Expert Advice Needed - I "did the math"

[JoLin]

Got some great baseline stuff this morning. Went out early while the bay was very calm, with no wind to speak of. Here we go:

The boat is a 21', constant deadrise (20 deg) v-hull. No "flattening" at the rear. She's also stern-heavy. Main motor is on a 30" setback bracket, and there's a 4-stroke kicker and bracket. Both batteries are also situated near the stern. Tabs will probably go on soon, but I want to concentrate on the prop first.

Current prop is 15x17, 3-blade aluminum in good condition. It's what I'd consider an "oem type" prop- no discernable cupping or ventilation.

Johnson spec calls for WOT range of 4500-5500 rpm

With normal load and motor trimmed for best speed, I'm pulling 5200-5300 rpm at WOT, at a GPS speed of 37.5 mph. I'm pretty close to optimum, but I think there's a little room for tweaking.

• I want stay with aluminum for now
• I want to maximize lift, hole shot and mid-range performance. Under normal (choppy) bay conditions, the boat and passengers are most happy at around 4000 rpm/26 mph
• I'm thinking maybe a 4-blade prop with a 1" reduction in both diameter and pitch - 14-ish x 16

Does it all sound right? Can someone recommend a specific prop brand/model?

Thanks, folks!

 

[JoLin]

Re: Expert Advice Needed - I "did the math"

A little more info. I went to this site to calculate my prop slippage:

http://heringpropellers.com/hering-propeller-slip.php

With a 1.86 gear ratio, 5300 rpm and 37.5 mph, I show slippage of 18%, which I know is higher than optimal (should be < 15%). How should I factor that number into my propeller search?

 

[JoLin]

Re: Expert Advice Needed - I "did the math"

<bump>

 

[Tail_Gunner]

Re: Expert Advice Needed - I "did the math"

Got some great baseline stuff this morning. Went out early while the bay was very calm, with no wind to speak of. Here we go:

The boat is a 21', constant deadrise (20 deg) v-hull. No "flattening" at the rear. She's also stern-heavy. Main motor is on a 30" setback bracket, and there's a 4-stroke kicker and bracket. Both batteries are also situated near the stern. Tabs will probably go on soon, but I want to concentrate on the prop first.

Current prop is 15x17, 3-blade aluminum in good condition. It's what I'd consider an "oem type" prop- no discernable cupping or ventilation.

Johnson spec calls for WOT range of 4500-5500 rpm

With normal load and motor trimmed for best speed, I'm pulling 5200-5300 rpm at WOT, at a GPS speed of 37.5 mph. I'm pretty close to optimum, but I think there's a little room for tweaking.

• I want stay with aluminum for now
• I want to maximize lift, hole shot and mid-range performance. Under normal (choppy) bay conditions, the boat and passengers are most happy at around 4000 rpm/26 mph
• I'm thinking maybe a 4-blade prop with a 1" reduction in both diameter and pitch - 14-ish x 16

Does it all sound right? Can someone recommend a specific prop brand/model?

Thanks, folks!

lets start the party here...Have you ever considered Trim tab's...As to a Baypro 3 and stern lift that i dunno..The baypro 2's do have a lot of stern lift but they are different props. You can call them at 1-800-944-4576 Stilletto

Here's a link as seen above http://forums.iboats.com/showthread.php?t=218877


HEAVY LOAD PROPS
MANUFACTURE RPM MPH NOTES
MWC 013052 15X15 6550 42.1 PUSHES 800LBS OF SAND BAGS LIKE THEY ARE FILLED WITH AIR
LEXOR 15.75X15 6500 41.9 “
MIRAGE PLUS 15.75X15 6500 41.7 “
MANUFACTURE RPM MPH NOTES
OMC 431930 15X17 6300 44.6 SLOWER HOLE-SHOT
LEXOR 15.5X17 6250 45.2
VENGEANCE 14.5X17 6300 44.8 LITTLE SLOW OUT OF HOLE
BALLISTIC 14.75X17 6100 44.1 NOT A GOOD LOAD PROP
MANUFACTURE RPM MPH NOTES
MWC 14.5X19 5850 47.6 SLOW HOLE-SHOT-NO VENTS
VENGEANCE 14X19 5850 46.9
MANUFACTURE RPM MPH NOTES
MIRAGE+ 14.75X21 5500 48.6 HOLE-SHOT BETTER THAN EXPECTED
LEXOR 15X21 5500 49.1 HOLE-SHOT AS ABOVE WITH GOOD MID-RANGE
TURBO 1 14.25X21 5550 48.0 VENTED BUT PRETTY SLOW HOLE-SHOT WITH MAX LOAD
VENGEANCE 13.75X21 5600 48.2 NO VENTS AND SLOW HOLE-SHOT, BUT GOOD MID-RANGE

Notice the MWC comment it is the first one...that is pretty high praise.

A few things you may want to know is just where does your outboard run its best rpm.... vary's wildly in O/B's i noticed...How is your motor setup and if you know all that...just wait around a bit im sure you will get some top advise.

