Heat Soak or Other?

[jetboater40]

Hi!

We own a 2015 Yamaha AR192 (purchased used 3 years ago).

During that time, I never utilised the boat WOT due the presence of our kids.

I did however, recently chose to pilot our jet boat WOT without our kids and noted the maximum speed was 30mph at 7500rpms.

Prior to that, I was on the water for approximately 15 minutes before the WOT (the engine was therefore warm).

The air temperature was approximately 20 degrees Celsius and the water temperature, 17 degrees Celsius.

I read that the AR192's top speed is approximately 48mph.

The boat is not damaged and is properly maintained each boating season.

All equipment is stock.

Do you think the issue could be heat soak?

A drop of 18mph seems a lot for heat soak.

Thanks!

 

[d_coyne1984]

I doubt that it is heat soak at those temps. I don't think heat soak is typically an issue until water temps start approaching 80 degrees Fahrenheit. I would question the impeller condition more. Another huge factor will be the condition of the bottom of the boat. Any kind of lake gunk buildup will drastically effect the top end speed.

 

[jetboater40]

Thanks!

I contoured the impeller with my hand while cleaning out the port from river weed.

I felt no imperfections or sharp edges.

The boat is docked for the summer at the marina and remains in the water.

River buildup is noticeable below the water line.

Can that buildup really decrease the maximum speed by 18mph?

 

[d_coyne1984]

Thanks!

I contoured the impeller with my hand while cleaning out the port from river weed.

I felt no imperfections or sharp edges.

The boat is docked for the summer at the marina and remains in the water.

River buildup is noticeable below the water line.

Can that buildup really decrease the maximum speed by 18mph?

I always had mine on the trailer or a lift so I can't speak from any real world experience on that. But any mussels clinging to the hull combined with scum will create a lot of drag and every little thing matters when you are pushing the top end of a boat. Adding 5 hp might only get you an extra 1 mph.

I believe the rev limiter is supposed to limit the boat to about 7800 RPMs so it looks like you are few hundred short too. I'm not really sure what would cause that other than you are possibly not getting to the throttle stopper in the engine compartment when the handle is at full, or the pump is gunked up. I would think spinning at 7500 RPM would be enough for the pump to clean itself out though.

 

[GTBRMC]

I believe @Julian has posted in the past about major performance loss from organic buildup on the hull. Maybe he can add his experience to the discussion. I will search for a link to his prior post(s) on this subject and will edit this comment with results.

Edit: Here is a comment from Julian on this topic, there may be others - https://jetboaters.net/threads/undercoat-for-freshwater-slip.17358/#post-300021

 

[jetboater40]

Thanks to you both for your feedback!

At the end of the boating season, the bottom of our boat looks like the one in the picture from @Julian.

I have the hull cleaned with acid in autumn prior to storing it in the hangar.

I will see next year if a clean hull makes a difference in the maximum speed!

 

[Julian]

Thanks to you both for your feedback!

At the end of the boating season, the bottom of our boat looks like the one in the picture from @Julian.

I have the hull cleaned with acid in autumn prior to storing it in the hangar.

I will see next year if a clean hull makes a difference in the maximum speed!

A furry/dirty hull massively impacts speed if it is algae fur like we have here in NC.