Offset Low Water Pickup Holes in Nosecone

[laser_ED]

I'm convinced Bob's used 5 holes in order to satisfy a very wide range of conditions and avoid lawsuits from overheated powerheads. 5 holes in most cases are completely unnecessary, so go ahead and plug em.

As far as prop slip goes, it's something to be used for amusement and nothing else. Unless your tach and prop are laboratory quality, the numbers you insert in the calculator are just guesses. I've seen boats set best top speeds with terrible slip numbers (over 15%) so slip calculations are a waste of time to me. If you put a prop on that runs 3 MPH faster but the slip is "bad", do we throw it in the dumpster?

100 % correct on the filling of the outside intake holes,all of my xr6 cases are 3 hole and one that is 2.and water pres is 8lbs on all 4.. The slip factor is broad average that changes with different conditions...With data now available to everbody,every race site requires a little tuning on the ecu and props..Water conditions play a bigger part in prop performance than most folks know....Water density is the culprit...the higher the temps the lower density,,the colder the water, higher density....In the bayous in the south the water is thick with{ stuff} and salt water is also thick,so slip factor is????????

[CI STV]

Thanks. I get it that slip is just a number, but, as with any other data, when you are looking at it in isolation, with all other factors being the same, it’s a pretty good indicator of efficiency.
Every prop I’ve tried on this boat so far with the 5-hole Bob’s 1.78 XR6, I’ve seen less than 8% slip, based on the data and the pitch stamped in the prop.
This was a variety of transom heights between ~1/2 - 1” above the pad. I have a F1 mid with the updosn and a PPE up/down sensor, which allows me to datalog engine height. I have a trim sensor and trim limit switch on it as well.
So far I’ve tried:
(1) a worked 20P Yamaha from Diamond which runs around 5-6% slip;
(2) a worked 24P Yamaha from Diamond that showed ~9-10% slip;
(3) a 22P Tillman that showed around 8-9% slip and:
(4) the above referenced Ronnie Gilbert 20P Yamaha copy
On all the data I have analyzed, I have not seen anything over 9% slip, except on the 24P, which showed ~10% slip. The 24P is badass though, and based on my datalogs, accelerated as good as the smaller props. I only ran that once though, and it was the first time I ran the boat wide open. This is the prop I’m going to run with the 2:1 case, so it will be interesting to see what it does
All of these props were work Yamaha style drag props, which aren’t really known for low slip numbers.
As far as tach accuracy goes, I have an AiM MXL2 dash/Datalogger and I can monitor RPMs from the ECU or the tach trigger wire. I’ve noticed that they tend to be a few RPM apart, somewhere in the order of ~10rpm difference @ 9,000+.
So I have fairly decent data to rely on, I think.
8 psi of water pressure? If I saw that, I’d probably panic, lol. I have the AiM dash set to alert me (by flashing red) if the water pressure drops below 12psi, if I recall correctly.
I’m definitely gonna to plug the 2 holes on the side. I’m thinking of drilling and tapping then for some 1/8NPT aluminum plugs and then epoxying those over.

[transomstand]

Thanks. I get it that slip is just a number, but, as with any other data, when you are looking at it in isolation, with all other factors being the same, it’s a pretty good indicator of efficiency.

That is a very common error. Prop slip actually has nothing to do with efficiency.

Prop efficiency is a complex calculation to determine how well a propeller converts torque from engine horsepower into thrust.

[CI STV]

That is a very common error. Prop slip actually has nothing to do with efficiency.

Prop efficiency is a complex calculation to determine how well a propeller converts torque from engine horsepower into thrust.

What is a very common error? How could prop slip have nothing to do with efficiency?
That’s like saying that a stator change in a torque converter that reduces converter slip has nothing to do with drivetrain efficiency. Bear in mind we’re talking about a fixed data point here, up at the top of the RPM scale, close to max speed, with the boat aired out and flying.

For the purpose of comparing prop “efficiency” (I’m open to using a less offensive term, if you have one in mind) under those conditions, I don’t need a complex mathematical computation to determine how much torque a prop of a certain measured pitch converts into thrust, when I can simply use the notional pitch number, rpm, gear ratio and speed to calculate “slip”?

To my simple mind, slip is a fair indicator of the efficiency (speed per RPMs) of a prop at any fixed data point during a run.

Obviously, a prop that has higher slip number can still be quicker (as opposed to faster) in a timed run over a fixed distance (elapsed time) than one that is notionally less “efficient” (lower slip number), but that has to do with hydrodynamics and where the engine makes the most torque in the RPM band, among other things, I think.

Sort of like how torque converters work, since after all, they transfer energy from fluid pressure to the transmission input shaft, sort of the obverse but same principle.
And, here’s the kicker, when they slip: they actually multiply torque, often by a factor exceeding double the input torque.
Maybe that’s the black magic that makes a good drag prop launch a boat out of the hole as violently as they do…

[CI STV]

Just to update this thread: I tested the lower unit this past weekend by making a couple of short (<400’ ) WOT runs with a 20P prop. I ran it up to 9800+ rpm and ~85mph and the water pressure was right around 18 psi at that speed, albeit I ran it with the bullet a bit lower (~1” above the pad) than what I think is optimum (1 3/8” above the pad).
I still need to do some more testing, but so far it looks like it’s making as much if not more water pressure than my old 5-hole Bob’s coned unit.
Here’s a screen shot from a datalog of one of the runs:

