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Old 08-18-2016, 06:22 PM   #1
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What hp??

So looking at options of engines and what i want in a boat and notice its all about HP? I dont think I am wrong but isn't a boat basically going "up hill" all the time, so under a constant load. Now since a trawler is not a planing boat and goes slow and is a big heavy pig why would anyone care how fast it's engine can do work? I would think they would be more interested in how much work it can do as in how much torque does it produce, but this does not seem to be the focus. What am i missing?
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Old 08-18-2016, 06:32 PM   #2
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I'm not an expert. I'm not even a well informed amateur. In general, I think you are right.

There are normally two type of hull designs in the "trawler" world. Full displacement hulls and semi-displacement hulls. For a full displacement hull, you actually need very little HP to push the boat at hull speed, or a convenient fraction of hull speed. There are rules of thumb that folks here can recite from memory that can give you and idea of how much hp that actually is. So in theory, that is all you need. A couple caveats to that.

-You don't want to have your engine putting out max hp all the time. That will shorten engine life.
- We don't always operate our vessels in flat water and no wind. Both of those can cause you to need more hp to maintain your speed.

For semi-displacement hulls like mine, it is little different. If you push the boat hard enough, you can run at a speed greater than hull speed but at a significant cost in fuel and noise. Many boat manufacturers, such as mine and others, have been putting in engines in SD hulls that I think are simply overkill. I have 380hp in my boat which is way more than I really need as I only ever run my boat at displacement speeds.

We can let the smart and knowledgeable folks give you a good answer now.
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Old 08-18-2016, 06:42 PM   #3
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We can let the smart and knowledgeable folks give you a good answer now.
Good point. Car engines come with max hp and torque ratings, at specified rpm. Boat engines seem to come with only hp ratings. As the late US physicist, Professor Julius Sumner Miller used to say, "Why is it so?"
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Old 08-18-2016, 07:27 PM   #4
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It's a big heavy WHAT?
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Old 08-18-2016, 07:34 PM   #5
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I'm not an expert. I'm not even a well informed amateur. In general, I think you are right.

There are normally two type of hull designs in the "trawler" world. Full displacement hulls and semi-displacement hulls. For a full displacement hull, you actually need very little HP to push the boat at hull speed, or a convenient fraction of hull speed. There are rules of thumb that folks here can recite from memory that can give you and idea of how much hp that actually is. So in theory, that is all you need. A couple caveats to that.

-You don't want to have your engine putting out max hp all the time. That will shorten engine life.
- We don't always operate our vessels in flat water and no wind. Both of those can cause you to need more hp to maintain your speed.

For semi-displacement hulls like mine, it is little different. If you push the boat hard enough, you can run at a speed greater than hull speed but at a significant cost in fuel and noise. Many boat manufacturers, such as mine and others, have been putting in engines in SD hulls that I think are simply overkill. I have 380hp in my boat which is way more than I really need as I only ever run my boat at displacement speeds.

We can let the smart and knowledgeable folks give you a good answer now.
Dave, I believe you have a QSB Cummins am I correct??? You can rate or derate that engine however you please. Would you be happier if that exact engine was rated at 220hp? De-rating can be done via ECM programming. The only thing that changes is that you will be running at 70% power all the time instead of 40%.

To the OP, I think it is a combination of the marketing department and what is available at the time. It is also that marketing department pandering to the ignorance of the uneducated buyer. More HP is better....right????
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Old 08-18-2016, 08:05 PM   #6
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Dave, I believe you have a QSB Cummins am I correct??? You can rate or derate that engine however you please. Would you be happier if that exact engine was rated at 220hp? De-rating can be done via ECM programming. The only thing that changes is that you will be running at 70% power all the time instead of 40%.

You are right. I have the QSB 5.9L Cummins. It can be rated from 230 to 480hp. It would be easy to derate mine to 330hp with an ECM change. The point though, is that I have a lot more horses available than I actually need to run at displacement speeds.
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Old 08-18-2016, 08:25 PM   #7
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I would run a low hp engine with high torque, trawlers need torque to turn bigger higher pitch props. I want to find something with a good old detroit diesel, maybe a 8v92 or 12v92. Plenty of torque and no turbo so it lasts forever.
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Old 08-18-2016, 09:55 PM   #8
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Any desired torque can be produced by any engine through appropriate gearing (transmission). But, the shaft speed at which that torque is produced is a function of horsepower. The power/torque curve is also an important consideration. Once I build an engine (gas) that put out lots of hp, but only at high rpm. The way I geared/proped it, getting up on plane wasn't easy, but once there, it had plenty of power.
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Old 08-18-2016, 10:15 PM   #9
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It's all about hp. How much the boat needs.
And matching the rated rpm to the boat/prop and gear.
That's it.
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Old 08-18-2016, 10:15 PM   #10
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I would run a low hp engine with high torque, trawlers need torque to turn bigger higher pitch props. I want to find something with a good old detroit diesel, maybe a 8v92 or 12v92. Plenty of torque and no turbo so it lasts forever.
A low HP engine, by definition, has low torque since HP is a product of torque and RPM. Large diameter props can be driven by low HP if high reduction ratios are used. But large (diameter) props are good (efficient) up to a point only, b/c it takes power to turn a prop, even one w/o any pitch.

At 36', my 26000lb Island Seeker is way overpowered with 50hp b/c it is a full displacement hull and can use only 13hp at cruising speed.

