Porpoising Seavette

lost2a6 · #1 04-14-2009, 08:52 PM

Are any of you guys that have a 23 SC running over 60 MPH? The reason why I ask is because my boat starts porpoising at 60 and then just gets worse the faster that I go. It will even do it with the outdrive all the way down.

I can put the tabs down some and it will stop but I would like to get it to run with out using the tabs. I was told by a local prop guy that the hull just wasn't designed to go that fast. I have tried several props with about the same results. I haven't tried a semi cleaver prop yet. I believe that the boat just has too much weight in the stern. What do you guys think?

Capt Chuck · #2 04-14-2009, 08:59 PM

Quote:

Are any of you guys that have a 23 SC running over 60 MPH? my boat starts porpoising at 60 and then just gets worse the faster that I go. What do you guys think?

Captainkass · #3 04-14-2009, 09:35 PM

that pic if too funny hahahahah

Snookerd · #4 04-14-2009, 09:44 PM

I have been up to 60 a bunch with the twin 225's. I have serious chine walk but not porpoising. I have an Armstrong bracket and the carbed motors are around 445lbs each

Finster · #5 04-14-2009, 10:07 PM

Steve, put some more up front and I'd be willing to bet that solves your problem. If you have the weight distributed right, that boat should chine walk at high speeds before it porpoises.

Fr. Frank · #6 04-14-2009, 11:30 PM

Steve, check the height of the cavitation plate above the hull with the cavitation plate as perfectly parallel as possible to the keel line. I'm wondering if you have wake flow over the top of your cavitation plate, which pushes the stern down.

What happens then is the wake pushes the cavitation plate down (and the stern with it) and so levers the bow upward, until the weight of the hull out of the water over-balances the downward thrust on the cavitation plate. Then the bow falls until the thrust on the cavitation plate is greater than the lofted hull weight, and the cycle begins again.

Ideally, a straight edge extended straight back from your keel should pass midway between the water inlets on the outdrive and the cavitation plate on an Alpha or Bravo outdrive. On a TRS or TRS II drive, the cavitation plate should be about 1" to 2" higher than the keel line.

At lower speeds this isn't a problem, but the higher speeds create a lot more downward pressure on the cavitation plate.

lost2a6 · #7 04-15-2009, 12:05 AM

Funny, Capt Chuck. I'm going to try 100lbs in the anchor locker to see what effect it has on it. The weight of the engine and drive is about 1200 lbs plus two batteries. Also the fuel tank is right in front of the engine. Plus everyone is sitting in the stern of the boat. When I slow down, the little rooster tail that follows up to the transom will actually splash over into the boat.

It doesn't have any hint of chine walk.

Frank, my cavitation plate is 2" above the keel which puts the center of the prop shaft 7" below the keel. According to the guys over at offshoreonly.com this is too deep, however I don't know because at cruise I can't quite get the drive at neutral position and the prop starts to cavitate. I feel that it still has a few more MPH in it if I can get it all dialed in.

Fr. Frank · #8 04-15-2009, 11:15 PM

What's your prop? Are you running the original TRS drive?

lost2a6 · #9 04-16-2009, 05:12 PM

Yes it is the TRS drive. So far I’ve tried a Lazer II, Ballistic, Mirage, and Revolution 4. The Mirage is the fastest however overall I believe that I like the Rev 4 the best so far. I want to try a semi-cleaver and a Bravo prop.

McGillicuddy · #10 04-17-2009, 01:05 AM

Everything you mentioned but the Rev 4 is a bow lifter, I'd hold out for Fr Franks opinion, but I think you skip over the the Bravo series and the semi-cleaver and jump into the cleaver category. Get the rear end up in the air, keep the pointy end down, "bend the throttle and let 'er eat" as has been oft repeated here. I have never run that fast so this is just theoretical not my experience...Heck I don't even like to drive that fast