Yanmar v Cummins v ...

mcarthur said:

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)

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You have data showing at 11kts you burn 2.5lpnm, and at 13kts you burn 3lpnm. And you want larger engines to get you to 15-16kts at 2.3-2.5lpnm.

Not going to happen. Your Perkins are not the most efficient engines in the world, but anything modern might burn only 10% less for the same shaft hp created.

And to go from 13kt to 16kt will require about 68% more hp using 2.5 exponent. So at 13kt/3lpnm you are burning 39lph. At 16kt probably 65lph, or about 4lpnm. Even if new engines are 10% more efficient, you are still at 3.6lpnm. Well above your desired 2.3-2.5.

Ski is spot on.

I'd further add that the Perkins 135s may be the perfect cost effective and fuel efficient match for your Cat's hull form. Especially when taking into account the time, cost and work involved to change out engines, transmissions, shafts, props and shaft supports.

If there where any fuel savings to be had, how many decades of cruising would it take to recoup the cost of changeover and is it really worth the drama?

smitty477 said:

"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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The above is dead on and be prepared for all because once you hit the domino they just keep on and it is pretty much all ahead full with no reverse....There ain’t no turning back.

Thanks Ski, Smitty, Sunchaser, Mule, Simi. I appreciate it.

We were perhaps being led along the garden path (ie. told fibs) when mechanics and others were saying we should see at least 20% better consumption in the newer, larger engines compared to the old.
The old have better longevity probably, and certainly ease of fixing away from major hubs, but we were being told that the fuel consumption would be much better. This was the basis for my "what we would like".

I think we'll go back to the drawing board on consumption - . Bigger engines = more top-end speed . But lots of expense in the changeover .

"but we were being told that the fuel consumption would be much better. This was the basis for my "what we would like"."

A super modern engine with High Pressure rail electronic fuel injection will certainly burn less fuel at the same load.

Weather its 10% better or 20% better will probably never pay for the change over.

The inability to repair these electronic jewels with out great expense and expertise is a hassle. Even bigger is the hassle of cleaning up most fuels to be acceptable to use.

At least a bank of 3 filters to get the fines .PBB explains in a past issue.

For good fuel economy I would install a method of measuring fuel use underway. The old Flow Scan was ideal , but you might have to use a calibrated tank and a stop watch.

Create a speed / fuel burn graph at say 200RPM intervals , and you will easily see where speed becomes expensive.

As an ex multihull cruiser I know its hard , but all multihulls MUST be as light as can be , pilling them down with extra gear immerses far more hull than a lead sled with the same ton of extra goodies.

The cheapest way to get better MPG is to leave a few tons of "stuff" on shore.


Since you can beach to change props , a set of props might change the numbers a bit , but even 5% might take 2-or 3 prop changes to get perfection.

Such an interesting thread. One question: do the currently installed Perkins work well? And, if so, how many hours (thinking, if they do work well, how long for?).

One comment: I note you do not want the expense and complexity of common rail diesels.

These considerations are a perfect example of the 'principle of diminishing returns'. If you add the 'principle of total costing' then staying with what you have is far and away the best choice, IF they are working well.

Keeping the Perkins and changing nothing seems to be a reasonable and inexpensive option.

WOW...with what Smitty477 had to say, I think I would leave things alone until I found a boat with the added HP....IMHO.

Why not replace the Perkins 135's with rebuilt Perkins 200's? Power boost with minimal mods.

Kit_L said:

Keeping the Perkins and changing nothing seems to be a reasonable and inexpensive option.

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Agreed. Maybe go on an exploration of weight reduction instead? There was concern about motor weight, but there have to be opportunities elsewhere to reduce weight, especially if you redirect your repower budget.

Thanks Kit, FF, Jeff, HighW, VitC.

The current engines are probably fine for a time - 3000 hours, but no maintenance records. While we are waiting the engine survey for pressure and oil analysis, the surveyor has pointed at the exhaust outlet - freshly painted antifoul: "see this, we just went across harbour at low speed to the liftout, and there's not even a hint of smoke in the air when we started or got going, or anything on the paint at all. You've got good motors".

I would guess that the fuel savings for common rail would eventually become a neutral changeover. We are intending the Atlantic and Pacific over the next years, so quite a few nautical miles to come.

