Propeller Pitch/Cavitation

[irishjim]

Thanks to the guys who replied to my last question on recommended propeller pitch for a Mercury 85hp Blueband.

The reason I ask initially is that the last time I had my boat on the water (lake) I experienced good propeller revs but with very little initial forward movement. I took this to be cavitation (may be wrong) and thought the cause may have been the rough/chipped edges of the blades....that is why I'm purchasing another prop in better condition.

Has anyone experienced the same issue and waht was cause/resolution. I guess you can tell I'm not an experienced/knowledgable person with boat handling

Thanks - Jim

[Y-Not]

Cavitation occurs mainly in turns so I would suspect the bush is flogged out in the prop. This will cause the engine to rev but allow the prop to slip on the bush loimiting forward propulsion.

[catchy_fishy]

Here goes a long but hopefully worthwhile response. It's c/o my Skipper's notes:

Prop Selection Selection of the corr prop to suit your boat is of utmost importance. Without the correct prop your outboard cannot operate properly. The following are pointers to incorrect prop selection

Sluggish to plane

Abnormal fuel consumption

RPM lower than specified

Poor top speed

Simplest way to determine correct prop selection is to fit a rev counter. At full throttle with corr prop the revs should not exceed manufacturers specs by more than 500rpm, with a light load. This ensures that at normal load it will operate at recommended RPM

Prop Theory
Props should be free of nicks, bent edges, pitting, etc to enable them to develop an even thrust across entire aft surface of the blade.
Wear of pitting from turbulance or an uneven bite produces hot spots on painted props. Non painted pros may be coated with a paste form of machine blueing to check for hot spots such as shown, indicating a leading edge needs slight rounding to eliminate upper RPM disturbance
A prop should deliver a max performance pushing the max load with the prop operating at recommended RPM and engine RPM.


5 bladed prop - reduces vibration on large vessels with max thrust for starting and stopping

4 bladed prop - used for low RPM on commercial vehicles with max thrust

3 bladed prop - used for small HP motors with better accelartion for planing at a lower top speed than a two bladed prop

2 bladed prop - best choice (per the book) is the best choice for speed racing at over 4000rpm. It will perofrm on the surface with enough biote and not cavitate.

A single motor will have a 12-pitch prop, yet for the same boat, and same load, when using a twin motor installation, a 13 pitch will be fitted.

Also always try and carry a spare prop

Hope there's something useful in there. If it were me, I'd go to a reputable dealer and get there advice telling them your full application - inland, surf zone, etc

[Panda]

[quote author=catchy_fishy link=1133750930/0#2 date=1134249192]Here goes a long but

Simplest way to determine correct prop selection is to fit a rev counter. At full throttle with corr prop the revs should not exceed manufacturers specs by more than 500rpm, with a light load. This ensures that at normal load it will operate at recommended RPM




I too am having some problems getting my new boat/motor set up right.

Is it normal practice to expect the motor to rev past the manufacturers recommended rev range with a light load?

My new 60 etec will rev to about 5,800 with only me in it on calm water. Manufacturer says it should operate between 5,000 and 6,000 at WOT. She revs out to about 5,600 with a load on. So I thought the prop was OK. But I am having problems with mid range power between 4,000 and 5,000. The motor struggles and eventually drops revs. A bit more throttle and she is back up to 5,500rpm where the motor seems quite happy.

Does this sound like the prop is too big? It originally came with a 17 pitch but wouldn't pull at all so the prop size was drooped to a 13 pitch.

All advice appreciated.

[rajawolf]

Panda,

Sounds more like a fuel or electric problem in that 4,000 and 5,000 rev range..