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Old 02-22-2016, 01:46 PM   #21
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I was reviewing the original 1970's design as a commercial trawler and the design only called for a 160 hp engine. Same exact lines with only slightly lower windage. Pulling trawls and working for a living.

Perhaps this is a reflection of modern trends that we "require" higher power and higher speeds? Take a look at the immensely seaworthy romsdahls and malahides and I bet the powering was quite low as well.
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Old 02-22-2016, 01:59 PM   #22
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"I was reviewing the original 1970's design as a commercial trawler and the design only called for a 160 hp engine. Same exact lines with only slightly lower windage. Pulling trawls and working for a living."

May I ask which exact engine was in the 70's design rated at 160HP?
What was the displacement and what was the rated rpm?


FWIW - I have had a few 4 stoke NA diesels that I would not fear running at or near full rated rpm, and I have 4 stroke turbo diesels that I will not run above 70% load and prop with caution. YMMV
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Old 02-22-2016, 05:51 PM   #23
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What is your calculated or expected cruise hp required?

Using a 2.5exponent, along the prop curve at 1500rpm it is developing 58hp. More complicated than that as once you reach near hull speed and above, the prop curve fails as hp needed shoots way up.

See if the engine mfr will send you the BSFC map. All the data needed is on the map, better than prop load rpm/hp/fuel curves.
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Old 02-23-2016, 07:11 AM   #24
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" All the data needed is on the map, better than prop load rpm/hp/fuel curves."

Most Mfg guard the BSFC better than the US gov guards the plans for the T 88 nuke.


Prop load rpm/hp/fuel curves are basically useless in attempting to set a boat up for useful power and best cruise efficiency.
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Old 02-23-2016, 07:33 AM   #25
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I was reviewing the original 1970's design as a commercial trawler and the design only called for a 160 hp engine. Same exact lines with only slightly lower windage. Pulling trawls and working for a living.

Perhaps this is a reflection of modern trends that we "require" higher power and higher speeds? Take a look at the immensely seaworthy romsdahls and malahides and I bet the powering was quite low as well.
I think your figures seem pretty good.
The "extra" HP thing is a marketing thing and not much else.

The figures for my Ford Lehman, SP135, seem similar to yours.
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Old 02-23-2016, 09:44 AM   #26
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Personally the last place I would skimp is propulsion if I were building a trawler. That said its really an optimization problem and naval architects get paid the big bucks to figure this stuff out. For the record our boat's ratio is just a smidge over 5 when fully loaded, but we have the luxury of two engines and can tweak loading a bit based on sea state, electrical and hydraulic needs.

Unless you added the expense of controllable pitch prop you are just going to wind up doing a bit of prop tweaking. Way cheaper than engine tweaking.

I'm not sure why the trend towards big oversized power plants recently, but probably a bit of marketing as well as some need for bigger alternators hydraulics etc...adding to the vessel's hp needs. With the new variable frequency drive DC air conditioning systems bigger alternators seems to be the trend in order to have dehumidified air under way.


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Old 02-23-2016, 10:02 AM   #27
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I too would go with a larger base engine. Diesels are designed for trucks and industrial equipment or cars. Cars don't typically run at high loads for long periods so 50+ HP per liter works for them. trucks and equipment OTOH run at high loads so ratings of 30 HP per liter is common. A heavy low RPM low rated HP diesel is a joy to own. Parts just don't fly off them.
IMO all the engines well known for long life are run at lower loads. The list posted above estimating life gives the right idea though nothing is certain.


Cat recommended 30,000 gallons of fuel between overhauls in the past. this because fuel used is the best indicator of how much work the engine has done. Hours of operation don't tell much. Work done is what counts.




FWIW Service and parts are more important than many people think. Buy well supported engine that many mechanics are familiar with and has a large installed base and an entire set of difficulties are avoided.
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Old 02-23-2016, 10:27 AM   #28
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Current can be ignored as far as reserve power is concerned. Windage and waves OTOH can add to the need for extra power to maintain speed.
Why do you say "maintain speed"? Why not just slow down?

