Maritimo propeller development
by Maritimo 8 May 19:30 PDT
Maritimo propeller © Maritimo
"High speed screw development definitely is a scientific approach to what is a black art," said Willaton in opening. "Propellers can be a tricky thing, and so we take the hull resistance figures from our in-house Naval Architect here at Maritimo, and then I work with our propeller manufacturers, where we also add into the equation our weight, shaft angles, and appendage drag to develop our own formula."
"Making sure we deliver the exacting standards set for a Maritimo Luxury Motor Yacht demands we get this critical aspect correct, so that the final, long-range, blue water pedigree affords our customers the opportunities to build memories in some of the best cruising grounds around the globe."
"We use five-bladed skew propellers, where rake and pitch are the critical decisions. This style of propeller are smoother and quieter, which is a direct result of their development for use on submarines, where any amount of disturbance and cavitation can be heard by enemy sonar."
"Modern diesels provide plenty of torque, which can help spin a large propeller overall, especially behind the 2, 2.2, and 2.5:1 reduction gearboxes we use. Making sure we can extract 100% load out of our set up means we can offer the best pace, efficiency and range outcomes at the desired 80% of load, which is where most boats do their fast cruise at," said Willaton.
"Today, we have inserts to help tweak the pitch, or bite of the screws, and this helps a lot for you can add or subtract small amounts of pitch with the propellor still on the boat. The other critical element in all of this is maximising the diameter to area ratio, which is reduced to under 100% by using a skew style of prop, which in turn spins faster and can offer more bite."
The one element the understated Willaton has not reflected upon to date is the level of experience that goes into the melding of art and science in this field. In this domain, Willaton is certainly princely. One marine journalist referred to Willaton as the 'Sourcerer of Screws', which was a play on both sorcery and sourcing, but a more regal name befitting a man of his experience could well be the 'Prince of Props'.
Options, equipment specifications, and locations all play their part in the overall mix, too. Stabilisers, swim platforms, watermakers, and so forth. They all have different weights, and the placement inside the craft affects the final outcome. Sometimes the strips, or inserts just cannot do the final tweaks, and a whole new screw is required. It is that intricate, and in a way delicate, in terms of balance.
The global supply chain, macroeconomic factors, and geopolitical elements are also involved. Many gearboxes come from central Europe, and often a boat would be expected to have a certain engine/gearbox combination arrive, which would mean a certain screw had to be fitted. Alas, a change of ratio in just the gearbox alone necessitated a complete rethink on what propeller had to be fitted.
"We held a small inventory of propellers for a while there, just to ensure there were no surprises when the drivetrain showed up," said Willaton, highlighting the company's prudent operating style. Here the black science side reigned supreme over the dark arts for the benefit of all.
More than ever, simple is just not part of the solution when it comes to making the progress of your new Maritimo motor yacht both effortless and efficient. The swan looks glorious above the waterline, however the feet are very, very busy, so it is good to have someone with the kind of thorough understanding that Willaton does.
As efficiency continues to be the number one mantra, being able to tap into every single horsepower being delivered by the best diesel engines available for marine propulsion requires skill, but also deep understanding. It is all about achieving the maximum area from the leading edge to the hub, and then going out to the tip. It means we are at about 92% of surface area to diameter on each of them, no matter which model Maritimo and which engine package.
"One thing is for sure. The old days of the cabbage ear aquafoils with overlapping blades are long gone," stated Willaton.
Obviously, the stainless-steel shafts themselves are machined to beautiful standard, as too the skegs and cutlass bearings, and Maritimo's famous, offset spade rudders. The underwater package is a classic case of the chain only being as good as its weakest link, so therefore, don't make weak links!
It was Bill Barry-Cotter who effectively pioneered the use of the reduction gearbox in this market segment decades ago to gain the inherent efficiencies a large screw can deliver upon. Today, no Maritimo screw is larger than 36 inches in diameter, and then it is a case of getting the pitch correct. On the bigger Maritimo M75 and S75, the 3-inch shafts are big enough to get on with the job, but not so big as to add significant drag, or additional weight. Boil the cauldron, drop in a bit of this, a touch of that, and a pinch of this, all the while reading the book of truths in screws. That is the tried and tested Maritimo way...
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