Most economical cruising speed

B

bryant

Active member
Joined
Feb 11, 2020
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Fluid Motion Model
C-24 C
Non-Fluid Motion Model
looking to buy a Ranger Tug R25
On a 2008 R25 with a 150 Cummings/merc engine what would be the most economical cruising speed?
Thanks
Bryant
 
I think you may be asking only half the question here. The "most economical" speed would be barely above idle, cruising at a blistering 3 knots. Heck you could get half way to Hawaii on one tank! But of course that's not terribly practical

Here's the fuel burn chart for my R27. Not the same boat, but I think the same principle applies to your R25. I typically have 2 modes of cruising; ~7 knots at 1700 RPM sipping about 1.5 Gal/Hr, or if I'm in a hurry ~14 knots at 3000 RPM guzzling about 5 Gal/Hr.

The thing to know with these semi-planing hulls is that anything between these two speeds is wasteful. If you notice in the chart, your getting about the same efficiency and range from 2000 RPM almost to WOT, so if your going up on plane you might as well enjoy the speed!

http://www.tugnuts.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=44924

main.php
 
My RT25SC has a Volvo Penta 150 and what I have experienced is from just under 2000 rpm to WOT at 3000 rpm my cruising range according to Garmin is the same. I just get there faster at the higher rpms. As Toki has indicated the best cruising range is just above idle, as you then increase rpm the cruising range decreases until just under 2000 rpm. On my boat I have not seen any sharp decrease up to 2000 rpm.
 
Speed is a function of RPM, wind and current. You would want to look at RPM for consistent fuel burn. Economy would include the first three I mentioned to give you MPG. With that said your chartplotter most likely has a MPG data block that you can put on your screen. Having traveled with a few 150 HP tugs 6.5 MPH seems to be an average for economical cruising. Keep in mind that those slow cruisers were also sail boat people at one time so 6.5MPH is fast. :lol:
 
6mph is the magic number for my R25. 1 gallon per hour. Fast enough to cover sufficient scenery that I don't get bored. Slow enough to be flat, stable and I don't have to slow up for most no wake zones.
Well shucks Mike. You know me. Anything over 7 and I get a nose bleed.
 
"Most economical" is a moving target (pun intended). There a are a bunch of fuel consumption and speed charts in my photo album. For some reason, I could not get them shown here. Take a look. The data covers a wide range of loadings and conditions. I checked the data this summer and they are still correct after 1,000 hours in the clock.
 
Agree with all of the above.

But depending on the total picture more can come into play.

If making a multi-day trip from Point A to Point B, faster can mean fewer marina nights and cost. You might spend more on gas going faster, but saving even more in marina cost.

Or something like that.
 
tlkenyon":210a1ibc said:
There a are a bunch of fuel consumption and speed charts in my photo album. For some reason, I could not get them shown here.
Click to expand...
There are two ways to embed images in your posts.

If the picture you want to include is in one of your gallery folders (on Tugnuts), click on the g2Imgbutton. That will open another window which will allow you to browse your galleries and select a picture. Be patient, it can take a while for the new window to populate. It should default to your album when it opens.

Click the box in the upper left of the picture you want to post. Scroll (way) down to the bottom of the window and click Submit. That will attach the picture to your post.

If the picture isn't in your gallery, perhaps something you've seen elsewhere on the web, or in another personal web collection (i.e. flickr, iCloud, google, etc.) then you click the Img button. The image/collection must be open to the public.

This will create the following string in your posting window - then you copy and paste the URL (http://whatever...jpg) of the desired image in the middle, between the ][ brackets. The trick here is to determine the direct URL of the image. Various image hosting sites show it in different ways. Sometimes it's right up in the address bar. Other times it's listed under "Share This Image", etc.

You can determine if you have the correct URL by hitting the Preview button. If it displays, you're good to go. If it's not correct, the form will give you some kind of clue so you can go back and dig around. You'll only have to figure it out once for each host.


