HP per tonne

[Wckoek]

Agreed, it was also good thinking to get a higher hp for margin of safety.
But the point is most pleasure craft do not run hard enough to clean out the carbon deposit of the engine, bring out its power potential and fuel economy.
Would 4 hp per tonne be "more than enough" or "just enough" in most cases on modern boats?

[StuM]

Originally Posted by Wckoek

Agreed, it was also good thinking to get a higher hp for margin of safety.
But the point is most pleasure craft do not run hard enough to clean out the carbon deposit of the engine, bring out its power potential and fuel economy.

To quote my Yanmar Owner's Manual.
(7) When operating the engine at low speed for long periods of time, race the engine once every 2 hours.
Note: Racing the Engine
With the clutch in NEUTRAL, accelerate from the low speed position to the high speed position and repeat this process about 5 times. This is done to clean out carbon from the cylinders and the fuel injection valve.
Neglecting to race the engine will result in poor exhaust colour and reduce engine performance,

Other recommendations I've seen when operating at low RPM/engine temperature is to run at high RPM under load for about 5 minutes every few hours.

[StuM]

Originally Posted by Wckoek

Agreed, it was also good thinking to get a higher hp for margin of safety.
But the point is most pleasure craft do not run hard enough to clean out the carbon deposit of the engine, bring out its power potential and fuel economy.

As for fuel economy,I can push my 11 ton on one engine at 5 knots at 2200 RPM (58% if WOT) burning about 2 lph. If I go to 2800 RPM (74% of WOT), I get about an extra knot but almost double the fuel burn (Any higher than that is when I fire up the second engine and run them both at lower RPM)

[StuM]

Originally Posted by Wckoek

Would 4 hp per tonne be "more than enough" or "just enough" in most cases on modern boats?

IMNSHO, 4HP per ton would be just about enough in "most cases".. It's those "other cases" that you need to be concerned about like fighting your way to a port in heavy weather and big seas where you will glad of everything you have available. Especially if it is a motor cruiser and you don't have sails and a keel.

So really, a lot depends on what your plans are. What's your "use case"? Where will you be using it? What tides/currents and weather conditions are you aniticipating? How far offshore do you intend to travel?

[Wckoek]

Originally Posted by StuM

IMNSHO, 4HP per ton would be just about enough in "most cases".. It's those "other cases" that you need to be concerned about like fighting your way to a port in heavy weather and big seas where you will glad of everything you have available. Especially if it is a motor cruiser and you don't have sails and a keel.

So really, a lot depends on what your plans are. What's your "use case"? Where will you be using it? What tides/currents and weather conditions are you aniticipating? How far offshore do you intend to travel?

Assuming we are talking about a 7 tonne 38 feet fuel efficient motor cruiser that is capable of offshore, CE category A, Van De Stadt's Stadtline 38 comes into my mind.
If we go with 4 hp per tonne, we may use 28hp, say for example a Yanmar 3YM30AE (29.1hp), which will burn around 0.66 gallons or 3 litres per hour at 75% rpm, with a 200 gallon of fuel capacity, running at hull speed, at ideal condition we could have a range of 2121 nautical miles.
If we go with 6 hp per tonne, we'll get 42HP, using the Yanmar 4JH45 (45hp) as an example, running at 75% would consume 1.5 gallons or 6 litres per hour, doubled that of the 30hp engine, and at hull speed, probably we'll only get half of the range, higher fuel consumption and 95kg more of weight.
Of course I don't know for a motor cruiser with efficient hull shape, whether 4hp per tonne is sufficient, or it will be more safe with 6 hp offshore.

[SecondBase]

Enough HP to push it 6 knots in flat water is enough for utility use and will allow you to keep on the ICW schedule if that matters to you. Is part of your engines mission to fight you uphill back into port in bad weather? If so 4hp per ton. If not the minimum hp to get to 6 knots.

The guy who went from 2hp per ton to 4hp per ton did engine manufacturers a solid favor.

As always "less is more".

[StuM]

Originally Posted by Wckoek

Assuming we are talking about a 7 tonne 38 feet fuel efficient motor cruiser that is capable of offshore, CE category A, Van De Stadt's Stadtline 38 comes into my mind.
If we go with 4 hp per tonne, we may use 28hp, say for example a Yanmar 3YM30AE (29.1hp), which will burn around 0.66 gallons or 3 litres per hour at 75% rpm, with a 200 gallon of fuel capacity, running at hull speed, at ideal condition we could have a range of 2121 nautical miles.
If we go with 6 hp per tonne, we'll get 42HP, using the Yanmar 4JH45 (45hp) as an example, running at 75% would consume 1.5 gallons or 6 litres per hour, doubled that of the 30hp engine, and at hull speed, probably we'll only get half of the range, higher fuel consumption and 95kg more of weight.
Of course I don't know for a motor cruiser with efficient hull shape, whether 4hp per tonne is sufficient, or it will be more safe with 6 hp offshore.

