

14082017, 10:02

#1

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Join Date: Sep 2016
Posts: 183

HP per tonne
Reading Pat Manley's diesel afloat he mention that people used to use 4 hp per tonne, while more often today's manufacturer uses 6 hp per tonne on displacement boats.
He prefer 4 hp per tonne and he mention that Diesel engines need to be pushed hard or you are storing problem for later, but he didn't specify which problem that will arise, sailboat diesel particularly needed to be push hard.
I would like to hear you guys opinion on this, and whether for a displacement motor cruiser 6 hp per tonne is more relevant?
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14082017, 10:31

#2

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Posts: 4,885

HP per tonne
The rule of thumb for sailboats used to be 2hp/ton but that's crept up to 4hp/ton. I'd have to doublecheck my old references.
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14082017, 10:40

#3

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Join Date: Jun 2017
Location: Subic Bay Philippines
Posts: 537

Re: HP per tonne
Running a diesel "hard" will prove to be beneficial in most cases as it gets the engine and all it's parts up to the correct operating temperature and cleans out carbon deposits through out the engine, most pleasure vessel diesel engines corrode away way before they wear out as they are always starting and stopping. Running for extended periods at load (with in the manufacturers recommendations) will extend the life of most engines,
All engines have a load rating as:
Continuous Duty
For use in applications requiring uninterrupted and unlimited service at full power.
Load Factor: 80% to 100%
Typical Annual Operation Hours: 5000 to 8000 hours
Typical Hull Forms: Displacement
Typical Applications: Freighters, tugboats, bottom drag trawlers, or deep river tugboats, dredges
Heavy Duty
For nearly continuous use in variable load applications where full power is limited to 8 hours out of every 10 hours of operation.
Load Factor: 40% to 80%
Typical Annual Operation Hours: 3000 to 5000 hours
Typical Hull Forms: Displacement
Typical Applications: Midwater fishing trawlers, crew and supply boats, ferries, purse seiners, and towboats. Or auxiliary applications like thrusters and cargo pumps in dynamic positioning.
Medium Duty
For moderate use in variable load applications where full power is limited to 6 hours out of every 12 hours of operation.
Load Factor: 20% to 80%
Typical Annual Operation Hours: 2000 to 4000 hours
Typical Hull Forms: Semidisplacement and displacement
Typical Applications: Ferries, harbor tugs, fishing boats (designed for high speed), offshore service boats, (noncargo) displacement hull yachts, or short trip coastal freighters.
Light Duty
For intermittent use in variable load applications where full power is limited to two hours out of every eight hours of operation.
Load Factor: Up to 50%
Typical Annual Operation Hours: 1000 to 3000 hours per year
Typical Hull Forms: Planing and semidisplacement
Typical Applications: Offshore patrol boats, customs boats, police boats, some nonnet fishing, fireboats, military and police vessels, or harbor tugs. Or auxiliary applications like emergency fire pumps and hydraulic power packs.
Pleasure Duty
For infrequent use in variable load applications where full power is limited to one hour out of every eight hours of operation.
Load Factor: Up to 30%
Typical Annual Operation Hours: 250 to 1000 hours
Typical Hull Forms: Planing
Typical Applications: Pleasure craft, harbor patrol boats, harbor master boats, some fishing or patrol boats, sportfishers, motoryachts, yachts,and cruisers.
Cheers Steve
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14082017, 10:47

#4

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Join Date: Sep 2016
Posts: 183

