Prop upgrade - how to choose

[region4]

I would most appreciate any suggestions for a new propellor. I could go used, or new ... old design or new ... just don't want to do it more than once!

Mercruiser 165 inline six (3900-4300 recommended WOT rpm), 1.65/1 gear ratio (I think)
Stainless steel 3-blade prop: 48 88440A-4 19 - has some cup, clean, sharp, but not perfect
1972 18' Jolly Roger ski boat (low freeboard, 19 degree deadrise V), clean hull
MPH per GPS

Load: Full tank (12 gallons in bow) + misc gear and one 180# person
3,000 rpm / 31mph
4,000 rpm / 41mph
WOT - 5,200 rpm / 49mph

Load: 1/2 tank(35 lbs less that full) + misc gear and two 180# people
WOT - 4,800 rpm - 47mph

Other interesting behavior ... Full on hole shot to plane is pretty quick (1/2 tank + misc gear and two 180# people). However, cavitation occurs when bow is pointing skyward, and engine must be throttled back to prevent over-reving. Once the prop 'regrabs', all is well, and full throttle can be reapplied. Above is without a skier.

From what I've read, each 1" increment of pitch equates to 150-200 rpm change, everything else being equal. This would lead me consider 21P at a minimum, or maybe as agressive as 22P or 23P. But since different propellor designs seem to have different 'grip', and diameters, it is so difficult to make an educated decision.

If I reprop to a more aggressive pitch, and bring WOT down to 4100-4300, can I expect a better top-end speed? And if so, which prop or props should I consider? Though I realize it is not good to run above 4300 RPM ... if I do, is the engine still putting out good torque, or does its output decline, for reasons like valve float and air exchange limitations?

Thank you in advance.

 

[region4]

Re: Prop upgrade - how to choose

Does it may sense to seek a used prop of the same vintage?
48 88442-21

 

[region4]

Re: Prop upgrade - how to choose

48 884444-23

 

[achris]

Re: Prop upgrade - how to choose

For skiing you do want a prop that pushes you passed that top end. That gives you the hole-shot you're looking for. Your current prop is pretty much a standard 'Vengance' style prop. But I believe it is pulling too many revs, hence the great response out of the hole.

If skiing is what you're primarily doing, consider a Laser II or even a High-five. Both these props have vent holes in the prop, allowing the engine to spin up without load before taking a bite... Hole-shot is unbelievable. I put a Laser II on my boat years ago, just to see what it would do... Talk about 'light the blue touch-paper and retire'... And the High-five is even more aggressive! One of the best props I ever used was the old 4 blade SS, 'Offshore' series. I believe it's now been replace by the Revolution 4. VERY difficult to make that prop ventilate (which is what you are getting, not cavitation).

Ventilation is when air from the surface is sucked down into the blade area. Cavitation is when the water on the low pressure side of the blades boils and the blade loses grip. The end result is the same, the engine revs, the boat stops. But the fix is very different in each case, which is why you need to know which one it is.

Very difficult to find a single prop to do all things. I think the closest would be the Laser II if you want to ski and cruise. Just remember to put the vent plugs in the holes before you go cruising... And I would definitely go up at least 2"... And yes, with the bigger prop, your top speed will increase..

Chris........

 

[region4]

Re: Prop upgrade - how to choose

Thank you Chris. Enlightening. Ventilation ... I now know how that sounds and feels!

Regarding prop selection ... I have no issue owning two props, and swapping as necessary.

Regarding the following vintage props ... am I foolish pursuing these? Are they truly inferior to recent designs? Cost certainly factors into my decision, as well as concern that changing designs and pitch, simultaneously, might lead to a WOT RPM that is unexpected. I was thinking that the 23P below would give me a good top end, for a good price.

48 88440 15.5 x 19, currenlty on boat
48 88442 15.25 x 21
48 88444 13.5 x 23

 

[hwsiii]

Re: Prop upgrade - how to choose

region4, "48 88440A-4 19 - has some cup, clean, sharp, but not perfect", it is my opinion that using this prop to try and make an intelligent decision to decide what pitch you might need to cut your RPM's to the manufacturers guidelines is definitely an error. This prop definitely has major problems that is causing it NOT to perform as it was intended.

