Two 14"x21" props, different MPH

[TBarCYa]

When I bought my boat it had a 14"x21" prop which I broke. It was replaced with a 14x19 which I ran for a couple weeks before realizing it is the wrong prop and purchasing another 14x21. The new 14x21 gives the same top speed (at 4800 rpm) as the 14x19 (at 5200 rpm) and i cannot figure out why it's different than the original 14x21.

This is a 2008 3.0L Mercruiser. Top speed on the dash was over 50mph with the original prop and only reaches around 45 with both of the newer props. Current top speed on GPS is 42mph.

When I changed from the 19 to the current 21, I checked the hub and it was cracked at all 4 corners but did not appear that the hub could have spun in the prop or that the metal part of the hub spun in the plastic. The current hub does not have any cracks.

 

[QC]

Re: Two 14"x21" props, different MPH

First, are props 1 and 3 the exact same model and manufacturer? Second, has she gained any weight? Tools, people, beer, stuff? Third are you trimming properly? The same?

Also, you need to verify with a GPS. My on board speedos have been very accurate, but many aren't, so I wouldn't trust that until verified . . .

 

[TBarCYa]

Re: Two 14"x21" props, different MPH

Props 1 and 2 are Mercury and #3 is Quicksilver.

Weight is a possibility as I do have some electronics and an anchor that weren't on board when the initial test was done plus I noticed that my towable (tube) was holding some water in the ski locker. Not much, but some. I had not tested how fast she was with all of my gear on board because she wasn't broken in. The initial 50mph run was the demo.

Trim wise, I always trim it out at moderate throttle until I stop gaining speed and then open her up. Typically, from full trimmed in to when I stop trimming out is about 10mph difference in speed.

The only speed that I cannot verify on GPS is the 50mph one as that was before I had the GPS installed and I had not tried to duplicate it until she was broken in. At that point, the original prop had been replaced.

 

[Silvertip]

Re: Two 14"x21" props, different MPH

You have just discovered that all props of the same diameter and pitch are not equal when it comes to performance. Prop design, prop material (stainless vs aluminum), cupped or not, amount of cupping, rake, numnber of blades, blade shape and probably other things that only prop designers know can affect how a given prop performs. That's why prop selection is such a crap shoot. That's why if you are happy with your previous prop, it is always best to buy an exact replacement from the same manufacturer, and not just go for the same diameter and pitch. Look at it this way -- when you buy tires for your car or truck you initially look at the same size do you not? The next choice is what you expect from those tires. Traction, noise, wear, handling, and ride are all design factors. Same holds true for props. Just because it fits doesn't mean its the best prop (or tire).

 

[kenmyfam]

Re: Two 14"x21" props, different MPH

You have just discovered that all props of the same diameter and pitch are not equal when it comes to performance. Prop design, prop material (stainless vs aluminum), cupped or not, amount of cupping, rake, numnber of blades, blade shape and probably other things that only prop designers know can affect how a given prop performs. That's why prop selection is such a crap shoot. That's why if you are happy with your previous prop, it is always best to buy an exact replacement from the same manufacturer, and not just go for the same diameter and pitch. Look at it this way -- when you buy tires for your car or truck you initially look at the same size do you not? The next choice is what you expect from those tires. Traction, noise, wear, handling, and ride are all design factors. Same holds true for props. Just because it fits doesn't mean its the best prop (or tire).

Absolutely.
My 2 14" X 17" props give a 500rpm and 5 mph difference. The 14" X 19" prop gives the same speed but the rpm is significantly reduced more than the "book" states. However it is cupped.

 

[Campylobacter]

Re: Two 14"x21" props, different MPH

I get 4 extra MPH with SS vs aluminum, presumably due to less flex at high RPM (both have same at WOT).

Obviously other factors also are also in play.

 

[TBarCYa]

Re: Two 14"x21" props, different MPH

For some reason I thought that Quicksilver was the OEMd Mercury.

I'm going to a prop shop today to see about getting the original prop fixed, maybe I'll talk to them about stainless props and try to get something with similar performance to the Mercury. I just don't know which is easier on the drive, a rubber hub or XHS.

 

[xxxflhrci]

Re: Two 14"x21" props, different MPH

Quicksilver is a Mercury prop. I run a 14 x 19 Quicksilver Silverado (stainless) on my 3.0. It turns 4600 rmps at WOT and does 40 mph. I love it. I used to run aluminum props, bang one up, get it fixed, run the other, bang it up, get it fixed. I've had the stainless one on for 9 years and it doesn't have a scratch on it.

About that prop that you had that let you rev to 5200....You engine won't take much of that.

 

[TBarCYa]

Re: Two 14"x21" props, different MPH

That's why I went back to a 21" pitch. Honestly, the 19" was a mistake because I didn't know the specs on the prop when I broke the original. I'll be putting that one on ebay to finance the new prop.

Regarding your stainless prop, have you damaged the drive at all with it? Everybody I talk to about stainless props warns me that I'm more likely to damage the drive than the prop. I tried looking it up but couldn't tell if it uses the rubber or XHS hub and I can't figure out if one is better than the other to prevent damage.

 

[QC]

Re: Two 14"x21" props, different MPH

All newer SS Merc style props have the plastic flow torq hub thing. It is desigend to break first. Don't worry about damaging the drive, worry about not hitting stuff . . .

 

[xxxflhrci]

Re: Two 14"x21" props, different MPH

Regarding your stainless prop, have you damaged the drive at all with it? Everybody I talk to about stainless props warns me that I'm more likely to damage the drive than the prop.

No, I have not damaged the drive. I was always dinging up my aluminum prop hitting even the smallest of sticks, trash, ect. The statnless prop just cuts them up. I think the whole "damage the lower unit" theory is over exaggerated. I'm guess if you made just the right direct hit on something solid, it might make a difference. However, I'd guess you'd bust something anyway.

 

[Nandy]

Re: Two 14"x21" props, different MPH

What happened to the original prop?

 

[TBarCYa]

Re: Two 14"x21" props, different MPH

All newer SS Merc style props have the plastic flow torq hub thing. It is desigend to break first. Don't worry about damaging the drive, worry about not hitting stuff . . .

It's tough not hitting stuff in the Delaware River and with gas prices, I don't make as many trips to the ocean as I used to. Some days the river is perfectly clear and others there's more submerged (but floating) wood than open water. I always happen to find the shallow parts too and I'm sure that doesn't help.

Nandy, the original prop lost a blade while I was anchored at a sand bar. There are no signs that it hit anything, but the blade broke off within 1/2" of the hub. It's really weird and looks to me like it has to be a defect but I can't find a dealer willing to try submitting the claim.

 

[QC]

Re: Two 14"x21" props, different MPH

Yeah, I am sure it is tougher than where I boat. Floating debris is pretty rare and the Colorado lake's water is pretty clear, especialy Mead and Powell . . . I can see color at 25 ft. there. Toughest thing for me is changing water levels on what's called the Parker Strip, but you can pretty much always see.

I used to tear up a ton of AL props below there though. Murky water, snags etc. I have torn up SS as well, but chunks nefer break off and I can repair little wiggles pretty easily. I think I'd run all SS even in the murky stuff if I boated there still. I am guessing cheaper in the long run.

 

[TBarCYa]

Re: Two 14"x21" props, different MPH

I put my mask and snorkel on to look at the lower unit last week and couldn't see more than 4' in front of me. It's clearer north of Philadelphia but where I am it's a mess.