Sometimes education comes unexpectedly. When special designs or capabilities come together in a unique new way, surprises can occur. This just happened: In preparation for the Miami International Boat Show, MTI was testing one of its 48 Race/Pleasure catamarans powered with Mercury Racing’s 1350s and M8 drives.
Dry sump. I’ve written before about the purpose of dry sumping – efficiency. Here we have a 48 MTI with two dry sump M8 sterndrives. Plus two dry sump, quad cam, four valve engines making 1350 hp each. Between engines and drives, dry sump transmissions. Big power; big expectations!
As people sometimes do, the owner tried propellers from another manufacturer. Whang! Blade gone. We warned that these engines produce big fat monster torque (BFMT); we learned this lesson the hard way, too; we designed a special prop series just to handle it. However, this was not the education – just its preamble.
What now? Change the prop? Idle home (six miles)? Nah, we’ve still got 1350 hp on tap! Let’s put ‘er on top and drive home on one! Before now, few would have even attempted to get a 48 up on one engine. But with BFMT on tap, the big MTI just slid on plane. Easy. Let’s see if she’ll run. 80 – 85 mph. Easy. Maybe too easy.
Let’s think about what’s going on: One engine, not working too hard, is pushing this rig right along. The other engine is off with the broken propeller free-wheeling in the slip-stream – transmission in neutral. On the working side, all is normal.
On the free-wheeling side, the prop shaft is spinning – so the sump pump is circulating its fluid through the drive. AOK. The engine is off – so there is no need for engine lubrication. AOK. The dry sump transmission tailstock is rotating, but its sump pump is engine driven and the engine is off (pump’s not pumping). Not OK. Without lubrication flow, the shaft seals and clutches heat up. To excess, the seals and clutches can become heat damaged and the transmission will not hold normal operating pressure. Who knew? The engine wasn’t even running.
Next trip: With good props installed, low fluid pressure in the now-damaged transmission triggers Engine Guardian.
We’ve paid our tuition and earned our education. Here’s the answer to the final exam question, “What should I do if I throw a blade?”
Let the “loafing side” engine idle, so the transmission fluids circulate – and idle home. Or change the prop.