Pacific Expedition Power Catamarans: Expedition Trawler Catamarans and Yachts

Expedition Catamarans:

Long-Range Expedition Power Catamarans Come of Age
"At Pacific Expedition Yachts we manufacture long-range power catamaran yachts that are faster, more stable and more economical to operate. These fast trawler style yachts are true, ocean-going homes designed for adventure – and they're easily enjoyed by couples and families in a secure, comfortable setting with a great deal of control and "no-roll" stability.

Nearly 20 years of power catamaran and sail catamaran as well as mono-hull yacht building experience are designed into each Pacific Expedition craft. These power catamarans employ world-proven semi-displacement multi-hulls that break away from the slower trawler designs yet travel up to 3,000 nm when desired. Pacific Expedition Yacths has integrated the best naval architecture and proven modern building materials and techniques into a massively strong, robust sea-going package. These power catamarans are designed to take you anywhere on the planet – all while delivering world-class comfort, safety, economy and performance.

Please take a look at the power catamarans on our website and learn more about our vessels that we feel are unsurpassed comfort, economy and safety at sea. We truly hope to meet you in person and have Pacific Expedition Yachts become an important part of helping you realize your cruising dream!"

John Shaw,
Founder, Pacific Expedition Yachts
tel: 1-800-613-1758

Pacific Expedition Yachts PE60 3D

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Pacific Expedition Yachts PE50 3D

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Pacific Expedition Yachts CE47 3D

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"The characteristics of semi-displacement power catamaran hulls are ideally suited to modern expedition fast trawler style yachts, offering excellent seaworthiness and economy up to 25 knots. Pacific Expedition Yachts deliver ocean crossing range in a multi-hull platform that provides much greater space and variety of layout options when compared with many alternatives. I cannot think of a better vessel on which to see the world."

Stuart Bloomfield
Bloomfield Innovation - Naval Architect

Hi all,

John and I, at Pacific Expedition Yachts, are very lucky to have a wonderful naval architect, Stuart Bloomfield, as a program partner who is responsible for much of the development to date of the new Pacific Expedition PE50/60 power catamaran range.

Stuart's experience is a tremendous benefit to the entire organization and ensures that the Pacific Expedition range of power catamarans are designed from the hull up to be some of the best in the world in all sea conditions.

Today, Stuart graciously offered to shed some light on the importance of catamaran resistance and trim and how it applies to real life boating issues.

So grab your java and take a look - and thanks again Stuart!

Pat and John

Catamaran Resistance and Trim - Stuart Bloomfield www.bloomfieldinnovation.com

Resistance is the term applied to the force that resists the forward motion of a boat; in other words it's the sum of the forces from the air and sea opposing the forward force provided by your propulsion system that you have to pay the fuel bill for. Resistance of ships is a complex topic and this article will concentrate on the specific area of resistance related to the trim of a vessel which is of great importance to the case we will consider, that of the 40' to 80' catamaran travelling at typical fast cruising speeds (10-25 knots).

Resistance can be broken down into a number of components: air resistance, skin friction (viscous drag, which is parallel to the hull skin), wave-making and other residual components (which are perpendicular to the hull skin, i.e. changes in pressure around the hull pushing back at the forward part of the hull and pulling back at the aft part of the hull). Air resistance is typically less than 10% of total resistance, skin friction is a major component (especially at lower speeds) and is proportional to wetted surface area (which is reasonably constant for non-planing hull forms), wave-making resistance can vary greatly depending on a number of factors including trim and residual resistance is relatively minor.

Wave-making resistance is represented by the wave pattern around a vessel and is proportional to the energy required to produce the waves, in other words the bigger the waves the more power you have to use.

As a boat moves through the water it has to push the volume of water where the hull will be out of the way, in doing so the pressure in the water will increase and at the surface this increase will cause the level of the water to rise, the momentum of the rising water will cause it to overshoot its equilibrium point and therefore it will bounce up and down (forming a wave) until the energy has been dissipated.

Wave trains around a vessel are made up of two types of waves: divergent waves (moving diagonally away from the sides of the hulls) and transverse waves (moving in the same direction as the boat). Both wave trains are influenced by the change in volume, width and depth along the length of the hull. Divergent waves are predominantly due to changes in the shape of the hull at the waterline (e.g. the bow entry, the stern and any shoulders). Transverse waves are influenced more by the shape of the keel and the trim of the boat. Conversely the wave system can also influence the trim.

