Pitch & diameter question = can someone check my prop math???

[cdoggy81]

Currently running a 15" x 17 pitch 3 blade aluminum propeller on my 19.5" bowrider (4.3GL). WOT is just at or under 5000rpm (depending on wind, chop, etc.) & top speed (GPS) is ~ 38-39 mph. It works great in the summer for water sports but now it is cold & all we do is cruise so I would like to get a new prop for better economy. I have a buddy who is wanting to give me his Laser II that is 13.75 x 23p but I'm not 100% sure how much it will help increase top speed & economy.

For every inch of pitch you add you drop 200 rpm.
It is my understanding that for every 1" in diameter you drop you increase your rpm by 500 or ~ 2.5" in prop pitch.
I have also heard when going from an alum prop to the same ss prop you pickup about 200 more rpm.

So if I am spinning 5000rpm with my 17p prop & I go to a 23p prop in theory I should only be able to spin 3800rpm.
Then if I decrease my diameter from 15" down to 13.75" it would give the effect of increasing my rpm by ~600 rpm. So now I am back up to 4400rpm. I should also pickup another 200rpm from making the switch to ss so that should put me back up to ~ 4600rpm which should be just about right.

Does my math look correct?
What are your thoughts?

I have another option for a used ss Mirage that is a 14.5" x 23p demo prop from my local mechanic for $100. I think it may be too much as I came up with 4250rpm WOT when doing the math. Plus a free Laser II is always better than a $100 Mirage)

 

[tpenfield]

Re: Pitch & diameter question = can someone check my prop math???

It is not an exact science, but more of an estimation process. . . your numbers are OK. I have done some prop calculations in the past by looking at the volume of water 'pushed' per revolution, but have never put the calculations to the test.

If I put your numbers into my "calculation spreadsheet", the proposed prop would try to 'push' 11% more volume per revolution than the current prop . . . so you would get a corresponding drop in RPM . . . which would put you in the 4500 RPM range.

The unknown about this is how the engine/boat will perform with the higher pitch prop. One thing to keep in mind is that the boat will probably very load sensitive with the smaller/higher pitched prop. So, as you add people and gear, it may not be so good.

The second option prop, will put about 25% additional volume (load) per revolution, which would probably be way too much.

If you can do the Laser II for free $$$, it would be worth a try.

 

[jestor68]

Re: Pitch & diameter question = can someone check my prop math???

A "free" prop isn't worth anything if it won't allow you to operate within your recommended rpm range. Your figure of about 3800 rpm with the 23" is about right.

 

[steelespike]

Re: Pitch & diameter question = can someone check my prop math???

I'm a little on the outside so only can make some observations.
We really need a confirmed hp ,rpm range and gear ratio. using a 1.62 gear ratio you have about 21% slip at 5000 and 39 mph.
Speaking very generically I think your rated rpm is probably 4400-4800.
17" sure seems like a low pitch for your setup. It seems to me you should be over reving more than you allready are. I think more typical would be 21-23" with speeds around 50.
Of course a 23" would probably not be best for water sports.
Rule of thumb a 6" increase in pitch would lower rpm about 1200.
If you look up some props I think you will notice that as pitch goes up diameter goes down.
While diameter can affect rpm it is usually a secondary consideration.
Disregarding pitch and rake a 17" prop should act like a 17" prop,or a 23 as a 23.
With a properly setup boat the difference in mileage between reasonable setups would likely be measured in 100ths.
It appears your setup at 21% slip could be improved.

 

[jestor68]

Re: Pitch & diameter question = can someone check my prop math???

VP SX current available gear ratios are 1.60, 1.66, and 1.79 for us lowlanders. There are two more for "high altitude"; 1.89 and 1.97.

VP only lists the max rpm as 4800.

 

[cdoggy81]

Re: Pitch & diameter question = can someone check my prop math???

Some notes
Rebuilt Volvo 4.3 GL (original 1999 vortec 190 hp I believe) with the SX outdrive in a 1.66 gear ratio.
Rated rpm is 4400-4600 i believe but the 4.3 v6 will run 4800 with no problem.
Current prop on there now is an aluminum quicksilver basic big eared prop.
Laser isn't exactly free as I need a Volvo hub kit for it (~$40-$50)


tpenfield - where can I find your "calculation spreadsheet"

that Solas Rubex 14.5 X 19 may just do the trick as I came up with about 4600 rpm.

Thanks guys!

 

[tpenfield]

Re: Pitch & diameter question = can someone check my prop math???

tpenfield - where can I find your "calculation spreadsheet" ?

Thanks guys!

It's on my hard drive , but I can send it along in a PM.

 

[QC]

Re: Pitch & diameter question = can someone check my prop math???

My contributions:

1) I think you should concentrate on pitch, and prop design, much more than spending anytime considering diameter

2) If your goal is "economy" you need to note that an appreciable change (more than 5%) is really not in the cards no matter what you do.

3) Top speed will improve if you decrease slip, and bring the RPM closest to "rated"

4) SS typically lowers RPM if all else is equal. The flex in aluminum allows a 19 to behave like an 18 for example, where the SS stays truer in pitch. Thinner blades results in better efficiency, so theoretically slip goes down and speed increases for the same RPM.

 

[cdoggy81]

Re: Pitch & diameter question = can someone check my prop math???

tpenfield - I'll be on the lookout for that PM

I checked my local CL & found a cheap SS 3 blade PROP 14.75" X 21p (part #3858028) that I may try out for $80. Not a ton of info I could find on it but I did see this:
Prop 14 3 4 x 21 s 3858028 | eBay

Thank you all very much for the info guys. I do appreciate it! I'll also report back how everything worked out.

