T-Pain Is On A Boat: Re: [Electric Boats] Re: Direct drive and thrust bearings vs flange mounted ballbearing

Friday, May 30, 2014

Re: [Electric Boats] Re: Direct drive and thrust bearings vs flange mounted ballbearing

My post was just an comment, based on what I learned.
As an aside, my data comes from Cross&Moore, a 50 person company in the UK who make marine stuff (and other (cn) things).
I spoke to one of the guys there, when they came to barcelona some years ago.
I consider them authoritative. Grin.

One right way to consider this, is that a boat of say 4000 kg, will be slowed from 6 knots, to about 0, when hitting a wave, within about 1 second.
This full impact, of about 40 kN, will transmit to the prop and the bearings.

Of course, the load is elastic , but its still an impact load technically.
You see this, as the revs on an engine drastically slow down in an impact against a wave.

The worse the prop is, as in smaller ==less efficient == less thrust, the smaller the shock load.

So empirical testing, in marine gears, shows that you need 5x the strength for a given auto application, if you want it to last.

In this as in other yachty stuff, one variable is the fact that 99% of leisure boats, are never really loaded, and those that are, are only loaded less than 1% of the time.
A fishing boat goes out every day, and will often be loaded hours on end, for days on end.

The vast majority of leisure boats always come inside to shelter, and never really go out in bad weather, and when they encounter it its only (so-so) for a short while.

So a lot of (maybe 95%) of marine gear is not really good for "real" sea use, but as its never tested in "real sea use", it does not tend to fail, thus the buyers are usually happy.
Thus, leisure marine gear trends to brands, and shiny kit, and pretty, rather than funtional.

Its a function of the users preferring shiny bits to reliability in use.
The users are right, in a sense. If they never go out in bad weather, why should they pay for it ?

This is why props are undersized, as are shafts, bitts, portlights, seals, anchor rodes, anchors etc.

Manufacturers (John Deere, lugger, etc) state 90%+ of all marine engines rust from inside out from lack of use.
I fully believe this, as well.

Best,
h-

On 30/05/2014 17:04, fitloose wrote:

Hi Hannu,

Thanks for your input. I put 3kN in as an max estimate and the calcs showed 4.04kN for dynamic axial load rating for the bearing

Looking at a 10m sailboat with locked 3 blade 14" prop giving a drag of about 45kg (100lbf) at 7 knots, and my 26ft boat with long keel protecting the prop and max bollard pull of around 150lbf,with max running thrust of around 100lbf - then it seems the safety factors are OK. I appreciate the difference between dynamic and shock load as you suggest. My prop is also in an aperture and shielded further by the deadwood.

I use a flex coupling between prop and motor shaft and flex isolation mounts for the motor mounting, so short of lots of R & D money I think I'm safe, if not completely without risk.

I suppose my arrangement is no different to the original 10bhp Volvo motor, where the gearbox bearing took these loads. I never did find out that bearing rating.

Real world testing so far has not shown up any problems. I also assume direct drive motors like the Elco take the same approach but I don't know their bearing arrangement?

John