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TOPIC: Porpoising ??

Porpoising ?? 3 years 4 months ago #107154

  • 65G3
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I, had a porpoising problem with my 65 G3 when the new Merc 50 was put on my boat , Dealer didn't set it up right , cavatation plate was 4 inches below the bottom of the boat . I fixed that problem !! Raised the engine So I tried putting sandbags up front under the bow (120 lbs. ) That stopped the problem ... got it up to 5300 rpm no porpoising !!! Also moved the fuel tank up front, thats about another 50 lbs when full !! At this point ,boat runs ok But now I hear I should put wedges between the transom and the engine !! How do I determine which ones to use they have 2 degree 5 degree and different sizes also . The G3 has a curved transom , How do I set up the wedges and what degree angle am I supposed to acheive ?? If I put wedges in can I remove the 120 lbs of sandbags ????

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Re:Porpoising ?? 3 years 4 months ago #107157

  • Ike
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If it is no longer porpoising don't do it. Most porpoising problems are due to improper weight distribution and trim and tilt of the engine. Will the wedges improve it? Probably not.

Most likely what happened is the engine weighs more than the boat was designed for, so you simply had too much weight hanging on the transom. You balanced it. So if it ain't broke don't fix it.

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Peter D. Eikenberry
newboatbuilders.com
newboatbuilders.blogspot.com
"Don't tell me that I can't. tell me how I can."

Re:Porpoising ?? 3 years 4 months ago #107160

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Ok Thanks I'm reading all the tips ..right now Its not porpoising with weight up front ! Whats strange is the boat didnt porpoise with the old 1978 70 HP 2 stroke engine I had on it and that engine weighed 242 lbs mabey because the 2 stroke prop is turning faster ?? I'm still puzzled !! The propeller shop guy said he thinks I should try wedges.. But which ones, its a try and fail game ??

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Re:Porpoising ?? 3 years 4 months ago #107162

  • Kerry
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Propeller design can matter. Uncupped props don't give trim much effect. If you are running an aluminum prop, check for cup. If it has little or no cup, see if you can talk your dealer into letting you try a cupped prop. Your goal is to get the weight back in the rear of the boat and run slightly bow up with neutral (level) trim.

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Re:Porpoising ?? 3 years 4 months ago #107180

  • MarkS
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I've gotta agree with the guys here John, had the same issues with my GT-150. (Not a G3, but very similar in the hull design - wight distribution dept.) Going through various props looking for the right one I found they had definite effects of the porpoising that I was experiencing with some of the props. Large amounts of cupping or swept back blade designs seemed to make it worse, but that was on my boat - yours may be different.

Good to see you on the boards again, don't be a stranger now hear!? :laugh:

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Mark
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