We probably will just leave the existing motors, given everyone's help: there seems little fuel consumption benefit, and lots of complexity and cost. We will have to do without the mid-range and top speed increase we desire (which was cruising 9kn, not 6-7, and top speed 17+kn, not 13-14).
Or we could buy Domino

Multi hull or lead sled long range cruise is always about the same efficient speed. SL.

SL is the sq rt of the LWL times .9 for fat heavy boats to times 1.15 for light skinny hulls.

6k or 7K is the common result for most cruisers, easy to plan for when route contemplating or at the A&P loading up.
120-140 miles a day is quiet and usually hassle free AP steering..


3000 hours is barely mid life , just well broken in.

mcarthur said:

".
. We are intending the Atlantic and Pacific over the next years, so quite a few nautical miles to come.

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I would love to know what 50ft design you have bought that can do that.
Not implying it can't be done, I'm sure it can, but shes obviously not your average boat.

Hi Simi - give me a few more days and I'll post about her more...

Wait..surveyor? You don’t own this boat yet? Let it go and buy something more ideal. I was going to suggest selling it but reading later in the thread it appears you don’t own it yet. This is an enormous undertaking that will never pay you back.

FF said:

Multi hull or lead sled long range cruise is always about the same efficient speed. SL.

SL is the sq rt of the LWL times .9 for fat heavy boats to times 1.15 for light skinny hulls.

6k or 7K is the common result for most cruisers, easy to plan for when route contemplating or at the A&P loading up.
120-140 miles a day is quiet and usually hassle free AP steering..


3000 hours is barely mid life , just well broken in.

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What he said.

refugio said:

Wait..surveyor? You don’t own this boat yet? Let it go and buy something more ideal. I was going to suggest selling it but reading later in the thread it appears you don’t own it yet. This is an enormous undertaking that will never pay you back.

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Thanks - yes, I'm hearing what everyone is saying about cost.

Regarding the boat, there are many other reasons she is a good boat for us. The engines - which are probably just in run-in mode - are the only serious question (ie. can we get a faster "power mode" speed, not long distance cruise speed).

I haven’t read all of this thread but what I have seen is someone looking to make a large increase in power and I haven’t seen anything about this particular hull and specificly whether or not it can benefit from more power. Anything close to a FD hull wouldn’t tollerate that and probably a hull anywhere close to FD but a SD hull probably wouldn’t tollerate that either.

But a hull closser to a planing hull would probably work well w more power. What kind of hull is this?

OK I see post #41 and it sounds like the OP has made good use of the posts on his thread. The hull is either a planing hull or a high speed SD hull. Another win for TF.

Definitely a win for TF - thanks all.

The boat is a displacement power cat, 12.3:1 length-to-beam waterline. It could definitely benefit from more power - probably "limited" to about 22-25kn given size of engine room and fittings (based on other boats). But I'm learning that it would likely be at a serious expense and equipment pain. Go TF

So now, to demonstrate my ignorance of diesels and show everyone I didn't grow up in a diesel's cylinder with oil on my hands ... is it feasible to convert, in-place, the existing Perkins M135 1006-6 models to 1006-6T or 1006-6TW? The first is a turbo version while the second is turbo + aftercooled. Visually, the 1006-6 looks different to the other two, so I assume it's not quite the same as adding an OEM or after market turbo-charger...?

Worth exploring,the different engine price is just the start of an engine swap. I had the old Perkins 6354 and think it came in 3 versions. Mine was the full catastrophe, ie + turbo + aftercooler. Question is, were they beefed up anywhere to shoulder the extra output?

"Question is, were they beefed up anywhere to shoulder the extra output? "

Most never are , the modern concept of a diesel operating life is lifetime fuel burn.

If you plug along at 2-5 GPH the engine will eat a lot of diesel before its overhaul or replacement time.

Roughly the same amount of fuel will be used if you consume it at a 15 or 20 GPH rate , before overhaul time.

This assumes the engine was neither under loaded or overloaded for long periods of time..

mcarthur said:

Definitely a win for TF - thanks all.

The boat is a displacement power cat, 12.3:1 length-to-beam waterline. It could definitely benefit from more power - probably "limited" to about 22-25kn given size of engine room and fittings (based on other boats). But I'm learning that it would likely be at a serious expense and equipment pain. Go TF

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22 knots with 36,000 pounds even in a Cat will be requiring some real HP.
And if it could reach 22 knots the actual realistic cruising speed would be much lower.

