overpropped/RPM ?

[davejnz]

Just installed my tinytach(works great).Its on my 14'Lowe1448 alum boat with a 87'20hp Johnson.My manual says the operating range is 4500-5500.My current prop is a michigan match 10/13 alum.After doing some test runs I was able to achieve 5070RPM with just me(218lb)and I've also added about about 120lbs to the boat in the way of a floor/casting deck/battery/etc..The maximum load capacity on the boat says 550lbs.So i would say I had a medium load when testing my WOT RPM.So,is this RPM O.K. or do you think i'm overpropped?I want to get a stainless steel prop and dont know if i should order the same pitch.Any suggestions?

 

[rickdb1boat]

Re: overpropped/RPM ?

You are overpropped. Try a 9 or 10" pitch prop. Should get you right where you need to be.. You want to stay as close to the higher number as possible and even a bit above to avoid lugging your motor and causing any damage. Running under a load like that will also cause carbon to build up in the cylinders quick. Not a good thing.. On a 20 HP, I would stick with an Alumnimum prop...

 

[davejnz]

Re: overpropped/RPM ?

What RPM should my target RPM be?wouldn't a 12 pitch bring me up to 5270?The reason I want a SS is that I fish in the saltwater flats of mosquito laggoon(redfish capital of the world).And it really eats those alum ones up.I replace 1-2 times a year where my SS friends are getting 3-5 yrs.Plus,dont the SS props bite better because my antiventilation plate is about 1" below the bottom but even with the alum rails on the bottom <br />of hull.When trimmed out in the level position(parallel to the water surface) I will ventilate occasionally during rougher water and hard turns

 

[Bass Runner]

Re: overpropped/RPM ?

I don't know if they even make a stainless for that small of a motor, check in the mega mall here on iboats they are having a sale on stainless now, as far as the pitch it thing you are right going with the 12" if the book say top of 5500rpm

 

[davejnz]

Re: overpropped/RPM ?

Actually,the manual just gives the broad range.I was told by a board member to prop it for 5200 with a light load.Ballistic(mighigan) actually makes a SS in 10/13 and a 10/12 for my low hp application

 

[rickdb1boat]

Re: overpropped/RPM ?

And what happens when you add more weight to the boat? A 12" pitch will get you to 5270 with the load you are talking about, but that is still too low. do you ever plan on having anyone in your boat or more equipment onboard. If so, your RPM's will drop significantly. A stainless will get you more bite if it's cupped right, but as was said, they are hard to find for small motors. I do not know who told you to prop it for a light load at 5200, but that is contrary to everything I know about props. Sounds like you have made up your mind, so good luck fishing and have fun....

 

[davejnz]

Re: overpropped/RPM ?

Thanks for your advice Rick/bassrunner,it is appreciatd.I haven't made up my mind,Do you think I should try to prop it for 5500?In my manual it says the motor is rated at 20hp@5000RPM.So I figured 5200 was about right.<br />I really want a SS and i think ballistic is the only one that makes one for lower hp.Just unsure if i should go lower than a 10/12

 

[rickdb1boat]

Re: overpropped/RPM ?

Prop it toward the higher end. These motors will run 5800-6000 with no problem, so go for 5500-5700 at least... Go with the 10" pitch... You do not always have to run wide open speed, but when you need it, it will be there....