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  #1  
Old 09-27-2020, 12:29 PM
Wiscowalleye Wiscowalleye is offline
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Default Prop Cavitation problem

Hey guys,

I have a 2020 Lund 1775 impact sport with a 115hp mercury four stroke. It has a 4 bladed spitfire 17 pitch prop. I got the boat new this spring and so far put 35 hrs on the motor. Over the past 10 hours paint has started chipping/pulling off the blade face along with metal. Iíll attach photos. The boat performs great but I donít think the prop should be doing this. The dealer tells me itís normal but thatís bs. I highly doubt that mercury would design a motor prop combo this way. What should I change to stop this? I was thinking of just getting the spitfire X7 in the same size and pitch because itís a stainless steel version. Any advice would be great I donít really know if I should change the pitch or not. The boat operates right in the middle of the motors recommended rpm range at WOT. Thanks in advance. Iíll attach photos for you to see.
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  #2  
Old 09-27-2020, 02:12 PM
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WallyWarrior WallyWarrior is offline
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I same have same prop w same size Yamaha motor. My prop probably has 150-200 hours on it and just recently started doing that on one blade. The wear on mine is much smaller than any of yours, but is the beginning of the exact same thing.

You spoke of prop cavitation in your title, are you actually getting cavitation? I am not and just assumed it to be normal wear on the paint.


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  #3  
Old 09-27-2020, 02:18 PM
staylor staylor is offline
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It does look like cavitation burn. Since your motor height doesn't seem too high from the pic then the damage you are seeing can be due to:

1. A bad prop- but the Spitfire has run well on many, many boats with no issues. It's cast so they all should be about the same in dimensions.

2. Lousy paint- always a possibility- and I've lost paint on several Merc Props- but never in the pattern you have on each blade.

3. Too much pitch for the rig and motor combo- and this may well be your problem. The prop pitch should be having the motor run at the top of its rpm range with a light load.

My GF has a 'toon with a 115 4S. The dealer set it up and noted that the prop suggested by the manufacturer ran awful when he tested it- motor could barely make mid-range rpm with 1 person on the boat. The dealer dropped down 2 inches in pitch and the rig runs great. No burn noticed after 3 seasons.

I would suggest asking your dealer to simply lend you a plain old Merc 3 blade Black Max in 15 pitch and see how the boat runs with less pitch. I think you'll like it better than running with the 17 pitch. If the lower pitch works OK and the motor is near the top of its rpm range with a light load then you can either stay with the 3 blade or get a Spitfire in 15 pitch.
Doug
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  #4  
Old 09-27-2020, 02:23 PM
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ltrain ltrain is offline
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That doesn't look like normal wear to me either, normal wear would be paint wearing off blades from the very ends. That like like the burning that happens when the prop cavitates as you suggested. Is the motor lugging or are you running near rev limit?
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Old 09-27-2020, 02:25 PM
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looks like Doug has you dialed in, I was typing as he posted
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2005 Yamaha f150
2020 Tohatsu 9.9 efi w/Trollmaster
Minn kota 72" 112lb 36v Terrova
Simrad nss evo2 12
BG Zeus2 12
Lowrance HDS 12 gen2
Lowrance autopilot
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  #6  
Old 09-27-2020, 04:29 PM
REW REW is offline
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I agree with this post from STaylor.

With a 4-blade prop on a relatively heavy hull with a 115hp 4-stroke, the normal pitch would be a 15 pitch.

Pitch down to a 15 and see how it goes for you.

I also agree that the pictures indicated cavitation burn on the prop, which indicates a less than fully efficient prop.

Best wishes.
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  #7  
Old 09-27-2020, 08:32 PM
muskyed muskyed is offline
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Spitfire's are a very efficient prop. Usually a burn like that is caused by a damaged prop, but yours looks ok, to high of an engine height, or transducers or hull damage, causing water disturbance at the prop area. Correct height for a lot of lunds is one open hole on top.
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  #8  
Old 09-27-2020, 09:08 PM
Wiscowalleye Wiscowalleye is offline
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Quote:
Originally Posted by staylor View Post
It does look like cavitation burn. Since your motor height doesn't seem too high from the pic then the damage you are seeing can be due to:

1. A bad prop- but the Spitfire has run well on many, many boats with no issues. It's cast so they all should be about the same in dimensions.

2. Lousy paint- always a possibility- and I've lost paint on several Merc Props- but never in the pattern you have on each blade.

3. Too much pitch for the rig and motor combo- and this may well be your problem. The prop pitch should be having the motor run at the top of its rpm range with a light load.

My GF has a 'toon with a 115 4S. The dealer set it up and noted that the prop suggested by the manufacturer ran awful when he tested it- motor could barely make mid-range rpm with 1 person on the boat. The dealer dropped down 2 inches in pitch and the rig runs great. No burn noticed after 3 seasons.

I would suggest asking your dealer to simply lend you a plain old Merc 3 blade Black Max in 15 pitch and see how the boat runs with less pitch. I think you'll like it better than running with the 17 pitch. If the lower pitch works OK and the motor is near the top of its rpm range with a light load then you can either stay with the 3 blade or get a Spitfire in 15 pitch.
Doug
Wow thank you so much for the info! The most I can get for RPM is around 5600 WOT. Since it’s a regular 4stroke not the pro xs mercury max rpm spec says 5000-6000 rpm. The boat weighs I believe roughly 1400lbs probably more with my gear and gas.. I wonder if they maybe propped it for a pro XS. Also do you think running it as is the rest of the year could damage my motor? I’d like to keep fishing this fall and get a new prop in the spring. Thanks again to everyone for the information I would have done the totally wrong thing and got a larger pitch prop not smaller. Lol

Last edited by Wiscowalleye; 09-27-2020 at 09:12 PM.
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  #9  
Old 09-27-2020, 09:25 PM
Wiscowalleye Wiscowalleye is offline
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Also here is a photo of the back end of the boat. I believe there is one open hole on the motor mount. And the cavitation plate runs even with the bottom of the boat so Iím pretty sure the motor is mounted correctly.
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  #10  
Old 09-27-2020, 09:27 PM
Boondock Boondock is offline
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Happens
Switch the the x7 17 spitfire and donít look back. Carry that one as a spare.
Worth every penny. Totally changes the handling of the boat
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