Strange ventilation issue

[Aqualift]

I have been running a 1965 Merc 500 longshaft on my 14 footer for the past 6 years. It has a 11P merc prop and I experience no ventilation when turning or under WOT. I get 5400 RPMs and 30 MPH. Well, I just rebuilt a 1968 Merc 500 longshaft and installed it on the boat. According to old manuals, it should have a gear ratio of 1.64:1. The same as the 65. Thus, I searched and found a new old stock Merc quicksilver prop in 11P. I got in a some test runs back in March and noticed slower speed, higher RPM's and severe ventilation when turning, and minor ventilation with WOT in a straight line. I checked the gear ratio manually and found that it was 2:1. That explained the above, or so I thought.

I was able to quickly track down a 13P merc prop. (all are hard to find for these particular years) I gave the 13P prop a try and I got back the same performance as my 65. The problem that I'm still experiencing is ventilation when turning. It's not as severe, but is very annoying. I can't figure out why the 68 does it. The motor is at the exact same height and tilt. Antivate plate about an inch below the hull. In fact, I raised the 65 to get more speed and rpms with no ventilation.

The only difference is the gear ratio. All the props are the same with no cupping.

Even my marine mechanic is stumped.

 

[nowgrn4]

Re: Strange ventilation issue

Is the pin that limits the forward movement of the lower unit in the same holes on both engines?

 

[Aqualift]

Re: Strange ventilation issue

Yes.

 

[steelespike]

Re: Strange ventilation issue

Did you actually measure the position in relation to the boat? perhaps the the setback is different?
Perhaps the front to back measurement of the gear case is different.
More space between the prop hub and gear case. Perhaps the gearcase is slightly different diameter or contour.
Maybe rougher?

 

[Aqualift]

Re: Strange ventilation issue

Setback is the same. I just remeasured the gearcase and it's no different. The one main difference besides the gear ratios are the water intakes. The 68 has six large intake holes and groves that span a big portion of the middle gearcase area. The 65 has five smaller holes and grooves that barely go halfway up. The 68 has a small gouge (mainly through paint and primer) on the left side of the bullet looking portion of the gearcase and the prop hub is 1/8" further out. I'm not sure if these differences would be enough to aerate the water and induce ventilation.

Would the increased thrust from the 2:1 gear ratio cause the non cupped prop to ventilate easier?

Next time I get up to the lake, I'm going to tilt the motor inward one pin hole to see what happens. Eventhough that postion slows the boat down, I'm curious to see if the ventilation is reduced by keeping the prop deeper in the water when turning.