# Tug boat and barge engine power calculation

Discussion in 'Diesel Engines' started by yodani, Aug 18, 2014.

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### yodaniSenior Member

I do not know how to calculate necessary tug power for my convoy . Could it be done with a regular prop calculator?

My setup is :

TUG - 16m long (water line) x 3.7m wide x 1.2m draft
Max prop dia 80cm
Max displacement - 41 cubic meters
Weight 30 metric tons

Floating hotel - 28m long (water line) x 5m wide x 60cm draft - weight - 70mt - flat bottom

Front tugging.
I did some calculations for each boat. Do i have to ad the two results and get the final power?

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2. Joined: Nov 2010
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### yodaniSenior Member

Tug calculation

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3. Joined: Dec 2012
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### Mik the stickSenior Member

Yes If you have a tug style boat which you use to cruise around on your local water.
If you are using it to pull other floating objects then no a tug measures its pull as Bollard pull, without being too technical if a Tug capable of 10 knots requires 300SHP fine. But if it is to tow a barge it might have perhaps 500-600SHP installed.
Probably the best formula for predicting required shaft horse power is Wyman's formula which will work up to SL = 2 as I present it.

W = weight in lbs
Lwl = waterline length (ft)

SHP = (W/1000) * (Kts/Lwl^0.5)^3

Plugging in the numbers for your tug gives 40SHP for 6.5Kts(12Km)
Hull speed(SL=1.34) is 9.7Kts which needs 134SHP
59SHP gives 7.36Kts
Dave Gerr's Propeller handbook explaines Bollard pull better than I could but if your tug was going at 6.5kts it only has 19SHP to overcome adverse weather tidal currents or anything else which could hold it back. My guess and it is only a guess not a calculation is if the tug towed the barge with 150shp installed tow speed might be 3kts.

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### Mik the stickSenior Member

It is late, in the morning I will post some info from Gerr's book which will help you. Now I understand the question NO you cant add the two horsepowers.

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Very simply..the amount of power (resistance) required to move the floating hotel at a certain speed is also the the power (pull) required for the tug(s).

So do you have a drag curve for the hotel?

If not, assume the hotel is a brick, and knowing the drag by such bluff 3D shapes and the WSA you can get a fair approximation of the resistance and hence force required to pull it.

In the absence of any real hard data..it is best you're going to get.

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### rxcompositeSenior Member

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### Mik the stickSenior Member

Well as I suspected my reply has got you answers from the very gentlemen who have helped me learn about boat design.

The only thing I would like to add is ( from Gerr's book) is average tow speed.
A 500SHP tug can expect to pull an average load at 1.43 times BHP
500SHP~526BHP 526^0.21 =3.7kts. times 1.43 =5.33kts.

I now know more about tugs and bollard pull than I did before.

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### yodaniSenior Member

Tug boat speed-

Thank you for your help and answers. I am now traveling with the floating hotel and have measured the speed of the convoy:
- 11.5 km/h downstream
- 7.2 km/h upstream
- average 9 to 9.5 km/h

Fuel consumption - 21 liters/hour plus 800g of oil with the 3D6 engine - 150 hp@1600rpm. The engine runs at 1300rpm (maybe because we have no RPM meter on it).

I will need to repower and my choices are :

Dosan 200 or 250hp@2200rpm
Scania 250hp @1800rpm - common rail

I am looking for best fuel economy and want to know if i can improve on my current setup. I would like to know if adding more power will be bringing my consumption down by gaining speed and efficiency.

A similar tug here replaced the engines with a Sole 205hp and consumes about 16 liters. The new regulation alows me to instal up to 250hp and keep the tug in the same category.

My instinct says to go for the Scania 250hp and to calculate its necessary gearbox and prop accordingly.

The floating hotel is more or less a brick with a fkat botom - 5m wide and 28m long and an average draft of 40cm.

Going for a kort nosel is not the best probably due to the debries in the water.

As many tugs are optimised for various loads, what I want is optimise this for my floating hotel as this is what we do all year long - pull floating hotels.

Daniel

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### Mik the stickSenior Member

Fuel efficiency

I don't think you can improve your fuel economy by in stalling a new bigger engine.

