Jet Yachts...

Richard Branson's Virgin Atlantic Challenger II. 72 Feet, aluminum and gulps more fuel than a 3rd world country.

One more pic. Branson must hold more records in a variety of different transportation forms than anyone. Doncha love it!

Destriero

What more do you know about Destriero?

Hello Donald,

Nice to see you as a member here! Since you probably know more than all of us together on Destriero, I would like to ask you two things:

She is claimed to have a semi-displacing hull. By definition, how can she be semi-planing in over 60 knots and what would be the difference if she had a planing hull? It has always puzzled me...

Secondly. Is the Yacht Club Costa Smeralda still the owners and what has happened or will happen to her in the future?

With regards
Lars

Destriero was only operated under charter by the Yacht Club Costa Smeralda. The vessel is owned by Bravo Romeo Limited.

Semi-dislpacement, as I use the term, represents the technology relating to the hydrodynamic speed range in combination with its displacement whereby the vessel's weight support is approximately 50% by buoyancy and 50% by dynamic lift. I do not believe that transverse section shape - round bilge, double chine or hard chine - should be descriptors of planing,semi-planing, semi-displacement or dislpacement hydrodynamics.

Yes! I only asked the question out of curiosity. Your interests seem to be very broad; from classics to the very modern and I was exploring to see if you might have some Destriero knowledge which would indicate that you might be a good resource as a technology detective.

By the way, Desrtiero was a most gratifying experience.

Thank you for this explanation. I am not sure I understand it completely, since Destriero had a fuel load of almost half her displacement and must have used more than 50 % dynamic lift most of her transatlantic journey?

But in general terms I understand what you are saying, that a hull can be considered semi-displacement regardless of details like chine and V-shape.

For me, a semidisplacement hull is almost the same as a displacement hull, with the difference that it has a straight cut off stern which allows the hull to release the water and add speed but still not with a shape allowing it to plane.

This is why I bought the idea of Destriero as a semidisplacement yacht since her dimensions in a multiple maybe allowed her to go 60 knots without planing?

I have a lot of information on her but nothing that really explains why she is called semidisplacement and not considered a planing yacht.

Finally, Bravo Romeo must stand for Blue Riband, or?

/Lars

The corporate name Bravo Romeo Limited has no dual meaning.

semi-planing vs. planing

I had always thought that the major difference between the two was in their resistance curves?

I percieved that a semi-displacement hull would have the sharp transom, allowing water to flow cleanly off the transom, and with more and more horsepower the hull would go faster to a certain point. This point being faster than a displacement hull with its theoretical hull speed limitation (1.44 x square root of LWL). Once the semei-displacement hull reached this "sweet Spot" in it's resistance curve, it takes exponentially more horsepower to produce speed gains given the same displacement.

And conversely, I thought that true planing hulls would produce more linear speed increases with increased horsepower (again given the same displacement)

Am I on the right track? Or was my perception off base.

By the way, Mr. Blount, I love what your company has done with the hull design and performance of the Rybovich sportfishing yachts !!

Thanks for your kind words about the Rybovich sportfishing boats.

You are on the right path. Hulls designed in a speed range where the vessel's weight is, for the most part, supported by buoyancy will have the aft end of it's hull lines buttocks curving upward toward the water's surface at the transom. This minimizes hull resistance below hull speed. If you try to power a displacement hull to higher than hull speed, the resistance increases rapidly.

As semi-displacement, semi-planing or planing speeds become a design requirement, the aft hull buttock shape must become straight, possibly with some hook (only for hump speeds) in order for the hull to begin to rise up relative to the surface of the water (not squat).

Using the square root of LWL x a constant multiplier (you used 1.44) is a representative way to describe relative boat speed for displacement vessels, but other factors become important for describing comparative speeds of different size boats when dynamic lift provides a significant component of support for the boat's weight.

For a relative example, Destriero at 60 knots has V/(square root of LWL) = 4.28 which would be the same as a Rybowich with LWL = 60 ft making a speed of 33 knots. A Rybovich of this size operates near 40 knots which would be V/(square root of LWL) = 5.16. In a relative hydrodynamic dimensionless speed comparison the sportfishing boat at 40 knots requires higher speed hull form technology than did the much larger Destriero design operating at 60 knots.

Keeping this thread going, I submit...

The New Millennium... 140 feet and 70 mph!

Power & Propulsion...

Twin Paxman 18Vp185 Diesels (5300 hp) Driving Vectored Lips Waterjets
Twin Lycoming Jet Turbines (4700 hp) Driving One-Fixed Lips Waterjets

(there's no replacement for displacement!)

Fortuna (41.5 m) this year, after five years in service, exceeded 70 knots (85+ MPH) during sea trials which is the reported design speed/bench mark condition for The World Is Not Enough as a finely finished yacht. I remain interested in the technical progress and challenges others, such as The World Is Not Enough, are attempting to extend the performance boundaries for high-speed, seaworthy motor yachts.

Why jet drive?

I wonder why a jetengine almost automatically implies a waterjet..
Given the problems you described with the vacuum and the extreme position of the flaps i might have thought that something like arneson drives would have been more logical.

