[hawkman]

Hi y'all. Still relatively new to boating in general and my 98 Pathfinder (60hp Yamaha 2s) so I'm looking to learn.

Took it out the other day for the first time this year and on some of the calmest water I've been on. I was surprised to have it start porpoising. Each bounce progressively worse so I quickly throttled down. I tried again and worked the motor trim as well as the trim tabs and seemed to get a good handle on it, yet I thought I should be able to have my bow higher and go faster.

This may be just a normal thing - brought on by a bow too high out of the water. In general then, what causes porpoising and how should a boat like mine ride? Or, rather, how should I position the hull properly, given calm water, a 120lb person near the bow, and me (230lb) in the back?

 

[Lagoonnewb]

Every boat is different and the conditions will dictate how much trim tab you need and how much you can trim out the motor. I would think you should be able to run with the tabs almost completely up and rely on motor trim but I’ve never been in that pathfinder. I know on my skiff I run with my tabs about halfway and then trim the motor up until it starts to porpoise and then trim down a little until it rides smooth. The more you can trim up your motor generally the faster you will go, find the sweet spot and try to remember where it is, you will feel it and it will get easier to dial it in faster

 

[SomaliPirate]

I'm running that same hull and power and I've never had an issue. Once on plane, I trim it up until I start to porpoise then tab down until she settles down. I don't know if prop could be an issue or not, but I'm running a 4 blade Powertech.

 

[Finsleft258]

Porpoising issues can be caused rocker in the hull, engine mounting too low, a bow lifting (high rake) propeller with not enough horsepower to carry the load, too much positive trim, poor weight distribution.

 

[Capnredfish]

I am betting on prop. Or, you want bow higher than it should be. Wrong trim setting.

 

[zthomas]

How fast are you going at WOT and/or at a given rpm?

With me (225 or so) and wife (125 or so) in a 98 17T with a Yam 50 I might get 28 or 29 WOT best case.

So it may be a matter of expectations. You just aren't going to get much over 30 no matter what you do.

On my digital Yamaha gauge, the little trim gauge thing on the right is generally between halfway and three-quarters of the way up when I'm cruising in open water between spots.

Like Somali, if the water is flat and ride quality isn't a factor, I push it to about the speed I want to go (usually 24 to 26), then trim up to where it just wants to start porpoising, then give it just a bit of tab to stabilize it.

 

[Smackdaddy53]

Similar size and a little more HP but I like to find my sweet spot by running with the motor trimmed out until it’s just starting to porpoise then both tabs down until it levels out. I get my best speed (33-34mph) with tabs about 1/4 of the way down and motor trimmed out past level.
Do you have a jack plate?

 

[crboggs]

Tuck the motor all the way down...get up on plane...raise the motor trim bit by bit until you start to porpoise...drop the motor trim by the merest amounts until the porpoising stops...now you can fine tune with your tabs while keeping your hull efficient maximizing speed and fuel burn...

Just my $0.02...

 

[hawkman]

Do you have a jack plate?

No. The transom is raised on a Pathfinder, which is also a tunnel-hull.

 

[Vertigo]

Porpoising on a planing hull is the result of the relationship between center of gravity, center of pressure and thrust axis. If the center of gravity is too far aft and the thrust axis is too low, the bow will rise as thrust raises the COG about the center of pressure, then, when forces balance, as gravity overcomes thrust, the bow will fall, and the cycle will repeat itself. It's all more complicated because the center of pressure moves around as the hull planes at different speeds.

Porpoising can be controlled by moving the COG (move weight forward), realigning the thrust axis (raising and/or tilting the motor) or applying force to change the center of pressure (trim tabs). I've owned hulls that were balanced well enough not to need trim tabs and I've owned hulls that should never be on the water without them.

 

[ckrawlin]

One more factor is trim tab mounting. A friend replaced his old skiff/motor with basically the same model, only new. Surprising, the new boat was prone to proposing, when the old hull did not. After months checking for rocker, COG changes, it was finally tracked down to trim tabs. The new boat had tabs that were flush with running surface. He moved them upward bt 1/4 inch, like the old boat, problem solved. Weird but true.