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Old 11-16-2017, 01:02 PM   #1
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How much engine do I really need?

I am searching for my next boat, a trawler, after having nothing about sailboats. My questions centers around how big a Diesel engine do I really need in a 32-34’ semidisplacement trawler? For example I see 6 cylinder Cummings diesels putting out between 220-370 HP in the same model trawler. Apparently the additional HP comes from adding Turbos and aftercoolers to the same basic engine and, to my mind, that is asking for trouble/additional expense with very little upside. Like I said my experience is sailboats and this would be my first pure power boat so help me out here.
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Old 11-16-2017, 01:22 PM   #2
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Unfortunately unless you custom build or spend tons on repowering, gearbox, prop, etc. the engine that comes with the boat is the Hp you will live with.

The Hp numbers you see are to get the boat into the double digits speed wise as they are semi displacement.

Can’t comment on derating an engine by removing all the goodies, but would think a gearbox ratio change and possibly prop would be needed....again an output of money some would say is unnecessary or cost prohibitive.

Have fun looking
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Old 11-16-2017, 01:24 PM   #3
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It depends on how much you anticipate using the "semi-" part. And then, how fast you want to go.

Me? I decided I did not want to lug around the weight and expense of 380hp and only ever using 30hp of it. I am also ex-sailor and am perfectly happy at 6knots. Forever.
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Old 11-16-2017, 01:41 PM   #4
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But how do you buy a production boat, and leave behind the “extra weight and 350hp”.

I think that will be the issue. I too only need 30hp and travel at 6kts. But the boat came with 120hp.....
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Old 11-16-2017, 01:44 PM   #5
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Check out the American Tug. Can't hurt to look at the AT site and read the specs. One very nice thing about the AT is, NO exterior teak to maintain. There is a broker up in Stuart where I bought mine.
I have a 2008 and it is documented as a 34ft. You can look at the 36ft which has the same internal and external dimensions as the 34ft but the newer boats add in the swim platform for OAL. The standard engine is a Cummins 380.
Boathealer is correct. If you get lots of HP nothing says you have to use them all. You can drive it at 6-8 knots very efficiently. Once you exceed the cruising speed, fuel consumption can jump up. The advantage of 'reserved' HP is, 'getting there faster' or outrunning the weather.
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Old 11-16-2017, 01:46 PM   #6
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But how do you buy a production boat, and leave behind the “extra weight and 350hp”.

I think that will be the issue. I too only need 30hp and travel at 6kts. But the boat came with 120hp.....
Yes, full-displacement. And yes, likely in the 100-200 range. But much better than 380!
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Old 11-16-2017, 01:52 PM   #7
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Adagio

All of us with diesels that are too big de-rate frequently. It is via throttle setting. Read Ski's take, posted today, on how he operates his vessel with a big Cummins. Look under Power and User's Opinions ------

As Sealife says, unless a new build you may not have a lot of choice on engine size.
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Old 11-16-2017, 02:02 PM   #8
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Like the man said, it all depends on how fast you want to cruise. A 32-34' semi-displacement hull trawler will need 150-200 hp to go 15 kts. A Cummins 6BTA, a Yanmar 6LP or 6LY will put out that much hp and won't unduly stress the engine.

The alternative is an older boat with a NA Lehman or Perkins. Those engines will weigh nearly the same as the Cummins/Yanmars above, so you aren't "lugging around any extra weight". The weight is needed to make 135 hp without a turbo. Also there are very few less than 20 year old boats in that size that don't have turbo engines and most have sea water cooled after coolers.

The sea water cooled after cooler is only a little extra maintenance: service it every couple of years if in sea water or every five years if fresh water flushed. You can do it yourself in about 4-6 hours and $100 in o-rngs (Yanmar prices).

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Old 11-16-2017, 02:37 PM   #9
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The Nordhavn had a John Deere naturally aspirated engine. I think it was 120 hp. Nothing fancy and could run all day at 7-8 knots. The Nordhavn carried 1000 gal of diesel fuel.
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Old 11-16-2017, 03:38 PM   #10
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Originally Posted by boathealer View Post
It depends on how much you anticipate using the "semi-" part. And then, how fast you want to go.

Me? I decided I did not want to lug around the weight and expense of 380hp and only ever using 30hp of it. I am also ex-sailor and am perfectly happy at 6knots. Forever.


