Please help!! Porpoising hydrosteam

[dompie99]

Been doing a lot of reading on this subject lately and it seems very difficult to develop "rules" to correct the instability because there are nearly an unlimited amount of variables to contribute to the cause.

One area of agreement is to keep the application of thrust inline (as much as possible) with the axis of rotation of the porpoise, so lowering the motor should (theoretically) make the condition worse.

I also now believe too many of us become fixated (myself included) on correcting the bow rise portion of the action while virtually ignoring the bow drop. There seems to be plenty of evidence for this in the failed attempts at correction by adding weight to the front of the boat (sometimes a great deal of weight), to find little or no change in the condition. As a matter of fact, I've found one example of where adding weight up front (a passenger) actually made the condition worse. Another example I've seen is where a prop with less bow lift was installed and it also made the porpoise worse.

This is NOT to say that no one ever corrected a porpoise condition by moving weight forward or reducing trim angle because obviously they have, it just isn't a universal fix for every boat.

I'm not even entirely sure we should refer to this as a porpoise because in my mind that describes a fairly gentle condition, like a front to rear rocking motion. What we see in high performance hulls is far more violent, more of a slapping or even hopping where the entire hull nearly leaves the water.

Seems that what is going on here that as the boat starts to plane and the hull rises the wetted area is reduced and moves aft until you reach a point where the center of lift or buoyancy moves behind the boats center of gravity. When that point is reached the hull loses all support and drops like a stone rewetting the hull moving the center of lift forward of the CG again and then repeating the whole process over and over. If you are able to survive the trip to reach the speed where aerodynamic force will support the bow, the porpoise will stop. Hardly practical.

This has become a VERY long winded way of telling you to move the center of gravity or mass rearward. You actually proved this theory with the rear seat passenger but didn't recognize it. Moving weight back in the boat or increasing setback should help the boat a lot and reduce the bleeding in your kidneys.

Very interesting Post indeed

[Motv18]

Knowing that it is paded im wondering if its chine walking and just going out of control from there? Thats a problem that seldom goes away.

[fs5]

i run a merc 55 on a slt boat and struggled with purpoising problems ...
turned out the bottom yoke where the bottom mounts bolt into was flogged out and had quite a bit of movement.
might be worth looking at if its an old mota.

[XstreamVking]

I have seen a mod done to the outer wing area on the bottom on a Viper. Hook was added to stop low speed hop. Once at speed, you get some air under the hull and this area is almost dry.... Not sure who did it, but saw it on here.

[transomstand]

I have seen a mod done to the outer wing area on the bottom on a Viper. Hook was added to stop low speed hop. Once at speed, you get some air under the hull and this area is almost dry.... Not sure who did it, but saw it on here.

I think there was a strake mod and a wedge mod, somebody I know has pics but I'm still waitin

[XstreamVking]

TS I agree about getting the wt aft. When I went to a 3.0 I moved the batt fwd to offset some of the motors wt. Made it wanna hop. Moved it back and it ran much better. All boats are diff, but I hit the trim and the throttle at plane off and go past the hop zone and get it on the pad. Then trim dn some once flying. These hulls were never designed to run slow....

[Sonik]

I think there was a strake mod and a wedge mod, somebody I know has pics but I'm still waitin

I'm still waiting on him to get ahold of me so we can compare notes on the phone before i start posting pics of Howard's bottom mod on the Viper he brought to Winona Pete....

[transomstand]

I'm still waiting on him to get ahold of me so we can compare notes on the phone before i start posting pics of Howard's bottom mod on the Viper he brought to Winona Pete....

No response anywhere, maybe he got pitched out and the boat ran him over

[texasvector]

It hasn't pitched anyone out yet. .. but all we done in it so far is sit in it and make motor noises heavily intoxicated

[texasvector]

Motorhead bishops old viking.... wedges more progressive than "pad like"

[texasvector]

Why you delete yo sh! T?????? Prolly working up a lengthy post.

[XstreamVking]

There was a bruiser colored viper on here that had a wedge mod on it. Was several yrs ago.

[thornl01]

Year ago, (when these boats were new) this issue was adressed with a small set of aluminum tabs added to the outer section of the transom. (hydrostream supplied them).

Nothing fancy, will get a pic of a set of these & post later.

Very very common problem. ( 15s, 17s, and 18s needed them)

LT

SC Melonbelly Associate

[transomstand]

Yes, I sold them. We never had much issue with porpoise back then, we usually added them for customers who wanted the boat to launch harder.

There were no rotten cores or rubber transoms yet, there are also many more choices of props, jackplates and gearcases, and many of the boats today have been changed by poor storage, repairs, different interior layouts, and worn out rigging and engines.

[transomstand]

It hasn't pitched anyone out yet. .. but all we done in it so far is sit in it and make motor noises heavily intoxicated

Who's Vector is that?