Myths & Mismatches.-

[Sea Rider]

Read repetitive posts of newbie boaters requesting info about which should be the correct OB height when combo undergoes its first dry installation process. The Automatic Rule of Thumb Answer is : “Even With Hull” (EWH)
To start with this rule ain’t such, all OB’s brands Owner’s Manuals state different height parameters, which range from 10 to 30 mm for portable to medium HP OB’s and from 30 to 50 mm for higher end HP-OB’s.



On dry installations this rule of thumb is useless, will not indicate at which lower leg height will water flow pass at speed whatsoever and that’s important to know in order for combo to perform top. One story is combo resting on top trailer on land, other much different when combo it’s on plane gliding on water level. It's about time tu cut this automatic nonsense response given to new boaters forever.

Things you didn’t know about that aren’t mentioned anywhere on this planet but are important to know, mentally dial & understand :

-All boat’s transom heights are not built exact same. Short ones can range from 38 to 40 cm, Long ones from 50 to 53 cm. It’s all down to what the boat manufacturer wants to put on each boat model.

-All OB’s brands lower legs are not built with exact same lengths, doesn’t matter if comparing S, L, sizes between brands. The given length will depend entirely on each OB manufacturer.

In conclusion : Both OB/Boats manufacturers doesn’t care the least which OB & Boat brand you buy. Just sit OB on transom and voil?…

As seen, there’s a complex OB/transom height divorce unknown for the majority of boaters that will pop up on some of you. So to avoid commercial conflicts, suits among OB/Boats Manufacturers, Dealers & End Boaters, OB manufacturers only state AV Plates/Hull Heights parameters as a precaution and wash hands as Pilatus did.

If you sit any OB brand on any boat brand transom, luckily some combos will achieve opt water performance, but many won’t due to lower leg, transom heights stated mismatches. So, some leg brands will work further down, others further up on water when OB it’s at plane. A combo just sitting on water means nothing, once on plane it’s everything.

Happy Boating

 

[achris]

Without being editted, sorry, but that^^^ is hard to read...

Bottom line (as a former shop owner/Dealer/Mechanic/boat owner and user).... The plates above the Anti-vent plate are irrelevant. The measurement above the plate don't need to be considered. So, the makers could put plates all the way up the drive shaft housing, once (the boat is) on the plane, they aren't relevant.

Engine makers also have multiple mounting holes on the transom brackets, for this very reason. I just recently had a fellow who'd bought a new boat.I looked at the installed height and we moved it up one hole. He said it performs so much better. That's the point of the holes. Moving an outboard up or down a hole or 2 is a very simple job, and it's no big deal if the dealer/installer put it in the wrong place. Where it gets critical is with stern drives. Not so easy to move them up and down.

To answer the last paragraph (even though it doesn't make a lot of sense the way it's written)... What happens when the engine is installed too low? Water hits the drive shaft housing higher than it should. A lot of water is sprayed up and hits the transom, creating even more spray. More engine in the water also means more drag, less speed.

The one time the EWH rule MUST be ignored is when one of those stupid hydrofin things is fitted. When one of those is fitted, the engine MUST be installed at least 1" above the bottom of the hull. If not, it will cause the boat to 'bow steer' at high speed. It will also run the risk of causing serious structural damage and broken parts on the engine. And no, I am not making this up!

Chris.....

 

[jimmbo]

The one time the EWH rule MUST be ignored is when one of those stupid hydrofin things is fitted. When one of those is fitted, the engine MUST be installed at least 1" above the bottom of the hull. If not, it will cause the boat to 'bow steer' at high speed. It will also run the risk of causing serious structural damage and broken parts on the engine. And no, I am not making this up!Chris.....

Well said. Those things are the worst of the dumbest ideas for a boat.

 

[achris]

Well said. Those things are the worst of the dumbest ideas for a boat.

They do serve a purpose, but they are not being sold to serve that purpose. They are being sold as a 'quick fix' to try and cover up other issues, usually the boat is underpowered, or overloaded.

Chris......

 

[Sea Rider]

Hey Gents, hold your horses, all comments were reffered to OB's and that is just half of the problem. What about other half, boat's transom heights, are all built with same height ? the answer is NO, have measured transom heights of different boats brands lot of times and don't match same, same happens with OB's. For the moment will leave this part right there to focuss in a misunderstood part that's way overlooked.

