Yanmar v Cummins v ...

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mcarthur

mcarthur

Senior Member
Joined
May 9, 2018
Messages
297
Location
Australia
Vessel Name
Blu Emu
Vessel Make
Ligure 50' aluminium power catamaran
We have an under-powered power cat that we're thinking of re-powering: taking out the 2x Perkins 135hp that weigh 600kg each even without gearbox, and replacing with 2x 300hp or so slightly more modern diesels.
We don't want common rail, as we'll often be in places where local fixing (or ourselves) may be needed.
We can't afford new engines :angel:.

We were looking at two used Yanmar 315hp 6LT's, possibly the Cummins 6BT 5.9 although it hasn't the hp we desired at 220hp or so and isn't the most economical compared to the Yanmar.

Are there other options we should consider?
 
If it were me replacing with a used engine, I'd probably stick with cummins. Parts are everywhere, they are pretty easy to work on and from memory, its not difficult to coax another few horses out of them. Not sure if you're looking at the 12v or 24v. Performance upgrades are different on each.

One thing to look at on cummins, depending on which generation, is the valve springs. If it were me, and the engine was well used, I'd think about pulling the valve springs. With the engine out its not that difficult without pulling the head.
 
John Deere. The 6068's are typically about 200HP with turbo, and 300HP with turbo + aftercooler. The common rail version is 400HP.

What brand and size is the cat? What did the designer indicate as max. HP? It is possible that the cat is simply overloaded (easy to do) rather than underpowered. Cats turn into dogs real fast if overloaded. If you provide additional info you might elicit some useful discussion. Re-powering is not a cheap exercise, even with used engines, so it would be nice to be confident of the outcome ahead of plunging in.
 
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I would go with Cummins in a heart beat over Yanmar. Yanmar may be fine engines but Cummins are pretty bullit proof.
 
My choice would be Cummins normally, but not knowing the type of cat you have, and considering the limited hull width and weight sensitivity of a most designs, I think it would be hard to beat the Yanmar 315 in that application. It’s HP to weight ratio and physical dimensions are favorable for powercats, but since prop size and pitch may be limited by hull clearance and form, actually putting the increased power to the water could be a real challenge.
 
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An additional thought. If it were me I would try and find some Yanmar 6BY, 260HP. Much lighter weight than the other options. But series 2 or 3 only, not the series 1. The block is the 3 litre BMW straight 6 (in the X5 etc) that is vey highly regarded. But Yanmar screwed up on the marinisation initially, hence avoid series 1.
 
Re-Power - more then doubling the power - couple points you may be over looking

Many consideration that need discussion.


Are you looking at a recreational M-5 rated diesel for weekends & day trips, or an M-1 Continuous rated Diesel to take to Hawaii or Panama Canal. -- or some where in between - like an M-2 or 3 , 4 ? or ? :confused:

What do you have for transmissions, are you reusing them ? & what are they rated for as to duty cycle M-1 through M-5 ? -- your increasing power, so you may, depending on usage, need new transmissions, depending on duty cycle, because for the doubling of HP power level your talking about & depending on usage, you may exceed their rating & they will not hold up. :facepalm:

Your going to have to re-prop them for sure to use that power as well & with more than doubling of the HP to 300, new bigger shafts will be needed, mark my words.


I think the Caterpillar brand might be to expensive. But some times a deal comes along with a used unit or two, that is to good to pass up. So shop around. Look on E-bay & internet.

You might look at the John Deere Marine Diesel line. They have a re-man line that is a big money saver. May need a core - so check what the core charges are, or locate a run out old 6068 core or two. :) The 6068 which are a real solid power plant that would give you 300HP with turbo + after-cooler per engine with not adding a lot of weight.

I have been happy with the JD engines myself & they make a lot of different size & power combinations & different rating options. Worth looking into in my opinion. Again, what is your usage ?

I believe Mitsubishi has a full compliant new diesel that is all mechanical.

Also shopping around for what is available as either a re-man unit or a good used one of what ever brand is available that is still mechanical & low hours & priced right would also be a possible way to go. In which case - you use what is available.

Cummings has some very competitive pricing on a couple of their units, so don't forget to look their way.

