Prop Question
 

Ok, I finally have our new Mariah in the garage. I'm still confused about the model but that will come later. My question is regarding two props that came with the boat. I have a one 3 blade stainless prop with an 18 pitch. The other is a standard, I'm assuming aluminum, 4 blade prop with an 18 pitch. Some one please educate me on the benefits of either of these props. Thanks in advance.

Congrats on the new boat!

The 4 blade aluminum will be good for hole shots, watersports, etc. It will provide good "bite" with the extra blade. It will also bring your top speed down due to being aluminum (having more flex) and having the resistance of a forth blade.

The 3 blade should be better for top end, and "just cruising" since it will keep your RPM's down (saving fuel).

The real question is... why are both only 18 pitch? A 20' boat with a 5.7 V8 should easily turn a 21 or maybe even a 23 pitch and gain significant top speed.

That makes sense, thank you. I was confused about the two props being the same pitch as well, but they came with the boat so no complaints here. I'm not sure if a person was buying two props they would have the same pitch. Would a higher pitch give me more top end speed?

You would think a higher pitch prop would increase your top end, but in reality the opposite is often true. If the pitch is too large, the motor cant turn the prop fast enough.

However, with that said, I'm with Indykoch, 18p seems too small for that boat/motor combination. You might be hitting the rev limiter before you get to your top speed.

I run a 21p 3 blade stainless, and it's a very good match to my boat (20' 5.7L Alpha 1).

imho.....yes, in this particular case. That 18" pitch likely has your motor exceeding the recommended rpm's at WOT. For the most part, lower pitch does equal lower top speed and vice versa but as Z202 pointed out, you can go too far and lose those benefits. I agree with the others in that your boat should be turning a 21" pitch 3 or 4 blade prop. Using a 3 or 4/5 blade prop kind of depends on what you use the boat for. If it's just cruising then a 3 blade works best. If you do some wakeboarding / water skiing / tubing then the 4 or even 5 blade prop gives more 'bite' and low to mid range punch at the expense of top speed. I would suggest though that you wait until you can get the boat on the water & give it a test run with those props you have. Note what the tach reads at WOT as well as your top speed (preferably with a GPS but okay with the speedo) then re-post your results & we'll try to guide you in right direction.

I need to clarify that I have a 5.0L Merc. not the 5.7L. Before I got the boat I was made to believe it had the 5.7. Anyway, I'm not sure .7 is a big difference. Like you said getting it out on the water will clear things up. Damn ice. Thanks for the help.