I'll ask a simple question : If nobody gives a you know what, then what's the sole purpose of the upper plate, mere adornment, looks cool, perfect to climb onboard from rear leg, what if it's cut even to match middle leg contour ? roll your wildsest theoretical tech answer to evaluate your individual boating expertise.

Happy Boating

 

[achris]

Differences in transom heights is acknowledged by the engine makers. That's why you get 4 or 5 holes to mount up to. Once up on the plane, those upper plates are irrelevant. They do keep the spray down at lower speeds, that's all. Their varying spacing are meaningless.

You are overthinking a simple process. Mount the engine with the lowest plate level with the bottom of the boat. Take it for a test run and adjust, if necessary, accordingly. There is no need for reams of theory on this.

Chris

 

[ahicks]

SR, it's OK to ask questions. You don't need to try coming off as an expert. It's plain to see you shouldn't be too concerned about that.

"roll your wildsest theoretical tech answer to evaluate your individual boating expertise."

Chris' patience dealing with a question like that is to be commended. I'm pretty sure his doing so is about sharing experience - not to be judged or have his boating experience "evaluated".

 

[Sea Rider]

Sorry for using the incorrect wording, it's not to evaluate, it's rather to see how you address things as each one addresses things much differently, For the question as of "At which lower leg height should water flow pass at plane remains unansewered seems will remain at Limbo Land foerever and was key to comprehend what posted and if in agreement with the post or not. For me it's a closed topic.

Happy Boating

 

[jimmbo]

Depending on what prop is used, the engine height can be raised until the prop loses bite or the water pump is starved of water, whichever comes first. Some gear cases have the water inlets much lower or alternate ways to feed water to the pump/engine are engineered, so it then becomes prop bite

 

[blecina]

From my experience it's much easier to move engine up than cut transom down, especially in aluminium boat. What needs to be remembered is that engines are not run in position from 1st post. When engine is trimmed further aft, the lower unit goes up as well. There is also a transom angle variable - if you have 15 degree angle you can mount bit higher than on 10 degree because of more setback. So generally speaking it's impossible to make all-round height, because each setup is different. Manufacturer provides "safe" solution that prevents engine damage and good boat control. It's user or dealer role to adjust this mounting properly.

 

[jimmbo]

When engine is trimmed further aft, the lower unit goes up as well.

That can vary depending where the tilt pivot point is. If the pivot is located forward of the transom/bracket/jack, the lift is greater than if the pivot is directly above the transom/bracket/jack.

 

[blecina]

That can vary depending where the tilt pivot point is. If the pivot is located forward of the transom/bracket/jack, the lift is greater than if the pivot is directly above the transom/bracket/jack.

To be more specific it is directly proportional to the distance between horizontal and vertical axis of movement.

 

[Faztbullet]

Transom heights have little to do where motor is mounted...its the hull design and what usage that matters. Sking/tubing motor needs to be deeper,just fishing cruising it can be higher. A deep V needs motor deep,a pad boat mounts higher(split water intakes),round bottom a medium mount,Tunnels and air trapping designs needs a high mount.Aluminum hull with ice runners a tad deep and flatbottoms at AV height. Starting at AV height works with 90% hulls on market to get a fellow started and then it fine tuning from there. Install a jack plate/gill bracket and engine can go even higher. Example: The 17' Bullet I had was on a 10" set back and propshaft was even with bottom pad or about 10" above transom..why??? at a regular setup turned a 26p 6400rpm, raised and went to 28p at 6100 rpm @79mph(GPS) full load

 

[ondarvr]

Sea Rider does a pretty good job of communicating even though English isn't his first language, so I commend him for that, I can't communicate in any other language, and neither can most people here.

​His ideas and comments are what can be confusing, he posts these threads from time to time about motor height that don't really have any foundation in fact or the correct setup procedures. Much of it seems to be gleaned from mounting motors on under powered inflatables, so it doesn't transfer that well to larger boats with much higher HP.

 

[jennyo]

Mod EDIT: CONTENT REMOVED!

 

[Sea Rider]

Was not going to respond to newer post about these boat/transom mismatches as you like watching your side of the coin, that's large boats with large HP-OB's with electric trim. The issue with big shot boaters is that they ignore or don't care the least at which lower leg height water flow passes by and this can make a night & day boating. if having prop aeration or back water splashes, it's temporal fixed it playing with trim and power. This lessens the issue to some extend but masks the problem which traduces in more fuel consumption plainly checkable in long distance cruises.

