Would wrong Prop cause porpoising???

[Jwhit612]

I am looking at an '88 Four Winns bow rider that I would like to buy. It's in great condition and runs really well. However, it seems to have a porpoising problem. Its a 16.6' boat and has the mercruiser 3.0 4-cyl motor. He said that when he bought it that it had a 29 pitch prop on it. He said that it would go 45-50 mph but had no low end power--obviously. So he switched to a 15 pitch prop which allows him to pull two skiers but at 3500 rpm you are only running at about 27 mph. I'm guessing it should have about a 19 pitch prop? Also, it is very difficult to keep it from porpoising like crazy. unless you keep it trimmed all the way down it will start porpoising. if you bump it up at all it starts porpoising. Also, for some reason he took the little sundial looking thing off because he said the prop was too big for it??? Not sure what that means.
I've attached a picture of the prop if it helps. What I need to know is, will changing the prop correct the porpoising problem? Also, what type prop should I replace it with. I do want to be able to pull one skier but would like to have the flexibility of cruising at a higher speed as well. Any thoughts?
Thanks in advance!
Jeff

 

[steelespike]

Re: Would wrong Prop cause porpoising???

I can get you started.Porpoising is a little unusual for an I/O.
Assuming the hull hasn't absorbed water or developed a hump the prop
probably has too much bow lift. Someone should be along with more specifics.
A 29 inch prop is literally off the scale. He's either a story teller or got his numbers mixed up.Assuming a 19" pitch at 4600 a 29" would be at about 2,600 surely not likely to produce 45-50 and even more likely to not plane.
If you can double check your prop size and maybe get us the brand we may be able to help with a selection for more top speed.A good prop shop may be able alter the prop if you explain the problem.
Your boat will probably run 40-45 when set up right.
The boat is no flyweight at 1960 lbs. and rated at 130 hp.

 

[Dhadley]

Re: Would wrong Prop cause porpoising???

A prop can and will contribte to porposing just like the right prop can help tame it. A 16 1/2 foot boat with a ton of cast iron in the back would sure seem likely to be prone to porposing anyway.