Noob Dumb Prop Questions

[aerospot]

I have acquired a 1976 18' Safe-T-Mate 4-Winns bowrider.
The aft face on the prop has rough blister spots that I suspect is caused by 'cavitation'.

I intend to use the boat mainly for water-sports, tubing, skiing, boarding, etc.
I want the most power out of the hole, yes?

It is a Mercruiser 140 I/O

What pitch / diameter prop would you recommend for power?
What pitch / diameter prop would you recommend for speed?

What about stainless steel props? Are they better or worse than aluminum?
If going from aluminum to stainless, should I consider a different diameter / pitch?

If there are threads already addressing these questions please guide me to them. No need beating a dead horse

Thanks!

Dave
Are 'anti-ventelation' or 'anti-cavitation' add-ons for the lower unit useful? If so, what would you recommend?

 

[walleyehed]

Re: Noob Dumb Prop Questions

Hey Dave! Welcome aboard!
There are not really any dumb questions here, and that's WHY we are here, so feel free to ask away.
First thing we really need to know is what pitch you have, the Prop Model/make and what RPM are you getting wih it?
The 140 I think is around 4200 on the recommended RPM range, not for sure on that, but this info will help in the determination of which way we need to go and whether or not a fin would help your situation.

 

[aerospot]

Re: Noob Dumb Prop Questions

I havent had the boat out yet. I purchased it last November and have had it stored in a garage all winter.
It is now in the driveway and I'm waiting for a nice day to take it out for its first run.
The prop that came with it has some rough spots in about the center of the prop blades on the aft side of the blade. Could this be caused by cavitation?

I'll get the numbers off the prop and take some pictures of the spots tomorrow.

Dave

 

[walleyehed]

Re: Noob Dumb Prop Questions

From what you describe, I'd say yes, that IS cavitation....which can happen for many reasons but the most common is a dinged leading edge on the prop, second would be mis-application on the prop design.
Let us know the numbers you get when you get the chance to wring'r out and we'll see if we can maximize the set-up for ya.

 

[aerospot]

Re: Noob Dumb Prop Questions

Last Saturday I took the boat out with my 8 year old grandson for a trial run. We put in early in the morning on calm water and spent around an hour and a half putting the boat through some paces.

WOT = 5100 RPM / 35 MPH

This after warming things up and slowly running at slower speeds.

It was strong out of the hole and was on step quickly.
I only regret not playing with the trim more. I put it full down after launch.
Would trimming up help with the pulling left. Would adjusting the fin behind the prop (thingamajig) ?trim? the steering? Which way adjusts what?

The prop doesn?t have any numbers on the outside of it. Do I need to remove the prop to find the numbers? It measures around 7 1/2? radius, 15? diameter.
Here?s a link to some pics;
http://borgpics.ulmb.com/2008-5-29 Boat Prop/index.html
http://www.borgportal.com/hobby_boating.html

 

[steelespike]

Re: Noob Dumb Prop Questions

I'm pretty sure your max rpm should be about 4,200 to 4,600 rpm.5,100 is too high.You may want to check your tach. Perhaps borrow a shop tach or have a tech go with you to verify your rpm.
And if you trim up you will get even more rpm and speed. and it will probably handle better.I'm not expert on props but I'll try to give you some round figures A 17 aluminum would get you down to about 4,700 but again when you trim you will be too high. Each inch of pitch will get about about 200 rpm.
increase pitch lowers rpm ,decrease increases rpm.
Increase pitch generally decreases hole shot and will increase speed if the motor can spin the prop.too much pitch will lug the motor(bad) and slow the boat.Too little pitch will over rev(bad)Like your present setup.
You can run the present prop for now just stay within the4,200-4,600 range.
Generally speaking a stainless prop is more efficient the same pitch as aluminum may lower rpm but increase speed.Stainless is tougher but more expensive to repair.

 

[Fireman431]

Re: Noob Dumb Prop Questions

Dave,
If your boat was strong out of the hole and running smoothly, why would you want to do prop adjustments? However, your listed RPM seem too high for your engine. Verify the tach reading.

