The Power Multihull sector has experienced a remarkable boom in recent years - few sectors in the boating industry can boast double-digit growth. In this buyer’s guide - which has become a special motor issue over the past two years - no less than 120 models are presented. Out of all these 15 to 100-foot power multihulls, almost all of which are habitable (we have deliberately stopped at 100 feet), the category we have chosen for this issue is the 40 to 50 footers. These models already have most of the attributes for a successful summer or long-distance cruise. Charter companies like Sunsail/The Moorings operate powercats from 43 (soon to be 40) to 53 feet. In this issue’s buyer’s guide, the seventeen 40 - 50 foot power multihulls are all catamarans equipped with two engines whose power ranges from... 90 to 860 HP. By filtering out the most extreme engines (including outboards), we arrive at a convergence of between 250 and 400 HP inboard. We will use this order of power as a benchmark to help us understand how to limit fuel consumption under way.
Two hulls are better than one!
These benefits begin, as we mentioned above, with powercats having a much more frugal appetite than their monohull counterparts. When under way, the drag of two thin hulls is much lower than that of a single wide hull, and this leads to much lower fuel consumption. If we compare a powercat with a boat of the same volume, the evidence is conclusive. Prestige Yachts, which recently arrived on the power catamaran market, would not disagree. The Prestige 590F burns about 30% more diesel than the brand new M48, which requires just 12 gal (45 l)/h at 10 knots and 20 gal (75 l)/h at 14 knots. Yet the powercat has a similar livability despite being 11 feet shorter. It should be noted that the M48 manages with two 320 HP engines while her big sistership requires two 600 HP units. However, the monohull has an advantage when it comes to its top speed - it can reach nearly 30 knots, while the M48 will not go faster than 20 knots. Also worthy of note is that while in the past, a top speed of more than 20 knots was a determining factor in the purchasing decision of boatowners, this trend is rapidly disappearing.
For the first models of our target category, the advantage in terms of consumption in favor of powercats remains. If we look at a Sedan version with minimal windage like the Fountaine Pajot MY4.S, the two 250 HP engines consume 6.6 gal (25 l)/h at 11 knots while the Bavaria E40 trawler, with its 2 x 300 HP, burns 8.7 gal (33 l)/h at the same speed. At 15 knots, once the schedule is set, the on-board instrumentation will show respectively 10.6 gal (40 l) and 13.2 gal (50 l)/h - that is to say a 20% fuel saving for the catamaran, which is considerably more welcoming for its passengers.
Let’s close this comparison between powercat and monohull with a brief incursion into the top of the range, well beyond our 40-50 feet. On board a Fountaine Pajot 67 Power equipped with a pair of 480 HP Volvos, fuel consumption at ten knots will be half that of a Galeon 630 S, heavily powered with 2 x 1,200 HP. The advantage is still with the powerboat, even when compared to much more modest single-hulled boats such as the Bénéteau Grand Trawler 62 equipped with two 730 HP engines: the difference is still 25%. We should also point out that even more convincing results were obtained by the LEEN trimarans – 2.1 gal (8 l)/h at 8 knots for the 56, which is an excellent result.
An average of 13 to 21 US gal (50 to 80 l)/h
Should we be satisfied with navigating a power multihull, which has the advantage of consuming less fuel, and at the same time be proud of our eco-responsible behavior? It’s not as simple as that... To begin with, the ban on CO2 emissions in ports and in many protected areas is unavoidable. As such, only electric propulsion - for the moment still hybrid - meets this requirement. Secondly, almost all powercats in circulation operate with an average consumption of 13 to 21 US gal (50 to 80 l)/h. This not only means high emissions, but also a considerable budget - and the latter is likely to increase dramatically in the near future. “Most charter weeks end up requiring a full tank of around 130 gallons (500 liters) for a Leopard 46 PC and up to 260 gallons (1,000 liters) for a 51 PC, provided the customer wants to sail at the maximum speed,” says Pierre-Yves Chanau, THL Marine’s European sales manager. “One and a half hours of daily navigation at a cruising speed of 14/15 knots is the weekly average recorded by the teams at our bases,” he adds. The anchorages are rarely more than 20 miles apart and there is no need to sail longer because the goal is to make the most of the stopover. A motor multihull has the undeniable advantage over a sailboat of arriving at the anchorage two to three times faster. The question of whether yachtsmen are ready to deprive themselves of this advantage is now crucial. The answer remains uncertain: shortening travel times obviously mean refueling and it could be the hit to the bank account which takes precedence over the pleasure...
A matter of friction...
