while it is true that efficiency is lost when going above hull speed, there are many variables like hull shape ( planning or displacement hull ) beam, draft...etc... and simply stating that going 1 knot over hull speed is " digging a huge hole and the most inefficient speed " is quite a drastic and inaccurate statement. 1 knot simply is not that drastic... simply stating that anything over hull speed is inefficient is more realistic in this case... just my two cents worth. on my 60ft waterline length 84,000 lbs displacement hull, single screw, my fuel consumption at my regular 7 knot cruising speed is a constant 1.4 gph with a ford lehman 120 and a 28" prop. on my 27 express single screw cummins 275 at 8 knots my fuel consuption is about 3.5 gph at 1400rpm.
Eric ----- Original Message ----- From: waterguy<mailto:[email protected]> To: UnifliteWorld<mailto:[email protected]> Sent: Wednesday, November 11, 2009 5:28 AM Subject: [UnifliteWorld] Re: more knots Regarding JC's $18 (CDN; =$17.11 USD) per hour cost, that's great, but relatively meaningless unless we know what your cost-per-gallon is. If it's $10 a gallon, you're doing great. If it's $1.50, notso- hotso. What's your gallon-per-hour consumption? To Russell: Hull speed on a 46 waterline length would be 9 knots (the formula is square root of waterline length x 1.34). If you're cruising at 10 knots, you're not planing, but you're pushing past hull sped and just digging a huge hole - the most inefficient speed. Either slow down a knot or so, or shove it up enough to get on plane. Those 671 turbos have a very, very narrow efficient running speed - about 200-300 rpm below wide-open-throttle (WOT). Run them slow, and at slightly-above-hull-speed, and you'll suck fuel like a wino with a quart bottle of Thunderbird. Plus, slow running is hell on two-stroke Detroit Diesels. They were designed to run at high RPMs. If you don't have fuel-flow meters, install them and experiment with them. Your interest isn't gallons- per-hour, it's miles-per-gallon. While you'll get the best MPG by cruising at hull speed or a smidge below, those engines won't like it and you'll pay for it in increased maintenance. What you're looking for with those engines is best MPG when you're on-plane. If you only ever want to cruise at hull speed, ditch those Detroits and repower with small (probably no more than 200 hp) four-stroke Diesels. Check with a marine power expert; you may not even need 400 hp to move at slightly under hull speed. As an example, consider the 42 Grand Banks. According to the McKnew- Parker Powerboat Guide (2008 ed.), standard twin 210-hp Cats cruise at 10 knots; twin 375-hp Cats cruise at 14-15 knots. So 330 extra hp on a full-displacement hull buys you 4 to 5 knots. Not very efficient. The difference is, your hull is a planing hull, which the GB is not. So running your 46 (which is the old Pacemaker 46 hull) at the most efficient planing speed with your 410-hp Detroits gives you a cruise around 16-17 knots (again according to the McK-P guide). One possibility would be to repower with four-stroke Diesels of approximately 400 hp each (your Detroits should be rated at 410 hp). Four-stroke Diesels will tolerate slow running better than two- strokes. That would give you the choice of a slow cruise (about 8.5 knots), but still have the reserve to run for shelter at a high planing speed if the weather gets nasty. Again, however, even four-stroke Diesels don't really like being underloaded; but they're somewhat better in that regard than two- strokes. Your best bet is to consult with a marine repower expert rather than listen to my half-baked rants. --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
