porpoising, trim and prop size-suggestions?

[sim]

Have had my recently purchased 89 Predictor out a few times in the last 2 weeks. Have to run it with the trim all the way down on the gauge. If I bring it up more than a fraction she porpoises a lot and it sounds like the prop is too high (engine/exhaust sound gets loud and raspy- great rooster tail, though).
Here's the essentials- 89 predictor, 92 mercury 115. Running a stainless steel 13x23- 3 blades and getting about 52 gps at 5800-6000 rpm where she tops out. Bow high and doesn't plane till 20-22 mph. Serious chine walk above 48.
Looking for a lower planing speed and less porpoising. Suggestions?
Would a larger diameter keep a little more prop down in the water and reduce some chine walk. 4 blades instead of three? Would a 19 pitch plane it out sooner?
Hydrofoil?
Thanks in advance for your help and suggestions.

 

[Craig]

I haven't been in a predictor for a very long time. Someone here should be able to help you out. You should be able to get some stable performance from that boat with no porpoise. Probably will always be some chine walk, but the guys on this forum with predictors should be able to help you reduce and control it. Hang in there.

-Craig

 

[daren34]

the guage could be wrong

run the trim all the way in and try it don't go by the guage

let us know
daren

 

[daren34]

how high is the motor mounted
you may need more lift to cancel out the chine walk
if you don't have one look for an elec. 6" jack plate
cmc makes one

daren


the trim guage sender is normaly adjustable

 

[Mark]

Your prop is slipping like crazy. I would say a 23" prop is too much for that rig. Maybe the motor is mounted high to get it to slip. Anyway, I would look at lowering your pitch and adding a fin. But get the kind that are not so wide so it won't drag when at high speed.
Powertech makes a prop called the SCD for small gearcase motors that virtually eliminates propoising. It is a stern lifter instead of a bow lifter. They use them on the flats boats where they spend a lot of time at lower speeds. The Merc Laser is a pretty good prop on that boat but will not get rid of the propoise. A Ballistic would be a good compromise between the 2 but will not work well with a Jackplate and high engine heights.

 

[wca_tim]

no hydrofoil - can do strange things to handling especially at higher speeds...

A prop with less bow lift could make a huge difference. most of what I read says that my diplomat should do better with a bow lifting prop, ie a good amount of rake like a laser 2, but i've found that a neutral (ie stern lifting prop) does a lot better in terms of stability at higher speeds and mid range porpoising. right now it's got a hydromotive on it and doesn't see any real propensity to chine walk up to mid 70's. It's stable enough that you can feel it start to slip off the pad and it's easily corrected with a little steering...

finally, a set of trim tabs will give you a lot more freedom and might seem like magic. Even the smart tabs that Nauticus sells would stop the porpoising and would also drop your planing speed... I added the tabs to be able to stay on plane at wakeboarding speeds... otherwise at 16-18 mph the boat was right on the verge of planing and very hard to keep an even speed. now can stay on plane at 14 if I'm careful... This is the direction I would go if i were you... the smart tab sx series... works well and can be had for less than a new prop... (ca. 150)

Food for thought...

note that my boat is a stern drive a a good bit heavier in the a$$ end, so not a direct comparison... more setback, jack plate / prop shaft adjustments, and other things would also presumably make a big difference in your case...

 

[supergenius74]

my mx-15 had a light bow and would porpus all the time, I had big' ol rude 140 on the back. I moved the battery to the front (made more room in the back anyways) I removed the jackplate, and ultimatly changed to a prop with less rake and it all helped smooth it out. I think that most of the small (less than 18 foot) checkmates have too light of a bow and moving weight foreward helps. My 21' has the opposite problem and I am constanly working on moving weight back. -shane