Skyhook – Your “Electronic Anchor” from MerCruiser – Dynamic Positioning

As a Captain and Marine Surveyor, I’ve had the pleasure to captain quite a few boats with this new technology, and I approve! The Axius joystick system is great for close-range piloting, and makes the job of parking in slips quite simple. Skyhook is an extension of that system, allowing the Axius-enabled drives to maintain a solid position while you’re fishing or just out at sea.

If you’re thinking about purchasing a boat with the new joystick technology, give us a call! I can help you to understand your systems as I survey your yacht.


Keep your boat in place with the push of a button!

Mercury Skyhook digital anchor allows you to lock your boat’s position using a GPS satellite antenna. Then, working with your engines and drives, Skyhook maintains your position and heading regardless of wind or current.

Key features

• Ideal for holding the boat over a fishing spot, waiting for bridges to open or holding position while next in line at a fuel docking station.

• GPS technology and electronic compass automatically control shifting, throttling and steering to maintain heading and approximate position.

• Simple to operate – just place the levers in neutral and push the Skyhook button to lock position and heading.

• Standard on Zeus and Joystick Pilotingequipped boats. Available on Axius boats with Premier Package.


Source: Mercury Marine

Hybrid Propulsion Technology (Yachting Magazine)

A word that pops up with increasing frequency in the world of marine propulsion is hybrid, and for good reason. The technology offers advantages over traditional diesel-only drives in some scenarios, on both yachts and commercial vessels.

Whether it is right for you depends on a number of factors, but before deciding, it’s necessary to understand what a hybrid system is and how it operates. Let’s start with an explanation of a total electric vessel, in which an electric motor drives the propeller shaft and the power for the motor comes from an electrical storage device.

Today, these storage devices are usually batteries, but research is active on other solutions such as supercondensers.

Such vessels usually have a limited range and are recharged from dockside outlets, but wind-driven generators and solar panels can provide underway charging. Also, some experimental sailboat installations use the propeller to drive a small charging generator when operating solely under sail.

The 100-plus-year-old, diesel-electric (or gasoline-electric) propulsion system has reliably powered everything from locomotives to transit buses to submarines to yachts. One of the largest diesel-electric vessels is the16-year-old, Lürssen-built, 315-foot Limitless. In a diesel-electric installation, a diesel engine drives a generator, much like an ordinary auxiliary generator set.

The generator powers an electric motor connected to the propeller shaft. Larger yachts add more engines, more generators and more motors, with an increasingly complicated switching and control system. On superyachts such as Limitless, the in-port hotel loads such as lighting, heating, cooling, cooking and guest accommodation support can rival the propulsion loads at sea, so using one power system for both situations can be a smart decision.

A hybrid propulsion system is basically a diesel-electric system with the addition of electrical storage devices and enhanced controls. Thus, each hybrid system involves an electric motor, a generator, electrical storage devices, a means to charge them and a control system. It also involves an engine, usually fueled by gasoline or diesel but increasingly by cleaner-burning liquefied natural gas (LNG) for commercial fleets.

There are two main types of marine-hybrid systems: serial and parallel. In a serial system, the electric motor drives the propeller shaft directly, just like on a total electric vessel. The engine, which has no direct connection to the propeller shaft, drives the generator both to charge the batteries and to power the electric motor. The batteries power the electric motor when the engine isn’t running, and the controls keep the whole system working.

In a parallel setup, the engine is connected to the propeller shaft through a combination motor/generator and a clutching system. The engine can drive the propeller directly without the motor being active, or the motor can drive the shaft with power coming from batteries or the auxiliary generators while the main engine is shut down.

In addition, the main engine can charge the batteries via the motor/generator when it is also propelling the vessel, but its full power is not required for propulsion. It’s a bit more complicated than a serial system, but is more flexible and delivers more overall efficiency.

It is for these reasons that parallel hybrids seem to be gaining in popularity over serial hybrids as the necessary specialty equipment becomes increasingly available. Now, a hybrid marine propulsion system is not the same one as on a hybrid car, although the technology and equipment are similar. The difference is how it operates.

The raison d’être for a hybrid car, pure and simple, is enhanced fuel efficiency. It recaptures the energy that would otherwise be lost in idling at intersections and traffic jams, in braking, in coasting downhill and in constantly varying speed on the highway.

