets first define the terms we're using.

quote:
Displacement
the hull is supported exclusively or predominantly by the pressure of water displaced by the hull. These hulls have a shape which does not promote planing. They travel through the water at a limited rate which is defined by the waterline length. They are often heavier than planing types, though not always.
Semi-displacement, or semi-planing
the hull form is capable of developing a moderate amount of dynamic lift, however, most of the vessel's weight is still supported through displacement
Planing
the planing hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed. These hulls have a shape that promotes the boat to rise higher and higher out of the water as the speed increases. They are sometimes flat-bottomed, sometimes V-bottomed and sometimes round-bilged. The most common form is to have at least one chine to allow for stability when cornering and for a supportive surface on which to ride while planing. Planing hulls allow higher speeds to be achieved, and are not limited by the waterline length the way displacement hulls are. They do require more energy to achieve these speeds. (see: Planing (sailing)).
All displacement hulls are probably really semi-displacement. That is, if you put enough power on them, at some point, the lift of the forward half will overcome the downforce suction of the aft portion and they will start to squat and just barely "plane". But with hulls generally classified as displacement, the amount of force required to do this is absurd and the hull will generally become unstable.
All planing hulls are displcement hulls when they're going slow. Sailing catamarans glide through the water at displacement speeds most of the time. But when they catch a good gust of wind, they usually start producing a moderate amount of lift. NOT ALL SAILBOATS ARE DISPLACEMENT hulls. Most of the small, competitive daysailer classes are actually planing hulls.
Note that "hull speed" is generally misunderstood. The general formulas out there assume a traditional displacement shape. Many unconventional boats (like catamarans) have unconventional hull shapes that are able to achieve far in excess of their supposed "hull speed" while still technically not producing significant lift. Kayaks are, perhaps, the best example of this. Their hulls would probably be best classified as displacement. Yet, their extremely small size, narrow beam, and sharp entry and exit angles allow them to travel at more than twice what one might predict their hull speed to be.
There are no absolutes in hull design.
Chuck Tribolet posted 10-17-2008 09:44 PM ET (US)
"There are no absolutes in hull design."
That's one of the most sensible things I've heard in a while.

Chuck
tmann45 posted 10-17-2008 10:41 PM ET (US)
"There are no absolutes in hull design."
Absolutely!
The "faster than hull speed" is made possible by the large length to beam ratio (slenderness) of the hull.
elaelap posted 10-17-2008 11:36 PM ET (US)
Different strokes:
http://i22.photobucket.com/albums/b309/elaelap/Hydrofoilspeedster.jpg
http://i22.photobucket.com/albums/b309/elaelap/LukaMikeKauaiwedding025. jpg
tmann45 posted 10-18-2008 07:08 AM ET (US)
Got any idea how fast the hydrofoil trimaran is going?
TransAm posted 10-18-2008 07:38 AM ET (US)
Sorry, I dont see myself relaxing, having a cold beer on one of those rigs. If that is part of your work-out routine, or something you do once in a while, fine. But I usually get on the water to relax, not bust my hump just to stay on the boat.
Hoosier posted 10-19-2008 06:00 PM ET (US)
OK, how would you classify this one?
http://gizmodo.com/388227/uss-independence-a-triple-hulled-weapon+ laden-monster-that-is-surprisingly-affordable
This is an adaptaion of a commercial high speed ferry design. I can almost see the hull lines of my classic Montauk in it.
Pierce posted 10-20-2008 12:44 PM ET (US)
Displacement hull. Very fast displacement hull though!
John W posted 10-20-2008 06:02 PM ET (US)

One type of hull design that has caught my attention are the "Seabright tunnel hull" boats designed by William & John Atkin (http://www.atkinboatplans.com/). They reach speeds of up to 20 mph or so with very little power. An example is "Rescue Minor", a 20 foot inboard skiff that goes 20 mph on 18 hp, runs in 6" of water, and gets 28 miles per gallon!
An article on the design, written by Robb White, was featured in WoodenBoat magazine in March 2006...there is also an article about the boat, with pictures, on Robb White's website here:
http://www.robbwhite.com/rescue.minor.html (Note that Robb White passed away some time ago.)
Here is the link to Atkins' plan for Rescue Minor, along with a link to pictures of the boat:
http://www.boat-links.com/Atkinco/Utilities/RescueMinor.html
Atkin designed various of these "Seabright tunnel hull powerboats" up to around 40 feet in length. They all were very fuel efficient and shallow draft...but they are not as fast as modern, power-hungry planing boats....the tunnel hull starts to push the bow down over a certain speed.
