Prop results,need advice

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mrtool2u

Petty Officer 2nd Class
Joined
Jun 15, 2003
Messages
185
Boat is a 19 foot aluminum,,,motor is a 1993 Johnny 120..<br />1st prop,3 blade stainless(not sure of brand) 12 3/8 by 21 pitch,approx 41-42 mph,5100-5200 rpm's with motor trimmed to top line. <br /><br />2nd prop,Michigam Ballistic(stainless) 13 3/8 by 19 pitch,,44-45 mph,5400-5500 rpm's with motor trimmed to top line.<br /><br />3rd prop,,14 by 17 pitch OMC 3 blade aluminum,,41-42 mph,,5400-5500 rpm,s.<br /><br />I had a new 13 3/8 by 19 Rapture but I did not want to try it,,,it looked like that if you put the hub together it would look deffinetly used and would not be returnable. Would the rapture of gave any better top end,,i have heard a lot of guys talk about the Rapture.<br /> If I went with a 17 pitch ballistic,would I possibly lose some top end.What other props would possibly give me better top end,,I do not care as much about bottom end.Thanks for any and all info.
 
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Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Prop results,need advice

While I'm not a fan of any of those brand props, all the (good) info you provided indicates the motor needs to be raised. <br /><br />Note your baseline water pressure and then raise the motor. Now compare notes. You should pick up rpm and performance with each but you may start to vent especially with the small diameter prop. <br /><br />Post your results and we'll go from there.<br /><br />Good luck!
 
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mrtool2u

Petty Officer 2nd Class
Joined
Jun 15, 2003
Messages
185
Re: Prop results,need advice

Dhadley,first off,thanks for your response,,I see that you help a lot of people out.My motor is already about 2 inches off the top of the transom,but the cav plate is about even with the bottom of the boat.I assume that I should go 1 hole at a time on the motor mounts.You said that you are not a fan of these props,,what would you recommend.I really want to get this boat to its peak speed to outrun my loudmouth brother,,especially when he says things like,,,I will wait for you at the dock,,,,oooooohhh!!!Any and all info is much appreciated.
 
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Dhadley

Supreme Mariner
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Feb 4, 2001
Messages
16,978
Re: Prop results,need advice

For now I would stick with the props you have. You can always spend money on props later. Besides, if you change props now you will lose the baseline. There are a couple of schools of thought when dialing in a rig. <br /><br />One is to go up one hole at a time to see exactly what each change does with each prop. You will eventually pass optimium for each prop and then can decide to lower slightly or not. <br /><br />The other is to raise it all the way and test. If thats too high you can lower it down to the optimum. <br /><br />Either way will work. We've done it both ways. In a learning process I prefer to see it moved up one hole at a time. <br /><br />Just be sure to watch the water pressure. Its not unusual for todays props to be able to "bite" when the motors too high to get sufficient water.
 
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tony_cliffy

Petty Officer 2nd Class
Joined
Mar 19, 2003
Messages
182
Re: Prop results,need advice

Dhadley,<br /><br />It seems people focus mostly on pitch when talking about prop adjustments but how does the diameter play into mrtool's reported performance? it seems a larger diameter gives more bite, kind of like increasing pitch. Does that suggest a 14x21 is approx. equivalent to a 13.5x23? It is hard to compare/estimate performance since so many manufacturers have different diameters.<br /><br />Or stated differently, if I had a 14x23 from one vendor and needed more RPM's, what would I expect from a 13.25x21? Normally ~400 RPM's to account for the pitch change, but do I have to "double" that since I also dropped diameter? I found this happened to me when I did this exact same move. I wonder if that applies here too given his varying quote diameters?
 
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mrtool2u

Petty Officer 2nd Class
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Jun 15, 2003
Messages
185
Re: Prop results,need advice

OK Dhadley,,,you said that the info that I supplied suggests that I need to raise the motor,can I tap into your infinite wisdom and ask,,can you tell that by just looking at results normally,,or did you assume that the motor was sitting low,,,,or,,,should there of been bigger differences in the results from each prop. Next question,,,,and I realize that each boat is a little differnnt,,,and I will not try to take away your first born if not happy with your suggestion,,but being fairly inexperienced with props,,what props would you recommend.Like I said,I realize every boat is different.Thanks for your input..
 
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Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Prop results,need advice

tonycliffy --<br /><br />Diameter plays a large part in the prop game. There are many things that affect how the prop bites that have to do with diameter but basicaly a larger diameter SHOULD mean you can run a higher X dimension (higher on the transom and less gearcase drag). It really depends on the prop technology. It SHOULD also increase the blade area. It SHOULD increase lift too.<br /><br />mrtool2u --<br /><br />Its just that fooling with props for this long the info you gave seems consistant and about what we'd expect. The fact that you can use 100% of the trim (and I'd almost bet it seems the boat is wanting more) is a good indicator that the motor can be raised. <br /><br />The info you gave was also consistant by running your numbers to see what slip % you have. <br /><br />As far as which props, I really like the Stiletto and Turbo for these type boats. If you are even considering a jackplate, either manual or hydraulic, I would suggest holding off buying a prop until you get one and then go with a Turbo. If a jackplate is not gonna happen, go with a Stiletto.<br /><br />Same with 3 and 4 blade deals. If youre not gonna run high, stick with the 3 blade. In either scenario, dont pay for technology you'll never take advantage of.
 
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mrtool2u

Petty Officer 2nd Class
Joined
Jun 15, 2003
Messages
185
Re: Prop results,need advice

Dhadley,I checked my motor and the bottom of the cac. plate is about even with the bottom keel of boat. Actually on the trim,,I can trim up to the next to the top line on the gague,any more,and I gain a few rpms but seem to lose speed.But I assume that this is considered the optimum line on gague,like I said,if I go that last little bit top very top line,the speed drops off.
 
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G DANE

Commander
Joined
Nov 24, 2001
Messages
2,476
Re: Prop results,need advice

mrtool2u<br /><br />When you raise motor by trim, you will reduce drag, by getting less of lower unit in the water, but you will too change the ideal angle of lower unit in water, reducing some of the gained speed and changing boats attitute ( right word ? ) Lifting motor by trim doesnt optimize hight the best, only gives a hint of what happens if you raise motor. If you can trim that high up, without ventilating, you could as well raise motor a couple of holes.
 
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Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Prop results,need advice

mrtool2u --<br /><br />Thats right. For that prop thats getting real close to optimum. <br /><br />The ideal trim situation is to let the prop and the hull design lift the boat (reduces wetted surface or drag) and the propshaft would run parallel (no matter how low or high)to the waters surface to produce maximum forward thrust.<br /><br />The ideal gearcase location would be a situation where the bullet of the gearcase is "hidden" completely behind the transom and out of the water. Thus creating no drag. <br /><br />Increasing efficiency is the goal of every set up. Getting the right engine rpm is a minor part of the overall process.
 
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