Engine HP-cruise speed

[hntrss]

Yesterday, on my way in I put my 300hp 3116 on the pin to check that I was getting max RPM fully loaded. 31 Duffy, 200 gals fuel, 3 men gear and 75 gallons of water. She went just a hair over 2800, which I feel was perfect for the load. Now the question--On the pins, on the GPS, I hit 21.8 knots steady. Does this mean that basically, if I repower with an engine that is producing 300hp at rated cruising RPM I can expect 21 knots? It makes sense, but things that seem to make sense hardly ever work out in the real world. I am thinking ahead for when I sell my 3116 to lobstercatcher.

 

[captjohn]

If everything is equal (like engine weight, same RPM's, same gear ratio, same wheel, same slip) I would say yes. But if the new engine is heavier you'll have to compensate for that (some more HP), less weigh, your going to be better off. Weight is going to have an effect on displacement (possibly wetted length, but probably not enough to make much of a difference). Where your going to have to do some homework is on the gear ratio and prop as the new engine is unlikely to develop the same HP at the exact same RPM's.

 

[BillD]

Yesterday, on my way in I put my 300hp 3116 on the pin to check that I was getting max RPM fully loaded. 31 Duffy, 200 gals fuel, 3 men gear and 75 gallons of water. She went just a hair over 2800, which I feel was perfect for the load. Now the question--On the pins, on the GPS, I hit 21.8 knots steady. Does this mean that basically, if I repower with an engine that is producing 300hp at rated cruising RPM I can expect 21 knots? It makes sense, but things that seem to make sense hardly ever work out in the real world. I am thinking ahead for when I sell my 3116 to lobstercatcher.

I'll take a stab @ this

Simple math says yes. burning 15 gals per hr. +- a few % equals 300 hp in any marine diesel engine.
Now you have to get the "rubber to the road".

21.8 knots with 300 hp is @ 2800 rpm with a particular gear and prop pitch.

1st thing is you'll need is an engine that is designed to produce 300 "cruise" hp.

The Cummins QSB 5.9 480 comes to mine. The engine is designed to produce 300 hp (15 gals/hr) over many thousands of hours.

There are many others engines that will do the same but plan on buying an engine designed for 300 hp cruise rpm.

You may have to change the gear to get the same shaft speed @ a lower cruise rpm or increase the pitch of the prop or some combination of the two.
Making the change could effect efficiency few percentage points either way??

How about some "more" info?

What is the boat geared and propped to now.

 

[Toolate]

I always wonder why you diesel guys talk in terms of HP instead of torque. Seems that the torque numbers for any given hp engine can vary widely (the ratio between torque and hp is not the same for all diesels-right?).

 

[eyschulman]

A suggestion become a member of Boatdiesel.com and go to the calculator section of the site plug in your boat specifics and info on intended motor and you will get good estimates of what is needed.

 

[hntrss]

I am on boat diesel, just testing the infinite knowledge pool of the DEBF. I think Bill D brings up a point in itself from his knowledge of Cummins. I wouldn't be willing to put 480 hp in the Duffy if that is what it took to get 21 knots. I run this motor at 2300, (of a possible 2800) and averaged out I think 15.8 - 16 knots is average speed at this rpm. I was really just wondering because she pretty effortlessly gets from 16-22knts. I could also run this engine much harder than I do, but its not in my nature. At 2300 she is not even straining and sounds real happy. By the way we are 2:1, 24 square 4 blade.
I could do some math but am guessing that 370 would bring me to an easy 18knots, which would be good enough for me. I do like my current fuel consumption compared to my past 2 boats. Burning slightly more than half the fuel for the same trip. (in a much smaller boat of course). Thanks for the replies!

 

[captainlarry84]

The 3116 is a good power plant. However it is rated at 300 HP @ 2800 RPMs. The recommended continuous RPM is only 2400 RPMs which drops the HP into the mid 200HP range so you cannot take advantage of the true 300HP. The new lighter diesels arm much different. For example the 370 Yanmar which I owned was rated at 370 HP @ 3300 RPMs. the continuous rated RPMs was 3100 @ 320 HP. Therefore your cruising speed goes up. Same with the new C-7 CAT at 2800 RPM motor with a continuous rating of 2600 RPMs allows you to use almost all of the HP. The C-7 is the old 3126 and they were also rated at 2400 RPMs. So the answer is yes. If you for example dumped your 3116 for a 315 Yanmar. You would save 1,000 lbs. & have a continuous RPM or 3600 out of the 3800, which pus the Yanmar at 280 Plus usable HP.

 

[hntrss]

See, as expected I can get all the answers I need right here. Thanks Larry.

 

[petrel]

The 3116 is a good power plant. However it is rated at 300 HP @ 2800 RPMs. The recommended continuous RPM is only 2400 RPMs which drops the HP into the mid 200HP range so you cannot take advantage of the true 300HP. The new lighter diesels arm much different. For example the 370 Yanmar which I owned was rated at 370 HP @ 3300 RPMs. the continuous rated RPMs was 3100 @ 320 HP. Therefore your cruising speed goes up. Same with the new C-7 CAT at 2800 RPM motor with a continuous rating of 2600 RPMs allows you to use almost all of the HP. The C-7 is the old 3126 and they were also rated at 2400 RPMs. So the answer is yes. If you for example dumped your 3116 for a 315 Yanmar. You would save 1,000 lbs. & have a continuous RPM or 3600 out of the 3800, which pus the Yanmar at 280 Plus usable HP.[/QUOTEo

How long do those engines last running 200 off the top of 3800? You lost me somewhere. Seems like the continuous duty engine was only 3100 WOT. Is the 3800 rpm engine also a commercial engine?