1/ Your tug is doing its job installing a new engine is a considerable cost. So unless your engine is worn out an engine change is not needed. A bigger engine would allow you to pull more or bigger hotels.

2/ Diesel engines use about 0.4lbs of fuel per horsepower per hour. The 205hp engine using 16 liters an hour means the engine is producing an average of 77hp. If your similar boat pulled the same load it would need about the same power and use the same fuel. As I explained your boats maximum speed is about 9.7kts needing 134hp. So without a tow your boat will use about 28 liters per hour of fuel at slightly less than full power. If you fit a 205hp engine it will use 43 liters per hour at maximum power. But you can't go more than 9.7kts so your stern will go deeper into the water as the boat tries to climb its bow wave. The extra power only makes bigger waves, so the extra power is useless unless you intend to tow something bigger.

3/ Finally some engines are more fuel efficient than others. World war two diesels use about 0.45lbs of fuel per horsepower per hour. If your new engine saved .05lb of fuel that would save you the price of 79 liters of fuel on 3000 hours of use. It will take a very long time to save the price of your engine.

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### yodaniSenior Member

Hi Mik,

Thank you for the great info, I apreciate it and am learning every day.

As I told you the speed of the convoy today is about 9km/h and an average fuel consumption of 21 liters/h.

My friend has upgraded the tug with a 205hp engine and is going a bit faster but consumes about 16 liters/h. That is a substantial difference of 5 l/h x 3000h = 15000l x 1.3€/liter 19500€

I think the whole efficiecy of the drive can be to blame, so a repower plus new gearbox and propeller would optimise the towing?

Also we will access a grant that will pay 70% of the repower so repowering is a must...

How do you think a common rail like Scania 250hp@1800 rpm would compare with a classic diesel? Could the better torque curve save me some fuel?

Cheers,

Daniel

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### Mik the stickSenior Member

Yodani I am certainly not an expert on diesel engines BUT it seems the old slow reving classic diesel designed to run for ever is getting harder to aquire. I think all new engines are a little more fuel efficient than they were 30 years ago and common rail diesels are an improvement in efficiency. Many diesels on the market give better fuel consumption than the 0.4lbs per horsepower I suggested to you.
I have attached a document which give fuel consumption/hp and ratings, the engines are mostly John Deere. I understand you are Romanian so I have no idea of availability of any engines in your area. My best advice would be to choose the best engine you can get for your money BUT MAKE SURE THE MANUFACTURER HAS A GOOD RELIABLE SPARES NETWORK which does not charge extra for spares because you live in a remote area.

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### Mik the stickSenior Member

brochure from mik

Read the M ratings and engine limits engine revs versus fuel use.

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13. Joined: Dec 2012
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### Mik the stickSenior Member

Unless your friends tug is the same as your tug pulling exactly the same load you should not compare fuel consumption figures. Depending on the set up it is usual for 3 to 5% power to be lost between engine and propeller.

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### yodaniSenior Member

Hi Mik,

Thanks for the message, as I said before my options here in Romania are - Dosan - Scania and Sole Diesel (based on Deutz). I think most "parts friendly" would be Scania as these engines run in their trucks and they are many on the roads today.

My friend is pulling the exact same load like me and before repower had the same engine with the same consumption but the tug is 2m longer.

I have a hard time interpreting the prop curve power consumptions because they are sometimes confusing as each manufacturer tends to do it based on different standards etc. I'll post my findings below.

Cheers,
Daniel

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### yodaniSenior Member

The Scania 250hp@1800rpm engine - @135hp prop curve - 16.8 liter/h @1400RPM (specific fuel consumption 200g/kWh)

Dosan 200hp@2200rpm - @135hp prop curve - 17.5 liters/h @1800rpm (specific fuel consumption 155g/hph)

And for the sake of it the consumption of the Dosan 320hp@2200rpm - @135hp prop curve - 16.5 liters/h @1500rpm (specific fuel consumption 146g/hph).

As you can see the most efficient theoretically would be the bigger engine - Dosan 320hp...

I have calculated with a fuel density of 0.84 as per Scania standard.

Formula - Prop HP x spec fuel consumption x 0.84

Cheers,

Daniel

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