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Old 27-03-2011, 16:32   #1
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small slow diesel?

hello guys and gals.

i see several threads about diesels for bigger boats but what about 25-30ft? i dont see many small diesels that are slow. reading threads most of the threads say you need a slow diesel to get the best GPh. but i dont see many small diesels that are slow. the small yanmars turn around 3600rpm or something like that. what is considered a slow rpm for a cruising diesel? i know you don't normally use the engine at its peak of 3600rpm but isnt that to high of a max RPM for cruising? i was looking at the smallest of the lugger line and have heard nothing of good praise about lugger (northern lights) but i was looking for more of around 30hp and lugger is 40hp. i see volvo pentas have smaller engines and they turn around 3200max rpm and i see a lot of people with volvos.

when looking at cruising engines. you need the hp required to move your boat properly at the cruising rpm you cruise right? not the max engine hp and then you cruise at a lower rpm resulting in a lower hp?

so my questions are
what is a good max rpm for a small cruising diesel
what are good brands

and what ever you want to add to this.

thanks
John
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Old 27-03-2011, 16:35   #2
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Re: small slow diesel?

a lot depends on the hull design. Look at Ranger Tugs, they use small diesels.
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Old 27-03-2011, 16:38   #3
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Re: small slow diesel?

When I used to own a farm tractor I ran it at 1800rpm all day every day.
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Old 27-03-2011, 17:26   #4
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Re: small slow diesel?

I have a 32 foot hunter, it has a Yanmar 2 cylinder, 18 hp diesel. Max RPM is 3200 rpm for ever and 3400 rpm up to an hour. I have found no real change in boat speed between 3000 and 3400 rpm. So, I choose to not run it over 3000 rpm. This results in a boat speed (using GPS) of approximately 5.5 - 6 knots. Hull speed should be somewhere around 7 knots. I have seen 7 knots under sail but wind speed was 25- 30 knots. I keep the bottom clean (boat was hauled in 2008 and has had a service cleaning the bottom ever month since then).

As for fuel, I was out 46 days in one year, and used 25 gallons of diesel that year. Based on engine hours, this was ~1/3 of a gallon per hour.. Since my fuel tank gauge does not work, I track hours and fuel consumption closely, and still rarely put in more than 8 or 9 gallons at a time.

Most of the time, the diesel is only used for getting into/out of the slip, however going from San Diego to Oceanside (or the other way) in one day can be a bit of a challenge without the motor, also returning from Catalina to San Diego is a 15 hour trip motoring most of the way (started at 0030 and arrived at our slip at 1530).

I have heard a friend who is a Diesel engineer (worked on really big diesels) and is also a sailor, he states that for real developed horse power with a diesel expect 1 gal per hour per 18 horse power. Based on this, and that my boat will not get to hull speed with my engine, I suspect that a better matched prop might be needed.

I am also curious as to why you want a 30 horse power diesel for an 25-30 foot boat it you want maximum gallons per hour?
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Old 27-03-2011, 17:39   #5
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Re: small slow diesel?

If I were going to spec an engine for a displacement speed cruiser, I would be looking at an engine that produced about 2 hp per 1000 lbs of boat displacement. This will usually push you to displacement speed using 2/3 of that horespower and the other third is in reserve for pushing against wind and seas in an emergency.

A 25-30' boat is going to weigh 10,000 lbs or less, so 20 hp would be enough. A Yanmar, Volvo, Beta Marine, etc would all do the job. In the case of the Yanmar you would be running it at about 2,400-2,600 rpm to produce enough power to reach hull speed. That engine will run for many, many thousands of hours at that speed. So will the others.

Many, many years ago small diesels were rated at 2,400 rpm max at wot and you cruised them at 1,800. Not any more. But engine longevity and fuel consumption aren't the worse for it.

David
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Old 27-03-2011, 18:16   #6
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Re: small slow diesel?

Your question was not answered and I'm unfortunately not that qualified to answer it either. Slow diesels run max power at about half of what we see in Yanmar speeds now. If you remember the old Popping Johnny John Deere engines of yesteryear it seemed like they fired and ran well at about 900 rpms. I just don't think they make a marine version that is a slow diesel.
Perkins is slow in comparison to Yanmar and so is my Mercedes but they are big and heavy for your use. I'd look at the one lung Volvo if you can find an old one. It would have a compression release lever and you could start it by hand.
kind regards,
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Old 27-03-2011, 19:14   #7
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Re: small slow diesel?

