berns29scarab

im currently spinning 15x32's on my 30 spectre to 6400 rpms at 106-107. have been offered a pair of 14.5 x30's..what should i expect. motors are 300x's and peak power is at 6800, limiters supposed to be at 7200 according to manual

throttleup

I would expect your RPM to increase around 600 RPM or so. Although the smaller diamter may effect your thrust and efficiency. Let us know how yoru testing goes.

Julie

Tantrum

If Julie is correct, and I have no reason to doubt her, 600rpm will put you at 7000.....to high in my opinion.
You also didnt say that your maintaining you numbers at full load, people and fuel.
Ive been told to prop the 300x's at 62-6300 in our heavier boats as that is where the torque curve flatten out.
If that is the case and I were you, I might try a set of 15x34. If your going to spend the money you may as well try and stay in front of me because Im working on ways to get infront of you
What do the prop pro's think about that ?
How much RPM would you loose going to a 34?

We are going to be on the Hudson this weekend.
You know what I have, bring your boat up one day and we can run all 4 sets on your boat and post results here to see what the pro's recommend.

berns29scarab

i talked to 4 diff people. 3 said to get the rpms closer to 6800 and get away from the torque curve and let the HP run the boat. and 1 says 64-6500 is perfect. it has always been my belief that the closer to peak torque you are thats when ya break stuff. no diff between cars and boats in that respect...any damage i've ever done to a drive train has been launching off the line when torque is at its highest . but hey i just run'um what do i know, other then SPORTSMASTER LOWERS HATE TORQUE. no probs with the 2.5's or shall i say not as many as we have with X's.

Tantrum

I agree it would be better to be above the torque curve for the sake of the drive. I just havent seen my boat be able to do that yet without underpropping it.

throttleup

I agree as well. Running at peak torque, is a recipe for disaster.

Julie

berns29scarab

well she ran 102.6 at 68-6900 but the midrange was awesome. felt like there was a 3rd engine back there. with my 15x32's 5000 rpms was 78-79 mph i lost about 6-8 in the mid...how can i keep the mid range acceleration and get my 6-8 mph cruise speed back ?

berns29scarab

ttt

throttleup

Yur peak cruise speeds are found at a lower RPM in the power band of your motor. So, if you stepped up in pitch you will decrease your RPM and attain better cruise speeds but top end may drop off. OR you could consider a 4B confuguration and pick up some cruise speed.

Julie

bbladesprops

Hey Bernie,

Told you the acceleration would be awesome. Your R's are perfect.

Get with Chris and do some serious testing.

When your ready, we'll make you that special recipe set of 14.5x32's!! Of course we'll need Chris's permission first. LOL

Brett