Water test on 115 HO etec

[Workboating]

I have a cmc jackplate. 5 inch set back, the motor is mounted all the way down on the jackplate and the JP is all the way down. JP was installed by the factory. During the test, the JP was all the way down, which lined up the cavitation plate even with or up 1" from the plane of the bottom of the boat.

 

[Workboating]

Ok, here's the latest test. I just did it 5 times to make sure. I don't know what happened yesterday. But at WOT, boat trimmed perfect, I got 5720 RPMs, speed 38.5, with a 13.5X17p prop. I did have 200 lbs less in the boat, but it still had close to operating weight. Should I go with the 19 p or 21p. I would think the 19 would get it close to 5500. Is that ok or should I go with the 21p that would drop it to 5300 plus a little.
Thanks.

 

[jlh3rd]

well...I finally decided to look up your motor myself and, the info that's been posted on here is not right for your motor. I should quit assuming.
Your rpm range @ WOT is 5500- 6000, according to Rubex....unless THEY are wrong.
So, depending on what you want, it's fine where it's at. So actually going to a 16" would get you closer to 6000 but cost you 1+ mph probably, but more weight in the boat would keep you above the 5500 minimum.
My pontoon boat I propped as close to my 6000, if not slightly over, with just me, so I could pull teens on tubes. Top speed suffered.
It depends on what you want.

 

[jimmbo]

54 56 i believe

I would prefer Real Numbers, not Beliefs... Find out.

 

[jimmbo]

I have a cmc jackplate. 5 inch set back, the motor is mounted all the way down on the jackplate and the JP is all the way down. JP was installed by the factory. During the test, the JP was all the way down, which lined up the cavitation plate even with or up 1" from the plane of the bottom of the boat.

You need to have the Rpm #s that are attained running at WOT, in the Configuration it will be running. It makes no sense to prop it for say, 6000(or what ever the Number you need), with the Motor lowered fully, then go screaming down the Lake with the Engine Jacked up. It also goes the other way, if propped with the Engine raised scraming away, and then you have to run it at lower speeds with engine dropped, again the prop will be wrong.

 

[Workboating]

Thanks jlh3rd, I attempted to locate that info, but had no luck finding it. I know how important it is to work with facts to get the correct results. I'm not a boat guy, this is a tool. I don't know where to find the info I need. I looked for all of that, but had no luck finding it. That's one of the reasons I came here to find out. I know there are people on here that are a wealth of knowledge, that could help.
Jimmbo, this is a Duckwater duck hunting boat. The reason for the jackplate is so I can travel in shallow water. Not fast, but still move. It's where the ducks are. If it's deep, the motor will be down, if it's not I'm not going to be moving fast. More like idle speed. I'm 62 yo, my need for speed left 30 plus years ago. Thank you both for being helpful, it is appreciated.

 

[jlh3rd]

ok...so, I think rubex is correct. Jimmbo is correct. You have to test the boat in one configuration.
And I'm sorry, I ass-u-me(d) ...you knew more, so I'll back up some.
Your motor operating manufacturers rpm range is 5500-6000 @ WOT.
Standard opinions on ventilation plate starting position is even with hull bottom, but yeah , can be maybe higher or lower.
Regardless, pick your testing position.
Rule of thumbs: this is with everything else being equal...@WOT....and +/-...
every 1" of prop pitch, up or down, gives a 200 rpm change , down or up.
17" - 18" slows rpm by 200 rpm.
Increasing diameter by 1" slows rpm by about 200, opposite is reverse.
Increasing by one blade ( 3blade to 4blade) decreases rpm by about 200
opposite is reverse.
Raising a motor can increase ventilation issues especially in turns, increase rpm, increase speed.
Lowering a motor can eliminate ventilation issues, lower rpm's but enable more "bite", but increases drag thereby reducing speed.
Adding weight to a boat decreases rpm, decreases speed. opposite is true.
Even different manufacturer props that are close to the same size can give different results because of blade shape or cupping...
...so...
It depends on what you want.
I propped my pontoon for pulling (torque) under load, not speed.
Also, for example, the Merc 4-14x14..Merc 3-16x13 and Solas 3-15.5x13 ALL gave the same results for my boat. Props are not magical. They are for fine tuning.

Now you see why we ask all these questions. There are many variables to consider.
pick a starting point with your boat, don't change anything, and correct #'s, then give us the max rpm @WOT so we can give options correctly.
If my minimum rpm @WOT is 5500, I would not operate my boat @WOT below that rpm. So when fully, legally loaded, I want to be above that.
That's just me.

 

[Workboating]

Thanks for your detailed response. My understanding is that my water tests today were good. 5720 RPMs @ wot, with a range of 5500 to 6000. Today the motor was in the all the way down position, (where I will run it whenever I'm running). The only reason for the motor to go up is to run below 1500 RPMs in shallow water. I might take off with the motor up in shallow water, but will lower it to the bottom as I come on plane.
A couple of questions. Do they make props in even numbers, or is it a special order item. Example: props seam to come in 13, 15, 17 19, and 21p.

 

[jimmbo]

The Pitch Values are picked by that particular Manufacturer. Some Props are even offered in One Inch changes. Props can be Tweaked, but results will vary by what is Done, and also by how Competent the Tweaker is.
Taking off with the Engine at higher Heights does offer a big chance of Ventilation resulting is loss of Thrust.
Anytime the Engine is Raised, there is a Chance of starving the Water Pump of Water, which can destroy the Water Pump Impeller, and overheating of the Engine

 

[jlh3rd]

Thanks for your detailed response. My understanding is that my water tests today were good. 5720 RPMs @ wot, with a range of 5500 to 6000. Today the motor was in the all the way down position, (where I will run it whenever I'm running). The only reason for the motor to go up is to run below 1500 RPMs in shallow water. I might take off with the motor up in shallow water, but will lower it to the bottom as I come on plane.
A couple of questions. Do they make props in even numbers, or is it a special order item. Example: props seam to come in 13, 15, 17 19, and 21p.

propping, even if someone knows what they are doing, can be....interesting, difficult, and expensive. I can't answer that question specifically, only by example. My new Merc. in 2021 came with a merc. 4blade - 14x14..an even pitch. I needed a 4 - 14x13, merc didn't make one...No one did that fit my larger diameter gear case. So my propping games began...I ended up with a 3 - 16x12 merc. prop. (which I destroyed )...🫤
If your inferring you want a 16" pitch prop to nudge up your rpm's, try googling that specific size prop including diameter and # of blades and see where it takes you. Also, don't forget it has to match the splines, or use a prop kit.

and you're welcome. I claim no expertise in stating my figures, most info I put out came from comments learned from old salts on here.