These conditions don’t exist in marine applications, so we must look for other reasons why hybrid propulsion makes sense. For commercial service, where design decisions are largely driven by pure economics, there are increasing numbers of hybrid vessels.

The reasons come from duty cycles on, for example, supply and support vessels for the offshore oil industry and tugboats for harbor service. These spend large amounts of time idling or standing by, more time actively maneuvering, and little time simply cruising.

Additionally, some commercial vessels such as sightseeing and dinner-cruise craft benefit from the elimination of diesel-engine noise, and when they operate in environmentally sensitive areas, the absence of exhaust gas is a plus.


While hybrid systems make perfect sense in some commercial applications, the picture is less clear regarding yachts. Several yacht builders have occasionally offered hybrid options with mixed results. Ferretti Yachts built a 74-foot Mochi Craft long-range hybrid, a lovely yacht that I had the opportunity to examine in detail with the builder’s engineers in 2008.

The technology was unassailable and the construction was exemplary, but the price premium was about a third above the diesel-only version. It was apparently too much for even the most green-minded shoppers, especially as the yacht’s introduction coincided with the beginning of the worst recession the boating industry has seen. One wonders, though, if such a yacht might succeed better today when the greater availability of equipment would help bring the price down.

Royal Huisman delivered the 190-foot sailing yacht Ethereal in 2009, a masterpiece of both hybrid technology and overall environmental awareness. She utilizes two 300 kW ­Combimac electric motors for silent running. (The boat also features twin 714 hp Caterpillar C18 diesels.) Only one engine is required to move this 190-footer along at cruise speed, and the propulsion system also charges her main battery bank.

Those electric motors work in concert with silent thrusters for ­station-keeping, giving swimmers fume-free bathing too. Custom-designed and built for tech guru Bill Joy, she’s a floating test bed for a host of his innovative ideas, as much a research vessel as a personal yacht.

Royal Huisman continues to incorporate some of the technology developed on Ethereal in its current builds, but none has duplicated the complete package found aboard Joy’s yacht.

On the other hand, Elco Motor Yachts has seen a rebirth of its company based largely on electric and hybrid craft. And David Marlow, head of Marlow Marine, is nobody’s dummy when it comes to delivering what customers want.

He added solar-power options to his company’s line of Explorer motoryachts, which range in length from 53 to 97 feet, and offers an electric-propulsion option for the Marlow-Hunter 27 sailboat, so I suspect a hybrid option would not be out of the question should a customer ask.

Greenline, which offers 33-, 40- and 48-foot production powerboats, is perhaps the most successful with the hybrid concept to date. The builder is reportedly working on 55- and 70-foot models. In a written statement, Greenline cites environmental friendliness as the main advantage of its yachts:

“We want to keep our most beautiful boating spots in the same pristine condition as when we first discovered them. We want to enjoy the untouched beauties of the boating world for years to come and pass them on to our children and grandchildren.”

Not only do the boats employ hybrid-propulsion systems, but they also include solar panels on the house top and other “green” measures to optimize efficiency.


Interestingly, just before the recession, when both the Mochi Craft and Ethereal first saw the light of day, many ­builders were gaining interest in hybrid-system potential.

The level of activity was such that the American Boat and Yacht Council saw fit to update its technical standard TE-30, “Electric Propulsion Systems.” While the standard does not address hybrid systems specifically, its guidelines on electrical safety should be adhered to by anyone involved in such systems.

Even in those heady pre-recession days, however, in-depth research would have been too expensive and time-consuming for any single company, so an international, multimillion-dollar effort was mounted. Under the auspices of the International Council of Marine Industry Associations, and with the cooperation and support of a host of electrical- and hybrid-equipment suppliers, a research program dubbed HyMar (for Hybrid Marine) was undertaken specifically to study the technology, the environmental reality and the economics of hybrid marine propulsion systems.

On the research team was Nigel ­Calder, sailor and author of the well-respected Boatowner’s Mechanical and Electrical Manual, and it was his Malo 46 sailing yacht that became the test bed.

In support of meeting clean-air treaties and reducing emissions from marine sources, one of the project’s stated objectives was “reduction of total fuel consumption by 30 percent relative to conventional diesel drive, tending to 90 percent on long-distance sailing boats.”

