Advice on changing prop on my Chaparral Sport

[Arynden]

Last year two of the blades on my boat's prop got a bit chewed up by some rocks. While it still works I was planning on replacing the prop to get better thrust and fuel efficiency. The prop I have is OEM and I decided to stick with the cheaper, coated aluminum models to replace it.

Current prop: OEM, think it's composite but may be aluminum, diameter 14.25", pitch 21", 3 blades
My engine: Mercruiser 4.3L Mercury MPI CAT Alpha
Boat: Chaparral 21 Sport H2O

I'm looking at getting a Quicksilver Black Diamond aluminum prop, same pitch, diameter, and blade number. Hustler is out of the question, because I'd have to lower the pitch for the power and I have enough problems without trying to figure out a pitch change. The one brand I kept hearing recommended was Quicksilver or Black Diamond.

The problem I'm having is since I've never changed a prop before I don't know which additional tools I'll need outside of the prop wrench (which I'm ordering). Nut and washers come with the new prop, but is it necessary to order the spacer too? I've only seen a prop changed a couple times, and in both cases I don't remember a spacer being used, just washers. I'm pretty sure my current prop uses tabs and not a pin. One of our friends said to make sure we have lubricant, but he has an older, smaller fishing boat with a pin prop, so I'm not sure we'll need that either.

Is the prop replacement a good choice? Do I need the spacer? What additional tools/items will I need on hand?

 

[steelespike]

Now is the time to consider if you need to improve your performance.Your prop wouldn't be a composite if its original.
You should read the stickys at the top of the prop forum.
You will need some grease to put on the prop shaft just a light coating so the prop doesn't get seized on the shaft.
Someone said "any lube is better than no lube"Can't help about a spacer.If there is a spacer sometimes they get stuck to the prop.
It would probably be cost effective to have a prop shop repair your prop.
You should get us your lightly loaded wot(wide open Throttle) performance. rpm,gps speed. Describe your overall performance.
It is important that the motor is able to rev freely within its rated rpm range.

 

[Sea Rider]

If prop is only chewed but blades are perfectly aligned, not bent, go for a wot test lightly loaded and report achived max wot revs to check if inside factory wot range for that OB. From there can go for a prop maximization to rev at or near max wot revs. Once there, can throttle throughout the whole rpm range with nice water performance and fuel consumption.

Happy Boating

 

[Arynden]

Now is the time to consider if you need to improve your performance.Your prop wouldn't be a composite if its original.
You should read the stickys at the top of the prop forum.
You will need some grease to put on the prop shaft just a light coating so the prop doesn't get seized on the shaft.
Someone said "any lube is better than no lube"Can't help about a spacer.If there is a spacer sometimes they get stuck to the prop.
It would probably be cost effective to have a prop shop repair your prop.
You should get us your lightly loaded wot(wide open Throttle) performance. rpm,gps speed. Describe your overall performance.
It is important that the motor is able to rev freely within its rated rpm range.

I read the stickys at the top which is why I provided the information I did, but they implied including a WOT was a good idea but not necessary. The reason I didn't include a WOT is because I don't know what it is. We've never needed to go faster than 45 mph and usually we don't go faster than 30-35 mph. The places we go aren't conducive to max speeds over a long area. Suffice to say that at those speeds, even with all our gear, we can still easily water ski, wakeboard and tube. Out boat is always somewhat loaded too, so achieving max speed is not something we're concerned with. It's just after the prop incident we could tell the boat was straining a bit when we tried to take off and we had to depress the throttle much farther to get our normal speeds, and it just felt slower in general.

Thanks for the info about the lube, so I guess I'll pick up some grease after all.

The prop itself isn't damaged badly. Like I said it's a bit chewed up around the edges, but the blades themselves are not bent out of shape, which is why they still function. However, due to the jaggedness of the metal edges (especially on one of the two blades which had a ball sized chunk taken out of it) and a slightly weaker thrust we decided to just remove and keep this prop as a backup.

 

[Sea Rider]

All OB's must works inside factory MIN-MAX WOT RPM (WOT = Wide Open Throttle) or full hammer down. To say that combo performs good is no indicative that you are taking all horses out of that OB. Probably you're lugging the OB at some point specially when boating loaded or pulling inflatable toys, and that you don't know. How is it that the boat doesn't have a tach on dash board, or has one but not reading accurate ?

Happy Boating

 

[ERock82]

I've changed a prop about 3 times. Here is what I used:

1.) Large size adjustable wrench (worked great. No need for a prop wrench and if you only have small size adjustable wrench, you'll use the large size a lot so worth it to just buy and have. It did not mess up my threads and wasn't hard to loosen.)

2.) a 2x4 or 2x3 about 1 and a half feet long. You wedge it in prop blades to keep it from moving. I keep this board on my boat all the time. Never know when you'll need a block of wood.

That's it on tools.

If prop nut looks worn or like it could be causing any trouble (loosening) than replace with a new one.

I never used a spacer.

