G23 Prop Fell Off .....

[Nickleej]

Provided he has a good MC dealer........ but ya, myself and my whole crew absolutely love the new star, and every one of them owns a Nautique. SL is pretty nice too, and a little cheaper. Could really stand about 1000lbs more ballast, and a better tower design. Pretty sweet boat though.

Are they moving away from Nautique and buying an XStar?


Sent from my iPhone using PLT Nautique

[nyryan2001]

Seems like most of the broke shafts happen under 75hrs. Not many reported north of that correct?

[greggmck]

For those of you who say the G23 is a "bow down boat" therefore prop angle is not a factor loading the shaft... Consider this photo. The gunwale of the G23 lowers from the windshield forward. This is a design goal to improve driver visibility. Most people see this as the G23 being a "bow down" boat, because the person thinks "look how well I can see in this boat"! When I drive boat xyz I have to sit up higher. Also if you believe a level boat can make a great surf wave, then you really need to take a physics class. The hull MUST push water to make a large wave. To do this it has to push the stern down into the water and move a mass of water to the side. Without this there is no wave. It is NOT possible to make a great surf wave with a level boat.

Also this is not caused by ONE factor. It is a multitude of factors: The asymmetric prop load during surfing, pushing large amounts of water AND exacerbated by a surf system which causes the boat to YAW, not ROLL or List (like MasterCraft or Centurion), AND a 2:1 transmission increasing shaft torque, AND the 17" prop load.




I'm beginning to believe this failure is fundamentally due to the keyway cut into the prop shaft. My previous shaft had a longer keyway cut into the shaft that extended almost back to the strut. With an asymmetric load caused by the angle of attack of the lower prop blades the keyway could be the origin of the failure. Looking at the shaft that failed below one can see the keyway cut into the section that still remains just in front of the strut. This would be the point of greatest load on the shaft from the asymmetric prop thrust.



And here is the new shaft. There is no keyway cut longer than the 3" prop hub, e.g. This prop is a 17x17 3" hub, not a 3.4" hub. The extra room is for the longer hub props.

[DealsGapCobra]

As for nose-up/nose-down, I maintain that it doesn’t really matter. The idea of offset thrust caused by flow angle is why I asked in an earlier post if anyone knew the angle between the shaft and the bottom of the boat. The theory that the flow angle will cause a cyclical load resulting fatigue is correct. This will be caused more by propeller shaft angle than bow rise because the water flow under the boat will be pretty close to parallel to the bottom of the boat. In my mind, there is no question that these failures are the result of multiple factors any one of which could be changed to solve the problem.

[scottb7]

I thought the keyway didn't bear the load because of the taper. Is this not true? I used to work at a fastener company and our quality dept would load test bolts and screws all the time to see breaking strength or whatever the measure is. You would think Nautique engineers would have also compared a taper shaft to a splined shaft by now wouldn't they?

P.S. Greg you may be right about the visibility helped by the downward angle shown in your pic. But I believe you said the G had low gunwale and I disagreed - yes in part - based on visibility but also on how much less water I take over the bow compared to my 2008 210. The G is out of the water more then the 210. Is that not a measure of gunwale, which is a measure to the upper edge of a boats side.

[scottb7]

Seems like most of the broke shafts happen under 75hrs. Not many reported north of that correct?

My 2014 G21 had the shaft break on the original owner at 175 hours. I currently have probably 420 hours and - knock on wood - haven't broke another one.

[greggmck]

As for nose-up/nose-down, I maintain that it doesn’t really matter. The idea of offset thrust caused by flow angle is why I asked in an earlier post if anyone knew the angle between the shaft and the bottom of the boat. The theory that the flow angle will cause a cyclical load resulting fatigue is correct. This will be caused more by propeller shaft angle than bow rise because the water flow under the boat will be pretty close to parallel to the bottom of the boat. In my mind, there is no question that these failures are the result of multiple factors any one of which could be changed to solve the problem.

