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Am I on the right track for prop selection?

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freebeerisgood

Member
I have a 26ft walk around cabin/10ft beam/powered by a 1998 7.4l MPI Bravo I w/~670hrs on it

Current prop is a Mirage Plus 3 blade/19p (48-13700) and I am making 4100 RPM WOT - It has always been that way since I put the engine in the boat in 2003, its running great but I would prefer to prop it correctly and get another 4-500 RPM at WOT.

Im looking at a Revolution 4 prop based on reviews mostly, with a desire to have better cruise performance and increased "bite", particularly for bad weather performance.

Im thinking that every inch of pitch reduced will give me ~200 RPM.....I need a 17P prop to give me about 4500RPM. Online, I read the Revolution 4 will turn 300RPM lower than my prop, at the same pitch. Looking at what is offered in the Revolution 4, it seems that the 15P is the best option to make the RPM my engine needs at WOT. That should give about 4600RPM??? Ballpark What can I realistically expect per inch? Is 200RPM accurate?

Am I on the right track?

Anyone recommend a 4 blade prop other than the Revolution 4?
 
It would help if you told us what the MAX WOT RPMs range is for this particular engine per the decal on the air cleaner.
 
Hi There. You should expect as you say 200-250 per pitch. Going to a 4 blade from a 3 blade you typically want to go down one pitch. That said, where you are currently in the engine's RPM range I would suggest going to a 4 blade 17p. Just my 2cents.
 
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freebeerisgood said:
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Im thinking that every inch of pitch reduced will give me ~200 RPM.....
That is a generalize estimate ONLY. It will vary with changes in gear reduction, engine horse power, hull weight, etc.

I need a 17P prop to give me about 4500RPM. Online, I read the Revolution 4 will turn 300RPM lower than my prop, at the same pitch. Looking at what is offered in the Revolution 4, it seems that the 15P is the best option to make the RPM my engine needs at WOT. That should give about 4600RPM??? Ballpark What can I realistically expect per inch? Is 200RPM accurate?

Am I on the right track?

Anyone recommend a 4 blade prop other than the Revolution 4?

Just make sure that your OEM Specified WOT RPM can be achieved. If we over-burden an engine, it may be over-burdened at any RPM.

Often we simply need to try several or more propellers until we get it right!
Click to expand...
 
re: Im thinking that every inch of pitch reduced will give me ~200 RPM.....
That is a generalize estimate ONLY. It will vary with changes in gear reduction, engine horse power, hull weight, etc.

I need a 17P prop to give me about 4500RPM. Online, I read the Revolution 4 will turn 300RPM lower than my prop, at the same pitch. Looking at what is offered in the Revolution 4, it seems that the 15P is the best option to make the RPM my engine needs at WOT. That should give about 4600RPM??? Ballpark What can I realistically expect per inch? Is 200RPM accurate?

Am I on the right track?

Anyone recommend a 4 blade prop other than the Revolution 4?

Just make sure that your OEM Specified WOT RPM can be achieved. If we over-burden an engine, it may be over-burdened at any RPM.

Often we simply need to try several or more propellers until we get it right! "



RPMs vs inch/pitch assumes a linear relationship. Its not linear... If you look at the engine's torque curve you will see its not particularly linear. Also the torque ( as converted to thrust by the prop) needed to move a boat varies most directly with wetted surface area, angle of attack and load ( among other things), and the effect of wetted surface/attitude varies with boat speed ( among other things..). I once changed props expecting a 400 RPM increase and only got 250. AS Ricardo states.. "Simply need to try several". Rarely pays to change more than one size (pitch step, usually 2 iches) at a time. Your guesstimate of a 15P 4 blade should get you in the specified range... You only need to be within the range, i.e., 4200 to 4600. In your case, as you concluded, you should try one more size to compensate for the 3 to 4 blade change.
 
Last edited:
sandkicker said:
re: Im thinking that every inch of pitch reduced will give me ~200 RPM.....
That is a generalize estimate ONLY. It will vary with changes in gear reduction, engine horse power, hull weight, etc.

I need a 17P prop to give me about 4500RPM. Online, I read the Revolution 4 will turn 300RPM lower than my prop, at the same pitch. Looking at what is offered in the Revolution 4, it seems that the 15P is the best option to make the RPM my engine needs at WOT. That should give about 4600RPM??? Ballpark What can I realistically expect per inch? Is 200RPM accurate?

Am I on the right track?

Anyone recommend a 4 blade prop other than the Revolution 4?

Just make sure that your OEM Specified WOT RPM can be achieved. If we over-burden an engine, it may be over-burdened at any RPM.

Often we simply need to try several or more propellers until we get it right! "



RPMs vs inch/pitch assumes a linear relationship. Its not linear... If you look at the engine's torque curve you will see its not particularly linear. Also the torque ( as converted to thrust by the prop) needed to move a boat varies most directly with wetted surface area, angle of attack and load ( among other things), and the effect of wetted surface/attitude varies with boat speed ( among other things..). I once changed props expecting a 400 RPM increase and only got 250. AS Ricardo states.. "Simply need to try several". Rarely pays to change more than one size (pitch step, usually 2 iches) at a time. Your guesstimate of a 15P 4 blade should get you in the specified range... You only need to be within the range, i.e., 4200 to 4600. In your case, as you concluded, you should try one more size to compensate for the 3 to 4 blade change.
Click to expand...


I appreciate the replies, from all of you.

Ive tried to find both a 17P and 15P to try before I buy, but its easier said than done. I may have to just purchase one, and I think either will keep the engine in the specified range.
 
Some not all marina's ON THE WATER if you ask nicely will allow to to test props. Just make sure you know what you are looking for before you ask them.

It makes sense to allow a potential customer to try a couple props and if one works he will make the purchase. If it is a 4 balded stainless steel that could be a good sale with good profit! They know it is a crap shoot sometimes and takes a little testing to get it right.

At least that is how we use to do it.

They may require a employee to ride with you or a deposit but if you get what you want out of it, it will be worth it.
 
Best bet is as jack recommended try before you buy. In your are there is PropGods that has a mobile prop trial service

http://www.propgods.com/Content2/Testing/tabid/62/Default.aspx

othewise propMD on line let's you exchange pitch on same prop for a small fee.

i was able to do the same with Propeller Works in Canandaigua NY, he took my card and gave me two props to try. Went from a 19" 3 blade to 18" 4 blade Michigan Vortex. Although aluminum really woke up the holshot and lowered my on plane rpm. A great value at $120.

Based on your numbers above I would think a 16" 4 blade would get around 4500 or so.
 
Dieter said:
Best bet is as jack recommended try before you buy. In your are there is PropGods that has a mobile prop trial service

http://www.propgods.com/Content2/Testing/tabid/62/Default.aspx

othewise propMD on line let's you exchange pitch on same prop for a small fee.

i was able to do the same with Propeller Works in Canandaigua NY, he took my card and gave me two props to try. Went from a 19" 3 blade to 18" 4 blade Michigan Vortex. Although aluminum really woke up the holshot and lowered my on plane rpm. A great value at $120.

Based on your numbers above I would think a 16" 4 blade would get around 4500 or so.
Click to expand...


Thanks for the link, Ill give those guys a call. I'd rather try a prop for a day offshore as opposed to the $95 an hour demo thing they do, but Ill see what they will do for me.
 
Yeah $95/HR is steep especially if you pretty much know what you need like you do. Hopefully they will let you try a few pitches for a day or two.
 
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