My confusing ideas, pics are excerpted from OB's Owner's Manuals, General OB's Installation Guidelines and Marine Literature, being the latest ones from 350 HP-OB's installations. Were tried under theory and practice in innumerable different boat type set-ups. Works spot on on small, medium and large combos powered with different OB brands as many boaters have proven it.

A boat is a a powered hull no matter if small, medium or large, must have a well dialed OB/transom height match in order to achieve the least water drag on lower leg, no back water splashes or prop aeration whatsoever, add prop maximization to that equation and will achieve excellent hole shot, good top end speed and nice fuel consumption. Did you know that fuel consumption and millage on a 2 strokes OB is 4 to 1 compared to a car millage.

These lower pics are examples of short, medium and large boats running portable, medium & high range OB's just seated on top transoms and voil? while guzzling at speed more unnecessarily gallons of fuel than if they had well height seated OB's on their respective combos.





This pic corresponds to a Sundancer boat with a 200 HP Merc, owner has played with all +,- trim settings along weight distribution throughout the entire deck with no possible solution. It's hard to plane out, once on plane & speed can surf at wake side or have a shower at back transom. Guzzles gallons of fuel due to excessive lower leg drag, LU sits way low. Owner ended selling the combo 2 month ago, new owner couldn't solve the excessive back splash issue, guess what, ended returning the combo to original owner. Very bad music along bad business..




Will you keep ignoring after watching these pics that there's NO transom & OB height mismatch between boat brand and engine brand fabricator, that all transoms have exact same height measurements and that all lower legs have same length ? Been doing OB/transom height optimizations for 10 years and prop maximizations for 4 years now on all kind of boats. Happen to run a small marine business with highly skilled fiberglass technicians and can't possibly delivered any combo to any new owner with a badly height seated OB.

To end this endless post due to having too much tech discrepancies between parties, let's keep following our own OB height installations & parameters and live happily with it. For me this post is closed...

Happy Boating

 

[Sea Rider]

Was not going to respond to newer post about these boat/transom mismatches as you like watching your side of the coin, that's large boats with large HP-OB's with electric trim. The issue with big shot boaters is that they ignore or don't care the least at which lower leg height water flow passes by and this can make a night & day boating diffrence. if having prop aeration or back water splashes, it's temporal fixed it playing with trim and power. This lessens the issue to some extend but masks the problem which traduces in more fuel consumption plainly checkable in long distance cruises.

My confusing ideas, pics are excerpted from OB's Owner's Manuals, General OB's Installation Guidelines and Marine Literature, being the latest ones from 350 HP-OB's installations. Were tried under theory and practice in innumerable different boat type set-ups. Works spot on on small, medium and large combos powered with different OB brands as many boaters have proven it.

A boat is a a powered hull no matter if small, medium or large, must have a well dialed OB/transom height match in order to achieve the least water drag on lower leg, no back water splashes or prop aeration whatsoever, add prop maximization to that equation and will achieve excellent hole shot, good top end speed and nice fuel consumption. Did you know that fuel consumption and millage on a 2 strokes OB is 4 to 1 compared to a car millage.

These lower pics are examples of short, medium and large boats running portable, medium & high range OB's just seated on top transoms and voil? while guzzling at speed more unnecessarily gallons of fuel than if they had well height seated OB's on their respective combos.





This pic corresponds to a Sundancer boat with a 200 HP Merc, owner has played with all +,- trim settings along weight distribution throughout the entire deck with no possible solution. It's hard to plane out, once on plane & speed can surf at wake side or have a shower at back transom. Guzzles gallons of fuel due to excessive lower leg drag, LU sits way low. Owner ended selling the combo 2 month ago, new owner couldn't solve the excessive back splash issue, guess what, ended returning the combo to original owner. Very bad music along bad business..




Will you keep ignoring after watching these pics that there's NO transom & OB height mismatch between boat brand and engine brand fabricator, that all transoms have exact same height measurements and that all lower legs have same length ? Been doing OB/transom height optimizations for 10 years and prop maximizations for 4 years now on all kind of boats. Happen to run a small marine business with highly skilled fiberglass technicians and can't possibly delivered any combo to any new owner with a badly height seated OB.

To end this endless post due to having too much tech discrepancies between parties, let's keep following our own OB height installations & parameters and live happily with it. For me this post is closed...

Happy Boating

 

[Faztbullet]

The picture of the Sundowner is a bad example as customer is trying to make boat do something its not designed for...Note the wake tower that was added so to pull skiers. The engine is mounted lower thus more drag in water and Av plate causing the shower effect. all due to boat having a V pad