SEDATION 4  Diesel Engines

Diesel engines are predominant in the marine industry worldwide.  Their reliability, cost-effectiveness and inherent safety are unsurpassed.  Unlike gasoline, diesel fuel is available everywhere and typically costs less.  Yanmar Diesel Engine Co. LTD is the leading supplier of marine diesel powerplants for small-to-medium size vessels, with  parts and service available world-wide.  They sell 2 models of outboard diesel motors, as well, at 25 HP and 36 HP ratings with various shaft-length options.  SEDATION 4 was initially designed for 3 of these engines; 2 for main propulsion units and 1 for the 15' Caribe dinghy. Unfortunately, they do not meet proposed EPA emission standards, so Yanmar America has ceased outboard sales for use in the USA.  I found a work-around and planned to buy them in Mexico, since Cozumel will be my primary port-of-call.  Then Caribe advised me that 36 HP was inadequate for acceptable performance on the large dinghy.  Sadly, I abandoned the Yanmar outboard plan and purchased a 4-stroke Honda outboard for the Caribe.
The Honda 50 performs admirably on the dinghy but I wanted more power for the yacht.  My boatbuilder's 38' Sail-Cat can attain 16+ knots motoring with a pair of Honda 50s, but that boat is significantly lighter than SEDATION 4.   The Honda 50s are very light (at 212 lbs. each) with a power-to-weight ratio of 4.24 lb/HP.
Suzuki makes a 140 HP 4-stroke outboard motor that provides a power-to-weight ratio of 3.007 lbs/HP.  These were installed on the 44' Power Cat and perform satisfactorily in that application, so I revised the boat's model to accommodate them.  Just in case some power-crazed future owner wants more speed, I designed the engine mounts to handle a pair of 225 HP engines like those on the 38' Power Cat. So far, so good.
Then I flew to the St. Petersburg boat show, in November 2003, and met the friendly folks at Cummins Marine.  They have partnered with Mercruiser Marine to offer a line of diesel inboard-outboard (I/O) powerplants.   The 120 HP model is based on a european-built Isuzu 1.7 liter turbocharged engine, marinized by Cummins and mated to a Mercruiser Alpha 1 drive.  The unit is compact and has the best power-to-weight ratio (5.44 lbs/HP) in its class.  THIS got my attention!  Click the image at right for the complete brochure.
Comparing the fuel-consumption curves for the Suzuki 140 and the Cummins/Mercruiser 120, it is obvious why diesel is the preferred choice for mariners worldwide.  Peak torque is attained at 2150 RPM burning only 1.3 gallons-per-hour (GPH).  The Suzuki 140 burns this much just idling!  At wide-open-throttle (WOT) the Suzuki burns over 11 GPH while the diesel only sips 7.1 GPH.  A diesel's torque curve inherently provides more thrust at lower engine speeds, therefore operation at WOT is unnecessary.  Proper selection of propellers permits operation at low RPMs with miserly fuel-consumption rates.
The "diesel decision" was a no-brainer.  My next challenge was to add another 464 lbs. all the way aft without compromising balance of the vessel.  The outboard motors were to be mounted forward of the transom, at either side of the aft deck, between the hulls.  The diesel I/Os are mounted to the transoms. This mandates shuffling of some mass like moving the watermaker forward, adjacent to the water tanks, and re-locating the holding tanks.  The batteries have always been placed amidships in the SEDATION 4 design but could be re-located somewhat forward by sacrificing storage area under the bathroom counters.  This helps.  Next the transoms must be modified to accept the outdrives' mounting flanges.  My transoms are vertical, by design, to facilitate rudder mounts and handling of the large Caribe dinghy.  Typical outboard- and I/O-transoms have a 13° slope.  Mercruiser specifies a 10° to 16° rake in their installation guidelines.  I chose 11° and designed a recessed sub-transom accordingly.  The section view at right illustrates the new configuration.
The Cummins engine mounts require two lateral stringers, spaced 22.5" apart, so I incorporated them into the sub-transom for additional stiffness, and added a cross brace at their forward ends to tie it all together.  These components will, of course, be cut on the ShopBot for a precision fit and a detailed lamination schedule is required to achieve optimum strength with minimum weight.  The outdrives provide seawater for the heat exchanger cooling and heated water is expelled, along with exhaust gases, through the propellers' hubs so no additional plumbing is required. Noise and ventilation are not an issue in the port-side Machinery Room but the starboard-side Workshop/Lab needs some re-engineering.