Prop pitch Calculation

[esoxesox]

Hello Boys

I am switching from a Renegade Offshore 17" pitch to an Offshore 19" pitch on my 20' CC (running a 1996 150 Johnson FastStrike). I was previously running a stock SS three blade prop with a 19" pitch. When I went to the 17" Renegade I lost a bit in top speed and could run the tach up a little higher than I liked. My hope is that the 19" Renegade will pull down the RPMs and push up the high end PLUS add the benefits of the 4 blade. I hoped to get it in to run it tonight but it might be a while before I can. Does anybody have a formula that will help predict the change in RPM and top speed? I'm dying to know how this new prop will change the boats performance.

Thanks...

Scot

[barnacle]

Going from a 17 to a 19 you should see about a 400 rpm loss. You speed will go up even with your RPMs comming down. When you prop a outboard it is very important that the engine can hit the proper RPM. If im not mistaken your engine is a 5500 rpm engine. YOu want to make sure it can hit 5500 rpm if it only hits say 5100 the engine will probley live but it very hard on the powerhead. If your shy a bit you can raise the engine up one hole on the transome to pick up some more rpms. Just be sure your water intakes do not get to high. When playing with engine hights its good to have a water pressure gauge installed to make sure the cooling system is ok at higher engine mounting. What rpms are you getting with the 17?

[esoxesox]

Joe, I could get it up around 6000 when it was trimmed just right. I'm hoping the 19" will bring it down around 5500. I really don't run it that high too often. The big pond is usually rolling a bit too much for that...now and then you gotta let her roll though. I have thought about installing a jackplate to move the engine back and give me the ability to fiddle with the engine height. Anybody have any experience with that? It seems like most guys, if doing anything beyond the transom mount, are putting brackets on their 20's.

Thanks for the note...Scot

[JohnS]

Scot, I am not sure of the formula but, I know MercuryMarine.com has a prop calculator on their website. You may want to plug in your number there and see what it says. Also, if you have a dealer in your area that is willing to work with you, at times, they are willing to let you try out different props before you buy them.

John

[JohnS]

http://www.mercurymarine.com/prop_selector

http://www.admboatworks.com/pitch.htm

http://www.rbbi.com/folders/prop/propcalc.htm

http://continuouswave.com/cgi-bin/propcalc.pl

http://www.hillpropellers.com/propcalc.html

[ May 14, 2003, 12:16 PM: Message edited by: JohnS ]

[esoxesox]

Hey John

Thanks for those sites...very cool. Looks like the change from a 17" to a 19" will get me ~ 5 mph more on the top end if the engine will turn the prop to the necessary RPMs. One of these days I'll get my sh!t together on the technical specs on my rig. I don't have the faintest idea what my slip % is...

Thanks again,

Scot

[barnacle]

THe offshore 19 should be perfect if your hitting 6000 with the 17. Your engine makes its hp at 5500. anything over that its just burning gas.