Porpoising Seavette

[jwclbi]

Lost, how fast is that seavette with the 502?

[lost2a6]

I've got 64 MPH out of it once, but it will do 63 all of the time with a full tank of gas and porpoising it's a$$ off.

[lost2a6]

A little up date, Brett at BBlades sent me a couple of test propellers. He sent me the Mirage plus 21 pitch and a Bravo 1 22 pitch 4 blade.

Here are the results with the Mirage plus: 3000 RPM=30 MPH, 3500 RPM=40.5-41 MPH, WOT=5200 RPM @ 64.7 MPH with drastic porpoising. Even at 30 MPH with the drive all the way down it still wanted to porpoise.

Next I switched to the Bravo 1, those results are: 2800 RPM=30MPH, 3500RPM=42.5-43MPH, and WOT=5200RPM @ 63.2MPH with not near as much porpoising. I can even cruise @ 30MPH and trim the drive up some and it doesn't porpoise. Even though I lost a little MPH on top end with the Bravo1 it is by far the best all around prop for my boat. So what Brent wants to do is to Lab finish a 24 pitch Bravo1. He said that I should get more MPH out of it than the Mirage plus and that I should see 66-67 MPH. I told him to get after it.

[McGillicuddy]

According to Merc Racing the Mirage Plus is a maximum bow lifter.

http://www.mercuryracing.com/propellers/mirageplus.php

They describe the Bravos and Trophys as 70 mph+ w/ Med bow lift and the 3 and 4 blade cleavers as 80mph+ w/little bow lift

Problem might be that their pitches are > 25" and your boat may be heavier than props were designed for. I'd hit the prop guy up for a 3 blade cleaver or a 4 blade trophy or bravo if they make such a thing for stern drives and see doesn't ride a little flatter at the crazy speeds your you're attaining. Again this theory is based on Merc Marketing not my experience. But as long as you got a guy loaning you props - heck, check 'em

[gofastsandman]

[QUOTE]
According to Merc Racing the Mirage Plus is a maximum bow lifter.

Judging from how my boat rides, the Mirage+ is a good stern liter. My Michigan Rapture would give me tons of positive trim. I would almost lose the horizon on hole shot not so w/ the Mirage+. I can feel the stern rising w/ the Mirage. It`s a nice feeling. I also think I learned why Denny`s 4 blade Apollo beat my Mirage+ WOT when he did his comparo.
4 blades lift more boat out of the water. Less wetted surface= higher top end.
Steve, do you have room for a 6-71 under the cover?

[lost2a6]

I have learned a ton of info on props these past couple of weeks. From what I have learned is by just adding the extra blade will give you stern lift such as going from a 3 blade to 4, or a 4 to 5 blade, etc. However when you add rake along with cup it will raise the bow. The Revolution 4 will lift the whole boat because it's a four blade with a good amount of rake and some cup. The problem with a 4 blade is you'll lose a little on the top end because of the extra blade creating extra drag in the water. If you look closely at the Mirage Plus it has a lot of rake and cup which makes it a bow lifting prop. The Bravo I has ran the best so far on my boat because it's a 4 blade with not much rake or cup, it will almost remind you of a cleaver prop.

I don't have room for a 6-71, I wish that I did. I had to already cut 1" off of my flame arrester. Although I bet that I could make a Paxton supercharger work. At this time I have about reached the limits of the TRS outdrive and the Mercruiser transmission, so I'm going to leave well enough alone.

[McGillicuddy]

Whoopsie, I missed the part about the bravo being a 4 blade...

[Fr. Frank]

Steve, I don't think a Paxton blower on a high compression motor will work well. My brother had an underdriven Paxton on his '76 Mustang II Cobra. It worked well enough until he had his heads milled and rebuilt the valve train and installed new dome pistons to obtain an 11.25:1 compression ratio. (At that time Paxton reccomended a max comp ratio on the stock Ford 302 of 9.5:1)
His crankcase pressure was so high at high revs he would blow immense quantities of oil out of the breather, and somehow he eventually hydraulic'd the motor with oil blow-by , and had it come apart completely. It even blew the oil pan loose on the bottom.

[PipeDreamsMarine]

Steve.. Have you tried a 4" drive extender, to set the drive back 4". My old Wellcraft Nova did the same thing and the drive extender solved the problem for me.