Cheers,


Bruce
 
Where can I obtain and speed and efficiency chart for my 2015 R-31 CB LE with a D4-300 diesel?
 
I find this question of best cursing speed to be confusing. It would depend if you were going up river or down river! I would think a better question would be at what rpm do I get the best bang for my buck!
I find that most diesel engines run most efficiently at 1800 to 2200 RPM's. If you check your diesel truck going 60 mph you will see it's only turning about 2000 rpm's most diesel engines whether large or small are designed to work all day under load at these rpm's. The next time your in a truck stop ask a owner operator truck driver what rpm he usually runs his truck, remember he is paying his own fuel bill that comes out of his pocket. Any way to answer your question of what speed, what ever 1800-2200 RPM pushes your boat. Merry Christmas to all. Bob
 
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.)

I'm not sure if I ever shared my data with Bryant. I'm sending this to him by email but sharing it with everyone else here.

What the previous commenters said is all pretty true. You can see from my plots that the MPG is pretty good at low speeds and drops steeply with increasing speed. But, once you get to ~10mph it levels off and there's much less sensitivity to speed. Two and a fraction MPG doesn't sound so great if you are thinking about your car, but this is a boat... enjoy. 2-1/2 MPG is good for a 5000-10000 pound boat. Talk to some of your gasoline boat friends.

 
Thanks for all the great info. In the past I have had several Day Cruisers (gasoline) and all had test results which listed the most economical cruising speed RPM /MPH/GPH so I was looking for the same comparison but as you all pointed out I am probably looking for the wrong information what I really need is fuel burn rate. Which is not available on my boat Garmin not sure why its not or even if it is supposed to be available and just not there. I did check out the charts several of you provided which give me a great starting point to check out and compare when I put the boat back in the water in the spring.
As rpmerrill responded part of my problem is that my engine does not communicate with my Garmin as far as MPG is concerned I am not sure if it should. His boat a 2012 R27 and does. I asked that question in a previous post but re-reading that post ,as with this one, I was not too clear as to what information I was looking for. As I said this is my first Tug type boat so it has been a steep learning curve.
Thanks for all the help.
 
Bryant,

When I look at rpmerrill charts they are very similar to my experience in my RT25SC with a D3 150 Volvo Penta. The big difference is my boat achieves these numbers at 1000 less rpm. My mpg in the flat area of the curve seems to be identical to his (~2.3 mpg), also my top speed is about the same at 22 mph. I have not plotted my performance but I suspect it to be very similar to rpmerrill overall curves based on my observations.

Given the above, I suspect that your 2008 classic 25 performance may be similar. Currently without a way of measuring your fuel flow I would go with rpmerrill data to do your planning. Just make sure you plan for sufficient fuel reserve.

Good luck.
 
1000 rpm differential .....

My redline is 4000 rpm. If the Volvo is different then there are either more rpm or less rpms to work with. I'm just guessing that the Volvo might have a lower top rpm and a different transmission ratio The transmission takes the rpms supplied by the engine and slows the prop down to an appropriate rpm to match the prop power draw to the engine output around the area of the engine red line. And of course a good top speed.

I used to design propeller fan air handling systems they all work the same. It is always a delicate dance to match the motor, the fan diameter and pitch, and the transmission speeds to get proper performance.
 
rpmerrill,

You are correct, the VP D3 150 has a max rpm of 3000. I was talking top end at wot giving 2.3 mpg and 22 mph. Obviously as I reduce rpm, the rpm difference would be less than 1000 at a particular speed and at idle would be approximately the same. However the curves you generated look very close to what I have experienced for mpg performance at the speed on your chart. With my hull the same as the Bryant’s 2008 classic 25 I would expect his performance to be very close to your curves unless the 150 hp Cummings diesel is either much less or much more efficient than the VP. This is just a guess on my part based on the same hull as mine with a 150 hp diesel.

By the way I was somewhat surprised form your charts that the classic 27 with a 180 hp diesel so close in mpg performance to my 25.
 
