Prop question (basic 101)

[Will S.]

2 props. 13 x 19K & 13 x 13K  which will have higher RPMs ?  I believe there is about 1000 rpm difference, but which way..

thanks. I know its early... Increasing pitch lowers rpm so the 13 x 19 will have the LOWER RPM... right ?

[MrGiggles]

Think if the prop was turned in a solid like a wood screw. The distance it would move in one revolution is the pitch. The higher the number, the lower the RPMs.

There is more to it than I know, diameter usually decreases with an increase in pitch, so that 13x19 is likely a much "bigger" prop. 

[Will S.]

10 minutes ago, MrGiggles said:

Think if the prop was turned in a solid like a wood screw. The distance it would move in one revolution is the pitch. The higher the number, the lower the RPMs.

There is more to it than I know, diameter usually decreases with an increase in pitch, so that 13x19 is likely a much "bigger" prop. 

Got it, Thanks..

[Will S.]

I have two props both same pitch & diameter. One aluminum & one old "painted" stainless steel".

Other than the slip & flex would one or the other have lower RPMs at WOT ?

I have not personally tested yet & cant for a while...

[fishinwrench]

Is it painted BLUE ?   

[Will S.]

2 hours ago, fishinwrench said:

Is it painted BLUE ?   

No, it is black. OLD prop

[fishinwrench]

1 hour ago, Will S. said:

No, it is black. OLD prop

Ok, cuz the blue props are a different animal.  

Theoretically the stainless prop, having thinner blades and a little more cup will bite and hold the water better.  They'll do the job of pushing the boat better.....but the difference in the tach reading at WOT is anybody's guess. 

Propping boats to obtain the best prop for speed, hole shot, hard turns, ect. is a practice heavy in witchcraft.  I absolutely despise "prop doctors" that allegedly tune props for various applications.  All they are creating is a boat part that can't ever be duplicated.   It's horseshit!   So who cares if it, by chance, turns out to be perfect.   Plus, alot of those "tuned props" throw blades.....and then you're stranded, and back to square one..... provided it doesn't kill you.

[Will S.]

5 hours ago, fishinwrench said:

Ok, cuz the blue props are a different animal.  

Theoretically the stainless prop, having thinner blades and a little more cup will bite and hold the water better.  They'll do the job of pushing the boat better.....but the difference in the tach reading at WOT is anybody's guess. 

Propping boats to obtain the best prop for speed, hole shot, hard turns, ect. is a practice heavy in witchcraft.  I absolutely despise "prop doctors" that allegedly tune props for various applications.  All they are creating is a boat part that can't ever be duplicated.   It's horseshit!   So who cares if it, by chance, turns out to be perfect.   Plus, alot of those "tuned props" throw blades.....and then you're stranded, and back to square one..... provided it doesn't kill you.

Yeah, I just dont know why my boat came with an alum & a stainless same itch & diameter. The stainless is on now & at 3/4 throttle the rpms run closer to 7000, much higher than the 5000-6000 range. and that was not WOT... I plan on getting accurate numbers after the weekend on a calm day. I have read so much on props my head is gaining its own rpms.

* I am not looking for hole shot nor top speed. I am past all that nowadays. I just dont want to stress my motor. I am fine just getting up on plane to get from one fishing spot to another.

[fishinwrench]

I suspect that your tach may be off. 

What boat and motor are we dealing with here?

[Will S.]

1 hour ago, fishinwrench said:

I suspect that your tach may be off. 

What boat and motor are we dealing with here?

09 Alumacraft MV 1860SPL CC AW 

08 Yamaha F90 Four Stroke

First day on the water since I bought the boat. I have not done a WOT yet because I did not notice the rpms until I was almost back top the dock and that was briefly. By that time I was exhausted from fishing & sunburnt. It is very possible I was trimmed up too much, seriously I was so spent, I just needed to leave. Momentarily read 37mph on my lowrance @ 3/4 throttle and that was about MY limit with my current medical issues. Again im fine at 25ish mph- whatever just be up on plane.

Weather permitting (wind), I plan to go out Monday to better pay attention to my prop position & rpms. Make sure I actually do have an issue before I dive into the prop things. Cant imagine the (original) previous owner would have two incorrect props..

Very possible "exhausted" operator error.

EDIT:  Here is a picture of my gauges when the boat is off. Just noticed where the rpms are sitting. Naturally I dont know if that is suppose to zero out when you shut the motor off. ( I went out to the boat & turned the key on & off but there was no change in tach reading-if that matters.)