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Prop

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Curt:

--- Quote from: RJP44 on November 28, 2016, 05:17:26 PM ---Although a different hull i still think you should see similar numbers

--- End quote ---

Back when I had a 17 foot Tracker with a 40 HP Evinrude, I also topped out at about 32 MPH and I think that also had a 14 pitch prop. I had also added power trim & tilt, so I was able to trim up while running, which helped a great deal compared to when I first bought the boat with only manual tilt.  :nod:

jak0253:
 Thanks for the information, this is new to me. I get 28-29mph @5200 rpm, that is alone, with a passenger 26-27mph. (I am no light weight 250lb) After reading your reply's it is noticeably worst with a passenger(he is about 180lb) to the point that at times he would have to sit on the deck to get the nose down. Once on plane it runs great. I do start with the trim all the way down and other than adding a battery (24 volt trolling motor) just normal gear and most is stored in the front compartments. Maybe I am expecting too much? My last boat had a 70hp two stroke. I assume the prop is original and the affixed sticker looks to be factory applied. There is no indication that the motor has been raised or lowered, I am the second owner (motor has less than 15hrs.) So should I change the prop what would you fellas recommend? Thanks again.

Sully401:
@jak0253  I feel your pain! I currently own a 2001 PT 175 with a 40hp Mercury Force and i had similar issues when i first bought the boat. With just myself in the boat and all my gear i get about 27-28 mph out of my 40hp motor. Now my tach does not work currently and im actually not sure if i am running top RPMS for the motor. But i also believe i do not have the correct size prop, i could be wrong. My motor did come with a spare prop that is much larger. But as mentioned abut the larger prop allows for me to get onto plane faster, but my top speed is only about 26mph on a WOT.

As for the boat not getting onto plane very well i think its a weigh issue. My boat struggled when i first got it and turned out i had too much weight focused on the rear of the boat. I now store my anchor up front as well as a tool box for emergency repairs and that helped quite a bit. I also cleared out alot of unnecessary items under the seats and that helped free up some weight in the rear as well. Once i did that i had a noticeable difference. Even just for experimentation try adding some gym weights up front to see if it makes a difference.

frugalangler:
I ran some numbers based on a current model 40HP Merc 4S, which has a 2.0:1 gear-case, and I don't think they've changed since yours.

Your prop slip is approx. 55%, and since your WOT RPM is low @ 5200, that correlates. You should be in the 5500-6000 WOT range on that engine.

Running the calculation for a 14" prop, which would most likely attain the WOT RPM (I used 5800 for this calc.) you should see a noticeable improvement in launch, as it's a much lower ration prop, but your slip will likely come down (I ran a 20% for this calc., it might even go lower) and you should see about 31MPH WOT speed.

Those numbers are estimates, but what it is showing is that apparently that is a incorrect prop. Now, if that gear case is not a 2.0:1 and is something much lower (bigger number, it's a ratio) like 2.75:1 it's possible that is close to a correct prop as that gear case calculates out to a 39% slip - but even so WOT RPM is too low and that engine is lugging.

The best thing you could do is to get set up with a good prop dealer that has a try&buy program, get what you might think is a good starting point prop, try it, and see what you get. Then go up and down 1-2 pitches and see if it's better or worse. if it is better lower, go one more and see if it declines, if so, go back to previous, if better, increment again and compare. Prop selection is not a cut and dry science, it's trial and error with some guidelines.

What you're looking for is the lowest amount of slip (efficiency of the prop) that ultimately gives the best hole shot and top end @ correct WOT RPM.

As pointed out, hole shot can be managed (or miss-managed) by correct or incorrect loading (there is not black and white here either, it's trial and error) but a rule of thumb I've been given by boat builders is for a typical boat 1# at the bilge/transom equates to about 10# at the driver console and as much as 50# at the bow. Moving a few pounds of weight forward makes a major impact on bow loading, so moving that 20# anchor to the bow could very well change your take-off characteristics.

As for prop selection, usually going for WOT #'s first and then tweaking for hole shot is the approach most take, as generally a prop that runs WOT very well will also be capable of a decent hole shot given weight balance is managed correctly.

So, I hear your question: What size prop should I start with? Given what we know or assumptions (gear case ratio = 2.0:1) I'd start around 16" pitch, but using a try&buy so I can adjust and not cost an arm and leg! Yes, try&buy sometimes has a slight fee, but is definitely less than buying multiple props and then losing on reselling them if not correct.

Here's a link to the slip calculator I use: http://bblades.com/propeller-slip-calculator/

Hope this helps, I'm sure @bulletdc can chime in and fine tune the advise.

Curt:
@jak0253 Frugal just gave you some great advice. Some of it is highly technical and might seem overwhelming, but it's the kind of info that's used to figure out an answer to your problem from a technical standpoint.

If you can find a good prop dealer who has a try and buy program, it'll help you a great deal to be able to dial in exactly what you need for a prop on your boat.

And @Sully401 gave you some good advice also, when it comes to weight distribution. Sometimes small things can make a big difference. And in light of the fact that, as you said, you're not a small guy, adjusting and moving some stuff around in the boat can be helpful.

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