Anti Ventilation Plate Adjustment

[captdave311]

Greetings,

I recently purchased a 1970's 18' Larson All-American runabout that has a ~1967 85 hp starflite outboard motor. The previous owner metnioned that when giving the engine some good power from a stand still, the boat leans and turns right. He suggested I invest in a pair of trim tabs (which seems logical). However, the friendly folks at West Marine told me this weekend that trim tabs aren't the place to start on this! Much to my surprise, the WM guy said to start by adjusting the anti ventialtion plate (commonly mis-called the anti cavitation plate, thank you very much outboard repair manual). I'm curious to know if anyone who has been down this road before had any experience or advice they would share with me.

I haven't tried it out yet, so I can't give detailed information about what it's doing...yet (check back after the 4th-o-July).

Thanks in advance!

 

[iwombat]

Re: Anti Ventilation Plate Adjustment

That's very sound advice. The ant-vent plate should be even with, or slightly above the bottom of the transom. After that, you can adjust the trim tab on the lower unit and maybe get it all dialed in w/o buying anything extra.


I had a hard right pull on mine until I adjusted it about 1.5" above the keel in an attempt to get higher RPMs. The RPMs never did increase, but steering sure got easier.

 

[ezeke]

Re: Anti Ventilation Plate Adjustment

Edit

 

[captdave311]

Re: Anti Ventilation Plate Adjustment

Thanks for the reply guys! I'll try fiddling around with the anti-ventilation plate. From what you posted, here are the 2 actions I'm planning on taking:
1. Make sure engine is adjusted to proper height when sitting in the water.
2. Try to adjust the anti-ventilation plate by loosening bolts and turning plate in direction of lean/pull.
If that doesn't work, I'll likely invest in a pair of trim tabs.

Please let me know if you think I've got it right!! I'll, of course, post the storyline when I've done something by this weekend.

Thanks!

 

[ddennis]

Re: Anti Ventilation Plate Adjustment

Looking at that pic it seems that the motor is indeed too low when lowered...May be wrong though. Post a pic with the motor dropped..

 

[captdave311]

Re: Anti Ventilation Plate Adjustment

dd...I'll get a pic posted, but it won't be until tomorrow.

 

[Silvertip]

Re: Anti Ventilation Plate Adjustment

You are confused about what the antiventilation plate is. That's the flat plate that sits horizontally directly above the prop. That plate is not adjustable. You must raise or lower the engine on its mounting bolts to position the plate so it is flush with or an inch or two above the bottom of the boat. The trim tab is the little fin that sits directly behind the prop. It compensates for the torque the motor imparts on steering.

 

[iwombat]

Re: Anti Ventilation Plate Adjustment

Ayup. Looking at your last post, you are indeed confused.

Anti-ventilation plate = horizontal fin.
Trim tab = vertical fin.

 

[captdave311]

Re: Anti Ventilation Plate Adjustment

Ah ha! I think I understand now...it makes MUCH more sense! I'll take a look at the motor in person tomorrow and post any questions that come of that. Thanks for clearing things up (I'm new to this, if you couldn't already tell)!

 

[iwombat]

Re: Anti Ventilation Plate Adjustment

If your anti-vent plate is below the keel, and there's any turbulence from your hull it'll be amplified by your outboard. Also, if your outboard isn't perfectly in line with your hull, that issue will be amplified as well. Set the trim tab to dead center and raise up your motor to even, or slightly above the bottom of the transom and I'll bet most of your hard pull goes away.

Then, adjust the trim tab into the direction of the pull to remove the remainder.

The best part about this repair - you gotta try it out on the water.

 

[captdave311]

Re: Anti Ventilation Plate Adjustment

Iwobat...I understand completely! I will give it a try and post my results (or follow-up questions).

 

[BF]

Re: Anti Ventilation Plate Adjustment

I think having the anti-vent plate even with or even above the level of the hull applies to new style outboards having through the hub exhaust. Yours does not (exhaust comes out the port behind the prop). I think yours should be run deeper. On some OB's of your vintage, they have a water pickup just ABOVE the anti-ventilation plate, and that should be in the water when on plane. (my early 70's Johnyrudes are like that).

 

[Silvertip]

Re: Anti Ventilation Plate Adjustment

All the more reason to have a water pressure gauge when playing with engine height.