Prop, Porpoise, Boat Balance Convincor

[Mark A]

I had tried a 19” Mirageplus for a demo and it was much better than the cheap prop but it too would porpoise with any trim so I did not keep it.

From all that I have read, a “stern lifting” prop should help and a 4 blade prop might be another option. Also mentioned is moving weight forward away from the stern. I moved the batteries forward as far as I could mounting them in under the rear bench seat. It seemed to help a little.

Any suggestions for a prop or other suggestions would be appreciated.

I wondered though the following:

1. If anyone had ever moved the fuel tank forward in trying to improve the dynamic balance of the boat. I replaced this tank when I purchased the boat and there is about 10 inches in front of the fuel tank which I believe would shift a good chunk of weight forward.

2. Would the factory or builder of a boat like this shift the tank forward when they built this boat and equipped it with a big block (heavier) engine?

3. The Mercruiser 6.2 Horizon closed loop cooled engine requires water be fed from the bravo drive and be fed from a secondary through hull fitting (through hull intake strainer) like shown here:

http://www.freshwatersystems.com/p-2823-bronze-one-piece-intake-strainer-34.aspx

Could the strainer be a factor being in the area of the bottom of the boat a couple of feet or so from the prop? If so, how else could water be fed to cool this engine? How do they do it on new boats? I found that there are stern mounted water pickups and wonder if that would be a better than the through hull strainer.

 

[Old Navy]

Before you move the tank, try some different props like a rev 4 or a small bravo 1. If that helps at all, put some weight in the front and see if it gets better. If that solves it then weight redistribution is the answer. Some on the board have had the same problem so do a search.

 

[winter]

Hard to tell from the avitar, but do you have hydraulic trim tabs on your boat? I know that these boats could have been ordered w/o tabs, though most ppl opted for it. If you dont have them, i'd go that route. The mods you are speaking of will likely rob you of speed. Tabs will help control the bounce while having no real effect of drag (loss of speed).

 

[bigsbetter]

yep, my Convincor porpoises also..if you don't know how to drive it. After I tried Hydromotive props, it may have been a lil better, or maybe just me hopeing so badly that it was better that 'I' thought it was. Here's a good theory.......My top speed was originally about 55mph, I could trim it all the way up at WOT and there was no porpoiseing!...At cruiseing speed it porpoised like crazy. So my bright idea was to install a SUPERCHARGER!! So then my cruiseing speed would be close to 50 mph and maybe it wouldn't porpoise so much! So I put the charger on, tested the boat by myself with no rear seats and no big heavy engine hatch, probably about 350lbs removed from the rear of the boat. No porpoiseing at any speeds! Weight at the rear has something to do with it. After reinstalling my interior and hatch the porpoise was back....at most speeds below 55mph. Any way, i thought about moving my 2 batteries all the way to the front of the cuddy in the space behind the mirror in the very front of the bow.???, but that's gonna be a lot of heavy wire. I think it's bumb that they porpoise, but I like the boat so I make it sound like a good thing to other people....I tell them when it porpoises a little ,That Checkmate design the hulls that way so they could get 2-3 more top end mph in their boats!

 

[winter]

.......I tell them when it porpoises a little ,That Checkmate design the hulls that way so they could get 2-3 more top end mph in their boats!

And that's the truth. If you don't want the porpoise, buy a Baja
All this is generally the same for my Persuader. It's pretty simple, if i'm cruising with my kids or friends, i drop the trim in a bit and all is smooth, though you can feel the drag on the bottom. When we need to get to business, the trim comes up and the hammer goes down. Porpoise goes away at 45mph

 

[hotdaddy27]

7... I think it's bumb that they porpoise, but I like the boat so I make it sound like a good thing to other people....I tell them when it porpoises a little ,That Checkmate design the hulls that way so they could get 2-3 more top end mph in their boats!

I love it...that is something I would say!!! LOL

 

[ezstriper]

more than likely these boats have less hook in the hull for the higher top speed but can cause this at lower speeds...Rob

 

[Craig]

There is a thread started earlier in this same forum titled "Need to tame Porpoise". There are some good comments there from Big Red and others.

