Correct Pitch?

[MWBROWN]

1984 Baliner Fish and Ski, 16 Ft. (1510) All original
85 Hp. Force Engine
HP range is 4500 to 5500 according to the book.
Compression is 120 to 125 in all cyl.

Original Prop was aluminum 13 x 19
I had it repitched when I purchased the boat to 20", since the prop needed repair. Top speed about 35 GPS with 400 lbs and full 20 gal tank. 5200 rpm.
I could to a bit better, but the porpoising was terrible.

I purchased a 13 x 17 with 2" cup, stainless, hoping to do a bit better, and
it seems just about the same. I also added smart tabs and got rid of the wing.
Also no real improvement.

I read somewhere that the Force engines do not like the cupped props.

Should I have the prop pitched to a straight 19, or should I go to 20 as I did on the aluminum.

Thanks in advance,

Mike

 

[Silvertip]

Re: Correct Pitch?

If porpoising is not corrected with smart tabs they are not adjusted correctly or you have the wrong actuators. I would leave the prop as is until you get the porpoising issue resolved as you cannot get optimum performance until then. Have you experimented with engine height (not trim -- this is physically moving the motor up a Proper setup can help with porpoising and performance.

 

[MWBROWN]

Re: Correct Pitch?

Thanks for the response.

This boat is terrible with porpoising. That is why the original owner put the wing on. I tried the boat once without the wing, and it was almost undriveable.
I tried all the positions on the tabs, and found the best position. I have not tried other actuators. They did help with planing quicker, and slower plane speeds. The boat was also much more stable. If I set them too stiff, then the boat looses speed.
I can trim up some to get more speed and RPM, and then the porpoising starts.

I can move up one hole, and 1 1/2 inches to see if that helps.
But, I have to get the engine running again first. CDI went bad, but I have one on the way.
I will try that then, and give some feed back.

Mike

 

[Frank Acampora]

Re: Correct Pitch?

If trimming up or out starts the porpoising, then you just simply have too much weight at the stern. Try shifting some around a bit. As far as props, Force engines prior to Merc (as is your '84) are not critial to cupping or not. A cupped prop will simply allow you to run the engine higher on the transom. You should be able to utilize about 2/3 of total trim travel before the engine starts to ventilate if the engine is set high enough on the transom. If you can use all the trim travel without ventilating, then you can raise the engine a bit.

Do not try removing the cup from your stainless prop. Cupping the prop has the same effect on loading the engine as a 2 inch increase in pitch. However, it does nothing for theoretical top speed. Cupping just lets the prop "grab" more and ventilate less. SO: if you have the option--that is prop not ventilating badly-- it is preferable to use an uncupped prop 2 pitches higher to acheive the same RPM and USUALLY a couple of MPH more. If the prop only ventilates in turns at a lot of trim, then, simply trim in during the turns.

 

[MWBROWN]

Re: Correct Pitch?

Frank,

Thanks for the reply.

I had the engine up when I had the aluminum 13x19 (pitched to 20) and it would cavitate in tight turns. So, when I purchased the SS prop I lowered the engine back down to the factory setting. I was only on the water twice with the SS prop alone, and twice more with the smart tabs, before I had the engine trouble. So my testing was not real complete. I am waiting for my CD module to show up, and when I install that, I will raise the engine back up. Then, I hope, with the SS prop, raised engine, and smart tabs, to pick up a few MPH, and some fuel economy.

I will say, with the SS prop, getting on plane was MUCH quicker, and with the smart tabs, I could stay on plane at 14 to 16 MPH instead of 20. GPS.

Merry Christmas,

Mike