FF said:

"Question is, were they beefed up anywhere to shoulder the extra output? "

Most never are , the modern concept of a diesel operating life is lifetime fuel burn.
...

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Thanks for that.

So you feel most/all of the elements of the actual motor itself would be the same between a naturally aspirated, turbo, and turbo+aftercooler versions?

Certainly some of the externals are different - looking at the pictures, there's different placement of some filters, oil filler, etc.

I'm seeking a feeling on whether the differences are purely cosmetic (e.g. had to change the position of the filter because of adding part X), or indicate an internal difference. The bore and stroke sizes are identical, which still could mean they changed the materials from unobtanium1 to unobtanium2 to cater for the higher pressure and power...

smitty477 said:

22 knots with 36,000 pounds even in a Cat will be requiring some real HP.
And if it could reach 22 knots the actual realistic cruising speed would be much lower.

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Definitely. We aren't interested in that speed but I gave it as an example that some power cats aren't quite as speed limited. For example, similar displacement & length boats such as this Queensland 55 can (apparently!) do 24kn with twin 670hp's, or 20kn with twin 435hp's. Personally, I can think of better things to do with my money and highly compressed dinosaurs than spend 240L/hr going 24kn . (that figure is about the same as business class or even first class airfares across oceans! While I realise that doesn't include hauling your "home" and possibly 6-8 other people at the same time, it's still a rather-large-load of cash!)

Or Domino, cruising 20kn at 23gph (87L/hr) with twin 300 hp's (proven over many nautical miles).

Or the Aquilla 44 maxing 21kn at 30gph (114L/hr) with twin 300 hp's.

"So you feel most/all of the elements of the actual motor itself would be the same between a naturally aspirated, turbo, and turbo+aftercooler versions?"

This seems to be most common , although sometimes the really big power engines will use different pistons , to lower the compression ratio at high RPM .

These can be a bit harder to start in below freezing weather.

The most reliable diesel engines seem to be the "Naturals" no turbo , no before or after coolers. Hard to wear out what is not installed.

Gas engines ARE different , the "real truck" (not pickup) versions may have salt cooled valves in a heavier duty (different metal) block and heavier crank and con rods..

mcarthur said:

Definitely. We aren't interested in that speed but I gave it as an example that some power cats aren't quite as speed limited. For example, similar displacement & length boats such as this Queensland 55 can (apparently!) do 24kn with twin 670hp's, or 20kn with twin 435hp's. Personally, I can think of better things to do with my money and highly compressed dinosaurs than spend 240L/hr going 24kn . (that figure is about the same as business class or even first class airfares across oceans! While I realise that doesn't include hauling your "home" and possibly 6-8 other people at the same time, it's still a rather-large-load of cash!)

Or Domino, cruising 20kn at 23gph (87L/hr) with twin 300 hp's (proven over many nautical miles).

Or the Aquilla 44 maxing 21kn at 30gph (114L/hr) with twin 300 hp's.

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" similar displacement & length boats....24kn with twin 670hp's,"
Yes of course - there is your 24 knots with 1,340 hp and about 3 gals/mi or about 11.4 L/mi.

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

Is that per engine or total burn?

mcarthur said:

Thanks for that.

So you feel most/all of the elements of the actual motor itself would be the same between a naturally aspirated, turbo, and turbo+aftercooler versions?

Certainly some of the externals are different - looking at the pictures, there's different placement of some filters, oil filler, etc.

I'm seeking a feeling on whether the differences are purely cosmetic (e.g. had to change the position of the filter because of adding part X), or indicate an internal difference. The bore and stroke sizes are identical, which still could mean they changed the materials from unobtanium1 to unobtanium2 to cater for the higher pressure and power...

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Don't forget fuel delivery. The injection pump is likely a limiting factor. Also cooling capacity, exhaust sizing, and power delivery - you'd need to change props and/or gear ratio, maybe beef up shaft size, etc. These components are (or should be) integrated into a complete system. Trying to get more max power is not as simple as it might appear.

smitty477 said:

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

Is that per engine or total burn?

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Consumption is total, adding the two.
Speeds are when both are running. I believe it's possible with the gearbox to freewheel with a shutdown engine, so the lower two speeds could possibly be done with one engine (at different consumptions; the hulls are hard chine and deep so there would possibly be little rudder needed to keep a single engine on track).