XPM-78 Designing the First Hull

Updated: Nov 17

The following article resulted from a long conversation with Dennis Harjamaa of Artnautica Design. Dennis is the creator of the LRC and XPM range of explorer yachts. Rugged and capable vessels sized from 58, 65, 78, and 85 feet LOA with displacements to 70,000 kg dry. This story is about the birth of Mobius, the first hull in the XPM-78 series. Over to Dennis:

"I thought I might share thoughts on how the Mobius hull design came to be. You may be aware that when the owners first got in touch, they talked about a much shorter boat, a 65" foot or 20m hull. At this time, she was going to be a twin-engine boat with a developable shape like the LRC58, but with one extra chine, so the max available width of the alloy plate was sufficiently broad to span the beam of all panels. The design goal was to match or exceed the capabilities of the FPB65. The look they were after was "lean and mean" to scare away pirates. Early on, I lost the battle regarding fitting a flybridge, haha!

"Developable plates" means a hull form created by curves in a single dimension. Plates can be cut from flat stock and formed around the frames at the yard without recourse to more sophisticated machinery such as presses or rolling wheels.

A book worth of emails was written by all parties as design ideas spiraled round and round (ref Mobiusworld). The choice to go with a single-engine setup was made once the thought of fitting a Gardner with its legendary reliability came along, if memory serves. At about the same time, we moved away from a chine hull to a round bilge and started adding waterline length. Allow any designer the freedom, and the waterline will grow. After a few rounds of power and speed calculations, the decision was made to "go the whole hog" and make the hull 24m long. So it is not too much of a stretch to state that Mobius is really a "20m boat" with a "24m hull".

Mobius has a single-cylinder engine setup with a Gardner Diesel engine and CPP Propeller.

Imagine our amusement when the FPB70 was introduced. The similarities with what we had were obvious. The design for Mobius was well and truly fixed by that time; the goal shaped the design.

An initial proposal of a 75-foot hull with two engines.

My clients decided on the interior layout early in the process. The extra length was given to the forepeak but mainly to the engine room and workshop. Great discipline, I must say, or perhaps for Wayne, it was all positive gains, with room for more toys in the workshop.

Comparison of XPM-78 design with a single engine (left) and twin-engine setup. Notice the increased beam in the twin-engine setup, mainly to provide additional buoyancy for the extra weight involved.

I did an initial 3D model of the structural grid for a first stab at the structural weight for the weight estimate. Now came the fun part, the final shaping of the hull, once the variables were tightened enough to be pretty confident of the design displacement, fuel, water tank volumes, positions, etc. This was when I checked the proposed interior layout fit within the structural envelope. Since we were looking at a relatively low-powered single engine, I tried minimizing drag from wave-making and wetted surfaces. I worked hard to maintain the shaft

angle as close to the horizontal as possible, sculpting the protective skeg with a low-drag foil profile and blending the tunnel into the lines as smoothly as possible. No hard edges for the water to go around here.

An interesting thing to point out is that the propeller tunnel depth has to be drawn so that only one propeller blade tip is close to the tunnel/hull bottom intersection at a time. Otherwise, some impossible-to-fix harmonic noise can happen. Another exciting feature of prop tunnels is that the average tip clearance between hull and propeller can be reduced from the customarily accepted 15-20% to as little as 10-12% of the propeller diameter without noise becoming a problem. I learned this when LRC58 Hull #2 BROADSWORD chose to go with a 90hp engine rather than the 75hp of my KOTI. KOTI has a 650mm dia propeller which the guys at WEST-MEKAN wanted to increase to 700mm to absorb the extra power. If anything, Broadsword's propeller is quieter than mine. Arguably the smaller the gap, the better the propeller's efficiency. Good stuff!

I also wanted to keep the boat's profile as low as possible to minimize the added wind resistance. I gave the hull a narrow entry and balanced lines for easy steering, good tracking in a seaway, and gentle pitching. Checking the position of the vertical center of gravity against the rolling characteristics was done after each change to try to achieve a gentle rolling motion. With powerboats, there is a delicate balance between the initial stiffness and speed of the roll. Stiff is safe but potentially snappy and uncomfortable. So you choose to roll fast but not far or slow and deep. The decision was made to err on caution and go stiffer rather than softer. A rigid boat can be made smoother by lifting the center of gravity; lowering the CG is much more difficult. A softer hull shape is easier on the stabilizer system to work against, so a smaller set of fins/rotors can be used.

Here the need for extra large freshwater tanks becomes obvious. There is little water onboard at the start of the passage. As fuel is used, water is made to compensate for the loss in displacement and the rolling characteristics. Tanks forward and aft allow for some trim control, weight aft in following seas, and forward when punching into it.

At this stage, I hired the services of Peter Lawson, a structural engineer here in Auckland. I used to work for Roger Hill, mostly known for his excellent catamarans. Roger would often hire Peter to crunch the numbers on structures, so I knew he was top-notch and had a practical touch with loads of boatbuilding knowledge. We devised the structural layout with Wayne's input, which I turned into a super accurate 3D model and ultimately flattened 2D cutting files."

Mobius levant at the dock in Antalya for sea trials in early 2022.

The complete history of this build can be found on MobiusWorld, accessible HERE. The owners, Wayne and Christine, are currently (October 2022) in the Mediterranean, enjoying the fruits of a great build and hull precisely made to reflect their combined experience of a lifetime at sea.

Dennis Harjamaa - Yacht Designer, Artnautica NZ

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