 

[lobstercatcher]

Yesterday, on my way in I put my 300hp 3116 on the pin to check that I was getting max RPM fully loaded. 31 Duffy, 200 gals fuel, 3 men gear and 75 gallons of water. She went just a hair over 2800, which I feel was perfect for the load. Now the question--On the pins, on the GPS, I hit 21.8 knots steady. Does this mean that basically, if I repower with an engine that is producing 300hp at rated cruising RPM I can expect 21 knots? It makes sense, but things that seem to make sense hardly ever work out in the real world. I am thinking ahead for when I sell my 3116 to lobstercatcher.

Yea you should get the same speed out of the same HP.There use to be tainted HP rating so there could be a difference . I thought I was getting a zf220 vertical 2.475 / 1

someone will want it who has one and that person isn't me! I can ask around to see if there is one locally that could use a spare. Are you really going to do it!

 

[lobstercatcher]

The 3116 is a good power plant. However it is rated at 300 HP @ 2800 RPMs. The recommended continuous RPM is only 2400 RPMs which drops the HP into the mid 200HP range so you cannot take advantage of the true 300HP. The new lighter diesels arm much different. For example the 370 Yanmar which I owned was rated at 370 HP @ 3300 RPMs. the continuous rated RPMs was 3100 @ 320 HP. Therefore your cruising speed goes up. Same with the new C-7 CAT at 2800 RPM motor with a continuous rating of 2600 RPMs allows you to use almost all of the HP. The C-7 is the old 3126 and they were also rated at 2400 RPMs. So the answer is yes. If you for example dumped your 3116 for a 315 Yanmar. You would save 1,000 lbs. & have a continuous RPM or 3600 out of the 3800, which pus the Yanmar at 280 Plus usable HP.

There might be a novis boat around that could use a engine like that

 

[hntrss]

Yea you should get the same speed out of the same HP.There use to be tainted HP rating so there could be a difference . I thought I was getting a zf220 vertical 2.475 / 1

someone will want it who has one and that person isn't me! I can ask around to see if there is one locally that could use a spare. Are you really going to do it!

Nah, not till I smoke it. It runs too good. Doesn't burn a drop of oil, transom is clean. Its worth more in the boat than out of it. The only thing is the cummins remans will not be available much longer according to Peter, so if that's the way I wanted to go, my hand could be forced a bit. I have used it for 200 hours this fall/winter and it hasn't skipped a beat, changed the oil after 150 and never added a drop. Makes no sense to sell it for scrap. Seems like most sell for 4-5 k with the gear.

 

[Hilda]

I run the same boat w/ 440 yanmar. Runs 18-19 knots cruise at around 2850 with full load of fuel, fishing gear and 4-5 persons aboard. Comfortable speed but could always use more getting out and back. Runs around 23 knots wide open. Its power vs. weight. If you want to go a lot faster you may consider another hull.

 

[hntrss]

No, I am happy with 16-17 . I like hearing all this though, makes you know that its not worth 150hp to gain 2 knots. If I could get an easy 18 knots I would love it ,but not if it means burning 30% more fuel. Thanks for the reply. My boat is real basic too. Not a lot of gear, no generator etc. So its likely lighter than most. Really impressed with it in a sea though. Its quite a bit smaller than my last 2 boats, but we have used it in some weather and its pretty good.

 

[BillD]

Nah, not till I smoke it. It runs too good. Doesn't burn a drop of oil, transom is clean. Its worth more in the boat than out of it. The only thing is the cummins remans will not be available much longer according to Peter, so if that's the way I wanted to go, my hand could be forced a bit. I have used it for 200 hours this fall/winter and it hasn't skipped a beat, changed the oil after 150 and never added a drop. Makes no sense to sell it for scrap. Seems like most sell for 4-5 k with the gear.

Did Peter give you a date when the Cummins factory closes in Mexico ????

 

[petrel]

It's on here somewhere where he said EPA regs were going to mean the end of factory recon 6BTAs and 6CTAs in the next year or two, but I guess you'll still be able to get recon QSBs, QSCs, and QSMs b/c they are the next Tier. Maybe in that Calvin Beal thread. What's the word from So Cal, where those engines are probably already illegal? Or do you talk directly w/ Cummins headquarters?

 

[hntrss]

No, he said something to the effect that the engines would no longer skirt the emissions rules. Which we all know is all they are doing, right? I have not fact checked this but would find it believable.

 

[hntrss]

Petrel , you beat me to it!

 

[lobstercatcher]

Nah, not till I smoke it. It runs too good. Doesn't burn a drop of oil, transom is clean. Its worth more in the boat than out of it. The only thing is the cummins remans will not be available much longer according to Peter, so if that's the way I wanted to go, my hand could be forced a bit. I have used it for 200 hours this fall/winter and it hasn't skipped a beat, changed the oil after 150 and never added a drop. Makes no sense to sell it for scrap. Seems like most sell for 4-5 k with the gear.

It would be worth that in that condition., Was I remembering right that you had the zf220 vertical 2.5 to 1

 

[BillD]

I'll believe it when I see that Cummins has stopped shipping by the hundreds the RECON mechanical engines to the USA

I'm glad my new Cummins RECON 6BTA is Tier I compliant