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johndmay
05-03-2000, 06:49 PM
Here's the deal. I repowered my 1988 Z-25 with a new 502, built up, and dyno'd at 550 HP @ 5200 RPM. We did our first test yesterday, using a 26P Bravo, 4 Blade.

Before, with the 365 HP Magnum and 21P 3 blade, it would top end at 5200 and roughly 62 MPH. It also porpoised, and with trim full up, I could calm it down with the trim tabs.
Now, the power is awesome, but the porpoising is worse, and I have to really push the bow in with the tabs to stop the bouncing.

My rigger is suggesting the the hull has a hook in it, and needs to be filled and filed/sanded for 2 to three feet ahead of the transom to make it perfectly flat. I'm sure he's right, as it's been on a trailer with rollers for most of it's life.

The next time I get it in the water (maybe Sunday) I'll try trimming the lower unit higher with the trailer trim, then using the tabs.

Another suggestion by the rigger is going to a shorter, high performance, bottom end on the lower unit...at least another $2000.

Any suggestions/comments from those more knowledgable than I on the above three scenarios...perfecting the hull...trimming higher...shorter shaft...all three....any other ideas???

Frank Civitano
05-03-2000, 08:21 PM
Also a 4 blade will make any flaw worst then what it was with a 3 blade, Are you running stock Trim tabs, and with that horsepower you should run Imco steering with K plains. Make sure your hull is true,

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Frank Civitano

johndmay
05-03-2000, 09:25 PM
I tried less trim, but it really seemed to plow. This engine is (I'm guessing) maybe 200 lbs. lighter than the last one. I have big Bennett trim tabs. It's stock steering (internal) and works fine. No chine walk at all. I'm not interested in dropping back to a three blade 25P. Top RPM's were 4700 in yesterdays test with the 4 Blade 26P. We didn't have a speedo, but I'm guessing we were in the low 70's. I guess what I'm really wondering is; can indents in the bottom of the hull really affect the porpoising this much?

RickR,GroveCity
05-04-2000, 12:49 AM
John
At 10%slip = 72MPH
At 15%slip = 68MPH
Pretty darn good for a 25' Donzi
Buy a GPS and testing will be much easier.
Yes a hook in the hull will cause many different problems.
Go to Lowes and buy a 5' straight edge($10)
I just bought a 1995 22 Classic with a hook (factory defect). The hook is on the port side and causes her to list when that part of the hull is wet. http://206.150.187.82/ubb/frown.gif
I plan on getting the hull blueprinted next winter. http://206.150.187.82/ubb/smile.gif
BTW; at WOT you want the tabs out of the water except to keep her on an "even keel"

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RickR mailto:riggerb@aol.comriggerb@aol.com</A>

seano
05-04-2000, 06:57 AM
John,

The guys are right about the tabs and such and I just wanted to mention that trimming the boat up with the trailer button is not a good idea---this will cause the univerals to wear out in no time flat---you'll be replacing them every season if they last that long!

BTW, blueprinting the bottom is definitely a good idea. Your rigger mentioned a "hook", but what you describe is actually symptomatic of a "rocker". A hook will act like the tabs are down---a rocker acts like the tabs and drive are up the boat will "rock" back and forth on this surface like a rocking chair creating a "porpoise". Also, you may want to consider a propeller which can provide more transom lift instead of bow lift. I believe a cleaver will help you here in the short term. And if you are considiring IMCO's shorty Bravo just make sure it won't raise the x-dimension to the point where the boat is too difficult to get on lane(cavitation/ventilation). Hope some of this helps.

Donzigo
05-04-2000, 07:14 AM
I have a 1987 Z-25 with a new MPI 454 installed 1o hours ago. I run approx. 60MPH or so with a bravo and 21P three blade prop. Never had any porpoising problems. Of course I don't have nearly the juice pushing the boat.

My contribution is this. I have often heard, even by the people with hotboat mag. at the Universal show in Orlando, that the Z-25 boats of the 86,87,88 vintage had hull problems, and were off a little bit. I can't verify it; but, thought it might be worth telling you. Maybe the factory has info in the archives.

boxy
05-04-2000, 07:29 AM
John I have absolutely no technical info to offer you, but next time you go flying by the dock, stop in. I'd love to see to the new powerplant up close.

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boxy
mailto:sboxma@sprint.casboxma@sprint.ca</A>
Smuggler's Cove
Ontario

johndmay
05-04-2000, 09:53 AM
Well, the votes are in.......and blueprinting the hull is the verdict, at least for a start. Thanks, everybody, for the input. I'll let you know how it goes.

And Steve (Boxy), I'll watch for you on the dock. It is one very pretty powerplant.


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John D. May
Manotick, Ont.
www.jackmay.com

Frank Civitano
05-05-2000, 05:53 AM
John,
There also was an Interesting artical in the ne Hotboat magazine? some guy was having the same problem you are ? He couldn't stop the porposing. Hotboat responded= Check your Outdrive for looseness, he says grab the drive and see if there is any play in the gimble ring at the top? Then have someone move the outdrive while you inspect the steering arm to see if there is any where on the inside?


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Frank Civitano