I had a Convincor (270/259) model for 3 years and that boat would poproise at slow speeds, meaining speeds under cruise, anywhere between 40-30 mph depending which prop I had on. The Bravo 1 props worked the best as far as helping porpoise. Regardless of what prop I had on, it was always controllable....constant adjustments in trim depending on what speed you were going. Also wrok the throttle, give it a little more in combo with trim adjustments to pop it back onto it's pad. Definitely not a boat that you can just set the trim and throttle and leave it there, but once you get to a good cruise speed you should be able to. My 270 would always cruise porpoise-free at 50mph, all they way down to 40mph..after that I had to work the trim to stay steady. Also there are other ways: if you want to cruise slower with no propoise, keep the boat guessing, don't always run in a straight line, and very the throttle setting.

One thing I disagree with is the need for trim tabs. Yeay they help, put that just seems like an ineffiicient way to run a boat to me dragging those things down in the water, makes for worse gas milage too. My Convincor had trim tabs, and I hardly ever used em!

-Craig

 

[bigsbetter]

Definitley a drivers boat! If you don't want to 'drive' then dont buy one. Constantly adjusting tabs and trim, I don't mind it, it gives me something to do. When the hammer goes down and the supercharger is buzz'n the trim goes up to the max and the tabs are all the way up and then there is never a thought about porpoise or chine walk and then I can finally relax and quit 'driveing' it!

 

[winter]

There is a thread started earlier in this same forum titled "Need to tame Porpoise". There are some good comments there from Big Red and others.......
-Craig

Yup, that was me I truely thought i had a set-up issue, but came to realize that it is just the way the boat was built. It came from the dealings i had with my hydrostream and the very uncomfortable chine walking issue that dern near killed me. Mates don't seem to have that problem thank goodness.

And as mentioned, i like to get the trim right to the edge of porpoise then correct it with small steering changes, un-noticeable to my passengers.

 

[bigsbetter]

.

And as mentioned, i like to get the trim right to the edge of porpoise then correct it with small steering changes, un-noticeable to my passengers.[/quote]

Yep, that's the way to do it. And that's all good for us except last weekend we took my neighbors out for the first time....everthings going great as usual, and then on the 15 mile trip back home up river, he asked if he could drive for a 'bit. I said sure and just let him take control while the boat was still cruiseing at about 35 mph. I felt bad 'cause we're about 60lbs diff. in weight and all that change in the boat...he did'nt know how to drive it. It was bouncing and leaning and everything!

 

[Mark A]

Somehow part of my post was cut out. Here are some details that should have been in the original post:

1986 Convincer with 320 HP 6.2 Horizon motor and Bravo 1 outdrive.

Looking to prop boat but have no good prop shops in the area.

Cheap Michigan Wheel 21” Aluminum on the boat now and have a severe porpoise.

There is no hook or rocker on the bottom of the boat. I believe the outdrive is at the proper standard height with the anti-ventilation plate close to level with the bottom of the boat. Other than when trimmed all the way in the boat will porpoise. Even trimmed all the way in I need to use some amount of trim taps to control the porpoise. So with a lot of trim tab, and accepting the boat porpoising I can get the RPMs to about 5000 and 50 MPH. (I think 21” is the proper pitch.)

I had tried a 19” Mirageplus for a demo and it was much better than the cheap prop but it too would porpoise with any trim so I did not keep it.

From all that I have read, a “stern lifting” prop should help and a 4 blade prop might be another option. Also mentioned is moving weight forward away from the stern. I moved the batteries forward as far as I could mounting them in under the rear bench seat. It seemed to help a little.

Any suggestions for a prop or other suggestions would be appreciated.

I wondered though the following:

1. If anyone had ever moved the fuel tank forward in trying to improve the dynamic balance of the boat. I replaced this tank when I purchased the boat and there is about 10 inches in front of the fuel tank which I believe would shift a good chunk of weight forward.

2. Would the factory or builder of a boat like this shift the tank forward when they built this boat and equipped it with a big block (heavier) engine?

3. The Mercruiser 6.2 Horizon closed loop cooled engine requires water be fed from the bravo drive and be fed from a secondary through hull fitting (through hull intake strainer) like shown here:

http://www.freshwatersystems.com/p-2823-bronze-one-piece-intake-strainer-34.aspx

Could the strainer be a factor being in the area of the bottom of the boat a couple of feet or so from the prop? If so, how else could water be fed to cool this engine? How do they do it on new boats? I found that there are stern mounted water pickups and wonder if that would be a better than the through hull strainer.

