Speed drops drastically as RPM's rise

[Kusogaki]

I have a 1993 Larson 190, with a 4.3 Mercruiser and Alpha1 drive. After cruising for several minutes at ~4,000RPM speed drops drastically as RPM's rise to nearly 6,000. I shut down, visually checked the drive/prop and everything looked good. Resumed at 3,200RPM to normal operation. Any ideas?

 

[Dave-R]

I believe you have spun the prop hub. Try another prop. You can test this by marking the hub and the prop, then run it. If the marks are not lined up , it's spun.
Worst case is you might have a bad coupler, which would require the engine to pulled. Dave-R

 

[smokeonthewater]

Other possibilities are too much trim and ventilating the prop..... Prop fouled with weeds.... Or spun coupler

Spun prop hub is pretty likely tho.

 

[cjjjdeck]

There is a hub inside your propeller that gives up it's life if you hit something so the drive doesn't get damaged. It sounds like that hub has either deteriorated or took a shot and needs to be replaced. I would check that first.

 

[Kusogaki]

Thanks all for the quick replies. I'll pull the prop off and inspect the hub tonight. If it all looks good, I'll start saving to replace the coupler

 

[Chris1956]

I doubt you will be able to determine if the prop hub is good, by inspection, unless it has a modular hub, you can disassemble. Mark it and test it, or replace with a known good prop.

 

[Kusogaki]

Thanks Chris - Perhaps I'm not sure what the hub actually is. I was going to look for the plastic sleeve that takes the shock if the prop hits something. I got that impression based on a search for "Flo tech hub kit". Another hub?

 

[Chris1956]

You are looking for the correct item, provided that your prop is of that type. A lot of the older props have a permanent rubber hub, which cannot be disassembled except with a press. If you have the old style prop, I would recommend you mark the brass hub and the adjacent prop blade component and retest it, if convenient. If not convenient, have the prop shop check and replace the hub.

 

[Kusogaki]

I removed the prop and see there is a splined hub inside of the prop that mates to a shaft on the lower unit. I presume "spinning the prop" is the rubber hub spinning inside of the prop which, as you mentioned, wouldn't be visible after the fact. I would also presume that to change to a flo torq hub I would need to get a new prop?


BTW - I marked the brass hub/shaft and corresponding blade for future reference.

 

[Bondo]

I would also presume that to change to a flo torq hub I would need to get a new prop?

Ayuh,..... Welcome Aboard,.... That is correct,.... The hubs are Not interchangeable,....