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Old 11-14-2013, 09:22 PM   #41
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Originally Posted by Vashon_Trawler View Post
I'm a little overpowered for a full displacement vessel. My boat is around 3.5 tons on paper... maybe 4+ tons plus fully loaded. Original power plant was a 18HP Sabb diesel. I now have a MD2030 29HP Volvo, which equates to roughly 7 HP per ton. Extra HP has been useful handling some nasty weather and rough water. I spin a 16" prop on a 1.25" shaft that is likely too much torque for the smaller 20HP Volvo MD2020. I'm guessing the old Sabb could handle the torque quite well.

Maybe my boat's PO installed a larger engine to handle the relatively large prop/shaft?
Hard to say without knowing your prop pitch and gearbox ratio.

I have a 16" prop with 16" pitch, 1.25' shaft, with my 36hp volvo. (5 hp/ton) It is perfectly suited to my setup as I can just reach WOT at max (3000) rpm with my 2.4:1 gearbox.
This translates into about 7.5 knots in flat water, which is more than I would expect out of a boat this size. Top speed slows considerably in rough seas.

I agree with the comments that a little more than the standard 3 hp/ton is required for smaller boats in rough water. When running into large waves, the horsepower gets eaten up by tons of water pounding against you. I'm glad I have 5hp/ton to compensate.
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Old 11-14-2013, 10:37 PM   #42
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AusCan, Vashon Trawler,
I have 5hp per ton too. Still overpowered but not by much. I'm convinced 4hp per ton is the magic spot for FD craft. Eight tons and about 40hp for me. I think I could have chosen a 32hp engine and been just as happy. My engine is actually rated at 37hp but I suspect it's an just an industrial rating. Westerbeke and Vetus offer exactly the same engine at 44 and 42hp respectively. I see you've got a 2.4-1 gear and a 16x16" prop. Mine's a 2.57-1 w an 18x14" prop.

My experience in rough going has been just the opposite of yours. Normal cruise rpm of 2300 and when the seas get to 6 or 7' and the wind 40 or so I always back off to 2000rpm. Never have added power under heavy head seas. I seem to make good headway and have covered considerable distances in these conditions.
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Old 11-15-2013, 08:23 AM   #43
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Originally Posted by AusCan View Post
Hard to say without knowing your prop pitch and gearbox ratio.
Great information in this thread. My prop is a 16x8 (pitch allows 3600 RPM at WOT per Volvo specs) on a 1.25" shaft. Transmission is 2.4:1. I can achieve 6.5 knots (sometimes a speedy 7 knots) at WOT and generally cruise at 5.5 to 6 knots (1/2 or so gallon per hour). Volvo says to cruise at 300-500 RPM below WOT, but I aim for around 2800-2850 RPM. This allows me to obtain slightly above 50% load. Anything higher is noisy, burns more fuel, and is a little uncomfortable.

If/when I repower, I will likely go with the same HP.
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Old 11-15-2013, 09:09 AM   #44
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What difference does it really make? 99%+ of us bought our boat oblivious of the rated HP per ton and I betcha will do the same in the future. More important are the things we normally consider when vessel purchasing are less esoteric areas such as:
  • Suitability for the mission at hand
  • The deal we got
  • Seaworthiness
  • Condition of the vessel
  • Propped correctly
  • Resale
  • Bling factor
Did anybody have HP per ton on their list?
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Old 11-15-2013, 10:29 AM   #45
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What difference does it really make? 99%+ of us bought our boat oblivious of the rated HP per ton and I betcha will do the same in the future. More important are the things we normally consider when vessel purchasing are less esoteric areas such as:
  • Suitability for the mission at hand
  • The deal we got
  • Seaworthiness
  • Condition of the vessel
  • Propped correctly
  • Resale
  • Bling factor
Did anybody have HP per ton on their list?
I must be the 1%.
Actually, I initially was concerned that my boat was underpowered, and this nearly put me off buying it. I mean - real men have more than 36hp. After a bit of research on this forum and elsewhere, I realized it was a good match, and went ahead with the purchase.
I think it was you, Eric, that alleviated my concerns. Thanks mate.
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Old 11-15-2013, 10:47 AM   #46
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With all the focus on how little it takes to move a displacement hull at hull speed, I'm confused about the effects of wind and current. Wouldn't "overpowered" be desirable in some conditions?
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Old 11-15-2013, 11:08 AM   #47
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So using 4 hp per ton is pretty accurate to calculate the needed hp for hull speed on a displacement vessel ...

