proper prop size

[will.i]

hello, i have a volvo penta aq 140 with a 250 or 270 leg not really sure which it is. they are pushing a 18' hourston boat. with the prop i have on right now i am not happy with the speed i am getting with the rpm. the rpm that i run at is about 4500 with lots of throttle left, the boat gets to plan at around 4000 rpm. i was looking in the manual and the high and low range for the rpm for this motor is 5100 and 900. i have a couple of questions that i need help with: first is it good to run the motor at the max rpm ie 5100 and second is there a way to lower the rpm with the prop size.
i am new to this and not really sure of the effects of the pitch and size of a prop. a friend of mine told me that if i increase the pitch i will lower my rpm. i say this because the throttle i have left " approximately half " will be used. not sure if this makes sense but i am throwing it out there anyway. any help or advice would be appreciated. thanks in advance

 

[Silvertip]

Re: proper prop size

Yes -- changing prop diameter and pitch does alter how fast the engine runs and yes, it is safe to run at 5100 or the manufacturer would not recommend it. But there are several things you need to consider before you change props and your reasoning for making a change are bad ones. First, the prop on a boat is essentially the transmission and it has only one gear. That prop must get the boat on plane so it must deliver lots of push at startup. It must also cruise effectively and it must also provide decent top speed. In a car, the transmission has multiple gears to accomplish this so you can imagine what it would be like to merge onto a freeway in a car with only high gear. So a boat prop must be a compromise of power and speed. Every engine manufacturer (outboard or I/O) specifies a maximum recommended wide open throttle rpm with an average load. If your recommended wide open throttle rpm is 5100 then you need a prop that allows the engine to rev at that rpm.
If you install a prop with more pitch, it slows the engine and makes it work too hard getting on plane as well as not being able to reach its WOT rpm which is also bad for the engine. If you install a prop that has too little pitch, the boat will have lots of power on start up but can easily be over reved at WOT. So you see, you need the right prop. To help us help you, you need to check your current prop for either a part number of numbers such as 14 x 17 (14 inches in diameter and 17 inches of pitch). Then make several wide open throttle runs with what you consider an average load. Note the maximum rpm. If the engine runs faster than 5100 you need a prop with more pitch. If it can't reach 5100 you need a prop with less pitch. If it s very close to 5100 or just a tick over 5100 the prop you have is proper. Without those two numbers we can't be of much help.

 

[steelespike]

Re: proper prop size

We need to know your prop size,max wot(wide open throttle)rpm and speed.
Prop should be marked with size.
Here is my opinion on operating at max rpm.All I/O engines are adapted automotive engines In an auto an engine would usually operate at max rpm for seconds at a time in a boat they could be at max rpm for hours at a time.
And in a boat it is like running up hill in 2nd or third gear at max rpm.
Well cared for and driven conservitively they will deliver many hours of trouble free operation.Nothing wrong with running at wot I did occasionally in my 1929 Dodge inboard that had only 2 valve jobs in almost 80 summers of operation.And one of those was due to improper adjustment.No question it was driven very consertively and extremely well cared for.
Run your boat as you would your car,ski or what ever don't be afraid of wot.
Prop pitch:very basically increasing pitch lowers rpm and can increase speed if the motor can turn it. decreasing pitch increases rpm,hole shot may lower speed.Sometimes lower pitch may require running at reduced throttle to stay within rpm range.
the whole idea is to run the pitch that operates within the max rpm range.
Sometimes it is necessary to change to a lower pitch for skiing etc,

 

[will.i]

Re: proper prop size

thanks for the reply very helpful. i am taking the boat out today and will play around with it and post my results. thanks again and tight lines to all

 

[will.i]

Re: proper prop size

took the boat out and with a wot i was at about 5400 rpm on the tac. which prop would be best if the suggested top rpm is at 5100 or is this too small of a amount to worry about. thanks again for all the help