CB-BLR

I have a Nordic Heat with an SCX upper, and an SC lower unit.
Prior to my current motor upgrade, the motor was putting out about 750 hp.

I was regularly spinning a 30 P5x to 5600 rpm and 92.5 mph, with a best of 93.3 mph.
I also ran with a 1/2 inch spacer and spun a merc lab 32 to 5550 rpm and 91 mph.

The boat is more stable on the top end with the Bravo, I am assuming because it has less transom lift.

I just upgraded my motor to 950 horsepower, and the math says that I should be in the 101-104 mph range.

Peak horsepower is at 6100 rpm.

Would the boat be faster switching from my current 1.504 gear ratio to a 1.36, and spinning a smaller prop?

What are the advantages or disadvantages?

Chris

bobsbillets

From my experience a smaller pitch prop will get on plane easier. So based on that, you would be better with the 1.36 and the 30 pitch prop. I had 34 pitch bravo 1 and 1.50 gears and would get major prop blow out. Went to 1.35 gears and 32 pitch and hardly any blow out. Speeds were basically the same. Hope this helps.

JasonSmith

The word I get is the Bravo propellers get less efficient the higher in pitch they get. They are harder to come by also in the higher pitch ranges, above 30 based on what I've seen.

Mr Gadgets

Chris,
That 32 Bravo looks like 20% slip.. are those numbers correct?
The P5 looks like 13% slip.

To answer your question.. It used to be that some props worked better than others in a family.. Larger pitch Bravo's, Mirages, etc. So people would swap the gear ratio to stay with the better prop. Or in old days.. they didnt make big pitch props.. so you had to swap the gears. I know of alot of guys that swapped the upper gear set (when you could do that) and they would end up going back to the 1.5, for what ever reason. Consenses around here in those days was, work on the prop.. stay with the 1.5.

These days I don't know if that holds true anymore. You can talk to the prop people and get a feel for that.
When you change lower gears to get the ratio, the price jumps for the gear set, about $300. Now if it works, then it can be worth it. But if they fail as often as they do, it just cost more to run the smaller prop..
Just another perspective..

Dick

Nordicflame

iTrader: (4)

Chris,
When talking to Brett at BBlades about 32 and 34 Bravos he mentioned that the 32 was pretty much the end of the efficient props and that the 34 was somewhat of a different animal. I took that to mean "not desireable"
I think with the low end torque your Whipple produces that a gear change would certainly provide you many more options as to what to run.
It's a question we pondered on the RTech Heat. We had a shop in Havasu add cup to a 32 until they couldn't add any more (3Xs) and we still banged the rev limiter at 6000.
I'm sure that a gear change would have been the answer there but sea level boating only constituted about 5% of the boating time with the remainder at 5000 feet. Stayed with 1.5 for that reason.

For my current boat (Flame) I have a set of 32's repitched and labbed to 33's by BBlades for my low elevation props. I haven't tried them yet but I'm assuming that they will be pretty close becuase we loose 3" of pitch here and my high altitude props are lab 30s.
Let me know if you want to spin the RH 33 in your testing...
Dave

bajabob

chris i switch to a 136gears in my baja. me and my buddy have the same boat one is runnig136 gear the other 150 we have the same prop23mirage now the 136 gear at 3000rpm is doing 40mph. now the 150 is doing 3600rpm at 40mph . so that 600rpm less i trun. now my other buddy has a 25baja this boat is a little heavey then my 24 . it has a 540with a pro charger 150 gear runnig a 30pitch prop at 5200rpm doing 78mph . now i have 136 gears with 23pitch mirage at 5400rpm doing 80.6 . it was a hot day . and we gps all are speeds .. i like the 136 but you have to prop it rigth . this is my finding on my baja !!!i can not say it will work on other boat hulls good luck

CB-BLR

Just to update you guys. I bought a new 1" shortie IMCO lower with 1.34 gears. The upper SCX is geared slightly different than a SC so the final ratio comes out to 1.34 instead of 1.36 for a XR or SC upper.

Anyway.. I went from spinning a 32 bravo with the 1.5 gears at 3.5" of propshaft depth to a 28 bravo with the 1.34 gears, and a 3" propshaft depth (1/2" spacer). The boat does everything better. It comes on plane better... my slip dropped, and I gained 3 mph... making my new top speed 100.2 mph at 6180 rpm.

Chris

dennis r

iTrader: (1)

Chris Imco makes a spacer for the SCX upper to the SC lower?

Mr Gadgets

If you have the SCX vertical shaft in the SC lower, you can space it. I suspect that would be the case???
Just thought of that Dennis, after a good night's sleep.
I will look at that option for you.
Dick

CB-BLR

Dennis,

I have a SCX vertical shaft in my SC lower... because of this, you can use the SCX couplers which you can get in 1/2 inch increments. As for spacers plates, you can use either the SCX spacer plates (which follow the outline of the lower unit) or the less expensive Bravo/SC spacer plates.

Chris

P.S. I will have a SCX vertical shaft for a SC/Bravo lower for sale here in the next week. I am taking it out of my standard SC lower, and putting in the stock SC vertical shaft so I can sell the standard SC lower.