 

[cdoggy81]

Re: Pitch & diameter question = can someone check my prop math???

"4) SS typically lowers RPM if all else is equal. The flex in aluminum allows a 19 to behave like an 18 for example, where the SS stays truer in pitch. Thinner blades results in better efficiency, so theoretically slip goes down and speed increases for the same RPM. "

Really? It was my understanding that the blades on a ss prop are thinner allowing the prop to spin easier / faster to a higher rpm due to less resistance. This results in better efficiency, decreased slip for the same RPM & increased MPH.

 

[Part-time]

Re: Pitch & diameter question = can someone check my prop math???

Like QC said above, an aluminum prop will be somewhat like a lower pitched prop compared to it's SS counterpart.
The SS prop will have a better top end because the pitch stays truer.
The aluminum prop will however get you out of the hole faster because it lets the rpm's get up faster.
Under load the blades of an aluminum prop will flex reducing the pitch, and I kind of wonder if the weight of the SS might also reduce the wholeshot...
Also keep in mind where you boat.
Myself, I only use aluminum because I'm on a river with a lot of risks of digning the prop on rocks and whatever else may be under there.
It's only $40 or $50 to get an aluminum prop repaired around here. $250 and up for a stainless.

 

[Tail_Gunner]

Re: Pitch & diameter question = can someone check my prop math???

Some notes
Rebuilt Volvo 4.3 GL (original 1999 vortec 190 hp I believe) with the SX outdrive in a 1.66 gear ratio.
Rated rpm is 4400-4600 i believe but the 4.3 v6 will run 4800 with no problem.
Current prop on there now is an aluminum quicksilver basic big eared prop.
Laser isn't exactly free as I need a Volvo hub kit for it (~$40-$50)



Ok here we go

tpenfield - where can I find your "calculation spreadsheet"

that Solas Rubex 14.5 X 19 may just do the trick as I came up with about 4600 rpm.

Thanks guys!

Well i think ..pretty sure actually you have some engine problem's..probably timing. That combination 18.5...190 hp..1.66 g3earing sould do a 20p or 21p ss to about 50plus mph and 4800 rpm with ease.....

 

[cdoggy81]

Re: Pitch & diameter question = can someone check my prop math???

PT & QC - now I'm all confused about ss vs alum...

TG - Pretty sure I have engine problems? Well that sux....
I thought with a 19'6" boat, weighing 3000lbs dry, a few people & a 38g tank I was doing pretty good with a 17p aluminum prop hitting 39mph.

 

[Tail_Gunner]

Re: Pitch & diameter question = can someone check my prop math???

PT & QC - now I'm all confused about ss vs alum...

TG - Pretty sure I have engine problems? Well that sux....
I thought with a 19'6" boat, weighing 3000lbs dry, a few people & a 38g tank I was doing pretty good with a 17p aluminum prop hitting 39mph.

Two people 40 gals 19' 3500 dry 210hp 1.66 drive 20p blade ss...52 all day every day. What brand and yr of boat...define a few people 4-5? or 2

 

[Part-time]

Re: Pitch & diameter question = can someone check my prop math???

PT & QC - now I'm all confused about ss vs alum...

Another thing to add to the confusion... The shape of the prop.
2 props of the same material, diameter and pitch can be very diferent due to the shape of the blades and the cupping.
Some props will tend to give you stern lift while others will give you bow lift.
If you want every last mph that you can posibly sqeez out, go with SS.
I use aluminum because if I strike a rock with a SS it will not only cost more to repaire the prop, but also because the SS is harder, I might break something that cost a lot more to fix in the bottom end.
Personaly I don't think there is anything wrong with your motor or timing.
Your just tached out from being "undergeared" with that 17p
I would look around to see if a marina could lend you a few props to try out.
You might also want to consider a 4 blade instead of a 3 blade. I find them smoother on a boat the size of yours.

btw, I ran a 14 3/4 x 21p on an 18' tempest with a 140hp I-4 mercruiser (don't remember the gear ratio) but it was a good all around and I had a 19' century with a 280hp sb chevy with a 4 blade 23p that ran over 50mph all day.

 

[cdoggy81]

Re: Pitch & diameter question = can someone check my prop math???

Thanks for the info PT.
I agree with you as well, my motor is fine

TG - I think you missed something. You are comparing my results with my 17p prop to what it would be for a 20p blade...

 

[jestor68]

Re: Pitch & diameter question = can someone check my prop math???

tpenfield - I'll be on the lookout for that PM

I checked my local CL & found a cheap SS 3 blade PROP 14.75" X 21p (part #3858028) that I may try out for $80.

You could be dropping your rpm by 600- 800 (4 inches of pitch) with that prop. The diameter difference of only 1/4 inch has no measurable effect.

 

[Tail_Gunner]

Re: Pitch & diameter question = can someone check my prop math???

Thanks for the info PT.
I agree with you as well, my motor is fine

TG - I think you missed something. You are comparing my results with my 17p prop to what it would be for a 20p blade...

No i have a similar boat about 6" shorter and at the time maybe 20 hp stronger..the 20 was a aggressive 4blade ss @ 52...it could run 56 with a 22 3 blade..That was the comparison..very conservative. Read Spike's comments again 17 is quite small for your combo. But carry on good luck

Here's a prop calc http://www.go-fast.com/Prop_Slip_Calculator.htm

Using a 17 @ 10% slip nets 42 a good clip for a 3.0 in a 18.