BUT do not over look your transmissions & shafts & props, :nonono: as need to match what your intended usage will be & what the duty cycle that goes with that intended usage will be & the power that feed through them or you'll have trouble real quick. :hide:

Good luck.

Alfa Mike :thumb:
 
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Thanks everyone for comments!

Some notes:
  • The boat is a 50' custom power cat, beam of 16'4, with 12.3:1 length to beam ratio, so pretty easily driven. 16,500kg/36,000lb which is just on the higher side of average.
  • We're hoping to do some long distance cruising, including Pacific & Atlantic (not at the same time :D). I don't know the M-ratings you're using (off to Dr Google!)
  • I expect we're up for transmission changes too. They are Hurth 450 at the moment. We'll need gearboxes that can freewheel too, as we'll be running on one engine when long passage making for the economy
  • good point re:shafts and props. We knew about the props but hoped the shafts would be ok - maybe not though.
  • Great idea re: the Yanmar 260. Lower grunt than we wanted, but we also value simplicity and reliability (doh!)
  • No maximum by the designer. I've done some analysis and similar boats are anything from 300-440hp per engine, so we are definitely an outlier at 135hp each.
  • the boat is aluminium, and the engines are in the centre of each hull in their own full-height bay. The Perkins are heavy (600kg) and long, so most others should fit in the bay. The shaft angle is pretty flat too, so should have options. One of our issues is how to get the engines OUT of their bays - we can demolish the seating and some of the galley to get access to the hulls from the main cabin, but that's still a sideways lift of 600kg onto the bridgedeck and then out the rear door...:eek:
  • As a non-technical person who's learning slowly :ermm:, what's the difference between the 12V and 24V cummins (apart from the number of valves :socool:!)
 
As someone who has owned, built and cruised cats both powered and sail, we need to see and get details of boat.
DWL
Displacement
Proposed cruise speed
Underwater shape, especially last 1/3rd of hull

I see some of those details above.
16500kg, she's no lightweight that's for sure, is that wet or dry.


What do similar designs have in them at your proposed cruise speed?
What does the designer or propulsion guys say when asked?
 
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Simi 60 said:
As someone who has owned, built and cruised cats both powered and sail, we need to see and get details of boat.
DWL
Displacement
Proposed cruise speed
Underwater shape, especially last 1/3rd of hull

I see some of those details above.
16500kg, she's no lightweight that's for sure, is that wet or dry.

What do similar designs have in them at your proposed cruise speed?
What does the designer or propulsion guys say when asked?
Click to expand...

Hey Simi,
Thanks for the reply.
But I'm left wondering what your response would be were I able to give you the exact information? My suspicion is that you would say something like "you may be able to go with a 240hp Yanmar gen 3. It could give you cruise of 8-10k and max of 13-16k" (I'm making the actual numbers up of course). That is, things I already know or can guess just as well myself :whistling:.

My question is around other options in the 240-360hp range of diesels, not CRD, with a pretty good track record (and economy). I don't need more guesses as to performance - and unless you're moonlighting from being a singularly amazing naval architect guru, you'd just be guessing like everyone else!

I don't doubt that you and many others have vast experience. Are there any other engines I should be looking at apart from those mentioned already?
 
My comment was more from the point of view that if you have bought a 20 knot ex ferry designed to carry 50 passengers and only plan on doing 8 knots with 4 people and a bit of gear you may easily get away with half the horses.

What's the point in running and paying for big motors if you don't need them?

What design?
Link to boat?
Curiosity more than anything
 
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Insequent said:
What brand and size is the cat? What did the designer indicate as max. HP? It is possible that the cat is simply overloaded (easy to do) rather than underpowered. Cats turn into dogs real fast if overloaded. If you provide additional info you might elicit some useful discussion. Re-powering is not a cheap exercise, even with used engines, so it would be nice to be confident of the outcome ahead of plunging in.
Click to expand...

Exactly what I was trying to get before afternoon martinis had their evil way with me.
 
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"We have an under-powered power cat that we're thinking of re-powering:"

This makes me think you want to go much faster. The goal?

Remember much added fuel will need to be onboard , which is heavy and expensive.

Do you have room for added tanks?
 
