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Old 24-07-2009, 08:33   #1
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Cummins Repower

hi all ,i am just repowering my sailboat from a perkins 6.3544m not sure if its 150 or 180 hp ,turbo with borg warner gearbox (gear ratio 2.67) to a cummins 6bta 180hp turbo but im realy confused about ratios my friend said that 2.67 ratio is too high for fuel consumption and should go for a 2 to 1 ratio for fuel consumption etc ,i am also a bit lost about what type of box to get, I may change the cummins to 210hp later, aparentely this is possible with just a bit of recalibration.also if i buy hydraulic i will need a shaft break .allot of variables! and im in venezuela and have to import the box so no mistakes, worried!.any advice would be greatly apreciated!!
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Old 24-07-2009, 12:03   #2
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Its all about matching the prop. If you are happy with the current prop/engine/ratio combination (you can achieve rated enginecruising rpm at wide open throttle), that is a starting point.

Print out the propeller diameter (Three blade) chart from Proper Prop Size

See where your current engine hp/prop diam/shaft speed lies, and where the Cummins would lie both at 180 and 210 hp, and that should give you a good idea of the shaft speed you want. Divide that into the Cummins rated cruise rpm and you will get the ideal ratio.
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Old 24-07-2009, 14:13   #3
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sorry being a bit dim

thanks for your rapid response to my question , but im still not getting it.printed out the grapth but basically the new engine in capacity is more or less the same as the old perkins and to make matters worse I only ever used the perkins once at reasonably low revs. so im still wondering about the gear ratios. why did the perkins have ratio 2.67?
thanks again
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Old 24-07-2009, 14:41   #4
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Cummins will recommend a ratio for you. I have installed a lot of the 6bt series engines. To protect your warranty you might want to have them recommend the ratio. Thos e engines are finicky about very tight, leak free fuel filters and lines coming to the engine. Once you get it right, it'll be a great engine though.
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Old 25-07-2009, 10:37   #5
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The 6bta is souped up with an aftercooler to a continuous 332 hp at 2600 rpm. That's a lot more power than you are replacing. Are you sure you don't want the 6bt at a continuous 204 hp at 2400 rpm out of the same block? Cruising sailboats have different requirements than sportfishermen.

Any idea of what prop you have--at least the diameter? If you let Cummins recommend a ratio, you will probably have to reprop. If you are jumping up to over 300 hp, you better make sure the shaft diameter is up to it too.
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Old 25-07-2009, 11:30   #6
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I have twin Cummins 6BTA, 332 HP twin engines on board that were installed about two years ago. Overall I am very happy with these engines and highly recommend them. Being computer controlled, they are quieter, more efficient and cleaner burning than were the old B series engines that they replaced. The valve timing and fuel metering is computer controlled.

As donradcliffe says, you probably don't need the turbocharged versions of that engine. If your boat is a full displacement boat and it is reaching hull speed with the old engine then the additional horsepower is a waste of money in terms of not only engine cost but a beefier transmission, possible larger diameter propeller shaft, larger propeller and fuel consumption.

Amongst the B series engines, there are 7 different marine versions each with a different horsepower rating. For a cruising yacht, I would go without the turbo/intercooler versions if you have that choice....for the sake of simplicity and user serviceability.

Have you consulted someone or ran the numbers yourself for what would be the maximum practical horsepower for your boat? I would start there first before making any other decisions.

Also, get the Walker Airsep's (which allows the engine to burn crankcase blowby which is filled with atomized oil) and a dual Racor fuel filter system, which allows you to change fuel filters by moving a handle, eliminating the necessity to bleed the fuel system. Both are very much worth the price.

Get the "SmartSystem"...it gives you lots of good engine information such as fuel flow, total fuel burned and other important information like transmission oil temperature and pressure. If you interface your GPS NMEA out to the SmartSystem it will give you fuel burned per mile.

Also, with this engine, when you turn the key to Run before pushing the Start button, you will hear an electric fuel pump come on before you start the engine...which is a really nice thing to have if your engine ever loses its fuel prime.

I added a shunt to the positive side of the alternator so I could see how much current it is producing. Don't let anyone tell you that you cant do this...(as I was told). If you add a "shunt shifter", made by Blue Seas, then you can read the output of the alternator(s). It really helps at times to know how hard they are working. Put your alternators voltage sensor on the battery side of the shunt to compensate for the small voltage drop across the shunt but not on the battery side of the alternator fuse.

BTW, if anyone else with twin engines is interested, the load on the alternators balance out very well without adding any fancy electronics to control the alternator field current....its usually within 5 amps of being equal. If you do have a large imbalance then Balmar sells a system that corrects a large difference.
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Old 26-07-2009, 12:40   #7
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hi, thanks for all the advice, will measure my prop.
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Old 28-07-2009, 07:31   #8
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hi,measured my prop and its 22 inches 3 blade,thanks
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Old 28-07-2009, 21:24   #9
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According to several experts, an engine that puts out 150 hp at 2600 rpm with a 2.67 ratio on the transmission should be matched with a 28 inch diameter 3 blade prop, while 210 hp at the same rpm and ratio would require a 30 inch prop and the 330 hp engine would require a 32 inch prop. Dropping the ratio to 2.0 would drop the matching prop sizes to 23, 25, and 27 inches respectively.

Looking at the Yanmar site PROP TABLE LH Series
they recommend a 2.0 transmission with a 23 inch prop for their 4lha-stp model, which is rated at a continuous 190 hp at 3100 rpm. Because the Cummins is a lower reving engine and you have a smaller prop, you should probably choose a ratio close to 1.7-1.8

Don't ask me why the Perkins had a 2.67 ratio with that prop--the PO probably got a deal somewhere.
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Old 28-07-2009, 21:39   #10
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Don't guess!

I highly recommend this book:


Amazon.com: The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers (9780071381765): Dave Gerr: Books

When I re-engined my boat this book provided the formulas I needed in order to determine the correct sized propellers. BTW, I nailed it! I got the right pitch and diameter down to the inch.
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Old 29-07-2009, 02:32   #11
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The Propeller Handbook ~ by Dave Gerr
is available on-line at GoogleBooks:
The Propeller Handbook: The Complete ... - Google Books
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Old 01-08-2009, 06:10   #12
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hi, i think don is right about the previous owner getting a deal on the engine I found out that the perkins engine is much older than the boat and the 2.67 ratio bears no relationship to anything! and im going to go for a zf gearbox ratio 1.9, thanks again this is my first post and its been a real help.
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