The numbers on the prop are stamped one of two places: Original construction numbers are on the center hub outer ring (around the retaining nut). If it has been reworked, it is usually stamped on the outer hub, between the blades.

Anti cavitation add-ons to the lower unit may be helpful to get on plane quicker, but I tried them once and they were useless to me. Might not be to you, though.

There are a few issues that need to be addressed before reccommending a prop change (if necessary):


1) Research the reccommended wide open throttle (WOT) for your boat/engine combo. There is usually a minimum & maximum. It should be listed in most manuals as: 4800-5200 or something similar. Youir ideal prop will put you within that range. Don't go smaller, which will lead to over-revving and a blown engine.

2) Trimming the motor up & down will vary the top end speed & RPM slightly, but more importantly, it will also affect steering performance. Don't trim too high up as that pushes the bow out of the water and can become unsafe. Usually with slight trimming the boat will feel & sound different. When the boat seems to be running ideally, you've hit the "sweet spot". You'll know it by the sound and controls after you become used to the boat.

3) Is running WOT really what you want to do all the time, or are you playing in the water with skiers/tubers? If WOT is not what you need, then adjusting the prop pitch will increase your "holeshot", but decrease your top end speed. Usually watersports require more torque than speed. Higher pitch=less top end. Say you currently have a 15 x 17 (diameter x pitch), then a 15 x 19 would drop approx 3-4 MPH. Not a big deal. But this also decreases the WOT RPM as well, say by approximately 3-400 RPM.

4) Stainless vs Aluminum: What bodies of water are you running it in? Lake with sand? Ocean with coral? Lake with rock? Aluminum tends to get scarred more easily on the rough bottoms than SS does. There is also less flex in the SS prop, but to be honest with you, I've never really noticed a huge difference in my boat between the two. But, that may just be my vessel.

Going to a local marine service center can be very helpful. Ours will lend out 3-4 props with a minimal deposit. The you can water test each one and see what fits your need specifically.

Good luck with your boat.

 

[aerospot]

Re: Noob Dumb Prop Questions

Having just purchased a timing light I am shopping Ebay for a dewll / tach. I'll use it to establish my tach numbers. I do not intend to run WOT often. The boat will be used for watersports mainly, perhaps some fishing as well. I have a shop manual and will look for the recommended RPM.

I will be using the boat on Lake Erie from the shore of Pennsylvania. The bottom is a mix of rock, sand and muck, no coral

I'll pull the prop this evening and see if I can locate any numbers. What does the sequence of numbers represent? Is the diameter / pitch included in the 'numbers' sequence? Reading posts and visualizing prop performance I assume that an increased pitch will reduce torque and increase top end speed. Reduced pitch would increase torque and reduce top end speed. Is this correct? I like the idea of 'borrowing' props and will see if any of our local shops would be willing to rent me a couple to try.

As to why the boat pulled left... How can I correct this so the boat will track straight hands off? Will rotating the fin behind the prop 'trim' the steering?

Strong 'out of the hole' is a bit subjective as I have nothing to compare this to. But having grown up water skiing I feel the boat has plenty of umph and could probably pull me up on slalom. "Hole-Shot" performance is what I am mainly after. Tubing, knee-boarding, wake-boarding, children training skis, adult pair & slalom skiing...

Thanks for the advice! This place Rocks!

Dave

 

[Fireman431]

Re: Noob Dumb Prop Questions

You shouldn't have to pull the prop to look at the numbers. Look directly into the center hub at the locking nut. Stamped around that should be 2 numbers written like these examples:

15 x 21
14.5 x 19
15 x 21p

(Check the image for reference)

The first number represents the diameter and the second number represents the pitch. Also, spin the prop and look between the fins. If the prop has been reworked (changed to a different pitch by the previous owner), it will be stamped there. Pitch is the theoretical distance a prop moves forward in one revolution. A propeller has a 24 pitch will theoretically result in 24? of forward travel in one revolution. However, in the real world, the propeller experiences ?slippage? so that its actual travel per revolution is less than the stated pitch.

