Porpoising Seavette

[jwclbi]

I agree with previous statements as speed will cause walking rather than porpoising. My 20sf will get just shy of 70mph on gps with 2 guys and half a tank of fuel, and will not porpoise, but will start to chine walk if im not careful. just mt 2 cents

[lost2a6]

Wow, 70MPH, that's great. What size engine are you running? I had a 20ft Sceptre with a 200 Yamaha and it would run about 55MPH, it also wouldn't porpoise but had a slight hint of chine walk when low on fuel.

[Caymanboy]

Yes, I am curious also, what power and what prop?

[jwclbi]

I'm running a 225efi short shaft merc and 14 1/2 x 19 prop. We also had perfect water and cool weather hitting 69.7 on gps at 6100 rpms. Friends have said when running like that with the boat trimmed there isnt a whole lot of boat in the water! However, I have noticed at those speeds there is considerable fuel burn!!

[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.