As a prop increases RPM's it always decreases prop slip, not increases it, and your prop is doing exactly the opposite. You either need to have that prop repaired or use another prop for your tests in order to have any semblance of an orderly test for your purposes. Unless the numbers that you gave us are way off, although it appears that you tried very hard to give us most of the information required to help you. A few pictures of the prop might help to enlighten us better. Although it could be another problem that we are not aware of at this time, but there is definitely a problem here.



H

 

[region4]

Re: Prop upgrade - how to choose

region4, "48 88440A-4 19 - has some cup, clean, sharp, but not perfect", it is my opinion that using this prop to try and make an intelligent decision about using this prop to decide what pitch you might need to cut your RPM's to the manufacturers guidelines is definitely an error. This prop definitely has major problems that is causing it NOT to perform as it was intended.

As a prop increases RPM's it always decreases prop slip, not increases it, and your prop is doing exactly the opposite. You either need to have that prop repaired or use another prop for your tests in order to have any semblance of an orderly test for your purposes. Unless the numbers that you gave us are way off, although it appears that you tried very hard to give us most of the information required to help you. A few pictures of the prop might help to enlighten us better. Although it could be another problem that we are not aware of at this time, but there is definitely a problem here.

View attachment 215146

H

Thank you for the insight.

Just yesterday I purchased, a new, unused, still-in-the-box, vintage 1970's 48 88442A4-21. It should provide an accurate benchmark, from which to extrapolate. I should have it on the boat, and complete a water test, within a couple of weeks, maybe sooner.

With regard to other unknowns ... I had earlier confirmed that the prop shaft is straight, measuring .002 runout.

Stay tuned.

Again, I stress how grateful I am for all this help. Most appreciated!

 

[region4]

Re: Prop upgrade - how to choose

Attached are 3 photos, one of each blade, for those with interest. The vantage point is not identical ... my apologies. I do think the leading edge inconsistencies are revealed, somewhat.

Interesting observation: After my first, of two, water outings, the painted prop showed cavitation erosion (of the paint only), on a single blade ... I cannot remember which blade, however. Before my second outing I treated the prop to an ultrasonic bath, and lost all my erosion markings.

I can only conclude that erosion on one blade, and not the other two, indicates significant asymmetry, which is bad news, and which is likely to root cause of my high revs.

 

[achris]

Re: Prop upgrade - how to choose

That's a nasty ding in the first picture...

 

[region4]

Re: Prop upgrade - how to choose

That's a nasty ding in the first picture...

I agree the ding is significant.

Interestingly, I had called a local prop shop. For ~$175, they can rework the SS prop. But for that kind of money, they recommended a new BlackMax aluminum, and a TorqFlo sleeve/adaptor.

He asked me ..."Does it vibrate? Is there a problem?" I told him "No, I didn't think so". I now know better, thanks to all the input above.

Looking forward to spinning the new wheel! I''ll report the results asap.

Thanks again for all your interest, and help.

 

[region4]

Re: Prop upgrade - how to choose

As I impatiently await the chance to try the 21P prop, I am trying to predict the performance. Is the following 'reverse engineering' logical?

Based on the following observations (with the dinged 48 88440 19P):
1. The boat can reach 49mph (at 5200rpm), this implies the engine is capable of generating that much power.
2. 3,000 rpm @ 31mph at 5% slip
3. 4,000 rpm @ 41mph at 6% slip

Substituting for the 19P, a true 21P (48 88442 21P), is is logical to input into the SLIP calculator the following values:
1. Slip = 5%
2. Pitch = 21
3. MPH = 49

and derive RPM = 4192 ??

