MJM… Designed for Offshore Performance

When several 34z owners say their level of joy in driving the boat is directly proportional to the height of the waves… when an experienced boat jockey pushes a 29z in 5-8 footers and proclaims it’s the best sea boat he’s ever driven: The question arises, “How come?  What makes these boats so special?  Maybe outlining a few contributing factors would be helpful.

Low Vertical Center of Gravity.

Like a sailboat or a good sportscar, the lower you can get the center of gravity, the more stable.  A light boat with a low center of gravity will feel and behave like a much heavier boat.  For the J sailors among you who have sailed a J/42 or J/46 with carbon rig and bulb keel, the boat seems like a 5 foot longer and heavier boat than the original J/40 or J/44. A good low-slung sportscar feels more glued to the road than a heavier, tall SUV.   The low, sleek lines of the MJM line of boats, built with a light, but extremely strong wet pre-preg, vacuum-bagged and oven-post-cured epoxy/Kevlar/Eglass/Corecell laminate, reduces the overall weight…but equally important, eliminates weight high-up in the design.  The vertical center of gravity is near the waterline, instead of several feet above it…and that makes a huge difference in performance at sea.

Narrow Beam

The narrower a boat, the easier it slices through waves with less resistance or drag.  Makes sense that an 11 foot wide (10 foot BWL)  34z is less likely to pound than a 13+ foot wide Cabo of the same length, even though the Cabo weighs twice as much (11,000 vs 22,000 lbs.).

Modified Deep V

Ray Hunt pioneered the deep-V hull concept with Dick Bertram to create a design that allowed boats to go through waves at reasonably high speeds.  But, there are a couple of problems with the “deep-V” which a Zurn "modified deep-V" overcomes. A deep-V hull generally has a transom deadrise of about 20 degrees which becomes increasingly steeper toward the bow.  This deep-V is like a long keel on a sailboat. It imparts directional stability.  The boat wants to keep going in the current direction. This might be desirable in a sailboat.  But, sailboats have huge rudders to easily take a new bearing. Conversely, powerboats have very small rudders. When you try to steer a deep-V hull with a small rudder(s) in a new direction, reaction of the boat is delayed… particularly running down large waves at over 15 knots.  This is not only scary,  but a problem for effective use of autopilots, and causes lots of work for the helmsperson.

Along these same lines, only more pronounced and worrisome at higher speeds in a seaway are Downeast “lobster-boat” type hull shapes which truly have a keel.  These boats weren’t designed to do more than 12-14 knots, or less than the speed of waves.  They were designed to stay in one place, oriented to the wind while pulling lobster traps.  As builders put more horsepower into these “keel boats” with higher cabins and flybridges, boats reach an unsafe point of design where in a tight turn at cruising speed (even in smooth water) … instead of leaning into the turn…lean outward as the boats trip on their keels while their superstructure wants to keep going in the original direction.   Not a good trait for a boat offshore in waves.

There is increased drag with a deep-V or keelboat.  They don’t really have an efficient planning surface, so require more horsepower to get up and overcome resistance/drag.  In fact, the bow rise can be such in the 10-15 knot speed range as the boat attempts to climb up on a plane, that you can’t see over the boat in congested waters.  And, you wonder why people want flybridges?

Furthermore, a deep-V hull tends to be less stable in waves with pronounced rolling when stationary or at slow speeds.

Doug Zurn has created a “modified deep-V” hull shape with a 17-18 degree transom deadrise that is carried forward 1/3 the length of the hull and is then warped into a sharper 55 degree deadrise at the cutwater.  The last 1/3 of the boat serves as the planning surface and provides the form stability lacking in a “deep-V” without sacrificing the ability to go through waves.

Carolina Bow Flare

The bow design results from MJM owner Bob Johnstone’s time in Charleston SC and trying to deal with running inlets in heavy weather, where the waves become steep and control is critical.  With breakwaters close at hand on either side, it’s important that a boat respond and not bury its bow to yaw off to one side or the other.  The purpose of the flared bow with lots of reserve buoyancy is to get over the wave ahead as you charge down the face of the wave you are riding into the back of the next one ahead… not submarine into it.  Doug Zurn nailed the shape perfectly.  Often it seems the wave comes to within an inch of climbing over the bow, only to fan out (Buddy Davis ad style) to either side.  The combination of this shape plus dual lifting strakes and chine flats, gets an MJM over the wave ahead without sticking the bow under with subsequent loss of control or the need to throttle back anticipating disaster or taking green water over the roof.

Many powerboats in the Downeast/Trawler category as well as jet powered boats (steering with a hose) don’t offer the instantaneous response to the wheel that makes running in large waves a pleasurable challenge rather than a frightening, anxiety-ridden nightmare.

IPS Pod Drives or Stern Drives

Of course, with stern drives as on the 29z and the new 40z,  or the new IPS or AXIUS drives on the 40z, there’s even more control as props change direction to replace the need for a rudder… similar to outboard motors.  Dual, counter-rotating props should outperform any twin-screw, fixed shaft installation with small powerboat rudders in terms of maneuverability in large waves as well as around the marina.