Help with prop selection / slow planing issues

[tunedportcj5]

Hello - I have a 1998 16' Pro Sports Pro Skiff with a 1996 50hp Johnson. I just purchased this boat a few weeks ago. It has some bugs and I am trying work them out.

At first I was having major cavitation issues at WOT (trimmed all the way down) and with any slight turn - it would come "unhooked". After some reading I determined that the anti cavitation plate was too high from the bottom of the hull. I lowered both the motor and the CMC hydraulic trim plate. It is now at its lowest settings and it is much better. The anti-cav plate is about 1/2" above the lowest point of the hull.

My main problem now is that it takes forever to get on a plane. It should jump out of the water with a 50! I have been testing with 2 different props. Both are 3 blade aluminum and are in perfect shape. One of them is a 11.75x15 and the other is a 11.75x17.

With the 11.75x15 it gets on a plane a little faster (30 seconds maybe) and has a max RPM of 6000 (too much). From idle to WOT it jumps right to 5000rpm and stays there until it finally planes out.

With the 11.75x17 its naturally slower to plane (30-45 seconds maybe) and has a max rpm of 5100. From idle to WOT it jumps right to 4000rpm and will eventually plane out.

I also feel that the WOT speed is low. I don't have an accurate speedometer but I am guessing around 30mph.

I also have eliminated any possible engine problems. It is firing on both cylinders, good compression (~135psi on both), carbs have been checked and cleaned, etc...

I am sure that the props do not have spun hubs. The 11.75x15 is brand new.

Whelp... thats my story. Does anyone have an idea of what might be going on?

Thanks!
David Smith
Jacksonville, FL

 

[Silvertip]

Re: Help with prop selection / slow planing issues

My previous boat was a 15.5 ft aluminum fishing boat with a 40 HP Evinrude. Equipped with a 15P prop that boat jumped on plane with two aboard, full tank of fuel and a load of fishing gear. I would be looking at power issues. Another issue is that a 17P prop should not pull 900 RPM from the WOT 6000 RPM you said you got from the 15P. Since RPM flares right from the moment you go WOT, I'm wondering if the prop hub is correct for your engine. If the hub is too small, excess exhaust gas is exiting through the gap between the prop and lower unit housing which creates air bubbles behind the blades. Venting a prop uses this technique but by drilling holes behind the blade.

 

[tunedportcj5]

Re: Help with prop selection / slow planing issues

I also meant to mention that I have a livewell pickup and a bilge pump drain at the bottom of the transom (not the bottom of the hull) I wonder if it could be sucking air through these holes?

 

[Silvertip]

Re: Help with prop selection / slow planing issues

Not likely! Most fishing boats are so equipped and it's not a problem. Having the AV plate 1/2 inch above the keel should not cause the problem but you might lower the engine one more hole if you can. Some boats have a really deep "V" and that may be an issue. Although 6000 RPM is a tab outside the RPM range for the engine, 5800 is perfectly acceptable and the engine actually is happier at that RPM with a load.

 

[tunedportcj5]

Re: Help with prop selection / slow planing issues

I have lowered it as far as possible on both the bracket and the transom. Do you think a larger diameter prop would help?

 

[Silvertip]

Re: Help with prop selection / slow planing issues

I'm not convinced you need to be looking at props at this point. Something else is going on as there is no way going from a 15P prop to 17P should drop 900 RPM. Do you have any idea what this boat weighs? Is there a locator transducer mounted at the center of the boat? If so, move it at least 12 inches port or starboard of the boat centerline. If the engine is mounted on a jack plate turbulence at the back of the boat should not be an issue as the setback would help that. Is there a trim pin that is preventing the engine from being trimmed down fully? As for prop diameter, 11-3/4 to 12-1/4 is a fairly standard prop diameter range for that engine. What you might do is have a prop shop add some more cupping to it. That would bring the RPM down a bit and help with bite. In my neck of the woods that engine is extremely popular and on boats in th 14 - 16 foot range a 12-1/4 x 15P OMC aluminum prop is almost the universal choice. I too looked for a little extra performance with my 40 HP and always came back to 15P.

 

[tunedportcj5]

Re: Help with prop selection / slow planing issues

Thanks for all of your help. I do have a fish finder transducer but it is way out of the way. I will take some pictures when I get home to help you vision what I am dealing with.

Thanks!