The purpose of this article is not simply to examine in detail the various devices that engine manufacturers offer. However, to cut a long story short, it is obvious that electronic regulation, programmable speed reducers, turbocharging systems and other level control devices have led to significant reductions in fuel consumption in recent years. Technology is constantly evolving - and for the better. On a Fountaine Pajot MY5, Volvo Penta’s interceptor system (christened IS by the manufacturer), enables the boat to be sailed flat, gaining two knots of speed and saving three to four liters (up to a gallon) per hour of sailing. In this case, it’s the technology that is influencing the consumption of the boat, and not the skipper. It is the manufacturers who are providing the know-how. Nevertheless, there are also many areas where we can limit the fuel bill. For those who want to play a role in today’s energy challenge, there are many useful practices that can be adopted for burning less fuel. Following the manufacturer’s recommendations for use is the first good practice to adopt. The characteristic of engines is to transform thermal energy, resulting from the combustion of fuel in a chamber, into mechanical energy that will be used to move a power multihull. Only 40% to 50% of the energy supplied by the fuel is transformed into mechanical energy. The rest of this energy is lost and evacuated by the hot gases coming out of the exhaust and by the water circuit that ensures the cooling of the engine. It is important to understand that diesel fuel has the ability to maintain its combustion cycle even when the throttle is reduced, especially when the boat drops its speed to return to cruising mode. In this case, the diesel engine’s consumption will fall quickly while still maintaining its efficiency. Adapting the engine speed to the optimum operating range recommended by the manufacturer therefore seems particularly judicious. On the other hand, the mechanical energy is not entirely restored: nearly 30% is lost through friction. These losses are distributed as follows: around the piston (about 45%), in the interactions between the conrods, the crankshaft and the engine block (about 30%) and around the valves and their actuation systems (about 10%). The remaining 15% are lost in the peripherals, such as the alternator or water pump belts and the transmission to the propellers via the propshaft or sail drive. Since 30% of the fuel is used to overcome friction between moving mechanical parts, lubrication with an oil that limits friction and wear of these surfaces becomes essential, not only to preserve the engine from overheating or breaking down, but also to reduce fuel consumption. The use of high-performance lubricants can significantly reduce the consumption of your engines. If your engine already has a few thousand hours on the clock, using oil and fuel additives to descale the combustion chambers or to clean the valves can be very beneficial. In the end, the energy used to propel the boat is only about 30% to 40% of the total energy supplied by the fuel! Can this number convince the most skeptical of the relevance of going further to limit consumption? If so, there are still many other possible actions...
Sailing “clean”, not too fast, and light
The power required to move a boat forward at a desired speed and perhaps into the wind depends on many factors: the size and shape of the hulls and appendages, the operating weight - and its distribution on board -, the windage and the condition of the hull. We risk repeating ourselves on this last point, but the influence of a dirty hull, even if it is just greasy, on the boat’s forward resistance is disastrous. Depending on the level of dirt, the speed can decrease by one to three knots and the consumption can increase by a gallon an hour for the same engine speed. This is the reason why manufacturers always try to make us test powercats with very clean hulls. A simple waterline and slightly “green” propellers are enough to show the differences with the theoretical consumption curves provided by the manufacturers. Calculations can give an estimate of resistance/speed/consumption, but real conditions with full water and fuel tanks, sea state and wind dunnage change the situation somewhat, as we will see later. These curves (see graphs in illustration) are however very instructive and allow us to confirm what we all know: speed on a powerboat is expensive. Very expensive, even. Two to three times as much power is needed to go from 70% to 100% of a boat’s hull speed, with consumption in line with this. Commonly accepted average statistics on this point show that for a monohull with a waterline length of 40 feet (12 meters), the hull speed is 8.5 knots. So the most economical speed will be about 6 knots. Powercats have a greater advantage in this regard. Taking the Longreach 40 as an example (whose waterline length reaches 40 feet thanks to its strongly inverted bows), its theoretical consumption at 8.5 knots is 1.9 gal (7.20 l)/h, while at 6 knots it is barely 1.3 gal (5 l)/h. It increases to 4 gal (15.3 l)/h to reach semi-planing and then soars to over 21 gal (80 l)/h when hitting a maximum speed of 25 knots. Obviously, it is important to relate the notion of consumption to the mile (a bit like the way we consumption for a car): at 6 knots, we consume 0.219 gal (0.83 l)/m; at 8.5 knots, we find 0.225 gal (0.85 l)/m; at 13.5 knots, consumption climbs to 0.299 gal (1.13 l)/m, reaching 0.85 gal (3.2 l)/m at 25 knots.