As planing boats go, the classic 16' and 21' Whalers are more fuel efficient than most, but Rescue Minor takes it to a much higher level of fuel efficiency.
John
Hoosier posted 10-22-2008 11:51 AM ET (US)
How about this one?
http://www.mshipco.com/?page_id=11
tomol posted 10-22-2008 03:16 PM ET (US)
Tony, great shot of that trimaran. I think the speed record for one of those big hydrofoils is something like 42 knots.
As an aside I just got word that the magical 50 knot barrier was just broken by a kite surfer.
But back to the catamaran lift/displacement discussion. When I used to race Hobie Cats in the 70's the rule of thumb was to keep crew weight forward to get the flat transoms out of the water at slow speeds when the hull behaved like a displacement hull.

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Author	Topic: Fast displacement hulls of the future?
cooper1958nc	posted 10-16-2008 04:14 PM ET (US) With fuel and carbon concerns increasing, powering boats as efficiently as possible is always an interesting topic. Recently I operated a pontoon boat and was pretty surprised at the speed this 22' boat would make on 50hp. Even loaded to the gills with 10 people or so, cruise speed on the 50 resulted in 12-13 mph. This is way beyond hull speed yet there was no bow rise and the pontoon hull is not designed to lift. Sailing cats also seem to sail way above hull speed, yet it is hard to say they are planing in any realistic sense. There is not much literature on this, but I would not be surprise to see major manufacturers going to fast displacement cats if they can be driven a moderate to low speeds with very little power.
20dauntless	posted 10-16-2008 05:13 PM ET (US) Glacier Bay Cats are all displacement hulls...you should be able to find more information on them by searching google or checking their site. Unlike displacement monohulls which are limited to 1.34 x the square root of waterline length, displacement cats can achieve much higher speeds.
boatdryver	posted 10-16-2008 05:55 PM ET (US) I tested a Glacier Bay 26 four years ago on choppy San Francisco Bay with a sales rep. I was very very impressed with the ride. But then the rep spied a container ship entering the Harbor at 10 knots and decided to cross the ship's wake at 22 knots. Well, the wake was much higher than bargained for and my buddy did a face plant on the deck-he looked like a road kill. The thing that turned me off, though, other than the sticker shock, was the fuel flow meter which showed 17 gallons per hour at about 22 knots. So even though displacement cats can exceed traditional (1.34xsquare root of waterline length) hull speed it appears they require a lot of energy to do so. Does anyone have personal experience with one of these? JimL
pglein	posted 10-16-2008 06:08 PM ET (US) Sailing catamarans are not displacement hulls. They don't have to hop up on top of the water like a flat-bottom or deep-vee to be considered "planing". If they're producing lift, they're planing.
TransAm	posted 10-16-2008 07:32 PM ET (US) Won't happen. Consumers will flat out reject it. Technology will overtake any need for such a thing.
cooper1958nc	posted 10-16-2008 07:37 PM ET (US) "Sailing catamarans are not displacement hulls. They don't have to hop up on top of the water like a flat-bottom or deep-vee to be considered "planing". If they're producing lift, they're planing." How do you know if they are producing lift? Round-pontoon boats will not produce much if any lift, and they run flat at like zero angle of attack. But they go far beyond hull speed on small amounts of power. "Won't happen. Consumers will flat out reject it. Technology will overtake any need for such a thing." Well, this is a type of technology, and further, technology can't change physical fundamentals.
