Lower Leg Plates.-

[Sea Rider]

As there is too much upper and lower plates controversy going on the forums as which lower plate dials top water performance have made a fond research at my boating Club about different outboards living there. 3 types were found : Old and New ones being bolt on transom OB’s in which the height can be varied by simply playing with different holes height and the Clamp OB’s which are not going to be discussed here.

As to follow a proper hydrodynamic tech conversation between parts will add real world pics and examples accordingly as the post evolves as you dislike following a conversation with simple flat drawings.

Please answer in simple terms the basic, what's there : Don't write an essay though..




(1) What are the rear openings found between middle upper and lower plates for ?

(2) What’s the front lip protruding at front of upper plate, why is it there for, what happens if It’s chopped even with lower edge located under it ?

PD : Don't quote my posts, simply write an answer on a new post as to follow properly..

Happy Boating

 

[Scott Danforth]

1 - idle relief ports
2 - leading edge of anti-splash plate. if you removed it, you will get a lot of splashing of water into the boat

 

[Faztbullet]

1) Vent for exhaust and exhaust cooling water..it is exit only.
2) Splash deflector.....If you mean chopping it even with leading edge it will result in spray being deflected upward.
3) Add that motor in pic is a good set-up...

 

[WesNewell]

(1) On my 60hp 4s merc this is where my sacrificial anode is also. Doesn't appear to one on that engine. Perhaps that's why the prop i so pitted.

And there's also the fact that neither 1 or 2 has anything to do with how high/low you should mount your engine. The lower plate and also the water intake below that is where one should be looking for proper engine height. For max performance, the lower plate should be just above the water line at wot max speed on plane. The only other concern is that the water intake should always be below the water line. On most boats, that means the lower AV plate should be even or slightly above the bottom pf the boat when av plate is inline with the hull bottom. It should never be below it. With a jack plate, add ~3/8" higher for every inch of setback.

 

[Faztbullet]

With this 2018 lower unit, which color line would be even with bottom of boat?????( I know the answer)

 

[Faztbullet]

What about this one? Which color???

 

[WesNewell]

Impossible to answer since one can't see what they are mounted to in relationship to the hull.

 

[Sea Rider]

OK guys, let some days to pass by to post your answers, so will continue with our conversation...

We all agreed that those rear leg vents are relief ports, part of the exhaust system spillway that OB's needs to count with to decrease the amount of total exhaust exiting through prop at say max wot rpm range. Those vents are present in modern 40´s bolt OB's and higher HP ones. Have also seen modern 60 HP ones without them, it's entirely up to each fabricator if adds those vents not.

One big favor, Mr.Trim was not invited to the water party, remains trimmed to 90 deg as to have hull, prop shaft and both plates riding parallel to water level at plane along a perfectly distributed balanced deck weight.

Question : when at full wot rpm range would you say that water flow at speed must cover or not those vents ? if not what's going to happen ?

Will not answer Fazt's posted pic as it's irrelevant as we will find latter when all the practice data is posted and confronted with the theoretical data and elucidate which plate achieves the best "Overall "water performance.

WN, I'm curious about (1) where did you get the information that AV plate must be set even or close to rear middle hull shape, read it somewhere, other forum, elder boaters recommendation, marina pier talk ? (2) what's your next move if you've played with all possible holes while raising, lowering the OB on transom and that weird parameter is not dialed ?

BTW, the prop shown on first pic has only paint pitting, overall alum prop material is spot on, that props needs be fully sanded, primed and re painted. Lower leg anode needs to be removed and sanded. Next month will take care of that as new owner has inherited that combo from his father in that condition.

Happy Boating

 

[Chris1956]

Gee, the idle relief ports are usually much higher on the midsection, as they need to be out of the water to allow the exhaust to exit, when engine is idling (hence the name). Om my Mercs, they are just under the bottom cowl.

The gratings above the A-V plate allow water to exit whenever the motor is running.

On my speedboat, I want the A-V plate about 1 inch above the water when on plane at 1/2 throttle or higher. Any higher and she will go a bit faster, but she requires more attention to exact trim, than I want to do.

 

[WesNewell]

WN, I'm curious about (1) where did you get the information that AV plate must be set even or close to rear middle hull shape, read it somewhere, other forum, elder boaters recommendation, marina pier talk ? (2) what's your next move if you've played with all possible holes while raising, lowering the OB on transom and that weird parameter is not dialed ?

