Need For Speed - Prop Question

[bgiles]

Hi there, need some advice, I have a 1993 Evinrude 120 on a 17 foot Checkmate. I just took the boat for the first ride of the season with a new jackplate installed.<br /><br />However, I wasn't really expecting an increase in speed due to the fact that before I installed the plate I was running approximately 6000-6100 rpm according to my tach with a top gps speed of around 53. My current setup is a Raker 13.5 x 22<br />with the cavitation plate approx 4 inches above the pad. While "fooling' around with different heights on the plate, it really didn't affect the rpm's or water pressure or top speed even with the plate pretty well at the top.<br /><br />So, that brings me to this, according to the Seloc manual the "wot" rpm is 6000 and I am achieving that, give or take.<br /><br />I'm thinking of trying to find a 13.5 x 24 Raker which will drop my rpm's roughly 200 but may afford me a couple of more mph due to the increase of pitch.<br /><br />What do you think, 13.5 x 24 or perhaps try a 14.5 x 24 and perhaps lose approx 400 rpm.<br /><br />I don't know if the 14.5 would even fit onto the shaft. I'm not sure that the 13.5 and 14.5 are compatible.<br /><br />Thanks in Advance for any suggestions /comments<br /><br />Bruce

 

[sangerwaker]

Re: Need For Speed - Prop Question

Had a '91 120 on my last boat...great motor.<br /><br />WOT is not necessarily the max RPM's you can spin. Best way I can explain is this...If your RPMs go up, and your speed does not increase, you're at WOT for your trim setting. If you're shooting a big ol rooster tail, you're probably over trimmedand wasting power. Here's a link to a prop slip calculator. I'd do the math for you, but I can't remember the gear ratio on that motor.<br /> Prop Slip Calculator <br /><br />You will also drop 400 rpms (roughly) by going to the 24 pitch. The general rule is 200 per pitch.<br /><br />The 14.5 will not fit on your motor. That prop is only for the larger gearcase V6's and some of the newer models.<br /><br />As a suggestion, try some other props if you haven't. The Raker's a good prop, but not necessarily the best for your boat. Find a good prop shop that will let you try before you buy. It's really amazing the differences in props. You might surprise yourself what actually works.<br /><br />If you're looking for top end, I'd try a Stilletto or better yet a Turbo. Typically three blades will give you the best top end.<br /><br />Good luck and post back!

 

[bgiles]

Re: Need For Speed - Prop Question

Hi Sangerwaker, thanks for the advice, I just came back from the garage and there is only 1/2 inch of clearance under the cavitation plate so as you stated, the 14.5 won't fit.<br /><br />I'll try and find a prop shop with a few different props and give them a go, like you said, you never know what will work out.<br /><br />I'm beginning to know way too much about this stuff, but that's what keeps us 'boat nuts" going.<br /><br /><br />Thanks Again<br /><br />Bruce

 

[walleyehed]

Re: Need For Speed - Prop Question

Bruce, you have a 13 spline shaft, and as posted above, that limits your diameter choices...the 14.5 is for 15 spline larger gearcases.<br />With your set-up, I would highly recommend you look into a Turbo by precision propellers..<br />I run several Rakers and they are good props, but my best time and again, is the Turbo.<br />When the set-up is as advanced as yours, we tend to reference the prop-shaft in relation to the pad bottom insted of AV plate.<br />What type of jackplate and how much set-back do we have now???

 

[bgiles]

Re: Need For Speed - Prop Question

Hi Kenny, thanks for the feedback, I have the 6 inch Detwiler "on the water" Jackplate. Currently the Prop shaft (center) is sitting 4.5 inches below the Pad.<br /><br />However, even when I raised it another another inch, inch and a half, leaving the prop shaft at 3 inches below, it didn't make any appreciable difference, same numbers as far as speed,rpm and water pressure.<br /><br /><br />Thanks<br /><br />Bruce

 

[bgiles]

Re: Need For Speed - Prop Question

Hi there, just a quick update, I spent the other day "fooling" around with a different prop on my pulse 170, 1993 Evinrude 120 here's the outcome.<br /><br />Original setup<br /><br />Raker 13.5 X 22<br /><br />WOT = 5000 - 6000 rpm, According to Seloc<br /><br />(Trimmed out to the max with Rooster tail, Tach reads 6100 rpm) Rev Limiter - 6700 rpms (Seloc)<br /><br />Top Speed (rpm where speed does not increase)<br /><br />52 mph GPS @ 5500 RPM<br /><br /><br />Other setup<br /><br />Raker 13.5 X 24<br /><br />Top Speed (rpm where speed does not increase)<br /><br />53 mph GPS @ 5500 RPM<br /><br /><br />Conclusion<br /><br />Really only gained about 1 mph on top end, but lost a lot on the hole shot. Could just barely get the boat on plane without cavitating like crazy and that was with only me (160 lbs). <br /><br />Added another passenger (170 lbs), the boat would not plane at all, the Raker 24 pitch was just simply "eating" up the water until the motor would start to over rev.<br /><br />Put the 13.5 x 22 back on when I got home, looks like I have reached the optimum setup at the current Horsepower.<br /><br />Bottom line<br /><br />Installed Jackplate - Helped Hole shot for sure and handling improved. Really didn't help me in top end speed. <br /><br />Increased Prop Pitch by 2 - Only really gained 1 mph and lost a whole lot of hole shot.<br /><br /><br />So, once again it proves, different pitches for different outcomes. High pitched props with 1 person for speed and lower pitched props for all around performance. <br /><br /><br />Take Care<br /><br />Bruce