Which prop ?

[batfalcon]

I have a "Bat 9 Falcon X" rib (5,3 m) powered by a Merc 115. The engine was delivered with an Aluminun 19x13 prop and worked fine with this rig, exept the fact that I added to many things (weights) during the past years, so I was forced to pitch down to 17x13 1/4. With this prop the engines tops at 5.000 rpm (don't remember the speed).
Recently I was given a standard Yamaha steel prop 19x13, which ffits fine and I tested last weekend.
I get 5.200 rpm (!!!) with this prop BUT I think that the speed had decreased. In fact the top speed now is 35,8 knots (according to gps).
I also have measured the speed at 4.500 prm which is 28,5 knots.
Any suggestions on which prop is the most recommened please ?

[batfalcon]

... that either none around this forum is able to answer my questions, or they are insignificant. Then again maybe none saw them ....

[Turbodiesel]

Is your engine 2 or 4 stroke? A 2 stroke should rev to 5500rpm and the 4 stroke should rev to 6000. It sounds like the Yamaha prop you have got is slipping, check the bush in the middle of the prop. If it has play in it then that could cause your problem.
The general rule for props is for each pitch of prop you will go up or down about 200 rpm.Maybe with all your weight you should try a 15" pitch. If its a four stroke maybe try a 13" pitch.

[batfalcon]

It's a 2 stroke.
The Yam prop is definetly not slipping. If it was the situation would have been worse everyday, which isn't happening.
Any other ideas ?

[Jono]

Talk to Julie at Steeldevelopments. She posts here, but it may help if you did proper speed/revs/weight checks to give her the information accurately. Also check your manual for max RPM. I thought that the 115 (Classic?) 2 stroker WOT maxed at 4750-5250….

[Manos]

...........or hire a 640SR LOLOL