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2016 90 HP Mercury Prop Help

T

tdelam

New member
Hi,

I have a 2016 90 HP Mercury 4-stroke. I was reading on the web site the max RPM is 5000-6000 I max out at 4900 currently with the stock prop installed. The site says 5000-6000 so that does mean I should be able to squeeze out 6000rpm with a different prop on it and get more speed? If so, any suggestions as to what I would need to get?

Thx
 
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Typically, I think u would want to be close to 6000 rpm with just u in the boat and little to no gear. What's your current prob size? I have a the same motor but older and carbureted on my nitro 17' bass boat. I hit 5800 rpm with just me in it and a bit of fishing tackle. I'm running only a 13.5"x15pitch prop and it gets on plane fast and still carries me 52 mph.
 
Bruce2012 said:
Typically, I think u would want to be close to 6000 rpm with just u in the boat and little to no gear. What's your current prob size? I have a the same motor but older and carbureted on my nitro 17' bass boat. I hit 5800 rpm with just me in it and a bit of fishing tackle. I'm running only a 13.5"x15pitch prop and it gets on plane fast and still carries me 52 mph.
Click to expand...

I wish I knew my current prop size. I am a pretty big noobie when it comes to props. Whatever is stock is what it has. With just me in it, I hit 5000 rpm and max 42 mph. I was hoping to get some more mph and some higher rpm's. If they're rate for 5000-6000 I should at least be able to get 5500-5700 safely and maybe 5-8 more mph, am i right? I don't know what prop size/pitch to get though. I am looking for advice :)
 
U can check your prop size and pitch by looking at where the prop bolts to your lowering unit. U may or may not need to unbolt it to check. It's just a pin and one crown nut. The numbers will be on the inside
 
Bruce2012 said:
U can check your prop size and pitch by looking at where the prop bolts to your lowering unit. U may or may not need to unbolt it to check. It's just a pin and one crown nut. The numbers will be on the inside
Click to expand...

My stock factory pitch is 17.

Does more pitch = less rpm? I would think so since theres more prop to turn. I was looking at a 4 blade prop tonight with a 21 pitch... I would think this drops the rpm.
 
So a very basic rule of thumb (and can vary a little) is - change the pitch OR diameter by 1" and you change the rpms by 200.

So up one in pitch equals 200 LESS rpms (and vice versa).

Also going to cupped blades shaves about 50 rpms off.

Going to a 4 blade shaves about 100 rpms off.

Going from Aluminum or composite to Stainless can shave 50 rpms off.

So if your current pitch is 17 and you only get 4900 rpms and it's a 3 blade and you want to go with a 4 AND get your rpms say into the mid-5000 range (call it 5500) then you need to gain 600 rpms - so moving "down" in pitch by 3 or 4" (12 or 13 pitch 4 blade) should come in there somewhere.

Also, motors have a little trouble breathing in the middle of summer due to heat/humidity, so if repropping (if that's a word) in the middle of summer I really would stick to the mid-rpm range of your WOT range - in the spring/fall you might gain another couple hundred rpms at the top end so if you really under-prop you could end up exceeding your max WOT number.

If this motor was on the boat from new then somebody over estimated the prop for the boat. Merc's (over 20 horses) don't come with a "factory prop" included so the dealer would have suggested one to add to the motor and they obviously picked wrong (unless this was a repower).

Selecting the correct pitch to get the motor working in it's range will not "always" result in more speed but will lengthen the life of the motor. So even if your top speed doesn't improve (or improve to the degree you wish it would) it is in your interest to get it propped correctly.
 
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