pglein	posted 10-17-2008 07:50 PM ET (US) Lets first define the terms we're using. quote: Displacement the hull is supported exclusively or predominantly by the pressure of water displaced by the hull. These hulls have a shape which does not promote planing. They travel through the water at a limited rate which is defined by the waterline length. They are often heavier than planing types, though not always. Semi-displacement, or semi-planing the hull form is capable of developing a moderate amount of dynamic lift, however, most of the vessel's weight is still supported through displacement Planing the planing hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed. These hulls have a shape that promotes the boat to rise higher and higher out of the water as the speed increases. They are sometimes flat-bottomed, sometimes V-bottomed and sometimes round-bilged. The most common form is to have at least one chine to allow for stability when cornering and for a supportive surface on which to ride while planing. Planing hulls allow higher speeds to be achieved, and are not limited by the waterline length the way displacement hulls are. They do require more energy to achieve these speeds. (see: Planing (sailing)). All displacement hulls are probably really semi-displacement. That is, if you put enough power on them, at some point, the lift of the forward half will overcome the downforce suction of the aft portion and they will start to squat and just barely "plane". But with hulls generally classified as displacement, the amount of force required to do this is absurd and the hull will generally become unstable. All planing hulls are displcement hulls when they're going slow. Sailing catamarans glide through the water at displacement speeds most of the time. But when they catch a good gust of wind, they usually start producing a moderate amount of lift. NOT ALL SAILBOATS ARE DISPLACEMENT hulls. Most of the small, competitive daysailer classes are actually planing hulls. Note that "hull speed" is generally misunderstood. The general formulas out there assume a traditional displacement shape. Many unconventional boats (like catamarans) have unconventional hull shapes that are able to achieve far in excess of their supposed "hull speed" while still technically not producing significant lift. Kayaks are, perhaps, the best example of this. Their hulls would probably be best classified as displacement. Yet, their extremely small size, narrow beam, and sharp entry and exit angles allow them to travel at more than twice what one might predict their hull speed to be. There are no absolutes in hull design.
Chuck Tribolet	posted 10-17-2008 09:44 PM ET (US) "There are no absolutes in hull design." That's one of the most sensible things I've heard in a while. Chuck
tmann45	posted 10-17-2008 10:41 PM ET (US) "There are no absolutes in hull design." Absolutely! The "faster than hull speed" is made possible by the large length to beam ratio (slenderness) of the hull.
elaelap	posted 10-17-2008 11:36 PM ET (US) Different strokes: http://i22.photobucket.com/albums/b309/elaelap/Hydrofoilspeedster.jpg http://i22.photobucket.com/albums/b309/elaelap/LukaMikeKauaiwedding025. jpg
tmann45	posted 10-18-2008 07:08 AM ET (US) Got any idea how fast the hydrofoil trimaran is going?
TransAm	posted 10-18-2008 07:38 AM ET (US) Sorry, I dont see myself relaxing, having a cold beer on one of those rigs. If that is part of your work-out routine, or something you do once in a while, fine. But I usually get on the water to relax, not bust my hump just to stay on the boat.
Hoosier	posted 10-19-2008 06:00 PM ET (US) OK, how would you classify this one? http://gizmodo.com/388227/uss-independence-a-triple-hulled-weapon+ laden-monster-that-is-surprisingly-affordable This is an adaptaion of a commercial high speed ferry design. I can almost see the hull lines of my classic Montauk in it.
Pierce	posted 10-20-2008 12:44 PM ET (US) Displacement hull. Very fast displacement hull though!
John W	posted 10-20-2008 06:02 PM ET (US) One type of hull design that has caught my attention are the "Seabright tunnel hull" boats designed by William & John Atkin (http://www.atkinboatplans.com/). They reach speeds of up to 20 mph or so with very little power. An example is "Rescue Minor", a 20 foot inboard skiff that goes 20 mph on 18 hp, runs in 6" of water, and gets 28 miles per gallon! An article on the design, written by Robb White, was featured in WoodenBoat magazine in March 2006...there is also an article about the boat, with pictures, on Robb White's website here: http://www.robbwhite.com/rescue.minor.html (Note that Robb White passed away some time ago.) Here is the link to Atkins' plan for Rescue Minor, along with a link to pictures of the boat: http://www.boat-links.com/Atkinco/Utilities/RescueMinor.html Atkin designed various of these "Seabright tunnel hull powerboats" up to around 40 feet in length. They all were very fuel efficient and shallow draft...but they are not as fast as modern, power-hungry planing boats....the tunnel hull starts to push the bow down over a certain speed. As planing boats go, the classic 16' and 21' Whalers are more fuel efficient than most, but Rescue Minor takes it to a much higher level of fuel efficiency. John
Hoosier	posted 10-22-2008 11:51 AM ET (US) How about this one? http://www.mshipco.com/?page_id=11
tomol	posted 10-22-2008 03:16 PM ET (US) Tony, great shot of that trimaran. I think the speed record for one of those big hydrofoils is something like 42 knots. As an aside I just got word that the magical 50 knot barrier was just broken by a kite surfer. But back to the catamaran lift/displacement discussion. When I used to race Hobie Cats in the 70's the rule of thumb was to keep crew weight forward to get the flat transoms out of the water at slow speeds when the hull behaved like a displacement hull.