A couple of critical points control how the wave trains behave and in turn how they influence the trim:

The entire wave system moves forward at the same speed as the vessel.

Wavelengths of waves are a function of the wave speed, gravity (which is fixed) and water depth (which is deep). They get longer as speed is increased.

When the wavelength of the transverse wave train becomes the same as the length of the ship there are wave crests aligned with the bow and stern and a trough amidships, this speed is known as the hull speed and is independent of hull form or whether the boat is a monohull or catamaran; this speed is 10 knots for a 55' waterline. As a boat increases speed above hull speed there remains a peak at the bow and the wave lengths increase resulting in the hull effectively having to continuously climb up its own transverse wave, this is sometimes referred to as the resistance hump because the rate of increase in resistance with increasing speed is greater at this speed than slower and faster speeds; as the speed increases further the longer wavelengths mean the hull is effectively closer to the peak at the bow and the slope of the water surface around the hull starts to get flatter (less steep near the crest) again.

So how does all this relate to trim and why are catamarans so much better than monohulls?

The job of the hull designer is to produce a hull that will push the water away from the bow at just the right rate so that the overshoot we referred to previously is minimized resulting in the smallest possible waves size, and the reverse at the stern. One of the primary factors influencing this challenge is the displacement length ratio, which is a measure of how heavily loaded a hull is per unit length. This is where catamarans have the advantage over monohulls because each hull only has to push half as much water aside as a similar monohull. A second factor, the length/beam ratio, also has some effect on the wave generation; again catamarans have a significant advantage over monohulls which have lateral stability requirements that often dictate wider hulls for safety. Catamarans pay the penalty in wetted surface, often having greater wetted surface than an equivalent monohull, so at speeds below hull speed monohull designs can have a resistance advantage, whereas at speeds greater than hull speed catamarans are generally a lot more economical.

Planing hulls operate in a similar fashion for both monohull and catamarans; they produce significant wave resistance but use the dynamic pressure of the water hitting the hull bottom to raise the hull out of the water thereby reducing wetted surface area. Planing hulls will always trim as they effectively act as the lower face of a hydrofoil or wing in the water; the trim is there 'angle of attack' which is required for lift; as speed increases trim will decrease to maintain pitch equilibrium. For our purposes we are primarily concerned with semi-displacement (also know as semi-planing and marketed as displaning) hulls which are more efficient than planing hulls at speeds below four times the hull speed.

As mentioned previously there is a strong interaction between trim and the transverse wave train. This can form a positive feedback system (more trim leads to larger waves leads to more trim...) resulting in significant increases in overall resistance if not handled correctly; for example several degrees of trim may increase resistance in the order of 100%, meaning you will use twice as much fuel to go the same speed and achieve half the range, not to mention the cost of those expensive engines. Optimal trim (for minimum resistance of well designed semi-displacement catamarans) is normally slightly bow down (less than one degree), but level trim is normally quite close to optimal and there can be other negative effects (on handling) associated with bow down trim. This (zero trim) often initially feels unnatural to sailors used to the feeling of a trimmed boat and it can even feel like the bow is trimmed down at first even when the decks are flat. The primary way to control trim is with a good hull design, but additional devices like trim tabs can offer good improvements even for well designed hulls. The things that work against a design are heavy (short) hulls, hulls spaced closely together (where the wave trains from the two hulls combine) and wide hulls (low Length/Beam ratio), sometimes these limitations can compromise the ability to minimize the transverse wave resistance.

A well designed and trimmed boat will have almost no transverse wave train, which can be clearly seen as a flat wake (no rollers behind the boat). In fact looking over the stern of the boat is a very effective way to tune the optimal trim setup.

Resistance is a very complex subject and we have only just scratched the surface but hopefully it has helped you understand some of the fundamentals of how and why different hull shapes and configurations are better suited to particular applications and you will know how to set up the trim on your boat so you are not burning fuel to tow the ocean behind you.

Stuart Bloomfield www.bloomfieldinnovation.com