Proper outboard engine height.

Wild and Free

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I hang out on several other boating and pontoon sites and this comes up almost daily and this is about the best and simplest description of where to start with boat performance issues. I copied and pasted the below info from a verado forum for you to quench your intellectual thirst for knowledge.Thumbs Up

Bill

THIS APPLIES TO EVERY ENGINE AND BOAT AND IS NOT VERADO SPECIFIC

Before you start messing with props, you must get the engine height right. New props are WORTHLESS if they are not given the opportunity to do what they are designed for. And your engine height is probably not at optimum now. It is due to a little known fact about how mfr's rig: they "bury" them a little. Most mfrs rig the engine lower than optimum so they can keep themselves out of liability due to spinouts or blowouts. And raising to optimum won't cause blowouts. Forget all that stuff about using rulers and how far the hull is above/below the prop/plate. It's easy to tell if your engine height is right: with normal load (50% gas) ,get someone to take it up to cruise, trim it normal, and go back and look at the big anti vent plate above the prop on the lower end. It should be OUT of the water at speed, getting splashed is ok, but out of the flow. If it's totally dry in all seas, you're probably too high. If it's "buried" under the water, you need to raise your engine and then start testing props. A buried cavplate is like dragging a bucket on a rope behind your boat: total drag that hurts performance and puts pressure (pull) on the transom. Sorry, not an artist but a really crude pic might help:
prop-1.jpg


An additional advantage in raising is the powerhead 1 -3 inches higher off the water is it reduces the salt/water mist, that gets in the motor and causes corrosion.
Courtesy of Ken at propgods.com, here's an E-tec that is "buried":

toolow-1.jpg


and here's the same engine mounted at the proper height:

correctheight.jpg


a Verado at proper height:
correctheightVSmall.jpg


and how the merc racing team sets the height - a perfect shot of the cav plates getting splashed, but not in the flow. This is not a "racing" setting, this is how everything from a 12 foot to a 42 foot boat should be:

merc350-1.jpg


Now back to propping....

Someone writes:

“I don’t think I really need to test my props by going to WOT, I don’t run fast and just like to cruise”

A common misconception is if you are not a full throttle user (WOT), that the props that came with your boat are the proper ones. Running at full throttle is not something that any of us do often, but just like your car transmission selects the right rpm, a good prop does that too. Too big a prop and you will be “lugging” the engine in cruise. But the testing requires WOT (wide open throttle) to see if you are propped correctly for long engine life.

Here is how to tell if you are propped correctly. This may rattle you to cavitate your engines but it'll be OK....

1. Load the boat down with 50% gas and a light load for fishing cruising, etc
2. Find a very flat piece of water a mile long and bring your trim tabs all the way up.
3. Trim your engines "in" and take it up to full WOT and start trimming up
4. Keep trimming up SLOWLY a bit at a time, letting the boat catch up to the setting...until the engine starts cavitating - losing speed ( you can't hurt it - all new engines have rev limiters)
5. Now drop it back down a bit until it "bites"
6. Now look at your rpm...- it should be in the top half of the mfr's recommended top end rpm. For example, the Verado’s range is 5800-6400 rpm. Therefore I should be at least 6100, 6250 would be ideal. But don't get hung up here, anything over 5900 rpm is ok, you won't be hurting the motor. A few more hundred rpm may get you a bit more speed though. But less than 5800 is going to do long term damage to the motors. It will cause the motor to add gas to the oil, raising the oil level and diluting the protection the oil gives the motor. Eventually the crank will fail and it will not be warranty.
7. Fill out the PDF sheet below so you can compare after you are completed.

compare the results on a prop slip calculator - at WOT and cruise you should be less than 10%---> go here: http://www.rbbi.com/folders/prop/propcalc.htm

It should be noted that prop changing is not a magic cure-all - all you can do is try to get your props up near the upper range of mfr WOT and that's about it. Too many people think changing props is like adding Nitrous or 50 more HP...not so....it's fine tuning in my opin, and for me never been worth more than a few mph or gph when changing a pitch up or down...However, many props are different.... some are smoother than others..others allow you to dock better ....it's not just all about hard statistics....I personally run a prop that gets a bit worse mpg and less top end but it "feels" better than all the others I tried....Don't get hung up on specs searching for the "holy prop grail".

Size depends on what you find above, but three of the best props for < 35 feet are Merc’s Tempest Plus, the Revolution 4 (4 blade) and the new Enertia. Most boats come with a Mirage plus as it is a good all purpose prop.

Discuss your results with our prop expert here - Ken Reeves of Propgods.com. See his site.
 

Attachments

prop.jpg toolow-1.jpg correctheight.jpg
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Petras

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yes sticky would be great... just got my new boat home last night and it has a manual jackplate on it so i will be doing some tweaking on it. this was a good read
 


sl1000794

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If you want the thread, you could bookmark it into your fishing bookmarks or if you just want the propellor calculator you could just bookmark that website.