Also there is no need to reach hull speed on a FD boat. I'm not even sure Willy can attain her HS of 7 knots. I wish the expression "hull speed" had never been coined. It's very misleading in many ways. A boat very similar to Willy at 5hp per ton (2000lbs) w less power probably would not be capable of HS. There's fast boats, slow boats and slower boats. Hull speed is just a mathimatical expression. And there's no need to go at HS ever. Most FD skippers wisely cruise a knot below HS and working the engines hard the FD boat is still closser to one knot below HS than actual HS. Providing the boat is not overpowered. You'd be better off forgetting about hull speed.
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Old 02-23-2016, 10:42 AM   #29
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Cat recommended 30,000 gallons of fuel between overhauls in the past. this because fuel used is the best indicator of how much work the engine has done. Hours of operation don't tell much..
For large industrial engines fuel consumption is many times higher than 30,000 gallons between rebuilds. Warranty programs for off highway equipment are normally based on hours as a negotiating value. On my last big job we had complete engine warranties for about 30 new pieces of equipment 16 of which were 400 ton haul trucks with 4000 HP engines.

Fuel consumption was a warranty item too at max HP ratings. Fuel mapping is closely followed as an indicator of engine and drive train performance. Lest I digress too much, as mentioned by Ski, the starting point for many engines is the BSFC. From this springs drive train performance and engine health measurement.

In our little boats most of this though is moot. But this and a parallel thread do point out why a good engine survey is important. You may be looking at a vessel where the previous owner ran it at full throttle most of the time.
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Old 02-23-2016, 11:24 AM   #30
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May I ask which exact engine was in the 70's design rated at 160HP? What was the displacement and what was the rated rpm?
Excerpt is at the bottom:

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I have had a few 4 stoke NA diesels that I would not fear running at or near full rated rpm, and I have 4 stroke turbo diesels that I will not run above 70% load and prop with caution. YMMV
Seems to me that its the manufacturer who would specify the continuous operating range, unless the engine is getting old and unreliable.

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What is your calculated or expected cruise hp required?

Using a 2.5exponent, along the prop curve at 1500rpm it is developing 58hp. More complicated than that as once you reach near hull speed and above, the prop curve fails as hp needed shoots way up.

See if the engine mfr will send you the BSFC map. All the data needed is on the map, better than prop load rpm/hp/fuel curves.
According to Gerr's modified formula "B", the max rating of 180 hp will yield 8.2 knots. For cruise purposes, output of 140 hp will yield 7.6 knots. My naval architect has a powering specialist on his team but I have not received the detailed calculations yet. However I doubt there would be too much of a difference.

BTW, the manufacturer either does not have a BSFC map for the engine, or is not willing to release it!
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Old 02-23-2016, 11:32 AM   #31
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Mako

What brand, model and HP rating?
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Old 02-23-2016, 11:49 AM   #32
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In a passagemaker magazine (June 2001) there is an article about a 60' 80 ton wood boat the Nathaniel Bowditch that was powered by an eight cylinder Gardner of 152hp. That equates to less than 2hp per ton. That's the lowest power per ton I've heard of. No speeds were mentioned and long ton or other weight spec was mentioned. If it dosn't specify long tons I assume 2000lbs per ton.

My own personal yardstick limits an acceptable power to weight ratio of 3hp per ton. I've compared lots of boats that have stood the test of time and three hp for every ton seems more of less the practical lower limit.

Early on Bob Coffer wrote;
"You can always throttle down but if you are under powered at some point you have no options.

Who knows where that line is? I know there have been times that I have wished for more."