[RBT]

I have run as few as one water hole, but 2 is more than enough. When we were chasing top end numbers there was speed to be found. Can’t say definitively for drags as I have just used the same gearcase profiles.
That being said, consider that the pressure at the water inlet is velocity squared times .0142 or simply 142 psi at 100 mph, 30mph is 12.78 psi. An IMCO gearcase for stern drives has one 3/8 hole to supply a big block.
On the gear ratio experiment. Firstly I would comment on prop slip, this is something I have never been able to understand why people put so much emphasis on. It is super hard to get an accurate measure on what the actual pitch is. ( I have had props run negative slip, and I have had props throw down giant numbers with slips over 20%)
Anyhow, I would like to point out that running a 20 and 1.78 gears doesn’t translate to 2:1 and a 24. The math if I can articulate it here is say your motor makes 100 ft/lbs at 5000 rpm, a 2:1 ratio case would have 200 ft/lbs of torque at the prop shaft with a prop speed of 2500 rpm. Same scenario with the 20 combo, 178 ft/lbs at the prop shaft with a prop speed of 2809 rpm. Real world testing is the low ratio with turn the power band peaky sand the taller ratio with flatten the torque curve and give you a longer flatter pull.

Hope some of that makes sense

[CI STV]

I have run as few as one water hole, but 2 is more than enough. When we were chasing top end numbers there was speed to be found. Can’t say definitively for drags as I have just used the same gearcase profiles.
That being said, consider that the pressure at the water inlet is velocity squared times .0142 or simply 142 psi at 100 mph, 30mph is 12.78 psi. An IMCO gearcase for stern drives has one 3/8 hole to supply a big block.
On the gear ratio experiment. Firstly I would comment on prop slip, this is something I have never been able to understand why people put so much emphasis on. It is super hard to get an accurate measure on what the actual pitch is. ( I have had props run negative slip, and I have had props throw down giant numbers with slips over 20%)
Anyhow, I would like to point out that running a 20 and 1.78 gears doesn’t translate to 2:1 and a 24. The math if I can articulate it here is say your motor makes 100 ft/lbs at 5000 rpm, a 2:1 ratio case would have 200 ft/lbs of torque at the prop shaft with a prop speed of 2500 rpm. Same scenario with the 20 combo, 178 ft/lbs at the prop shaft with a prop speed of 2809 rpm. Real world testing is the low ratio with turn the power band peaky sand the taller ratio with flatten the torque curve and give you a longer flatter pull.

Hope some of that makes sense

Yep. All of that makes sense to me. I do recall that the IMCO lower on my old American Offshore 2600 had a single pickup hole and the first time that I ran that 120+ mph, it literally built enough water pressure that it blew the “suction” hose off the sea strainer and nearly sank the boat. I had to put a pressure relief valve in it after that.
I’m really not placing much emphasis on prop slip, but I use that to help me figure out what height to run the prop at, etc. That and I’m comparing it to how the same prop ran on the boat before with my other gearcase with 1.78 gears. I get what you’re saying with the gearing though.
At the end of the day, with drag racing, acceleration is king, so it’s the combination of prop, gearing, etc. that gets you to the finish line the quickest that matters. And there’s only one way to really figure that out: testing.
Having good data is critical though.
I’ve run the 24P before with the 1.78 gears and based on the datalogs, it had more slip but slightly better acceleration than any of my other props, which surprised me, tbh.
But I know I need more pitch now, simply because if it can hit 9800fpm in 370’, I’m pretty sure I’m going to need more pitch to run 800’, which is what we’re doing down here.
BTW: The boat used to run ~104mph in 800’ before with the same 20P prop and the 1.78 gears, but it was on the 10K limiter well before the finish line. I’ve bumped the limiter to 11K, so I’m hoping that it can pull the 24P through the finish line with the 2:1 gears without hitting the limiter now.

[LakeFever]

I have run as few as one water hole, but 2 is more than enough. When we were chasing top end numbers there was speed to be found. Can’t say definitively for drags as I have just used the same gearcase profiles.
That being said, consider that the pressure at the water inlet is velocity squared times .0142 or simply 142 psi at 100 mph, 30mph is 12.78 psi. An IMCO gearcase for stern drives has one 3/8 hole to supply a big block.
On the gear ratio experiment. Firstly I would comment on prop slip, this is something I have never been able to understand why people put so much emphasis on. It is super hard to get an accurate measure on what the actual pitch is. ( I have had props run negative slip, and I have had props throw down giant numbers with slips over 20%)
Anyhow, I would like to point out that running a 20 and 1.78 gears doesn’t translate to 2:1 and a 24. The math if I can articulate it here is say your motor makes 100 ft/lbs at 5000 rpm, a 2:1 ratio case would have 200 ft/lbs of torque at the prop shaft with a prop speed of 2500 rpm. Same scenario with the 20 combo, 178 ft/lbs at the prop shaft with a prop speed of 2809 rpm. Real world testing is the low ratio with turn the power band peaky sand the taller ratio with flatten the torque curve and give you a longer flatter pull.

Hope some of that makes sense

Makes sense other than the bold part, can you unpack this further?

[RBT]

Makes sense other than the bold part, can you unpack this further?

I am not great at articulating my thoughts in print, short of making a spread sheet showing the differences which I can’t do at the moment it is hard to expand upon. More later