New guy prop question

Scott76

Cadet
Joined
Jul 29, 2017
Messages
16
Bought the boat this year, first boat. Sorry in advance for dumb questions


Boat has been good and I'm happy with it, but this other prop came with it and I'm wondering if it would be an upgrade.

Boat:

19' Renken bowrider. 'Shallow' V.
Merc 120hp 2.5 / Alpha One
15x19 Aluminum currently


Other prop is an aluminum turning point 14.25x23.

Usually 2-3 people in the boat. General cruising, some skiing. Boat sees around 4K at WOT, haven't done much WOT however as I have been getting used to it and I've had to repair a few things along the way.

My general understanding is that the 23 will give higher top end at the expense of acceleration. If that's the case I'll stick with what I've got. What I don't understand is how the pitch affects it, and what if any benefits the Turning Point design provides.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,033
You would probably lose about 800RPM if you put that 23P prop on. I will guess that the loss of 800 RPM would cause your boat to lug, which is bad for the motor.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Go for a wot spin with 3 up on flat calm water cond and check which prop makes engine to run towards max wot rpm range factory stated, stay with it, check engine specs for that water test.

Happy Boating
 

SkiDad

Lieutenant Commander
Joined
Jul 18, 2010
Messages
1,518
i would sell that 23, (it's way too high) and get a different prop maybe another 19p and keep one for a spare - Keep which one you like best on there. You want you engine to run around 4200-4300 at full throttle.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
If your engine is good shape, with an accurate tach, and you are getting 4000 with the 19, you are currently already over propped. I am assuming the WOT range is 4000 - 4400. You need to find what it the range is. Going to a 23 will probably keep the boat from even getting on plane, and even if it does manage to plane, it will be slower than what you are currently getting. A 17 would probably be a better choice.
A 23 in prop at any given rpm will be faster than a 17 or a 19, but the engine still has to be propped to run within a certain rpm range at WOT, meaning you can not select a prop that raises boat speed at less than WOT if it can't get to the specified rpm range at WOT, unless you want to see the engine die a Horrible Twitching Death.
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Listen to above advice...but also worth noting that it's a common mistake made to think that it's always the case that a higher pitch will make the boat go faster.
If your higher pitch prop isn't allowing the engine into the peak power rpm range, it will enevitably slow the boat down. Max HP will not be permitted, causing a slower top end. Not to mention the harm it can do and terrible fuel consumption.
When I first got my own boat it had a 17"p prop on it. It was mainly used for water skiing by the careful, but not too experienced previous owner.
When buying the boat, I knew this was likely too small a pitch and used it as a bargaining tool !
Sure enough, the V8 mpi hit the limiter very easily without any trim at 5100 rpm or so.
I half heartedly and without much research asked the local Merc dealer what prop he recommended. He insisted on a 23" pitch...I didn't argue, but was disappointed to find that this was a bad idea. The boat barely managed 4300rpm from memory and only ran about 50mph or just over. Anymore than 2/3 people in it and it would be slow as to plane.
He apologised and then insisted on a 21" pitch. This did a lot better and managed well over 60 mph (66 at its best from memory, but only just got her into the rpm range of 4600-5000 rpm.
I now have a 19"p performance prop (all be it of a slightly bigger diameter than average) and she runs as fast as the 21" but has none of the draw backs like slow planing and lugging at cruise speed.
Just a thought.
 

Scott76

Cadet
Joined
Jul 29, 2017
Messages
16
Makes sense. I'm going to try a smaller prop. Should I be looking for a 15x17, or a smaller diameter?
 

Scott76

Cadet
Joined
Jul 29, 2017
Messages
16
Disregard, did some looking and answered my own question. Going to give a 14.5x17 a whirl. Thanks!
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Don't sweat or lose sleep regarding the Dia., leave that to the prop engineers.
 
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