"no options"?
Not so at all. I used to think that. But it's really not true. If you guess 10 or 20hp too little power you just run a little slower. But it's not proportional. If you gain 30hp and gain .25 knots on the same boat if you were to loose 30hp you may only loose .10 knots. Those numbers are just for example.
There's no line. It's all a big grey area. We don't fall off the edge of the earth if we can't reach hull speed. If you guess too low you loose a bit of speed but very little as the power requirement is on a very nonlinear curve. If I had chosen 30hp (instead of 37) for Willy I'd probably be going the same speed as I go now w 5hp per ton (37hp). And if I worked the smaller engine the same as the larger I'd probably be going 6 knots instead of 6.15.
So you don't "run out of options" .. you just go a tad bit slower.
And Bob if you wish for more and get more almost immediately you'll be wishing for more again. The answer is not to wish for more.
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Old 02-23-2016, 12:08 PM   #33
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I've compared lots of boats that have stood the test of time and three hp for every ton seems more of less the practical lower limit.
Perhaps some romsdahl or malahide owners will chime in, but if I recall correctly they are generally powered about 3.2 to 3.5 hp per ton.
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Old 02-23-2016, 12:11 PM   #34
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Mako

What brand, model and HP rating?
AGCO SISU 49CTIM 180 hp
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Old 02-23-2016, 01:24 PM   #35
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Good engines and well regarded company. They have pretty well blanketed the farm implement business with this same base engine.

I can see why you are reluctant to go bigger in CID/HP as the next size up is much larger, more money and heavier. In this HP range properly marinized engines other than your selection are few - JD, Lugger and Perkins Sabre/Cat 3056 ( I have two) amongst those few. Are you using a keel cooled setup?

Prop selection seems the remaining question, are you there yet? Good job Mako and interesting post.
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Old 02-23-2016, 02:40 PM   #36
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If expecting to cruise at 140hp on a 180hp 2300rpm engine, that puts it about 2080rpm on the 2.5 prop curve. Engine should be happy there, but probably not the best for bsfc nor the best for noise/vibration. That engine looks to be a 4cyl without balance shafts, and that has a second order vertical vibration that may be obnoxious. That's why it is rare to see 4cyl in boats, where you see lots of six cyl. The six has near perfect balance.

A good rule of thumb is to size engine so under cruise conditions you are around the same rpm as full power torque peak, but obviously not at full power. You are way above that rpm.

Sizing engines for planing boats is different, you need the power but not the weight, so in those cases you expect the engine to run hard. Not the same for spec'ing for a trawler.
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Old 02-23-2016, 05:22 PM   #37
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Why do you say "maintain speed"? Why not just slow down?

Also there is no need to reach hull speed on a FD boat. I'm not even sure Willy can attain her HS of 7 knots. I wish the expression "hull speed" had never been coined. It's very misleading in many ways. A boat very similar to Willy at 5hp per ton (2000lbs) w less power probably would not be capable of HS. There's fast boats, slow boats and slower boats. Hull speed is just a mathimatical expression. And there's no need to go at HS ever. Most FD skippers wisely cruise a knot below HS and working the engines hard the FD boat is still closser to one knot below HS than actual HS. Providing the boat is not overpowered. You'd be better off forgetting about hull speed.
I never mentioned hull speed and agree I wish it had never been coined.

I had a full displacement boat probably similar to yours in many ways and trying to come north along the CA coast I assure you I needed more power to maintain speed. Each wave stopped the boat and it would then reaccelerate just in time for the next wave. More power would have overcome the waves.
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Old 02-23-2016, 05:57 PM   #38
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I am enjoying this discussion because I know nothing about it. Most of the discussion has gone completely over my head.

When you discuss hp/ton, what figures are you using? I assume max rated hp, but is ton the displacement, dry weight...?

Any idea what some hp/ton ratios are with some well known FD production vessels such as Nordhaven, KK, or Selenes?
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Old 02-23-2016, 06:43 PM   #39
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Mako

As a reminder to us ditherers, what is the WLL and weight of your vessel?

To Ski's point about 4 cylinder harmonics and vibration, any thoughts? Four cylinder JDs, Luggers and Perkins Sabres are no problem according to owners in this regard, but on your planned engine --?
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Old 02-23-2016, 07:37 PM   #40
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when waves are high, I have to lower power to reduce violent boat movement.
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