Prop pitch is frequently misunderstood. Many people mistakenly believe that switching to a larger pitch prop will make the boat go faster. If a prop with too large a pitch is used, the motor simply doesn't have enough horsepower to turn the prop adequately and the RPM level drops. Not only does the boat go slower, but the motor can load up and the plugs become fouled. If a prop with too low a pitch is used the prop can exceed the manufacturers recommended RPM level and damage the engine.

The ideal situation is to select a prop with a pitch that allows the motor to reach the maximum RPM suggested by the engine manufacturer without going over. If the prop selected doesn't reach the recommended RPM level, the boat will sacrifice speed and lift.

Here's a simple rule of thumb to follow when experimenting with prop pitch. Remember that at wide-open RPM increasing the prop pitch reduces RPM levels by roughly 200 rpm's per inch of pitch. In other words, when switching from a 23- to a 25-pitch prop, the maximum RPM level will drop approximately 400 rpm's. The reverse is true when going down in pitch size.

Increasing the pitch usually increases the torque, but will decrease top end speed. It will also change your RPMs to rev a little higher (if i have that correct...my mind is starting to go fuzzy).

If you use your boat for fishing, cruising and skiing, one prop probably won't do all three things equally well. It is best in circumstances like this to have two propellers; One to accommodate one set of circumstances and the other to perform best under the different load. It is imperative, however, that the wide open throttle RPM fall within the range specified by your engine manufacturer.

The advantages of props with extra blades are they provide quicker take-offs, and allow the boat to be kept on plane with fewer engine rpm's. However, they are slower at top-end speeds. A quality three-blade prop is normally two or three miles per hour faster at top end than the same pitch prop featuring multiple blades.

As far as the boat pulling left, that is due to the torque spin of the prop. Loosen the bolt on the zinc fin aft of the prop and adjust it over to the 1:00 to2:00 position. Also, do you have hydraulic steering? If so, check your fluid.

 

[steelespike]

Re: Noob Dumb Prop Questions

Keep in mind I/Os are tail heavy and can be a little sluggish on hole shot.
A number of things can help.trim tabs are an excellent addition.Some vent the prop; makes it wind up easier at low end but doesn't affect top end.
A higher pitch will no doubt be slower on hole shot.
Just reread your prop size.So we don't know pitch yet really but a 2 inch increase in pitch will still drop rpm about 400.

 

[aerospot]

Re: Noob Dumb Prop Questions

I had to pull the prop and scrape paint to get any numbers.

All I can read is Michigan PM 215 CUP

I could not find any other stampings on the prop.

I'll be buying one soon. I'll search and find a shop to rent me a couple at their recommendation and go from there.

Thanks Guys!

 

[72SideWinderSS]

Re: Noob Dumb Prop Questions

Old ------- New ----- Dia./Pitch
P 215 C = 031023 = 15 1/4" X 15"

 

[aerospot]

Re: Noob Dumb Prop Questions

Old ------- New ----- Dia./Pitch
P 215 C = 031023 = 15 1/4" X 15"

Wow! Thanks!! This gives me some insight.
However, I managed to chew this prop up pretty good this past weekend. Not destroyed but needing repair.

What do you think about a 15" X 19 pitch?
What changes in diameter / pitch would I need to consider for a 4-Blade prop? Stainless Prop??

What exactly is 'venting' and how does it effect the 'hole shot'? Any previous threads that explain this?

I appreciate the education. Not as 'Dumb' as I was a week ago. (Other than my encounter with rocks last night)

Thanks!
Dave

 

[Fireman431]

Re: Noob Dumb Prop Questions

If you go from a 15" to a 19" prop, your RPM's are going to drop by 800 (reread earlier posts). That means your WOT now will be 4300. Not near where it should be. You need to find out what the recommended WOT RPM is for the engine and get the prop that puts it in that range.