Engine mount failures

[watchkeeper]

Originally Posted by tunnels

AAHH THE TRICK IS TO KNOW WHAT NEEDS TO BE STRONG AND HOW TO ACHIVE STRENGTH WITHOUT ADDING EXTRA WEIGHT .

iTS POSSIBLE TO MAKE BOATS LIGHTER SIMPLY BY CHANGING TO A BETTER MATERIALS AND THEY ARE STRONGER !!SAME WITH OTHER MATERIALS AS WELL . THE CAR INDUSTRY IS THE PERFECT EXAMPLE OF STRENGTH FROM LESS ! OR STRENGTH FROM GOOD INNOVATIVE DESIGN .

Whats stronger than 5000 series or options in marine grade alloys for hull structure.

[tunnels]

Originally Posted by watchkeeper

Whats stronger than 5000 series or options in marine grade alloys for hull structure.

Its the way its used! ,clever design and shape , How many flat panels do you find in a cars construction ? Every panel or small part has shape or shapes to them .

[MikeJohns]

Originally Posted by watchkeeper

...........
This is not an issue of liability, rather the more important issue of identifying the cause of failure to prevent the same with future pilots including 2 currently in production..........
Class is CZ CCS

The cause of failure is that the long effectively cantilevered threaded studs are taking all the thrust.

The important figure to work to is the fatigue strength of steel threaded studs which for 24mm diameter is 45 MPa. Threads de-rate the fatigue limit hugely and it's as close to immaterial as to the stud strength.

So as I already said;
The stud is fixed at the top and pinned at the bottom so it's very simple cantilever beam theory, you signed yourself as an engineer and designer so I left you to do the calculations since they are not hard.

Work out your extreme fibre stress ( M/Z) keep it below 45Mpa and work out the allowable thrust.

Looking at the picture you posted you can take approximately 4 stud diameters for the cantilever length which is actually close enough to 0.1m so M (bending moment) is 100( T/4) Nmm where T is the total thrust.
Section modulus (Z) of a 24mm stud is (PI r^4)/4

Z is 1357mm3

So stress = M/z using the fatigue limit of 45 =100 (T/4)/1357
Solve for T and you get 0.6kN per mount. As the absolute fatigue design limit for thrust alone without any safety factor....... above this it's just down to your SN curve as to how many cycles to when it finally fails.

Start to see why they fail yet? Every mount of the same type and size regardless of manufacturer will fail quite reliably :-) That's why it's not a specific Isoflex part fault, no other mount would fare any better so there have to be other reasons why they are not failing on similar vessels that we'd have to look closely at.

If you can't fit a thrust bearing then I'd prefer a fully shimmed bolt down (rather than jack up) mount for at least one mount per side.

[powerabout]

Originally Posted by Frosty

Cat makes many HP ratings from one such engine. Is it possible that those that have not failed have a lower HP rating , you cant tell by looking at it.

OR you have been supplied engines that have mistakenly been the wrong HP rating confusing the mounting design specifications.

I have no idea how you could determine that prospect. I suppose contacting Cat with engine numbers may be interesting and if confimed would be the reasons.

Good point Frosty
And fuel consumption would be a good pointer to high horsepower
If Cat did the commissioning test, boost, temps, egt, exhaust backpressure, fuel flow etc the guy would know immediately.

[Frosty]

I wasnt really refering to exhaust back pressures and the like but pump calibration and injectors. An engines fuel consumption would be a very rough yard stick to calculate HP as the writer said,--his drivers are conservative, confusing matters further.

Obtaining mathamatical proof of failure to lay in front of the owner is not going to happen. The figures used in the mount calcs must be conservataive if not wrong for the reason above.

[MikeJohns]

Originally Posted by Frosty

I wasnt really refering to exhaust back pressures and the like but pump calibration and injectors. An engines fuel consumption would be a very rough yard stick to calculate HP as the writer said,--his drivers are conservative, confusing matters further.

Obtaining mathamatical proof of failure to lay in front of the owner is not going to happen. The figures used in the mount calcs must be conservataive if not wrong for the reason above.

Frosty the main problem is that the threaded rod is just not fatigue resilient. The notch sensitivity of a threaded high tensile rod reduces the allowable ultimate fiber stress to close to an order of magnitude from yield for a 24mm stud. The bigger the threaded stud gets the lower the ultimate fiber stress you are allowed.

[powerabout]

Originally Posted by Frosty

I wasnt really refering to exhaust back pressures and the like but pump calibration and injectors. An engines fuel consumption would be a very rough yard stick to calculate HP as the writer said,--his drivers are conservative, confusing matters further.

.

Thats why I said the Cat engineer would know immediately
They are able to tell you the horsepower very close just by measuring the parameters and input to laptop.