Jack plate on 18 ft aluminum with 90 hp

[bryanzzz]

Hope I'm posting in the correct place. Have a 2014 aluminum 18 ft, with what I would call semi-vee hull and 90 Yamaha 4stroke. Anyone installed a jack plate? Only looking for fuel economy and anything else would be a plus. Do not have a water pressure gauge and would rather not install one, would not get radical with adjustments, factory install is 7 3/4 prop to pad, thinking more like 6???? Thanks for any advice.

 

[jrttoday]

on a glass hull and depending on design of that hull, at 6" back.... standard starting point for cavitation plate is 1" above hull bottom at transom. How the boat is loaded WILL affect how much bite your prop gets and how high you can set it. Higher it's set (up to a point) determines how much efficiency you realize from installing a JP.

If all you do is install a jack plate and not raise the motor, it might look good but you haven't really done anything.... at least, not towards fuel economy. JPlates increase efficiency in two ways that work together, by allowing the prop to surface reducing drag on prop which allows the motor to achieve WOT with less effort. And by moving it back 6", the prop is in cleaner water - less bubbles equal better bite. Do one and not the other and WOT means "waste of time" not "Wide Open Throttle"

Personally, I will not operate my outboards without a water pressure gauge; and especially with a jack plate. By raising the motor, you're more at risk of being trimmed to high for adequate water supply. And with an outboard, an ice bag is equal to an iceberg!

 

[Chris1956]

You might take a look at the antiventilation plate at high speed. If it is above the water surface a bit, you may already be getting max economy. non power Jack plates need to be set at a compromise between driveability and max speed. Max speed does not always impart max economy.

 

[metriccrescentwrench]

I agree with the above replies as far as a water pressure gauge, it makes a lot of sense to install one when you install the jack plate.

All hulls are different, but on mine I have a 6" jackplate and run the prop centerline about 2-1/2" below the pad (which puts the AV plate somewhere around 7 inches above the pad). Also, I filled the top water intake hole on the gearcase, (some guys fill the top 2). Even with that, it's quite easy to trim high enough that the engine loses water pressure - that water pressure gauge is an indispensable tool.

 

[jrttoday]

How's the power in relation to your boat? A cupped prop is a simple solution to increasing economy and perhaps, a better solution. It all depends... You're not looking to get radical? A jack plate IS a radical change in hydrodynamics.

Going faster doesn't always mean less economy; with the hull riding high, less drag equals less work for the motor and less prop slip. Getting somewhere faster also means less operating time on the motor. It's all a delicate balance.

There might be some mathematical genius that has it figured out, but I don't need that kind of headache!! There's a "sweet spot" on every boat; trial and error works for me... and a good understanding of what I'm getting into beforehand.
good luck!

 

[bryanzzz]

To quote Joe Dirt "Dang" you guys gave me so much info I will not even look at jack plates. HA, HA. Seriously, thanks for input, looking for a winter project and keep in mind my rig is aluminum, without pad, thanks again.

 

[NYBo]

Your rig should be pretty easy on gas as it is. Any increase in fuel economy from a jackplate would take a long time to recoup the cost, if ever.