You are on the right point but the flow of the water relative to the prop rotational axis will be related to BOTH the shaft angle AND the pitch of the boat. To the best of my knowledge these failures occurred exclusively due to extensive surfing. Not wakeboarding, not cursing. Surfing does cause the bow to raise significantly which also increases the propeller angle of attack. Knowing the shaft angle alone will not determine the relative angle of the waterflow to the propeller. Also pitch (bow high) is NEEDED to make a surf wave. If the hull is parallel to the waterflow the boat will have the smallest wake possible for that hull design. The stern of the hull must sink lower than the bow to displace more water then when level to make the surf wave.

During surfing the asymmetric load on the propeller is greatest compared to all other use modes. In effect, the propeller blades at the bottom of the rotation produce much more thrust than the top blades which have a much lower angle of attack relative to the water flow. To visualize this load imagine if you were able to put a long pipe on the propeller shaft extending to the rear of the boat and hang a 1,000lb weight on the end of the pipe while running the engine for many hours. The effective downward rotational force on the shaft will cause the shaft metal to compress at the bottom and expand at the top. These metal fatigue forces are focused in the shaft right at the strut which is designed to absorb any non longitudinal load on the shaft. So the shaft is failing due to metal fatigue NOT torque shear. And the point where the keyway is located closest to the strut is where the shaft will almost certainly break.

Many have posted opinions based not upon any understanding of engineering principles, but one point that was posted about Mastercraft using a splined shaft has merit. A splined shaft has symmetric strength compared to one with a keyway and this is likely very applicable to this particular failure.

Regarding many who have expressed concern about this failure in a G23. My perspective is that if you don't surf much it is unlikely this will be a problem for you. I have over 430 hours exclusively surfing in my 3 G23s. I have had the first shaft failure last week. I know three other competitive surfers who also have had failures. But they also live in Arizona and surf 12 months a year too.

[DealsGapCobra]

It’s actually the left and right sides of the propeller that have the larger effect and I maintain boat “pitch” is a secondary issue but as we are basically discussing the same issues it doesn’t really matter. Point is there are multiple issues here and, it seems, that only the G boats have the problem. My interest is in the ways that these boat differ from everting else out there.

[greggmck]

It’s actually the left and right sides of the propeller that have the larger effect and I maintain boat “pitch” is a secondary issue but as we are basically discussing the same issues it doesn’t really matter. Point is there are multiple issues here and, it seems, that only the G boats have the problem. My interest is in the ways that these boat differ from everting else out there.

I think there is a misunderstanding. There is NO left and right force that can fatigue the shaft. Both the left and right blades experience the same relative waterflow and therefore generate the identical thrust. It is the top and bottom blades that cause the resulting downward force that causes the shaft to fatigue and fail. I have discussed this with several engineers very experienced in propulsion design. Perhaps you are thinking about prop torque. But prop torque causes the boat to roll to the opposite side of the prop rotation. This force is directly longitudinal to the shaft. It does not cause ANY asymmetric force that would fatigue the shaft.

Here is a picture to help one understand the issue. Both boat pitch and shaft angle combine to cause the top blades to become more PARALLEL to the water flow, reducing the relative pitch and thrust of the top blades. While the bottom blades become more perpendicular to the waterflow enabling them to generate much greater thrust than the top blades.

[DealsGapCobra]

I see no reason to debate this here, the point is that there is a cyclic load on the propeller caused by differential thrust, I believe we can agree on that.

[Shockthis]

No wonder people don't get anything done at work .

[GMLIII]

No wonder people don't get anything done at work .

[Skidave]

I have the solution:

Twin engines setup. 1) prop force load sharing. 2) redundancy if a shaft does break. 3) easier docking; thrusters not needed. 4) edge above the competition. 5) higher price [ok, this is a disadvantage for the consumer]. 6) lower ballast fill times because you already have extra weight.

We will call it the new G Squared series.

Sent from my SM-J320V using Tapatalk

[dlowry81]

No wonder people don't get anything done at work .

Agreed...where can I find a job that allows me to afford a G23 and create diagrams for PN all day? If you're hiring hit me up.

[fairlydangerous1]

2018 with 48 hours cracked but didn't loose prop. I shut it down just when the shudder started.

Supposedly new shafts from February 2018 are not an issue. Mine was built last year. Is it really that hard to figure out?! I don't hear this happening with other manufactures.