Porpoising Seavette
 

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. :eek: 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?

Re: Porpoising Seavette
 

Re: Porpoising Seavette
 

that pic if too funny hahahahah

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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

Re: Porpoising Seavette
 

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.

Re: Porpoising Seavette
 

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.

Re: Porpoising Seavette
 

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. :eek:
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.

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

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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.

Re: Porpoising Seavette
 

Over 60? Yikes!!!!

I am more of slow and steady kind of guy. I try to keep it to around 55 or so

I got a pair of rev 4's last falland have yet to try them out. I am looking for some more stern lift.

Peter

Re: Porpoising Seavette
 

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 :D

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I feel that a cleaver might suffer at cruise due to not having enough blade surface area, however I'll try anything. At cruise I'm running 3500 RPM, 40 MPH and getting 2.4 MPG so I don't want my cruise to suffer just to get more MPH.

Re: Porpoising Seavette
 

Actually, the Mirage and the semi-Cleaver and Cleaver props are all stern lifters. The others mentioned are bow lifters. I use the old-time (early 1970's) definition of a semi-cleaver being a through-hub exhaust prop with true straight back cleaver blades. A full cleaver is over-the-hub exhaust.

Here is a cleaver. http://www.mercuryracing.com/_media/...leaver_med.png

Here is a semi-cleaver:

http://i278.photobucket.com/albums/k..._0231041-1.jpg

and here is an almost-semi-cleaver from PowerTech, the OFX3:
http://www.boattest.com/images-galle..._CFL4_CFS4.jpg

I truly don't know which will give you the best cruise performance, but I lean toward the almost semi-cleaver/semi-cleaver (or thru-hub cleaver). This may cost you a little top-end performance, but will cruise at 75% power better than a full cleaver.

75% power is defined in terms of hp produced, not in percentage of max rpms. For example, for my 90 Optimax, 75% power is 4600 rpms, which is actually 82% of max 5600 rpms. For my old t/260hp 26' Nova II, 75% power was 4400 rpms, 91% of the max 4800.

It's important to remember that any cleaver-type propeller blade is at its most efficient when it is run as a surface-piercing propeller. The amount of blade surfaced for best performance is a matter of constant trial and error.

Cleavers also ALWAYS cost you out of the hole performance, because of the cavitation induced under acceleration.

For the record, I ran thru-hub "semi-cleaver" props on my 26' Nova II (and both RH drives) after trying a lot of props. That was partly because I ran offshore a lot, and cleaver blades don't shock the drive-train as hard on re-entry after launching off the top of a swell.

Re: Porpoising Seavette
 

Thanks for clearing that up Fr. Frank, I figured if I spouted off I'd learn something yet again...Didn't think economy at cruise was an issue to a dude set on running over 60mph :D On the other hand you can do more often if there's more gas left to burn ;)

Re: Porpoising Seavette
 

You have a point, here. But SeaCrafts CAN be made to run over 60, and I have seen a 20' SeaCraft with a slightly modified hull approach (but not quite reach) 70 mph.

I didn't do the hull work, but I did tune the rig on a 20' SF with a 240 hp. Merc Bridgeport on a hydraulic jackplate. We ended up running a custom prop from 2nd Effort in Lk. Hamilton, FL. The boat was REALLY, REALLY Squirrelly from about 55 to 62-63, and then settled down until reaching 68, which was WOT. Without trim tabs and the hull mod, there is no way it could have been pushed past 62.

The hull mod was done by Rybovich/Spencer in WPB, and included a flat planing pad on the rearmost 5'-6' of the center V, much like the early Steve Stepp 22' Velocity hulls. With a similar modification, I believe a 20' or 23' can easily be pushed past 70 mph with modern outboards.

I think your hull, Steve, can be pushed to near 80 mph with a similar mod, and a change in drive and engine, ..say to Mercruiser EU662-SCI.
http://mercuryracing.com/_media/imag...662sci_med.png

Re: Porpoising Seavette
 

I just cannot imagine running a small boat that fast. You sure could not do that in our waters here, there is just way too junk in the waters. Right now, it would be dodging the tribal crab pots and kelp. Me, I am just a simple boaters who likes to cruise at 25-27 MPH. :cool:

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I can still hear that thing coming in from about 3 miles in the ICW at Longpoint = The Sweet sound of Horsepower

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Those speeds are not that ridiculous.

You may recall that the early '79 to '84 Thunderbird Formula 233SE (later called the 233LS) was available for order with twin 350ci/260 hp engines and (in '83-84) "Scimitar" gearcase sterndrives right from the factory, and could reach 74 mph "out of the box" in this config. (Post 1984 this was only available on the 242LS, 242SI or 272LS)

The same hull was also available with a single 454ci. 330, 370, or 440 hp big-block Mercruiser and TRS drive.

Standard power, however, was a single 198 hp Mercruiser or twin 170hp 4 cyl. Mercruisers

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Rev 4 better be a stern lift or I will be ticked off.

Prop Gods and others recommended the Rev 4 for stern lift.

In fact here is what he posted on his web site for me last October

Re: Porpoising Seavette
 

Peter, you are absolutely right. The Rev 4 is one of the best sternlifter's out there. My boo-boo.

Re: Porpoising Seavette
 

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

Re: Porpoising Seavette
 

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.

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

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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!!

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Lost, how fast is that seavette with the 502?

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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.

Re: Porpoising Seavette
 

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. :cool: :D I told him to get after it.

Re: Porpoising Seavette
 

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 :cool:

Re: Porpoising Seavette
 

[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. :o
Steve, do you have room for a 6-71 under the cover?

Re: Porpoising Seavette
 

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. :D 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.

Re: Porpoising Seavette
 

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

Re: Porpoising Seavette
 

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.

Re: Porpoising Seavette
 

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.

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I don't know if they make one for the TRS drive, I would love to try one if one is available.

Frank, I know about the compression. I could probably get away with a little boost however ideally I would need to cut the compression back to around 8.5:1 for it to work right. That will probably never happen but it doesn’t hurt to talk about it. :D