anti ventilation plate question

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Good Morning Boater,

Glad to read you enjoy reading tech boating related issues. Check this, is my UK version, the US version was so plagued with nonsense discussions that is no bueno to read.

http://www.rib.net/forum/f50/abc-sib-rib-installation-guidelines-58373.html

Read complete thread to have full knowledge of what I'm trying to school. Which type of boat and lenght are you running including engine HP ?

If going for the transom optimization, seat a mate up front to better ballance boat, Don't shim engine prior making the test, need to determine if in need of a transom opt or not.

Happy Boating
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
This is another tricky example of correct engine/transom height marriage of a hard hull deep V keel boat. Don't see AV plate even with lower keel, it's 2" under. Where do you expect for water flow at plane and speed be passing by : AV Plate, middle or WFDP height ?



Just want to show that AV plate even or sligh under keel is pure you know what.. This nearly cast on stone method does not takes into the least consideration, hull shape, height, width, boat weight nor transom height to blindly assume that formula is bullet proof and everyone must follow blindly.The answer is well documented, it's the only way to assert boaters the misunderstood topic I'm trying to school since immemorial times.

Happy Boating
 

boater1234

Master Chief Petty Officer
Joined
Jan 6, 2010
Messages
869
Well,I have good news,my local lake is open now,it was closed due to the water being to high and the ramp was completely covered in water so they shut it down until the water dropped like it has now,I'm going to drop it on the transom and water test it with me and my friend at 200lbs. There will be times I go by myself to,I do understand these light boats are very sensitive to weight and I completely understand I have to shift most of the weight to the middle and front of the boat. I used to put an extra cooler in front of my 1436 with 2 cinder blocks in it to counter the weight,lol. Worked like a charm. Most if the weight will be in the front. Like I said before,every jon boat has been like this,below the keel and it has ran great,so I may be over paranoid over nothing. I will find out next week. I'm busy all 4th of july weekend,got a big fireworks show to put on,lol. I will read the thread though,ty for your help as of now.
 

boater1234

Master Chief Petty Officer
Joined
Jan 6, 2010
Messages
869
So curious question Sea Rider,do you think I should go back to the 20hp since it will be just me or me and a friend at the very most? That would be near 40 less lbs in the back of the boat. Overall I believe I wouldn't lose to much top end speed from a 25 to 20hp. I don't know for sure though. I have to test the 25 first to see what type of speed I get. I do know the 20hp efi suzuki is a little power house for it's size. Keep in mind the boat is really light.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
I'm beginning to lose details on this long thread. Which boat is it, a hard hull, an inflatable. Need boat type and size and max rated HP on transom.

Happy Boating
 

boater1234

Master Chief Petty Officer
Joined
Jan 6, 2010
Messages
869
1648 alumacraft flat bottom jon boat,290lbs. It's a light boat. Rated at max 35hp which is stupid because no one makes a new 35hp engine. Most people go with a 15-25hp on these boats that I've seen.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Did you fully read the posted link, assume not. This is the short cut, try to achieve same cond.

https://www.youtube.com/watch?v=oBeZjaA76VM

If water passes under small water deflector plate or middle leg it's ok must not pass close to lower AV plate or will achieve excessive aereation at tight close turns and choppy sea cond. If water flow passes above small water deflector plate will need to shim emgine untill over transom splashes disappears.

See which of both stated engines matches best the sweet engine/transom heigh as seen on vid. Assume that's a Jon boat, right ?Let me know.

Happy Boating
 

boater1234

Master Chief Petty Officer
Joined
Jan 6, 2010
Messages
869
Went to home depot and bought a 1/2 inch shim as well as a 1/4 inch with it just in case I need to use them. They are oak shims. I was told I could spray the shims with rustoleum clear coat paint for water abd uv protection. Does that sound ok if I need to waterproof and protect it from the uv rays? Yes it's a jon boat with a tiller engine. I'm going to mount it on the transom and go for a wot spin here soon since my local lakes boat ramp opened back up. If it performs well then I won't mess with it. If it's slow and shoots water back up the transom like your saying then it needs lifting up with shims. Every jon boat I have ever owned ran great with it the same as it is now so it may run great again. During the fall here in Florida I'm going to mid the front of it a bit to counter the weight from the back,that should help tremendously. I'm using a 12 pitch prop which most use with light loads. To me this test Will be lightly loaded at or near 600-700lbs. With me,friend,and motor is all the weight. This boat with this 25 should fly near 30+mph give or take a mph. I do know with a 20hp if I decide to go back to that I would lose some top end speed,maybe 3-5mph but I would also shed 40lbs of weight in the stern that's why I asked you your thoughts on going back to the 20hp. With 2 in the boat 24-27mph is a reasonable number I know I could hit but 30+ mph sure is tempting,lol. Only one way to find out,run it. The only reason I said I was going to shim it was so many people with these small boats have raised the motor to dead even with the keel and got great results,that's why I stated that.
 

