ziemer

iTrader: (1)

I agree! My full cabin '90 with stock 502/415's is right at 80 (ok, 79.3)...I think those small blocks will help a lot!

ttrm007

Kenny,
What RPM's are you turning at 83 with those 28's? My math says about 5200 if you have 10% slip. Mine hit the biggest power at 6200. If I can spin them that fast, I'll be dancing but I think that is a stretch? What drives? I think I may have asked that before but don't have it written down. I'm collecting a bunch of different numbers for 30's for comparison purposes. Something to occupy my mind during this long build.

ttrm007

Ziemer,

How about you? What drives? What RPM's at 79.3 and what props are you running?

Thanks,
Tom

ziemer

iTrader: (1)

Hydro Q-IV 28's... Right in the neighborhood of 5000-5100. Last time out port motor was tickling the rev limiter, which is 5150 (I think) and the stbd right behind that. Turning them in on B-1 drives.

ttrm007

Looks like these boats are very consistant .09 to .12 prop slip. That is really the variable we are talking about here along with the key question of how much prop can your power turn.

Thanks for the data. Here are some examples of 30' velocities and speed numbers.

502's 28", Bravo 1 HP500, 1.5 Bravo XR, 28" 5 blade hydro's HP500, 1.5 Bravo XR, 31" 5 blade hydro's 540 HP 383's Bravo 415hp 502's
RPM 5000 5300 5300 5200 5100
Drive Ratio 1.5 1.5 1.5 1.5 1.5
Prop 28 28 31 28 28
Slip 0.12 0.09 0.09 0.1 0.12
Max MPH 88.38383838 93.68686869 103.7247475 91.91919192 90.15151515
Real MPH 77.77777778 85.25505051 94.3895202 82.72727273 79.33333333

kenny j

I have bravo drives, 1.5 gears, turning 28 4 blade bravos, I'm spinning them way high, I'm up over 5600 Rpms, my slip is kinda high. We're guessing my HP at 550 a side?(the last owner built the 502's to HP500 specs, but has A/M steel heads, different intake, carbs, crossover tubes and dry exhaust). He had the motors built in 2003 and couldn't find all the paper work on specs, so I'm just guessing on the HP) I think my boat would be happy with a set of 30 bravos. I have a set of 30 and 31 spinillis, but my data is real inconsistent with those props, I need to go out one day and with good conditions and run all the props in both directions.. My boat is a solid 2mph slower with the props turning out... I was hoping my boat would like the spinillis, but I don't think I have the power for the 31's, and the 30's i was told the diffuser rings need cut off to see better top speed with them... I'm gonna try a set of 30 bravos once the boats out for the season... It was my first year with the boat last year, so at top speed I was focusing all my attention to trim gauges, steering my a$$ off, and holding my butthole closed... So the exact rpms I'm not sure, it was defiantly over 5600, into the 5700's. my friend has a GPS app on his phone, the while its taking video the speed is displayed on the screen, and he has a suction cup mount that hold the phone video taping the tac's... I'm gonna use that when I get my hands on those 30's

I'm sure my info dosnt help much, being I'm cloudy on HP and R's. I'm wondering if as I'm climbing the R's by trimming and tabbing, that maby I'm reaching max speed of 83.1 at like 5400-ish, and then over trimming it to far and pushing the R's into inefficiency? not gaining any, or even loosing speed? everything is hard to see at top speed with a 30' boat dancing around like a bass boat.! I slow down and look at the max speed after.? I know someone else could drive this boat better than I can.. What do you guys do as far as trim and tabs to get it up on the pad? I'm sure it's hard to type but like do you get on plane and throttle up, then bring up tabs and trim till top speed? Or do you get rolling 3/4 throttle with the tabs set, the. Trim and throttle together to top speed?
When the last owner took me on the test drive I coulda drank a cup of coffee at full speed... When I'm running in the 80's my friends are flying around the cockpit like it's a bouncy house.! And I'm steering so hard I look like in doing the chicken dance at a wedding.?!

kenny j

Oh, plus you gotta calculate in an extra Mph or 2 for "bad ass factor" on your boat.!!! ;-)

ziemer

iTrader: (1)

I like to get in the 4000-4500 range with the trim set around 5 ish and the tabs around 4. Hammer down and bring the drives up to 7-8 and slowly bring tabs up to optimal position. I can't get the tabs much above 3, as it wants to porpoise with the drives turning in. 3 or so is around parallel with the bottom so that's not a bad thing. With the drives in, the ass will feel a little light and you definitely have to drive it. However, you don't want to "overdrive" it either. If it's starts walking, bring the drives in a bit and start over...

Make sure drives are aligned, and adjust the tie bar if need. I believe I'm sitting around 1/8" toe in (props are 1/8" closer than nose of the drive) Also make sure your indicators match the position of the drives. So when you're at 5 on the indicators, both drives are at the same level. Same with tabs. My drives were out about a 1/2" on the toe and it made a huge difference.

I feel like with my stock power, I've got mine fairly high on the trim almost over trimmed to get the boat out of the water, but with big power, you could probably get away with less trim... maybe 6, 7 max...

ttrm007

Thanks much for the feedback guys. You reminded me that once I get all this work done, I'm still not going to be able to drive this beast!

bigboat28

Is your boat ready to go yet?