I've got oscillation

[MikeL]

Can someone give me a suggestion to point me in the right direction? My 191 W/T with a 115 yamaha and T 8 kicker has a tendency to porpose at speeds above 30 MPH even with the trim completely down. It comes up on plane almost imediately and the bow stays down until around 30 and then if it is pushed up by a wave it will start to oscillate. More throttle exacerbates the situation. On calm water, the top speed is 40MPH. Any waves at all I have to reduce to 30 or less. Is a jack plate an answer?

[Cincinnati]

Do you have a Doel Fin or other type of fin bolted to your cavitation plate? That can cause this.

[Mike in Woodlands]

Try triming up a bit after you get on plane. I have had many boats (including my Logic 210 with a Honda 130 which is definitely underpowered) porpoise when trimed down too much when fully on plane. I do have a fin on my motor and that does exacerbate the problem.

[k9reno]

Now dont quote me on this my memory is not what it used to be. I seem to remember from the old site before it went down that the frist model yr of I believe the 191 had a problem with a number of the hulls porposing (forgive the spelling). If I remember it was due to a problem with the material not sticking to the mold right at the factoy. The factory guys were able to corect it by welding material to the boat to correct the chine. Some one here with a better memory might be able to give more acurate info.

[Putershark]

These stern "Lift Devices" do not cause this running condition matter of fact, they help to negate it by trying to drive down the bow "when" engine trim is correct. What happens is after installing such, many people still keep using the same amount of engine trim (or at the wrong time) as before, and do not take into consideration that they now have an even "greater" effect / force (more lift) on the stern and the hull’s running angle of attack. Many forget such, and then try to connect the dots into thinking that the fin is “now” the cause of the Porpoising... Nope, you just have more lift or drag (if the angle of attack is too great in the other direction) on the stern and you have to relearn (or learn) new trim and power habits since the boat now handles and reacts different than before and this even changes as your load changes and as you burn off fuel.

An easy visual comparison of the before and after effects of installing most plates, would be to compare the effects of your hand stuck outside of your truck window at highway speeds flat (this kid still likes doing this!) with just two fingers extended and change the angle of attack. Then feel how much less of a change in the angle of attack raises or lowers your hand when having “all” of your fingers extended and more like an aircraft wing in both shape, and surface area... Well, you just bolted on more surface area to your lower unit, and these changes (both positive and negative) have a lot more effect and much faster then before…, This is also why you can keep the hull on a plane at slower speeds and lower engine RPM’s.

As one of my flight instructors use to preach that also applies to boat operation "Power is my Pal and Trim is my Friend" and once you get the true “feel” of both, and how your boat reacts to even the slightest changes in either one… You will see that you can adjust and or correct running condition issues in most sea states OR trim out for the most hull speed at any given speed “above” that speed when the hull is on a plane. Case in point, would be in using a higher than normal bow running angle of attack to help knock down waves and spray to help keep you dry. Now you will slow down for the same throttle position over your normal speed trim position, but more than likely needed to help keep from beating both you, and your equipment to death Now if you hull is also equipped with trim tabs, you can even dig one side down to help raise up the other side of the hull to help stay dry in crossing seas and winds.


Now I do not remember if this year model hull may or may not have any intermittion mold casting issues like some of the others we have seen corrected. But I would think that a call to Dick at Merritt would help since I would think he (if anybody) would remember if they did have any

Hope this helps!

[haweater]

i've been experiencing the same porpoising issue with my '04 191 f/s so it was good to see someone else with the same problem. mine has a suzuki 140 engine which, according to my dealer, should run comfortably at 6,000 rpm. i can't get it past 5,000 rpm even after added transom wedges. the wedges helped a little but the more you trim down the more the boat plows through the water which only serves to slow the boat down. i'd be curious to know if anyone found a solution to this problem. i'd like to max out the engine once in a while if i ever find some smooth water.
thanks

[Putershark]

Originally Posted by haweater

i've been experiencing the same porpoising issue with my '04 191 f/s so it was good to see someone else with the same problem. mine has a suzuki 140 engine which, according to my dealer, should run comfortably at 6,000 rpm. i can't get it past 5,000 rpm even after added transom wedges. the wedges helped a little but the more you trim down the more the boat plows through the water which only serves to slow the boat down. i'd be curious to know if anyone found a solution to this problem. i'd like to max out the engine once in a while if i ever find some smooth water.
thanks

2 separate really issues...
1. Can't get past 5000 RPM (too much pitch / bite with the current prop)
2. Porpoising (most likely due to engine at the wrong trim angle for that speed) here again a prop change both in pitch and in blade config, can make a huge difference in overall performance. Many seem to forget that not only does the prop develops thrust, but also lift. Changing the running angle of attack on the prop will change where the water is striking the hull once on a plane... I would also check how the boat is loaded weight wise forward and aft as well as starboard and port