The analysis is very similar for all types of powerboats. This can be seen even more clearly on the graph of the Leopard 46 PC equipped with two 250 HP Yanmar engines. At 7 knots. 2.1 gal (8 l)/h is sufficient, while at 9 (its hull speed) 4.2 gallons (16 liters) are needed. The other lesson to be learned from this curve is the increase in consumption during the phase when the boat tries to break away from its hull speed - the curve then becomes almost vertical. Unlike gliding monohulls, power multihulls don’t really break away, they just reach a semi-planing speed (around 13 to 14 knots) which relieves the forward drag. After this point, the curve becomes more linear again, even if the slope is steeper than at lower speeds. It is better to avoid stagnating in this speed range where the boat is pushing against the water and requiring a big effort from its engines. Finally, the difference between a half-loaded powercat (blue curve) and a fully-loaded boat (purple curve) is significant. A two-ton difference in fuel and other supplies translates into an increase in consumption from 16 to 20 gal (60 to 75 l)/h at an average speed of 15 knots. If you don’t plan to do a lot of sailing, there’s no point in filling all the tanks to the brim and taking all your possessions on board - unless you’re sailing at about hull speed, where the difference in consumption becomes negligible.
The relevance of powerful engines
Many skippers claim that a bigger engine, with more torque, will lower the engine speed and consequently the fuel consumption. Is this a preconceived idea? Comparing the fuel consumption curve of a Leopard 46 PC equipped with 370 HP engines with that of the same powercat equipped with 250 HP engines, we can see that up until hull speed, large eight-cylinder engines do not consume more than four cylinders ones. Semi-planning at 13- 14 knots will be reached with only 10 gal (38 l)/h against 10.6 gal (40l)/h for the smaller engines. On the other hand, at 18 knots, the big engines will only need 16 gal (60 l)/h while the small ones will peak at 20 gal (75 l)/h and will not even be able to take a fully loaded boat beyond 17 knots with 26 gal (100 l)/h while the power reserve of the 370 HP will only need 21 gal (80 l)/h at this speed. It will also be able to take everyone and their luggage at more than 20 knots at the same 26 gal (100 l)/h. Therefore, a more powerful engine is only an advantage at high speeds. Of course, mechanical wear and tear is limited with a lower rpm, but by staying within the hull speeds or below 15 knots (which corresponds to the most common usage), this eventuality can be mainly avoided. Going above these normal parameters can be reserved for extreme sailing with strong headwinds and heavy seas of course. But is this the general rule? When cruising at reduced speed, a smaller engine will be more suitable, as diesel engines are not well suited to running at less than 50% of their maximum power.
Other factors that significantly impact consumption
As mentioned above, in the best-case scenario, less than 50% of the energy consumed will be used to move the boat, i.e. to generate the power that will activate the propeller. As it is the propeller that pushes the boat forward, the choice of this material is of prime importance. The loss of performance can be more down to the wrong choice of propeller than to the wrong choice of engine. Boatyard technicians usually test two or three propellers before deciding which one offers the best compromise between speed, acceleration and fuel consumption. When we do our tests, we sometimes come across a lack of tuning. The motors with the highest gear ratio at the base or shaft are those that drive the propellers with the largest pitch (angle of incidence of the blades) and the largest diameter. When choosing a propeller for your boat, you can therefore select the diameter, the pitch and also the number of blades and their surface area. Three or four-bladed propellers are generally installed. The three-bladed propeller is the most common in pleasure boating because it is versatile, lighter and often faster even if it generates more vibrations than a four-blade. Four-blade propellers mean a larger surface area without increasing the diameter. A larger diameter would penalize engine torque and impair acceleration, unless you’ve gone for a more powerful engine. A diameter of more than 23 inches is suitable for a 400 HP engine whereas a 150 HP engine will handle just 21 inches. The propeller’s pitch is also a determining factor when it comes to performance. A short-pitch propeller (either 19 or 21 inches; pitch is measured by calculating the theoretical forward motion in one revolution) allows for faster acceleration, but it will result in higher fuel consumption and lower efficiency. The best propeller for a boat is above all a matter of compromise, as practice sometimes tends to contradict the theory - only the empirical method (tests and measurements) can determine the right choice. The ideal is of course to find what suits you. The propeller that will allow your boat to obtain a good top speed will not necessarily be compatible with modest fuel consumption at cruising speed. We will deal with this specific point that requires a detailed technical approach in a future issue.
A more ”nautical“ approach to motorboating
There is no secret. The faster we want to go, the higher our fuel consumption. If we decide to limit our fuel consumption by not exceeding hull speed, we are back to the performance of a sailboat. This observation must undoubtedly be considered as a new way of boating: closer to that of those who sail. Here, it is good seamanship that is in charge: avoid heading into the sea and the wind, go down the leeward side of islands, delay or bring forward your departure to take advantage of the best conditions, such as early in the morning when the sea, not yet under the influence of the thermal breezes, is often flat. Take advantage of going downwind, thermal breezes, favorable currents... A power yacht will still be quicker overall - no more sails to hoist and stow, and the possibility of sailing against the wind. In a few years, this approach, which will undoubtedly lead us all - forced and/or voluntary - to practice more responsible cruising speeds, is already being experienced by those who have switched to hybrid propulsion. Let’s bet that in 10 or 20 years, in the era of “genuine all-electric“, there will be little difference between sailing and power multihulls…
