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Old 01-29-2021, 11:14 PM   #21
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I was told CAT recommends 80% of WOT. For me WOT is 2800 rpms
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Old 01-29-2021, 11:42 PM   #22
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Pryos are one of the best tools you can have to tell you how hard you are working the engine.
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Old 01-29-2021, 11:55 PM   #23
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Yup. True. Maybe I should find pyros that will fit the 3208.
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Pryos are one of the best tools you can have to tell you how hard you are working the engine.
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Old 01-30-2021, 08:20 AM   #24
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Look at the HP curve and prop required HP curve for any engine and boat.

The prop required HP falls away very rapidly, far quicker than the HP curve does.

Diesels need to be loaded , but the engine load depends on the RPM of the engine.

IF the engine can create 100HP at say 1500, and your boat runs great at 1500 ,if the fuel burn is less than say 4GPH at the 1500 you are all set.

6GPH might be over the 100hp so would require extra care. Can you get another 200-300rpm with no black smoke? your fine.

An EGT gauge is a worthwhile investment for any long term cruising , tho folks have been using the no smoke 300 rpm up for a long time.

In theory you might be over propped as the eng mfg sees it. He wants the smallest load at all times so prefers a very high speed as prop check, so if you pull back at all the engine loads drop with the prop curve.

BUT the engine producing your 80 HP at say 2000 rpm will burn more fuel and wear at a higher rate. The pistons simply get more miles at 2000 than 1500 , and the engine load may not be high enough to properly seat the rings.The extra noise sucks.

Some folks prefer the higher engine loading at low RPM and install a cruising prop, others fear the practice as their brother in law may borrow the boat and try to tow water skiers.

IF you chose the cruising prop a throttle stop is some insurance the engine will not be overloaded.
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Old 01-30-2021, 11:07 AM   #25
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FF thanks for the detailed response! To make sure I understand what you're saying I've put some questions in red in the quote of your post. If you could address my questions that would be very much appreciated.

I've attached the curves for a 210 HP 3208 pulled from boat diesel.

Boat diesel's power required calculator estimates this boat needs 55 HP at the props to make 8 kts and 218 HP at the props to make 12.5 which was the max speed WOT on sea trials.

One thing I'm not sure about is boat diesel's power required calculator giving total HP required at the props? Or HP per engine at the props? Big difference.

I'm looking for the sweet spot for this boat and our enjoyment. A decent average RPM that gives an acceptable, to us, noise level and good fuel economy. Along with that and understanding I'm likely under loading the engines, a good end of day speed.

Typical so far has been 6 hrs underway per day. Between 1400 and 1800 RPM. Then about 40 min from the end run them up to 2000 for 30 min.

As I said up thread, she seems to be both over powered and under powered. A nice quiet hull speed cruise doesn't work the cats hard enough. Run 'em up and she doesn't really get up and go.

Quote:
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Look at the HP curve and prop required HP curve for any engine and boat.

The prop required HP falls away very rapidly, far quicker than the HP curve does.

Falls away in what sense? With engine and boat speed? When you say prop HP falls away quicker than HP are you talking about engine manufacturer's HP data at a given RPM?

Diesels need to be loaded , but the engine load depends on the RPM of the engine. Understood. Run the RPM up, load the engine(s) up.

IF the engine can create 100HP at say 1500, and your boat runs great at 1500 ,if the fuel burn is less than say 4GPH at the 1500 you are all set. What is special about 100 HP? Is that a general guideline? Specific to CAT 3208 NAs? Specific to this boat's engine + hull configuration?

6GPH might be over the 100hp so would require extra care. I haven't had the boat underway a lot. I don't have accurate fuel gauges or any fuel monitoring. Best rough wild guesstimate I have at this point is about 5 GPH when I tend towards the lower end of the 1400 - 1800 RPM range and about 6.4 GPH when I run more consistently at 1800. Generally where I boat I can't set an RPM and use it for an entire cruise.

Can you get another 200-300rpm with no black smoke? your fine. I can run 'em at WOT with no black smoke. The only time I've seen black smoke was departing a fuel dock, in wind and current, with bone heads crowding me to get in. I had to back her at near full throttle. She belched black smoke that time.