Here is my take on the choices discussed:


Cummins: Rock solid but heavy for a cat
JD: Same
Yanmar BY: 3 liters is too small
Yanmar 6LY: Good compromise between size, weight and power


David
 
What speed do you now get with the Perkins 135's? Cruise speed and rpm, also full power speed and rpm?

With those four data points, some estimation can be done regarding what more hp will do.

I sense you may be going in the wrong direction as the fuel burn rate going faster and using larger engines will probably cut your range down to an unacceptable number.

135hp ea should get that vessel to its most efficient speed which is likely about 10kts with engines at modest power level. Speed above that with any engine the burn rate goes through the roof.
 
"We have an under-powered power cat that we're thinking of re-powering: taking out the 2x Perkins 135hp that weigh 600kg each even without gearbox, and replacing with 2x 300hp or so slightly more modern diesels."

One key question raised above is whether you need or desire the much larger hp and whether that will ever practically get 'used' in your cruising plans.
If you really want to go from a usable 135 hp to the area of 300 hp it will likely require these changes in addition to engines...
- Shafts, and props (if you have the clearance)
- transmissions
- Struts and cutlass bearings
- controls and gages
- fuel delivery lines and filters
- raw water intakes , valves, baskets and hoses
- Engine mounts
- Exhaust system hoses and thru hulls
As you review the requirements some of these it may raise a problem or two in the planning and/or costs of this project.
 
Further to Ski's post above:

See how fast your boat will go at wot with 270 hp from the existing Perkins. That is about how fast you can cruise with two Yanmar 6LPs putting out 150 hp each, probably the limit for continuous power cruising from those engines. You might pick up a knot or two due the lower weight of the Yanmars.

Then look at that speed against a combined 17 gph fuel burn and ask yourself if that is ok. Then go forward from there.


If the speed/fuel burn is reasonable to you at those numbers then you can consider more hp if more speed appeals to you. Cats have a more linear hp vs speed curve and more hp may push you significantly faster. But 250 hp each continuously from two Cummins 6BTAs is about all that you can hope for. 500 combined hp might pick up another 5-10 kts of speed but at a combined fuel burn of about 28 gph.


David
 
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I'm a big fan of the Cummins due to simplicity, longevity and cost/availability for parts. You are correct that the 6BT is 220 HP (that is what we have in our boat), but they also make a 6BTA which adds an aftercooler and comes in versions that put out well over 300 HP.
 
Thank you all.
I've done a little research on some of the options and put together a graph from the manufacturers prop curves and power curves. Obviously they are theoretical for each engine model, but they are the only semi-objective data I think can actually be used to compare.

I have graphed the results and added the speeds of our boat at a few different total hp's, stopping 200rpm short of WoT since you'd almost never push it for very long at all.

Compare diesels.jpg

The Cummins 6BTA wins on fuel consumption, and seems a reasonable match for the prop at the low end at least.

The other contender, the Yanmar 315hp is much lighter though - 408kg vs 508kg, or 200kg difference for two motors. There would only be a little saving on weight going Cummins with the Perkins being 595kg each.

Pricing secondhand seem similar.
 
Simi 60 said:
Then to totally mix things up 72ft of aluminium with 200hp x 2

Sher Khan
Click to expand...

Hey Simi, thanks for that. I missed Sher Khan :angel:.

But I have armchair-looked at quite a few power cats...lots of investigation through individuals web pages and discussion groups of people actually travelling on them reasonable distances, manufacturers sales sites :banghead:, and then when nothing else was available, a combination of tealeaves and rubbings cats against balloons (no reason, just fun :D)...

A comparison of length vs power-to-weight:

Compare powercats - length v power.jpg

A comparison of length vs engine size:

Compare powercats - length v enginesize.jpg

Interestingly, there is an 84% correlation coefficient on a linear relationship of boat length to engine size if the outliers are ignored (our boat, the Aspen 120 which is single-engine asymmetrical hulls, and the Tennant 66' Domino).
 
135 HP upgrade to 300 HP + requires more than just the bigger engines.

That Cummings -- the 6BTA -- which adds an after-cooler and comes in versions that put out well over 300 HP.--- sounds pretty good.