It all depends on the vessel. The Maersk E class ships move 94,000 tons along at 19 knots on about .9 hp per ton.
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Old 11-15-2013, 11:19 AM   #48
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With all the focus on how little it takes to move a displacement hull at hull speed, I'm confused about the effects of wind and current. Wouldn't "overpowered" be desirable in some conditions?
Angus - You're right - It is nice to have a bit up the sleeve to push against the added force of the wind.
But current is another story. Extra hp doesn't do as much when pushing into the current. If your calculated max hull speed is 6 knots and you are running against a 5 knot current, you will only do 1 knot over the ground before getting to max hull speed. More horsepower will not help much to get over that big bow wave. Best to just wait for the tide to change.
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Old 11-15-2013, 12:46 PM   #49
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Thank you very much AusCan.

With a FD hull you can't go faster to buck a tidal current any more than about 1/2 of a knot. The FD boat is more efficient at her cruising speed of about .75 to 1 knot less than hull speed. But it's efficiency gets even better as you go slower. I cruise w 20hp on Willy and w 10hp I would be going a lot faster than half speed. So when the going gets rough and you slow your speed a bit (300rpm for me) there is more power available to overcome wind and waves. Wind and waves reduce my speed so little I still make good progress even at reduced throttle.

So over 8 years or so I've never needed more than 20hp. So a 25 to 27hp engine will do me just fine. But I can't really make an objective case for needing over 20 .. based on my experience. And that would be just 2.5hp per ton.
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Old 11-15-2013, 07:32 PM   #50
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1.* Nobody's saying the engine can't do it, just that it can't do it very long.* A 1950s engine is a 1950s engine.** To think it can deliver the same longevity under hard use as a newer generation engine*is not a smart assumption except in the eyes of your diesel shop, which will love you for it.* The diesel shop we use has seen a lot of failed Lehmans over the years, and they told me not long after we bought our boat that in almost all cases the root cause of the failure was running them too hard, too fast, and too hot for too long.* Failed head gaskets were the most common specific cause of the failure they said.

Lehman created the FL120 in the mid-1960s.* At that time there were not a lot of engines on the market that would lend itself to this kind of conversion so their choices were limited.* Also, people viewed engine operation in the '60s considerably differently than they may do today.* Cruise rpm ranges for these engines were conservative.* Our owners manual itself does not call out any specific cruise power--- it simply says (verbatim)*"Cruising rpm varies with the conditions and type of engine fitted.* Please check with your dealer for his recommendations."* Our manual includes a separate page from the GB dealer who sold the boat new in 1973.* The page contains a list of operating and maintenance recommendations and intervals.* Under "recommended cruise power" it says 1600 to 1800 rpm.*


2.* You're right but....* I see "rpms" used in print*to mean a range of rpm.* What's plural is not the word*"minute" but the whole acronym.* So it means "more than one specific rpm."* But.... just because I see the term "rpms' used by various motoring journalists doesn't mean it's correct.* It may just mean that we're all wrong
There where options but maybe a money question. I know that the OM 352 engine is a proven power-plant for over decades and still produced in for example Brasil.

The OM 352 engine introduced at the 1964 Geneva Motor Show by Mercedes-Benz

The Mercedes Benz OM352 Engine is a 5675 cm3 (5.7 litres, 345-cubic-inch) displacement inline 6-cylinder 4-stroke Diesel engine. It is one of many motors in the 300 series of engines, which were developed during and after World War II, while specifically the OM352 was revealed in 1964. The OM352's lineage can be traced back to the OM312, itself an 80HP inline-six engine.



[1] The OM352 has many applications, including marine, military, municipal, and agricultural vehicles, as well as stationary settings. The engine has differing trim and power levels, affording designations such as OM 352A a variant fitted with a turbocharger, or the OM 352LA, which is fitted with an intercooler and a turbocharger. (List of Mercedes-Benz engines.)
The engine is water-cooled, and is produced using cast-iron cylinder block, with cast-in cylinders. The engine utilizes diesel fuel delivered in a direct injection method from a Bosch PES style inline injection pump. The cylinder head is a single unit for all cylinders, and the cylinder head cover and air intake are shared by a single cast aluminum alloy. Interestingly, exhaust ports 2&3 and 4&5 are siamesed together, presenting an exhaust manifold with only four outlets.
The crankshaft is a precision forged unit running in seven three-layer bearings, with counterweights bolted onto the crank webs, much like any other diesel motor of its vintage. The middle bearing is also the thrust bearing. The connecting rods are of a split design, with bronze bushings for the piston pin.
The OM 352 was also one of many Mercedes engines licensed in 1979 for manufacture by Atlantis Diesel Engines (ADE) in South Africa. These were known as ADE 352 and are virtually identical to the Mercedes engines.Recently the OM 352 was licensed by TATA for use in their 713S Trucks. These are manufactured by TATA but however have slight differences like the weights no longer being bolted to but rather part of the crank and no holes for oil sprayers in the block. These modifications are likely to reduce complexity and cost of engine production.

