What is the Elastic Contraction of a Propeller Shaft at Full Power?

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SUMMARY

The discussion focuses on calculating the elastic contraction of a propeller shaft in large ships transmitting 50,000 horsepower at full power. The shaft, measuring 300 feet in length and 24 inches in diameter, experiences an elastic contraction of 0.985 inches when subjected to the calculated forces. The modulus of elasticity (E) assumed for the steel shaft is 30,000,000 lb/in². Participants express skepticism about the provided answer, suggesting a possible misprint, and discuss the complexities of torsional stress and strain in propeller shafts.

PREREQUISITES
  • Understanding of elastic deformation and modulus of elasticity
  • Familiarity with basic physics equations related to power and force
  • Knowledge of material properties, specifically for steel
  • Concepts of torsional stress and strain in mechanical engineering
NEXT STEPS
  • Research the properties of steel and its applications in marine engineering
  • Learn about torsional stress and its impact on mechanical components
  • Explore advanced calculations for elastic deformation in rotating shafts
  • Investigate the design considerations for hollow versus solid shafts in propeller applications
USEFUL FOR

Mechanical engineers, naval architects, and students studying marine propulsion systems will benefit from this discussion, particularly those focused on the structural integrity and performance of propeller shafts in high-power applications.

Chacabucogod
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A propeller shaft in the largest and most powerful ships transmits about 50,000 hp (1HP=33,000 ft*lb/min). Assume that the propeller transforms this power into a forward push on the ship with an efficiency of 70 per cent and that the ship's speed then is 30 knots (1 knot is 6,080 ft/hr).
The shaft is 300 ft long from the propeller to the thrust bearing in the engine room, where the thrust is transmitted to the ship's hull structure. The diameter of the (solid, circular) shaft is 24 in. Calculate the elastic contraction of the shaft at full power. (Answer:0.985 in).

There is no number for E provieded. So I assumed it's 30,000,000 lb/in^2. Maybe that's where the problem lies.

Homework Equations



\Delta l=\frac{FL}{EA}
Power=F*v
Power=1.65x10^9 \frac{ft*lb}{min}
speed=182,400\frac{ft}{hr}50,000*33,000*0.7=v*182,400*\frac{1 hour}{60 min}
F=379.934x10^3 lb
\frac{379.934x10^3*300*12}{144\pi *30,000,000}
Ans=0.105 in
 
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Propeller shafts are made of steel, so using E = 30*10^6 psi is OK.

I think the problem's given answer is suspect, maybe a misprint (0.0985 in instead of 0.985 in). I found no problems with your formulas or arithmetic.
 
There is a complex stress in torsion that causes Euler strain and fractures as I recall from a long time ago. The demonstration was of the 45° angles in a twisted piece of chalk.

A practical propellor shaft of those dimensions would be hollow and filled with an incompressible (in my case, sand).
 

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