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

[rearwheelin]

Luck that's what I was saying back in 2008 . Very taboo subject. image.jpg

[Luck as a Constant]

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

Luck that's what I was saying back in 2008 . Very taboo subject. [ATTACH=CONFIG]128299[/ATTACH]

Yesway

I don't know how anyone could come to any other conclusion. Just based on physics this seems to be the only plausible theory


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[Shooter]

You guys that run a thrust bearing at the strut, what do you do with the cable contraction?

Been stewing on this a couple days.

It is an interesting line of thinking that Mike was eluding to. The thrust has to be actually transferred into the body of the boat somewhere. My brain is thinking that transfer happens in one of three places.

At the motor: This would mean that the stuffing tube was relatively straight and there was a gap at the drive dog. This does have a risk of flex shaft whip. There is also a risk of actually knotting up the cable if it's off a tick or if the gap at the coupler is too large. If the transfer is happening at the motor then the pressure is applied at the motors tail bearing or if the tolerances were just right on both bearings. Or....you place a thrust bearing between the motor mount and the coupler. Pressure is then placed on a bearing designed for pressure transfer.

In the stuffing tub: This is either a nice smooth J or S bend. So the transfer happens where the flex smashes itself into the side of the tube as the prop pushes it forward. Makes the transfer between two pieces of metal or against the teflon liner. This is the most common I would guess. Down side, it feels like we get a relatively small pressure point. I don't know exactly what that looks like. It may be that transfer happens the entire length of outside bends in the tube. If you follow. Works 99% of the time though.

At the strut: Requires a thrust bearing or washers at the strut. You still have a J or S bend but only need just enough to stop any flex cable whip. Not sure what happens with cable shrinkage though. Hard to imagine it just doesn't happen.

It's pretty easy to argue that having a thrust bearing or not depends on how you built your boat. Only you are going to know if you have a mild J bend and have thrust at the motor. For anyone to say "you gotta have it" or " you don't need a thrust bearing" isn't really fair. You have to look at the boats build and take your best guess if you need one.

Wow...awesome summary.

So I was talking to one of the gas guys last year and they claimed to have gained a couple mph by moving the thrust point to the strut via thrust bearing (bearing behind the prop). Sliding square drive at the motor allowed for cable contraction. The advantage is that you reduce the friction loss due to the flexshaft pressing on the stuffing tube wall while transferring thrust (Terry's second paragraph above).

[ray schrauwen]

Brian Buaas(sp) knows the benefits of this type of setup. Seen his boats with this setup at the Nats.

Wow...awesome summary.

So I was talking to one of the gas guys last year and they claimed to have gained a couple mph by moving the thrust point to the strut via thrust bearing (bearing behind the prop). Sliding square drive at the motor allowed for cable contraction. The advantage is that you reduce the friction loss due to the flexshaft pressing on the stuffing tube wall while transferring thrust (Terry's second paragraph above).

[kfxguy]

It seems to me all the forward force delivered by the prop is directly transferred to the motor, since the only thing keeping the cable from sliding in and out IS the motor, and therefor, that energy is transferred thru the motor mount to the hull. Essentially the motor is using the mount to drag the boat thru the water...
At the motor makes most sense to me


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Think about this tho....when the prop is pushing on the cable, it's bunching up in the stuffing tube (more on this in a sec) and its contacting it in different spots adding drag that we normally don't think about. It's probably not much but it all adds up. Then it pushes at the motor. Some motors I have bought have a spring washer at the rear bearing. They break because (well I've had some break) because of the flex pushing on it. So what I've done is move the spring washer to the front instead. That solved that failure point. I have an idea that I've not yet tried. I was going to form my stuffing tube. Then cut some 1/2 "bearings" out of the next smaller size brass and solder them in every couple inches to help guide the shaft straight. May be a bad idea, may be a good idea...won't know till I try it.

[Luck as a Constant]

I dunno if you're agreeing with me or not Travis..?
Either way, i stand firm on my theory. Regardless of any other small areas of resistance .
Just trace where the majority of the props thrust is converted to.
Better yet, if you didn't glue down the motor mount, short of the motor just spinning and thrashing about in there, would the boat really go anywhere?
The whole drive would just slide up into the bow until the prop or motor caught s point it couldn't move forward anymore and then the boat would move forward


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[keithbradley]

Oh boy...

I just noticed this thread and gave it a quick read.

[Luck as a Constant]

Oh boy...

I just noticed this thread and gave it a quick read.

i didn't think anyone was outta line... unless i am?
its one of those subjects that will never be fully agreed upon until theres hard evidence towards one..

[rearwheelin]

I have always ran the the Teflon octura washer stack for thrust . They last a good while and never fail . They don't seem to have much friction when pressing the dog against them and rotating the prop on the bench once they have a run on them with some sort of lubricant. I build my stuffing tubes as a guide for the flex rather than a place to absorb thrust forces . The whipping of the flex is only amplified when thrusting with the flex. I have seen some big failures from Teflon over heating and grabbing the flex destroying setups . Some of the best running boats I have seen thrust with the flex so bottom line is it's build skill and understanding what you are doing.

[keithbradley]

i didn't think anyone was outta line... unless i am?
its one of those subjects that will never be fully agreed upon until theres hard evidence towards one..

No, it's just interesting to see some of the stuff that gets tossed around in these types of threads.

[rearwheelin]

Here is a pic of a flex thruster gone wrong. See the post here .

http://forums.offshoreelectrics.com/...752#post291752
image.jpg

[Luck as a Constant]

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

I suppose if you have no gap at the strut, and the thrust washer were between the drive dog and strut tightly then the force is then put at the strut?

Also, the square drives ... The prop and Teflon washer seem to ride against the strut. I guess there too the forces are put at the strut?
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[Luck as a Constant]

Thinking about it now... The point where all the forward moving force is contained, could be transferred to different spots depending on where the thrust bearing is placed?


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[rearwheelin]

I suppose if you have no gap at the strut, and the thrust washer were between the drive dog and strut tightly then the force is then put at the strut?

Also, the square drives ... The prop and Teflon washer seem to ride against the strut. I guess there too the forces are put at the strut?
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That's correct. .5 mm gap is good never tightly ! I check the shrinkage by putting a wrench on the collet and turning the prop forcefully with thick leather gloves . The flex don't shrink much at all ! In fact every flex I have used there is a inner and outer winding that wrap in opposite directions so it resists shrinking !

[rearwheelin]

Thinking about it now... The point where all the forward moving force is contained, could be transferred to different spots depending on where the thrust bearing is placed?


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No. That would depend on your strut adjustment . Now , running a gap yesway. If you have your motor angled down in the front of the boat and your thrusting with the flex watch it go nose down.