115 HP Johnson with V4 gear case PROP selection please help

[scottcab]

Hello all,
I am trying to figure out a prop to help optimize my boats performance. I am running a 1992 Concept CC 17'6" with a 1.6l (115hp) 1985 OMC seadrive powerhead on a johnson mid and lower. My current numbers are 6000-6200 rpm at 44mph at optimum trim angle (about 3-4 seconds) with a lighter load and 5400rpm 41mph with maximum load for boat. The service manual for the PH states optimum WoT range is 4500-5500rpm. I have been told that each "step" in pitch will probably lower my rpm by 300-400rpm. I am currently running a stiletto advantage I 13 1/4" X 17 pitch. I plan on keepin the 17 pitch for heavy load days.

For lighter load days, I have been looking into the raker 13.5 X 20 pitch prop. This prop boast high efficiency and top speed with the ability to have the boat ride with the bow a little higher. Does this sound like a good matchup? How much do y'all anticipate the rake, diameter, and pitch change to effect speed and performance and rpms. Also would changing to this prop help improve fuel efficiency? Also, how would this prop perform for low-med. speed trolling?

Thanks for your time and expertise
Scott

 

[glenn property of pam]

Re: 115 HP Johnson with V4 gear case PROP selection please help

try the prop forum thats where they keep the people who deal in the dark art of prop slipage etc.

 

[emdsapmgr]

Re: 115 HP Johnson with V4 gear case PROP selection please help

A change of one prop size (ie: from 19 to 17 pitch) will make about 300 rpm's difference. Your description of your findings is pretty accurate. Your engine is factory rated to run up to 5500, but those engines will normally make good, reliable hp up to 5700. Your choice of a 20 raker should get your light load speed/rpm's pretty close to optimum. They are solid props and work as advertized. I wouldn't run that engine even up to 6000, keep your throttle back to keep your rpm's down. The exhaust bleed holes in front of each prop blade will really help holeshot-you may even see the rpm's drop when the prop hooks up as you come up on plane.

 

[Haffiman]

Re: 115 HP Johnson with V4 gear case PROP selection please help

Just be aware that it is a slight difference in pitching of Raker and std SST props. Rakers comes in even numbers 18-20-22.., std props in 17-19-21. A rough rule of comparison is to go 1" down when choosing Raker. If a 19" pitch SST works, the equivalent would be an 18" Raker. To get the optimum out of the Raker, the engine has to be lifted on the transom compared to 'normal' props.

 

[angleness]

Re: 115 HP Johnson with V4 gear case PROP selection please help

try the prop forum thats where they keep the people who deal in the dark art of prop slipage etc.

good luck.

 

[scottcab]

Re: 115 HP Johnson with V4 gear case PROP selection please help

I just got a great deal on a 13.5 X 20 pitch raker brand new in the box. Shipped for $125 :-D Hopefully this will be the perfect prop for me. When it arrives in a couple days I will test run and post the changed numbers for reference purposes for the iboats community.

 

[scottcab]

Re: 115 HP Johnson with V4 gear case PROP selection please help

I got the 13.5 X 20 OMC Raker prop today. I was sadly disappointed at the lake. The boat took longer to plane off and seemed to be having a harder time making speed/power. The boat top speed came in at 43mph (1mph slower) and would only make 4500rpm max. I am thinking that it was probably just a little too aggressive of selection. Also when running on plane and trimmed up, the boat felt really light. Almost like I could have used a little more boat in the water. I know the idea of the prop is bow lift, but I am afraid that there isn't enough boat in the water. The prop is in brand new condition, so I will sell it to recover my investment. Although I hate the idea of playing the prop game, what do yall recommend I try for a light load prop. Maybe I should stick with the Stiletto prop in a 19 pitch? Does anyone have a prop they recommend?

 

[Texasmark]

Re: 115 HP Johnson with V4 gear case PROP selection please help

My opinion is that you got your bow lift that Raker is supposed to give you. Interesting that in doing so the engine still lugged as much as it did. As mentioned, one would have thought that even though the pitch change was 3" relieving the engine of the extra boat drag should have translated into several more mph and surely not the rpm drop that you experienced. The engine is lugging and that is why your performance suffers. In lugging hole shot is poor and maintaining a steady speed at or just above planing speed is difficult.

I'd do as you suggested and go up a couple of inches with the current brand of prop. That ought to work for you given what you have told us. As far as hole shot with the higher pitched prop, ports in the prop help immensely in that situation not familiar with your current prop as to whether or not it's ported; if so fine. BTDT several times. If your new prop isn't ported and you want better hole performance, take it to a prop shop and have them drill ports in it for you. I'd start with 1/4" dia and go up. I just ported my 24P Ballistic with that size holes and it made a really bid difference in hole and it was obvious the engine liked the unloading during the load of the hole shot on the engine. The bigger the holes, the more the unloading and if too large, you unload too much, the engine revs excessively and instead of helping the hole it actually takes longer....but the engine loves the experience. Ha!

Mark