Prop Pitch ?

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stonybrookpictures

Cadet
Joined
Jun 7, 2005
Messages
15
I have a 20' Down East style Flat Bottom Round Chine Center Console (Eastporter 20) W/ 93 Mariner 90 HP. I dont know the pitch of the of the prop but I think it is
13 3/4" Diam.
15" pitch.

I rarely push the boat over 4000RPM, it just goes to fast.
Should I get a lower pitched prop to keep my speed the same but push the RPM's up? My manual says the motor wants to run @ 5000RPM
 
WillyBWright

WillyBWright

Fleet Admiral
Joined
Dec 29, 2003
Messages
8,200
Re: Prop Pitch ?

Probably closer to 5500 would be ideal. For every inch of pitch reduction, RPMs will rise by 200-250. Props for that come in 2" pitch differentials. So a 13" should give you 400-500 more than you're running now. However those are more commonly used on pontoons than planing boats. But you do have a 20' boat and that's kinda big for a 90.
 
Texasmark

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,788
Re: Prop Pitch ?

Mr Stony,

My '94 Mariner sales brochure says that the 3 cyl 90 (assuming the same as '93) is rated for WOT of 5500 rpm's. Also says that the gearbox is 2.3:1.

If you are happy with your current setup; boat speed, handling, engine noise, performance, and fuel economy, leave it alone. Just because you choose to run at 4000 doesn't necessarily mean you are hurting anything......and by your comments, you are indicating that you have a lot of whoopie left in the engine when putting along at 4000.

A 15p on that engine is not much prop (pitch). Now if the gearbox were 2:1 like a lot of higher hp Mercs then you might be pushing it; but I really doubt that too.....depends on the boat and load).

If you really want to know where you are, find a nice smooth spot on the water, with no one around (or any other thing that bothers you ) and put the pedal to the metal trimming the engine for max WOT speed. Come back with the rpm number (you can slow back down after you get it....grin) and we'll talk some more. I'm going to bet you will be up around 6000 or so. Wouldn't rev much over 6000 if it wants it. Once you hit 5500 you passed the test with your current rig.

Anything right at 5500 is perfect, a little over ok too. Now if you can't get to but 5000 then you might want to change something; but I doubt it......doubt that 5k is as high as you will go.

Let us know.

Mark
 
S

stonybrookpictures

Cadet
Joined
Jun 7, 2005
Messages
15
Re: Prop Pitch ?

I'll try that but its going to be scarry. I have never opened it up all the way, but at 4500 I feel like the boat is going to fast. We dont get a lot of flat days here on Narragansett Bay. The breeze is almost always 10-15 out of the SouthWest, with steep short chop
 
Texasmark

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,788
Re: Prop Pitch ?

Well man, if you don't feel right about it forget it. Again I only know what you've told me and I feel like you are on the other side.....pitch not as high as it COULD be (not has to be).

Sooooo you aren't hurting anything.

One more thought. I'm getting the feeling that you are going slowly due to water conditions. That means you seldom if ever go to WOT and you are interested in thrust and boat attitude control at the lower speeds.

How's your hole shot? If it's pretty snappy, like 5 seconds or less from dead stop to 25ish mph then that is a second data point saying you are in good shape with your current prop. If it is like 2 seconds and you are happy with everything else you are in great shape.

Mark
 
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