Anti-Ventilation plate on older motors.

BigB9000

Lieutenant Junior Grade
Joined
Dec 5, 2007
Messages
1,154
This has come up recently on some threads and my local craigslist.

The older outboards, have the anti-V plate at 20"
Is this proper for a 15" transom?

Or, is the large lower plate on an old lower unit even the anti-V plate?

Should it match the bottom of the boat?
 

Attachments

  • 0626101207.jpg
    0626101207.jpg
    25.8 KB · Views: 0
Last edited:

GA_Boater

Honorary Moderator Emeritus
Joined
May 24, 2011
Messages
49,038
Re: Anti-Ventilation plate on older motors.

A 15 inch transom would take a 15 inch or short shaft motor, a 20 inch transom would take a 20 inch or long shaft motor. Age has nothing to do with it, but you have to match the motor to the transom. Short and long shaft motors are both made today, although larger horsepower motor are usually only long shaft or extra long shaft or 25 inch.

The lowest plate right above the prop is the AV plate and should be about even with the bottom or a bit higher. I would say in your pic the motor is low.
 

kfa4303

Banned
Joined
Sep 17, 2010
Messages
6,094
Re: Anti-Ventilation plate on older motors.

+1. The anti-vent plate should be level, or slightly higher than the keel. While you can raise a 20" motor on a 15" transom by building a riser or jackplate, it's generally not advisable to cut down a 20" transom to accommodate a 15" motor. Your motor appears several inches too low and tucked too close to the transom. Most folks find the 2-3 pin position away from the transom to be best, but you'll have to do some experimentation on the open water to be sure. This may help too.

MINI_JACKER.jpgjack-plate-mii-camo.jpgIMG_0610.jpg1-Correct Trim Angle.JPG
 

BigB9000

Lieutenant Junior Grade
Joined
Dec 5, 2007
Messages
1,154
Re: Anti-Ventilation plate on older motors.

Obviously, the angle on mine is all wrong, but they all sit in the water about the same..
and that is the shortest they come.
So, these motors aren't made for a 15" transom?

That all hang lower:
01.jpg


evinrude-6hp-side.jpg


Utility-Raceboat-2.jpg


http://www.riverhawkboats.com/images/timmatthewriverhawk1.jpg

http://media-cache-ec0.pinimg.com/736x/ae/18/d3/ae18d34643b104d055c575035520b321.jpg

http://3.bp.blogspot.com/_074AjHs8w...bs/S0nWljC7Uuk/s1600/Minnesota+Monday+052.JPG


They all look like mine, 2-3" below the hull
 
Last edited:

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: Anti-Ventilation plate on older motors.

kfa has done a nice job of explaining things on his post # 3. A 20" engine on a 15" transom will work-but it's not advisable. When on plane, the engine will have significantly more drag than normal. You will see ton's of water spray from the lower unit at the back of the boat, as it tries to plow through the water. This will cause: abnormally high fuel consumption, much lower than optimal top speeds. Either get an engine with the correct lower unit length, or modify the transom to allow a higher mounting of the engine.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Re: Anti-Ventilation plate on older motors.

Does nobody remember "older motors"? They originally were designed for use on wooden boats with a keel. They were designed to run the plate somewhat below the bottom of the boat. In fact, the boat bottom was supposed to be about mid-way between the water pump bulge and the plate. That is why they appear to be too long now. Of course, we are living in 2014 and probably haven't even seen a wooden boat in who knows how long. Obviously, you usually are free to jack the motor up a bit to use on many boats.
 

Attachments

  • 2 - 6 Transom Height and Angle, Keel.jpg
    2 - 6 Transom Height and Angle, Keel.jpg
    139.1 KB · Views: 3

kfa4303

Banned
Joined
Sep 17, 2010
Messages
6,094
Re: Anti-Ventilation plate on older motors.

^This. Motors, hulls and transoms of this era weren't yet standardized so many manufacturers tried to find a happy medium. Better to have a leg that is too long and raise it as needed, rather than have to cut into a transom, which is generally a no-no. For a while the standard transom height was 15", but most makers are now making 20" motors almost exclusively as they can be raised as needed using the devices in my previous post. Virtually everyone where I live, myself included, use a transom riser/jackplate of some sort to improve draft and overall performance. My '66 20hp has an 18" leg, but I made a small riser out of scrap aluminum and lumber in order to bring it up to the desired height. Ideally, the anti-vent plate should just barely brake the surface at WOT (it looks a little high in the pic due to the angle).

CMC-50012-5-INCH-verticle-extension-for-jack-plates-transoms-installed-500.jpgFinished plate 2.jpgmotor height 2.jpg
 
Last edited:

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
7,952
Re: Anti-Ventilation plate on older motors.

Does nobody remember "older motors"? They originally were designed for use on wooden boats with a keel. They were designed to run the plate somewhat below the bottom of the boat. In fact, the boat bottom was supposed to be about mid-way between the water pump bulge and the plate. That is why they appear to be too long now. Of course, we are living in 2014 and probably haven't even seen a wooden boat in who knows how long. Obviously, you usually are free to jack the motor up a bit to use on many boats.

As a youngin' I had a wooden boat and that era was my introduction into optimizing. We all knew back then where the cav plate needed to be and shimmed accordingly. I only saw one mid way and that was on an acquaintenance's boat who thought dropping the motor would make it go faster.....LOL.

My current woodie is a 1952 - "old' enough for ya? Its factory built. The transom is 16" - its a typical V-hull rounded / flat at the transom (not a deep V). The keel is ~1" wide by ~3/4" deep, which has absolutely no impact on the motor height. She carries a '53 'Rude short shaft Big Twin and the cav plate is right where it should be.

If ya have an oldie transom that isn't 15-16 or 20-21 I suggest that it was modified or was a homemade boat.
 

Jasonlewis

Recruit
Joined
Mar 27, 2015
Messages
1
Off topic and this thread is ancient. The link is removed as questionable
 
Last edited by a moderator:

Watermann

Starmada Splash of the Year 2014
Joined
Jan 12, 2013
Messages
13,747
BigB you should go out and run the ole gal and see what comes of it if you already haven't that is. Your boat looks awesome BTW! :thumb:
 
Last edited by a moderator:
Top