cummins 18 knot cruise

[Tavis]

Could anybody with 6btas tell me what your RPM and fuel burn is at 18 knots? Thanks!

[wmachovina]

250 cummins, 16-1700 rpm, figure about 9-10 gph total . 18kts

[Tavis]

Thanks,
Cost comparing running a boat from Florida to Puerto Rico, shipping vs running her down at 18 knots...

[bob lico]

the beginning of torque curve is 1900rpm never run less then this. boat is also plowing water at less then 25 knots so the closer you approach this speed the more dramatic the increase in efficiency .31 bertram is terrible at 18 knots.10knt. in 10 to 12' seas 12knt in 6 to 8' seas but 4' and less let her raise up on strakes and ride the wave crests. you can actually feel the boat lift up on middle stakes at 27 to 28 knts fuel economy goes to 16gallons a hour at 28 knts.Basicly a up and down scale because the boat goes to inner stakes at 32-33 knots cruise for maximum boat efficiency but increase rpm increases fuel burn by a much larger margin then the decrease of water drag.

[In Memory Walter K]

20 kts @ 2300 rpm @ 7 gph per engine.

[Tavis]

Thanks,

That is real good advice, I guess I just figured wrong that I needed to slow up a bit to deal with the constant 4' slop I am sure to encounter crossing the straights. I like things simple and lower RPM, so been zoning my search for 6bta 250 hp range. What type of power does the hull need to ride in that 27-28 knot range?

[wmachovina]

I have 250 cummins in a 73 sf with a bulkhead gen, and ac. At 2400 I get on average 25-26kts. 2600 max recommended rpm I turn 2650 with a clean bottom 28-29 kts. To get the higher cruise you need 330s, or better shaft angle/prop re-engineering, ceramic tiles from used space shuttle.laminar flow bottom paint and afterburners ,$$$$$$$. I relish the days I. Can go from S Fl to the Abacos at 26. They are few. Most times I'm happy to do 20-21 and burn bout 12 at 1900-2000

[bob lico]

the " next step up from yours" that is 315 hp cummins but pull all the way back from the 200 hundred rpm. off the pin (2800 rpm factory recommendation ) back to 2250/2300 rpm enjoying crazy fuel economy for a 31 foot boat utilizing 681 lbs of torque per motor by maximum cup on prop with 5 degrees of rake and minimum pitch to cut back wot and increase more usable cruise speed and fuel economy . 27-28 kts. all day. calculations come out to 298 to 300hp at 2250. running at 80% and changing oil at a regular intervals with cummins rebuilt calculation at 35,000 gallons should put engine life at 50 plus or minus 5 years -----my grandson will be my age!!!!! i know hard to believe but i show people all the time. getting your ass kicked at 18 knots then increase the boat speed to 27kts and she gets up on strakes (like ray hunt design it to due!!!!!) and the ride smooths out keeping her beam to the seas.

[bob lico]

some day if you ever get a chance watch history/military channel on tv . watch second world war PBY seaplane take off in roaring pacific ocean waves after rescue of downed pilot . watch how it smoothes out as the plans accelerates .look very closely at bottom of plane!!!! ray hunt we know where you came up with the idea for the 31 bertram!!!!!!!!!

[Tavis]

Thanks again gents for all that info.

[wmachovina]

Bob, I certainly respect your prop knowledge and would love to have you recommend a prop change for me if you think it would work on a 250. Right now 20x23 4 blades no cup. Standard shaft angles, engine sit about 7 inches from the fwd bulkhead,nexgen to port. Performance nos above,, Bill