The HyMar report concludes that “hybrid systems are capable of delivering higher fuel efficiency for propulsion on a cruising duty cycle,” but the reported savings are nowhere near the initial hopes. The report goes on to state, “It is unlikely that hybrid systems will demonstrate enough savings in terms of fuel costs to justify the higher capital costs.” It admits, with surprising candor, that “energy efficiency is more complex than appears at first sight!” In other words, a well-designed hybrid system can offer a number of operational and environmental benefits, and may even be more fuel-efficient than a diesel system in some applications, but most yacht owners will never see an economic payback, and some will see no savings at all.

Those thoughts were echoed by Marnix Hoekstra, naval architect and co-owner of Vripack, one of the world’s largest and most respected yacht design firms, when I spoke to him at the recent Monaco Yacht Show. He strongly supported hybrid systems for use in yachts, much like in commercial vessels, when special operational requirements or desires dictate a need.

He also was quick to point out that most yachts aren’t taking advantage of other measures, such as better hull forms and more sophisticated propellers, that should be implemented before considering hybrid propulsion as an energy-conservation measure.

With a smile, Hoekstra concluded, “If an owner wants to be totally green, he should not build a yacht at all. Short of that, though, there are many things that can be done to improve the impact on the environment. Hybrid drive is only one of them.”

Read More: Yachting Magazine

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Should I Buy a Boat with Blisters?

One of the most frequently asked questions that a marine surveyor gets is, “Should I buy a boat that has blisters?” This is a question that I’ve wrangled with for many years, and after a great deal of research involving thousands of boats. This essay will answer your questions a bit more directly than some of our more detailed blister essays, which many of you found hard to understand. That’s understandable because this is a very complex subject. But be prepared that the answer is populated with a lot of ifs, ands, and buts.

Let’s face it, an awful lot of boats have blisters, so that finding one that doesn’t (or won’t get them) can be a difficult proposition. The short answer is that if at all possible, you should try to avoid that, if for no other reason than the potential expense you may face in the future. That expense may not result from the absolute necessity to repair the blisters, but the position you may find yourself in when it comes time to sell the boat. Particularly with newer model boats, say 1 – 3 years old, it is not unusual for buyers demand a reduction in price, or that the blisters be repaired.

For older boats, its usually much less of a problem, for the fact is that moderate blistering on an older boat rarely impedes the sale. Unfortunately, another fact of boating life is that there is a great deal of misinformation on this much talked-about subject. One common misconception is that blisters seriously weaken and/or damage boat hulls. In 30 years of surveying and examining around 4000 hulls, I have seen less than 10 cases where blisters have resulted in serious structural degradation of a hull where it was weakened to a point where some type of failure was immanent.

What is a blister?

First, let’s understand that all fiberglass hulls absorb water to some degree because both the gel coat finish on the exterior, and the fiberglass reinforced plastic is porous. Since water is a solvent, it will react with the plastic resulting in the water and solvents in the plastic mixing to create a weak solvent solution, usually with styrene. This then softens the gel coat somewhat and, combined with a bit of gas or fluid pressure, results in the blister.

Moisture Meters

Here’s a subject I get a lot of questions about, and one that I want to address it upfront. Since I have already stated that all fiberglass boats absorb water to some degree, and often without causing blistering, it follows that the use of a moisture meter is useless. If you doubt this, please see the essay Illustration of Water Absorption From Hull Interior on this site. It shows a hull that has been completely water saturated for 10 years, but has not developed blisters. Moisture meters measure only the surface moisture, and since gel coat and paint is very porous, the moisture meter is only going to tell you what you already know; its wet. It cannot tell you anything about the propensity of a hull to blister. While these instruments have their uses, predicting whether a hull is prone to blistering is not one of them. See also Moisture Meters on Boat Hulls.

Are blisters harmful?

Yes, but. This is a question of how much harm. Blisters form at the interface between the gel coat and what is called the skinout mat, which is a layer of chopped, short-strand fiberglass that is used to prevent the coarser weave pattern of heavier fiberglass cloth from telegraphing through to the finish surface. You’ve probably seen boats with a checkerboard pattern showing on the surface, and this is the reason why. Now, fiberglass fabric, being made of bundles of very fine glass fibers, is very porous also, most especially the outer layer of mat. Once the gel coat absorbs water, the fibers in the mat that are unsaturated with resin then spread the water around via the capillary effect.

