Another prop question: Response LX early 2000s

[PaulL]

I've had the Response LX for about 5 years.  It came with a 4 bladed stainless prop on it, and has the 330HP Mercruiser black scorpion direct drive.

The Mercruiser has a rev limiter at 5180 RPM, or thereabouts (it stutters around 5180, maybe it's 5200).  We're running pretty hard into the rev limiter, I think we can hit it on about 3/4 throttle.  We get 72kph (45mph) at the rev limiter with 1 person aboard, about half fuel, not much spare weight in the boat.  It feels like we're giving up quite a bit of top speed there.

I've looked at the prop and it doesn't have any clear identifying marks for size.  It has J1102 stamped on the side of the hub (where the blade merges in), and arguably it might be O J1102, but not sure why there'd be a gap between the O and the J if it was an OJ prop.  So maybe the O is something different.  On the end of the hub I can see CUP written clearly, then maybe the edges of some numbers.  I'll probably have to take the nut off to see what else it might say on there.

We mostly water ski, sometimes wakeboard (we have a wedge), we don't really ballast and the boat isn't big enough to carry a lot of passengers.  We have more than enough hole shot - we seem to be able to pull as many tubes as we like (6+ at a time) with no real bogging, and basically nobody who we ski with is happy with full throttle starts.  Probably the only use we have for all that hole shot at the moment is beach starts, in which we can idle to the end of the rope and then go (as opposed to the 3 coils of rope we took when I learned to beach start behind a 50HP Evinrude...).  Seems pretty clear that we could do with more pitch.

I don't have any actual problems with the boat, other than it feels like we could go faster.  My brother's boat (when he comes visiting) does about 47mph, and those extra 2mph are annoying.  Other than when he visits, I don't notice.  I guess I might get a bit more fuel efficiency with more pitch.  We like our current wake for slalom, we have an acquaintance with a very similar boat (early 2000s Response LX, Indmar powered I think) with a 3 blade, he reckons our wake is much softer (good thing).  I haven't skied behind his boat to compare, he did ski behind ours.

I see that the recommended prop would be an ACME 515 or similar, which is a 3 bladed 13 inch diameter, 12 inch pitch, with a little bit of cup.  We're at maybe 1000' (300 metres) above sea level, lake use only in New Zealand.

I have a lot of questions about 4 blade v's 3 blade.  There seems to be a lot of hearsay about 3 blades being more efficient, 4 blades being more balanced, but my reading tells me that's not entirely accurate.  Efficiency comes down to surface area, and 3 large blades have similar surface area to 4 small blades.  The newer 3 bladed props seem to be pushing into more surface area, so that might not make much difference.  Balance is probably comparing an older 4 bladed cast prop to a newer 3 bladed CNC prop, and I suspect there wouldn't be much in it.  Wake shape is clearly different for every prop, and would be hard to tell without trying, I do feel like it should be related to surface area somehow, so the newer 3 bladed props might not be all that different to what we have.

My options are:

1. Stay as we are.  I don't have any actual problems other than a desire to tinker

2. Get my existing prop repitched.  Apparently inexpensive and easy enough to do.  For some reason that feels like the wrong answer to me, that it would end up with the prop a bit less balanced or it might fatigue the metal

3. Reprop to a 4 bladed newer prop with a bit more pitch.  We'd get a bit more top end, perhaps similar wake and hole shot?  It seems like you must have to give up some hole shot to get more top end, that's the basics of gearing, so I'm dubious, I think we'd lose a bit of hole shot.  But we have plenty anyway

4.  Reprop to a 3 bladed newer prop with a bit more pitch.  As above, with maybe also some wake compromise?

Is there any authoritative information on the impacts, or is this all really black magic and you talk to the prop experts then you try it and see?  I guess I might need a spare prop anyway, so I could perhaps just buy a new one and try it.  What are people's opinions?

 

[UWSkier]

Malibu went with Acme 515 and 525 on their direct drives in 2002 and has never looked back, for good reason.  They're great props.  Super smooth, tons of power, and good wake performance.  I have a 515 on my boat and run right around 4950 RPM (Monsoon, not Black Scorpion but similar performance).

[justgary]

I'm guessing it's a 13x13, maybe a 13x12.  It should be stamped on the end under the nut.  I was going to offer to buy it from you until I saw the part about NZ.

See if your friend will swap props with you for a few runs.  That way both of you can see if the other guy's prop is better.

[PaulL]

3 hours ago, justgary said:

I'm guessing it's a 13x13, maybe a 13x12.  It should be stamped on the end under the nut.  I was going to offer to buy it from you until I saw the part about NZ.

