Porpoising with a replacement engine

[clay603]

I have a 1988 Bayliner Bass boat on which I used to have a Force 125. The Force gave up the ghost and I was fortunate enough to find a 1981 Evinrude 140 to replace it. The Evinrude has great power, starts easily and runs very smoothly at all speeds. My problem is that when I take the boat above 3500rpm, it begins to porpoise. I have tried to trim out the engine, but it won't smooth out, only gets worse. The only way I can get rid of the porpoising is to back down on the RPM to about 3000. Then the boat runs smooth as glass. When the Force 125 was running, the boat would not porpoise, even at top speeds. I suspect that there is a trim/tilt adjustment that can be made with the engine mount. I am including pics of the engine mount so someone may be able to help me out. Picture 2 shows that there is one lower adjustment for the tilt(?) but it is more difficult to see the adjustment hole in Pic #1.

 

[Stachi]

Re: Porpoising with a replacement engine

that engine has hydraulic tilt/trim....is it functioning ? Is it even hooked up ?

 

[bassman284]

Re: Porpoising with a replacement engine

If you're porpoising, you want to trim in, not out.

 

[UncleWillie]

Re: Porpoising with a replacement engine

... I have tried to trim out the engine, but it won't smooth out, only gets worse.

You want to trim IN the Engine.
Trimming OUT raises the Bow and produces the porpoising at speed.
You want to LOWER the BOW.
Trim Down or add more weight to the bow.
The Whale Tail exaggerates the effect.

 

[Jlawsen]

Re: Porpoising with a replacement engine

1. Remove the whale tale.
2. Start with the motor trimmed all the way down.
3. Once on plane, trim motor up in small increments until max speed is achieved. If the boat begans to porpose slow down, trim it back in a little and then try again.
4. If it works, great, if not you may need a prop change to one that lifts the stern more and the bow less.

Porposing - Cause - When the boat is on plane the motor attempts to hold the bow up. If it can't the bow comes back down, the boat slows and the cycle repeats. This most often happens when a boat is underpowered or is prop'd wrong. However, it can also happen when the boat has adequate power and is trimmed to far up and lifting the bow higher than where the motor can hold it.

When a boat is properly trimmed out it will feel like it's almost riding flat on the water. If you're used to it riding higher this might feel a little strange at first.

 

[rogerwa]

Re: Porpoising with a replacement engine

With the heavier weight of the 140, it will be sitting nose up from the get go and will porpoise easier because of the heavier rear end.. Also, if you cannot solve the porpoise, ensure you don't have hook in your hull.

 

[CoffeeHound]

Re: Porpoising with a replacement engine

Take that whale-tail and hide it someplace other than on that motor . Trim the motor to where it is almost hitting the hull then go to the water for a test run !! When properly setup and trimed,, that motor @ WOT from standing start should almost bring the bayliner out of the water

Send pictures with the engine in the down position so we can see where the Av plate ligns up with the hull !!

 

[Alumarine]

Re: Porpoising with a replacement engine

If the above doesn't work you could also try moving the pin to the hole closest to the transom.

 

[Chris1956]

Re: Porpoising with a replacement engine

Move the tilt pin to the lowest hole. Trim motor all the way down and slowly increase speed until she starts to porpoise. Are the shaft lengths of the old and new motors the same, or is the new motor 25" and the old was 20"?

The whale tail should help neutralize the porpoising, so leave it on for testing. However, I would observe the whale tail while under way. If it is under water, it may be contributing to "porpoisegate". If it is at the surface, it is probably OK. You may want to remove it later and see how she handles.

 

[clay603]

Re: Porpoising with a replacement engine

that engine has hydraulic tilt/trim....is it functioning ? Is it even hooked up ?

Yes, the tilt/trim is hooked up and it is functioning. But thanks for the heads up on this!

 

[clay603]

Re: Porpoising with a replacement engine

Thanks for the help, everyone! I am going to take the following steps and will let you know the results later this week (can't get it back on the water until Wednesday or Thursday).
1. Move the tilt pin to the lowest hole. and follow Chris1956's suggestions. As far as I know, the shaft lengths on the motors are the same, so that should not affect the actual running.
2. If step 1 doesn't work, then I will try removing the whale tail and try everything again.
3. If Steps 1 & 2 don't work, then I will move the tilt pin back to its present position and just remove the whale tail and see what happens.
4. Check to see if changing the prop makes a difference.
Again, thanks, guys, for the suggestions. As a newbie, it really helps when the more experienced hands are there to guide us.

 

[Thalasso]

Re: Porpoising with a replacement engine

I would replace what looks like silicone on the bolts also. You are or will have a water logged transom.Use 4200or 5200 sealant

 

[clay603]

Re: Porpoising with a replacement engine

Porposing - Cause - When the boat is on plane the motor attempts to hold the bow up. If it can't the bow comes back down, the boat slows and the cycle repeats. This most often happens when a boat is underpowered or is prop'd wrong. However, it can also happen when the boat has adequate power and is trimmed to far up and lifting the bow higher than where the motor can hold it.

When a boat is properly trimmed out it will feel like it's almost riding flat on the water. If you're used to it riding higher this might feel a little strange at first.

I previously had the boat up for sale and when a prospective buyer asked me to take it to a guy to have him look at it, he grabbed the lower drive unit and started lifting it. He really lifted the back of the boat quite a bit on the trailer and then he told the guy that the transom was weak and that was what was causing the porpoising. Two other professional repairers have told me that the transom is solid and does not need to be redone, and one even said that if he lifted the engine with that much force on any boat, the transom would flex some and he could say that the transom was weak. Have you ever heard of the transom being the cause of porpoising?

 

[generator12]

Re: Porpoising with a replacement engine

Clay, look up "Smart Tabs". Google it for reviews. This installation (about $125 or so) DEFINITELY helps with porpoising. Not disagreeing with what has been posted regarding engine attitude, weight, etc., but keeping the bow down is one of the selling features of these things, and they do help. Huge difference on my Four Winns.

 

[Texasmark]

Re: Porpoising with a replacement engine

You can test the transom by putting the engine vertical on the trailer and getting up on the anti vent plate and jumping up and down while someone else watches the top of the transom for movement. No movement, no problem.

Raising the engine helps to reduce porposing. If the Anti vent plate is above the bottom of the boat....straight edge (yardstick) up against the hull right in front of the engine running fore-aft (front to rear) at the center of the hull, engine tilted so that anti vent plate is parallel to ruler, anti vent plate above ruler one or so inches and you will have a hard time porposing. Also trim positions have less dynamic effect. Below it can help cause it.... BTDT

Your Evinrude (OMC issue) prop is not a bow lifting prop. It's a good prop but not bow lifting.

Leave your tilt pin in as far as it will go.

As others said, speed and trim position effect porposing. When it occurs, you can either trim in till it stops or speed up till it stops.

I doubt you have a hull hook as you would have known it with the Force.

That boat doesn't need a whale tail on that engine. You have plenty of whoopie without it.

Mark

 

[clay603]

Re: Porpoising with a replacement engine

Well, I finally got it back into the water and all it took was moving the tilt adjustment pin to the lowest hole. At WOT, ran smooth as glass unless I hit the wake from other boats/waverunners. Took the whale tail off and tried it, but it took almost three times as long to hit plane as with the whale tail. So, I put it back on and within about 3-4 seconds, boat planed out and ran smooth at WOT. Stayed on plane even as low as 10 knots. Appreciate all the help.