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

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QBhoy

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Hi all....
At the risk of sounding like a show off (totally not)...is it at all possible for a boat to be so perfectly prop pitched, it performs better than the Mercury website prop slip calculator suggests is possible ?
Won't bore you all too much with details, unless interested....long and short is that I'm not an expert, nor a novice but having spent a few bob on different props on this particular boat, and taken true GPS speed readings against true RPM, the evidence would suggest I have an impossible speed for my boat, engine and prop selection. Is this at all possible or are the mathematics involved in prop slip calculations simply not surpassable ?
Thanks all...
 
There are more prop calculators, designed for different boat/engine types. The maths for a planing runabout isn't the same for a displacement hull.

Selection of a prop is about compromise. If optimizing top speed you have to accept other drawbacks - and properties to balance are many. At bit simplified you may think of a car with only one gear. Which would suit you? 1st, 2nd, 3rd or..?
Engine mantufacturers state a recommended rpm range at Wide Open Throttle (WOT). This is important because the engine need to reach this range to performe right, not spinning too high (too low pitch) or lugging at low rpm (too high pitch) and with a given load your only means of reaching right rpm is prop pitch. Ultimately only tests will show the exact prop because actual slip and your presonal preferences can't be calculated. Prop type, design and material will come into play too.

The weight/power ratio will dictate the possible top speed. A hull (deep-V, modified-V or flat) weighing X being pushed by Y hp will be able to do Z knots. No propeller will change this but the right prop will enable it - you'd need to change ether weight or hp to get a higher top speed. Of the twp parameters the easiest and free way of course is to reduce weight, commonly by removing dead weight.

State your hull shape, ready-to-go weight and hp and I'll see what my calculator comes up with.

If you like there is a lot of info in this booklet
 
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.
 
No clue what prop calculator you are using but without having displacement weight and hull factor you will never know.

Propeller will not work without slip.

My old Casio calculator using Crouches method works for me...........

I am certain Clements Marine love people phoning them asking for prop calculations but not selling anything, and I am deeply distrustful of the #'s they come with as they never make a decent allowance for weeding ending up with overloaded engines.
 
Not a prop calculator, prop slip calculator. Think that's where the confusion was. You won't require weight in a prop slip calculation, as far as I know ?
 
I doubt your rev counter is 100% accurate, and it would only take 200 rpm to make a 2mph difference either way. You also mention 'semi surface piercing props' I don't know what they are, but is it the case that any surface piecing is going to change how efficiency is calculated, and normal rules would not apply?
 
Definitely something doesn't add up. Suppose that's why I was asking. The variable of the Rev counter, I hope I have accounted for....although not totally scientific, previously the boat has the wrong prop (a 17"p) and fairly hit the rev limiter at 5150 rpm or just under. I then read that the rev limiter is 5100 rpm or there abouts, making me presume it was fairly accurate. Also, having been around engines a long time, I'm fairly sure it's accurate or not too far off. Certainly not hundreds and hundreds rpm out. Anyway, I have little to complain about, the boat runs flawlessly and seems to be perfectly pitched and suites to all my needs. Just that the maths doesn't add up and curiosity had the better of me. Thanks for all the interest and opinions !
Allan
 
CLB
You might just have nailed it....if the propeller is surface piercing a little, it's bound to effect the rule ? ?
 
The subject of prop calculation is very complex. For a hydrodynamicist to calculate an accurate propeller pitch and size he must know the EXACT hull resistance at various speeds. Usually calculated from model testing on larger vessels. Then the EXACT shaft horsepower and torque at the propeller has to be known at various RPM taking all bearing losses etc in to account. Then there is the question of prop design. Surface piercing props are a special case. Add to this the fact that when hulls plane everything changes and it can be seen that all of the prop calculators around make a lot of assumptions that may be wildly out for a vessel like yours. Even RPM/pitch calculations of your boat rely on being right about the prop RPM and exact measured pitch. Just be happy you have a good result!
 
QBhoy said:
with a 19"p laser 2 semi surface piercing propeller on a 1.47-1 ratio.
Click to expand...