Dclagget and rpmerrill,

I have a D3-200 R27 which has a WOT of 4000 (or at least it did when it was brand new with a pristine bottom and not overloaded with gear! 😀 ). The difference is not just Yanmar vs. Volvo. My understanding is the D3 is physically the same across all power versions. The difference in power ratings (and WOT RPM) is in software (tuning of fuel flow, and turbo boost etc.). My transmission gear ratio is 2.03 so perhaps that is different. Perhaps the props are also different. I have a 17x17 prop.

Incidentally, my performance curve is similar to the rpmerrill 180hp graphs, with just a bit higher fuel consumption. At WOT my fuel consumption is 10.2 GPH getting about 2.0mpg (again with a clean bottom and not overloaded with gear or people).

Curt
 
Curt,

My understanding is also that the D3 is basically the same motor across hp ratings. However for some reason the D3 - 150 hp is limited to 3000 rpm so it is propped accordingly. My D3 is connected to a VP HS45AE-C “Reverse Gear” transmission with a gear ratio of 2.43:1 and 2.03:1. I don’t have the prop specifications handy which also is part of the drive performance. The prop on the boat was factory installed.
 
Going beyond my expertise... (probably an hour or so ago)... maybe shoulda stopped then?

Displacement hulls have a max speed that is a function of their length. So a 25 would have a "max speed" that is lower than a 27. And it would be harder, in the displacement mode, to get to, and thru, the transition to (what we call) planing mode. A very small difference but they should be very close in performance pretty much all around. Maybe the 150hp engine has to work a bit harder/closer to its limit than the 180, but I'd bet it is a very small difference.
 
Rpmerrill and dclagett,

Understood. No issue here. I’m out of my expertise here as well. Perhaps BBMarine or someone else knows the secret to how they do the magic.

The specs show the performance (hp and torque) of the 150 and the 200 as the same up to about 2500rpm and then the magic happens. Torque and horsepower are extended up to 4000rpms on the 200 while dropping off on the 150 up to the max 3000rpms. I would guess most of difference in the 25/27 boats (at semi displacement speeds) amounts to weight and thus the reduced horsepower and torque requirements for the 25 (and thus reduced max rpm/horsepower curves). I suspect the 150 is actually not working “harder” and may actually have even a longer life due to the lower rpms. The 200 in the 27 has to “lift” more weight and thus the higher rpms. There I go, way beyond my expertise!

Sorry to the OP for hijacking this thread!

Curt
 
There are involved some "hard facts", some "rules of thumb" and many variables.

Fact: 150HP x 5250/3000 = 262.5lbf.ft (don't confuse lbf.ft with ft.lb which is a measurement of energy). Extrapolating, this is he same torque as 200HP/4000 rpm. The curves published seem to show that both variants have peak torque around 310 lbf.ft but the 200 variant carries this torque over a much wider range. That makes it a more flexible engine. In terms of wear and tear the 200 variant can see sustained higher stresses in terms of component load and logically things like pistons and bearings will travel further if the engine is operated above the 3000 limit of the 150 variant. There are other factors that are far more important in terms of engine life however.

Bear in mind that comparisons with the same engine block used in a car (which it is) are not that useful. Once accelerated to a certain speed the load on a car or truck engine is much reduced to that required to simply maintain that speed. A boat engine can be thought of as permanently driving uphill.

There's a "rule of thumb" for a full displacement hull that hull speed (kt) = 1.34 x sqrt waterline length (ft). After that adding more power simply pushes more water. When you get into semi-displacement hulls and then planing hulls, multihulls, aerated hulls, wave piercing technology etc. what happens after hull speed brings in all the variables. I would love to see the tank test curves for our boats but I doubt they are freely available. For us casual users we know what makes a difference. The biggest is weight and where it's distributed. Experience will be your best guide here but remember that what is the best weight distribution at 12kts may not be the best at WOT. Decide on where you want to cruise and trim the boat accordingly, bearing in mind comfort and safety. In the end, just have fun.
 
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