Opt Engine Transom Height.-

[Sea Rider]

Been doing a exhaustive research on several engine owner brands printed manuals regarding a forum myth near cast on stone and word of mouth recommendation passed from boater to boater through the years that states that all brands AV plates must be even or slight under keel. The research found that this stated myth mentioned to satiety is not so. Each engine brand states different heights ranging from 0 to 40 mm heights during setup installations + poor guidelines that does not say or explain much such as :

-It is your responsibility to choose a boat suitable for the engine, During operation, check to be sure the anti-cavitation plate remains under water at all times.
-The propeller axis must be parallel with water surface, Boat performance depends on outboard mounting height. Make sure the transom height matches the length of the outboard to be installed.
-Test outboard and boat performance at different heights until the best performance is achieved.

Most boaters take for granted that on initial setups all is needed is to sit a given engine brand on a given transom boat model and achieve top water performance on first outing. But years of doing lots of initial setups installations have proven that this is not so.

What does each OB brand states about AV Plate :

Tohatsu, œBe sure that the anti ventilation plate of the OB is below the water surface when running with throttle wide open.

Yamaha, œThe info is intended as a reference only. Proper mounting depends in part based on proven experience and the specific engine boat combination. Ideal is to find a spot on lower unit height that achieves the least water drag resistance when boat is on plane at speed.

Honda, The anti ventilation plate of the outboard motor should be 2.0 inches below the bottom of the boat.

OMC, The AV plate must be adjusted parallel to boat bottom, not higher than, nor more than 2 inches below it.

ETEC, Generally, the anti-ventilation plate should align with the bottom of the hull.

Suzuki, The Av Plate must be 0 to 1 inch below the bottom of boat.

Bear in mind that S-L lower units of different brands are not same in length, vary slightly from brand to brand. Same applies to boat transom heights differences varying from brand to brand. All of them concludes that top prop thrust is obtained when engine sits at 90 angle on transom and boat rides at speed parallel to water surface.

Continues on 2

 

[Sea Rider]

2-Have learned from many initial setup experiences that on real sib boating world will depend entirely on hull shape, transom height & chosen engine brand.

Strangely none of them mentions at which LU height must water flow (WF) should pass when engine is in operation, for what concern us all, operation is at plane at speed while cruising. So based on your particular experience and own frustrated water trials it’s entirely up to you to find at which best LU’s height produces a neat combination combo of (1) Least lower unit drag resistance (2) Top performance on overall all water conditions. (3) Top engine prop water efficiency & fuel economy. If you can achieve those 3 best boating conditions on first outing water trial, congrats in advance, have definitely won the boating lottery.

A local 800 Mt long rowing water course is used to fine tune engine transom heights for spot on perfect match after initial setup trials for all kind of tiller driven and remote installations in virtually all type of boat brands.



All are tested under these spot on conditions :

-Flat calm, glassy no wind water environment.
-70-80% of max HP or max HP engine transom rated for.
-Engine and prop in 100% working conditions.
-Well trimmed and weight evenly distributed on deck.
-Fresh high octane fuel.

Continues on 3

 

[Sea Rider]

3-As no engine manufacturer has provided a precise clue at which LU height must water flow pass at speed for least LU drag and best combo water performance will agree with Yamaha ‚statement, moons back under trial and error have dialed my sweet engine/transom height spot to be located between upper water deflector plate and lower AV plate. Right in middle there is a sharp edge in which water flow is evenly cut achieving the least water drag resistance on the entire LU. Seems that picking the best engine/transom height spot is a personal choice that must necessarily be picked under these parameters which you must have into total account.

At plane, water flow must pass under these 4 possible LU heights :




(1) Above Water Deflector Plate. If so will achieve water splashes over or out back transom, will need to raise engine accordingly to avoid this situation which will slow you down making near impossible for underpowered boats to plane out.




(2) Under Water Deflector Plate. Best spot on height. Will achieve top wot rpm, fast hole shot, excellent water performance on flat waters and slight choppy waters, excellent tight close turns at any speed due to having more water bed height for prop blade to bite & grip better.




(3) Between Water Def Plate & AV Plate. Good performance, less than 2, much better than 4.

(4) Around AV Plate. Worst spot performer, will produce prop ventilation on fast straight accelerations and excessive ventilation on chop or tight close turns at any speed due to AV being exposed, consequently having a poor water bed height for prop to bite & grip best compared to (2)




Continues on 4

 

[Sea Rider]

(4) Trim setting : If you vary trim angle from ideal 90°, you will experience best prop thrust loss due to varying prop’s angle of attack, achieve unwanted drag resistance on AV plate as per example. Bear in mind that water is 100 times denser than air, salt water even more. More acceleration more thrust loss and more drag on AV plate.




