40 foot displacement hull cruising at 15 knots -- Impossible?

[StuM]

Originally Posted by captlloyd

Yes, think container ship. Those buggers move right along.

With a 200-400m length, the classic formula gives them hull speeds of
35 - 49 knots.

[oleman]

My at sea experience has been on 26,000T to 27,500T and larger tax payer owned war ships. All of those ships are designed to cruise in the 28 KT range and a flank speed exceeding 30 KT. The real limitation on fossil burners is running out of fuel. On a nuc power surface ships the limitation is running out of rations for the crew, ship has to rendezvous with a supply ship. Every few weeks.
These ships are obviously displacement hulls, the REAL limitations are available power, crew comfort, and operating costs other other than fuel.

One ship I was on went from SASEBO Japan, to San Francisco, USA averaged 27+ knots without a single stop. That was the now decommissioned USS Truxtun(CGN-35). That ship was powered by 2 EA GE reactors rated at around 80,000 HP each with motion provided by twin steam turbines and screws.
Even on a large heavy ship bouncing across 25' seas at 32 kt is not a fun ride.

[newhaul]

Originally Posted by ranger42c

That doesn't quite cover it all.

Gas engines have a hard time pushing heavy boats at high RPMs, too, once the overall weight and/or hull form exceed the available gas horsepower. (40' and 20K-lbs is pushing gas engine limits, for semi-displacement and planing hulls.)

So diesel is also used in larger boats because there aren't any gas engines big enough.
).


-Chris

I know where there are a couple meranised rolls merlins that are like new .

[Elie]

Any boat provided with enough power will start surfing. Once in a surf(past the bow wave) accelerating to higher speeds is possible and proportional to the amount of avalable power. The lighter the boat, the shallower the bow wave and therefore the easyier is it to pass this wave.
Many racing(and some modern cruising) displacement sailboats will surf at speed largely in excess of hull speed. All IMOCA class sailing vessels that go around the world go at more then twice their theorical hull speed limit. But an heavy cruising sailboat like the Wessail 32 or a Morgan Out Island 41 will need a hurricane to go beyond their hull speed limit. Large following waves, light displacement, wide sterns, long water line, are all favorable factors for passing the bow wave. By own sailboat(moder 42ft lwl) had been clocked at over 14 knots in big waves.

[hamburking]

I think you could do it with a flat bottom boat that would induce planing. I've exceeded hull speed with both a Cal25 and Tanzer22. You need a big wind. Choppy waves help induce the turbulent flow needed under the hull. You would need light weight, and lots of sail. I think it could be done, but I also think it would be a bad idea. Also a bad idea...300hp engine on a sailboat.

If you want to go faster, get a longer boat.

Foils might be the solution you are looking for. I think these are in development for cruising boats now, but I would not hold my breath.

[Pete7]

Originally Posted by hamburking

If you want to go faster, get a longer boat..

Or buy a boat that was designed to go quick rather than a displacement hull.

Pete

[StuM]

Originally Posted by hamburking

I think you could do it with a flat bottom boat that would induce planing. I've exceeded hull speed with both a Cal25 and Tanzer22. You need a big wind. Choppy waves help induce the turbulent flow needed under the hull. You would need light weight, and lots of sail. I think it could be done, but I also think it would be a bad idea. Also a bad idea...300hp engine on a sailboat.

The OP didn't mention a sailboat and from his other posts, it looks like he is considering powerboats. (Specifically, something to chase down and "claim" abandoned boats. )

[Training Wheels]

Originally Posted by Elie

Any boat provided with enough power will start surfing. Once in a surf(past the bow wave) accelerating to higher speeds is possible and proportional to the amount of avalable power. The lighter the boat, the shallower the bow wave and therefore the easyier is it to pass this wave.

Many racing(and some modern cruising) displacement sailboats will surf at speed largely in excess of hull speed. All IMOCA class sailing vessels that go around the world go at more then twice their theorical hull speed limit. But an heavy cruising sailboat like the Wessail 32 or a Morgan Out Island 41 will need a hurricane to go beyond their hull speed limit. Large following waves, light displacement, wide sterns, long water line, are all favorable factors for passing the bow wave. By own sailboat(moder 42ft lwl) had been clocked at over 14 knots in big waves.

I don't know, but I'd guess IMOCA boats are planing hulls.

