Which Prop?

[steam_mill]

A couple of weeks ago I purchased an old Merc 75. Early eighties. It runs well. Carb work, water pump etc.<br /><br />I had the dealer throw in an extra prop. <br /><br />The boat is a 12' aluminum. Figure no more than 150 lbs.(I'm guessing). <br /><br />The prop that came with the motor is a 9" pitch two blade. The second prop is an 8" pitch prop three blade.<br /><br />Both props are in great shape. The three blade is still painted. No knicks in either prop.<br /><br />I don't own a tiny tach and have no immediate plans to buy one.<br /><br />With the 9", the boat seems to go slightly quicker and not rev as high. With the 8", it revs higher and is slightly slower. Pushes the boat better with more on board (as expected).<br /><br />Any suggestions as to which prop I should use? I'm afraid of lugging with the 9" and over reving with the 8".<br /><br />I realize it is only one inch which should be no more than a couple of hundred RPMs.<br /><br />Comments? Which one would you use?<br /><br />Tx.<br /><br />Joe

 

[Coolmaker]

Re: Which Prop?

They make a vibration tach, that many boat dealers have. You just hold it on the motor and it will give you the rpm's. See if you can borrow one.

 

[steam_mill]

Re: Which Prop?

I was actually hoping for real world application.<br /><br />I think I'm okay with the smaller prop.<br /><br />Opinions?<br /><br />Tx.<br /><br />Joe

 

[LubeDude]

Re: Which Prop?

Originally posted by steam_mill:<br /> I was actually hoping for real world application.<br /><br /><br />Tx.<br /><br />Joe

Well, it sounds to me that you did your homework and did some searching on the forums, but dont want to listen to the advice others have gained from the forums.<br /><br />1. You need a tach, or we cannot help you, "Real world" or not.<br /><br />2, You need a tach to do this correctly!

 

[steam_mill]

Re: Which Prop?

LubeDude:<br /><br />I do know how to do this correctly. And yes, it is with a tach. I often read what you say regarding oils and quite enjoy it.<br /><br />Now, the tone of your post is not appreciated. Yes you may be right, but the delivery of the message is not very nice. <br /><br />In addition, being in Canada there is not the quick access to all items such as Tiny Tach. I was hoping there may be some sort of a cheat or maybe someone has a similar setup and could recommend one prop over the other.<br /><br />On the contrary, I do want to listen to what others have to say. That is why I posted.<br /><br />Tx.<br /><br />Joe

 

[ddaigle]

Re: Which Prop?

Run the 9 unless you have a big load. Its not going to matter much on a 7.5 anyway. Dont get freaked out over 1 inch of pitch.

 

[walleyehed]

Re: Which Prop?

Joe, do you have any shops around that sevice Briggs engines???(Lawnmowers, tillers) If so, they have a tach we use in the shop called a "Syrometer"...very inexpensive and is the vibrating type.<br />It's tough to tell someone which prop to use without a tach for reference, but "If" the engine does not appear to lug with the 9 and responds well with it, that would be the one to use. If you are having quite a struggle getting on plane, assuming your weight is not all in the back, you may need to run less pitch.

 

[LubeDude]

Re: Which Prop?

Originally posted by steam_mill:<br /> LubeDude:<br /><br />Now, the tone of your post is not appreciated. Joe

Didnt know you could hear the tone of my voice on the forum, but you heard wrong. It was just an incorrect observation on my part, sorry for any missunderstanding. My tact isnt the best at times.<br /><br />But you do need a tach, or at least borrow one as CoolMaker suggests. Lugging or over reving even a small engine is just as bad as a big one.

 

[Motor Boater Bill]

Re: Which Prop?

How good is your musical pitch? The F note three octaves above middle C is 5220 RPM. It works like this: F3 is 174 cycles per second. Divide by two for a two cylinder engine (two detonations per revolution on a two stroke engine). That gives you 87 Revs per second, times 60 equals 5220 RPM.<br /><br />F# is 5550 RPM.<br /><br />If you can get your hands on a pitch pipe or a harmonica, you can work this out.