Once you have dialed sweet engine/transom height spot you can go for an engine prop maximization to optimize your current setup needs or transporting loads. This post applies too to better water performance of underpowered engines to some extent. For most applications, the minimum outboard horsepower rating required should provide 70-80% of the maximum recommended horsepower for good overall performance, not top performance though.

Bottom line: To get the max HP out of your engine, must go for a wot spin and “visually” check water flow passing through LU to “determine” at which of the 4 possible LU heights is WF passing by. If later need to correct height flaws raising engine by means of wooden shims, jacks or even chop transom down a bit, it’s a different story. The entire post will need to be confirmed under your own “personal practice” not assuming, guessing, asking at the forums.

http://www.youtube.com/watch?v=oBeZjaA76VM&feature=youtu.be


Happy Boating

 

[Sea Rider]

No one with any comments on this exhaustive AV plate research ? So, AV plate even with lower hull is OK with you guys ?

Happy Boating

 

[undone]

It seems like you have posted this before, that's possibly why no one has responded. All you found was possibly the correct setup for that boat, motor and prop combo for how you want to use it, not a universal solution on how to set motor height.

 

[Sea Rider]

What I'm after is that all engine brands mentioned does not speecify that AV plate height must be even with boat's botttom as this seems the general consensus rule of thumb, near cast on stone issue mentioned on the prop & non repair forums when a new poster seeks engine set up instalations advise.

Practice and not plain theory indicates that the perfect engine/transom heigh can oly be achieved under trial and error, not on a dry set up installation..

Happy Boating

 

[undone]

This seems to be an almost exact repeat of a thread you started awhile back, almost word for word. And as said then, there is no hard and fast rule that the AV plate needs to be even with the bottom of the hull, that's just a good starting point. But for people that don't want to do any testing or adjusting it's a safe place to put it.


http://forums.iboats.com/forum/engi...ard-discussions/664186-avp-theory-vs-practice

 

[Faztbullet]

This works good for clamp on engines but does not apply to all boats as hull design and hp are a big factor, the futher back the engine the higher the motor can be raised(jackplate set back or built-in step). My old Bullet the center of propshaft was 1" above pad which put the AV plate about 10" above pad and my 409DC Javlin has a 12" setback and AV plate is about 7" above pad.....below is link explaining.

http://www.bbcboards.net/stratos-javelin-boats/83680-method-measuring-prop-pad-pictures.html

 

[Frank Acampora]

Several factors that you apparently have not included in your research: 1.The anti-ventilation plate is designed with the primary purpose of preventing air from being entrained at the top of the propeller. THUS with many propellers the best operation will be with the plate at the surface or slightly below it at planing speeds. However, propeller design may influence this height and quite possibly with a given prop, height may need to be lower.
2. Since most planing hulls do not ride level to the water surface there is a void behind the transom which is filled with water rushing back in. THUS: water level behind the transom is higher than water level at the transom. Because of this factor, the A/V plate can be run higher than the transom bottom on most hulls.

3. However, since nothing is ever that simple, the only sure way to achieve best performance is through experimentation. Something that I suppose almost everyone on the forum would agree with. (I know, bad grammar, ending a sentence with a preposition)

SO: What is your point?

 

[JimS123]

There's no myth. Its all people that don't know anything or don't care.

The engine manufacturers make several different sizes....short shaft, long shaft, etc. We're all stuck with that - the length is what it is. Now, the boat manufacturers are another story. They make boats for a certain shaft length, but the transom height is not always in conformance with the 15" or 20" "rules". Are we to assume that they have design engineers that optimized the performance with a certain hull and shaft length and they made the transom height accordingly? Hummmm!!!

Being a design engineer myself, plus a boatman that spends his free time optimizing his own crafts, I have run the AV plate from 2" above to 2" below. My experience has been with OMC engines with brackets that are adjustable in 3/4" increments. Agreed, it depends on the hull. "Optimizing" is the fun part, and I can confirm that my boats were getting the most out of what I had.

OTOH, my tinnies with a 25 HP tiller outboard, clamped on with no bolts to adjust run just fine. Is it worth my effort to shim and experiment to get the best height? Personally, I don't think so. Its good enough for me. I don't need another 1.5 mph or another 0.1 mpg.

Bottom line is that most people just don't care. So, just hang it on and get what you get. If you're in the 99.9 % of the guys with a boat you probably won't even read this thread.