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Old 09-03-2018, 11:47 PM   #1
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Yanmar v Cummins v ...

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 .

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?
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Old 09-04-2018, 12:29 AM   #2
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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.
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Old 09-04-2018, 12:30 AM   #3
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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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Old 09-04-2018, 12:36 AM   #4
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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.
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Old 09-04-2018, 12:39 AM   #5
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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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Old 09-04-2018, 12:41 AM   #6
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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.
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Old 09-04-2018, 12:42 AM   #7
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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 ?

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.

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, 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.

Good luck.

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Old 09-04-2018, 01:28 AM   #8
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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 ). 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...
  • As a non-technical person who's learning slowly , what's the difference between the 12V and 24V cummins (apart from the number of valves !)
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Old 09-04-2018, 03:59 AM   #9
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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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Old 09-04-2018, 04:24 AM   #10
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Originally Posted by Simi 60 View Post
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?
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 .

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?
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Old 09-04-2018, 04:45 AM   #11
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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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Old 09-04-2018, 04:50 AM   #12
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Quote:
Originally Posted by Insequent View Post

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.
Exactly what I was trying to get before afternoon martinis had their evil way with me.
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Old 09-04-2018, 07:34 AM   #13
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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?
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Old 09-04-2018, 07:44 AM   #14
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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


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Old 09-04-2018, 07:54 AM   #15
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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.
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Old 09-04-2018, 08:01 AM   #16
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"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.
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Old 09-04-2018, 09:45 AM   #17
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Yanmar 6LY: Good compromise between size, weight and power
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Old 09-04-2018, 10:13 AM   #18
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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.


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Old 09-04-2018, 04:03 PM   #19
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What I was getting at
Cruising boat has 120hp

https://www.boatsonline.com.au/boats...-custom/199968

This one has 300hp

https://www.boatsonline.com.au/boats...owercat/221879


Charter boat has a whole lot more again

https://www.boatsonline.com.au/boats...tamaran/210520

I'd rather run the first one
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Old 09-04-2018, 04:18 PM   #20
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Then to totally mix things up 72ft of aluminium with 200hp x 2

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