Most economical cruising speed

[rpmerrill]

Don't worry about hijacking the thread, Bryant is my best friend.
Really, the big issue for him is what kind of network was on the 2008 R-25 with the Cummins and was it capable of linking into the Garmin GPS.

 

[BB marine]

On a 2008 R25 with a 150 Cummings/merc engine what would be the most economical cruising speed?
Thanks
Bryant

Your best fuel economy regardless of engine and horse power in the Ranger Tug or Cutwater hull design would be hull speed. Using Chimo's formula 1.34 x square/root of water line length ( Tug and Cut hull length is very close to its model size) 25' 27' in my case 26'. The hull speed's 6.7 kts, 6.8kts and 6.9kts very close to the same. The weight of each boat being different doesn't really effect this enough to figure it in . C26 being the heaviest ( 6800 lb published dry weight) , R 27 (6200 lbs Published dry weight) then R25 (5800lbs published dry weight) None of these are reality but good for reference. (I estimate my boat at 9000 lbs when doing prop calculations.) It should take take 20 hp to push my boat at hull speed 1.1 gph, 18 hp using 8000lbs just under 1gph. I use 18.3 hp, per gallon of fuel burn with my D3 220 hp ( max burn 12 GPH - divided by 220 hp = 18.3 )

Reality is --- I burn 1.6 gph 29.3 HP to make hull speed GPS Average 6.5kts or 7.5 mph 1850 to 1900 rpm. 4.6 mpg. Some of the deficiency probably propping. As many have already pointed out best fuel economy/speed is Hull speed with these hull designs.

I am running 17 X 17 .070 cup now, 6.5 kts requires 1900 rpm 1.6 gph 4.6 mpg, 10.5kts requires 3250 rpm 7.2 gph 1.66 mpg, 12.5 kts requires 3400 rpm and 8.3 gph yielding 1.72 mpg, 18kts top speed Max 4030rpm @ 12gph yielding 1.72 ( fuel , water, and boat equipped for long term cruising)

Bryant is a friend and neighbor of mine.
Part of his problem is that the Merc/Cummins doesn't communicate with the Garmin. So he has data scattered between the two displays and can't get an MPG readout. (We're looking for anyone who might know it if is possible to get engine data to display.)

Honestly, the Garmin communication is not necessary to get an accurate true fuel economy. All needed is GPS speed and engine fuel burn rate. Set the throttle at a comfortable rpm/speed record the fuel burn rate. Speed divided by GPH = MPG
I find that when I set the rpm at example 3200 rpm the Garmin read out, speed, MPG and GPH is continually changing as I'm cruising. Current, wind and waves influencing results. Its a good reference, but total fuel used divided by average speed at a given RPM is a better stat.

 

[Chimo]

Brian, you obviously love doing these calculations 😉. Why not calculate your efficiency? Fill in your numbers. You are running a 17ins pitch prop. At 100% efficiency it would require 4,289 revolutions to complete a nautical mile. I don’t know the ratio of your reduction gear but let’s use 2.1 as an example. That means 9,007 engine revolutions, about 976 rpm to give you 6.5 knots. If you’re actually turning 1900 rpm to get that speed your efficiency is 51%. I obviously have your reduction gear ratio wrong because that number is a bit low. Rule of thumb would have 2-3% loss in the reduction gear. A fully submerged propeller can be in the 85% efficiency range but again, more variables as you know. 3 blades typically more efficient, 4 and 5 blades smoother etc. The rest of the losses are in the hull and running gear, tabs, rudder etc.. This is where positioning of weight becomes critical. It’s fascinating stuff. Tank testing and real life are the only true tests but such thoughts are fun during COVID.

I have to admit that I don’t know the prop dimensions on or R25SC but the reduction gear ration is 2.43. The best curves I have seen for one of these boats were produced by Nellie II and 6.5 knots or 7.7 Mph were at 1500 rpm. That seems closer to what I would have expected.

 

[BB marine]

Chimo, your numbers are close, the difference is in the gear ratio. The factory prop installed was a 17 x 17 .105 cup. For all practical uses a lightly loaded C26 this prop probably gives a average performance. It probably gave the best WOT speed for the C26 non-planing hull with a 220 hp power plant. When the boat is loaded with gear, living necessities, water, fuel, add some wind and waves the performance changes drastically. I am now running a 17 X17 .070 cup prop with a 2.03 to 1 gear. Removing the .035 from the cup reduces the efficiency (increases slip) which allows the engine to operate at the manufactures recommended RPM.

Using Chimo's calculations 72913.4 in per NM divided by 17" = 4289.02 revolutions
4289.02 X 2.03 = 8706.71 engine revolutions to go 1 NM
At 7.5 mph @ 100% efficiency my engine is tuning 1088 rpm

50% loss from input (engine) to output (prop) is not unusual. If I could make 7.5 mph @ 1630 rpm (50% efficiency) I would be happy with that. I have never seen that.

I'm running at 70% loss input (engine) to output (prop.) The variables that effect this would be - hull design ( this is largest contributor to the loss and the hardest variable to fix), weight and weight distribution, bottom condition, trim tabs ( they are adjustable and at hull speed should not be used fully retracted ) and prop.

The hull design,( is what it is). It performs close to what was advertised for marketing purposes. The day I took delivery it was 10% less than advertised (I was ok with that.) Now that I have it equipped the boat the way we plan on using it, an additional 10% less is experienced.

The variable that may help. "Propping" This is give and take. My goal is maintain WOT Manufactures Recommended rpm 4000 rpm +/- 100 rpm (proper engine loading) and better overall performance ( speed , fuel economy ) at 3300 rpm (high cruise) and better overall performance ( Speed , fuel economy) at 1900 rpm to 2200 rpm ( low cruise) .

The give and take may be sacrificing WOT performance. ( I don't run WOT often) ( it isn't that good anyway)

The prop I am running right now is operating with 39% to 42% slip. Before I reduced the cup to increase WOT rpm the prop was running at 31% slip. The rule of thumb is prop slip should be around 30% for a fast semi-displacement hull. My goal is to try to prop the boat with a prop that will allow me to hit Max rpm but have less prop slip and increase efficiency. Prop slip about 30%. I'm looking at going with a four blade 18" X15.5 .030 cup. Acme prop.

Chimo based on your gear 2.43 to 1, estimated 3000 rpm WOT and a speed estimated speed 19 mph you probably have 21P or 23 P prop 25% to 29% slip.

There are a lot of variables that go into answering the original question of the thread. I general the answers that have been given Hull speed 6.5 kts to 7.5 kts will hold true. To get the best takes research. Ranger/Cutwater already did that. My hope is there is still a little more on the table to make it better for my use.

Chimo thanks for adding to my collection of Calculations ! :shock:

 

[bryant]

The top speed i got this summer was 17 MPH at WOT 3000 RPM. Not sure what the MPG were as my GPS (a Garmin 5212 does not display that information. Still trying to find out if that information should be available on my boat. Going to be interesting this summer seeing how my boat matches up to the charts you guys provided.
Bryant

 

[mikey]

anyone know the gear ratio of a 2014 c28 D4 260?