Prop or shaft release system?

[daiquiri]

Since I didn't get any reply to the following question posed in another thread, I assume it was either formulated in a messy or incomprehensive manner or was simply too difficult... So I would like to try one more time, putting it in another way...

The question:
Does someone here knows if there is some kind of safety system available, which can release the prop (or the portion of shaft aft of the log) in case of a violent hit against an underwater rock?

The ultimate goal would be to:
1) prevent shaft breaking and/or
2) prevent the transmission via shaft of the impulsive force and torque which could damage the drive system or (worse) the shaft log, opening the path for the water into the hull.

Can you please indicate me some manufacturer of that stuff or can you please suggest me any other solution in that sense?
Thanks!

[kistinie]

Kiwi prop idea is to brake the blades made of composite rather than the shaft.
Blades are not too expensive and can be replaced underwater

[FAST FRED]

There is really NO system made for a boat with a std prop/shaft installation.

If its really worth your effort I would replace the existing shaft with a shaft machined as a copy of an outboard drive that spins the prop you need.

The outboards come in 2 to 300Hp and most have prop hubs that feature a breakaway system. Sheer pins are cheap , but if it doesn't save the prop from some damage , at least a spare prop could be installed if the water isn't to cold.

Good luck,

FF

[kapnD]

While most of us are worried about building the drive system strong enough to withstand a blow, you want to intentionally build in a weak link?
Check out the Volvo IPS system where the whole underhull portion is breakaway.
There are also available vaious versions of the "drivesaver" synthetic puck device that inserts inbetween the output flanges of the gear. It is supposed to break apart as well as absorb vibrations and misalignment.
Also interesting is variable pitch propellors, built much stronger than conventional fixed models and blades that can be replaced individually, reportedly without hauling out.
Both outboard and stern-drives are built not only with shear pins and breakaway hubs, but will kick up on impact.
Placing the propellor in a tunnel and behind a keel will help protect it from hits, or a jet drive where there is no exposed prop at all might be your answer.

[CDK]

Originally Posted by daiquiri

The question:
Does someone here knows if there is some kind of safety system available, which can release the prop (or the portion of shaft aft of the log) in case of a violent hit against an underwater rock?

The ultimate goal would be to:
1) prevent shaft breaking and/or
2) prevent the transmission via shaft of the impulsive force and torque which could damage the drive system or (worse) the shaft log, opening the path for the water into the hull.

Can you please indicate me some manufacturer of that stuff or can you please suggest me any other solution in that sense?
Thanks!

Somewhere in the drive line there usually is a coupling to reduce vibration, allow misalignment or remove the prop shaft. There you can achieve what you want by modifying the coupling element in such a way that it conveys to prop shaft torque but not the momentum of the flywheel at the moment that the prop hits an object.
With the rubber disc coupling, you can replace the bolts by thinner ones with a sleeve of bushing around it or by using aluminum bolts instead of steel ones. Or you can use a shear pin instead of a key to connect the coupling to the prop shaft.

[Ad Hoc]

Hi D

Normally you want this part of the propulsion train as "solid" and "stiff" as possible. If you are really concerned about the prop being hit, do the time-honoured 'design'...protect it with a skeg. The skeg takes the hit, not the prop.

[ABoatGuy]

A bent prop, even a very bent prop, can still be semi-functional and get you home, but a break away prop will leave you dead in the water calling for help.