Interested in why you'd buy it - I'm still trying to work out what it might be better for, and if you're interested in it you may know.  :-)

I should work out how to post images.  It's in OK condition, but has small patches of surface rust that I suspect mean it's not in as good condition as you'd expect.  In my mind a stainless prop shouldn't have rust on it, so something's wrong there.

Quote

See if your friend will swap props with you for a few runs.  That way both of you can see if the other guy's prop is better.

He offered last year.  But they live far from us, and winter our end of the world - so that'll be a summer/christmas thing.

Edited August 3, 2020 by PaulL
Update/extension

[formulaben]

On 8/1/2020 at 8:17 PM, PaulL said:

Get my existing prop repitched.  Apparently inexpensive and easy enough to do.  For some reason that feels like the wrong answer to me, that it would end up with the prop a bit less balanced or it might fatigue the metal

Adding cup adds pitch and will not at all compromise the strength of the blades.  According to ACME, a rule of thumb is .050" of cup is roughly 1" of pitch.  Most prop shops I've been to can re-cup the blades for a very reasonable price.  If it already has a low amount of cup I'd go there first.

[justgary]

On 8/2/2020 at 6:55 PM, PaulL said:

Interested in why you'd buy it - I'm still trying to work out what it might be better for, and if you're interested in it you may know.  :-)

I should work out how to post images.  It's in OK condition, but has small patches of surface rust that I suspect mean it's not in as good condition as you'd expect.  In my mind a stainless prop shouldn't have rust on it, so something's wrong there.

He offered last year.  But they live far from us, and winter our end of the world - so that'll be a summer/christmas thing.

I'd buy it because it is stainless and they don't really make them for inboards any more.  I boat in salt water that has a sandy bottom.  Maybe a few oyster shells, but no rocks.  I don't really need the same kind of prop strike protection that a lot of inboard boaters seem to need.  My buddy has that prop on his '04 Response and it works really well and runs with no vibration.  I'm still using the OEM CVP three blade stainless that everyone will tell you is a time bomb.

Anyway, Have you checked your speed with a GPS?  That may make a difference on the actual speed, but I agree that you should not be hitting the rev limiter, at least not before full throttle.  It does sound like you need more pitch.

The basic difference between three and four blades is exactly what you said except that efficiency is not about surface area, it is about shape.  Four blades generally run smoother than three blades because the total force contributed by each blade is less.  Four blades generally run slower than three blades because of higher drag.  Vortex shedding from the tips is significant,  and each blade adds its own vortex (and other drag elements).  People try to argue this about props, but when you offer up the fact that we quit using biplanes and triplanes 100 years ago because one wing is more efficient than two or three (or more) they don't question the efficiency.  A prop is simply a wing that rotates.  More blades will *not* be more efficient.  Yes, a one-bladed prop is the most efficient, but it is somewhat harder to balance. 

Hole shot may improve with more blades by helping to control slip and cavitation at very high angles of attack, but this is quickly overcome by drag as speed increases and the angle of attack approaches a nominal value.

I do have to agree that four or five blades look good, though....

[PaulL]

Ah, vortex and other drag elements associated with leading and trailing edges makes sense.  I presume a 4 blade has a little less diameter so you get a slightly reduced edge size on each blade, but that wouldn't fully balance out the impact of a whole additional blade.  

 

OK, so moving to a 3 blade would give me more top end for both reasons of pitch (not running into rev limiter) and efficiency (3 blades vs 4, plus probably a bit better shape due to being CNC rather than cast).  But probably at the cost of a bit of of smoothness.

Getting some cup added to my existing prop would give me a bit more top end for reasons of effective pitch, but without the other drawbacks.

[Woodski]

@PaulL -  Given today's state of the art in prop manufacturing, the CNC machined 3 blade offerings from Acme and OJ are very hard to beat particularly if skiing is the primary sport of choice.  They are extremely well balanced due to manufacturing process so that concern is now a thing of the past.  They have plenty of blade area for good holeshot performance and several configurations to fine tune to your power level, hull drag and altitude.  4 blade holeshot performance was better when comparing cast to cast 3 v 4 blade props, today you are not necessarily making that comparison.  Wake performance is very good for the newer options.  I would not be 100% confident adding cup to a cast 4 blade would give you more top speed, in theory yes, but my experience in doing some testing did not bear that out, the boat tended to hit a wall and did not care what the engine was turning.  My analysis was drag increased past the power curve, and for a small block GM, that is pretty flat.

BTW - which island and what lake in NZ?