Diesel Engine choice question

I am looking at getting a trawler with a full displacement hull (40 ft). If I have the choice of getting it with a 250HP, 350HP or a 480HP single diesel engine, will I get much better operating characteristics out of the larger engine? Aside from fuel consumption and cost of the larger engine, any other benefits or issues I may face by going up or down in engine size. Thanks!

Post the engine brands and models of your choices. 250hp is more than necessary for a 40' displacement hull. At cruise you might need about 60-80hp so a 120-200hp rated engine would be plenty. No advantage going with a 350 or 480, but there is some brand specific stuff that might favor one or the other.

Both 350 HP and 480 HP seem like extreme overkill for a 40' FD trawler. I agree with Ski, 120 HP to 200 HP would be far more practical.

Ski in NC said:

Post the engine brands and models of your choices. 250hp is more than necessary for a 40' displacement hull. At cruise you might need about 60-80hp so a 120-200hp rated engine would be plenty. No advantage going with a 350 or 480, but there is some brand specific stuff that might favor one or the other.

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+1

Not only post the engines but I'd be curious to see what the 40' boat is.

At 480hp, that sure sounds like a planing hull.

Gabe n Em said:

At 480hp, that sure sounds like a planing hull.

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Yeah, I am a bit surprised that a FD hull would even offer it.

What boat are you considering. There are very few full displacement hull trawlers in the 40' range: Krogen, Nordhavn, Willard, Hatteras LRC. That is about it. All of those use much lower powered engines than 480 hp.

I suspect you are talking about a semi-displacement hull.

David

Thanks for great info. Looking at those names but also a Helmsman 38E or 43 Pilothouse, and they seem to offer quite a range in terms of engines, so that sparked my question.

My 45' trawler requires 70 HP to cruise at 8 knots. The semi planning hull could also do 14 knots, but that required a 450 HP engine. Figure out what you want for a cruising speed (displacement or planing) and then pick the appropriate engine.

Ted

Helmsman don't offer a displacement hull. It is a semi-displacement hull, although most owners prefer to cruise them at displacement speeds to avoid using 20+ gallons per hour.

If this is a new boat and you can order her with a choice of three different engines, then I would get the one that is the most popular. All three choices will cruise as well with similar economy at hull speed, but the most popular size engine will have the best resale value. If we are talking about used boats more than a decade old then the condition of the individual motor is more important than its horsepower.

Markpadilla said:

Thanks for great info. Looking at those names but also a Helmsman 38E or 43 Pilothouse, and they seem to offer quite a range in terms of engines, so that sparked my question.

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Ok. The full displacement hulls won't benefit a bit from the higher hp engines you asked about. So your question only relates to the Helmsman and similar semi-displacement hulls.

First the semi-displacement hull is not as efficient as the full displacement hull at displacement speeds. So just to go as fast as a full displacement hull takes more hp, sometimes 50% more.

The Helmsman 43 is listed at 35,000 lbs dry but could easily weigh 45,000 lbs with full fuel, water, crew and gear. A rule of thumb (confirmed from data in Bebe's book) is that it takes 1.5 hp per thousand pounds of displacement to push a full displacement hull to its displacement speed. Displacement speed is 1.34* sqrt(lwl). So it could take 68 hp to do that and over 100 hp for a semi-displacement hull. That really means a 200 hp engine so you don't have to run it at red line all day and also have some headroom for pushing into head seas.

So, Helmsman offers the Cummins QSB 6.7 250 hp engine for that boat. It would be a good choice but you can't expect to cruise at more than displacement speed with it. But they also offer the 480 hp version of THAT SAME ENGINE.

There is no mechanical difference between a 250 hp QSB 6.7 and a 480 hp one, just computer chip tuning differences. You can buy a 250 hp version and later if you think you want more hp, Cummins for a price will change the chip to let it produce 480 hp. It will probably take a prop change or maybe a prop and transmission change, but the base engine stays the same.

I am guessing 480 hp won't do you much good. It takes about 100 hp to hit displacement speed or about 8.5 kts. With 300 hp which is probably all you should cruise the QSB at, you won't go that much faster, maybe 12-13 kts. A good comparison would be the Mainship 400 with the Yanmar 370 hp engine. Mainship's old website says that boat and engine will do 13 kts at 3,000 rpm which roughly equivalent to the QSB 6.7 at 300 hp.

