Thrust Bearing Source

Most marine gearboxes have the thrust bearing built in, and rely on the engine mounts to transmit those forces to the hull. CV axle / separate thrust bearing systems are for reducing vibration (you can use softer motor mounts) and for making drivetrain alignment easier (the CVs actually like a bit of misalignment, so you only have to worry about getting the thrust bearing itself rigidly fixed in the right spot).

If you're swimming in cash, just spring for an Aquadrive or the Evolution system.

If you're on a really tight budget and are OK with a lathe and mill, just buy a thrust bearing from McMaster-Carr or whoever else happens to have a shop in your area. Fabricate a hefty bracket to connect it to the stringers, and use a CV axle from a front-drive car to hook up the engine.

 

And again...check out the guys who build electric and steam powered launches. They all use thrust bearings ...always simple shop built setups .

 

For the shortest possible drive line, use the 1310 H-joint, made by Spicer.

 

If you don't have a thrust bearing, isn't all the force required to move a boat transmitted directly to the engine in a thrust type load? Don't most engines lack any serious thrust capabilities?

As far as a 25' boat not needing a thrust bearing, what's length got to do with it? You can easily load 10,000/12,000 lbs in a Carolina Skiff and they're rated for a 200 hp motor. That seems like quite a thrust load to put on an engine not designed for thrust loads.

 

All marine G boxes have a thrust bearing . Thats what makes them marine . Electric guys use thrust bearing because they are not using marine G boxes.

 

Gearbox and engine can easily handle thrust, but to cope with it engine mounts must be quite rigid, while flexibility would be better to reduce transmission of noise to the hull.
By making engine mounts longer, more thrust can be taken while still maintaining some degree of axial flexibility, but for maximum noise and vibration suppression soft "silent blocks" are used combined with the inevitable thrust bearing.

 

I'm going with what the OP asked about. The OP didn't mention a gearbox or marine anything other than that he is building a boat. Maybe he's putting a car engine and transmission in it, maybe something from a lawn mower.

In general, even though engines may have a nominal thrust bearing, as possibly automotive transmissions might also have, I am under the impression they are not built to handle the thrust loads generated by a prop. Even just riding the clutch with your foot lightly resting on the pedal on a manual shift auto or truck will destroy the thrust bearing in them.

I also realize some (ship) engines are built to handle the thrust, but in general, aren't most marine engines just marinized auto engines, and engines of any size and use are incapable of handling large amounts of thrust?

 

Marine gears are designed to take thrust and therefore have a thrust bearing built in. They transmit thrust through the engine block via the bell housing and there is often a bracket to combine the engine/gear/rear mount connection to help accept some of the thrust loading. The mass of the engine block is what transmits the thrust.

Car transmissions are not designed to take thrust. Think about the splined connection in a driveshaft of a rear wheel drive car. The splined connection is there to allow fore and aft movement of the driveshaft as well as the flexibility of the U joints at each end. The thrust is from the tires and is transmitted to the car through the rear axle.

Just like a remote gear box or remote v drive connection is connected to the engine via a jack shaft so the gear can take the thrust instead of the engine, in a car the drive shaft transmits the power to the rear differential which is part of the rear axle which takes the "thrust."

Engines do not have thrust bearings and most SHIPS do not have thrust taking engine/gear installations but instead do use thrust bearings. Thrust bearings are typical on large boats and ships.

What I have been trying to communicate is that for a 25 ft powerboat a thrust bearing is typically unnecessary complexity, cost and expense with (for a 25 ft powerboat) very little positive return. IMHO the reduction in vibration/noise over a well chosen thrust taking installation will be minimal, if noticeable at all. A 25 ft powerboat is going to generate plenty of structure borne and airborne noise and vibration regardless of the drive, just from wind and water interaction underway. Unless there is a serious need for an extremely smooth drive train at displacement speeds I just can't see the justification for a thrust bearing installation.

Steve

 

Its a shame that the original poster didnt post a reason why he needs the thrust bearing...for instance an offset engine or engine forward installation

 

Search for hydradrive and marine joint, made by a Norwegian company called power train.

 

Yes they do and they can thrust a boat without any modification.

What happens when you put your foot on the clutch on a car

 

The place where a thrust bearing might reside on a typical engine would be at the front of the crankshaft. Thrust bearings are usually tapered roller bearings, not usually found at the front of an engines crankshaft.

When you put you foot on a clutch you are disengaging the engine from the clutch plate. A clutch interrupts rotational energy between the engine and the transmission so you can be in neutral or shift gears. It has nothing to do with forward thrust. Thrust, as I mentioned in my example of a rear wheel drive car, is from the tires to the axle through the differential.

The only thrust one might find in a car would be the Batmobile's jet engine.

Steve

 

Here's a thread that's more informative. http://www.boatdesign.net/forums/diy-marinizing/automobile-transmission-why-not-5204.html The short story is you need a thrust bearing. More than likely there's not going to be one in your engine that will stand up to propellor shaft thrust for very long. A marine transmission will probably have one, an automotive one probably won't. Buried in the thread is a suggestion on how to cheaply make your own.

 

Thrust bearings on a car engine are center crank or sometime fore or aft of the crank . It does't matter but they are there. They are white metal bearings looking a little like a horse shoe. ALL engines have them.

When you put your foot on the clutch the clutch release bearing pushes on the fingers of the pressure plate releasing the clutch . This force is pushing the crank forward.

This thrust bearing is capable of much more thrust and some professional boats have been built with this bearing alone with no problems.

One boat I have in mind was driven from the front of the crank.

All inboard boats in uk used car engines in the early days.

A car gear box does NOT have a thrust bearing. The Main shaft of the box needs to be changed.There are tapered rollers that take thrust both ways .

http://science.howstuffworks.com/transport/engines-equipment/bearing2.htm


ALL Thai fishing boats tourist ferries etc do this, they keep clutch and gears with a carefully adjusted piece of string as throttle.

 

they're rated for a 200 hp motor

A prop is very lucky to create 20lbs thrust for each shaft HP.

200 x 20 is 4000 , the weight of the boat does not matter to the shaft.