Another Prop Question

[pair of jacks]

Thanks for prop gods tip. Ken was very helpful. Ordered a 15 1/8 x 15 pitch enertia. I'll let you all know the outcome.

[eggsuckindog]

Originally Posted by Mikem8560

Egg I think I read you have a 200 merc on your 20 what size prop do you have?
I just got a 2001 with no prop.

A few years ago I found a 19 Stilletto used - and it became a different boat - I couldn't be happier.

They are tough to swing as I can only get 5500/5600 and would like 5800 - my XRi is maxed there but the others are 5600.

It needs to set up very high, lots of cup and bite in it, if you load heavy I might even suggest a 17, I considered dropping down but raising it again got me 200 rpm

[Fr. Frank]

Originally Posted by Commander Coo1

looks like you have about 15% slip at WOT which seems pretty high. The Enertia got me about 2% slip and the Rebel was "0%". you may end up needing to come down in pitch unless you pick up a decent amount of speed. My setup weighs about 4000lbs loaded and tops out at about 43mph/37kts.

No prop swings without slip, period.

An extremely well matched prop-motor-hull setup will have around 10% slip. Average minimum slip used to be 15-18% for most dealer setups. Occasionally, you may see a setup down around 8%, but that's pretty rare. If I recall right, ESD gets about 51-52 mph at best trim WOT with that 19" Stilletto on his XRi 200, which with the most common gear ratio on that motor being 1.81:1, puts him near that magical 8% ratio. (Correct me if I'm wrong on that speed) OTH, if he has the available/optional 1.65:1 gear ratio, then his slip is a more normal 14%.

Here are three examples:
The most efficient setup I have ever seen on a regular (non-race) hull was on a '69 16' Whaler with an '85 90 hp Yamaha turning a Yamaha 21P SWS semi-cleaver SS prop which got an astounding 6.7% slip at 5800 rpms/46 mph.

I once rigged an 21' Allison XTB mod-V race hull with a Yamaha Excel 220 with a blueprinted hull, nose cone, hydraulic jackplate, and a custom one-of-a-kind Second Effort Marine 31"P through-hub prop which got all the way down to 4.5% slip at 6470 rpms/108 mph.

Back in the early days of PowerTech, I had them design and build me a custom prop for my '72 Seafari running a blueprinted Mercury XR4 150 (190 hp) with a 1.65 gear ratio. It was a 15"Dx16.5"P 3-blade, and at 6050 rpms and 53 mph, I was achieving only 7.5% slip.

If you see a slip ratio being reported of 8% or under , I would suspect the accuracy of the propeller specs, such as prop labeled as 17"P actually being 16"P or so. It is not uncommon for aftermarket props to be as much as a full 1" different from their labeled pitch.

The amount of slip will also vary on the same hull-motor-prop setup depending on the efficiency of the trim and rpm setting being used. While I achieved as low as 7.5% slip at 6050 rpms on my Seafari, when I was cruising at 3800 rpms, my slip ratio was up to 14%, achieving 30 mph.

[eggsuckindog]

Your right on frank and my slip at 4000 rpm is only 11%, I was amazed and actually have done it more than once. Thats why I say for a 20 you can't ask any more from any prop and their pretty cheap new compared, same as a Trophy and Yamaha bought the company. They are aggressive so err on a lower pitch, I lost 300 rpm vs my old one, got some back raising the engine.

I sent DonV a lead on a used 17 not long ago, not sure if he bought it

I think the gear ratio is 1:87

[Commander Coo1]

Fr. Frank,
It isn't really all that uncommon to see a prop produce low to negative slip numbers. Suzuki, stilletto, and BRP Rebels just to name a few manufacturers/props are "under rated" in their pitch disignation and may produce low calculated slip numbers on a smaller vessel.

Surely the Rebel 15p I ran could not have actually measured as a 15p propeller. But unless my GPS, EMM/NMEA2k RPM readout, multiple test runs in opposing directions or 1.85 gear ratio are incorrect, I am getting the calculated slip numbers that I previously reported. I know 0% slip is basically impossible, but you can't assume that less than 8% slip is nearly impossible on a small fishing boat.

I have seen WOT slip as high as 12% (BRP Viper 19p) and as low as -4% (same Rebel 15p prop running at 4500rpm) on my setup using the same measurement methods. props can produce unexpected results and produce theoretically impossible numbers at times due to how the manufacturer designates their propeller pitch.

pair of jacks,
I think that the Enertia will do well for you, I really have been enjoying mine since I got it. Much more predictable performance than the Rebel I have! I look forward to hearing how it performs!
-James

[76Red18]

Originally Posted by Commander Coo1

Fr. Frank,
It isn't really all that uncommon to see a prop produce low to negative slip numbers. Suzuki, stilletto, and BRP Rebels just to name a few manufacturers/props are "under rated" in their pitch disignation and may produce low calculated slip numbers on a smaller vessel.

Surely the Rebel 15p I ran could not have actually measured as a 15p propeller. But unless my GPS, EMM/NMEA2k RPM readout, multiple test runs in opposing directions or 1.85 gear ratio are incorrect, I am getting the calculated slip numbers that I previously reported. I know 0% slip is basically impossible, but you can't assume that less than 8% slip is nearly impossible on a small fishing boat.

I have seen WOT slip as high as 12% (BRP Viper 19p) and as low as -4% (same Rebel 15p prop running at 4500rpm) on my setup using the same measurement methods. props can produce unexpected results and produce theoretically impossible numbers at times due to how the manufacturer designates their propeller pitch.

pair of jacks,
I think that the Enertia will do well for you, I really have been enjoying mine since I got it. Much more predictable performance than the Rebel I have! I look forward to hearing how it performs!
-James

ALL props will slip. You do know the good Father was throttle man or driver in quite a few races back in the day don't ya? He knows a little about slip numbers I would imagine.
I do agree that a few makers do understate their pitch numbers which means that the prop calculator is just a tool to get you close.

[Commander Coo1]

All I really want to get across is do not expect to have normal 10% slip numbers and pick up any RPM going from a 14.5x15 prop to the 15.75x15 Rebel, it could happen but dont count on it. I wasted quite a bit of time trying to get a Rebel from my dealer only to find out it holds down the RPM quite a bit. Although I have never driven a race boat, I spent a lot of time propping my 20' boat with an E-TEC 175 on it and that is my experience.