Roy was Right - October, 2021

(Caution, this gets a bit technical and contains numbers and graphs)

I won’t name names, but I have heard from more than one of the four Ness kids (who know a lot about diesels) the following theory that goes something like this.

“Dad built a good boat, but the pitch on the propeller isn’t right for that hull and engine. The engine lugs and smokes more than it should because she hits hull speed at too low of an engine RPM. We’ve always hit cruising speed of 8.6 knots at 1750 RPM, which is turning too slow. If the prop were pitched flatter, we wouldn’t hit cruising speed till 2000 RPM and the engine would be happier. We kept telling Dad for years to get the prop re-pitched, but he’d wave us off and grumble that it was just fine as is.”

All summer, Lori had run the Perseverance just as instructed, and indeed had consistently seen speeds between 8 and 9 knots (~10 mph) when running the diesel at 1750 RPM. Our best estimates for fuel economy worked out to between 2 and 3 mpg (statute), consistent with what the Nesses had claimed.

While that fuel economy doesn’t sound too good to people used to getting 20 mpg out of their minivans, some perspective is in order. Over the entire season, we burned 230 gallons (less than a quarter of the 1100-gallon fuel capacity) to cover 616 statute miles at 2.7 mpg, spending 66 nights on board. That’s about the same amount of fuel that we used in our Ford Flex just to get us to Escanaba and back 6 times. Everything I’ve learned about other cabin cruisers or trawlers indicates that owners generally expect to get less than 1 mpg, particularly if they are built with planing hulls and can go fast.

So, while the Perseverance demonstrates extremely impressive fuel economy as is, it is likewise very important to be nice to the diesel and operate it within its sweet spot RPM range most of the time, like the Ness kids were suggesting. Because of this, we decided to investigate what would be involved in getting the propeller reworked to a slightly flatter pitch – a fussy job requiring experts to perform. Essentially, specialists would machine and bend the 4 blades of the current 30” diameter “Nibral” alloy propeller such that each single rotation of the prop shaft would ideally “screw” the boat forward 22” instead of the 24” chosen by Roy.

The nice folk at Nestegg Marina in Marinette, WI heard us out on this theory, but suggested that the first step should be to check the accuracy of the boat’s tachometer. In short order, a timing light revealed that the tach (part of the original 1993 Cummins engine control cluster) was reading low… by a lot. See attached graph of Tachometer Error for the actual data, but in short, the gauge reads about 14% low. I don’t know how long it has been like that, but some old sea-trial test data that I dug out of Roy’s old brief case suggests that it was probably reading low back before the roll stabilizers were installed in 1995.

Digging deeper into Roy's brief case yielded correspondence, vendor data and calculations regarding his propulsion design. One early page of hand calculations, back when he was figuring out how big a boat he was going to build, listed the theoretical maximum speeds (hull speed) for displacement vessels ranging from 20 to 60 feet in a table. The simple and well-known equation (see attached sheet) he used is famous for accurately predicting the “elbow” in the curve of speed vs. required horsepower, essentially the cruising speed for a displacement boat (the equation doesn’t work for planing hulls) that if exceeded, will start requiring infinite power. His finalized design goal was consistently stated to be “8.7 knots at 1800 to 2000 RPM for fuel economy of 2 to 2.7 mpg.” This was based off diesel performance data for the 180 hp B6T5.9M Turbo 6-cylinder Cummins he had chosen, and for the 44’ waterline length of the 49.3’ hull he had designed. He communicated these data to the transmission, prop shaft, and propeller vendors and selected from their recommendations.

So what’s this Tachometer Error mean? When Lori was running the diesel at an indicated 1750 RPM and our GPS speed was at about 8.6 knots, the engine was actually turning at about 2000 rpm. In other words, EXACTLY as Roy had calculated. And at the targeted cruising speed, the fuel economy was EXACTLY where Roy had hoped it would be.

Now armed with this analysis, our plan is to fix the Tachometer and cruise at a slightly lower RPM so that the engine is turning at ~1850 RPM. Roy’s equations suggest that that will increase fuel economy substantially while only dropping our cruising speed by a tiny bit. Based on available data and adjusting for known errors, I’m predicting 8.4 knots (9.7 mph) at 3.3 mpg. We’ll certainly be running some sea trials early next season to confirm, but if true, will allow the Perseverance to have more than a 3000 nautical mile cruising range

I’ll be forwarding this missive to the Ness family, not really knowing whether they’ll be irritated or grudgingly pleased that their Dad absolutely nailed yet another of his goals.