General Prop Question

[gwgautney]

Just trying to figure out how props work. If you have a 15X17 Prop on a Evinrude 175 and you put a 14 1/2 X 19 what would be the results generally speaking. I am just curious. I am in the middle of a remodel on my boat where I am installing a tachometer where it didn't have one before so I can't give you the readings as of now. I am just curious what the effects would be.

 

[roscoe]

Assuming the props are similar in design,
Your wot rpm will drop 200-300 rpm.
Speed may or may not increase by a mph or 3.


It really depends on where you were rpm wise, with the 15x17 prop.

If you had high rpm, say 5900, the slight drop would hardly be noticeable.

If you were on the low end of recommended rpm, say 4500, the additional drop from the new prop may lug your motor.

 

[Texasmark]

The rest of the story is the unseen variables which are things like where the engine torque is at the new rpm, how much wetted area your boat now has as compared to what you had meaning that your new prop will increase your speed disproportionally, with less wetted area and assuming similar prop characteristics what is your new slip....slip being the difference between a screw turning in a piece of wood and a screw turning in a glass of water which is not a solid medium. Then you throw in some prop characteristic changes, like changing material, or cupping, or high rake blades, or thinner blades, or number of blades and more things change.

So my man, the only way you really know is to run it. Everything else is pure speculation, may-be educated speculation, but speculation none the less.

HTH,
Mark

 

[gwgautney]

Ok sounds good once i get my paint and wiring done I'm going to compare the 17 to the 19 I'll let y'all know what comes out of it. Thanks for the info

 

[Chris1956]

gwgautney, The model number of your motor suggests it is a 1998 (the EC is key). It is a small point, as it is probably identical to the 1997 motor.

 

[tblshur]

my recent personal experience, i have 140hp on bass boat went from a 19 to a 17 , improved my hole shot not sure my tach is accurate but top end rpms sounds smooth. didnt get a chance to run very far but gps mph seemed to drop maybe 5mph not sure. hope this helps:joyous: my guess your top end speed will increase and hole shot will suffer.

 

[gwgautney]

My hole shoot is excellent wouldn't hurt to lose a little thanks Chris i need to change that i originally went by the plate on the motor which must show actual manufacturer date

 

[Sea Rider]

Gwgauntney,

Get the factory min-max wot rpm parameters stated for that engine, once you have them handy, trim engine well, go for a wot spin on flat calm, no windy waters with you alone and very lightly loaded boat. Check achieved max wot rpm and compare them to factory specs, will know if inside safe wot rpm readings. Stay with the prop that gives the highest rpm, but that's without over revving.

Happy Boating