200 BR starts to "porpoise" when i trim out

[Fasterblaster]

It seems anything over about 25 mph and up and down we go. It stops if i trim the leg right in.

I'm not experienced, but this is what i do: get to my cruising rpm of about 3200 and speed will be over 20 mph and then trim out; the boat speed will increase and I'll trim out a bit more. It feels like i am "freeing up" the boat. If i go too far and the rpms increase i have to trim back in a bit. The gauge reads about 3/4 trim, although i don't have much faith in the accuracy of my gauge.

If theres any swell at all and i'm doing 30 mph, away we go, up and down, up and down, until i trim back and slow down. the wife really hates this.

Is this the prop?
Would trim tabs or one of those leg mounted foils help?
Is this the problem with a 20' boat? ie: 2 months in and its time for a bigger boat

as always, thanks for your comments

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[Alex D]

On stern drive boats, porpoising often starts when you are over trimmed. Just trim out and then trim back in small steps until the porpoising stops and that's the best trim setting. if you have tabs sometimes trimming out and stopping the porpising with a little bit of tabs can also be the way to go.

 

[tphinney]

my old 220SD didn't like anything over 1/2 trim on the gauge. sure i could go higher be the slights ripple would start the boat to propose.

if your trimed right she'll porpose a bit when you hit a wake or wave, but it will settle right down agian once you clear the wake/wave.

give it s shot at 1/2 trim instead of 3/4 and you should be good.

 

[Searay Jim]

I agree with everything said so far. It just sounds like you're trimming out too far.

Another thing I'm picking up on. You say you throttle up to 3200 RPM, your cruising RPM, then start trimming and the boat "frees up". Sounds normal of course, however, with most hulls, you can/should/could be trimming out as you get on plane so you're not at a point where the drive-line and hull are so far apart that the boat is "bound" and waiting to be freed by trimming out.

I know a BIII will act different, but even with mine, I trim as I throttle up and by the time I get on plane, I'm trimmed out where I need to be for that speed. In my case for normal casual cruising, 2800-3200 RPM (32-36 MPH or so) is where I spend most of the time. Not until I get up near 4000 RPM (48 MPH or so) do start to trim a tad bit more, then at WOT (5000-5200 RPM) do I trim even more, but even there I can trim too far and as you described, a little wake or chop and it'll start porpoising a little. I tuck the drive by just bumping it as gently as possible and it'll smooth right out.

I can also make it porpoise now at lower speeds by trimming too far out. I say now, because it used to not do that but I bumped the read prop on a ramp on the first outing this year and took a tad bit of blade off the edge. It's not much, but it's enough to make a difference. Used to, I could trim all the way out and it was solid. I think the slightly less blade surface has caused it to not hold like it did.

The good news though, I don't have any cavitation burn issue at all this year with it.

Sorry for the long post, but the points are, yes, the prop could be the cause, a different type of prop could improve it and lastly, you might should be trimming a little sooner, as you're getting on plane because you really shouldn't be having the boat "bind", and then trimming to free it up. Many performance hulls won't take any trim at all until 60+ MPH, but they also don't plane out as nice and quick as our pleasure boat hulls. Ours can pop right out and if trim is timed right, you'll jump up to cruising speed quick.

Ok, no more espresso for me, this morning anyway.

 

[Morpheus]

get to my cruising rpm of about 3200 and speed will be over 20 mph and then trim out; the boat speed will increase and I'll trim out a bit more.

I think your issue might be right here.

What I do is get up to cruising speed then trim out just a little bit at a time until I see the speed jump up a few mph that is the sweet spot anything over that your going to start making things worse instead of better.

I don't have a 220 like you do but on our 240 when I trim out it's very very very little before I see the RPM jump.