(1) That only applies to engines mounted directly to the hull. If you have a jackplate or setback it would need to be higher as I've explained before. Where did I get this info? A lot of research and from personal experience with different engine mount heights. Mainly with the last new boat I bought. The procedure is documented here.
https://www.thehulltruth.com/boating-forum/820886-prop-selection-advice.html
(2) There is no next move for my boat. I've raised the engine as high as it would go mounted directly to the transom and it's in an ideal position now. If I ever repower it, I will have an adjustable jackplate added at that time so height will be easy to change.

 

[QBhoy]

What about this one? Which color???

Green

 

[QBhoy]

With this 2018 lower unit, which color line would be even with bottom of boat?????( I know the answer)

Red

 

[QBhoy]

Hi. Glad you started this searider. Finding it interesting. I always presumed in the average case, the cav plate or AV plate (green and red in pictures above respectively) would follow the line of the lower hull of your average boat.
After looking closer, I’m sure my boat is a bit different. The lowest point of the V at the very rear of mine is almost in line with the upper plate with trim at level....but, the exception is that this is not the lowest part of the boat from memeory. I have a flat running pad a few inches forward of the transom. The boat runs on this when at high speeds. This is in line with the lower AV plate, if not perhaps lower and just in line with the water intake ports. She will go through an impeller each season easily. See the video below. You can see it running at the upper plate cruising speed then at higher speed it runs on or below the AF plate with the prop surfacing.

https://youtu.be/MF-v2LYlogw

 

[Faztbullet]

Question : when at full wot rpm range would you say that water flow at speed must cover or not those vents ? if not what's going to happen ?​

Water DOES not cover those at speed.....and if not covered will have no effect on engine or performance. Also some 4 strokes don't have these due to exhaust system design of 4 stroke.

Will not answer Fazt's posted pic as it's irrelevant as we will find latter when all the practice data is posted and confronted with the theoretical data and elucidate which plate achieves the best "Overall "water performance.​

Why is it irrelevant ??? It shows different motors/units use different set-ups and heights depending on hull design.... and the correct answer is blue line. There is a lot more data, pictures,videos, websites,that debunk your ideas and suggestions.

Mr.Trim was not invited to the water party,​

And why not as 90% of the motors have trim and that effects attitude of boat dynamics.Trim is part of engine height set ups as when trimmed correctly you get best ride, less steering torque more efficient running angle . This leads to less hull and lower unit in water which decreases drag which improves mileage and speed....hint parallel is not a perfect running angle.

 

[Faztbullet]

Qboy… since you cant adjust your mounting height all you can due is play with props till you find one with best lift and reset your trim limit to raise drive a tad higher. When we sold Checkmates at dealer I(service mgr) would take each I/O to lake with several dozen props to find best overall performance. When trimming I used the trailer button to override the limit switch. When found best speed and angle would note gauge reading and adjust. All our boats would outperform the other dealers selling same brand and we was nations largest Mariner and Hydra-Sports,Lotus,Ebbtide,Checkmate dealer at the time. I was questioned several times at boat shows how we made our rigs outperformed other dealers.

 

[Sea Rider]

Qboy, if you happen to boat with a IN-board engine, sorry, we're discussing Outboards in which you can vary OB/transom different heights.

This issue was started moons back and there's no light at the end of the tunnel besides what each boater wants to try on its own. it's a personal choice. Some important issues that everybody is bypassing and are important to consider when sitting any OB on transom on a dry installation.

(1) It's NOT MENTIONED in any OB's Owners Manual basic Installation procedures that AV plate must set even with rear hull whatsoever, have personally read all OB brands researching 20 to 350 HP-OB's and all states heights between 20 to as much as 50 MM with AV plate to lower rear hull. This height depend entirely on the model sitting on transom. Smaller gaps for portables ones, much larger gaps for larger HP-OB's

(2) In real world wet boating these parameters heights means nothing, when boat has undergone the Hydrodynamic Lift term for Hole Shot won't know if water flow passing at speed is passing right were you guys like or not.

(3) Not all boats brands transoms are built with same height, varies from 40 to 43 cm on large ones, the ones we're discussing as of right now.

