Using Turboprops for Ship Propulsion: Pros & Cons

[Anand Sivaram]

TL;DR

Turboprop for ship

Normally ships use either marine diesel engine or turboshaft gas turbine connected to the main water propeller.

I was wondering why aircraft like turboprop could not be used. Unlike turbojets, turboprops have good efficiency when both the vehicle speed and propeller tip speed are much lower compared to the speed of sound. Ships anyway are slow moving. That way, the whole propulsion could be air based.

For example C130J has four turboprops of around 3500KW each. Whereas the largest turboprops like the Soviet Tu-95 or An-22 use four contra rotating turboprops of 11000KW each.

In a ship 6 or 8 of these turboprops could be used and that could easily get upto 60MW of power.
Is there any particular reason why no one is using it?

 

[russ_watters]

Is there a difference between turboprop and turboshaft?

 

[BvU]

I think he wants to put big propellers on deck ... airboat-like

 

[Baluncore]

A displacement boat hull has a speed limited by bow wave and wake generation.
Greater efficiency can be obtained from slow water propellers than fast air screws.

Big slow diesels drive ship propeller shafts directly, gas turbines require a high reduction ratio gearbox to drive a water propeller.

A gas turbine in a fast aircraft has ram-air pressure to assist the first compressor. For a slow ship that would require an extra super-charger or compressor stage.

 

[Anand Sivaram]

@russ_watters :
>> Is there a difference between turboprop and turboshaft?
They are very much related. Turboprop < 10% thrust is generated by the jet stream output, whereas turboshaft is used in helicopters and for power generation.

@BvU
>> I think he wants to put big propellers on deck ... airboat-like
Exactly, something like C130J etc.

@Baluncore
I think efficiency could be a factor for water propellers.
But, one of the main advantages of turboprop vs. aircraft piston engine driven propeller is that turboprop is much reliable because of less number of moving parts compared to the piston engine. Also, the power to weight ratio is better for turboprop compared to a piston engine.

 

[jrmichler]

It's about thrust.

Small airplanes get about 4 lbs thrust per horsepower.
Helicopters get about 10 lbs thrust per horsepower.
Ships get over 30 lbs thrust per horsepower.

Thrust per horsepower comes from propeller diameter and density of fluid. Helicopters get more thrust per horsepower than airplane propellers because the rotor diameter is several times larger than propeller diameter. Ship propellers get more thrust per power than aircraft propellers because water is 800 times more dense than air.

Propeller efficiency is a minor variable. Most properly designed propellers (air and water) are over 80% efficient.

 

[russ_watters]

@Baluncore
I think efficiency could be a factor for water propellers.
But, one of the main advantages of turboprop vs. aircraft piston engine driven propeller is that turboprop is much reliable because of less number of moving parts compared to the piston engine. Also, the power to weight ratio is better for turboprop compared to a piston engine.

Those don't apply to turboprop vs turboshaft/GTE, though do they?

In any case, efficiency will drive everything, and I'm sure it's possible to find the efficiency difference with a little reasearch.

And I'm sure you will find that air boats are many times less efficient - maybe even orders of magnitude less efficient - than water propeller driven boats. The reason for this is that power is thrust(force) times velocity, and because air is less dense than water you have to move it at a much higher velocity to generate the same force.

 

[gmax137]

notwithstanding all the excellent replies, I have to wonder if the Pratt and Whitney guys down in Jupiter Fl have lashed a "spare" turboprop onto an airboat :)

 

[russ_watters]

It's about thrust.

Small airplanes get about 4 lbs thrust per horsepower.
Helicopters get about 10 lbs thrust per horsepower.
Ships get over 30 lbs thrust per horsepower.

Thrust per horsepower comes from propeller diameter and density of fluid. Helicopters get more thrust per horsepower than airplane propellers because the rotor diameter is several times larger than propeller diameter. Ship propellers get more thrust per power than aircraft propellers because water is 800 times more dense than air.

Propeller efficiency is a minor variable. Most properly designed propellers (air and water) are over 80% efficient.

Dang, I didn't notice you beat me to it. Excellent post -- and you had the numbers I just guessed about.

 

[cjl]

Is there a difference between turboprop and turboshaft?

A turboprop is a turboshaft with a propeller attached. They're functionally identical, which means any ship powered by a turbine engine is, effectively, already powered by a turboprop (just with a much slower, underwater prop).

 

[Anand Sivaram]

From Wikipedia pictures for both turboprop and turboshaft. Looks like there are some subtle differences.

https://en.wikipedia.org/wiki/Turboprop#/media/File:Turboprop_operation-en.svg
https://en.wikipedia.org/wiki/Turboshaft#/media/File:Turboshaft_operation_(multilanguage).svg

In a turboprop the exhaust gases contribute to around 5% to 10% of the total thrust generated.
Whereas in a turboshaft the exhaust gases are not used.

Also, I found an interesting power output figure.
https://www.airliners.net/forum/viewtopic.php?t=771465
It says the following:
"For example a PT6-114A has a shaft horsepower of 675hp, and exhaust thrust of 124 lbs. Add both together using a formula and they come up with an equivalent shaft horsepower of 725hp."
That means actual shaft (turboshaft) power = 675hp, exhaust = 50hp and total = 725hp, then exhaust is 7.4% of the total.
Turboprop is used mainly in aircraft. I doubt the terminology is used in shipping or power plants where as turboshaft is used.

I was mainly wondering about using air based propellers for shipping. Now, I understood that the efficiency is the main problem. jrmichler has given the following numbers.

>> Small airplanes get about 4 lbs thrust per horsepower.
>> Helicopters get about 10 lbs thrust per horsepower.
>> Ships get over 30 lbs thrust per horsepower.