Prop Damage and WOT RPM.

[WriterCJ]

Hi all,

Fairly new to this -- the pic below is of a 10"x13" prop fitted to a 1989 Evinrude 20HP ELCEM on the back of my 17ft fibreglass boat.

I dinged the prop on the bottom returning to a mooring at low tide, and did a bit of damage.

My question: is it safe to stick with this prop until the boat comes out of the water in a month or so... or am I risking more damage to the lower unit by doing so?

Also trying to work out whether to stick to the current 13" pitch or drop to 11", but working through some engine issues, so can't really make a call on WOT RPM and boat performance until that's sorted.

Once I'm sure the engine is running properly, will I still be able to perform a valid WOT RPM test to determine optimum pitch with the current (damaged) prop?

Thanks for your help.

Cheers,

Calvin!

 

[Brewman61]

Re: Prop Damage and WOT RPM.

If you're not noticing any vibrations or other unusual symptoms, especially as you run at higher speeds, then you might be okay. I've seen people running props damaged worse than that. Impossible for someone to say for sure, just by looking at a photo, though.
It's your boat, and you'll be stuck with the $$$ of any damage that an unbalanced prop could cause.
Time to think about a spare prop. Get another prop. as a spare, then have this rebuilt. My prop guy would charge less than $50 to fix that prop.

 

[WriterCJ]

Re: Prop Damage and WOT RPM.

If you're not noticing any vibrations or other unusual symptoms, especially as you run at higher speeds, then you might be okay. I've seen people running props damaged worse than that. Impossible for someone to say for sure, just by looking at a photo, though.
It's your boat, and you'll be stuck with the $$$ of any damage that an unbalanced prop could cause.
Time to think about a spare prop. Get another prop. as a spare, then have this rebuilt. My prop guy would charge less than $50 to fix that prop.

No obvious increase in vibration when running, so thinking it's OK to run until I pull the boat out of the water. I'm planning to do exactly what you suggest but want to work out if I need to drop from 13" to 11" pitch before investing in a new prop.

Going to get hold of a tach to measure WOT RPM -- then use it to inform my decision re. prop pitch -- but wondering if WOT RPM will be an accurate guide to pitch choice given the current prop damage?

 

[steelespike]

Re: Prop Damage and WOT RPM.

Your not likely to cause any damage in the short term but a very expensive repair
if you do.Especially compared to a new prop cost.
A 20 hp on a 17 ft?Is that a missprint.I also don't find a 10 X 13 for a 20.??
By the seat of my pants a 20 in that application would feel a lot better with an 11".

 

[WriterCJ]

Re: Prop Damage and WOT RPM.

Your not likely to cause any damage in the short term but a very expensive repair
if you do.Especially compared to a new prop cost.
A 20 hp on a 17 ft?Is that a missprint.I also don't find a 10 X 13 for a 20.??
By the seat of my pants a 20 in that application would feel a lot better with an 11".

Thanks Steelespike.

It's not a missprint :redface:

The 20HP came as part of the package when I acquired the boat and the current prop (which was unblemished until I ran into something ) I think is the Evinrude OEM 10" diameter x 13" pitch aluminium prop (part #0386933 ) -- as listed in the BRP online catalogue for the 1989 E20ELCEM.

I know the engine is on the small size for the boat, but finances dictate that, for now at least, I'll need to persevere until I can find the right deal on a used 40-50 HP.

The real question is whether to stick with a 13" pitch or drop to an 11" -- I'm thinking 11", but would love to get an accurate WOT RPM reading with the 13" as a guide before I buy.

Wondering how, if at all, the damage to the existing prop would impact WOT RPM...?

 

[RicMic]

Re: Prop Damage and WOT RPM.

The damage is going to have some effect on rpms, more drag= lower lower rpms, probably not much, I have to think with that pitch on that boat your rpms are going to be way low anyway. If it was me, I would take that prop off and with a file and hammer, straighten up the edges a bit, to get you through the rest of the season.

 

[WriterCJ]

Re: Prop Damage and WOT RPM.

The damage is going to have some effect on rpms, more drag= lower lower rpms, probably not much

I thought that initially too -- but then read somewhere that prop damage can increase cavitation/ventilation, causing more turbulance/bubbles around the prop, decreasing the resistance of the water and thus INCREASING RPM.

Confusing doesn't even begin to describe the whole prop / hydrodynamics thing [head spinning just thinking about it].

I have to think with that pitch on that boat your rpms are going to be way low anyway. If it was me, I would take that prop off and with a file and hammer, straighten up the edges a bit, to get you through the rest of the season.

That's the plan . Going to measure WOT RPM properly before ordering a replacement prop. Will get this one fixed up as a spare.

 

[steelespike]

Re: Prop Damage and WOT RPM.

The 13 is pretty far up the chart; only 2 props with more pitch.
You will surely need a smaller pitch With the rpm you will be able to tell
better

 

[WriterCJ]

Re: Prop Damage and WOT RPM.

The 13 is pretty far up the chart; only 2 props with more pitch.
You will surely need a smaller pitch With the rpm you will be able to tell
better

I'm thinking this Solas Amita 3 aluminium prop at 10.5" x 11" for ?100 delivered looks about the best bet -- but want to get an accurate WOT RPM reading first just to be sure.

Cheers,

Calvin!