A previous blog spoke to some of our incremental changes that improved Vanguard's through-water performance. Specifically:
Stowing the Varifold propeller fully to prevent windmilling.
Aligning the rudders in the flow stream
Docking the stabilizers when not needed
Reducing parasitic electrical loads en route.
Whereas each change made a marginal difference, the effects were cumulative. They proved worthwhile, gaining 1KN for free or about a 12% improvement.
Having landed in Florida, we needed to recharge the drained power batteries. Ordinarily, we would do this at sea when underway. However, when tied to the dock, we loaded the starboard engine on this occasion. The total power demand was about 30kW at 1800RPM during the charging cycle, which is 25% of the full engine load. Examining the exhaust outlet revealed a considerable amount of gas escaping via the muffler water drain instead of the larger gas outlet. So, even at this relatively light load, exhaust gas is entrained in the water flow around the hull and forward of the propellers. As the propellers are in partial tunnels, once/if this gas gets pushed towards these tunnels, it must pass over the propellers before exiting at the stern. I'm no naval architect, but regardless, this does not sound like a smart idea. The hunt was on.
We already know the Halyard wet exhaust mufflers are oversized for the application. Ordinarily, this would be harmless, but it does mean that the drain is considerably larger than the seawater cooling inlet. A 90mm drain hose and 50mm water inlet hose mean. The factor of over 3 differences in cross-sectional area. We also know that the overboard standpipes allowed exhaust gas to pass up the hatch drains before fitting no-return valves. So gas was in these standpipes under low but positive pressure. We surmise the muffler drains fully and runs dry, allowing exhaust gas to exit from the exhaust and water drains.
A conversation with Halyard technical support confirmed that this should not be happening. (They responded almost immediately, so credit to their service.) Their proposed solution was twofold.
Firstly, restrict the drain water flow to >1.0 times the inlet flow area but not so much that water exits from the exhaust outlet. This would increase the water dwell time within the muffler and allow for more effective degassing.
Secondly, connect the top of the overboard standpipe with the outlet of the exhaust elbow to allow the standpipe to clear any gas buildup. A small-diameter exhaust hose would be sufficient for this purpose.
Our next journey begins at the end of January. In the meantime, the shipyard in Fort Lauderdale will arrange for contractors to make these adjustments, and time will tell us the end result. Mistakes happen, and in this world, little is perfect, especially the first time around. Our role is to have eyes that see, recognize what's happening, and improve on the facts. I'll report back in a later blog.
Wishing our followers a Merry Christmas and a productive New Year wherever you may be.
Chris