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.

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

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.

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.....

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.

Sealife said:

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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Yes, full-displacement. And yes, likely in the 100-200 range. But much better than 380!

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.

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).

David

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.

boathealer said:

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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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

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.

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.

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.

David

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."

djmarchand said:

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
David

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I believe that is Tony Athens favorite engine?

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.

Regards,

Pete

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.

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.

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.

healhustler said:

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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Did he have trim tabs ?

If buying an older boat which may require re-powering during your ownership, it is much easier to swallow the bill for new little 50hp baby rather than 2 x 300 hp diesels.

If you don't plan on exceeding hull speed and normally cruise efficiently one knot slower, there is no need to exceed 80 HP. (Have a four-cylinder, naturally aspirated John Deere.)

markpierce said:

If you don't plan on exceeding hull speed and normally cruise efficiently one knot slower, there is no need to exceed 80 HP. (Have a four-cylinder, naturally aspirated John Deere.)

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Problem is Mark thanks to decisions of a past administration that excellent US built engine is no longer available for sale in the US.

But obviously many other 80hp engines are.

I’ve heard of Bob Sinter. The Northern Lights people seem to have figured it out.

Larry (healhustler) seems to have arrived at the same conclusion. “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.”

A real trawler person waits for the tide to change or is a planner (like Sunchaser) and arrives at the approprate time not to need more power than what is required to drive the boat. For SD that’s about exactly 7hp per ton of displacement.

While I appreciate Bob Sinter's thoughts and I believe he is right for constant rpm generator engines, I do not believe he is right on his load requirements for propulsion engines.

I can not understand why it is ok to run an older Lehman at 50 hp (40% of 120) but it is not ok to run a 370 hp Cummins of the same displacement at 50 hp (14% of 370). What is the difference?

Yes the Cummins has three times the cooling capacity and a turbo and aftercooler. But the thermostat takes care of keeping the engine warm and the turbo and after cooler might as well not be there at the low rpms it takes to make 50 hp. Yes the fuel injection pump and injectors of the Cummins can provide three times the fuel as the Lehman and at low power that injection system is operating well down on its operating curve so its spray pattern might not be as good. But then lets talk about the QSB whose common rail injection system solves all of that.

Why can't you run a Cummins QSB 380 at 50 hp? Dodge pickup trucks do it all day long at a steady 60 mph.

David

I'm with you David. Good analysis. I have yet to see an engine in trawler service suffer from low load running. Yes, they can wet stack, but that is easily resolved with a clean-up hard run.

I have had generators glaze the cylinders from light load. But those are a different situation. Constant rpm.

On a propulsion engine, as you go down in load so does the rpm. So the specific load (considering rpm) is actually higher. A FL 120 or Cummins 370 making 40hp at 1200 is 33% for the FL, 11% for the Cummins, but that is only referring to max hp at rated rpm. What matters more is what can the engine make at full power at 1200? Maybe 60hp? So now the load looks much more reasonable.

On a gennie it is always 1800. Once the AC's cycle off at night, there is almost zero load and it runs at say 5% load all night. Even then, most do not suffer. Some do, but on a percentage basis the number is still very low.

If I am spec'ing an engine for a boat I would shoot for 50-70% comparing cruise hp to rated hp. But that is not the only consideration. On planing boats, the weight and size penalty of a larger yet lightly loaded engine may be a bigger consideration. On a trawler, the vibration of a four cyl may push the choice to the smoother six. But there are not many low hp sixes out there unless you go to an obscure brand which creates its own problems.

So sometimes all things considered, the best (least bad) choice is to go with a larger engine in the trawler. The Deere 4cyl have balance shafts, so that that can be a good option.

But all this only applies if you are building or repowering. The point I guess I am trying to make is that if shopping used, don't be freaked out by something with way more power than needed for hull speed. Other than a few minor operating considerations (power up occasionally) and a bit more maintenance (aftercoolers) you should not have any problems, nor burn but a little more fuel.

My addition may be pointless. I understand your concerns about too much engine.

I,m in that boat also.

Too much engine for my 32'er. My engine now is old (40yrs) V555 Cummins @217 hp, yet it has done fine at 7 -8k. for the last 30 years. Originally powered to plane but cannot because we carry too much, er, stuff.

If you buy a boat with what you consider to be too much hp it doesn't mean it has to be used. Yes run it up for 10-15 minutes once daily on a long run to clean it up if concerned.

NOT WOT. Not needed. Just bring it up to a point where it is working harder on a slow PLANE or just on to the lower portion of the hump.

For a good normal cruise speed stay just below,10-15%, the hump speed , and it will be loaded sufficiently to be fully at operating temps which is what is important. That has also "derated" the engine.


As pointed out you will have a tad more mtc. with a seawater cooled after cooler but not that big deal unless ignored.


Stop worrying so much about the engine hp unless you are in the position to spec. exactly what you want or do a repower. Just learn to run and maintain it smartly and whatever you have will serve well.

I strongly suggest you add one of the most important guages, EGT. It will tell you ,with some experience, just how hard you are operation g the engine, overloading or easily.

JMO.

David and Ski, doesn't the Cummins(6 series) have a coolant cooled oil cooler? Does that help in what we are talking about? When everything gets up to 180 degrees I would think that would help. My little 4LH Yanmar had a seawater cooled oil cooler. I had a bitch of a time changing oil on that thing because I could never get the oil temp up to help the oil flow. That would not have been good to run that engine slowly all the time.

Let's face it, most folks buy used boats and live with the engine that came with it.
If some buy a custom new build, they usually follow the recommendations of the builder.
Repowering? We all hope to sell the boat before that happens.