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Ive got a 13.75 by 15 aluminum prop on my 2010 merc 50 horse bigfoot. Its on a lowe roughneck 170rv. It performs well out of the hole and top speed is 30mph. It sounds like the rpms are high at top speed but i dont have a tach. I want to switch to a stainless prop and keep the same performance, what size and pitch do i need on the stainless prop?
You really need a tach to dial in a prop, otherwise we would just be guessing and that can get really expensive really fast. Google "tiny tach" and look into getting one. For less than 40 bucks you can get one and obtain a baseline WOT rpm that we can use to dial in your setup. Drop the pitch too much without knowing your baseline and you'll be out of a powerhead.
x 2 on the tinytach. Also, figure the estimated weight of your rig fully loaded, figure out your max rpms with current prop, and give this information to a prop shop. They should be able to help you out.
I installed tach and im turning 4600 loaded down and 4900 empty
Your RPMs are definitely not high. The recommended RPMs for that motor is 5500 to 6000. I would see if a local prop shop or boat dealer has any SS props you can test. I would prop it to run real close to 6000 with a moderate or light load. That way you will still be turning within your RPM range when you add another person and their gear. I'm thinking you will need an 11 or 12 pitch to gain that many RPMs. You will usually see an increase of about 200 RPMs for each degree of pitch decrease. But in addition to that when you change from aluminum to stainless you typically want to drop down one pitch to maintain the same RPMs due to the fact that SS props usually have more cupping and less flexing than aluminum. Since you are making such a drastic change in pitch I would test a couple first. Some places will let you test things for free and some charge a small fee.