Better prop than a laser 2 for my bowrider ?

[QBhoy]

Hi guys...
Posting from the UK...not much experienced advice available on my particular subject...
Question is;
Might there be a better prop to run on my Campion Chase 550i, 18ft bowrider with 5.0 MPI with cam, running about 300hp with an Alpha one leg ?
Being honest, she runs really well lightly loaded at about 65mph, trimmed well with not much in the water at all. Decent acceleration, although I haven't ever opened her up from a standstill.
Is the tempest and trophy + really going to be any better ? Or am I right in thinking she won't go much better than that ?
The line is fine between cavitation and perfect trim and you can forget trying to turn when running flat out.
All advice appreciated !
Allan

 

[alldodge]

To be able to determine what is working or not working we need more info. We need info at WOT:
Speed = 65 mph
Drive ratio = ?
Prop pitch = ?
Assuming 4800 RPM advise if different

If I use 65 mph, 1.81 ratio, 28 pitch prop, and 4800 rpm = 7.56 percent slip which is real good

 

[QBhoy]

Hi alldodge

Drive ratio is 1:47:1
RPM is a smidgen over 5000rpm
The mpi Max rpm is 5000rpm
Prop is a 13.75x19"P as far as I know, but I believe they all differ in tolerance slightly.
I've tried he prop slip calculator and the maths don't totally add up, suggesting pretty much no slip. I think the prop is slightly surface piercing when she runs on the flat pad at the back of the chase models hull. Perhaps throwing the figures...God knows. The speed is GPS.
That's why I ask the question....I can't see it running better. Pretty sure the rpms are accurate too.

 

[alldodge]

maths don't totally add up,

Agree, with your figures your at -6.21 slip and that doesn't happen. So since your speed is gps, your Tach is off. Turning 5K rpm with the motor and unless the motor is designed to turn more then recommended, there is an issue.

Sounds to me you need to get a calibrated hand held Tach

 

[QBhoy]

Yeah...agree.
I've posted a similar post on a UK forum and all I got was smart arse answers.
Just for the record, I'm not an idiot and an engineer to trade...so a firm believer in the facts and maths. Haha.
The only thing that puts me off thinking the Rpms are out is that the boat came with a 14x17p and hit the limiter very easily at about 5100 rpm. Which I believe is what it should be. Added to this I have a decent ear for a good tick over at about 700rpm.
The prop wasn't new, but as good as from eBay. It may be possible that it was reworked ??
To further confuse the matter, I did try a laser 2 21"P. This only ran at 4400rpm trimmed at around 60 mph GPS. Totally throwing the old 200rpm gain for each progression in pitch ! All I can imagine is that with the 21", the engine wasn't in the suggested torque range of 4600-5000 rpm and it picked up revs and speed being in the range with the 19".....go figure !
Just wondering what other dynamic variances or exception to the rule there could be other than the standard calculation of drive ratio, rpm, prop size and speed ???

 

[QBhoy]

Also worth noting that speed over ground, with regards to tides etc don't really come into it. Was on fresh water lake Loch Lomond.

 

[QBhoy]

https://youtu.be/9EDpd61DxX0
Wee video with 4 on board. Was running about 63ish.

 

[alldodge]

The normal set up for the 5,0 isn't with a 1.47 drive, and unless your 100 percent sure, this may be the variable. I'm just and technician and work from facts, your the engineer so your even more so looking for the facts to calculate the issues. From the vid I did see near the end that the boat was putting out quite a rooster tail. Since an 18 footer with and arch will create more drag then a bass boat, I think your loosing power by gaining rpm.

My thought is to determine what true numbers are. Suggest starting by determining where the straight and level with the CAV plate is with the bottom of the hull. This position may be a bit more drag then when trimmed up a bit more but I'm looking for true numbers. Take a prop and run WOT and get rpm and speed. Then change prop and do the same. Getting this info we will be able to determine which is work more efficient then another.

Don't know if I'm explaining this clearly enough, it looks good in my head, just don't know if I'm conveying it

 

[QBhoy]

Totally know what you mean...
The rooster tail was a show off thing for the video. She looses speed doing that and has melted an impeller too
100% sure on the drive ratio and quite common on older 5.7 and newer 5.0 mpi installed in this kind of set up....light and fast boats...from what I can tell.
It's pretty late over here, so off to bed. All the advice really appreciated.
Thanks and bye for now !
Allan