Oh, and be careful with the cotter pin. Don't make it so hard to ever remove again. It is such a pain to get off if twisted up a lot. Just do the basic bends on the pin. I had a mechanic guy twist that on so much it took me an hour just to remove.

 

[roffey]

I have changed the prop on my boat numerous time and check it twice a summer, you need to inspect the prop for fishing line. As stated above grease the shaft before to put the prop back on, makes it easer to come off. A prop wrench is cheep, like $12 and are made specifically for the job. I keep a spare prop and prop wrench in the boat just in case, I hate paddling, LOL. There is no cotter pin on the 4.3 so no worries there.

My 2 cents worth.

 

[steelespike]

The wide open test run can help establish how well the outfit is setup.
Also tells if the tach is accurate. Important
A good prop shop can repair your prop and you won't be able to tell if its original or repaired.
Once we establish your wot rpm results you can drive at what ever speeds you prefer.
If it turns out the wot is too low or too high it should be fine tuned.
If too low it will lugg the motor regardless how fast your run if too high
you can control it with the throttle.
The tach is often the weak link for accurate test results.
​

 

[Arynden]

All OB's must works inside factory MIN-MAX WOT RPM (WOT = Wide Open Throttle) or full hammer down. To say that combo performs good is no indicative that you are taking all horses out of that OB. Probably you're lugging the OB at some point specially when boating loaded or pulling inflatable toys, and that you don't know. How is it that the boat doesn't have a tach on dash board, or has one but not reading accurate ?

Not really sure what OB stands for, can you elaborate? The boat does have a tach, but the boat is only a couple years old and is checked up every year so should be operating fine.

Edit: Oh hahaha, I realize you probably meant an outboard engine. I would have figured it out sooner if you had typed O/B. Our boat's an I/O sterndrive model.

I've changed a prop about 3 times. Here is what I used:

1.) Large size adjustable wrench (worked great. No need for a prop wrench and if you only have small size adjustable wrench, you'll use the large size a lot so worth it to just buy and have. It did not mess up my threads and wasn't hard to loosen.)

2.) a 2x4 or 2x3 about 1 and a half feet long. You wedge it in prop blades to keep it from moving. I keep this board on my boat all the time. Never know when you'll need a block of wood.

I never used a spacer.

Oh, and be careful with the cotter pin. Don't make it so hard to ever remove again. It is such a pain to get off if twisted up a lot. Just do the basic bends on the pin. I had a mechanic guy twist that on so much it took me an hour just to remove.

We do need a block of wood to wedge the blades, I just neglected to mention it, but thanks for bringing it up.

Sounds like a spacer is not something others are needing or using, so we'll pass on it.

It never would have crossed my mind to twist pins that are just supposed to be bent. It's scary hearing that your mechanic thought it was okay to twist them. However, there's no cotter pins on ours.

I have changed the prop on my boat numerous time and check it twice a summer, you need to inspect the prop for fishing line. As stated above grease the shaft before to put the prop back on, makes it easer to come off. A prop wrench is cheep, like $12 and are made specifically for the job. I keep a spare prop and prop wrench in the boat just in case, I hate paddling, LOL. There is no cotter pin on the 4.3 so no worries there.

There's no fishing line or anything else on the prop right now, although we have had floating line get tangled up on it once before - took forever to get that junk off. We are getting the prop wrench since that is one tool we figure we'll need again and the one designed for this prop is under $9. All our boat tools stay on the boat, and the boat is not near the house, so we wouldn't be able to use standard tools elsewhere during the season.

The wide open test run can help establish how well the outfit is setup.
Also tells if the tach is accurate. Important
A good prop shop can repair your prop and you won't be able to tell if its original or repaired.
Once we establish your wot rpm results you can drive at what ever speeds you prefer.
If it turns out the wot is too low or too high it should be fine tuned.
If too low it will lugg the motor regardless how fast your run if too high
you can control it with the throttle.
The tach is often the weak link for accurate test results.
​

Tach should be fine. The boat was bought new from the factory in 2014 and handles beautifully until our little rock encounter. The only time we really opened up the throttle was our second day with the boat, but we didn't try to remember it if we even bothered to check it at all. I doubt the cost to repair our current prop would be worth the trouble. I can get a replacement from iboats (almost identical) from a good brand for $115 (it's advertised as being $160 because of the hub assembly kit that comes with it ).

 

[steelespike]

From your description your probably safe to replace your prop pitch for pitch.

 

[roffey]

Arynden, like you I keep a full set of tools on the boat. It reads like you have all your ducks in order and have a grasp on what to do and how to do it. You are set grass hopper, go forth and have fun.

 

[Mohawkmtrs]

...and make sure you tap down every other tab after the new prop is installed.

 

[roffey]

I only bend over one, not saying that the way to do it but I have not had an issue yet.

 

[Mule Laker]

Tach should be fine. The boat was bought new from the factory in 2014 and handles beautifully until our little rock encounter.

I think they mean the in-dash tachometer's are frequently inaccurate from the factory.