An EGT gauge is a worthwhile investment for any long term cruising , tho folks have been using the no smoke 300 rpm up for a long time. The work boats I spend my career on had pyros. Great info on how hard you're working the engine.

In theory you might be over propped as the eng mfg sees it. He wants the smallest load at all times so prefers a very high speed as prop check, so if you pull back at all the engine loads drop with the prop curve.

BUT the engine producing your 80 HP at say 2000 rpm will burn more fuel and wear at a higher rate. The pistons simply get more miles at 2000 than 1500 , More miles in the sense of how far the pistons travel? Higher RPM, more ups n downs, more piston distance? and the engine load may not be high enough to properly seat the rings. That is my concern with running at lower RPMs. The extra noise sucks. Yes it does. And as I said up thread, higher RPMs means more noise, bigger wake, deeper hole, more fuel burned, not enough more speed to justify fuel burn.

Some folks prefer the higher engine loading at low RPM and install a cruising prop, others fear the practice as their brother in law may borrow the boat and try to tow water skiers. PO told me I'm over propped. Engine survey at sea trial verifies that. Max observed RPM 2480.

IF you chose the cruising prop a throttle stop is some insurance the engine will not be overloaded.
Attached Thumbnails
Prop and power curves.jpg  
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Old 01-31-2021, 08:25 AM   #26
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"Falls away in what sense? With engine and boat speed? When you say prop HP falls away quicker than HP are you talking about engine manufacturer's HP data at a given RPM? "

The power required to spin a prop depends on its RPM, lower the RPM a bit and less power is needed to turn it.

"What is special about 100 HP? "

Nothing is specific about 100hp, it is just an example

"I can run 'em at WOT with no black smoke"

GOOD! you are probably not overloaded at that RPM , so wont be at any lesser RPM.

The peak RPM and load is just that the max , operating at 80% is what the mfg would recommend for a fast boat to not go beyond , it is NOT a recommendation for a cruiser to obtain underway.

80% or less of the engines available power at the cruise RPM selected is what is needed .

Hope this helps .


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Old 02-05-2021, 02:40 PM   #27
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I have a single 3208 in my gb42 and at 6000hrs runs great. No oil usage. Burns 4 gals/hr at 1800 rpms which gives me 8 kts. 1700 gives me 71/2 at 3 gals/hr which I like better. With the single I have great access both sides. Space for alternator does limit to small frame 100amp. Starts at a touch of the button. Don’t know that you have to run high rpms at end of day with natural aspiration.
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Old 02-05-2021, 03:52 PM   #28
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Quote:
Originally Posted by DNT99611 View Post
Pryos are one of the best tools you can have to tell you how hard you are working the engine.
Interesting you mentioned Pyros. I didn't think mine were working because the gauges never start to read. My marine electrician says they are working fine. Then I thought there placement was the problem being in the stream of cooling water. No, says my CAT mechanic.

Ok back to more of the original questions. Mine are the inter-cooled 325 hp. I have 2100 hours on them. They are 1991 vintage. Stb engine had a major overheat for PO and it was sleeved and rebuilt. I get a fair amount of blue smoke in the morning that clears up on heating. This winter the smoke seems a little worse. I run her at 1600 - 1850 typically. I get about 1 gallon per mile at 10 knots in my 64,000 lb 55 Californian. The only real service problem has been the port oil heat exchanger. Since I have a second generator in front of the Port engine, it was extremely difficult to replace. My guy said it was only the third of such failures he had seen in 20 years. I just did a calculation that showed in the last 2.5 years I have averaged 7.54 gallons per run hour. So, sometimes (almost never) I am idling in my slip. Running 5 knots through the marina, or just running the generator at night. Seems pretty good though. I'm thinking I should double check that to be sure. Seems awfully light. I love the 3208s. My brother's boat has the 375 HP models. They don't really smoke at all. He also gets about 1 gallon a mile at 10 knots in a 48'. For me its CATs or newer Cummins.
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Old 02-05-2021, 06:03 PM   #29
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I have two Cat3208TA 375's with about 4400 hrs. They start cold (in the winter) in less than 5 sec cranking, burn a quart of oil about every 100 hrs with a 15-40 fully synthetic oil. Typically run at 1300 to 1500 with a daily run above 2100 rpm to clear the turbos. The boat a GB 49 classic runs about 9 knots in the 13-1400 rpm range at 1 1/2 miles per gallon and about 1 mile/gal at 1750 at 10.5 knots. The lower rpm is very pleasant to live with. 1800 range is acceptable. The engines warm to about 80C with no problem at the lower rpm.
I change water pumps --- the hardest part is removing the guard for access. Also fitting the tees and aligning is hard on the Jabsco pump on the bench if you replace to whole pump. So far I had good experience and the boat size allows reasonable access to both sides of the engine. I replaced the stock 50 amp alternator on one engine with a 100 amp model with Wakespeed 100 regulator same design and the other engine has a 160 amp alternator double belt and a sheave extension off the crank shaft to drive the larger alternator.
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Old 02-05-2021, 09:04 PM   #30
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My 3208 NA's have about 3800 hours on them, installed in a 1994 Ocean Alexander 43 foot trawler. These have been great trouble free engines, following a Cat dealer service by Blanchard Cat in Charleston some years ago.

