messed up prop runs better than new one? need help

[chasebull]

i rebuilt a 24 ft partybarge and took my 1987 70 hp johnson on it, controls and all. the motor was originally on a 14 1/2 tri-hull. so i put it on the pontoon with original prop to see how it did and it surpassed my expectations and would really plane out and move across the water. but the other day i backed it into a truck and bent one of the fins. i replaced the prop with the same as it had on it, 13.25 17 pitch and it is running terrible. it won't plane out and is plowing in the water and the back has a weird wake behind it. (sorry for the non tech terms) so i came home and figured i would try to fix the old prop and put it back on to see what it would do. nothing has changed on the boat at all except for the prop. when i started straightening out the fin i had bent i realized that they were all at different angles and in comparing it to the new one, i think only one was in the correct position, the other 2 were much flatter. in looking on this forum it seems that an 11 pitch is what is recommended for my application. so here is my question. is it possible that the boat ran so well before because the 2 fins were flatter and closer to the 11 pitch than the original 17? thanks in advance.

 

[jimmbo]

Are you certain that only the propeller got bent when you ran it into the truck? The transom didn't get 'adjusted' too? If two of the blades on the old prop were 'bent' much flatter there should have been a very noticeable vibration. Now you said you replaced it with the same dia and pitch as the old prop. OMC did made a few different props with the same dia and pitch, but with totally different blades. Did you get an 'exact' replacement, as same part number?

 

[chasebull]

i am fairly certain nothing else was bent except the one fin but i will double check that later today. as for the replacement, it was the same size and pitch but again, i will have to check the fin size. here are some pics i took. the first is the one i just bent. the last pic has the one i bent on the left.

 

[Maclin]

Seems the old prop was really about an 11" pitch. If the markings on it show it is a 17p maybe it had been re-pitched. And a 17p for that combo would be a bad choice, even for a 15' trihull. Needs more like a 13 tops, and more like a 10 or 11 for a pontoon.

 

[SkiDad]

i agree a 17 would not be a correct prop - 11 or 13 is more normal. I would have a lot marine prop shop hook you up - they can probably measure your old prop to see what it was. Or you could have them repair the prop that you liked.

if you remember your old RPM and speed that would help the guys on here give you some other ideas.

 

[jimmbo]

Well I can tell it's not an OMC/BRP prop. I'll go out on a limb and speculate. Your old prop whatever it is, was slipping enough it allowed the engine to rev higher and push enough water to move the boat. Your new prop is probably grabbing the water better and holding the rpms down. There is a slight chance the transom bent a bit and it's like trimming the engine down a bit