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Old 17-11-2018, 22:37   #1
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60 ton boat, 120 hp engine, how does that work?

I met this guy once and he had a 60 ton steel trawler, similar in style to a fishing trawler but built for him as a pleasure boat. I've been thinking about it ever since.

The boat was absolutely beautful and perfect in every way. He knew an enormous amount about boats and had been a pro fisherman for years.

He said it had a 120hp engine and a reduction gear number I should have paid more attention to but didn't. It cruised at six nots. It also had a very big prop.

How does that work? I would have thought 120hp engine would be too small for a 60ton boat but clearly not!

Is it all in the gearing? How would a boat like that behave?

Thanks for any explanation!
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Old 19-11-2018, 07:52   #2
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Re: 60 ton boat, 120 hp engine, how does that work?

This all depends on the engine. If he had an old Hunstead or Wickman engine he would not have needed 120 HP. Many years ago I helped rebuild a Hunstead 80 HP diesel in a square rigger which weighed at least 60 tons. This engine however developed 80 HP at about 220 RPM and when at idle, it was running at less than 60RPM. The flywheel alone must have weighed a ton or more. Here is a link to video that may help you.
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Old 19-11-2018, 08:00   #3
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Re: 60 ton boat, 120 hp engine, how does that work?

2 hp/ton, perfect fit.
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Old 19-11-2018, 08:02   #4
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Re: 60 ton boat, 120 hp engine, how does that work?

Most "trawler" style boats are grossly over powered. This seems to be an American thing similar to car engines.

Our 38' trawler cruises at 7kts at 1800rpm with a 55hp engine and uses 0.75 gph. Yes, it is all in the gear reduction and propeller.

My great grand father's 40' north sea trawler (real trawler) had a single cylinder, 30hp engine.
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Old 19-11-2018, 08:09   #5
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Re: 60 ton boat, 120 hp engine, how does that work?

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Originally Posted by MartinR View Post
2 hp/ton, perfect fit.

The rough Rule of thumb for determining Horsepower Required for a Sailboat : one horsepower per 500 pounds ( ± 50lbs ) loaded.

Westerbeke Corporation suggests 2 hp for every 1,000 lb of displacement for coastal cruising, and 2.5 hp per 1,000 lb of displacement for offshore.

In their book "The Gougeon Brothers on Boat Construction" the authors suggest a good rule-of-thumb is 1 hp for every 500 lb of displacement.

Some people suggest 1 hp for every ft. of boat waterline length.

Some people suggest 3 - 5 hp (continuous rating) per long ton (to calculate the boat's weight in long tons divide the displacement by 2,240). For offshore conditions some people suggest the engine be larger than the size calculated for coastal cruising, possibly + 25%.
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Old 19-11-2018, 08:53   #6
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Re: 60 ton boat, 120 hp engine, how does that work?

Here is the link that should have been in my earlier post
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Old 19-11-2018, 09:40   #7
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Re: 60 ton boat, 120 hp engine, how does that work?

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The rough Rule of thumb for determining Horsepower Required for a Sailboat : one horsepower per 500 pounds ( ± 50lbs ) loaded.

Westerbeke Corporation suggests 2 hp for every 1,000 lb of displacement for coastal cruising, and 2.5 hp per 1,000 lb of displacement for offshore.

In their book "The Gougeon Brothers on Boat Construction" the authors suggest a good rule-of-thumb is 1 hp for every 500 lb of displacement.

Some people suggest 1 hp for every ft. of boat waterline length.

Some people suggest 3 - 5 hp (continuous rating) per long ton (to calculate the boat's weight in long tons divide the displacement by 2,240). For offshore conditions some people suggest the engine be larger than the size calculated for coastal cruising, possibly + 25%.
I would think for offshore you need less horsepower than for coastal. The main reason for excess power would be to be able to claw off a lee shore. For long ocean passages, you want to save on fuel, and in that respect a smaller engine is preferable.
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Old 19-11-2018, 09:52   #8
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Re: 60 ton boat, 120 hp engine, how does that work?

