How to set trim on a pontoon boat.

[jeffinsgf]

I believe the term for what Skyco calls breakout is "cavitation". In a tight turn at high speed, if the prop is trimmed high, it can throw all the water away from the prop, leaving it spinning with no resistance. This causes an immediate loss of propulsion and an over-revving of the engine.

 

[N80]

Noise has nothing to do with it

Mr. Jimi, even I can hear when an engine is working well and not so well.

The folks who sold the motor to my parents put a specific prop on it which they said was good for the type of boat they have and what they do with it, which is riding around real slow waving at other people on pontoon boats doing the same thing.

The engine manual says top rpms are in the 5000-6000 range. I don't know what type prop it has but it aint SS and I'd hazard to say they don't need a SS prop for what they do.

 

[N80]

Jeff, cavitation seems right but this would happen regardless of how the motor was trimmed sometimes.

So far not an issue with the new motor, but again, haven't really 'tested' it yet.

 

[Dyer retired]

Jeff, cavitation seems right but this would happen regardless of how the motor was trimmed sometimes.

So far not an issue with the new motor, but again, haven't really 'tested' it yet.

The new motor may have a longer shaft on it? N80, do you have a handheld GPS? I use one to get an accurate speed on my sport boat when playing with the trim. John

 

[jeffinsgf]

Yeah, I'm going to agree with Dyer. Your old motor's shaft was too short -- maybe not by much, but if it cavitated on any sharp turn regardless of trim, it needed a little more depth.

However, just about any small boat can be forced to cavitate. If you throw a super sharp turn and catch any amount of cross wake -- which is pretty darn likely, you're destined to see cavitation.

 

[Skyco]

Yup- prop breakout/cavitation/ventilation, all common names for a similar effect, basically the prop loses "traction" with the water and starts to churn air, boat props don't do real well at pushing air

ps "breakout" is probably the more southern redneck version of the technical term

Here is an excerpt-
Ventilation and Cavitation
Ventilation occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving.

Anti-ventilation Plate: Large plate cast into gearcase housing directly above propeller. Helps reduce surface air from being pulled into blades.

Cavitation: The aeration (bubbling) and boiling effect of water caused by creation of a low pressure area. Generally caused by a solid shape (propeller blade) passing through the water, in such a position and speed, that a low pressure area is formed due to the inability to move through the water in nonresistant manner. An example is, a propeller blade that has a rough edge would not cut efficiently through the water, thus creating a low pressure area. If the pressure drops below the vapor pressure, a cavitation bubble will form in that region. These bubbles will collapse when they reach the higher pressure region of the blade. This causes a rapid change in pressure and can result in physical erosion. You may notice burns (erosion) at some area on the face of the blade.

Common rules of thumb:

Ventilation can lead to excessive slippage.
Cavitation can lead to ventilation and/or slipping.

Which came from- About Boat and Marine Propellers (props)


BTW I do hope your parents were sold the "high thrust" version of the engine. Many outboards come in two varieties, the high thrust version has different gearing and is designed to swing a larger prop more slowly...ideal for heavier slower loads such as a pontoon boat

 

[N80]

They bought this engine without consulting me, which is fine since I know nothing about it. In fact, they bought the boat without consulting me too. And they have enjoyed it.

And I've enjoyed it too. They live nearby and I get to use it. I'm not much into boating but as they say, its better to have a friend (or family) with a boat than having one yourself.

In any case, the old Merc on the back got finicky and hard to start. It was often beyond my dad, who has early Alzheimers, to get it going. My Mom got frustrated and a friend of theirs referred them to a boat dealership and they sold her this Yamaha F60EFI. It is labeled as a mid-range on their website. So apparently not a high-thrust.

Four Stroke Midrange Home

But again, they putt around at pretty low speeds in the afternoons on weekdays when no one else is on the lake. It is unlikely to hit its max RPMs unless I'm driving it. So even though its not a high thrust deal, its probably more than they needed. And I have no idea what they paid for it. I'm sure it wasn't cheap.

 

[kenmac]

Different pitch props are used for the applications or, uses. go from a 23 pitch to say a 19 pitch & you will have better take off or, out of the hole power with less top end speed like changing the rear end gears in a vehicle. Changing from a 23 to a 19 will also make higher RPMS at a lower speed. If you have a 21-23 etc, prop . You probably won't have to worry about over revs. on the engine. I have a 23 pitch on the bass boat for speed but, it won't take off as fast as it would if I had a 19 or 20. SS prop is the way to go. but, if they are just putting around , they should be ok with aluminum prop. Like I said , You can install a trim gauge . I just to the sound of the motor

 

[Skyco]

But again, they putt around at pretty low speeds in the afternoons on weekdays when no one else is on the lake.

We frequent Lake Murray on the weekday afternoons/evenings, a little fishing but mostly just chilling out. We trailer our center console from a few miles away but also have a midsize sailboat moored on the other side of the lake. I've often said if I was in a position where I could only go on the weekends I would sell the boat. Going on the weekends definitely isn't chilling out, if anything it would raise the stress level....and don't even get me started on boat ramp etiquette

 

[hazmat]

On a planing hull (17' whaler) I always set trim by trimming up to get the rpms up and steering balanced but not so far it was out of the water. When trimmed up for max speed, we'd often get ventilation in a tight turn - lots of tight turns when you're trying to eject your brother off the tube . So we'd trim it down a little.

They make motors with different shaft lengths 15, 20, 25, 30, but the mounting point is also somewhat adjustable - you kind of have to trust the dealer set it up right when "rigging" the motor onto the boat.

I imagine the pontoon boat isn't all that heavy & is probably running in "semi-displacement" mode - ie you are kind of planing at cruising or max speed. Assuming you are moving at greater than 6-7knots, you are no-longer in pure displacement mode (where the wake never leaves the boat).

So - lots of rambling on, but since it is power trim, play with it a bit see what works for you. You probably won't affect the trim of the boat much (bow up or down), but perhaps the engine will run happier.