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Prop vs RPM

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For all the years I've had my boat, it has always operated at 3000rpm and 30 mph [GPS agrees with fish finder]. Occasionally, the rpm gauge gets whacky and shows a false indication. I tap the gauge and it eventually returns to normal.

But this year, it is maintaining a consistent 25mph @ 3100rpm. If I push the throttle to get the speed to 30mph, then the rpm is at 3400rpm.  I had prop repair last year and it was freshly painted. The paint was pealing off. I raised the outdrive the other day to check for slime thinking it was causing drag. It was clean but I immediately noticed tiny like bubbles on the prop. Not air bubbles but bubbles within the paint [not there when I picked it up]. This immediately tells me poor water flow over the prop.

Will this cause a 4000rpm change to keep the boat moving at 30mph?

I haven't had a chance to change the prop to see if there is a difference.  

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Either the prop shop adjusted the pitch or cup, or you have a hub that is starting to slip. Either is a problem. 

The pitting sounds like cavitation. Left unchecked, it will destroy the prop. I would start by checking with the prop shop that did the repair job. 

brick

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Will this cause a 4000rpm change to keep the boat moving at 30mph?

No.

 

The "bubbles" in the paint are not cavitation. It is a bad substrate during PowderCoat/E-Coat, and will have

little impact on performance.

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Yesterday I swapped the prop and took it for a run. It only improved minimally.  It just didn't feel right.  

I have kept an eye on the outdrive for the growth of scum. Only a very thin layer. So I didn't think the hull was any worse.  With the lake so cold I haven't jumped in to check...until yesterday. I put on my wetsuit and slowly climbed into the water. Oh so cold.  I took my cleaning pad with me and I was surprised at how thick the slime was. So I started cleaning it. I only got 70% of the hull cleaned before I had to get out. Shivering. I then took the boat for a run and it was back to normal.

The water is so cold I didn't really think it would grow this fast in just 30 days. Can't wait till August. 

I do believe the prop contributed a little bit. After looking at it, it has more bubbles on it than a glass of champagne.  

I'll finish cleaning the boat this afternoon and then pull it out before headed on vacation. The gelcoat needs lots of work. I put two new BIG scratches on it. 8 years of hard work just wiped out. I love my boat.

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I can tell you that my pontoon lost a lot of speed over the summer from growth.

my Chap is painted wiTh antiflulant 

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