We've now selected a new engine for Zen Again. We have ordered a Craftsman CM3.27 (3 cylinder, 27HP) and will keep the existing propellor.
Craftsman Marine is a quite new supplier of marine diesels. They marinise Mitsubishi industrial engines in the Netherlands. I've read plenty of positive comments about them and can only hope we'll be equally pleased. Here's why we chose the Craftsman:
- Fits in our engine compartment (and is smaller overall than existing engine)
- Lighter (113kg compared to 123kg new Yanmar and 138kg existing engine)
- 430mm mount spacing which matches our mounting beds. It turns out our existing Yanmar has custom mounting brackets which extend their standard 370mm spacing.
- Regular service items (dip-stick, filters, impeller etc) all on starboard and forward sides
- Nice IP67-sealed instrument panel with hour, temperature, oil pressure, voltage and RPM gauges
- Preheater (just in case we ever go anywhere cold!)
- Nice looking engine with hoses NOT painted!
|Craftsman Marine CM3.27|
|PolyFlex Engine Mount|
We also plan to install a flexible coupling to provide a little "elastic" in the drive train. We've chosen a PolyFlex...
|PolyFlex Flexible Coupling|
Our existing shaft half-coupling is rusty and its bolt hole pattern doesn't match modern gearbox couplings. Again PolyFlex offers these, and drilling out to match your shaft diameter is included in the price. So we'll get one of these...
We looked into the possibility of a smaller engine with a new propellor but that proved difficult due to issues matching available gearbox ratios to new propellors, and also a possible clearance problem between the P-bracket and an AutoProp propellor.
It will be interesting to see whether the new engine can drive our 16" Varifold propellor up to the engine's full rated RPM of 3600rpm. The existing engine can only get to 2500rpm, but we suspect that's because it is producing less than its rated 27HP.
We used Nigel Calder's excellent book "Marine Diesel Engines - Maintenance, Troubleshooting and Repair" to calculate engine and propellor parameters for Zen Again. Section 9 "Selection and Installation" provides all the necessary information.
For a yacht with the following characteristics:
- 6,800kg (15,000lb) displacement;
- 8.65m (28.4ft) waterline length;
- 3600rpm maximum engine revolutions; and
- 2.72 gearbox ratio
the calculations showed:
- 15HP needed for a "racer", but 30HP recommended for a "cruiser";
- Recommended propellor size of 16.5" (ours is 16"); and
- Recommended propellor pitch of 14" (ours is 12").
So, if anything the boat should be slightly "under-propped" rather than "over-propped" with a new 27HP engine. And so the new engine should work well. Time will tell!