ondarvr

Supreme Mariner
Joined
Apr 6, 2005
Messages
11,527
The shim doesn't do anything except make it easier to keep the motor vertical as you tighten the clamps, it can be removed or left in place, it makes no difference.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
If you need to shim engine up to achieve same cond as on posted vid, you should glue shim onto transom to sit there forever once proper shim height is dialed. The guy on the vid used a 1/2 inch shim to be at the sweet spot and that vid was shot at only 1/2 throttle as engine was undergooing its break in period.

You need to ordder a TT or Hardline induction tach with programmable firing sequence to test both engines at wot, starting with you alone and lightly loaded boat, then with 2, one sittting up front to balance boat nicely. Trim must be set to 90? Depending which engine suits you boating needs the best, you can always maximize propulsion by playing with prop pitches, but need a tach for it, ear taching is no bueno...

Happy Boating
 

boater1234

Master Chief Petty Officer
Joined
Jan 6, 2010
Messages
869
3/4 inch shim is what he advertised on YouTube. I got oak shims like jim told me to get. I'm going to test it out soon and report back.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Yep sorry, the issue about buying 1/2 and 3/4 pre cut shim sizes is that probably will need a middle size right between both shims. On the example seen on vid which is the way to go, engine still needs to sit a tad lower till water flow skims flat right under upper plate. Probably 1 Cm less height will do the trick, can file it down at a wooden carpentry. Shim must be the same width of the transom.

Other tech issues to have into account is that all brands short or long shaft OB's are not same lenght, usually some bit longer than others, or shorter than others, depends on which side you want to look it from. The vid is a exercise dialing sweet engine/transom height for a Tohatsu OB which are natural born water splashers compared to Yam or Susy. Will post other example about ideal engine/transom height of other OB propelling a deep V keel hull boat.

Happy Boating
 

boater1234

Master Chief Petty Officer
Joined
Jan 6, 2010
Messages
869
I bought a 1/4 inch oak shim as well as a 1/2 inch oak shim at home depot. They are perfect as far as width on the transom. If I need a 1/4 I have it,half inch,I have it,3/4 inch,I have it,if I need a full inch I just double the 1/2 inch shim and I'm good. I have it all covered. I do notice both of my suzuki s have large splash plates as well as very large anti ventilation plates,I don't know if that's good or bad but I noticed they are bigger then any outboard I have owned.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Much better than having shorter ones on old outboards, a larger AV plate should provide better boat handling when AV plate rides parallel to water level at plane, provided your engine is 90? trimmed and boat well balanced.

Happy Boating
 

boater1234

Master Chief Petty Officer
Joined
Jan 6, 2010
Messages
869
What in your opinion would the best hole to run the outboard on. Most people say from all I have read the 3rd or 4th hole from the bottom up seems to work best for these type of boats. I do know I need to play with the engine a bit and start with the pin all the way in and work my way out till I get the best performance possible. From some reading I did it says jon boats with tillers run and handle better with the prop being a hair deeper in the water,I don't know if there is any truth to that but I do know when it gets rough out on the water all my jon boats handled great and ran fast. I know when you have a 2 foot chop there isn't much you can do but slow down to avoid a disaster. With my 1442 jon boat I had and a 25hp 2stroke merc the boat did near 30mph with me,ex wife and daughter on it. Motor was straight on the transom also. Same boat with a 15hp 2 stroke Yamaha,that boat did near 23 mph with the same weight,motor on transom also. Last boat was 1436 with me and my friend and the 20hp 4stroke suzuki,that did near 27mph with a 11 pitch prop,motor on transom again. Now this boat 1648 with a 25hp efi suzuki should top out at 30+ mph with the 12 pitch prop I have on it. I do need to buy a tiny tach soon. If I get 30 mph or better with me and my friend with a plywood floor then I'm happy and will leave it alone. I'm going to use some 2x6s under the front plywood floor to weigh the front down to even the weight as best as I can.
 