[downunder]

The Artnautic designs vessel linked below is as fuel efficient as you can get,

- a poor mans version of the Dashew FPB.

Could be worth talking to the designer Dennis Harjamaa about its speed capability with larger motor.

https://plus.google.com/u/0/100095003579874099740

Dickey Boats Art Nautica LRC 58 built in New Zealand | Dickey Boats

This is the closest thing to a Dashew FBP. >.8L/NM fuel cruising

The designer can do a range of sizes apparently .

Long Range Cruiser 58: a fuel-efficient boat. - Dennis Harjamaa Yacht Design, Elegant Yachts, Aluminium, Fuel-Efficient, Sailing, Cruising Boats,arnautica

https://www.facebook.com/Artnautica-...02724/?fref=ts

[Comix Bay]

Originally Posted by captlloyd

Yes, think container ship. Those buggers move right along.

Those ships have the bulb on the bow, how does that effect the speed. Have also read that some keep fuel in the bulb. There are a few fishboats tied up at the Comox wharf that have bulbs.

[jsc7]

I have seen the Dickey boat. It maxes out at around 11 knots and has a 4.5 length to beam ratio. If we increase that to 5 to 1 and put in a more powerful engine could it reach 15 knots cruising speed?

[Solandri]

Hull speed happens due to how the hull fits between the bow wave it creates as it moves forward.



Hull speed is the maximum speed at which the hull will be level (equally supported at front and back). As you exceed hull speed, the second peak of the bow wave falls behind the stern. The stern of the boat then starts to drop into the trough of its own bow wave. From that point, the boat is basically trying to climb uphill out of its own bow wave. The faster it goes, the further down into the trough it dips, the steeper the angle of the hill it has to climb, and the harder it has to work move forward.

So it's possible to exceed hull speed. It's just that it takes a lot more energy to eek out a little more speed than when you're below hull speed and the hull is level.

The bulbous underwater bow found on many tankers and cargo ships, as well as some wave-piercing hulls move the start of the bow wave forward. Effectively, they make the overall length of the ship longer for the purposes of calculating hull speed, thus allowing the ship to eek out another knot or two.

Slender hulls work for a different reason. As a displacement hull is moving through the water, it is pushing the water down, then letting it up after it passes. This action on the water causes it to form waves (duh). Since a wave has energy, that energy is lost by the ship and thus has to be replaced by the engine or sail for the ship to maintain speed.

The size and energy of the waves that are created depends on the length and width of the area of water which is depressed. I won't get into the math behind this wave drag because it contains lots of fun integrals and series. But the bottom line is that for the same displacement, a slender hull generates less wave drag than a beamy hull. It's a trade-off though because a slender hull has a greater wetted surface area for the same displacement (a square has the smallest surface area for a given displacement). So it has less wave drag, but more friction drag. The trade-off between these two forms of drag varies with length-to-beam ratio and speed. A lot of proper boat design is optimizing these trade-offs based on the expected cruising speed of the boat (which in turn is based on sail area or engine power).

So a slender hull is still subject to hull speed, it just requires less energy to overcome it than a beamy hull because it's not creating as big a wave.

It gets even more fun with catamarans. Since they have two hulls, the wave coming off of one hull interacts with the other hull (and vice versa). This can reduce overall drag if you space the hulls the right distance from each other at the expected cruise speed.

[downunder]

Originally Posted by jsc7

I have seen the Dickey boat. It maxes out at around 11 knots and has a 4.5 length to beam ratio. If we increase that to 5 to 1 and put in a more powerful engine could it reach 15 knots cruising speed?

Correspond with the designer. I am sure he will respond.

[StuM]

Originally Posted by Solandri

The bulbous underwater bow found on many tankers and cargo ships, as well as some wave-piercing hulls move the start of the bow wave forward. Effectively, they make the overall length of the ship longer for the purposes of calculating hull speed, thus allowing the ship to eek out another knot or two.

A more accurate description of how bulbous bows cause wave interference and reduce drag can be found here.

What's The Importance Of Bulbous Bow Of Ships?

[Seabeau]

I suggest you investigate the designs of the early British inventors who developed the first planning hulls. In some cases they used multi-steps in the aft sections of their hulls to be able to reach plane with the use of the extremely heavy but weak engines of their time.