Steve.
 

guywhofishes

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So cavitation is only an issue when accelerating onto plane but not after?

- - - Updated - - -

Also - the plate is "knifelike". Don't understand the dragging a bucket reference when the plate should be cutting through the water.

Not disagreeing... Trying to understand.
 

sl1000794

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So cavitation is only an issue when accelerating onto plane but not after?

- - - Updated - - -

Also - the plate is "knifelike". Don't understand the dragging a bucket reference when the plate should be cutting through the water.

Not disagreeing... Trying to understand.

Maybe when you are running on plane the outboard/inboard lower unit is trimmed up so the plate is no longer parallel to the boat's travel and is essentially pointing slightly down and acting like a diving lip on a crank bait.

Steve.
 

TFX 186

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So cavitation is only an issue when accelerating onto plane but not after?

- - - Updated - - -

Also - the plate is "knifelike". Don't understand the dragging a bucket reference when the plate should be cutting through the water.

Not disagreeing... Trying to understand.

Guy
It's more of a push/pull on the motor. If trimmed too much one way it pushes your motor up. If trimmed too much the opposite way, it pulls your motor down. Hit the sweet spot and it should run smooth. I kind of liken it to when you were a kid and would stick your arm out the window going down the road at 65. Tilt and trim until it's smooth sailing. :cool:
Fish On.
 

Still_Learnin'

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If you're going to be raising your motor, DO NOT RAISE IT WHATSOVER UNLESS YOU HAVE A WATER PRESSURE GAUGE INSTALLED.

Make sure you know what your engine's specific water pressure should be at wide open throttle (WOT). Not doing this is a great way to blow your motor up in a hurry.
 


shorthairsrus

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If you're going to be raising your motor, DO NOT RAISE IT WHATSOVER UNLESS YOU HAVE A WATER PRESSURE GAUGE INSTALLED.

Make sure you know what your engine's specific water pressure should be at wide open throttle (WOT). Not doing this is a great way to blow your motor up in a hurry.


Newer engines will let you know if your to hot -----and/or a laptop plugged into the emm will too
 

Still_Learnin'

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Newer engines will let you know if your to hot -----and/or a laptop plugged into the emm will too

This is true. However, the engine alarm (even on a newer engines) can be faulty. Or tough to hear at high speed. And in most cases, the alarm won't sound until you've reached a harmful engine condition. Not saying that the alarm won't save you from blowing your engine. Just that its not a good idea to solely rely on the engine alarm to know if you are pushing it too hard.
 

guywhofishes

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Guy
It's more of a push/pull on the motor. If trimmed too much one way it pushes your motor up. If trimmed too much the opposite way, it pulls your motor down. Hit the sweet spot and it should run smooth. I kind of liken it to when you were a kid and would stick your arm out the window going down the road at 65. Tilt and trim until it's smooth sailing. :cool:
Fish On.

i get that - but plate is up and out of the water in the pics.... Playing no role when on plane. I trim for max speed and it's always "knifing" perfectly at that point - but I don't think it's clear of the water.

so cav plate up and clear of the water is really how they are supposed to be in regards to height?
 

TFX 186

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i get that - but plate is up and out of the water in the pics.... Playing no role when on plane. I trim for max speed and it's always "knifing" perfectly at that point - but I don't think it's clear of the water.

so cav plate up and clear of the water is really how they are supposed to be in regards to height?

That's a good question Guy. I am always driving and have never really paid attention to how much the cav plate is out or in the water. I guess I am gonna have to have my wife drive and I am gonna investigate. That might be scary! On my old boat, I could feel in the steering when i trimmed up to the sweet spot. My new one isn't near as obvious in the steering when trimmed just right.Maybe Still Learnin can answer the question about the plate location in relation to the water. I am definitely gonna check this out!
I do have a couple questions regarding these jack plates. How are these guys able to tell where their motor runs the best? Do they just experiment with the adjustments when driving? Do they have these water pressure gauges installed on these boats that run jack plates?


Fish On!
 
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Captain Ahab

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i get that - but plate is up and out of the water in the pics.... Playing no role when on plane. I trim for max speed and it's always "knifing" perfectly at that point - but I don't think it's clear of the water.

so cav plate up and clear of the water is really how they are supposed to be in regards to height?


Maybe the big white Johnson is plowing a little deep?
 


TFX 186

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Guy
You must have a big white Johnson.........on your boat!:;:huh Ha
 

deleted_account

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Had some time to kill so ive been googling this and it seems that the "cavitation plate" isn't actually that at all, its a anti ventilation plate and is designed to make it less likely for the engine to suck air. Proper level of plate should be at water level when on plane.

I read it on the internet so it has to be true.

- - - Updated - - -

also, penis
 

guywhofishes

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I learned it's entirely dependent on motor, hull, etc.

some setups are best with av plate even with water, some under, some above
 
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