Has there ever been an attempt to use both diesel engines ánd jetengines to produce electricity and let the electricity drive for example the arneson drives?
This option is used in dieselelectric trains to avoid transmission problems...

It usually doesn't, except in the case of some yachts, where speed is a primary consideration. Turbines generate a favorable amount of power for their size and weight. They also produce excellent torque, which jetpumps demand, as the venturi increases backpressure against the impeller prior to expulsion. Jet pumps work on the principle of accelerated flow, or the Venturi effect. The venturi reduces the volumetric area in which water can pass. Because water can not be compressed, water must accelerate in order to pass the same amount of volume through the expulsion orifice.

This is a subject better discussed privately (as we have). Certainly, surface piercing drives are an option, but they slightly increase draft over a jet-pump application. One of the protocols for the M-140 was operating in the shallow waters of the Caribbean and keeping draft to a minimum. Typically, surface piercing drives don't offer the low-speed maneuverability that would be favorable for handling a yacht of this size. Because of their "surfacing" nature, these drives are not deployed as deeply as conventional shaft driven props and are less efficient for achieving plane. Once on plane, surface drives can offer reduced drag and increased RPM's, as the prop is not running completely submerged. Surface drives are best left to high speed apllications, which is not normally associated with "yachts" (atleast luxury yachts), where the majority of time is spent idling, cruising or maneuvering. For most yachts, they don't achieve on-plane speeds that would truly benefit from surface drives.

Lurssen's new "Air" is using this platform, diesel electric power connected to Pod Drives. But this is not new, large cruise ships and freightors have adapted this technology in recent years. We'll have a full feature on Air in the coming weeks.

As for using diesel generators, driving electric motors that are connected to surface drives... I don't recall this ever being done before. I would imagine the power demands would make this prohibitive for any length of time. Further... in the quest for increased speed, weight is a primary consideration. This sounds heavy.

new territories

The inner workings of these technics are kinda virgin territory for me (bloody obvious from my questions)) but fascinating nevertheless.

Another part of waterjets i dont know is how the waterintake works?

Let me begin with some jet-pump fundamentals...

To better understand how the intake gullet works, we need to take a closer look at the jet-pump and the impeller. The impeller itself will only scatter water, and is highly inefficient. But, when placed within a "shrouded" environment, it becomes a ducted propeller. This shrouded configuration produces greater efficiency than its open, non-shrouded counterpart. The reason is simple. The duct controls water and forces it backwards as opposed to a propeller which allows water to slip outwards.

Impellers (and jet-pumps) work on the principal of positive and negative pressure, or a push/pull concept. As a blade rotates, it pushes water back (and outwards due to centrifugal and accelerated force). At the same time, water must rush in to fill the space left behind the blade. This results in a pressure differential between the two sides of the blade: a positive pressure, or pushing effect on the blade face and a negative pressure, or pulling effect, on the backside of the blade. This action occurs on all the blades around the full circle of rotation.

Thrust is created by water being drawn into the impeller and accelerated out the back. However, due to the spiraling effect (vortex) of water leaving the trailing edge of the blade it must pass through stators (straightening vanes) to "true" its trajectory. Stators also increase velocity by "catapulting" water, similar to the way a "kick" works on the trailing edge of a propeller blade. To further enhance velocity, water passes through the venturi before finally exiting the pump as thrust. As we discussed earlier, the venturi works on the principle that a restriction or reduction in line size will cause water to accelerate if the same volume is to be realized at the other end of the restriction. This is where you get the "jet" in pumps. Finally, a steering nozzle is used to vector or deflect thrust for yaw direction.

Impeller design and efficiency is strongly linked to the other components that make up the jet-pump, i.e., the intake gullet, its volumetric area, the laminar transition of the intake housing, stator blade area (including angle of trajectory), venturi rate of compression, venturi "bowl" area, exiting orifice dimension, mass and weight of the hull, and pump placement or depth within the same.

The intake gullet is the recessed area within the hull leading up to the entrance of the jet pump. This area plays a vital role in jet pump efficiency. There are a multitude of factors that determine its length, size, shape and depth. Much of this has to do with the operational parameters the vessel was designed for, such as hull speed and displacement.

For instance, a larger vessel with greater displacement may choose an intake gullet design with a more gradual rake leading up to the jet pump entrance. This maximizes the amount of water available for acceleration. In this scenario, intake gullet vacuum is not as critical because the weight of the hull (and the depth of the pump) will keep the intake cavity primed. In contrast, a light, high speed hull that rides closer to the water's surface, may use an intake gullet with a more aggressive rake and a reduced intake gullet area. This decrease in cavity size, increases the vacuum (or negative pressure zone) at the intake, which helps reduce ventilation brought on by a higher speed planing hulls that operate near the water's surface.

Several variables effect water flow to the pump, including the speed and density of the impacting water due to the forward motion of the hull when underway, and the amount of negative pressure created by the intake housing under operation, which is directly relevant impeller pitch, rpm, and blade area.

Ultimately, the best intake gullet design would be variable in size. In other words... larger for acceleration and smaller for high speed operation, to maximize intake vacuum when aeration is present.

There are a myriad of other factors to consider as well. In the end, it depends largely on the application.