tomcei

Not long ago I blew out a lower bravo XZ. I installed the replacement and found out that it's the wrong gear
It was a 1.5 and now it seems to be a 1.34/ 1.36 (something close to that) The lower came from a newer XR.
Anyway, I was running a 26b1 and was pretty happy. top speed was 74 @ 5300 with nice accelleration. I now have great cruze speed but lost accelleration and about 1000rpm(4300rpm @ wot)
What prop could I use to off set this gear? I was thinkig a 22 or 24 b1.
What are your thoughts?
Thanks,
Tom

29goingon42

I have a 1.36 and they will have good mid punch and top end as opposed to your 1.5's. You do need to prop way down. When I switched I had to go from a 4 blade Hydro 26 to a 4 blade Bravo 22 labbed and cut down to get back to my WOT of 5600-5800. You may also see it being a little sluggish coming out of the hole, I did.

Good luck.

tomcei

I was hoping I could find a smaller prop and the original performance would return.

If it doesn't I will regear the lower. I would much rather swap props

bbladesprops

Nope,

This is another issue that has been discussed for years on end. People will fight me till they are blue in the noggin. More engine revolutions create more throttle response and hole shot. Tall gears when you run out of prop pitch only!

Don't tell anyone. Especially the competition. shhhhhhhhh.

tomcei

I agree . What pitch would you recomend as a quick fix to bring me back where I was?

throttleup

Think of your drive as a transmission in a car and the prop as the rear end. If you car goes 60 mph in 4th gear at 3000 rpm with 4.1 rear end gears it's the same thing as having a 5th gear overdrive with 4.86 rear end gears. You are still going 60mph at 3000 rpm.

Same thing with a boat. If you are going 80mph at 5000 rpm with a 30 pitch prop and 1.5 gears and you change you gear ratio to 1.36 a proper prop pitch change is necessary. The difference between 1.5 and 1.36 gears is 10%. So a lowering your prop pitch by 10% should put you back to where you were.

The difference with a boat is when you change drive gear ratio you have changed the prop rpm at a given engine rpm. That's where the difference is. With a greater prop rpm you now have a higher prop tip speed. A 15" diameter prop turning 3500 rpm means the tips of the blades are traveling 312mph. If you change the gear ratio to 1.36 the prop tip speed, at the same engine rpm, is 343mph.

Now, enter hydrodynamics. The tips of the blades are traveling through the water faster, with higher (lower numerically) gears and this is what causes differences in performance. All these variables come into play when changing gear ratio. When the prop pitch is changed to a lower pitch the lift on the stern of the boat has changed as well.

That's why some boats will gain speed with a gear ratio change while other lose speed. Most of the time the set up of the boat is not changed. Only the gear ratio and props. It can take a lot of work to dial in a boat when making a gear ratio change.

In your case, if your rpm was at the high end of the rpm range I would go with a 24 Bravo 1. If it was on the low end of the rpm range I would go with a 22 Bravo 1. You may find you will have to modify either the 22 or 24 Bravo 1 to get the rpm right where you want it.

Matt

tomcei

If the math works out on the prop size( 22-24 pitch, which ever) and now I have a smaller pitch and the rpm and the mph went back to what it was, would the bow lifting effects be deminished with the smaller pitch?

throttleup

In theory you should have more bow lift because you will be running a lower pitch prop.

Matt