When the length over water of two ships is about equal, the actual weight or water displacement of the ship is a very good indicator for fuel consumption. The length over water defines the maximum displacement speed. Two ships of equal over water length have therefore an equal potential top speed.
But if one ship is heavier and bulkier than the other, well, then it displaces more water. As in the example of the previous post: a Nordhavn 68 displaces 93 metric tonnes, while our boat design displaces only a third of that. If our design needs (roughly speaking) 100 horsepower to sail at 18.5 km/h in less than ideal circumstances, then the boat that’s three times as heavy should need 3 x 100 = 300 hp.
Yet, as we learned from the real world feedback from an actual Nordhavn 68 owner, his boat uses much more horsepower and diesel. It takes 400 hp to get to 10 knots. That’s a third more than it should take. How come the theoretical models do not work in practice? They do not work, because they start on the wrong foot.
A boat may displace 93 tonnes or 31 tonnes. But that is displacement measured at rest. Nobody is interested in fuel consumption and efficiency while a ship is in the harbor or at anker! What we want to know is water displacement on the move!
As a boat travels through the water, it needs to manage the liquids it encounters. Liquid management. A bigger boat disturbs more water, due to its bigger size. This is covered by the displacement at rest. But not all underwater-ships are designed equal. Some move through the water more efficiently than others.
Less efficient underwater-ship designs disturb more water as they move faster. More water disturbed equals more water displaced. In order to displace more water, well, one needs to spend more energy.
More efficient underwater-ship designs do not disturb the water they sail through that much. As a result, their efficiency stays much closer to the displacement at rest numbers. Less water displaced equals less energy lost. Underwater-ship design is the essential key to a comfortable sea state and an acceptable fuel-consumption. Crossing oceans comfortably is all about managing the liquids one travels through!
The “inside-out” design of the Nordhavn 68 prioritizes interior space over liquid management. The “outside-in” design, that we apply, prioritizes liquid management over interior size. That explains why the LM65h, at 10 knots, stays very close to the actual displacement at rest calculations. The way the Nordhavn does its liquid management is obviously not as good. Extra width, depth, weight, and an overall focus on maximizing the interior space do come at a cost. A cost that is higher than the 300% explained by just displacement at rest. As the Nordhavn needs 400 horsepower, instead of 300 hp, there is an additional 100% or 100 hp efficiency loss. That’s over 20 liters of efficiency loss per hour.
In fact, the additional 100 hp lost in the more traditional motor yacht design of the Nordhavn is just as much as our design needs at 18.5 km/h. In other words: the efficiency losses of an “inside-out” designed ship are at least equal to the total power our “outside-in” ship needs at 18.5 km/h. Let me repeat that. Let it sink in. Just the inefficiencies caused by the traditional manner in which motor yachts are designed are enough … to power a ship like ours, that actually focuses on liquid management rather than interior space!
Wave creation is a good indication of sub-optimal liquid management: thousands of liters (and hence thousands of kilo’s) of non-functional waves may be produced. An optimized underwater-ship design would have used more of that energy for actual propulsion.
Shall we calculate how much energy the Nordhavn design is throwing away? It’s quite easy, actually. She weighs 93 metric tonnes. Sailing at 10 knots should be possible by applying 300 hp. The ship needs 400 hp though. That’s 100 hp out of the window. Producing those additional 100 hp is enough to create 31,000 kilo’s of waves and under water turbulence per hour. Producing those 100 horsepower asks for an additional 21 liters of fuel-burn per hour. And that’s on top of the additional 200 hp and 42 liters per hour the boat already needs due to its additional displacement weight.
Small ship creating big waves equals small ship creating huge inefficiencies: