Prop selection- choosing the right prop

[londee]

So I have adjusted my outboard height as far as it can and its pretty close to where it should be (caitation plate just visible).

I have tested the following props with a light load (with normal fishing gear, half tank of fuel and usual crew on board)

Prop 1:
Piranha 13.75 15B
RPM (WOT): 4700
Speed: 47- 48 kph

Prop 2:
S/S Yamaha 13.5X16K
RPM (WOT) 5000
Speed: 54 kpm

Outboard:
Yamaha 90 hp 3 cyl 2 stroke
WOT recommendation: 5000- 5500
5 inch jack plate fitted

Boat:
Quintrex Reef Raider

My thoughts are to go for a 17 inch pitch prop. Might fit the piranha prop with new blades and see what happens. Planing to do other test run with the Yamaha 13.5X16K first with a bit of weight and see what happens.

Has any one go any thoughts, am I on the right path?

[Darren Mc]

Looking at your results with the SS Yamaha 13.5X16K I would say a 13 3/4 X 13p Solas New Saturn SS 3 blade would get you up to around the 5500rpm mark, maybe 5600rpm? If you go up to a 17p your WOT rpm will drop even lower.

[Blackened]

G'day

Imo, go for a 3 blade stainless prop in a ~ 13 1/2 x 15''. Solas props are ok, imo search for a mercury/quicksilver prop, get a hold of a vengeance. You can get the hub kit to suit.

By dropping 1'' in pitch from 16-15 you should increase rpm by ~ 300, this will bring you up into the mid part of your WOT range with your "light load", one up it will bring the rpm up further.

Make sure your figures are true and taken with the engine trimmed all the way out, as if the prop's just about to blow out.

Dave

[londee]

Bit confusing,

I increased speed and RPM by increasing my Pitch (from the 15 inch Piranha to the 16 inch S/S). Is this a good comparison or more like comparing apples with pairs! All the information I'll read suggests dropping in pitch, as suggested, but my gut tells me to go up a pitch.


Make sure your figures are true and taken with the engine trimmed all the way out, as if the prop's just about to blow out.Think I might double check before purchasing other prop just to be sure

[Blackened]

G'day

You will not increase RPM by increasing your pitch, your piranah measures 18.75" diameter? I know this isnt true as it wouldnt fit on the engine.

There are other variables at play, like boat weight, load, trim or even wind and current.

You're welcome to do as you wish, I have done extensive prop testing on my current rig and have settled on a 16'' vengeance prop which is PERFECT (50rpm below recommended max at WOT.)

Not to mention the prop testing etc on my previous boats including the thundercat, everything I've done in the past has been consulted with a marine engineer to bounce my ideas off and then to settle on the correct prop.

Props are an art, the slightest difference like rake or cupping can give you dramatically different results in performance, educated guessing will get you close to what you'd want to achieve but trial and error with ACCURATE figures is what will finally give you what you seek.

Understand that outboards only have one forward gear, the prop acts in exactly that way, to be the perfect final ratio for that hull / engine combination. That's why cars have 3, 4, 5 gear ratios, to keep the engine in the optimum rpm range all of the time for a given speed.

Dave

[Horse]

Dave is on the money. All props are not equal. If you don't want to keep trying props it can be worth dropping a line to Ken at the Propgods Forum http://www.propgods.com/forum/
The guys there know the props.
Currently you seem a little over propped with the 16K and a 15 would suit you better but not in one of the full bladed designs. Is the cav plate just visible or clear of the water. The jacking plate would see a pretty high cav plate well above the hull line

[Darren Mc]

I don't think 1inch of pitch less will be enough, I think at least two would be better. 3 inch's is slightly to much but if you increase the diameter slightly that should compensate for that wouldn't it. I like to be propped slightly on the lower pitch side as it's much less load on the engine and much nicer to drive on to the plane , at slower speeds and with the occasional heavier load etc. Unless of course your trying to ring every bit of top end speed out of it then that's another story. Is that at all close to the mark guy's or not?

[londee]

Is the cav plate just visible or clear of the water.Visible, if I go higher (and I have) I get a lot of cavitation in heavy seas, where it currently is I will still get a bit now and then but not much.


If you don't want to keep trying props it can be worth dropping a line to Ken at the Propgods Forum http://www.propgods.com/forum/Great idea thanks, will drop him a line regarding my prop and might also ask about outboard height (could cavitation at a higher setting be caused by my current prop!).


Props are an art, the slightest difference like rake or cupping can give you dramatically different results in performance, educated guessing will get you close to what you'd want to achieve but trial and error with ACCURATE figures is what will finally give you what you seek. I agree, its not as simple as I thought


piranah measures 18.75" diameter? I know this isnt true as it wouldnt fit on the engine. Thanks mate, I have looked at the prop again it is stamped 13.7515B (have edited) :-[

I will post what response I get from the Prop Gods when they reply::)

Thanks for all the suggestions... more food for thought

[Shelby35]

Sorry to hijack but what does WOT stand for.
Thanks
Byron

[Blackened]

Sorry to hijack but what does WOT stand for.
Thanks
Byron

G'day

Wide Open Throttle (max throttle)

Dave

[Shelby35]

Cheers
Byron

[tenzing]

hi londee
Even with all the science, it appears there is still a bit of an art to selecting the right prop for YOUR boat.
I ended up with the TENTH combination of prop and motor height as the best compromise. It wasnt the prop that anyone had picked first up ( except for a very patient fellow ausfisher)
I tried a couple through my motor dealer, then ended up getting a heap of help from Steve at Solas. I also had a long conversation with Yan at propellor repair at the gold coast ( a very knowledgable gentleman who had adjusted my original prop)
My advice is to keep a log of each prop and its speed and fuel usage at each rev and its WOT. also note the conditions etc so you can compare apples with apples.
I also used this prop slip calculator which will show you how efficiently the prop is moving you through the water at various revs .
http://continuouswave.com/cgi-bin/propcalc.pl
the first things all these blokes said was that there is no perfect prop for every situation , so you are looking for the best compromise.
It gets hard to remember later if you dont write it down
Good Luck
Brendan

[WalrusLike]

Um forgive the dumb question....

but is all this prop swapping a rich-bloke-only sport? Do you loose say, $200 with each swap for instance?

In my situation with the stock prop can anyone guess a cost to end up with the right stainless prop? Assume say 4 swaps?


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[londee]

Um forgive the dumb question....

but is all this prop swapping a rich-bloke-only sport? Do you loose say, $200 with each swap for instance?

In my situation with the stock prop can anyone guess a cost to end up with the right stainless prop? Assume say 4 swaps?

Some company's will allow you to test and swap different props at no charge (after you pay for the initial one)


http://continuouswave.com/cgi-bin/propcalc.plThanks for the info, will check it out

Prop God reply:

Height looks low, but you said you've already raised it.

I'd suggest a powertech SCD4 14 pitch.

Its an easy prop to turn. its a semi cleaver design. (similar to the yamaha aluminum style)

Acceleration would be strong, and it would hold well at high engine heights.
Probably keep the same top end, maybe increase 1 or 2 kph.

[tenzing]

Um forgive the dumb question....

but is all this prop swapping a rich-bloke-only sport? Do you loose say, $200 with each swap for instance?

In my situation with the stock prop can anyone guess a cost to end up with the right stainless prop? Assume say 4 swaps?



(Using Tapatalk on iPhone so can't easily 'thank' or 'like')
My suzi dealer supplied a couple to trial. I then bought one from Solas ( if you do this you absolutely MUST ring for a price - the cheapest way by far)
I then swapped it a couple of times for no extra charge (Remember if you ding it its yours!)
You cant do that with many things in life.
My prop even has a guarantee, so if I return it undamaged ( maybe within 5 yrs) say if changing motors , I can change for a new appropriate one. Hows that?
BTW freight was included (not return cost of course). In one case they only had one in Perth- overnight airfreight, at my office 8am next day. Those guys mean business!
Good luck with it
Brendan

[Mister]

What is this cavitation plate thing being referred to?

[WalrusLike]

Cav plate is a horizontal plate above the prop designed to reduce cavitation.

It's actually a anti-cav plate. It's the biggest horizontal plate on the leg above the prop. It's also what hydrofoils bolt onto.


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[Horse]

"What is this cavitation plate thing being referred to?"

Its actually an anti-ventilation plate but only tossers call them that::)

[gofishin]

Cav plate is a horizontal plate above the prop designed to reduce cavitation.

It's actually a anti-cav plate. It's the biggest horizontal plate on the leg above the prop. It's also what hydrofoils bolt onto.... Walrus, think you will find that 'Mister' was having a lend ;). This incorrect term is probably commonly used for the same reason that 'cavitation' is commonly used when 99% of the time it is ventilation that people are referring to!


"What is this cavitation plate thing being referred to?"

Its actually an anti-ventilation plate but only tossers call them that::) I resemble that remark Horse .... ;D

PS. Is 'Mister' really 'SatNav' without the bullet numbers???:-X

[Horse]

Spot on gofishin. Cavitation occurs from the low pressure essentially boiling the water around the prop while ventilation is the addition of air allowing the prop to slip. Only the most pedantic use the term as most know it as cavitation

[WalrusLike]

Well it IS a fishing site..... wouldn't want a bait floating forever without at least one gullible fish rising to the bait.

:)

So maybe that's why I am a lousy fisherman... they outsmart me!


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[Fed]

The first time I came across cavitation on my props I thought they were corroding away, they damage looks a lot like what you see on the surface of anodes.
I saw my local Mercury dealer & he sat me down and explained all the ins & outs of it, who would think that little bubbles could cause so much damage.
Apparently the damage is done by the bubbles collapsing on the low pressure surface of the blade but I haven't worked out how to watch it with a microscope under water beside a spinning prop yet so I guess we have to take the Merc dealers word for it.
I saved this picture a while back because I thought it was cool.
95753
Hold the line Mister otherwise they will all be talking gibberish in another 50 years.

[Mister]

Great to see some actually know what cavitation and the causes of cavitation is and even the tossers that continue to get a little lost with the correct terminology can't argue with that. Hence the anti-ventilation plate has nothing to really with cavitation but no doubt there will be those tossers 50 years on who still will not (or is that don't want to :)) know the difference.

[WalrusLike]

.., but no doubt there will be those tossers 50 years on who still will not (or is that don't want to :)) know the difference.

Dunno mate.... calling folks tossers because they don't have a particular bit of nomenclature in an arcane area is a perilous activity.

It's kinda tempting fate for the next time in life you are dealing in an area that you are not an expert on.

I point this out not because I feel offended... far from it.... but rather because it is part of the increasing tendency to belittling others that I see in the society around me.

Re cavitation... with my physics interest I was aware of the actual cause of cavitation being a lowering of pressure and had wondered how a plate nearby could affect that.... maybe pressure waves reflecting (I thought??)

I had heard that its actually an anti ventilation plate but had forgotten it... Unlike some, I am not perfect. :)


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[Malcolm W]

so if you cut the anti-ventilation plate off an out board leg, would every outboard ventilate?.

Is it there to turn back the cavitation bubbles on the prop as in the photo above and stop them breaking the surface?

so why don't the outboard manufacturers make them larger to cover the whole prop such as a foil?

Or is it merely there for guide to the depth the outboard should run?

Interested to know what people think.

[WalrusLike]

..,,

Interested to know what people think.

At the risk of being laughed at for my ignorance.... I am willing to try guessing.

I would suspect its effect is to impede the creation of a whirlpool draw down from the surface which would be a bad thing.

If you notice a pool skimmer with low water level it will sometimes suck air down in a twister from the surface. My guess is the anti-ventilation plate (popularly called the cav plate) :) is to get in the way of that happening.

I could well be wrong. :) I frequently am.


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[Mister]

I would suspect its effect is to impede the creation of a whirlpool draw down from the surface which would be a bad thing.

If you notice a pool skimmer with low water level it will sometimes suck air down in a twister from the surface. My guess is the anti-ventilation plate (popularly called the cav plate) :) is to get in the way of that happening.

That's close enough, the anti-ventilation plate (wrongly called the cav plate :)) is designed to stop air from the surface interacting with the prop, cavitation is a result of pressure and totally different thing to ventilation. The anti-ventilation plate does not stop or cause cavitation.

Even dolphins and the like experience cavitation which also has absolutely nothing to do with being a result of induced air either.