As mentioned previously, Cat measures wear item maintenance in terms of fuel consumed rather than engine hours, which hours can vary all over the map depending on the application....smart!

So my 40'000 lb boat gets 2 miles per gallon at 8 knots, and i know this absolutely for certain due to long trips up and down the ICW with known mileage markers, and known fuel fill invoices.

Doing a bit of math, I burn 2 gallons per hour per engine at 8 knot cruise, . Cat recommends booking an appointment for rebuild at 30,000 gallons.....the math says that equates to about 15,000 hours. Over the 26 service years, that is about 140 hours per year for my boat hours, so that means.....107 years to rebuild at this usage rate!

Don't forget this engine was among the highest volume diesel engine ever built of all time. These engines were used in millions of military vehicles, ships, excavators, gensets, bulldozers, school buses, dump trucks, highway haulers, commercial fishers, etc. As such most were bought by industrial professional purchasing managers looking for best commercial value. You are standing on the shoulders of giants given the rigors of the selection process resulting with the huge attendant production volumes.

search Cat 3208 on ebay to get a sense of the huge supply network still in place today, and excellent value of replacement parts available.....just for fun check out volvo engine equivalent parts pricing for a true education.

That said, before buying the boat, get an engine survey performed by a Cat mechanic. All big heavy machinery needs proper maintenance to meet design life. Proven maintenance records and invoices are gold. All bets are off if any good piece of equipment is not maintained.

Great engines, easy to work on, reliable as hell, simple, and you can get parts and help anywhere due to their justly earned popularity.

IF....IF they have been well maintained and not murdered, these are among your best bet engines in the world.
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Old 02-08-2021, 11:22 AM   #31
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Originally Posted by Portage_Bay View Post
How hard to run 'em? Ski I welcome your input as well.

I've got twin 3208 NAs 1983 vintage 4000 hrs. Californian 42 LRC. I haven't had the boat very long for many hrs so still learning the engines.

I've heard I need to run 'em hard to make 'em happy. I've heard that about so many engines and it isn't necessarily true so I'm taking that with a grain of salt.

I prefer 1400 RPM. The boat is smooth, quiet and just slips through the water. 1600 is OK but noisier and starting to make too much wake. Still decent fuel efficiency. 1800 and she's getting noisy and starting to dig a hole, fuel efficiency is starting to suffer. 2200 where the PO said he ran them it's quite a racket and she's digging a pretty deep hole.

I know they are over propped. I don't know by how much.

So, last outings I've stayed in the 1600 - 1800 range and for the last 30 minutes of the day run 'em up to 2000 with a 5 - 10 min lower RPM cool down entering the marina and getting tied up. Since I've been doing that they smoke less on startup.
Think these engines will run forever at 1500 and is incredible for fuel flow, sounds like one of your interests. The difference between 1500 and 1700 is significant and 1800 even more so. Especially if you are not seeing a corresponding increase in speed (due to hitting hull speed), stay in your "smooth range" and enjoy the ride.
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