 

[JoLin]

Re: Expert Advice Needed - I "did the math"

Thanks, Tailgunner-

A couple of answers-yes, as noted in the post tabs are on the list, but I want to prop it well first.

What is best rpm? As noted in my post, boat and passengers are comfortable with speeds in the mid-20's, roughly 4,000 rpm. Maybe I'm not sure what you mean by "best", but to me it's the speed at which I'd normally travel in the waters where I boat.

My motor is set up to the point where I'll see a bit of ventilation/cavitastion in a sharp, high speed turn. That works for me and that's where it'll stay.

Yes, I read those test results, but... lighter, faster boat, balanced load, different goals. I didn't feel that they applied well enough to my situation to use.

 

[Silvertip]

Re: Expert Advice Needed - I "did the math"

I would do the tabs first and then do the prop. You'll get more stern lift with the tabs than the prop can ever provide and hole shot will be greatly improved as will the ability to stay on-plane at slower speeds. I'm a Smart Tab owner and swear by them.

 

[JoLin]

Re: Expert Advice Needed - I "did the math"

Silvertip,

That's the funny thing.... I KNEW that if I asked about the prop, somebody would tell me to get trim tabs, and if I asked about trim tabs, somebody would tell me to get the right prop first

I have certain goals to attain as stated in my post. Stern lift and mid-range performance are the 2 most important, along with gaining just a couple hundred additional rpm at WOT with normal load. A prop is central to the boat's performance at all speeds and in all conditions. I know that I want and need a different prop, and I'm now willing to pay the price of stainless. I also believe I need tabs. Those are are next on the list.

Based on some posts here concerning the Bay Pro II, I'm looking hard at the Stiletto line. I think I need a 15 pitch in stainless, but the Bay Pro II doesn't come in that size for my motor. Fiirst order of business is to confirm that I'm on the right track in terms of pitch, SS and 4 blades. At the same time I'm asking for recommendations of brand and model.

1. Does it appear to you that a 4-blade, 15 pitch prop is what I'm looking for?2. Got any prop recommendations for me?

 

[45Auto]

Re: Expert Advice Needed - I "did the math"

I'm with Silvertip on this one. Doesn't make sense to me to optimize the prop - then put tabs on it - then not have the best prop with the tabs and have to re-prop again.

Why not put the tabs on if you plan to anyway, then see what you need for a prop and do it once???

 

[JoLin]

Re: Expert Advice Needed - I "did the math"

45auto,

We're getting into a "chicken and egg" sort of debate. Honestly, what will change in the propeller selction if I put tabs on first? The boat will still be stern-heavy, I'll still want to optimize lift and mid-range performance, etc. All my original goals will still apply. If I get the right prop, I may not need tabs, or I might only need a smaller set. Remember, tabs also mean more drag in the water. Personally (and good-naturedly ), I think you guys are putting the cart before the horse. I'm not trying to start an argument with you- I'm just trying to get some help in selecting a prop.

The main components of a power boat are the hull, motor and prop. I bought the hull type I wanted, with a good motor. The existing prop does jot suit my needs. Once I have those primary components configured that way I need them, I'll decide what size and type of tabs I need (if I still need them) to optimize the package.

Based on the original post, am I on the right track in terms of pitch and number of blades? I think I am, but confirmation would be nice.

Got any specific propeller recommendations for my boat?

Thanks!

 

[Tail_Gunner]

Re: Expert Advice Needed - I "did the math"

stay tuned..i certainly am... what year is that johnson O/B.....Umm if your tach is accurate within 50 rpm and your using gps to verify your speed......

http://boatpropellers.iboats.com/Jo...r=2001&**********=376845310&*******=031666073

Your two hundred off (RPM) from a baypro 2 16 pitch...go into the O/B forums and ask just what is the best rpm for that O/B.

 

[JoLin]

Re: Expert Advice Needed - I "did the math"

Tail Gunner,

THAT's an interesting-looking prop! Nice price, too.

Can you tell me where we're going with the "best rpm" question for my motor?

As soon as I ask "what's the best rpm for a 1995 Johnson 175 carb'd powerhead?", I'll be asked "best rpm for what?"

I'll have to reply "I dunno, Tail Gunner told me to ask."

In the meantime, yes, the tach is accurate and yes, speed was measured via GPS.

 

[Tail_Gunner]

Re: Expert Advice Needed - I "did the math"

Tail Gunner,

THAT's an interesting-looking prop! Nice price, too.

Can you tell me where we're going with the "best rpm" question for my motor?

In the meantime, yes, the tach is accurate and yes, speed was measured via GPS.

http://forums.iboats.com/forumdisplay.php?f=24

I would go there and just explain your tuning up the boat a little and if indeed the rpm your quoting is the optium range..On O/B's i have seen some big variation's from very credible people .. advising to go way up in the rpm range...for myself i dunno...maybe Wh or Dhadley will chime in

 

[JoLin]

Re: Expert Advice Needed - I "did the math"

Okay, done. As it turns out, I need a new prop anyway. Hit a sandbar last weekend, blew my PT&T, and I think I spun the hub. Had an odd speed-up / slow-down on the way to the ramp to haul it out to repair the trim unit.

