Prop spin ?

[mda2002]

Hello , I have a 2004 Key Largo 206 with a 2004 mercury sws 150. According to the prop spin calc on boat ramp. com I am at 17% spin at WOT. rpm at WOT is 5400 and my speed according to my GPS is 40 mph. AT 4100 rpm Im at 11.7% spin and 31 mph. What can I do to decrease the spin.

 

[PAkev]

Re: Prop spin ?

I'm not a prop guru but will explain the best I can.<br /><br />The term "Prop Spin" is also known as "Prop Slip" which is the inefficiency of theoretical forward motion as per prop dimensions.<br /><br />For instance a 3 Blad 14Dia. X 19Pitch with 10% slip will have slip which reduces forward motion approx. 2in per turn. Not bad but reducing pitch to a 17 pitch will probably increase performance with less pitch. <br /> <br />What prop size are you turning?<br /><br />Going down in pitch often decreases slip.

 

[Ron G]

Re: Prop spin ?

Im not for sure but also, i think your rpms need to be up a little..

 

[mda2002]

Re: Prop spin ?

My current prop is 15" with a 17" pitch, I think that would be 15x17.

 

[Ron G]

Re: Prop spin ?

WHats your motor height now?Something dont sound right to me?maybe DH or Kenny can help more.ButI would think a 17 would have your RpmS on up there alot more.Tell us were your motor is at,is there a jackplate?

 

[gatorred]

Re: Prop spin ?

i had one spun the prop would slip by turning motor over by hand with prop in gear out of water it was a 6 hp johnson bigger motor's the prop shop marked my center with scrib,thus looked at blade shape now he said that was leading to slip from over powering it i hope this helped

 

[Ron G]

Re: Prop spin ?

I was thinking that to but if it was slipping that bad your rpms would shoot up,do they?

 

[mda2002]

Re: Prop spin ?

Im not sure of my motor height , no jack plate , Hull has a notched transom, transom height is 25" .RPMs dont shoot up. I know the WOT range is supposed to be 5000 to 5600 according to the manual. The boat and motor only have about 20 hrs. Also I am havin a cutting out problem at 5100 rpm and above and was told by the dealer that it may be a rev limiter, he told me to trim the motor down and see if that helps and if not bring it in.

 

[gatorred]

Re: Prop spin ?

blowout are part blowout? this was what was on my bass boat the dealer was telling me

 

[Ron G]

Re: Prop spin ?

Weres your cavation plate on the motor compared to the bottom of the boat?

 

[mda2002]

Re: Prop spin ?

I will check tomarrow and get back. Thank you for the help.

 

[walleyehed]

Re: Prop spin ?

if max RPM is 5600 by manual, I would guess the rev limiter to be in the 6200RPM range.<br />The dealer is reaching for straws..<br />It could be a fuel restriction...be careful.

 

[mda2002]

Re: Prop spin ?

Walleyhed:<br />I was thinking the same thing. I replaced the inline fuel filter with a see thru one and installed a , dare I say, inexspensive water seperater. I was starting to have this problem in lower rpm ranges, around 4500 rpm, but it cleared up after changing the inline filter. It's now doing it in higher rpm ranges , above 5000. Could it be that the filter on the seperater doesn't allow enough gallons per minute? I am going to buy a racor that allows 60gpm.

 

[mda2002]

Re: Prop spin ?

Ron:<br />Checked cavitation plate and its inline with the bottom of the boat.

 

[Ron G]

Re: Prop spin ?

I'm no guru for sure.Listen to kenny he's one of the best. <br /> Make sure you have no fuel restrictions first,Hows your water preassure? How big is the boat and how much weight are you pushing?

 

[mda2002]

Re: Prop spin ?

I don't have a water pressure gauge so that is unknown. the boat is a 20' KeyLargo. Not exactly sure on the weight but I would guess it to be close to 2900lbs loaded. I did make a mark on my prop to check if it has spun a little and will find out in the morning.

 

[Ron G]

Re: Prop spin ?

Good,I think you rally need a water preasure gauge,Thats an important gauge to see how your motors doing. Try to get the cutting out fixed first so as not to do any damage to the motor,then you can you try raising your motor up one hole to see if that will help,it should bring your rpms up a little and get ya a little more speed.Post up above in the mercury section to find out about the motor i wish i could help more.Then we'll go from there.

 

[mda2002]

Re: Prop spin ?

I checked the prop selector on mercurymarine.com and their recomending a 21" pitch. Does this sound right??. I would think this would kill my rpm's

 

[Ron G]

Re: Prop spin ?

As you are right now i think a 21p would kill your motor,Right now your running a 17p at wot at 5400rpms.are you sure your tac is correct?going to a 21 in theory would put you at 4600 rpms i dont know that much about motors but that would be killing it!Im no guru but something is off here i think,maybe one of the gurus can chime in and see what what they think.