Are you 100% on this? A small difference in GBox ratio would make a big difference to the speed + I have a Lazer II prop on my Arrowflyte that says 20P on it (because that's what it started out as) but it is actually 19" pitch as I had it pitched down. Yours could have been pitched up previously if it's secondhand.

Also, I don't think the Laser II is meant to be a surface piercing prop.
 
QBhoy said:
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...
I've only just seen this thread.
There isn't such thing as an alternative calculation, and indeed, maths can't lie. According to your numbers, you should have a negative prop slip.
Of exactly 4.6% btw, though I suppose you just rounded your "-4", therefore there's nothing wrong in your calculation - though you are wrong in assuming that you should make 60 or 61 mph, because that would mean a slip of 2% or 0.3% respectively, both extremely optimistic, and practically impossible to achieve.

Now, the problem is that a negative prop slip is a complete nonsense, because if you would have discovered a propeller that increases the boat speed without actually demanding more rpm/power, you would have solved the energy problems of the planet... :D
Therefore, something is positively wrong.
But mind, not in your calculation: there must be at least one of your numbers which is not correct - and you might be unaware of that.
My guess on what is more likely to be wrong goes to either the tacho or the actual prop pitch.

Re. the latter, don't be surprised if a stock prop labelled as a 19" is actually a 21" (which would imply a much more reasonable slip of 5.4%).
Even with props like the laser II, which isn't particularly exotic, production variances can be much bigger that you would think.
And some Merc props are known for being regularly different from the rated pitch - one case I'm aware of is the Bravo I, whose actual pitch is normally 1" shorter than its mark.
Not to mention, as was already said, that someone could have machined the prop to make it longer (if you bought it used), but I'm assuming that the prop is stock.
Btw, only a prop shop with a proper equipment can confirm you the actual pitch of a prop, regardless of what it says on the tin.
Funnily enough, if you are sure that all the other numbers are correct, by reverse reasoning you can only come to the conclusion that your 19" prop is actually longer, by a couple of inches or so.

PS: just for the records, whether (and how much) the prop is working as surface piercing is totally irrelevant for the slip calculation, whose math remains exactly the same.
 
Thanks for the recent posts. More good points made.
The prop was used, but only for an hour on another boat. I assume it is a 19"p. I did have a laser 2 21"p on it previously for a short period. Only managed 4400 rpm, which is below the recommended rpm range, according to the power curve.
TheOrs....the Laser 2 certainly is surface piercing, when trimmed up on a performance boat like this.
Thanks again guys, perhaps the tachometer is out.
 
QBhoy said:
I did have a laser 2 21"p on it previously for a short period. Only managed 4400 rpm, which is below the recommended rpm range, according to the power curve.
TheOrs....the Laser 2 certainly is surface piercing, when trimmed up on a performance boat like this.
Click to expand...
If you remember which speed you achieved at 4400 rpm with the 21" prop, you could cross-check the slip of the 19".
Anyway, a difference of 600 rpm is a bit too much: somewhere between 400 and 500 would be consistent, IF (and it's a BIG if!) the pitch of the two props is exactly as marked.

Re. the Laser II being a surface prop or not, TheOrs was actually correct: it is definitely NOT designed as a surface prop.
This doesn't mean that it can't work as such, though.
That actually depends mostly on how high the outdrive is installed, i.e. on its so called "X dimension", rather than on trimming.
Non-surface outdrives like the Merc Alpha and Bravo are in fact designed to be installed with the cavitation plate aligned with the hull bottom, when trimmed neutrally.
But the builders can install them higher (within reason), and that's how many sportboats meant to reach 50+ mph are rigged.
Just as an example, in my old Fountain, the X-dim was 19", as opposed to 13" 1/2 of the standard Bravo installation.
As a result, the cavitation plate was 5" 1/2 above the hull bottom, with the prop axle under the water by just less than 3", hence working pretty much as a surface transmission, even if its prop (a 4 blades Bravo I) was NOT, as the Laser II, designed as a surface prop.

Beware of trimming with raised outdrives btw, because it's easier to cross the border between speed optimization and cavitation - at risk of damaging the prop.
And also tracking while steering at speed becomes more critical, with higher risk to spin the boat.
Something you definitely don't want to experience in general, let alone at 60mph...! :encouragement:
 
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