porpose problem

[150aintenuff]

this was a new issue that turned up for me yesterday.. never had problem before....

:back ground....... 150 mercury 2400 lb boat 21P Rapture michigan wheel tuned blue printed (not thinned or modified from spec) top speed 52.2 MPH


Problem....

trimming out slowly as usual the hull lifts and holds for 2-3 min and then slowly starts to porpose slightly at first and in 10-15 seconds the porpose is so bad it takes full negative trim to stop it.. instantly re trim and 2 min later same problem.... this is a new issue and nothing has changed with prop or setup... nothing has ever been hit or bent on hull since removing hook 3 years ago... hull is not lifting up on to pad untill 5400 used to pop up @ 3800... prop looks like new minus for some fine scratches due to 5 seasons of use.... is the blades losing cup or is there other problems with my hull

BTW the hull is .190 GAUGE Aluminum fully welded with a full length ( 12' long 14" wide flat pad bent in to 14* progressive Delta V hull ( semi vee pad) similar in desigh to a S O B style hull... motor bolted to transom on 3rd hole up... cav plate 2 1/4" above bottom of pad....(NOT Propshaft) 20 lbs water pressure any higher and prop wont lift hull if prop would bite could go up more.... need jack plate but kicker motor is on starboard side so would need to add hydro steering before adding plate... cant afford one so unless better option comes along no jack plate suggestions... YET....

I have ran out of ideas and am getting ready to go prop shopping... the prop was ideal for my hulls usage but would like to find and try a 22 trophy and a 21 tempest if anyone has one here locally or willing to loan I could try...

[150aintenuff]

?????????????????????????

no replys ???????

[Reese]

Are you saying that all of a sudden this just started happening??? Same prop, same setup, same trim, same weight distribution?

[150aintenuff]

yep.... this last week end... had a very light ( 5-6" MAX porpose before) but nothing like this.... at full throttle if I kepted the trim the same it would porpose so hard it would burry thge nose deap enough to pull the prop 2/3 of the wqay out of the water.......

[Rickracer]

Props don't suddenly cause a porpoise. If I were you, I'd take a straightedge to the bottom of your hull, I bet something is going on there.

[Reese]

Sorry Nick, I'm with Rick on this one...can't figure out how something like that can change so suddenly without anything being different???
If ya figure it out please post the findings.

[150aintenuff]

neither can I.. hence the post.... something is definatly up some where... I am taking the trailer down this week end and pull it out of the water as it is moored for the fishing season... and it has only been in the water for 2 weeks in 45 degree water so algea is zero...

the fact that it holds for a while and falls in to porpose is what is getting me...

[wideglide55]

Maybe the prop needs rehubbed.It could be starting to spin in the hub when you load it all the way.

[150aintenuff]

top speed was within the range of normal loaded to unloaded (47 loaded -51 light) but will look in to that as well

[150aintenuff]

OK FOLKS.... heres the skinny.......

Pulled the boat out, checked a few things and rann the piss out of her this weekend

Bottom : straight as an arrow....
Pad section: nice and flat
transition (could be smoother but good enough for this hull)( it is better than 90% ok)

prop: slightly off of the gauge given to me when it was blue printed(1/16" max) but runnable and all blades are evenly off to the eye.

speeds: loaded 4 guys full fuel (50 gal) LOTS of gear and a 100 lb kicker 43.5 MPH GPS only can trim to nuetreal trim any more porpose problem starts. RPM 5150

Moderate load 2 guys and 35 gal of fuel LOTS of gear and 100 lb kicker ( average fishing load for this boat) 47.5 GPS can trim 2/3 up and then porpose starts RPM 5200

light ( 1 person 20 Gal of fuel no gear no kicker ) just me (190 lbs ) 51.3 GPS @ 5350 RPM
Can trim to full nose first rooster tail about 5 feet over cowl starts @ 4500 and by 5000 rooster is about 1 foot over wake nose is high and boat finally settles down on to the pad.. mild porpose in straight aways (6"-12" total range of motion)

seems to me prop is fine as the light handling is fine.. the boat did sit for a longh while last winter could it be motor mounts or is it time to just step up and get a 12" jackplate and a different prop bear in mind the prop hasnt been touched since it was blueprinted in 1999 and the engine is untouched from 1989 and only modificatiuons done were to the air flow into carbs but engine is stock...... its running about 300 RPM shy of where it should as it was built and setup for 5600 RPM and up until last august it would get to 5600 no problem... but since getting a tank of bad gas 5300 is all she will get...

[Reese]

Still hard to figure out....but the fact that it porpoising and not hitting it's original rpm seems to suggest something is going on under the hull. I would check the bottom again and push up on several areas to see if they deform under pressure. Sometimes the area measures flat but will bow under a load or pressure????

[150aintenuff]

the hull is .190 gauge aluminum... it dont move... even wave jumpin.. the entire weight of the hull can be supported on a standard floor jack and the hull wont flex... in a word she is a tank.... the mota would not hit 5600 as a result of the bad gas as she has done it since August 24th of last year...It was a day i wont forget as it was the only time I have ever been towed in by the coast gaurd.
I assume there might be some buildup in the carbs and I have yet to go through them...

[150aintenuff]

guys if this helps I have pics of the hull and a profile as seen from the transom if that would help lend itself to a few more ideas.....