Last edited:

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Boater,

If you don't fully read the posted link sent on post N? 41, at this pace, you'll be wild guessing and losing precious boating moments till next summer. There's all the ABC data you need to know about which hole is best. Useless testing all the trim holes, trim don't play well while matching sweet engine/transom height. Float boat alone, trim engine to a hole (usually 2-3 out transom) for engine to achieve a perpendicular 90? deg position while AV plate rides parallel to water level when boat is on plane.

The engine seen on Jon boat vid sits as described and performs top while opt height seated, don't give for granted what other guys says about Jon boats installations. What works for them probably won't work for you, to each his own set-up.

Happy Boating
 
Last edited:

DECK SWABBER 58

Lieutenant Commander
Joined
Aug 14, 2009
Messages
1,913
Clicked on this thread because this is a topic I have researched here a lot for motor mounting on both a jon boat and a pontoon.

boater 1234 please don't take this the wrong way but my goodness their has been 4 pages of posts and the boat has not even
been in the water yet. This is not rocket science, an inch or two is not that big of a deal. On my new jon boat with a 9.8 Tohatsu 4-stroke
I made sure the plate was "about" even with the keel, put your boat in the water, go fishing and have fun. I think you might be over thinking
this just a little bit.
 

boater1234

Master Chief Petty Officer
Joined
Jan 6, 2010
Messages
869
I never take things the wrong way,you are correct,I need to get it in the water. You also contradicted yourself in a way,you said one or two inches is no big deal but then you go on to say well I made sure mine was even with the bottom of the boat. If one or two inches was no big deal then why make sure it's even with the bottom of the boat as you say,makes 0 sense. I do like I said agree with you,I'm mounting the motor back on the boat to go test it. I'm a heavy fellow the the back of the boat is going to sit low in the water so imo I think getting the av plate even or closer to even with the bottom can't hurt and only help it. I know I need to test it for sure but I'm going to shim it because I feel it can't hurt it,may not help much either but hey less drag is better than more drag,right? I will even out the weight as much as possible. I haven't seen anyone yet who said raising the motor will hurt it but I have seen tons of threads were it says the starting point for most boats is even with the bottom. For me your probably right,I am over thinking this whole thing and I'm going to do it the way I believe it will work best,by making the av plate at least even with the bottom of the boat. It can't hurt and only help if anything. I'm done posting till I test the boat.
 

boater1234

Master Chief Petty Officer
Joined
Jan 6, 2010
Messages
869
Well,I went back to the 20hp. It's close to 40lbs lighter,easier to start manually and easier to tilt on the boat. Top end is fairly close on both of them. With a 12pitch prop on the 25 and one person,i hit 30mph,with 2 I was at 28-30mphdepending on water conditions. Really not to much different between 1 and 2 people,never tested 3 but I would guess that's were the 25 would come in handy with the extra power over the 20,I will almost never have 3 full adults in the boat anyway,it will be rare. Now with one in the boat and motor with gear it was near 28mph most of the time,got on Plane just as fast as the 25,it was like a 1-2mph difference between the two.2 people and gear was between 27-28mph,so it was a very slight difference of 1-2mph. Got us on plane in seconds. The 20really surprised me how powerful it is. It was so much easier to work with the 20 and it also sits much higher in the water in the back due to being muchlighter. I have a 3 gallon tank instead of a 6 plus the 20 over the 25,I'm probably shaving nearly 50-60lbs off the back of the boat. The ani ventilation plate also is a bit higher on this motor compared to the 25,don't know why but I won't complain,lol. The av plate is right around 1/2-3/4 below the keel instead of being close to 1 1/2-2 inches below so if I want to raise it just an inch I would actually be above the keel a bit which would probably put the av plate skimming the water as it should. The numbers I'm gettin are outstanding though so I may just leave well enough alone. Maybe over the winter I may tweak a little and play with the height to see if I can any difference at all. It sips gas, a 3gallon tank lasts a long time,I can go up and down the gulf coast,say 10-15miles back and forth for 20-30 miles round trip and it uses maybe a half tank if that,maybe a quarter,thats wot most of the time to. So all in all I'm happy and this is my motor for many yrs. The only motor I would trade this out for is a really nice flawless 25hp 2 stroke yamaha,I love that motor,light at 105lbs and a power house as well as bulletproof. So we shall see how the durability of this motor is over the yrs.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Need to rise the engine only if having back or over transom splashes, if none, Just leave it there. If water flow skims right under AV Plate will have excessive aeration at close turns and while on choppy windy water cond. At that height only goof for straight water courses at speed on flat calm waters. Now you know that all lover legs are not built same, some are shorter, some longer, depends on brand.

Happy Boating
 
Top