(4) Not all OB's brands have same length no matter if all are L sizes, can vary a bit between brands.

Going back to the posted pic example showing the vents located in middle of both rear plates. Made a water test to that combo on a flat straight, no wind rowing water course with 2 up. Water flow was skimming right over AV plate, this 140 HP-OB just released from full maintenance achieved 5960 wot revs.

As OB was sitting at the upper side of the swivel bracket holes was impossible to lower the height for water flow to skim right under upper plate, my fiberglass techie made a spot on transom height modification.

On exact same second test, OB lost only 60 wot revs as there is a higher water bed for prop to grip much better than if passing by around AV plate. What was achieved : (1) Noise reduction, exhaust noise when vents are air exposed is now dumped and canceled into the water flow. (2) Combo performs much better on tight close turns, on choppy, wavy water cond as there's O prop aeration in comparison to as before transom height modification.

This combo doesn't need a prop maximization, current Yam prop is revving fully towards the upper wot rpm range with 2 up and will lose slight wot rpm with 3 up which is intended to be the max boaters that will be on board for this coming summer.

Hey folks, to get the idea of what I'm trying to discuss here, forget that all of you have electric trim OB's, are shot at 90 deg or running manual trim OB's set at that given angle which is a very basic manual trim installation.

​
If you like for water flow to skim right over AV plate once on plane with deck weight evenly distributed, New Question: how will these pair of doel fins perform : Excellent, SO-SO, Awful and why...

Happy Boating

 

[Faztbullet]

Since you don't understand dynamic's you opened a new can of worms. You wasted your time,fiber tech time and customer time and money.That prop in picture is designed to run with prop tips surfacing that's why they are pointed. You buried the prop reducing its performance.

(1) Noise reduction...nope you misunderstand function of vents as more water than exhaust exit thru these at speed/RPM.
(2) In real world wet boating these parameters heights means nothing, when boat has undergone the Hydrodynamic Lift term for Hole Shot …Need you to understand proper terms..Hole shot is time to get boat from dead stop to plane, nothing to do with AV height at this time
(3) Not all boats brands transoms are built with same height, varies from 40 to 43 cm on large ones, the ones we're discussing as of right now.... No as only boats with 40-43CM transoms are johnboats 30Hp and below or race hulls in this part of world. . That 140 you did work on is not a 15" shaft as would like to see pic's of performed work.​

 

[QBhoy]

Qboy… since you cant adjust your mounting height all you can due is play with props till you find one with best lift and reset your trim limit to raise drive a tad higher. When we sold Checkmates at dealer I(service mgr) would take each I/O to lake with several dozen props to find best overall performance. When trimming I used the trailer button to override the limit switch. When found best speed and angle would note gauge reading and adjust. All our boats would outperform the other dealers selling same brand and we was nations largest Mariner and Hydra-Sports,Lotus,Ebbtide,Checkmate dealer at the time. I was questioned several times at boat shows how we made our rigs outperformed other dealers.

Yeah. That’s what I’ve been doing really. Almost there with props using the enertia just now. Maybe one more adjustment to try.
At full chat I have the trim slightly higher than the max trim setting and just into the trailer mode. Same button, just needs pressed further in. Anymore and she will drop speed and rooster.

 

[QBhoy]

Searider, I think you have to take trim and it’s effects into this. It is directly proportional almost to the level of the water slow on on the gear case surely ?

 

[Sea Rider]

Will end this post right here, Fazt being the ultra supreme boater and his posts being law will keep on demonizing, bashing, ignoring, discrediting my posts done under full practice, not plain theory. It's useless to continue posting wasting my precious boating time. So.....

-If you like boating by the parameters set when Ole Evinrude fabricated his first outboard, keep doing so.

-If you think that will have an outstanding OB/transom height match by lowering the OB when prop aerates or rising the OB when back splashes are produced at back of transom, keep doing so.

-If you like going playing with other brands props and not test OEM ones in other pitches is better, keep doing so.

-If you like boating at speed with 90% of the hull out of the water and all boaters must follow those insane and dangerous boating parameters, keep doing so.

Since Ole's times to 2019, outboards, hulls has evolved for better boating than the old boating days, but the prop forum remains boating in other planet...

Bye, bye...water is calling...