 

[Mark A]

I boat in Narragansett Bay in Rhode Island, Salt water. The water conditions do effect the porpoise as well. Everthing is very sensitive and at times it actually can perform fairly well but that is tough to achieve. A buddy of mine does better then me with it. He has owned Checkmates, Chriscraft, Donzis and Formulas. Swears the Checkmate Convincor is the best 25' hull he has ever driven but that the porpoise issue with mine is not right. He thinks it is all the prop. I think a good prop would help but shifting weight forward might be a better fix. I have replaced the fuel tank and do not see this as being too difficult. I believe the boat is too stern heavy/too bow light. I added weight (appx 200 LBS) to the stern in going to a fresh water cooling and Bravo drive replacing the alpha that came on the boat. I also removed the refrigerator in the cuddy which is about 50 lbs or so. Riding with no passengers on the rear bench and everyone moved up front helps a lot.

I really want to try moving the fuel tank forward to see what happens but wonder if anyone has tried that before.

 

[Mark A]

Also looking for prop advice. Has anyone tried the enertia?

 

[winter]

I think you may have a bit of a problem if you can't fix the porp. with some trim. First and foremost, lose the alum. prop. Exactly what you need to put on is best left to those who own the same set-up you do. Once you have your prop squared away, then see what's left. I really doubt Checkmate would have allowed a boat to reach production if you couldn't somehow trim adjust your porpoise to non-existance. I'll be playing with props next summer, but i did find that when i removed my lower unit hydrofoils, the porpoise got worse. I hate to rec. hydrofoils to anyone, but in my case, they did help.

 

[bigsbetter]

Hydrofoils DEFINITLEY help! It seems every time I mention a hydrofoil on this site, I get a lot of frowns. Others say they're not meant for performance boats, or that you should be able to control the porpoise with tabs. We'll I do have one on my Convincor and it does help. I hit something with mine and now I need a new one. I'll take the 1-2 mph drag if it helps the ride.

 

[Wide Open]

You hit the nail on the head. Those hulls (baja included) have NO hook. That's what helps them lift at speed which is the only way they can ever achieve any top end performance. You've moved some weight in the boat, and that is most likely the reason for your porpoise issue. Different props will certainly do different things. However, I don't think you're going to find you "fix" in a different prop. But who knows, give it a try if you have some different ones to try.....can't hurt anything.

 

[winter]

Hydrofoils DEFINITLEY help! It seems every time I mention a hydrofoil on this site, I get a lot of frowns. Others say they're not meant for performance boats, or that you should be able to control the porpoise with tabs. We'll I do have one on my Convincor and it does help. I hit something with mine and now I need a new one. I'll take the 1-2 mph drag if it helps the ride.

LOL, yeah, thats the perception. I always looked at those as only for boats that liked to cruise at 12mph while pulling tubes, or for those people that didn't know how to properly keep a boat trimmed. I took mine off and i lost about 1mph and the porpoise got worse. So they went back on. OTOH, mine are the type that splay upwards once they leave the outdrive, so i doubt it really has any effect on top end. they do help get the boat on plane faster though.

Dunno, all this talk of moving the weight to the front of the boat goes against my grain. I know it will help the ride, but it doesn't help much for top end if you are trying to lift the bow out of the water. You then have to compensate by adding more trim and then you start working agaisnt the proper angles for higher speed. I guess there is a fine line, but then again, this is my first I/O. Porpoising wasn't a problem in any O/B boat i had before, just chine walk. That's a whole 'nother issue

 

[wca_tim]

Have you tried:

1. a labbed bravo

2. any hydromotive quad iv (awesome prop and probably most likely to elimiinate or drastically cut down on your problem).

 

[winter]

Have you tried:

1. a labbed bravo

2. any hydromotive quad iv (awesome prop and probably most likely to elimiinate or drastically cut down on your problem).

What's your take on a labbed Mirage? The previous owner of my boat sent his back to Merc for massaging and many $$$ later swears speed and ride wont get any better