The specs on Bukh are interesting,the smallest is a 2 cyl, 500# rated at 24hp @ 3600 rpm. Same engine is rated @ 19 hp if turning 2400 rpm. The largest 48 hp 3600 rpm puts 31.5 hp out @ 2400 rpm. Obviously you save a lot on fuel doing lower revs but lots more horses in the stable if needed. Engines are available 24,32,36 and 48 with or without saildrive. For those not familiar Bukh engines are used in a lot of lifeboats on European shipping.
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Old 27-03-2011, 21:37   #8
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Re: small slow diesel?

my top picks for the diesels are yanmar and volvo.

what company has a better reputation? what will last longer? easier to repair? availability of spare parts? easier to use?

or any information about the company's

thanks
John
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Old 27-03-2011, 23:38   #9
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Re: small slow diesel?

Small, low speed diesels seem to have been driven from the market place by the new high speed diesels. These were good engines but all were large and heavy for the horsepower that they produced and were produced in fairly low volume. The high speed diesels from the small tractor manufacturers like Yanmar, Kubota, Mitsubishi are almost half the weight and, with the economy of scale, cheaper.

Volvo made the MD Series of 1, 2 and 3 cylinder engines that were designed from the water up as marine engines. They could be rebuilt, all except the crankshaft, in the boat. With their heavy flywheel and compression releases, they were easily hand cranked. Had the two cylinder in a Westsail and it proved reliable and extremely parsimonious in its consumption of fuel. Hand cranked it for years after the dyanamotor starter fell off. It wasn't overly powerful, however, and the hp claims may have been a bit inflated. If you could find a used MD1, that wold be a good engine for a smallish cruiser. Might want to check if Volvo still has parts for these engines, though.

Another mfg of low speed diesels is/was SABB. They may no longer be in business but they were the ultimate basic, heavy, slow speed marine diesel built in single and 2 cylinder varieties. They were the mainstay of the small fishing boats in the Baltic and North Sea and lifeboats. Designed to be hand started with variable pitch props, their ability to push heavy boats belied their low rated horsepower.

Good luck in finding a new slow speed diesel but afraid you'll have to find a used engine.
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Old 28-03-2011, 01:47   #10
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Re: small slow diesel?

Volvo has a reputation for charging an arm and a leg for their parts.
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Old 28-03-2011, 02:09   #11
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Re: small slow diesel?

Quote:
Originally Posted by roverhi View Post
Small, low speed diesels seem to have been driven from the market place by the new high speed diesels. These were good engines but all were large and heavy for the horsepower that they produced and were produced in fairly low volume. The high speed diesels from the small tractor manufacturers like Yanmar, Kubota, Mitsubishi are almost half the weight and, with the economy of scale, cheaper.
True. This side of the pond the likes of Sole and Lister have all but disappeared or the low volume for canal boats means they cost the earth in comparison to the more common Volvo, Beta or Yanmar. However, slow turning and lowly stressed they seem to last a lifetime in a canal boat.

Pete
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Old 28-03-2011, 02:14   #12
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Re: small slow diesel?

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Originally Posted by SkiprJohn View Post
Volvo has a reputation for charging an arm and a leg for their parts.
There is only a small saving by buying consumables from a motor factor rather than the genuine Volvo ones. The free advice from the local Volvo dealer has also been invaluable in the past.

Pete
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Old 28-03-2011, 17:30   #13
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Re: small slow diesel?

I have a 6 cyl perkins in a 34' Mainship trawler. My 8.5 kt cruising rpm is around 2050. It's a comfortable speed, keeps the engine at temp, no smoke, and reasonable fuel burn. Slow is a relative thing...

Tom
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Old 28-03-2011, 18:02   #14
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Re: small slow diesel?

Roverhi is correct in my opinion. The demand for higher turning, lighter and relatively efficient diesels have forced the older, smaller slow turning engines out of business. If you want a 'relatively' new, like 50 year old engine, raw water cooled with a replaceable cooling jacket, an 18 HP, single cylinder Farrymann, (german make I believe because I had to have a german girlfriend translate the manual) was dependable and would run for an afternoon on the wiff of a greasy rag! To really go retro, try and find an old rebuildable Easthope single banger. Had a big flywheel in the front that you would grab and throw around after hitting the decompresion lever to start her. They were popular in Monterey trollers of the 40's and 50's as well as many of the PNW trollers fishing inland waters in the same period. Very lean on fuel, dependable, cheap and had the distinctive 'put-put-put' sound. Boy, has that dated me!! cheers, Capt Phil
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Old 28-03-2011, 18:38   #15
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Re: small slow diesel?

I did not find Volvo parts any more expensive than Bukh, Vetus or Yanmar. All depends on where you are perhaps.

The newest small Volvo is slower revs than the earlier model, give it a look. The smallest from Vetus may also do the job. I think Vetus is Mitsubishi, Volvo is Perkins.

The Volvo has a 115 A alternator, if it counts for you.

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