BRACKET OWNERS

[macojoe]

If the cavation plate is even with the bottom of the boat then you are fine. Just have to reprop for the best fit.

I have a old prop from my I/O and tryed it and I belive it is best for all around, I think it is a 15??
I have a 17 pitch now and it is great on the top end but when I do a hole shot it spins a little and then in the turns.

One more trip with CB, I will bust it off all toghter and when I replace it I will go with something closer to the other prop.

Hey,
I don't have a bracket but I have a qustion concerning prop diameter and pitch. I have a 77 200 HP V6 Johnson with a 78 V20 cuddy. It currently has a 15 x 17 prop but I only get 4400 rpm at wot. I am busy refurbishing and I am going to rebuild the fuel pumps which may be the problem but the prop seems to be a bit on the big side. I think the manual calls for 4550 to 5750 at wot which means I am only a little bit shy but it also says the 15 x 17 is the biggest prop I should use.

Any insight woud be appreciated.

Thanks in advance,
Buenos

[chesapeake724]

b_d,

It sounds like you need to prop down, to like a 15x15. This will allow the engine to wind up a bit more. IIRC the ballpark calculation/rule of thumb is for every 2" of pitch you go up, you drop 3-400 rpms at WOT. Conversely, by dropping 2" of pitch, you should gain 3-400 rpms at WOT, putting you in a more optimal operating range.

Thanks for the quick response. I'll rebuild the fuel pumps since I was going to do it anyway and then go down, so to speak. ;D

Thanks again,
Buenos

Just FYI, I think you guys are discussing propeller ventilation, not cavitation. I would not know cuz my propeller does not turn presently :'(


Propeller Ventilation

Propeller ventilation is caused by air from the surface or exhaust gases being drawn into the rotating propeller blades. This results in the propeller slipping more than usual due to the reduced water load on the blades. The obvious symptoms of this are a sudden increase in engine RPM as well as a possible loss of speed. This commonly occurs when trying to turn the boat at high planing speeds or if the outboard or sterndrive is trimmed out too high.

In racing conditions this can also occur when following another boat too closely. The small bubbles in the water created by the leading boat can cause ventilation of the propeller of the following boat, with a subsequent loss of speed. This is why you rarely see high speed race boats following directly in the line of the leading boats, even if they are far enough back to be out of the spray and wake.

Propeller Cavitation
Propeller cavitation is less obvious than ventilation, but can be far more damaging. When the propeller blade passes through the water at an increasing speed, the pressure that holds the water to the sides of the blades is lowered. If the water is sufficiently warm, and depending on the speed of the boat, formation of water vapour (boiling) can occur. These bubbles that are produced then immediately collapse, releasing energy that can cause a cavitation burn on the propeller blades. This is one of the great advantages of stainless steel propellers. Due to their superior strength they can withstand cavitation damage better than aluminium and can also be produced with thinner blades to reduce the occurrence of cavitation.