Re: HP per tonne
Quote:
Originally Posted by Captsteve53
Running a diesel "hard" will prove to be beneficial in most cases as it gets the engine and all it's parts up to the correct operating temperature and cleans out carbon deposits through out the engine, most pleasure vessel diesel engines corrode away way before they wear out as they are always starting and stopping. Running for extended periods at load (with in the manufacturers recommendations) will extend the life of most engines,
All engines have a load rating as:
Continuous Duty
For use in applications requiring uninterrupted and unlimited service at full power.
Load Factor: 80% to 100%
Typical Annual Operation Hours: 5000 to 8000 hours
Typical Hull Forms: Displacement
Typical Applications: Freighters, tugboats, bottom drag trawlers, or deep river tugboats, dredges
Heavy Duty
For nearly continuous use in variable load applications where full power is limited to 8 hours out of every 10 hours of operation.
Load Factor: 40% to 80%
Typical Annual Operation Hours: 3000 to 5000 hours
Typical Hull Forms: Displacement
Typical Applications: Midwater fishing trawlers, crew and supply boats, ferries, purse seiners, and towboats. Or auxiliary applications like thrusters and cargo pumps in dynamic positioning.
Medium Duty
For moderate use in variable load applications where full power is limited to 6 hours out of every 12 hours of operation.
Load Factor: 20% to 80%
Typical Annual Operation Hours: 2000 to 4000 hours
Typical Hull Forms: Semidisplacement and displacement
Typical Applications: Ferries, harbor tugs, fishing boats (designed for high speed), offshore service boats, (noncargo) displacement hull yachts, or short trip coastal freighters.
Light Duty
For intermittent use in variable load applications where full power is limited to two hours out of every eight hours of operation.
Load Factor: Up to 50%
Typical Annual Operation Hours: 1000 to 3000 hours per year
Typical Hull Forms: Planing and semidisplacement
Typical Applications: Offshore patrol boats, customs boats, police boats, some nonnet fishing, fireboats, military and police vessels, or harbor tugs. Or auxiliary applications like emergency fire pumps and hydraulic power packs.
Pleasure Duty
For infrequent use in variable load applications where full power is limited to one hour out of every eight hours of operation.
Load Factor: Up to 30%
Typical Annual Operation Hours: 250 to 1000 hours
Typical Hull Forms: Planing
Typical Applications: Pleasure craft, harbor patrol boats, harbor master boats, some fishing or patrol boats, sportfishers, motoryachts, yachts,and cruisers.
Cheers Steve

Is the load factor equivalent to engine rpm?
I mean displacement boat, both motor and sail in this case.
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14082017, 14:49

#5

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Join Date: Nov 2006
Location: San Francisco Bay
Boat: Fantasia 35  s/v Feeling Good
Posts: 1,080

Re: HP per tonne
I have recently been informed that hp is not the correct parameter for selecting the size of the engine for a displacement vessel. The correct parameter is torque.
I have a 35+ year old Volvo MD17C that develops a max of 35 hp in a 12 ton (cruising load) sloop. This is kind of on the light side at only about 3 hp per ton. Considering the engine's age, parts availability and cost I am looking at replacing it with a Beta engine. However, the few times I have run the Volvo at full power it seem to be way over powered with a big wake and a lot of squat. Thus, I was considering going to a lower hp engine. But, when I spoke with the Beta rep. he said that engine torque and not hp was the spec. to match and suggested that I go with the Beta 38 to match the torque of the Volvo.
I confess that, except for the above, I know squat about the technical considerations for selecting engines for boats. So, don't use this info as your only source when selecting an engine.
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14082017, 15:09

#6

Registered User
Join Date: Jan 2014
Posts: 600

Re: HP per tonne
Quote:
Originally Posted by Wckoek
Is the load factor equivalent to engine rpm?
...

John Deere defines load factor as:
Quote:
Load factor is the actual fuel burned over a period of time divided by the fullpower fuel consumption for the same period of time. For example, if an engine burns 160 liters of fuel during an eighthour run, and the fullpower fuel consumption is 60 liters per hour, the load factor is 160 liters / (60 liters per hour x 8 hours) = 33.3 percent.

Later,
Dan
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14082017, 15:49

#7

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Join Date: Aug 2005
Location: Kona, Hawaii, Carlsbad, CA
Boat: 1969 Pearson 35 #108 & 1976 Sabre 28
Posts: 6,027

Re: HP per tonne
Did more than 10,000 miles in a boat with 2hp per ton. Never a problem though we did motor sail going to weather mainly because it got us to where we were going burning very little fuel. Would easily push the boat at 5kts in flat water. Boat was over propped so was never able to try the engine for top speed.
Made several deliveries on the same design boat with 4hp per ton engines and felt the boats had too much power. Witnessed one idiot skipper stop the boat on a reach at hull speed before he crashed into the bridge at the end of a channel.
Suppose if you are the type that turns the engine on any time boat drops below hull speed and/or hp as a substitute for good seamanship, 6hp per ton might be justified.
The bigger the engine, the larger the prop that it will spin and the more drag under sail. Without an investment in a feathering prop, you'll hurt sailing performance way more than a smaller engine/prop. If you haven't checked, feathering props cost a boat unit or two so will make the larger engine even more costly.
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14082017, 23:03

