As if the art of Prop selection wasn't strange enough !

Hi Mapism
I kind of agree about the laser 2 not being a full surface piercing, in the traditional meaning of a true SP prop, but it is generally considered as such by the Mercury website and many U.S. Forums. There is even a strengthened flex hub kit for this application designed for the laser 2 and tempest props in surface piercing use like my boat.
Anyway, with the 21" I didn't get GPS readings but the fairly accurate speedo suggested about 62 mph from memory.
You are also right that in most general cases 400rpm per 2" of pitch is considered and is what I had hoped for, but I'm assuming that because the engine is now operating into its full power band and Max HP rating (at between 4600-5000) it's giving me that extra rpm that was lacking with the 21" only achieving 4400rpm Max ?
Like the title, it's a black art with so many variables to be considered. Haha
 
Well, fwiw 62mph @4400 with a 21" prop also gives a negative (=impossible) slip number, i.e. -4.2%...
...which would also suggest a wrong tacho reading.

Anyway, yep, your last sentence says it all.
As the old saying goes, there are three ways to find the right prop: test, test, test. :)
 
Hi all...especially Mapism who's knowledge and advice was superb....
Just a wee update and a solve to the problem told.
After writing to the manufacturer quoting serial number etc. I was informed that the boat was a special edition model and the engine installed was not only unusual but also tampered with when fitted. Fancy camshaft to go with exhaust was thrown in. Doesn't totally explain the maths but thrown in a slight variable to the rule.
Thanks again guys.
 
Probably only relevant to shafts but if the thrust isn't in the direction of travel would that not alter the slip calculation?
 
Nope.
If there's a thrust component which is "wasted" vertically, as it happens with shafts (but only on P/SD hull, mind!), the propulsion is less efficient than say with outdrives, AOTBE.
But the slip calculation as such remains exactly the same.
 
QBhoy said:
Excellent responses guys but to be honest, I am aware of most factors stated above, having had a fair bit of experience with boats over the last 30 years and obviously researching the ass off it recently. Haha
Unless I'm missing something, I think I have most variables covered.
The boat is in fresh water (Loch Lomond) so therefore no tide to affect such things like the difference in speed over ground etc. wind has been all but ruled out, given the calm conditions.
The boat is a Campion Chase 550 Chase performance edition. Specs below;
18ft bowrider
Mercruiser 5.0 MPI 260hp (Max power curve at 4600-5000 suggested)
Corsa Marine full stainless through hull exhausts (when feeling anti social)
Running 64 mph GPS lightly loaded and well trimmed up (just the leg in the water almost) @5000rpm with a 19"p laser 2 semi surface piercing propeller on a 1.47-1 ratio.
My prop calculator reckons I have less than 0 prop slip at -4.
Surely the maths can't lie. If the 19"p screw is rotating at 5000rpm and pulling forward at a rate of 19" every rotation of the 1.47-1 gearbox, the max speed suggests it should be only 60 or 61 mph ?

Any alternative calculations made are welcome guys...thanks again.
Click to expand...

Hi and thanks for the interesting reading! I was struggling with a similar case on one of my earlier (speed)boats where the calculation suggested a slightly negative slip. It turned out that the prop was heavily cupped and this increased the total pitch above the nominal blade pitch stamped on the prop. Also the gear box ratio would be good to double check.
 
MapisM said:
Well, fwiw 62mph @4400 with a 21" prop also gives a negative (=impossible) slip number, i.e. -4.2%...
...which would also suggest a wrong tacho reading.

Anyway, yep, your last sentence says it all.
As the old saying goes, there are three ways to find the right prop: test, test, test. :)
Click to expand...

When you do manage to get hold of a few props to test, make sure you make a note of every detail in terms of RPM / Speed / Wind direction and speed / tide flow and direction....everything...and try to test with the same amount of kit and fuel on board for best comparisons. Back when I was offshore racing, Peter Little and Dave Arthur in Class 3 were renowned for their thorough testing regime and persistence to get the absolute maximum from the boat in all conditions and got a lot of success because of it. Dave kept a notebook with all the information in so they could go to any race venue, look at conditions and select the prop that would be best on the day, with confidence that it would be spot on..... Scott Collings and Roy Smith also.
 
Guys
Fully aware that this is a response to an old post and often frowned upon, so please delete if not allowed or not of interest to anyone even. Just thought I’d put some sense to the problem we encountered and prove the expert suggestions worth...
Since the post, the wee Campion now has a VesselView kit and displays exact engine data amongst other things.
Turns out the prop was indeed a 21” and reworked. Likely with cup too. The leading edges of the prop were much thinner than a comparable standard laser.
Since also been verified by another 21” prop which gave similar results in numbers.
Been a while since I took some numbers, but an average run will see
64mph gps
5000rpm (sometimes more if some chop and more air under the hull)
Gear ratio 1.47-1
Prop slip around 4/5 at this, which is very rare.
On one occasion since, she ran an incredible 66 gps. She hit the limiter at 5200 (presumably), although I wasn’t watching the rpm at the time. Too busy making sure she didn’t take off. Not too shabby for a hefty 1350kg bowrider with luxury (heavy) finish.
What a machine.
Thanks for the advice guys and hopefully you find it of interest.
 
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