Should I change prop pitch/ go to stainless.

[rkX4TR]

I am going to be adding a jackplate with 6 inches of setback to my setup. My current setup is a 15-foot v haul, with a 115 merc tower of power, 19 pitch aluminum prop. Currently turning around 5000 rpm. This is a bit low right now, but that should come up with the jackplate height. Currently experiencing some chine walk at 50mph, im worried the jackplate will make this worse. Im wondering if a prop change might help, or maybe trim tabs are the way to go. I know its going to be hard to give advice withought me having any feedback yet with how the jackplate handles. But thanks in advance.

 

[jimmbo]

I really don't see the purpose of adding a Jackplate on a small V Hull, but it is your choice. Raising the Engine often reduces Chine Walking. I am assuming you have the inline 6 115. WOT Range is 5000-5500rpm.
One thing to keep an eye on is, making sure the Engine is not being starved of Water when raised up. That goes for high Trim Angles.
A properly selected SS Prop, is often better at raised Engine Heights, than the AL props

From 1984 to 1998, I had a 15ft V hull with an 84 115, It came with a 19 Al Prop, and turned 5500, in 1985 I put a 19 Mercury 19 SS and it turned 5500, but had better bite on the water in turns. Engine was on transom but raised 1 3/4"

 

[Chris1956]

Have you checked the accuracy of the tach or speedo? It would be hard to get to 50MPH on a 19" pitch prop @ 5KRPM, I would think.

If you try 21P prop and can still get 5KRPM, you should be closer to 50MPH, IMO.

 

[rkX4TR]

Have you checked the accuracy of the tach or speedo? It would be hard to get to 50MPH on a 19" pitch prop @ 5KRPM, I would think.

If you try 21P prop and can still get 5KRPM, you should be closer to 50MPH, IMO.

the tach is old, seems accurate but i guess I cant know for sure. the speedo is a new gps model.

 

[Chris1956]

Do you have a 2::1 gear ratio on that engine?

 

[jimmbo]

With the 19, mine was turning 5500 and the speedo was reading about 49-50mph.
The I6 115 used 2:1 from 72 on. In 71 it had 25:14 ratio according to the Brochure(could be a Typo though)

 

[Chris1956]

Here is prop slip calculator. It may help.

https://www.mercuryracing.com/propellers-overview/prop-slip-calculator.html

 

[Texasmark]

What are you trying to achieve with a jack plate?
The amount of hull in the water, and hull type varies the "drag" on the prop's ability to propel the boat. The less hull in the water the faster you go with the same engine and prop.
Just because you have a 19" pitch, the characteristics of the prop go a long way in determining your performance. Turning Point Hustler aluminum props are as close to the performance of SS to the best of my knowledge...may be others out there but I'm not aware of them.
A plane Jane aluminum prop is lacking in most everything needed to get any kind of performance out of your rig.

I bought a new 1988 Ranger bass boat with a "pad" ( semi-V hull with dead rise all the way back including the transom area with about 4" of offset....the pad stopped about 4" preceding the transom.) The pad was a narrow flat surface along the "keel" area and the purpose was to reduce the amount of hull in the water to have a significant effect on reducing hull drag and allow for an increase in top speed.
As you got up to speed, the water pressure against the pad lifted the remainder of the hull out of the water for reduced drag.
I had a New Mercury instruments in my new boat and my 115 Tower of Power ran at 6k RPM on a light chop with only me aboard with my 19" Mercury SS Laser prop. I had trim at maximum....defined as sudden increase RPM and decreasing speed. Part of the prop may have been out of the water as I could hear the engine singing and the prop's blades beating the water making a humming sound....lots of fun!
Had that boat 7 years and sold it to my SIL. Only thing I did, had to do, with the engine was replace the gasket on the rear of the block water jacket cover.

 

[aspeck]

The decision to go stainless over aluminum is for various reason. If I was boating a shallow, rocky river or lake I would keep the aluminum prop ... better to lose a prop blade than bust the shaft. Aluminum props are more forgiving.

I boat a deep lake. So rocks are not an issue. A cupped SS prop gave me more speed and MUCH better slow speed handling (my Renken SeaMaster is the WORST boat I have every handled at slow speeds ... I would rather dock a pontoon in a current and wind than my boat before I got the SS prop).

So, depending on the SS you get, you can expect less slip and possibly higher RPMs. If I was sold on the jackplate, I would not worry about the prop until AFTER the plate was installed. Then check your performance and RPM's and go from there.

 

[Scott Danforth]

do you have a high-speed nose and pickup on the foot?

why do you need a jack plate on a 15' boat?

are you racing? or running flats? because if you are running flats, a tunnel hull would be better than a V-hull, then you can run in 2" of water

 

[Chris1956]

Scott, My 16 footer/ Merc 1500 did have better performance with a Jackplate. It was not a lot better, but it made the boat harder to drive IMO than the extra speed benefit. I therefore removed the Jackplate.

I still have the engine raised about 1-1/2 inches on the transom, and the factory trim tab (anode) is mostly out of the water at speed.

 

[Scott Danforth]

I still have the engine raised about 1-1/2 inches on the transom, and the factory trim tab (anode) is mostly out of the water at speed.

are you running the top pickup, or a bottom pickup? what is your water pressure?

I have been in many switzercrafts and hydrostreams with merc 150's and a few 225's back in the 80's and 90's. my buds '74 super hugger was also a 16' boat with a manual jack plate with a 1500. he had to run a bottom pickup

but the OP has a 15'. without knowing the boat or the weight, we cant answer how his boat would respond to a jack plate or a stand-off

we can only ask the questions.

locally down here, plenty of 16' hydro cats running 225's on hydraulic jack plates running the flats. just a bit un-nerving running thru water where the birds are standing