Where we boat the 100km tidal river runs at 6 to 8 knots in places without 330 HP or 220hp in the IG32 we would be going backwards and be restricted by tide to when we go out
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Old 11-16-2017, 04:16 PM   #11
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Having some additional HP to overcome currents was why I asked for a 380hp engine in our new build of a 43 SD hull. We will spend the majority of our time at hull speed, but I wanted the ability to ramp up to 8 or 10 on a longer run, or make better time due to weather. Destinations are far apart in so Cal so having some more speed if needed is nice to have.
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Old 11-16-2017, 05:16 PM   #12
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Thanks to everyone for your responses. I can see where local conditions may lead you to a higher horsepower engine. Being a former sailor I find 8 kn to be very fast especially if I can go in any direction I want ��.
I think with everything else being equal I would pick the lower HP alternative and avoid the expense and maintenance of the higher HP version.
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Old 11-16-2017, 05:55 PM   #13
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It sounds like you have your boat all picked out and are just debating engine options. That being the case and if you don't want to go fast, then pick the Cummins 6BT 210/220 hp engine. It has a coolant cooled after cooler just like Dodge pickups and requires no special maintenance.

Make sure it is propped right so that it turns 2650 or a bit more at wot and you can run it at 2,400 continuously if you need to, producing 150 hp. But make sure that at that rpm you are well over the hump. Staying within the hump will overload the engine which might be the case with an American Tug 34 or even a Nordic Tug 32 depending on how loaded you are.

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Old 11-16-2017, 06:03 PM   #14
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One serious thought, one not so serious.
The manufacture's published data generally involves a lightly loaded boat. WE all load our boat with everything we can squeeze in. This will have an effect on fuel efficiency.
Consider the excess part of the engine as ballast or an "internal keel."
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Old 11-16-2017, 06:52 PM   #15
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Quote:
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It sounds like you have your boat all picked out and are just debating engine options. That being the case and if you don't want to go fast, then pick the Cummins 6BT 210/220 hp engine. It has a coolant cooled after cooler just like Dodge pickups and requires no special maintenance
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I believe that is Tony Athens favorite engine?
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Old 11-16-2017, 07:47 PM   #16
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Adagio43-Be careful of paralysis by analysis. As Sealife stated, the engine in the boat you buy is the one you will live with. Unless you are in the very distinct minority of boaters that can specify every parameter of their new boat, either via a (probably limited) options list on a new build, or a custom design, the boat that ultimately makes you fall in love already comes with an engine attached.

Go aboard a bunch of candidates, charter if desired until clarity occurs, sea trial a bunch, and get great surveys. Anything else is simply noise on the airways. You know what people say about opinions. And most of them are stinky.

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Old 11-16-2017, 11:05 PM   #17
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I'd say the difference in maintenance with a turbo and aftercooler is likely negligible and perhaps no difference after a year or two of overall trawler maintenance. It will be far more important to find a boat with good bones... Or you will end up spending a lot more on other issues.
Go to the March Trawlerfest in Stuart; good opportunity to see a lot of different trawlers.
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Old 11-16-2017, 11:30 PM   #18
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The real issue with turbos and aftercoolers is life span between overhauls. If you run flat out the exhaust gas temps will slowly eat the metal parts involved with the cylinders and valves. However, it rarely hurts to run your engine at lower rpms, along with less fuel burned. Derating isn't easy in many engines. Some have different pistons, cams, injectors for turbo engines.
The issue of having the excess hp for use in strong currents is important in the PNW, but maybe not in Florida.
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Old 11-17-2017, 01:07 AM   #19
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I hope our prolific member OCDiver pipes in here. He spent the time and money refitting his 45' Cherobini Independence from a 380 Cummins to a small John Deere for "trawler speed only" aspirations. Before the refit, I was with Ted coming across the Okeechobee Waterway and asked him to nail the big 380 to see what the boat would do. It got to 9 knots or so and was looking at the sky. The wake was beyond huge, but that was pretty much it. At a sedate 7.5-8 knots, the boat was the ideal cruiser. Even though the hull is a semi-displacement design, the 380 was still not enough to get it on top. Now with the modest HP John Deere, it's the perfect trawler. From time to time, money, determination and the sacrifice of a couple of knots can make a better boat out of a good one.
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Old 11-17-2017, 01:12 AM   #20
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I hope our prolific member OCDiver pipes in here. He spent the time and money refitting his 45' Cherobini Independence from a 380 Cummins to a small John Deere for "trawler speed only" aspirations. Before the refit, I was with Ted coming across the Okeechobee Waterway and asked him to nail the big 380 to see what the boat would do. It got to 9 knots or so and was looking at the sky. The wake was beyond huge, but that was pretty much it. At a sedate 7.5-8 knots, the boat was the ideal cruiser. Even though the hull is a semi-displacement design, the 380 was still not enough to get it on top. Now with the modest HP John Deere, it's the perfect trawler. From time to time, money, determination and the sacrifice of a couple of knots can make a better boat out of a good one.
Did he have trim tabs ?
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