 

[hwsiii]

Re: Prop upgrade - how to choose

region4, I have seen a prop like this before and in my opinion there is no way to predict with any accuracy at all what you can expect the numbers to look like. There is an assumption here that by changing 2" in pitch and with a prop that has the correct angles of attack on each blade that it will lower the RPM's by 1,000 yet keep the same speed, the other prop I saw like this when it was replaced with the same exact prop but brand new lost over 5 MPH and 14% of the speed it ran with the bad prop.

But that does not mean that will happen in this case, it could do exactly what your numbers show, I just have no confidence in my ability to predetermine that outcome to ANY degree of accuracy.

H

 

[region4]

Re: Prop upgrade - how to choose

Well, I will soon know, as I have the 'new' 1970's 48-88442A4-21 in hand, as as shown on the attachment (21left / 19right).

I did measure the diameter of both props:
48 88440 19 : 14 x 19P
48 88442 21 : 13.75 x 21P

Bottom Line: If I spin my motor, even accidentally, at 5200rpm, she may blow. Stay tuned.

 

[hwsiii]

Re: Prop upgrade - how to choose

region4, I hope for your sake that the numbers in your substitutions is exactly what that prop does when you test it.

H

 

[region4]

Re: Prop upgrade - how to choose

region4, I hope for your sake that the numbers in your substitutions is exactly what that prop does when you test it.

H

I hope so too. But, nothing ventured, nothing gained. Nobody said education was cheap. It's all fun.

 

[smokeonthewater]

Re: Prop upgrade - how to choose

I had a merc 165 inline 6 that I spun at 5000 rpm for hours on end... it never missed a beat... I had a slightly hotter cam in it and through hull exhaust and had it in a 26' Carver flybridge cruiser... sounded like a pissed off Cessna LOL

just sayin

 

[smokeonthewater]

Re: Prop upgrade - how to choose

BTW I pulled that 165 out of an 18' trihull and IT had an old school 21p stainless prop on it... got it from a friend and he said the boat did great with that prop

 

[region4]

Re: Prop upgrade - how to choose

I performed a water test, with the new 48 48442A4-21P, as follows:

Load: full fuel tank, one 180# person, misc gear, ideal calm water with just a ripple; same as before
1000 rpm / 6.0 mph
1500 rpm / 8.3
2000 rpm / 15.8
2500 rpm / 24.6
3100 rpm / 30.9
3300 rpm / 32.3
4000 rpm / 40.3
5000 rpm / 49.2 (WOT)

Observations
1 - Reading a tachometer accurately is definitely challenging. How can I verify my tachs' accuracy?
2 - Hole Shot is still quick, prop ventilates when bow is skyward, throttle is reduced until prop 'regrabs', then throttle can go wide-open with no ventilaton detected (same as with dinged 19P).
3 - SLIP seems to decrease up until 4,000, then it seems to climb. Could I have a coupler issue? If so, how does one detect that? Or, perhaps, the hull is not getting any bow lift, and needs a Laser II 23P, or 25P, with its higher rake, and thus bow lift, to reduce the hull drag?

 

[smokeonthewater]

Re: Prop upgrade - how to choose

when my coupler went out it would slip at heavy throttle settings and work fine at lower settings... within an hour of the symptoms starting the boat wouldn't move and there was gooey melted rubber slime slung all over behind the engine.... YMMV

 

[limitout]

Re: Prop upgrade - how to choose

I performed a water test, with the new 48 48442A4-21P, as follows:

Load: full fuel tank, one 180# person, misc gear, ideal calm water with just a ripple; same as before
1000 rpm / 6.0 mph
1500 rpm / 8.3
2000 rpm / 15.8
2500 rpm / 24.6
3100 rpm / 30.9
3300 rpm / 32.3
4000 rpm / 40.3
5000 rpm / 49.2 (WOT)

Mercruiser 165 inline six (3900-4300 recommended WOT rpm), 1.65/1 gear ratio (I think)
Stainless steel 3-blade prop: 48 88440A-4 19 - has some cup, clean, sharp, but not perfect
1972 18' Jolly Roger ski boat (low freeboard, 19 degree deadrise V), clean hull
MPH per GPS

looks like you need to go to a 23p and retest,