But you won't need to run the 42HP at 75% to get the power you are getting from the 28HP at 75%!

If you have your 28HP running at 75% RPM , it will be using somewhere around 40% of available power or about 12HP and 3lph.

To get the same speed from the 42HP Yanmar, you will need about the same 12 HP and 3 lph. The difference being that you will run it at only about 65% RPM and will have more power in reserve for bashing into heavy weather.

It's when you need to run at high RPM % that your fuel consumption really goes up.

Let's assume you are pushing into strong winds and rough seas and you need another 10 HP to make reasonable headway (i.e. 22 HP total).

With the 28 HP engine you will need to run at about 92%. using roughly 6.5 lph
With the 42 HP engine you will need to run at about 82%. using more like 5.5 lph.







.

[Wckoek]

There is an argument that leisure boat doesn't push their diesels enough and causes carbon buildup.
Maybe a median, between 4 and 6 hp, 5 hp would be good for efficient hull.
Not so efficient hull maybe 6.

[a64pilot]

First torque and HP are actually the same thing, expressed differently.

Don't believe me? Then explain why HP and torque curve always cross at 5252 RPM on any dyno chart that hasn't been doctored.

Then there is a lot more to determining power required to move a hull than just weight, some hulls are more easier driven than others, you racers have to know that better than I.

There is never a simple formula, however there are many general rules that get you close.
Over powered? Then enjoy the excess if you ever need it and the rest of the time enjoy the lower noise, RPM and vibration that goes with an engine not being run hard.
Underpowered? Then maybe you won't have enough power to claw away from that leeward shore you shouldn't have gotten close to, but due to a lapse in judgement you did. You travel slower and get to listen to a louder engine, but likely burn less fuel, cause your going slower is all, but do have a lighter engine that takes up less space.

My opinion is the reason HP of sailboats is climbing, is largely marketing. Put the specs of two boat together when looking and average person will buy the boat with the larger motor, cause bigger is better, right?
Maybe more than used to be that new boat owners are motoring more than they used to?

[barnakiel]

On a nice and clean hull 2hp seems to produce displacement/hull speed in flat water.

I think I can see many modern boats having about 5hp.

Our boat has 4hp which is heaps and enough when she is clean.

All depends on how much wind / wave you expect in your face.

Cheers,
b.

[David M]

It depends on how important it is to you to reach or to slightly exceed hull speed.

Wckoek is exactly right about a Diesel needing to be run at maximum RPM for a half hour or so to burn out carbon deposits. Yes it is bad for fuel economy to do this but it is really good for the engine to do this.

Idling a Diesel around all the time is really good for fuel economy but is really bad for engine longevity.

[barnakiel]

I think chasing the hull displacement speed is a mistake. If you want to go fast, get a planing hull. If you do not want one, but do want more speed, get a longer hull.

Many sailboat hulls suck big way (hydro-dynamically) when they reach hull speed. The change from 1lwl to say 1.34lwl is quite a jump. Aim for about 1lwl or thereabout in a regular boat.

Cheers,
b.

[hpeer]

Good grief, my small boat is 8 ton with a 13 HP Volvo MD7. To think I've been happy all these years and 10,000 miles. Many more miles than that counting the PO.

The big boat is 20 tons with a 72 HP Yanmar. I see I'm guilty of running too low an RPM for extended periods. Thanks for that advice. But really, I get around just fine at these low RPMs, 1,800 - 2,200.

Also, elsewhere there was a thread on what RPMs folks really use, sorry can't recall where. Anyway, it seemed the majority of folks ran well below the recommended range because of vibration issues.

[a64pilot]

My 25,000 lb 33 ft waterline 40 HP boat I run at 2000 RPM and get about 6.5 kts. Motor is rated for 3600 RPM.
I run 2000 RPM more than anything else for noise. At higher RPM it sounds like we have a weed wacker in the salon.
Running lower RPM also pretty much doubles available range. Going just a little more than 1 kt faster almost doubles fuel consumption, and of course cuts range almost in half.

I'd rather have more motor though, I'd like to have about 50 HP I think.
I doubt much would change, motor is same weight and size and if I ran the same speed I believe fuel consumption would be very close. But I would have more reserve power if I needed it.

[StuM]

Originally Posted by barnakiel

On a nice and clean hull 2hp seems to produce displacement/hull speed in flat water.

I think I can see many modern boats having about 5hp.

Our boat has 4hp which is heaps and enough when she is clean.

All depends on how much wind / wave you expect in your face.

Cheers,
b.

When I first read that, I thought - No Way!.

Then I realised that you meant hp/ton (at least I hope you did)