I would look at that one real close.

But your old trans & shaft & prop are going to need replacement & the exhaust system needs to be bigger & bigger raw water intake for cooling.

So lots of stuff to upgrade when you make the jump to more HP..

Good Luck.

Alfa Mike
 
Wow, I thought I was the only nerdy engineer here on TF (well maybe Ski when he is bored) who pores over hp/fuel curves and plots them up.


But take a look at your data again. There is no way that the Cummins 6BTA uses almost half as much fuel as the others at the same hp. I would expect that the Cummins 6BTA, the Yanmar 6LP and the Yanmar 4LH to all be within 5% of each other. The Cummins 6BT and the Perkins maybe a bit worse.



I will bet you mixed up wot fuel consumption with prop hp fuel consumption for the Cummins, particularly since Cummins is one of the few who gives both prop and wot fuel consumption curves.


Now if you really want to be nerdy, join boatdiesel and use their power calculator. Calibrate your boat's current speed vs power to what the calculator figures and adjust the hull type to get them to approximately match. Then you can plug in higher hp engines using that hull data and get a pretty good indication of how fast the increased hp will push you. Then use your corrected fuel burn curves to find lph burned.



David
 
Yep, the engine burn rate graph is off. As David pointed out, all those engines in mid range will burn within 10% of each other if making the same power.

Still looking for cruise rpm and speed, and full power rpm and speed for the package you have now. That is very powerful info to guide for engine shopping.
 
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Ski in NC said:
Still looking for cruise rpm and speed, and full power rpm and speed for the package you have now. That is very powerful info to guide for engine shopping.
Click to expand...

MacArthur
Yes, this information as requested by Ski is the key real world data. Given the large go forward cost you are potentially undertaking investing some money in fuel measurement equipment NOW will answer many questions.

BTW, guess I'm missing something, but how can you show the +135 HP points on your curve for the current 135 HP engine?

My guess is that for long passages you will be running around 9 knots to keep fuel burn in check. Thus, the big HP increases you desire will be for closer to port endeavors.

Lastly, do you really want to be a blue water cruiser on a used and unknown set of engines? Cummins remans addresses this issue.
 
Please let me know where I've gone wrong for the for the first graph (post #22). I looked at the manufacturers' engine graphs:
The Cummins 6BTA data did confuse me as the hp power numbers in the table don't match the graph. But the graph appears to match a prop curve of 2.5 or so, so I used that instead of the numbers. I did use the numbers for consumption though as they did seem to match the graph.
 
Current boat:
  • 1300rpm, 6kn, 1.2L/nm
  • 1800rpm, 7.5kn, near 2L/nm
  • 2100rpm, 11kn, about 2.5L/nm
  • 2400rpm (as close to WOT as we get), 13kn, near 3L/nm

What we would like in the future is to long distance cruise at 8-9kn at, preferably, about 1.2-1.4L/nm (Domino does near 1L/nm at 10kn, but we don't have a Malcolm Tennant design).

When coastal hopping, especially out of waves, we'd like to get moving at 15-16kn (not WoT!) at about 2.3-2.5L/nm. Obviously it all depends on currents, wind and wave conditions, but those are flat water "wants".

In talks with various experienced "salts", the feeling is that the old Perkins are fantastic work horses, but the newer engines are lighter, smaller and have more power for less fuel.

(I'm already on boatdiesel - I didn't know about the calculator though, so I'll go have a look...thx)
 
Oh, and I forgot to list the power catamarans I got data points on for consumption for the other graphs. I found at least three points per model per engine configuration on almost every boat (e.g. boat model + engine size + speed + consumption):
  • FP Greenland 34
  • FP Highland 35
  • FP Maryland 37
  • FP MY 37
  • FP Cumberland 44
  • FP Summerland 40
  • FP MY 44
  • FP Cumberland 47 LC
  • Aspen 120
  • PDQ 34
  • PDQ 41
  • Leopard 43
  • Leopard 51
  • Aquilla 44
  • Aquilla 48
  • Lagoon 43
  • Tennant Domino 66
  • Tennant PH8 54
  • Roger Hill Tenacity 66
  • Roger Hill Lola 66
 
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