#8

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Join Date: Jun 2017
Location: Subic Bay Philippines
Posts: 537

Re: HP per tonne
I think GORD MAY answered this question in a very thorough way back in 2005 as:
The rough Rule of thumb for determining Horsepower Required for a Sailboat : one horsepower per 500 pounds ( ± 50lbs ) loaded.
Expect you'll want a lot more than 28 HP (> 55 HP).
PS:
Just came accross this in my files:
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. That would mean a 38 ft. sailboat with a displacement of 15,000 lb should have a 30 hp engine for coastal cruising and a 37.5 hp engine for offshore.
In your case: 15 Long Tons = 33600#
Hence: 33. x 2 = 67.2 HP ~or~ 33.6 x 2.5 = 84 HP.
In their book "The Gougeon Brothers on Boat Construction" the authors suggest a good ruleofthumb is 1 hp for every 500 lb of displacement... that would mean 38 ft. sailboat with a displacement of 15,000 lb should have a 30 hp engine.
In your case 33 x 2 = 66 HP
Some people suggest 1 hp for every ft. of boat waterline length... that would mean a 38 ft sailboat with a waterline length of 30 ft should have a 30 hp engine.
In your case 41 x1 = 41 HP
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)... that would mean a boat with a displacement of 15,000 lb should have a 20  33 hp engine. For offshore conditions some people suggest the engine be larger than the size calculated for coastal cruising, possibly + 25%... so in this example the engine size would be increased to 25  41 hp respectively.
In your case up to 5 x 15 = 45 HP
How do you measure horsepower?
Power is a measurement of the ability to do a unit of work in a specific time. It is calculated, not measured. Horsepower and watts (kilowatts are 1,000s of watts) are units of power. Power is calculated by measuring engine torque at a specific crankshaft rpm and multiplying the 2 figures together.
Engine Horsepower/Torque Formulae
Horsepower = ( Torque in Lb/Ft x Crankshaft Speed in RPM ) ÷ 5252
Power = Torque X RPM
Torque = Power ÷ RPM
RPM = Power ÷ Torque
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14082017, 23:55

#9

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Join Date: Aug 2010
Location: Central California
Boat: M/V Carquinez Coot
Posts: 3,423

Re: HP per tonne
An 80HP engine pushes my fat, 14ton boat up to hull speed with 200 RPM below WOT. A good fit for me.
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14082017, 23:58

#10

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Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43
Posts: 6,820

Re: HP per tonne
> Torque in Lb/Ft
Just for clarity's sake. That is, of course, "Torque in FtLb"  or LbFt (foot pounds or pound feet, i.e. ft x lbs,), not lb/ft (pounds per foot, lb ÷ ft ).
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15082017, 00:23

#11

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Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43
Posts: 6,820

Re: HP per tonne
Are we talking about a sailboat auxilliary or a motor vessel's primary propulsion?
The OP states "displacement motor cruiser". Most responses seem to be talking about sailboats.
If we're talking about sailboats, then:
I've got 80 HP ( 2 x 40HP) with a loaded displacement of around 11 ton, so that works out at over 7 HP/ton.
But I've never had to use all that in anger.
I use 2 engines at low revs for maneuvering, but generally a single 40 HP gives all the grunt I need for motoring, so that's about 3.6 HP/ton.
For "displacement motor cruisers", you often see 710 HP/ton.That will give decent cruising speed at economical revs and spare power when needed.
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15082017, 01:13

#12

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Join Date: Sep 2016
Posts: 183

Re: HP per tonne
In the aforementioned book, Pat said that you should never cruise lower than 75% of your maximum continuous RPM, say would be 2400 rpm in a 3200 engine, and that best rate of fuel consumption usually comes on that range from 75%80%, would say 24002600 rpm which is quite accurate on the engine specs I refer to as well.
You get the highest torque around 300 rpm less than maximum rpm, 2900 in a 3200 engine.
75% of RPM would generate 50% of maximum rated power, so 4hp per tonne (loaded) in cruising would use 2 hp per tonne power at cruising.
Quote:
Originally Posted by StuM
Are we talking about a sailboat auxilliary or a motor vessel's primary propulsion?
The OP states "displacement motor cruiser". Most responses seem to be talking about sailboats.
If we're talking about sailboats, then:
I've got 80 HP ( 2 x 40HP) with a loaded displacement of around 11 ton, so that works out at over 7 HP/ton.
But I've never had to use all that in anger.
I use 2 engines at low revs for maneuvering, but generally a single 40 HP gives all the grunt I need for motoring, so that's about 3.6 HP/ton.
For "displacement motor cruisers", you often see 710 HP/ton.That will give decent cruising speed at economical revs and spare power when needed.

