what causes a boat to porpoise

[mhyder63ford]

i have a 1988 bayliner cobra br had a 125 force on it changed it to a 115vro johnson. not matter what we do to the trim it wants to start jumping out of the water, about the same time its starting to plane out, no china walk just like a bucking bronco (HELP)

 

[180Fisherman]

Re: what causes a boat to porpus

Re: what causes a boat to porpus

A boat porpoises when the bow is being forced up into the air. It will will reach a certain attidude and then the bow will fall back down causing the up and and down motion. As to why your specific boat is doing this I can't say. Maybe try shifting a bit more weight toward the bow.

 

[Silvertip]

Re: what causes a boat to porpoise

is the engine mounted at the same height the Force was? Engine height can play a part in the porpoise issue.

 

[JimS123]

Re: what causes a boat to porpoise

Basically two root causes for porpoising......motor trimmed out too far, or the hull has a hook in it. Speed also plays into the equation. Even a boat with a hook in the hull will run smooth if you run her slow enough or don't have too much HP on the back.

Secondary issues are running the motor too low.

Check the hull for trueness, and make sure you are in the proper rpm range at WOT. Then raise the motor 1 notch and check it again. A Doelfin hydrofoil may fix the problem if your hull is not true.

 

[Barnacle_Bill]

Re: what causes a boat to porpoise

Basically two root causes for porpoising......motor trimmed out too far, or the hull has a hook in it. Speed also plays into the equation. Even a boat with a hook in the hull will run smooth if you run her slow enough or don't have too much HP on the back.

Secondary issues are running the motor too low.

Check the hull for trueness, and make sure you are in the proper rpm range at WOT. Then raise the motor 1 notch and check it again. A Doelfin hydrofoil may fix the problem if your hull is not true.

I think you mean lower it a notch or 2. If you trim a motor up too much it will cause porpoising.

 

[mhyder63ford]

Re: what causes a boat to porpoise

the boat had trim tabs on it with the force motor on it and the force motor had a sting ray on it also. took the trim tabs off and ran the motor as close to transome as possible and it didnt change any thing. we also tried it in every hole with no chage

 

[mhyder63ford]

Re: what causes a boat to porpoise

and what do you mean by a hook in it ? im a newbie lol

 

[cr2k]

Re: what causes a boat to porpoise

A "hook" is where the bottom of the hull kinda bows up a little before the transom and then it hooks back down to the bottom edge of the transom.

This causes a proposing by trying to keep forcing the bow down and it bounces up. Trimming too far up (out) causes a proposing by lifting the bow and it can't stay so it falls back down.

If trimming in doesn't help and you do not have a hook you may just need to balance better with more bow weight.

 

[MRS]

Re: what causes a boat to porpoise

the boat had trim tabs on it with the force motor on it and the force motor had a sting ray on it also. took the trim tabs off and ran the motor as close to transome as possible and it didnt change any thing. we also tried it in every hole with no chage

Did you try running with the trim tabs only without the sting ray on? Not good to run both at the same time....

 

[QC]

Re: what causes a boat to porpoise

1) Probably had a foil and tabs because it porpoised

2) weight balance is huge and is where to look if the bottom is flat and the motor height is right

3) Rocker (convex) is more likely to cause porpoise, not hook (concave) . . .

4) Put the tabs back on, they will help you manage porpoise if everything else is right, or even if everything else is wrong.

 

[JimS123]

Re: what causes a boat to porpoise

I think you mean lower it a notch or 2. If you trim a motor up too much it will cause porpoising.

I said the motor was either trimmed out too far, or positioned too low on the transom. So, it needs to be trimmed in and raised up.

Now that the OP told the whole story, it looks more and more like a hull problem.

A "Hook" most definately WILL cause porpoising. I can attest to it from personal experience.

Also, a fin and tabs together CAN offer a synergistic affect. More personal exoperience.

 

[Capt'n Chris]

Re: what causes a boat to porpoise

A "hook" is generally created from the boat sitting on malfitted trailer for long periods of time or can be placed there purposely by hull design. A "hook" will cause the bow to "plow" if it is bad enough. A "rocker" will bring the bow up. If these are not part of the hull design, your boat will always run errant unless the hull is reconstructed. Oftentimes the boat value precludeds that type of effort and expense.
(1) Properly redistribute your weight
(2) Proper mount your motor referencing you AV with the running surface of your hull. begin with flush.
(3) Trim and mark your zero trim angle for running reference. Best "at speed" efficiency is at zero or +
(4) Verify that your motor is turning at or very near the max rpms with the load that you carry trimmed at zero +. If it does not, you are propped incorrectly.
(5) The correct prop in most all, but rare instances will correct running posture, however, is the least understood by boaters.
(6) Avoid the use of "add on" regalia which can give the false impression of correction.

 

[Mark42]

Re: what causes a boat to porpoise

Too much weight in the rear will cause porpoising. Did you add extra batteries or fuel tanks in the rear? Or remove things from the front?

The trim tabs and fin may all have been added to support added weight in the rear.

 

[mhyder63ford]

Re: what causes a boat to porpoise

no the only thing that i did to the boat was first take the 125 force of of it, then we put a 115vro johnson on it the shaft is the same and every thing we left the trim tabs on for the first time out tried the trim in all 4 holes, we couldnt get the boat to full throttle . so we went back home took the trim tabs off and back to the lake it didnt change any thing the boat just starts to plan out then all hell brakes lose never get past half throttle and we didnt have any problems with the force on their but it did have the stingray on it and the johnson does not

 

[itsaboattime]

Re: what causes a boat to porpoise

Will the stingray work on the Johnson?? If it will, I'd try it.

I do agree with cap'n Chris. "add ons" don't fix the problem, they cover it up.

I would try adding some weight to the bow, maybe a person or a battery. I would experiment with weight distribution.

 

[ddrieck]

Re: what causes a boat to porpoise

Does your Johnson have hydraulic tilt -n- trim?? If not you can add an aftermarket trim unit that will allow you to trim to different angles that "WILL" help you with your problem. If you only have a fixed trim lock system "pin and lock" you may never get rid of the problem do to it happening at different speeds. With the hydraulic option you can trim in and out at different speeds to minimize or completely get rid of your troubles.

CMC tilt trim unit available here for a good price:

http://transom-jacks.com

 

[QC]

Re: what causes a boat to porpoise

Tell us more about the trim tabs . . . Seems very odd that there was no difference with and without tabs. Are these helm adjustable like Bennett's? Fixed? Do you know how to use them?

 

[180shabah]

Re: what causes a boat to porpoise

...Also, a fin and tabs together CAN offer a synergistic affect...

With proper installation a 'foil has ZERO effect at planing speeds - cause it is NOT in the water.

 

[mhyder63ford]

Re: what causes a boat to porpoise

the trim tabs were the abs plastic type no adjustment on them

 

[NSBCraig]

Re: what causes a boat to porpoise

So you swapped motors and couldn't get up to speed but your pretty sure your anti-ventilation plate is at the same height as the old motor?

Then the first thing to look at is your prop. Do you have an even close to the right prop on your new motor?

Also by abs tabs do you mean nauticus smart tabs?

If so I'd put them back on and try to figure out why your not getting enough throttle (again look to a prop change it's the usual suspect)

As for hook and rocker like capt chris said sometimes they are there for a reason some manufactures build them in from the get go so unless you really know what your running surface is supposed to be like it can be hard to figure out.

Cant hurt to put a straight edge on it and give her a look though so you know how it is now.