It's been an interesting week

 

[Tail_Gunner]

Re: Expert Advice Needed - I "did the math"

Well if your motor does indeed respond best @ 5500 to 6000,then you have some issues. WalleyeHead generally recomend's that you run at the top of the range or prop at the top..
__________________________________________________________________

Current prop is 15x17, 3-blade aluminum in good condition. It's what I'd consider an "oem type" prop- no discernable cupping or ventilation.

With normal load and motor trimmed for best speed, I'm pulling 5200-5300 rpm at WOT, at a GPS speed of 37.5 mph. I'm pretty close to optimum, but I think there's a little room for tweaking.

___________________________________________________________________


To get optimal i would think 14 or 15 pitch 3 blade would be a start...I have no ideal on the type of gearing you have..but that smaller prop would really dig you out of the hole...and if your sure your tach is on.....tash does not give out info off the cuff....it appear's the motor has been lugging around a bit....

The two or three guys really in the know havnt chimed in yet.....but i suspsect tash is right.. And if so when thd dust settles you will have a completely differnet and new experience with you boat...

Under reving a engine is not good

http://boatpropellers.iboats.com/pr...r=1995?**********=390930016&*******=938205848

175 OP
4500-5500
1.86


There's I-Boats spec's

 

[JoLin]

Re: Expert Advice Needed - I "did the math"

Okay, just to clarify:

JOHNSON advises that WOT rpm range is 4500-5500 rpm. That's the manufacturer spec.

Tashasdaddy said the motor has more power available- it'll keep producing up to 6000 rpm. Not all that unusual- happens in the automotive world, too. Couple of things to remember...

1. My powerhead is 13 years old, and I don't know how many hours it has on it. It runs well, but I am not willing to consistently push it beyond the manufacturer's specs.

2. Given that it produces power up to and beyond 5500 rpm, that's what I would target as the maximum WOT rpm. 5500 rpm.

So, given that I want to get close to 5500 rpm at WOT, I believe I should go from 17p to 15p. 1" for moving from aluminum to stainless and 1" to gain a couple hundred rpm (my wot with the current prop is 5300)

Make sense?

 

[Tail_Gunner]

Re: Expert Advice Needed - I "did the math"

Yes it does, unless someone can shed anylight onto the rpm question...Beside's all that i-boats doe have a exchange policy and they do have a 14p....

http://boatpropellers.iboats.com/Jo...r=1995&**********=448080948&*******=938205848

Good luck........

 

[Randybeall]

Re: Expert Advice Needed - I "did the math"

Ok, JoLin. Your last post makes most since. First talk to your prop supplier and you can exchange props for whatever freight costs are in many cases. My opinion, for what ever that is worth, go with the stainless, drop the 2 inches in pitch. Worst case is that you over rev and have to exchange for the same pitch in stainless. The stainless prop with proper cup will give you better efficency and may increase load enough to keep the rpm range correct. When you put the trim tabs on, prop alone won't get what you want unless you tuck the motor in and kill time to plane, you may find that you loose 200 to 300 rpm and start the prop process over again. Have you considered a nitros oxide injection to get the hole shot thing.

 

[Tail_Gunner]

Re: Expert Advice Needed - I "did the math"

Randall the discussion evolved into the optium rpm @ wot with a 500 rpm difference which is huge...not even talking pitch or bow lift or stern lift..at this point it secondary...

 

[JoLin]

Re: Expert Advice Needed - I "did the math"

By the time Randall chimed in I had already ordered the Titan 4-blade stainless in 15 pitch. I can understand the rationale of propping for maximum rpm regardless of what the manufacturer says, but at some point the law of diminishing returns comes into play.

If I propped for 6000 rpm I?d be running something like a 13-pitch. My wife and I normally cruise at around 25 mph. I?d be pushing what, high 4000?s? I have a big ol? carbed motor. Increased fuel usage, noise and concerns about the motor being overstressed would all detract from my boating experience instead of enhancing it. I?m a new boater in crowded and sometimes rough waters. I have enough other things to worry about

I know everyone thinks I should have installed Smart Tabs first. You?re probably all right and I was probably being pig-headed and ?old school?. I apologize to anyone I might have offended by blowing off their advice. The question became academic when I hurt my prop on July 5th. I need a new one and I don?t feel like replacing it with the same type, adding tabs, and then propping it again. I want to get the boat back in the water and try to keep it there for the rest of the season. I'll keep looking at eBay for a cheap spare.

It?s been said time and again that every boat is a compromise. I think I found a good prop compromise for my boat and boating style. I think the Bay Pro would have been a good choice, but so is the Titan- and it?s cheaper. Thanks, Tail Gunner.

John