Originally Posted by

**_QB_**
Chris,

For the record here's a rundown of what I have done trying to model Skookum Maru's fuel use.

There are a bunch of commonly used simple formulas for power required to move a full displacement boat -- Keith's, Wyman's, early Gerr, the 2008 Gerr, some others. They all give power as a function of speed through the water, waterline length, displacement, and an empirically derived constant or two. In the operating regime of a boat like S.M., these formulas can differ by +/- 25% from each other. The 2008 Gerr formula you used seems as good as any.

For displacement, I use 33,000 pounds, 14.7 long tons, because that's what I remember the travel lift operator telling us at the Bellingham haulout. (Beebe's book has S.M. at 13.4 tons; Bet Oliver's book says "21 gross tons", which is unbelievable.) For LWL, 38.5 feet, based on Monk's drawings, and Beebe's book says the same.

Plugging all that into Gerr's 2008 formula, I get that SM will make 6.4 knots on 30hp, and takes 38 hp to make 7.0 knots. So more power than you estimated, because of greater assumed displacement.

Now that is for power delivered through the propeller to move the boat (shp), but there are some transmission losses on the way from the engine. I used 5%, i.e. 95% efficiency, for the marine gear and bearings (Gerr somewhere suggests 96%) ; so engine power (bhp) required is going to be a little more, about 40 hp for 7 knots.

To model fuel consumption, you need a prop curve and engine performance curves. For the prop curve I just go to Gerr again ("The Propeller Handbook"), with an exponent of 2.7 and a coefficient that assumes the prop curve and maximum engine power coincide at 1800 RPM. For the engine curves, I referred to a GM brochure for the 71 series. Like your 4-53 chart it requires some extrapolation and interpretation. I fit a quadratic to the fuel required curve there, and went with the "continuous gross power" rating of the 3-71 as 82 bhp @ 1800 RPM.

Going with all that, I find that running S.M.'s engine at 1300 RPM should give 6.58 knots through the water, using 2.08 gallons of diesel fuel per hour (to make 34.1 engine bhp and 32.4 shp). Whew! That fits pretty well with what we observe in the real world. Which is nice.

But about that observed 2.1 (or so) gph... That's figured based on pump metered gallons to fill the tanks, and tach time. I suppose pump meters are pretty accurate. But the 3-71's hour meter is driven by the mechanical tachometer. I do not know, and have not seen any documentation, about exactly how that varies with engine speed: how many tach hours per real clock hour, as a function of RPM? It was on my list to run at a constant RPM on some multi-hour cruise leg and try to determine that, but I never got around to it. I also thought about getting a FloScan fuel flow metering system. But after talking to a factory rep, I wasn't sure it would be particularly accurate on a 3-71. That engine has the same fuel pump as a 6-71, and it flows something like 35 gph no matter what, returning about 95% back to the tanks. The forward and return flows need to be measured pretty precisely to compute their difference reliably. Maybe the FloScan can do it, but it's about $1K to try, and I never got around to that either.

Anyway, all that was an interesting exercise, but it required making a lot of assumptions along the way, and it all should be viewed with the appropriate amount of skepticism.

Chris, I'll send you links to my spreadsheet and supporting documentation if you'd like.

--Paul _QB_