Does Prop Slip Increase Max RPM?

[swire]

I'm trying to figure out a new prop for my top end and want to make sure I mess up my calculations.<br /><br />I have a 1998 90hp Force on a 19 1/2 fish & ski. One of the props that came with the boat was a aluminum 12 1/2 x 23 Black Max. The max RPM for the motor is 5250 but with that prop it was only running at 4800. In theory dropping down to a 21 pitch prop would give me 300 to 400 more RPM and get the moto very near it's max RPM. However, I just ran the boat performance analyzer and compared that prop to my ski prop which is only a 17 pitch. It turns out the 23 pitch prop had a slip of 24% as the max boat speed was only around 34-35mph. With the 17 pitch prop I can usually get 32-33mph which gives me an 8% slip.<br /><br />So my question is how much does slip affect the RPMs or does slip just affect boat speed? Does a 24% slip mean the motor is turning 24% easier and that if the prop actually did bite and there was a theorectical zero slip would the true RPMs be 24% less? I would hate to buy a high end 21 pitch prop only to find out it has a good bite and that I'm still over propped.

 

[WillyBWright]

Re: Does Prop Slip Increase Max RPM?

21 is too steep for a 90 on a 19' boat. Even 19. 17 would probably make the best speed, 15 for skiing. I don't know why the comp prop didn't perform well. Unless the boat is so underpowered that the fourth blade really loaded it down. Sell that 23 on eBay or something. No 90 will ever twist that.

 

[swire]

Re: Does Prop Slip Increase Max RPM?

With the 17 pitch prop I can easily over rev the engine. I just got in a 3 blade aluminum 19 pitch prop but haven't had the weather to get the boat out and try it. I've been researching props and boat setups in these forums non-stop lately and plan on a lot of tweaking trying to maximize my setup which is why I'm trying to figure out if the %slip can skew my max RPM numbers.

 

[Bondo]

Re: Does Prop Slip Increase Max RPM?

which is why I'm trying to figure out if the %slip can skew my max RPM numbers.

Ayuh,........<br />ALL the #s in the equation ARE relevant to each other...............

 

[Texasmark]

Re: Does Prop Slip Increase Max RPM?

If the screw were turning in a solid (semisolid) there would be no slip and 1 rev would move you forward the # of the pitch. So speed would just be pitch x rpm's. <br /><br />With no slip there would be more loading on the engine (for a given set of circumstances) and the rpm's would therefore decrease.<br /><br />What messes this calculator up with the real world is that the engine may not agree with the calculator.....theoretically all this happens, but what about the engine's ability to perform......like I think your 23 has the engine on the back side of the torque curve so you are getting the wrong idea as to where you need to be. The fact that your boat speed increased with a lower pitch tells me that.<br /><br />For the calculator to be right the rpm's have to respond to the physics of the calculation. If it's (the engine) overloaded it can't. This is independent of the loss caused by propeller slip which is based upon environment and propeller.<br /><br />One last thing. You said you want to optimize the rig. Gotta get off the aluminum to do that and look at other things; trim, engine height, boat load distribution and all.<br /><br />HTH<br /><br />Mark