Dear Mark, i.e. waterguy. Excellent description of GPH and GPM in accordance with STW and SOG time-travel rates as they may be calced by combining or separating the use of GPS / Floscan meter / Velocity-over-water speedometer... etc. Understandable to the ength degree! I owned a Uni, currently own a Tolly, and have looked at several Uni's this year for possible purchase. During my decades of boating I've explained similar to your explanation in email below over and over again to boat owners. It really is simple math and simple reasoning. Your depiction should help some boaters better understand the actual, simple reasoning incorporated into GPH, GPM, STW, and SOG in relation to personal time preferences, Money $pent, safety from deadheads, and reserve fuels (i.e. speed) to get out of harm's way if required. May I add... at hull speed there is also a pleasing calm that occurs in one's mind as the scenery (be it land views or the joy of watching sun glistening waves) has time to "sink-in"; not to mention the peacefulness of greatly reduced engine noise and the reduced wear on entire driveline! Happy Boating!! - Art
Art From: [email protected] [mailto:[email protected]] On Behalf Of waterguy Sent: Thursday, August 16, 2012 1:20 AM To: [email protected] Subject: [UnifliteWorld] Re: Planing speed/rpm I have a 1971 36 SS with 440's and an older Floscan fuel consumption meter. I've found that at hull speed (around 7.5 kts; 1,200 rpm), I'm burning about 2 gph per side. According to the Chrysler engine manuals, max RPM for these engines is 4,000, and they recommend not exceeding 80% of max rpm for sustained operation (3,200 RPM). At 3,000 RPM, I'm cruising about 20 to 22 kts and burning about 8 gph per side. To answer your "additional" question about the difference in speed vs fuel consumption, what you want to figure out is miles per gallon, not time vs. speed, unless your time has a value (e.g., could you be at work earning money vs. boating). So at hull speed, burning a total of 4 gph, you're getting 1.875 nautical miles per gallon. At 20 knots, burning 16 gph, you're getting 1.25 kmpg - 33% worse. So over a trip of 100 miles, you burn 53.3333 gallons at hull speed over 13.3333 hours vs. 80 gallons at 20 knots over 5 hours. You use 26 gallons more fuel. At $4 per gallon, that's a difference of $106.66, but you save 8.3 hours on the trip time. Fine calculation of the miles per gallon figure requires a fuel usage meter and some method of measuring speed. You can use a GPS that has a readout of Speed Over Ground (SOG), but only if you know there's no current or tidal flow to increase or decrease your SOG. If there is, you need a speedometer that reads out your speed through water (STW). My boat has a depth sounder that has a little paddlewheel in the transducer that provides a STW reading on the sounder. Newer sounders and speedometers have a NMEA 0183 output that will interface with the newer flow meters and give you an instant readout of MPG. Personally, I like to cruise at hull speed, unless I've got to be somewhere. Saves gas (read $$$$), and it's less stressful - at high speeds, it's hard to see a floating log in your path (a real hazard in Puget Sound and Northwestern waters) before you're on top of it. Plus, if you do hit a deadhead, it'll do less damage to hull, props, shafts, rudders, etc. at low speed. And it's nice to have a big speed reserve on hand in case the weather blows up and you have to run for a sheltered harbor. -- You received this message because you are subscribed to the Google Groups "UnifliteWorld" group. To view this discussion on the web visit https://groups.google.com/d/msg/unifliteworld/-/Pf61OvfQ_6IJ. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/unifliteworld?hl=en. -- You received this message because you are subscribed to the Google Groups "UnifliteWorld" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/unifliteworld?hl=en.