I have never been in a situation where bad conditions have made me want to speed up, usually one slows in adverse conditions so I don't know why some specify extra power for that reason? Outrunning a storm in a high speed vessel is another story.
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Old 08-18-2016, 11:00 PM   #11
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Warwgn,
Pushing a boat is work being done.
The definition of hp is work done in a specific amount of time.

Driving a boat through the water is a function of;
1. Hull shape
2. Weight
3. HP available
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Old 08-18-2016, 11:44 PM   #12
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My 80 HP diesel is only working at 40% load (1.7 GPH versus 4.0 at full throttle) at normal cruise speed of 1800 RPM at 6.3 knots (one knot below hull/maximum-speed).
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Old 08-19-2016, 01:02 AM   #13
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My 80 HP diesel is only working at 40% load (1.7 GPH versus 4.0 at full throttle) at normal cruise speed of 1800 RPM at 6.3 knots (one knot below hull/maximum-speed).
Mark, you see to me saying that "load" is based on fuel consumption. I have always considered it was based on % of max hp. For example, at my typical 1400 rpm I am at about 40% of the rated hp for my engine. That is about 46% of max rpm (or would be if I wasn't slightly over propped).
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Old 08-19-2016, 05:52 AM   #14
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HP is used as it is the easiest to work with.

A genuine marine engine may have 4 HP ratings , from water skiing to 24/7/365

Fuel burn is easy to approximate , as is frequently the installed efficiency, if it matters.

A 50Hp engine creating 40Hp will be more efficient than a 450HP engine making 40 HP.

BUT at 2.5 GPH vs 3GPH does anyone care that the big engine is 20% less efficient?

Unless its an ocean transit most could not measure the difference.
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Old 08-19-2016, 07:50 AM   #15
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Boat ads list HP but engine makers provide detailed fuel, torque, HP curves.

HP and engine size is an obsession for some but IMO as long as you have enough more doesn't matter much. Fuel used is mostly dependent on work done not engine size. Big or small a four stroke diesel will produce in the range of 20HP per hour per gallon of fuel . To put it another way moving a 30,000# boat at 8 knots will take about the same fuel regardless of engine size.
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Old 08-19-2016, 08:25 AM   #16
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Twisting force is torque, HP is a measure of work over a period of time.

I can show you a huge 3 cylinder diesel that make 1000's of pounds of Tq yet has less then 100hp, it runs at 125 to 400 RPM, it is an old power-plant diesel that would fill a 2 car garage.

So while related are not the same.

A low HP engine does not mean a low tq engine.

Large displacement low RPM engines generally have higher Tq. then smaller high RPM engines down low in the RPM range. That's why Tq. makes you smile and HP makes you giggle.
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Old 08-19-2016, 08:30 AM   #17
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Mark, you see to me saying that "load" is based on fuel consumption. I have always considered it was based on % of max hp. For example, at my typical 1400 rpm I am at about 40% of the rated hp for my engine. That is about 46% of max rpm (or would be if I wasn't slightly over propped).
I think fuel being burned is a very good way to measure load. Read Tony's Tips article "Props move boats. Engines turn props". I think that is the name or similar.
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Old 08-19-2016, 08:34 AM   #18
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Warwgn,
Pushing a boat is work being done.
The definition of hp is work done in a specific amount of time.

Driving a boat through the water is a function of;
1. Hull shape
2. Weight
3. HP available
That does not really follow in the case of a trawler, using that I could just put a 250 hp outboard on the back and be good to go. I get what your saying and technically speaking your statement is correct, but...

I am looking at torque as a bodybuilder, and HP as a sprinter, tell each to run 100yds, then put a 100Lb weight in a backpack and have them run it again. The body builder wont really notice much and will have about the same time, but the sprinter will suffer and time be reduced.

I think that may illustrate my question or point or whatever, but wouldn't it matter more for a trawler to look at the work load requirement, meaning torque required to turn a prop, turn a gear, turn a generator, turn a hydraulic pump, not really how much time it takes to do the work but how much work needs to be done.

I may be way off since all my engine knowledge has been based on automotive, but I drive a 8000Lb Suburban with large tires and pull a boat so the small block that was in it had 230 HP and 360 FtLbs about. It was ok with out towing, but hook up a load and it was a dog, so I built a big block that puts down 633 FtLbs and it has no issues at all now. (ok true it puts out a lot of HP at higher RPMs, but I use it mostly under 2500RPM where the HP is low and tq is still high)

So I still fail to see where HP is more relevant to trawlering than torque.
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Old 08-19-2016, 08:44 AM   #19
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you cant separate torque from hp

T = HP * 5252 / RPM
HP = T * RPM / 5252
RPM = HP * 5252 / T


When you tightened those head bolts you exerted 130 FT # of torque or so but did very little work ( HP) because the RPM was near zero.
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Old 08-19-2016, 09:09 AM   #20
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you cant separate torque from hp

T = HP * 5252 / RPM
HP = T * RPM / 5252
RPM = HP * 5252 / T


When you tightened those head bolts you exerted 130 FT # of torque or so but did very little work ( HP) because the RPM was near zero.
Exactly my point, a trawler needs a lot of force but low RPM. I need a lot of power but don't want to work to hard for it, that is why a diesel is the right power plant, it puts out lots of torque at low RPM, so you can put a large high pitch prop on it and still be able to turn it at low RPM using less fuel and less wear and tear on the engine.
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