New prop Question

[Secretpursuit]

http://www.veradoclub.com/smf/index.php?topic=137.0

Attached link has a very good article on how to properly prop a boat. The key starting point being to set the proper engine height first for outboards( cavitation plate wet but not submerged). Two high you caviaite, too low your wasting engery / speed. Then proceed to determining the correct pitch. Followed this out when re-powering this spring, worked well.

 

[lorneparker1]

So it looks like I needa 13 pitch from my 17. So i had to buy a new prop as bending it to 15 wasnt going to get up to the higher end of the 5000-6000. Going to pick it up today and see what happens!

 

[Kristo]

with a 13p my guess is that it should jump out of the water and you will be able to get on step with a very heavy load top end would suffer though. I would have guessed that a 15p or 14p three blade would be a great all around prop...

 

[trendsetter]

With a 13p prop your top speed will be 29-30 MPH if you can reach 6000RPM. However your motor will be under a lot less stress than before. Just make sure that you don't over rev the motor past the 6kRPM limit.

As Kristo said a 15p would've probably been a better all around fit but if your boat is really heavy then that 13p should be ok. If you find the 13 is too small you can always get it bent up to a 14p with some added cupping.

 

[lorneparker1]

Well got the prop on today and in the water. 13 pitch is 5700rpm at wot. with another guy in the boat with me. top speed before was 35 and change and now its 34 and change, but is out of the hole quicker and sounds like shes living the dream compared to before. !

 

[trendsetter]

what are you using to measure speed..gps or a speedometer? reason I ask is it's impossible for that motor to push you more than 30 mph with a 13p prop.

gear ratio is 2.31:1, 13p prop, 5700RPM = 30.38 MPH with 0 slip. Most props will give 8-10% slip at wot.

 

[lorneparker1]

Gps. and its 24 not 34 with someone in it (typo). So i would assume it is 27ish by myself. Which is a pretty big decrease in speed however as i mentioned the motor sounds like its working way less Lorne

 

[trendsetter]

ahh, that makes more sense.


Now you can calculate the slip for that prop which by those numbers is about 20%. Ideally you want 10% or less at WOT. This is probably because the blade design is not meant for a heavy boat such as yours.

Is it possible to return the prop and try another one? With a heavy boat that is somewhat underpowered a 4-blade with more surface area can help with slip.

If you cant return it then the new prop will still be much better for your motor. Just less fuel efficient and a little slower than it needs to be.

HTH.

 

[Charlie415]

If you get it cupped to 14 it might help out. But I like the idea of a 4 blade.