Aloha Jay Jacobs
Thomas, coincident waves only came up in the trial because of the unexpected testimony by my expert on oceanography and wave formation, Rae Strange.
Since there were no survivors or eye witnesses, no one will ever know what happened for sure, but I went into the trial concerned that the other side might offer as an explanation for the loss of the Aloha that the boat was navigated too close to the western edge of Bonita Channel and a big wave came over the Bar and sank it. This is what happened a year prior to the incident when three men were washed overboard.
Concerning Rae Strange’s testimony, “Waves are formed in only one way … by the wind,” I believe is an accurate overall statement. Your commentary that waves can be formed by passing large ships is correct, but I think most boaters would categorize “waves” created by a large passing ship as a wake rather than a wave, and know to keep well away from the ship.
I also agree that the state of the tides and currents can and do influence incoming swells. An outgoing tide of 3 to 4 knots or greater, in confined areas such as the mouth of a river or the entrance to the Golden Gate, can cause incoming waves to “rise up” to some degree.
An alert skipper will keep a close eye out to avoid one of these “stacking” waves from affecting his boat.
There was no evidence of tsunami conditions anywhere in the Pacific on the day of the incident.—
CALDER PROP THEORY IN PRACTICE
Engine manufacturers universally specify that boats be propped to run at rated maximum rpm at Wide Open Throttle (WOT). In the last issue (In Praise of Big, PassageMaker Nov/Dec 2014), Nigel Calder suggests that this longstanding mandate needs to be rethought. Specifically using a combination of larger props and slower rpm will improve overall performance with few drawbacks. We, Ken Fickett owner of Mirage Manufacturing and I, the owner of a Mirage Great Harbour 47, actually departed from the prop mandate with success.
We ventured off the mandated path not out of a desire to research and explore. We did it to correct problems created by an earlier attempt to meet our engines’ specifications requirements. This departure involved going to slightly larger props and increasing the pitch rather than changing a transmission. A little history . . .
East Passage was commissioned in 2004. The engines were twin 71hp Westerbekes. As construction was starting, Westerbeke changed engines specs insisting that we use hydraulic transmissions instead of the previously used mechanical transmissions. Their reps claimed we would see no difference, but we did. Maximum running rpm were way below rated maximum. Westerbeke’s literature clearly specifies that the engines be propped to maximum rated rpm and clearly states that improper use of the engines will void their warranty—an unpleasant combination.
So we cut down the props to 11inch pitch by 23-inch diameter from 13 by 24. We got some of our rpm back but lost top end speed. Still couldn’t meet specified rpm but Westerbeke signed off on warranty requirements.
Fast forward to early 2014: We repower. In go Yanmar 74 hp engines. The old props get the engine to rated rpm easily—too easily—the engine would run over rated maximum. The reduced pitch produces rpm speed but not enough forward thrust. Speed is off. We seem to be regressing.
The potential solution comes in a pair of props on the shelf at Mirage. Acme developed a set of increased pitch props (15 by 24-inch) for the larger line of Mirage boats. These propellers are not only larger, have increased pitch but also are inherently heavier because they are machine milled props. We expected reduced rpm, increased speed and increased fuel efficiency at running speeds. As the owner, I was certainly willing to pay for the swap.
Confidence was high because I—now this is Ken Fickett talking—grew up in the commercial boat business in Miami. Workboat owners have long increased prop pitch to improve performance and fuel efficiency. Furthermore, my experience with light aircraft teaches me that best performance, in cruise, the type Nigel Calder seeks, comes from running “over squared,” higher manifold pressure, lower rpm.
I have built all my trawlers with increased pitch with this experience in mind. They don’t quite meet rated maximum rpm. This practice has been called over-propping. We call it propping right. Now, with Calder’s data, perhaps we can remove the stigma of that term. Our changes worked and here’s how: With our Westerbeke engines we cruised at 2850 rpm producing a standard 7.3 knots. Fuel burn average was 3.5 gallons per hour. New prop set up allows us to cruise at 2300 rpm producing 8 knots of speed. Preliminary fuel burn calculations indicate we are burning 3.3 gallons per hour.
We expect the burn rate to drop with more running hours in the calculations. But let’s assume no drop. We are burning 5 percent less fuel while going 10 percent faster with 20 percent less rotational wear. Slow speed maneuvering is not affected. Of course the engines’ manifold pressures are up. And we have no more sense of where the sweet spot actually is than Calder does with his extensive data collecting.
Manifold pressure gauge ports are non-existent on the small Yanmars. Overheating would be our telltale, and they run rock steady, only going up one degree during WOT runs.
WOT running has never been a habit of ours. We are mindful of Yanmar’s admonition about time limits for sub WOT high rpm running and WOT running. You still need to make sure your cruise rpm is 10 to 15 percent off WOT rpm. Some will say that we are leaving horsepower that the engine is capable of producing unavailable, and they are right but the inability to get the last few percent of max horsepower is way overshadowed by the other benefits.
Bottom line: We are convinced our set up is proper for this fulldisplacement cruising boat and feel vindicated by Nigel Calder’s work. Perhaps it can really be a new day for propeller assessment.