Testing WOT & Prop Size

[985crabs]

A couple of weeks ago I'm out and the water is really flat and I felt like burning old gas in my tank. I pushed my motor further than I ever had before. I got up to about 46 mph @ 4800 RPMs. I'm not sure if it had anything left or not. It didn't seem like it, but I was scared to try. When I bought the motor the dealer told me WOT was 5500 RPMs and I was disappointed I wasn't getting that. I've since looked in the manual and it says WOT is in a range between 4500 and 5500 RPMs.

If I want to really see what its got to I just firewall it and see if I can get to 5500 RPMs without going over? The last thing I want to do is blow my powerhead. I'm running a 14.5 x 19P stainless prop. Is that pitch keeping me at the lower end of the range? I'm going to buy a spare prop and hardware to carry at all times, just in case, and I'm considering a 17P to see if it makes a difference. As it is I get a good hole shot and can plane quickly which is probably more important to me than top end speed. I just want to be sure I'm not putting undue strain on the motor by running the wrong size prop on it.

Any thoughts?

[gtightline]

Heck I think that's pretty good at 4800 rpm...I get around 46-48 mph at 5600-rpm on a 1998 115 hp 2 stroke. I'm running a 13.25 X 17 P.....Aluminum.

[gnrphil]

My thoughts are you need to be as close to max rpm as possible, but that's just me. Your in your operating rang but if your loaded your going to be working the motor harder, a 17 pitch prop would in theory raise your rpm's to 5200 and maybe a drop in diameter would get you a little more ( don't quote me on that though).
have fun with the prop selection thing although it can get confusing and expensive, Ken @ prop goods is the man to speak to.

[Juan]

I'd be proud of 46 mph at 4800 rpm but I tend to agree with gnrphil. For the best performance and efficiency, you should be as close to factory recommended rpms as possible. IMO, you could try a 17 or even lower pitched prop to get you there..
I know it's a different set up but it took a 14 1/4" X 15p to get mine to the factory recommended 6,000 rpms.

[freezerfiller]

I wouldn't touch it.... If you were lacking hole shot, maybe, but since you aren't, why bother?

[gnrphil]

Crabs, just a thought, are you sure your RPM's are correct? what gauge have you got. And yes 46mph at 4800 rpm's is awesome.

[985crabs]

I've got the Yamaha multi-purpose, Tach/trim/oil/and every other alarm gauge they installed when I got the motor. Speed is from the GPS.

I looked on the Yamaha site and used the prop selector there. The recommendations were right in line with what Juan said, 14 1/2" x 15P (if I selected the right motor model, they didn't list mine exactly). When I chose another 150 hp motor it went up to a 17P. I'm going to get one or the other just because I want a spare on the boat.

I usually cruise at around 40 mph in the 43-4500 RPM range (I may have misstated my numbers originally, I think I was going 44.6 mph at 4800 RPMs not 46.4). I rarely ever push it past that. So does it matter what I'm turning at WOT if I don't go to WOT? Am I straining this motor at the lower RPMs?

[gtightline]

I would say no. If you are planing nicely I don't think you would be putting any strain on the motor....You should open it up just to see what your WOT numbers are. As long as your not plowing through the water there shouldn't be any strain on that motor.If you get a good hole shot, and the boat planes quickly, then I would say you are good to go....just my 2 cents.

[gnrphil]

as I said in my first post that's just my opinion, you are within the operating rang so should be ok, I'd love to cruise at 40 mph @ 4300 rpm but my boat won't do 44 mph at WOT  .

[gnrphil]

Here's Ken's phone and E Mail
941 735 5808
[e-mail]
Or find him at www.bandofboaters.com/?_=1439213270588

[985crabs]

Thanks guys. I'm going to push it to the stops next time (conditions permitting) and see where the limits are. Then I'll be in a better position to make a determination and, perhaps, call Ken.

[mosquito13]

I just repropped my newly rebuilt 150 two stroke. Got a heck of a deal at the CCA event. Powertech worked with me on the repropping. Tons of good info and videos . Had a quicksilver thunderbolt 15.5x17P . They were real ez to deal with and matched what I wanted to get out of my new prop.  www.ptprop.com/index.php?option=com_frontpage&Itemid=1

[Warpig]

If I could get 44MPH @ 4800 RPM, I think I'd fall into the leave well enough alone camp here.

There is also a prop forum on Propgods. Tons of good info on there. When I was looking to change mine, I did some reading there and put up a few posts that were answered by Ken. That place is great. Here's a link to the forum.

www.propgods.com/forum/default.aspx?g=topics&f=1