Damaged prop bad?

[lister]

Mercuriser 3.0 I4 with Alpha one outdrive - 18 ft fiberglass deck boat
So yesterday I took a big chunk out of one of the blades on my prop on a rock I didnt see. It's a semi circle that looks like a bite about the size of half a poker chip.
I didnt notice any vibration coming back to the marina, but I was just wondering if I should change it out? Maybe theres vibration causing a short or long term problem I dont know about?

 

[Scott Danforth]

get it fixed

 

[lister]

because..... ?
I mean should I not use it at all until its fixed, or plan on doing that at the end of the season when its out of the lake?

 

[harringtondav]

Fix it now, and get a spare. New Michigan Wheel alum. props can cost less than a rebuild. I have a nice SS prop, but I only use alum. while I'm on the Miss. river. Too many dead head logs. An aluminum prop is like a fuse, it may get ruined with a bad hit, but it's less likely to bend the prop shaft than SS. A new Merc. prop shaft is about $500, Sierra equivalent is $290. If you can't replace it yourself, add at least another $500 for a shop to do it.

 

[RaceCarRich]

Props can be changed in the water. I’ve never done it but they make floating prop nut wrenches.

 

[porscheguy]

Aluminum props cost about the same to fix or replace. Do one or the other. It may not be vibrating now, but why give it the chance to start?

 

[Silverbullet555]

Props can be changed in the water. I’ve never done it but they make floating prop nut wrenches.

I have done it in the water. But I bent the prop shaft so it didn't matter.

OP, get it fixed or replaced. Right now it's a prop. Keep running it an it could create issues in the drive

 

[lister]

ok ill replace it thanks
Hopefully the shaft is fine, it was barely above idle when I did it, just about to pull away although it did stop the engine with a clunk
no vibrations so hopefully its just the prop. I'll try it in shallow water

I guess ANY prop that says its for an Alpha one will work? Fairly standard fitting?
like this

https://www.ebay.com/itm/Michigan-M...2532113924?epid=625521801&hash=item3d20222604

 

[GA_Boater]

because..... ?

.............

...................You asked the question, so you must have some concerns. You don't always feel any vibration until more damage is done.

How deep is your slip, can you change the prop there? Or get a new prop and slowly motor to a place you can beach her with water you can stand in to change the prop.

 

[Scott Danforth]

because..... ?
I mean should I not use it at all until its fixed, or plan on doing that at the end of the season when its out of the lake?

because an imbalance in the prop will take out bearings which will cost you more than if you simply get it fixed.

 

[Sea Rider]

It's a pity that current prop was not tached on a wot run to know max wot rpm achieved as loaded, that way can order exact same one if performed spot on or one with less pitch to pull wot revs up if in need for engine to rev middle to max wot rpm range factory stated to suit better your boating needs.

Happy Boating

 

[lister]

Heres the old prop, it was a 19 pitch, my new one is 21 and I think I like it better, and its stainless.
I DID take it out of the water to replace, the way my lucks going lately I would have dropped it all

 

[Sea Rider]

Heres the old prop, it was a 19 pitch, my new one is 21 and I think I like it better, and its stainless.

Why did you liked the new prop better, has better hole shot, better top end speed ? Don't know if engine is revving inside safe -max wot rpm range factory stated for that engine which is a must know...

Personally would't dare to play with props without a installed tach, could be over revving or lugging the engine badly as usually loaded without even knowing and that's definitely no bueno..

Happy Boating

 

[lister]

Well I like to keep my engine right around 3000 rpm when cruising across the lake.
The difference between the two props is about 3-4 more MPH. yes that might mean a little more strain on the engine but its 135hp I would think it can handle it.
As for my imminently failing coupler, well.....we'll see

 

[jimmbo]

Well I got news for you. You do not prop a boat because you like how fast it goes at 3000 rpm. You prop it so the engine can run in the specified rpm range at WOT. What was the WOT with the old prop?

 

[lister]

Dont know I've never ran it at WOT. I baby this thing like its made of glass.
All I can tell you is theres PLENTY of go left even at 3000rpm. At WOT I'd guess it would be around 5500 rpms and 45 mph? I guess we'll never know I dont intend on taking it over 3000 which is about 70% throttle

Whats the danger of a too aggressive pitch on a prop? Is the difference between 19/21 really that big a deal? Should I change it for a 19?

 

[jimmbo]

Too much pitch can place enough load on the engine so it can't make it to the recommended RPM range at WOT. That results in high bearing loads, high piston temperatures, basically a strained engine, plus it is not making all of its HP. Propping to low causes the engine to over rev, which has its own problems, and also results in less than rated HP

Since you don't know what rpm the engine can attain at WOT, you really are flying blind on props. You may discover that the last 30% of the throttle results in less than a 10% increase in RPM.
If you bought this unit new, and have never ran it wide open, it never got Broke In properly

 

[lister]

Well thanks for the info.
I'll have to roll with it this season and change it for a 19 when its out of the lake.
Is a change by 2" typically bad or often fine?
If i go wot with this prop i should expect a maximum rev of the manufacturer specifications then?

According to this page, a valid option is :
https://www.rubexprops.com/boat-prop...203.0L%20135hp

15

21

R

1571-150-21 (spline / pitch /etc )

Although having said that theres just about every other combination there too so....... lol


This document
http://www.operatorsguides.brp.com/...rsManuals_EJ/attach/Accessories\766315 EN.pdf
says
"One pitch size usually results in a change of 200-300 RPMs."
And the +/- range of acceptable RPMs is well over 300rpms (more like 900) so surely one size isnt that big a deal as far as engine life

 

[jimmbo]

You need a tach to know what your revs are, you also need to know what the manufacturer specs as the WOT range for your engine and then you select a prop so it is in that range preferably in the upper half of the range. On I/Os the range is usually 400 rpm wide. Coincidentally, in most cases, a 2” change in pitch will result in a 400 rpm change. To further complicate matters, several props(different brands and models) can end up getting the engine speed in the correct range, but will yield different boat speeds, ride, and handling.
Prop selectors are recommendations, not cast in stone. A tachometer is the only thing that can tell you if you are propped correctly regarding engine load

 

[lister]

Thanks again. Guess I'll just switch back to a 19 then. dammit, ignorance costs so much money sometimes

So for mine looks like:

3.0L 135

All

4400-4800rpms

I'll test next time