I mean both sail and motor cruiser, and would be interested in seeing if there is a difference between them both in practice, it seemed that the vast majority of sailboats have overpowered engines.
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15082017, 01:36

#13

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Join Date: Sep 2016
Posts: 183

Re: HP per tonne
Quote:
Originally Posted by Captsteve53
I think GORD MAY answered this question in a very thorough way back in 2005 as:
The rough Rule of thumb for determining Horsepower Required for a Sailboat : one horsepower per 500 pounds ( ± 50lbs ) loaded.
Expect you'll want a lot more than 28 HP (> 55 HP).
PS:
Just came accross this in my files:
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. That would mean a 38 ft. sailboat with a displacement of 15,000 lb should have a 30 hp engine for coastal cruising and a 37.5 hp engine for offshore.
In your case: 15 Long Tons = 33600#
Hence: 33. x 2 = 67.2 HP ~or~ 33.6 x 2.5 = 84 HP.
In their book "The Gougeon Brothers on Boat Construction" the authors suggest a good ruleofthumb is 1 hp for every 500 lb of displacement... that would mean 38 ft. sailboat with a displacement of 15,000 lb should have a 30 hp engine.
In your case 33 x 2 = 66 HP
Some people suggest 1 hp for every ft. of boat waterline length... that would mean a 38 ft sailboat with a waterline length of 30 ft should have a 30 hp engine.
In your case 41 x1 = 41 HP
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)... that would mean a boat with a displacement of 15,000 lb should have a 20  33 hp engine. For offshore conditions some people suggest the engine be larger than the size calculated for coastal cruising, possibly + 25%... so in this example the engine size would be increased to 25  41 hp respectively.
In your case up to 5 x 15 = 45 HP
How do you measure horsepower?
Power is a measurement of the ability to do a unit of work in a specific time. It is calculated, not measured. Horsepower and watts (kilowatts are 1,000s of watts) are units of power. Power is calculated by measuring engine torque at a specific crankshaft rpm and multiplying the 2 figures together.
Engine Horsepower/Torque Formulae
Horsepower = ( Torque in Lb/Ft x Crankshaft Speed in RPM ) ÷ 5252
Power = Torque X RPM
Torque = Power ÷ RPM
RPM = Power ÷ Torque
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"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

500 lbs is around 226.8 kg. A quarter tonne 250 kg would be 551 lbs, so that is close enough if not on the higher figures of the +50 lbs.
If using Westerbeke's formula, say a 5 tonne boat around 11000 lbs, would be 22 tonne for coastal, 27.5 tonne for offshore, quite a it more than 20hp for 4 hp per tonne figure. Gougeon was the same to Westerbeke's coastal formula.
The waterline formula may not be accurate in this case, there are really heavy boats for their waterline and there are light boats.
But of course I'm not sure if the 4 hp per tonne formula works equally for off shore.
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15082017, 01:51

#14

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Join Date: Sep 2016
Posts: 183

Re: HP per tonne
The book mention run 75% RPM above to minus 300 from the maximum rpm and for sailboats, no more than 75% RPM.
If planing hull 300 rpm less from the maximum.
If twin engine forced to run slow, use one engine whenever possible. So using single engine for motoring fits the books as well.
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15082017, 01:53

#15

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Join Date: Jan 2013
Location: Adelaide Australia
Boat: Cuddles 30ft Motor Sailer
Posts: 218

Re: HP per tonne
The 4 hp per ton is a good rule of thumb for an average full displacement hull, but it isn't a one size fits all thing. If you have a sleek narrow hull and low windage and always have a clean bottom, then 23 hp/ton may be plenty. A wide motor cruiser with a flybridge and lots of windage may need 56 hp/ton.
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