Porpising- Weight Distribution & Motor Height

[Traxion]

Background- Aluminum hull, 17.5', 115 Evinrude. 17P Stilletto, 5100 RPM, holeshot OK, top end 37.5 mph, bad porpising at even moderate trim angles. Overall prop grip is poor in turns, etc. Transom was just replaced, issues got much worse after the replacement.

I've done quite a bit of searching and have learned a lot about porpising. In my aluminum walleye rig, I've got a lot of weight in the rear of the boat. Main motor, 8hp 4s kicker (100#), 2 batteries (110#), oil tank (15#). Additionally, my fuel tank is mounted in the floor approx. midline or a little back of the boat (30 gal). I have absolutely nowhere to move any weight forward, so other than gear on board or filling the front livewells, I am pretty much stuck as it is.

My question really revolves around motor height. Many people have had good luck moving the motor up. I could be a tad low, my memory is poor and I have to reverify where the cavitation plate is. I want to say I am even with the keel of the boat. Will I have success in moderating or completely getting rid of my porpising issue if I raise the motor a hole or even two? I am mounted as low as it can go now.

I'm having difficulty figuring out where to start, I am needing to reprop so I'd rather play with motor height & my current prop (which is OK but not perfect by any means) before I spend the $$$ on a new prop. What is the best way to go about this, test motor height first, or should I try different props?

 

[steelespike]

Re: Porpising- Weight Distribution & Motor Height

If you have venting issues allready you will have to solve those before you can raise the motor.What year is the motor? Your rpm is at least 400 rpm low possibly because you can't trim up enough.Trim tabs could solve your porpoising but at a slight reduction in mileage ( hard to tell without a flow meter)and speed.A prop designed with stern lift might help.4 blade props have stern lift and will resist venting possibly allowing higher motor height and additional trim.
My guess is the stilletto has bow lift.It might be possible to modify the prop for less bow lift
but might agrivate your other issues.(venting)
Perhaps someone with more technical knowledge will be along to help.

 

[Traxion]

Re: Porpising- Weight Distribution & Motor Height

Motor year is 1995. I need to be running a 15p right now, things I know would improve with less pitch. To what point, I don't know. I need more RPM for sure, even trimmed to the max with pretty serious porpising I can't get any more than 5100. I couldn't get any more than that before the transom replacement either, so I know I need less pitch.

Precision Prop has recommended a 4 blade prop (RXB) in a 13p. I think that could be really close, however I am hesitant to spend the coin with the issues I am having. I'd really like to avoid tabs if at all possible.

Edit- Looked at the cavitation plate and the motor is close to an inch above the bottom of the boat.

 

[walleyemanII]

Re: Porpising- Weight Distribution & Motor Height

You may want to look at this web page about engine height before changing any prop
http://www.veradoclub.com/smf/index.php?topic=137.0

 

[Traxion]

Re: Porpising- Weight Distribution & Motor Height

I am real close to perfect height according to that thread posted. Cav plate is visible while running.

 

[steelespike]

Re: Porpising- Weight Distribution & Motor Height

I'm surprised the Stiletto can't handle the height.Is it a little tired?
While it is likely you need less pitch I don't think a badly porpoising boat is the best indicator of
speed or rpm.Your boat is spending half its time with the whole bottom in the water.
It won't cost anything to lower the motor a hole or half a hole. Maybe with better bite the prop can hold the bow up better.
In your position it might be cost effective to find a dealer that will let you try props till you find one that works.Usually requires a deposit with the understanding your going to buy a prop.
While I wouldn't casually recommend changing brands,material and design;you might try the Solas Amita al 4 blade in a 13.They have been pretty predictable and usually have good results.
certainly a more cost effective test source.

 

[Traxion]

Re: Porpising- Weight Distribution & Motor Height

The Stilletto is only about 3 years old. So it should be fine. One thing I just thought of that I neglected to mention is the alititude I run at, usually around 3000' but can be lower than that depending on location. Also have a goofy SE 300 hydrofoil on there, tried the boat with and without and it behaves better on holeshot and low end with the plate with little difference in top end.

As far as motor height, I can't go any lower. All I can go is up, motor is mounted in the highest hole. So I am stuck there.

I guess I could try the Amita in AL, gotta be close to the RXB Powertech is recommending.

 

[captkevin]

Re: Porpising- Weight Distribution & Motor Height

Smart tabs might be an easy fix for you.

 

[steelespike]

Re: Porpising- Weight Distribution & Motor Height

I think the Solas will resist porpoising some and may let you raise the motor a little to help more.
At 3000ft you lose about 9% power.The foil is ok if its clear of the water on plane.With the 4 blade you may not need the foil.

 

[Traxion]

Re: Porpising- Weight Distribution & Motor Height

I think I am just going to bite the bullet on the Powertech. I think it should get me really close from my research. Thanks for the help.

 

[steelespike]

Re: Porpising- Weight Distribution & Motor Height

Good luck;Please let us all know how you make out.