# Tug boat propulsion

Discussion in 'Boat Design' started by manon, Nov 24, 2012.

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### manonJunior Member

Vessel Particulars
L oa=26m
L wl=24.86m
B = 8.0m
D= 3.0m
T=1.8 m
Required bollard pull= 10.0 Ton
speed = 10.5 knots

Now, for this purpose, I want to select 2X600BHP engine, which will give the required bollard pull. Question is, for which condition I will calculate the propeller particulars? 2X600 BHP will give approx. 15 knots speed, but I need this power to over come the required bollard pull. Propeller is fixed pitch.

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### kach22iArchitect

Aren't over half the tugs made Diesel/Electric hybrids?

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### gonzoSenior Member

If you satisfy the minimum bollard pull and speed target, it should be OK. Usually the requirements are the lowest value. If it pulls more or goes a bit faster is not a problem. I think that ideally you would use a lower pitch or higher reduction for a lower speed and higher bollard pull. That way you have a reserve on both aspects.

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### manonJunior Member

dear Kach22i & Gonzo

Gonzo, my confusion arises more and more in every steps, because I have never been dealt with propulsion of TUG boat. It seems three conditions: 1. Bollard pull test condition (10 ton min) 2. free running condition (10.5 knots min) 3. Towing condition (not sure at this stage of my work).

Around 2X600BHP is sufficient to pass the bollard pull test of 10 ton and this is power is more than enough for 10.5 knots. So I should go for 2X600 BHP, that is good. Please help me about the towing condition. Problem here is worse, since I have to use FP propeller. So, should I fix the pitch, gear ratio at the towing speed condition? So far I understand, bollard pull of 10 ton does not mean that she has to pull 10 ton at towing condition (correct me here please). I would like to calculate exact power need at the towing condition (towing speed around 3 knot?)

With best regards
manon

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### johneckSenior Member

You are correct that with a fixed pitch prop the power absorbed and the thrust will be less when the vessel is moving faster. The key with a tug boat is to not overload the engine at bollard, so the exact pitch setting is really dependent on the engine torque characteristics. it is not uncommon to allow the engine to lug and not reach rated RPM at bollard. As long as you can can get full power you will be able to get maximum thrust. Then when towing, the engine can turn faster and you still might be able to get nearly full power to maximize thrust. When free running the engines will be lightly loaded, but there should be enough thrust to achieve a high speed.
If the specification requires several conditions to be met, you will need to check them all with a specific propeller to ensure that you can meet them. In many cases the vessel resistance is unknown and thus it may be difficult to determine how fast it will go.
There is also no good means for measuring thrust produced in a towing condition, so there will probably be no way to prove that this conditon has been achieved.

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### manonJunior Member

Dear Johneck,
Thank you so much for your suggestion.

I am still puzzled about one thing. I will get maximum thrust at free run condition with above particulars (propeller dia 1.2m, blade area ratio=0.585, pitch =1.228 m, gear ratio =2.874) as 34.66 kN. This is the optimum case so far in free run condition. And in towing condition (towing speed 3 knots), the optimum particulars become propeller dia 1.2m, blade area ratio=1.002, pitch =1.051 m, gear ratio =2.635, produced thrust = 40.53 kN. Which propeller should I select than?

Regards,
manon

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### manonJunior Member

uploaded file gives the propulsion and engine power details. Hope I am in the correct path.Can any one please check it for me please?

#### Attached Files:

• ###### Summary Report.doc
File size:
20.7 KB
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### johneckSenior Member

I don't fully understand the summary. For instance, why are you using a design RPM of 1530 vice 1800 rated? These engines will only really produce full power near rated RPM, so by designing for rated power at 85% rated RPM it seems that you are selecting something that won't work.
In general, the key is avoid overloading the engine while getting as much usable power as possible in all conditions. So if you were to design for the free running condition, you will totally overload the engine while towing. Therefore you should select the maximum load condition as the design point and you will still be able to run at the other point, just not absorb full power.
Just as another point; in general propellers are more efficient with bigger diameters and lower RPM. It seems tome that the RPM is a little higher than it should be for the numbers given. How did you select a gear? I am not sure what the Hydrocomp tip speed allowable is, but this application is high for a tug boat.

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### manonJunior Member

Johneck,
you are absolutely correct, there is mistake. I was supposed to give 85% at design power and 100% at design rpm.

Every thing is changed now, I am uploading the second calculation. Now I have a different RPM, gear ratio and thrust result. Do you think this is good enough? Will the propeller or engine be overloaded during her operation?

It will be very helpful for me, if you could provide some guideline about, when the engine or propeller becomes overloaded.

I am highly appreciating your effort and guideline. Thank you so much.

regards,
manon

#### Attached Files:

• ###### New Microsoft Office Word Document.doc
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20.7 KB
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### johneckSenior Member

I think that you are closer; however I still think that the correct approach is to use the 3 kt point as the design condition. I don't think that the free running speed is as important as getting the most out of the engines in the towing condition. With the 1070 pitch, I would predict that the engines will lug to about 1500 RPM. That is probably not acceptable. I would set the pitch to about 920mm. That will allow the engines to nearly reach rated power and RPM at 3 kts. The tug will not go quite as fast, but this is probably a reasonable compromise.
The blade area at 0.9 is a bit low. So if the wake is good, it will probably be OK, but if not then I would be worried about cavitation and vibration.

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### manonJunior Member

I think I have set the design point at 3 knots, which is the towing condition, since I have selected the service option in hydrocomp as towing. (That is why it is showing us the towing pull at service condition, in other service conditions it does not calculate pull. please see the screen shot).

I have worked out according to your suggestion, it really work and gave me higher pull than previous one. please have your kind look to the attached report and make comments and suggestion.

regards,
manon

#### Attached Files:

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• ###### New Microsoft Office Word Document1.doc
File size:
20.7 KB
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### johneckSenior Member

The numbers look reasonable. My only comment is that the engine cannot turn up to 2035 RPM as shown for the freerunning case. It will probably be limited to about 1850 by the governor, so the max speed will be slightly lower.
Also, NavCAD is only a means to roughly estimate the resistance, especially when going that fast in a short stubby towboat. As we say in the US "your mileage may vary". So be careful about putting too much stock in the max speed prediction. I was just on a pushboat recently that pretty much put its foredeck underwater at about 10.5 kts, so going faster was not even an option. Obviously this all depends on the details of your specific tug.

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### manonJunior Member

Johnace,

Do you remember this propulsion problem of TUG boat that I have posted? Now we are at the position to finalize and order the propeller manufacturer. Calculation file is attached here that has been sent to the manufacturer. The problem is, they are saying that, this calculation is wrong! Well, I do not understand here, why and how they can say this calculation is wrong totally?

What I am thinking, they might point at the high blade area ratio (here BAR means EAR at hydrocomp)

Can you please guide me a bit more?

#### Attached Files:

• ###### Tug Boat With 2X600BHP _450kW_ engine and 1800rpm and 4.05GB.pdf
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### johneckSenior Member

Have they said what they consider to be wrong?

It is not a great design point, so they may be concerned about how well it will work, but the calculations appear to be correct. The part about sizing for max speed at 2143 RPM is misleading, so maybe that is where they have a concern. The other issue is building a 4 bladed propeller with a 1.1 DAR. I'm not sure what it would look like, but that might also be a problem.

The prop would work much better as a 1.4m diameter with a DAR of 0.90. I show about 58kN thrust, about 11% increase. Probably still should have 5 blades for that DAR.

For tugs, the prop should be sized for the towing condition and the boat wil go as fast as it goes when free running. I would not even suggest trying to estimate what that speed might be.

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