So does that answer your original question? In summary the higher hp engine isn't any different mechanically from the 250 hp engine, so fuel economy and longevity will be the same at or below displacement speeds. It takes a chip and maybe a transmission and certainly a prop to make and use 480 hp and that will maybe get you another 4 kts.

David

Any make but VOLVO...run from them as you would the plague.

Mark P said:

Thanks for great info. Looking at those names but also a Helmsman 38E or 43 Pilothouse, and they seem to offer quite a range in terms of engines, so that sparked my question.

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As many of us thought, semi-displacement. Most of the time you will run them at displacement speeds so don't really need a lot of power. I have a North Pacific 43. It has a Cummins 380hp engine. The boats were offered with a 330hp version of the same engine. I don't think I benefit at all by having the extra 50hp, but it doesn't hurt me either. As David mentioned above, the different is just in the ECU programing.

If you are looking at the Helmsman, I would also suggest looking at the North Pacific Yachts in the same size ranges.

ancora said:

Any make but VOLVO...run from them as you would the plague.

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It is not the first time that I ask that question and never received much answer, why is that?
I do not own a volvo nor have any involvement with this brand just try to educate myself.
Only thing I know about these engine is that I have friend who got a new boat built (not a trawler) and put a Volvo in it (don't know the exact model something around 450ch) and it is nicely running and quite fuel conservative for the speed.

May you elaborate?

L

For what it is worth, my boat weighs north of 65 tons. She as a CAT 3306 and uses 80 hp to move her at 8 knots with a clean bottom, burning 4 gph. At WOT she burns 15 gph and I move at 9.5 knots. I travel at 7.5 knots and around 3 gph, using 60 hp so I run her up to WOT for 20 minutes every 8 hours of run time or so. If you are going to be traveling at trawler speeds, a 40 foot trawler could get by easily with a 100 hp engine. My boat could do just fine with 150 hp to 175 hp.

Semi-planing is a different story altogether.

Lou_tribal said:

It is not the first time that I ask that question and never received much answer, why is that?
I do not own a volvo nor have any involvement with this brand just try to educate myself.
Only thing I know about these engine is that I have friend who got a new boat built (not a trawler) and put a Volvo in it (don't know the exact model something around 450ch) and it is nicely running and quite fuel conservative for the speed.

May you elaborate?

L

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I've never owned Volvo but I have friends that do. Most talked about is the high cost of parts. On the plus side, Volvos are used extensively in Alaska's Bristol Bay fishery where they run the hell out of them and they're are loved for their durability. Plenty of power for their size and weight.

djmarchand said:

I suspect you are talking about a semi-displacement hull.

David

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+1

Lou_tribal said:

It is not the first time that I ask that question and never received much answer, why is that?
I do not own a volvo nor have any involvement with this brand just try to educate myself.
Only thing I know about these engine is that I have friend who got a new boat built (not a trawler) and put a Volvo in it (don't know the exact model something around 450ch) and it is nicely running and quite fuel conservative for the speed.

May you elaborate?

L

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I hear it a great deal also I am the 3rd owner of my boat with Volvo TAMD great engines but parts can be pricey if your not willing to shop

With a full displacement hull, the fewer HP is needed. Eighty HP would be sufficient. But is the boat a true FD?

djmarchand said:

So, Helmsman offers the Cummins QSB 6.7 250 hp engine for that boat. It would be a good choice but you can't expect to cruise at more than displacement speed with it. But they also offer the 480 hp version of THAT SAME ENGINE.

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Off topic, but I find it odd that Cummins has a "QSB" 5.9 and a "QSB" 6.7.

Think it's because there's no alphabet letter between B and C?



-Chris

QSB 5.9 and 6.7 are very similar aside from bore and stroke, so I guess they kept the "B" family designation. And yep, they are starting to run out of letters.

Have had the QSB6.7 for a grand total of 2 weeks, and it has been great- knock on wood. Quiet, no smoke.

ancora said:

Any make but VOLVO...run from them as you would the plague.

