Opt Engine/Transom Height

[Sea Rider]

Was wondering how many of you assumes, have experienced over or back transom water splashes, prop ventilating on choppy waters or at tigh close turns at speed ? Have you been in need to adjust trim angle to avoid any of those mentioned issues, did it work for you ?

Happy Boating

 

[Sea Rider]

Being this theme new for some sibbers and oldies as well and probably not knowing much of what they really need to be acquainted for, wanted to share this tested & proven guidelines with them in case they want to try it out for themselves during initial setups or optimizing old setups to fine tune top combo water performance.

Have done a exhaustive research on several engine owners 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 + some very poor guidelines that does not say or explain much.

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

Yamaha, The 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 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 the boats bottom. The AV must not be higher than boat bottom, nor more than 2 inches below it.

ETEC, Make sure the transom height matches the length of the outboard to be installed. Generally, the anti-ventilation plate should align with the bottom of the hull.

Bear in mind that S-L LU of different brands although being same size are not same in length, vary slightly from brand to brand. Same applies to transom height differences from brand to brand. All of them concludes that top prop thrust is achieved when engine and AV plate sits at 90 angle on transom while sib rides parallel to water surface, provided sib weight is evenly distributed on deck.

 

[Sea Rider]

Have learned that on real boating world will depend entirely on tube diam, transom height, engine brand if sitting any of the mentioned brands on any given sib brand.

Strangely none of them mentions at which LU height must water flow (WF) be passing when engine is in operation, for what concern us all, operation is at plane at speed while sibbing. So based on your particular experience and own frustrated water trials. Is entirely up to you to find at which best LU height produces a neat combination of 3 in 1 combo : Least lower unit drag resistance, top performance on overall all water conditions and engine top prop efficiency on your current setup. If you can achieve those spot on top boating conditions on first outing water trial congrats in advance, you have definitely won the lottery.

 

[Sea Rider]

My local boating club has a 800 Mt rowing water course, if in need to fine tune engine transom height for a spot on perfect match after initial setup, we use this nice facility. Most tested sibs were wooden, alum, flat air decks versions, tiller driven engines and manual trim up to 30 HP engines. All sib/ribs were water tested under these strict lab conditions :





-Flat calm, glassy no wind water environment.

-Sib tubes & keel, min 3.0 PSI with gauge.

-Tiller driven engine trimmed to 90 angle.

-Mostly S-L 18 HP Tohatsu engines.

-Engine and prop in top working condition,

- Driver/spotter at throttle, balancer mate up front.

-Fresh high octane fuel.

- As no one engine manufacturer has provided a real clue at which LU height must water flow pass at speed for best combo water performance, will stick to Yam statement, moons back under trial and error have dialed the sweet engine/transom spot, is located between upper water deflector plate (WDP) and lower AV plate. Right in middle you have a sharp edge in which water flow is evenly cut achieving the least water drag resistance on the entire LU.

It's impossible to get rid of the water drag resistance as water is 100 times denser than air, but could be minimized while passing through that sharp large edge.

 

[Sea Rider]

At speed water flow will pass under 4 possible LU heights :



Right above water deflector plate. If so will achieve water splashes over & out transom, will need to raise the engine accordingly to avoid this situation which will slow you down making near impossible for sib to plane out on those conditions.



(1) Right under water deflector plate. Best spot on height. Will achieve near top wot rpm, fast hole shot, excellent water performance on flat. choppy, windy waters conditions, excellent tight close turns at any speed due to having more water bed height for prop blade to bite & grip top.





(2) Right in middle, good performance, better than 3, but less than 1.

(3) Right over AV plate. Worst spot, will produce prop ventilation on fast straight accelerations, choppy waters and excessive ventilation on tight close turns at any speed due to AV being exposed out of water and having a poor water bed height for prop to bite & grip top compared to (1)




A prop is a pull/push screw water device, pulls from one side and pushes from the other, needs to work with a correct water bed level for prop angle of attack to do his homework efficiently providing engine sits at 90 angle on transom.

 

[Sea Rider]

Trim setting : If you vary trim from ideal 90 angle you will experience best prop thrust loss due to varying the prop angle of attack and achieve unwanted drag resistance on upper or lower AV plate as seen on pic.




Once you have dialed sweet engine/transom height spot you can go after a prop maximization to optimize combo performance to suit best your particular needs. This applies too to better water performance of underpowered engines to some extent. For most applications, the minimum outboard horsepower rating required should provide 70% of the maximum recommended horsepower for that particular boat to move up to medium loads,

Does not really matter which sib/engine brand you have purchased, try to match engine/transom height for water flow to pass skimming right under water deflector plate. Have tested all our sibs/ribs to match that spot on condition, then state of the art factory height modified to match Tohatsu & rebadged engines which are the one that gives the most water issues compared to any other engine brand tested on same sib/rib










As stated earlier for best engine/transom match to happen, must go for a water wot spin and visually check water flow height passing through LU to know which of the 3 possible LU heights conditions you have. If in need to raise engine by means of wooden shims or chop transom down a bit, is a different story.

Bottom Line : Bear in mind that sibs have more hull drag than ribs, so the overall idea of this post is to take the max HP out of those portable engines for sibs to perform at their best.

Happy Boating