"90 Javelin F/S 378 17'8" long. 120 Johnson, 6" jackplate, 3 blade 20P Raker (3/8" vents). At medium rpm it'll get on plane but then porpoises badly when I try to trim up the least little bit from full negitive. Not to confuse this with the set-up forum, just wondering what the general cause of this is.......
Thanks,
Jeff
Porpoising occurs when the motor cannot keep the bow of the boat lifted. The bow rises, but the motor is unable to keep it there, and it falls back down. This process repeats continually until you trim down.
This is a function of motor size, prop selection, boat length and weight, etc etc etc.
I had a Ranger that did it. The cure was to add hook to the hull. Ranger sent me some jelly coat putty to build two small wedge shapes in the back of the hull.
Bass Tracker TX 17 1995
So like adding small permanent trim tabs?
It seams to to better when I have someone sit up front, maybe add some ballest up there?
Weight up front will kill your speed. What happens when you're at full throttle, or more than "medium rpm," and you trim up? Porpoising, like poolboy said, is a thrust vs. trim thing. If you start porpoising, do one of 2 things....either trim down or increase throttle. Trimming down won't try to fly the nose so high, so it will ride better. Adding throttle will push the nose up harder, allowing it to stay there, instead of pushing it up and not being able to keep it there.
You might need to get a little cup added to the prop to carry the bow better.
Well, played hookie yesterday and went out perch fishing. On the way out it was 1 footers and the boat porpoised no matter what I tried, what ever speed, even think the prop blew out a few times when the bow would fall into a swell. So on the way back in on an unusually smooth Erie and Maumee I throttled up to max and it still wanted to porpoise a little bit. Tried to trim up and it went ape sh*#. I'm no expert by any stretch, but I think mdtritn hit the nail on the head with it needing more umph(cup) on the prop. It ran 46mph(speedo) @ 5500rpm @ full negitive trim. specs again are '90 Javelin F/S 378 17'8" long. 120 Johnson, 6" jackplate, 3 blade 20P Raker (3/8" vents).
Looking for conformation of thoughts before I send the prop in for mods.....
Thanks for your input fellas!
Jeff
No expert here, but is the motor raised way up on the jackplate? For the prop to blow out it would have to be very close to the water surface and at full down trim it shouldn't be. It sounds like you are porpoising because at full down trim is set up to run at WOT. I have a 17'6" boat with a 115 and run quite a bit on a lake that has a speed limit. I have to run at mid throttle to stay under the limit. My boat is prone to porpoise especially if there is wake or chop on the water. To stop it I just trim down a bit and problem solved. It really sounds like for whatever reason you cannot trim the motor down enough becuase of position or a defect in the trim cylinder. To give you a reference, when my motor is trimmed full down and the boat is sitting idle the prop is aiming slightly toward the bottom of the lake.
Tight Lines,
Andrew
Yes the motor is raised on the plate, I set it to 4" below the bottom of hull. The previous owner had it at 5" or so below and it acted the same way. I was thinking of lowering it to where it would be without the plate on it for testing. See how it ran from the factory, I know it's not the same with the set back but height could be close to the orginal design.....
Is the center of the prop shaft 4" below the bottom of the boat?
Mountain Home ChamP 181
as it sits now, yes
You need to get a 4 blade prop either a renegade or a trophy. Where you are running your going to have swells and rough water more than smooth water and a three blade is going to keep blowing out. When the nose is dropping in a swell that is not porpoising. A lot of that is boat control and with a three blade blowing out you don't have it. You need to learn to keep the bow up when going over a wave and figure out where and how the boat rides the best, but you have to be able to control the boat.
I rarely will be on Erie, that was just a perfect calm day, very rare! What I was getting at is that it won't stay on top of the swells since I'm trimmed down. As soon as there is no water under the bow it goes down, motor/ prop won't keep it up and on top.
One thing I did notice and I'm sure it most of my problem, aside from it being a Fish & Ski, is that it has built in hook to the hull. There are 1/8" - 1/4" wedges molded into it. Guess I just accept it's flaws and live with it, shut up and drive it, or get out the belt sander.....
Thanks for everyone, for your replies
Jeff
I had a 2072' flat bottom from BP tracker marine and had extreme porposing with a 125 from factory had it a year and never could use it. put up to 500 lbs on front deck and still extreme, sent back to BP and they said the motor was too big (they sold it that way) they put on a 75 merc and said that would solve it guess they never tested because it didn't work. finally an old timer said NO HOOK, after over a year they put a bead of weld accross bottom where transom and hull joins and that solved the problem not much faith in tracker on any boat they sell so yes i would go with no hook you could add the trim tabs either permanent or hyd or electric for fiberglass boats.Originally Posted by diemaker
Hull design, weight distribution, trim angle, motor size and height. All these things can add to a boat wanting to porpoise.
If the little hooks are at the rear of the hull they are there to lift the hull on take off and keep the bow down. Look at the Ranger hulls and the newer Tritons they both have hooks. It still sounds like the prop needs some cup to carry the bow. You can't run trimmed down in ruff water because the bow will drop and the motor can't pick it back up.
Like stated above either cup prop more or get a 4 blade. Last motor that was on my boat had a 4 blade and it would never bounce now with a new outboard and a 3 blade mine will bounce at low speed full trim down so I just drive a little faster and it goes away. Some day I might change to a 4 blade.
http://www.scvwl.com
2013 Ranger 620VS Yamaha 250 and Yamaha 9.9 kicker
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