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I can't speak for the newer Volvo inboards, but my Volvo MD2030 is rock solid and reliable (knock on wood). Then again there is a "Perkins assembled in England" decal under the Volvo sticker. Volvo parts are notoriously super-duper expensive. Perhaps I should remove the decal and paint the engine blue--this might increase the resale of my boat?

The fuel consumed by any of those engines at the same boat speed will be similar. Load not engine size determines fuel required.

Higher HP versions of the same block may add items such as turbo and or after coolers in addition to a reprogramming a chip. reprogramming is probably not cheap as the maker will want the price difference between power settings.

I have found that extra power above the calm conditions requirement is nice to have when hoing into a strong headwind or smashing into head seas. YMMV

All, thanks for great info. Need one more piece of information to keep the Admiral on my vessel happy (it is cheaper to keep her happy than have her leave me at the next port): If I go with a Helmsman 43 (or similar vessel weight about 45K loaded) with a semi-displacement hull and a 480HP Cummins QSB 6.7, is 12-13 knots a cruise speed or WOT? Typical trips, we will be running from Hilton Head to Charleston SC, or from Hilton Head to St. Augustine (against current), Key West, etc., how long on either of those trips can I run the vessel WOT with adverse effects?

ancora said:

Any make but VOLVO...run from them as you would the plague.

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We had a Volvo TAMD40A (I think that was its designation) for a number of years on our previous boat, and it ran very well. In the five years of ownership it ran flawlessly, even including a nasty overheating issue (not the engine's fault) with no resultant damage. When we sold it at around 4200 hours it still ran well, and still is today. It's also a very quiet engine.

Mark P said:

All, thanks for great info. Need one more piece of information to keep the Admiral on my vessel happy (it is cheaper to keep her happy than have her leave me at the next port): If I go with a Helmsman 43 (or similar vessel weight about 45K loaded) with a semi-displacement hull and a 480HP Cummins QSB 6.7, is 12-13 knots a cruise speed or WOT? Typical trips, we will be running from Hilton Head to Charleston SC, or from Hilton Head to St. Augustine (against current), Key West, etc., how long on either of those trips can I run the vessel WOT with adverse effects?

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First of all you can't run the engine for more than a few minutes at wot without adverse affects. I am not saying that it will blow up after ten minutes, but every minute at wot is putting unnecessary wear on the engine. You should only run at wot once or twice a year to confirm you can get up to rated rpm. Otherwise, don't do it. You don't run your car at 120 mph for long distances, even in the Nevada desert, do you?

I am the one who thought that boat could do 12-13 kts and that was with the engine producing 300 hp. You can do this for a long while, certainly thousands of hours if the engine is propped and maintained right. It will burn about 16 gph at that speed and load.

David

13 or 14 Kts is neither on plane nor bow down economy speed just marketing BS. IMO 9-10 Kts is the top end of economical cruise speed for that boat, Anything more you will be pushing water at little benefit.

Woodland Hills said:

If this is a new boat and you can order her with a choice of three different engines, then I would get the one that is the most popular. All three choices will cruise as well with similar economy at hull speed, but the most popular size engine will have the best resale value. If we are talking about used boats more than a decade old then the condition of the individual motor is more important than its horsepower.

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That’s interesting Hills,
You would buy the most popular. And that turns your crank to have the most popular things? Like teen aged girls HaHa.
I can see not buying a boat w a Volvo because of expensive parts. I have a very unpopular engine in my boat. I don’t know of anybody else on TF that has one in their boat. Had it for 12 years and haven’t bought any Mitsubishi parts other than filters and the like.
Volvo engines must be really bad to require enough parts to get the reputation they have. If they were as good as my Mitsu knowbody would know the parts were expensive. Also needing engine parts could have a lot to do w when the majority of Volvo’s were sold into new boats. Or who buys Volvos. Many Volvos get more or less forced on buyers by builders installing Volvos in their new boats. Just some thoughts.

But your buying the most popular engine because of it’s popularity makes me wonder if you really like your things. Assuming you buy most other things (cars, TVs, clothing, your boat) because they are the most popular and that may not be what you do. Perhaps this just applies to boat engines.