Porpoising

[mbrown2097]

Hello all,
I am having trouble figuring out how best to trim my boat out while running. It seems like when I trim up the slightest amount, I start porpoising (bouncing). If I trim back down, things smooth out. My gutt tells me to just leave it trimmed down but Intellectually I believe that I ought to be trimming up some. Am I doing something wrong?

 

[tpenfield]

Re: Porpoising

Not doing anything wrong . . . If you want to be able to trim up some more, you could move some weight forward. Trimming up and having weight toward the stern have a similar effect. The porpoising is just a sign of an imbalance.

 

[mbrown2097]

Re: Porpoising

3 batteries, 10 galllons of gas, 1 gal of oil all in the stern. Really nothing much in the bow. I guess that makes sense then. So I really shouldn't worry with trimming up because the weight distribution is a pseudo-trim?

 

[tpenfield]

Re: Porpoising

You will always want to trim to the wave consitions and loading of the boat. But the weight positioning can help out and avoid unnecessary amounts of trim.

 

[Texasmark]

Re: Porpoising

Porpoising is caused by a hooked hull or a trim/speed imbalance as stated. Take my boat as an example. I'm at mid throttle, trimmed in somewhat, boat running smoothly but bow is cutting the water, like I do when it's rough, which limits speed.

Without touching the throttle, I trim out slowly and as I do, the bow comes up out of the water, speed increases, rpms go up, the boat gets a floating feeling, and as I continue to trim out it starts porpoising. The more I trim out, without touching the throttle, the worse it gets.

So, if I want to go faster, I just bump up the throttle at the current trim position until it quits. Simple as that. When slowing down at high speeds, it will porpoise then too because I violated the same balancing "equation" for lack of another word when I reduced the boat's speed. So, with the throttle where I chose to set it, I just tuck her in till it stops. Simple as that.

My boat has the cross seat aft of center usually with 430# of occupants, a livewell, a 22 gallon fuel tank, 2 TM batteries and 1 engine run battery, tools, anchor, and other stuff in lockers just ahead of the engine, and a 305# engine....so I AM stern heavy so that is not necessarily the main concern....it's hull to water attitude vs speed.

HTH,
Mark

 

[joe009]

Re: Porpoising

you could try a hydro foil it did wonders for my boat

 

[mbrown2097]

Re: Porpoising

Thanks Mark,
I'm very much a newbie at this and appreciate your insights. I have 150 merc that'll run 60ish and I tend to cruise at 35 or so. Sounds like I need to push her a little more to avoid the bounce (if triming up). My knee jerk reaction to bouncing is to slow down, sounds like I need to throttle up.

 

[QC]

Re: Porpoising

Weight balance as Ted and Mark suggest is a good thing to get straight no matter what you do. Ultimately I set my boats up so they porpoise slightly at full throttle and full up trim (just before it blows out). Then, I control the porpoise with trim tabs.

A hydrofoil on a 150 is flippin' blasphemy

 

[JoDo]

Re: Porpoising

I am relatively new to boating as well (4th season) I frustratedly fought this same problem. I have a friend who is a very experienced boater and when I explained the problem to him, much like you explained yours, he simply said "Then don't trim up" Once I got it out of my head that I don't need to trim, I just need to ride smooth things got better. Also we have a 20' cuddy with not much in the cuddy. We usually only have 3 to 4 of us in the boat so I threw a 70# bag of tube sand all the way in the nose of the cabin along with the anchor. That helped a lot as well.

 

[JJwithaSearay]

Re: Porpoising

Along with all the suggestions above, trim tabs would help a ton. No problems on my cruiser that is very tail heavy. I've also used Nauticus SmartTabs on a runabout. Not as good as power trim tabs but money well spent. About $100 and easy DIY install. Just do a search on this forum for Nuaticus or Smart tabs. there are tons of comments from people that use them.

 

[Texasmark]

Re: Porpoising

Back the truck up folks!!!!!! What are we talking about here? A 3 ton water yacht and a 700# tin lizzie aren't the same critters and will not react to stimuli identically. So let's keep it apples and apples.

HTH,
Mark

 

[mbrown2097]

Re: Porpoising

For the OP, we're talking about an 18' fish and ski with a 150 merc on it. It'll get up and scoot (with a bounce )

 

[QC]

Re: Porpoising

he simply said "Then don't trim up" Once I got it out of my head that I don't need to trim, I just need to ride smooth things got better... I threw a 70# bag of tube sand all the way in the nose of the cabin along with the anchor.

Shifting weight (anchor) I agree with. Adding weight . . . Never!

... trim tabs would help a ton.... No problems on my cruiser that is very tail heavy.

See below

Back the truck up folks!!!!!! ... A 3 ton water yacht and a 700# tin lizzie aren't the same critters and will not react to stimuli identically. So let's keep it apples and apples.

Amen Bro!

For the OP, we're talking about an 18' fish and ski with a 150 merc on it. It'll get up and scoot (with a bounce )

You are the OP . . . Listen to Mark.

 

[Texasmark]

Re: Porpoising

Thanks Mark,
I'm very much a newbie at this and appreciate your insights. I have 150 merc that'll run 60ish and I tend to cruise at 35 or so. Sounds like I need to push her a little more to avoid the bounce (if triming up). My knee jerk reaction to bouncing is to slow down, sounds like I need to throttle up.

One or the other: Less trim or more throttle. Not rocket science folks, just balancing things out.

Mark