Going from 3 to 5 blade prop

[QBhoy]

Haha.
Yeah, it goes well.
It's a campion Chase 550i performance edition with a cam and Corsa Marine switchable exhaust from the factory. Only one in the country I'm led to believe. I wrote to campion for more info on it. They actually asked for the serial number to verify what I claimed I had. Were then good enough to tell me the story behind the special edition and how a limited number of these were sent out, one to each dealer around the world apparently.
It then featured on their site after asking for pictures. Lovely and feel like a lucky chap.
It will run at least 60 all day long with any number of people I've had on board.
it has ran a few mph faster lightly loaded and faster again with the 21", but not as happy overall.
Not sure what HP it is and there are various beliefs and theory about the worth of the exhaust, but I'm pretty sure the cam makes a difference, not to mention the uk higher octane fuel. It's a 5.0 mpi which is 260hp in standard form anyway.
Allan.

 

[MapisM]

I'm not quite sure how your calculations are derived - for prop slip I'm simply calculating the ratio between the theoretical top speed and the actual top speed.

Well, the typical slip calculation gives the "slip percentage", so to speak, not the actual vs. nominal speed.
In other words, the numbers in your "Prop slip" column, instead of 0.87, 0.92, etc. should read 13%, 8%, and so on.

But that's just semantic, sort of.
In fact, I must apologize, because I obviously made a mistake.
Before posting my comment #11, I threw your numbers in my Excel sheet for prop calculation - and btw, even if I didn't know your gear ratio, I correctly guessed the 1.62.
But I must have inverted some number between the 19" and 21" H5 props, because I had the weird difference which I mentioned.
What annoys me is that I can't even understand what mistake I made, because I didn't save my spreadsheet with your numbers... :ambivalence:

Regardless, sorry, you are absolutely correct.
Indeed it seems that H5 props are affected by the very same characteristic I mentioned for the B1, i.e. an actual pitch lower by one inch or so vs. what it says on the tin! :encouragement:

 

[Freeloader]

Hi MapisM,
No apology necessary! You did have me worried for a moment, though, as it's a while since I have done these calculations and I took it that I must have missed something. I thought that I hadn't got the format of the prop slip right but couldn't remember quite how to express it as a percentage, so I've learned something this afternoon! It is strange that Mercury sell props with an effective pitch different to their nominal sizing. There is probably some scientific way that they calculate the pitch but it would surely be more helpful to the end user for them to label their props with the effective pitch.

QBhoy,
60mph in a 5.5m boat must seem incredible! You are very lucky to have all that calm, open water on the Loch to be able to exploit all that power - on the South Coast it's a very rare event indeed. You are aware, of course, that there is a 60mph speed limit on the Loch...

 

[QBhoy]

Freeloader...
Speed limit is apparently 50, last I heard ?
Loch Lomond is still capable of throwing a fair chop and swell at the unsuspecting boater, to be fair. Not often it's flat calm.
I'd imagine it might even run better in the sea (in decent conditions) given the difference in buoyancy and all that.
I won't be trying it anytime soon though. Not sure wee boats like that are made for the UK coastal waters. Wouldn't do well I'd imagine ! Haha. We have slightly more suitable flybridge princess for that kind of thing ?

 

[QBhoy]

Correction. You were nearer the mark on speed limit. It's 56mph. I'll look out for the radar gun. That will be the next rediculous idea to come out the Loch Lomond park authority think tank !

 

[TheOrs]

Just to say; regarding prop slip/rpm/speed calcs.

If your numbers are not stacking up and your prop is secondhand, bear in mind it may not be what it says it is. The Laser II on my Arrowflyte states quite clearly '20P' but it's not because I had it altered to 19P. I have no idea how common this is.

 

[piratos]

Just to say; regarding prop slip/rpm/speed calcs.

If your numbers are not stacking up and your prop is secondhand, bear in mind it may not be what it says it is. The Laser II on my Arrowflyte states quite clearly '20P' but it's not because I had it altered to 19P. I have no idea how common this is.

well with o/b and sterndrives it is very unusual, and not easy to do because of the Propeller cupping.
Merc used to say that calculating slip you should add 1 Inch to the indicated pitch, which means your 20p is like an 21 uncupped Propeller. The cupping is one of the reasons why same size props act different.