>From what I like to think I know of this subject I must say thank you the >numbers quoted reflect my 29' ulrichsen with twin 318s.. My uniflite is still >waiting to be launched and will have more electronics. I came to close the >same by reading gas gauge and traveling distance on my charts (definitely not >the most accurate way) my uni is a 32 1978 but what I don't get is how does >one come up with hull speed??? I am familiar with the calculation for a >displacement hull, can the same formula be used on a semi displacement or >planning hull??. Things that make you go hhmmm.
Sent from my iPad On Aug 16, 2012, at 8:52 AM, "Art Burtis" <[email protected]> wrote: > 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. -- 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.