Blistering involves only the gel coat and surface mat in 99% of the cases. This is due to the fact that the structural fabrics, such as roving, get saturated better. Its also because the water is less likely to penetrate beyond the mat and, even if it does, woven fabrics do not have the weak gel coat factor and are much too strong to allow whatever pressure may develop within a void to cause a separation. The incidence of blisters occurring within structural laminates is extremely small.

Boat hull blisters

If the resin used to make the hull is of a lower quality that will react with water, a process known as hydrolysis, which means nothing more than becoming saturated with water and dissolving, then the hull is poised to develop blisters. Many other factors also come into play here, such as how well the mat layer is bonded to the gel coat.

Since the vast majority of blisters occur between the mat and gel coat (depicted in illustration above), this bond has to be fairly weak for the blistering process to occur. If the bond is strong, then blisters will not occur, even though there is a lot of water absorption. This is a very general, even generic, description of the blistering process. There are frequently numerous other factors involved which I will not address here.

Aside from the damage it causes to the surface, most of the damage done by the blister is to the gel coat and the skin out mat, which is not a structural part of the hull laminate. Remember, the mat is only there to prevent the fabric pattern from showing through to the surface. The obvious question is now, “But isn’t the water dissolving the rest of the plastic resin in the laminate?” The answer to that is “No, its not.” At least not to any considerable degree.

You may have noticed that I have not used the word “osmosis” that we hear so much these days. Technically, water passing through the porous gel coat is not osmosis; its just water passing through a porous material. However, the blistering process may involve osmosis, a process which concentrates solvents within the space formed by the blister void. This concentration of solvents does indeed dissolve the plastic, but fortunately the amount of fluid involved is so small that it does not seriously threaten the laminate.

Of course, the larger the blister, the more concentrated solvent is present, the more damage it will cause. Therefore the amount of damage, and therefore structural weakening caused by blistering, is directly proportional to size and number of blisters. This explains why only boats with very large blisters can end up with serious structural weakness problems.

 - Boat hull blisters
This photo represents a typical case of extensive blisters, small enough to be called pimples. They are dime-sized and smaller, but no matter how many of them there are, they are very unlikely to threaten the structural integrity. But they do make sanding and painting the bottom very difficult, and will cause a slight speed loss on sailboats.
Hull bottom blisters
This is one of the few examples we’ve seen where large blisters threaten the integrity of the hull. However, the problem here was that the builder used chopped strand mat that was over 1/4″ thick on a foam cored hull. The mat absorbed huge amounts of water, creating these enormous blisters. Thus the real danger to the hull was less a matter of blisters than the way the builder built the hull. In other words, too much of the structure was invested in a very weak material. This hull lacked strength to begin with, proven by the fact that it was also badly delaminated.Vessel: Irwin 65′

Since the vast majority of boats develop only dime-sized blisters, the amount of damage or structural degradation resulting is very small, even when the bottom is extensively blistered. Even boats with numerous blisters up to about 1″ in diameter, usually show no significant weakening of the plastic. The illustration above shows the relationship of blister size to the laminate thickness. Here it can be seen that even if some of the plastic is dissolved under and around the blister (indicated by dotted line) in proportion to the overall laminate thickness, its not much, even when the amount of degradation is above average. The amount of blistering would have to be truly severe to have even minimal effect.

These conclusions are based on two completely different types of evidence. First is the fact that physical inspection, probing and sounding rarely reveals softening or degradation in the area immediately peripheral to the blister. Second, the fact that significant structural weakening will make its presence known (before failure) in the form of delamination, surface deformation and stress cracking. The good news is that I know of no reports of these conditions occurring as a result of blistering, unless the blisters are extremely large. We’re talking here blister 4″ and larger, at which point the problem becomes rather obvious. From these facts I conclude that well over 95% of all hull blistering cases do not cause significant structural damage to the laminate.

Getting back to our original question, “Should someone buy a boat with blisters?” can be answered from several viewpoints. If you insist on a boat without blisters, fine, then go try to find one. If its an older boat, you may have little choice, since blistering tends to run in certain builder’s lines and you may have to look at quite a few before you find one. All things being equal, you’d certainly want to choose a boat without blisters. Unfortunately, unless the seller is kind enough to tell you, you can’t find out until the boat is hauled for survey, at which point you’ve already invested some money in it. Its a fact that most blistered boats are sold without regard to the blistering, and this is one of the reasons why. In my experience, the number of cases where blisters cause the boat to be rejected, or give rise to price renegotiations is considerably less than 5%.