thrust bearing.....what's your take?

[Peter A]

I gotta say that all this talk about transference of thrust forces is confusing to the point of it seeming to be fiction. The forward motion of a powerboat happens by the prop interacting with water. Even if you have a gap between the drive dog and strut/stinger, the shaft is held there and the working force is at that point, as Keith already said.

Regardless of where the flex is contained and what angle it's at, the boat is being pushed relative to the axis of the prop hub, not the axis of the motor shaft or flex.

Gyroscopic forces from the motor may have some effect but that may even be on the plus side, but don't forget that the turning of the motor is under load and that may also cancel out some or much of such issue. At the end of the day little of it has a large impact on how our boats run. Hulls, setups, strut/stinger angles and depth have way more effect and are much easier to work out and deal with.
And (disclaimer) all of this from the finite genuisnesses of my own mind!

[Fluid]

A club member built a retrieval boat using a flex shaft with the strut parallel to the water surface. The prop is a big one....70 mm. The motor axis is about 20 degrees from vertical, and the cable has a big gap at the strut. Know what? The boat moves horizontally, not vertically, and efficientally too. How is that possible???


.

[Fluid]

What happens on an outboard?

Most OBs take the thrust at the leg....



.

[rearwheelin]

Picture answer.

A club member built a retrieval boat using a flex shaft with the strut parallel to the water surface. The prop is a big one....70 mm. The motor axis is about 20 degrees from vertical, and the cable has a big gap at the strut. Know what? The boat moves horizontally, not vertically, and efficientally too. How is that possible???


.

Something like this image.jpg

[Cooper]

I would have to say I have entertained the idea of installing a thrust bearing between the prop and stinger. But that was only for a two piece shaft setup. Only reason I didn't was figured it was way more complicated and ineffective as a simple Teflon washer. And then I also make sure no part of washer is larger diameter than smallest part of strut/stinger/prop hub. Again my take for all out top performance (speed) the simplest, lightest, most balanced (complete drive system) possible is always going to out preform complicated multiple parts transmission. If a drag car could have such a simple transmission as we do a lot more records would be broken. Remember the prop and strut are basically everything for a boats tranny. Simple reliable and effective, no reinventing the mousetrap. :).

[rearwheelin]

Agree . Just keep in mind the stuffing tube setup needs to really strong when thrusting with the flex ! Well greased or oiled ! In monos a flex thrust setup builds more heat than a hydro/cat stuffing tube because the tube is water cooled:)

[tlandauer]

It's been an interesting thread. I have learned something and I got to say that thank heavens I didn't read this as my first thread when I joined this Forum. A lot of this is just middle school physics. I guess at the end of the day I would not change whatever that is working for me. Still, as said, interesting thoughts and theories, loving every bit of it!
P.S., called my insurance agent and explained what I have been doing wrong for the last 35 years.

[rearwheelin]

When your on a roll it's hard to stop lolimage.jpg

[rearwheelin]

I am stepping out out of my element and build a boat designed to push with the flex :)

[tlandauer]

I am stepping out out of my element and build a boat designed to push with the flex :)

LOL.....
and I am going to build a paddle boat!

[Jeff Wohlt]

Long debate on this one! I always put the thrust on the motor. Why do flex shafts snap when they have no room to wind up? Why do we leave a gap at the strut and drive dog? Motors today are much better at taking thrust because they are built different than our old brushed motors. It still pushes hard on your motor arm against the rear bearing...you do not think that thrust pushing a 5-10# hull matters on a motor? If not why need a stuffing tube......a flex wants to wrap in a knot from thrust.

[rearwheelin]

I have never busted a flex running thrust washers at the strut with almost no gap ! All my boats but one used .150 cable . My most recent build is a T29 that thrusts 100% at the strut using .150 flex. I have 15 4 minute runs and no busted cable. CC 1515 , 4s . I just don't buy into the shrinking of the flex thing , they will a little but not that much , in some cases not at all !

[tlandauer]

No to stir anything, and I believe everyone's point of view, let's say the flex doesn't shrink as much as people tend to think, with the thrust bearing at the strut end, any shrinkage however small the amount would then be 'absorbed' by the motor rotor being pulled, NO???
I am just trying to understand, please don't take this wrong.

[Cooper]

Long debate on this one! I always put the thrust on the motor. Why do flex shafts snap when they have no room to wind up? Why do we leave a gap at the strut and drive dog? Motors today are much better at taking thrust because they are built different than our old brushed motors. It still pushes hard on your motor arm against the rear bearing...you do not think that thrust pushing a 5-10# hull matters on a motor? If not why need a stuffing tube......a flex wants to wrap in a knot from thrust.

Absolutely, I've pushed a lot heavier boat over 115mph. Both bearings on the motor work with the load, taking in account quality of a motor is being used. I use to use the rule of thumb that a gap between strut and dog should be at least half the diameter of flex being used to a max of the flex diameter. But I go by trial and error, load about three Teflon washers, if one is squashed then good and remove one. I've taken accurate measurements from multiple runs and adjust accordingly. If I proped up and just rip into throttle max it is usually gonna end up with lost props and a swim. Radio throttle curves and a light finger on the throttle help. Again this is for pushing the limits on equipment. Sport is whatever is gonna hold up the best and last the longest :).

[Cooper]

Oh and to add the thrust debate,, that is why bulkhead and lots of reinforcing for the struts are necessary