Common output power can vary from 92 kW to 141 kW (126 to 192 hp) depending on the fuel delivery and air charge options. Higher output can be achieved through special modifications, however engine longevity may be affected with such non-factory modifications.
Improved engines developed after the OM352 include the OM366, which looks extremely similar in appearance. Due to emissions requirements, the 300 series engines were eventually retired. Mercedes Benz 900 series engines are the successors of motors such as the OM352
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Old 11-15-2013, 08:41 PM   #51
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Auscan, my mate`s Resort 35 has a 4cyl Perkins disguised as a Volvo (the Volvo marinising parts cost heaps), puts out about 60hp, at best does about 6-7 knots. I thought 80hp would be about right for it, but you do ok with 5ft less boat, and 36hp.
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Old 11-15-2013, 10:32 PM   #52
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Reiziger wrote;

"Interestingly, exhaust ports 2&3 and 4&5 are siamesed together, presenting an exhaust manifold with only four outlets."

I think most all engines of the period had siamesed ports. I remember Ford flat head V8s had three exhaust ports on each bank.

Perhaps there is a reason we don't see them (OM 352 engines) in trawlers.
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Old 11-15-2013, 11:20 PM   #53
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Auscan, my mate`s Resort 35 has a 4cyl Perkins disguised as a Volvo (the Volvo marinising parts cost heaps), puts out about 60hp, at best does about 6-7 knots. I thought 80hp would be about right for it, but you do ok with 5ft less boat, and 36hp.

Bruce - I think the Resort 35 has a lot more beam, especially aft, and is not quite so slippery through the water as my smaller version.
Hull efficiency obviously plays a big part in hp/displacement requirements.
My penalty for having an efficient hull is a tight engine room which could not accommodate anything bigger than what I have.
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Old 11-16-2013, 12:51 AM   #54
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Bruce - I think the Resort 35 has a lot more beam, especially aft, and is not quite so slippery.....
Hull efficiency obviously plays a big part in hp/displacement requirements.
My penalty for having an efficient hull is a tight engine room..
Steve, You are right, beam would be a factor. The Resort version hull is very much FD. Its ER, under the cockpit, is tight too, I helped him R&R the engine once.
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Old 11-16-2013, 07:08 AM   #55
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Angus - You're right - It is nice to have a bit up the sleeve to push against the added force of the wind.
But current is another story. Extra hp doesn't do as much when pushing into the current. If your calculated max hull speed is 6 knots and you are running against a 5 knot current, you will only do 1 knot over the ground before getting to max hull speed. More horsepower will not help much to get over that big bow wave. Best to just wait for the tide to change.
Thanks, that makes sense.

I asked because I was caught in a situation with a single 135 Lehman trying to turn the bow into a strong wind in a narrow fairway to be able to head up channel. We were caught abeam to the wind and being pushed fast down the fairway toward a grounding or worse. We got out of it by backing as far as we safely could, giving full throttle forward and hard rudder supplemented by the bow thruster. It still took 3 tries and a slight lull in the wind to get out of that, so I would not have wanted any less power . . . and would gladly have taken more.
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Old 01-03-2014, 11:09 AM   #56
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Wow! It took me a half hour to read this whole thread....very spirited to say the least. My Bristol is 42' and Net tons 35000. I have a Lugger 6414T completely rebuilt; its ratings are 120hp continuous to 170hp max. Did I get the right engine? The one the factory installed was a Cummins 160. I'm hoping for 8 knot cruise @ 1400 rpm and 2 1/2 GPH. You can see a picture of the engine at www.sibsie.com page 20.
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Old 01-03-2014, 02:43 PM   #57
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My Bristol is 42' and Net tons 35000. I have a Lugger 6414T completely rebuilt; its ratings are 120hp continuous to 170hp max.
Your boat displacement is a little under 13 tons per original specs. The net tonnage you mention is a measure of volume only.
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Old 01-04-2014, 07:44 AM   #58
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Most 42 ft boats will at best be 40 ft on the LWL, measure yours.

Usually the SQ RT of the LWL is the place to start looking for the sweet spot LRC..

6 K or so would be a start.

8K economical cruise would take a 64 ft LWL, or 3X the fuel burn of 6K on a 42 ft boat.

Its only currency , and a big wake, for 8K, for a couple of hundred hours a year , far cheaper than the 64 ft boat.
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Old 01-04-2014, 09:10 AM   #59
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This has been an educational thread - and a bit if time travel to some of Marin's old posts.

Good work!
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