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Most "trawler" style boats are grossly over powered. This seems to be an American thing similar to car engines.

they are trying to get that extra knot that costs hundreds more horsepower. example: the now out of production nordhavn 35 which had a 350 hp yanmar. ridiculous but it could do 10 knots.
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Old 19-11-2018, 10:20   #9
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Re: 60 ton boat, 120 hp engine, how does that work?

I have an 8 ton 33’ sailboat with a 13hp olvo MD7A. It’s a tad weak now and again but generally no issue. So that’s just a bit under the 2hp/ton rule.

My guess would be that as you go up in tonnage the HP requirement would come down a bit.
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Old 19-11-2018, 10:31   #10
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Re: 60 ton boat, 120 hp engine, how does that work?

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I would think for offshore you need less horsepower than for coastal. The main reason for excess power would be to be able to claw off a lee shore. For long ocean passages, you want to save on fuel, and in that respect a smaller engine is preferable.

===


The extra horsepower is useful when battling heavy seas and head winds, conditions that can easily reduce your forward progress by 50% or more. Under powered boats that I've experienced have difficulty just keeping their bow into the wind in winds over 35kts or so. Some extra power is also very useful when maneuvering in tight situations.
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Old 19-11-2018, 11:20   #11
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Re: 60 ton boat, 120 hp engine, how does that work?

As this is probably a 'displacement' hull the criteria for horsepower is based on the waterline length and has very little to do with the weight assuming that the hull has a fair entry and has not got a bluff bow or a planing hull.
To run the hull at 6 knots would require very little horsepower if the hull length was say 50 feet. Because the resistance to movement through the water for boats running at 'low' speed is based around the square root of the waterline length of the hull. If the water line length (LWL) is 49 feet then the square-root of that is 7 which would be a very economical speed. At that speed the wave created at the bow would be just finishing at the stern.
The maximum speed of a displacement type hull is 1.33 times the square root of LWL which is this case is 7x 1.33 = 9.3 knots. Putting any more horse power in would just make the stern sink further and create bigger waves.
If you draw a graph of the speed against the horsepower you would see that at low speed you could put in 20 horsepower to do 4 knots yet at 8 knots you would need perhaps 100 horsepower.
Now if you had a planing hull running at low speed (below the displacement hull speed) they would roll like pigs in any seaway. A true displacement hull like a fishing boat is usually fat bellied to minimize the roll and provide a greater moment to make it more self-righting.
Power for planing hulls is a different ball game than that for displacement hulls.
A Semi displacement hull is another and different category usually used by pilot boats which can go through a range of different speeds above displacement speeds and yet not get to planing speeds.
Commercial vessels usually run at less than 0.6 root LWL to minimise their power requirements.
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Old 19-11-2018, 12:19   #12
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Re: 60 ton boat, 120 hp engine, how does that work?

The word torque has not been mentioned yet I believe. Diesel engines produce gobs of torque but not great amounts of horsepower. These boats are able to swing big props because of torque. Torque is muy bueno.
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Old 19-11-2018, 12:33   #13
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Re: 60 ton boat, 120 hp engine, how does that work?

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This all depends on the engine. If he had an old Hunstead or Wickman engine ...
I know you're right and I know it all has to do with RPM and torque from any given engine but is there a fairly simple relationship or equation we can use to compare very different engines of the same HP (given the right matching propeller and gearbox of course)? Somehow I missed mechanics 101.

And thanks for the video - beautiful!
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Old 19-11-2018, 12:35   #14
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Re: 60 ton boat, 120 hp engine, how does that work?

MOJO is 85,000 lbs displacement and it takes about 60-65 hp to push her at 7.5 kts. It's all about efficient hull design. See MOJO's particulars at www.mvmojo.com
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Old 19-11-2018, 12:37   #15
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Re: 60 ton boat, 120 hp engine, how does that work?

We have a 65 tonne (71 ton ) ex working trawler.
She has 350hp of nta855m Cummins at her disposal but to get her to her near zero wash hull speed of 7.5 knots we only use about 85hp @ 1150rm and rarely go past 8.5kn using 120hp @ 1250rpm.


Our sister(from a different mother) ship when converted at the same time was re engined with a 6lxb Gardner @ 152hp.
I imagine she runs near full noise vs our fast idle.
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