Boat Porpoising...HELP!

[Hashi]

Hi Fellow iBoaters,

I just recently purchased a 2008 20' 6" Tahoe Q7i with a 5.0 MPI, 260 hp MerCruiser. Approximate weight is 3,100 lbs. The other day, I went to the lake I also go to and it was very choppy with white caps. My boat porpoised so much that I eased up on the throttle a lot because I started to get a little scared that I would start taking on water. I've never seen my boat go up and down so much that it started pushing water out from the sides of the bow. What a hairy experience! I noticed other boats going much faster and seemed to be cutting through all that chop with little porpoising, if any. When I bought this boat, I thought this boat had a decent vee hull and the weight would limit porpoising. What I can do to minimize this problem.

Thank you in advance for your expert opinion.

Rich

 

[troypolla]

Re: Boat Porpoising...HELP!

Boat porpoising is caused from too much lift from the prop, or in another words its trimmed out or up too much. Try to trim in or down some until it stops my boat had this problem until I learned how to fix it easily. Usually in choppy water I run slightly trimmed in more anyway.
Hope this helps.

 

[m&m252]

Re: Boat Porpoising...HELP!

i had a 20ft with a 200hp outboard and placed a set of marine store brand trim tabs /fixed, and help with the porposing and steering /i had the same problem of a scary situation with my wife and daughter . she told me to get a bigger boat...

 

[Hashi]

Re: Boat Porpoising...HELP!

Thanks guys...Prop was trimmed all the way in. Guess I have to invest in some trim tabs. But, I really think its in the design in the hull.

Rich

 

[steve201]

Re: Boat Porpoising...HELP!

one thing....if your pushing water and bouncing into chop..and taking water......you need to slow down some....in choppy water head into the wind and waves, slow down and keep your head about you in navigating your way to your destination......tabs isn't going to help when your in chop.....

 

[SpinnerBait_Nut]

Re: Boat Porpoising...HELP!

As steve says, slow down.
If you don't have the power to get up on top and stay there, then by all means, slow down.

 

[Boatist]

Re: Boat Porpoising...HELP!

I think you may have trimed in too far. Your boat is on the lighter side of 20 foot boats. Mine is ever lighter 2800 lb and 21 feet Aluminum hull.

When running in big chop with white caps the first thing is to Slow down. Deep V boats will slice thru the chop and be a little smother than a Flat botom. You want to trim in more than when running on flat water to force the V of the bow to slice thru the wave some. However if you trim in to much when you go over the wave the bow will dive down into the next wave. What you really want is for it to come down somewhat slow. Instead of going all the way to the bottom of the trof you want it to stay somewhat level and hit the next wave Halfway up.

Also do not take them head on but at a 30 to 45 degree angle. Small chop you can sometime increase your speed to get the bow into the next wave before it bottom between waves. In big waves and swells this can be danergous.

Also need to know your boats and crews ability. My boat if the swells are over 8 feet or less than 7 seconds appart we do not even launch. In big swells you can do a lot with the steering wheel to avoid the biggest pikes and white caps. Hard to do in small close together chop with wind blown whitecaps.

Better if you can stay on plane but in the bigg stuff you can not safely go that fast. One help is to get helm adjustable trim tabs. This will allow you to stay on plane at a much slower speed. Like my boat natural planning speed is between 18 to 20 MPH depending on load with the engine trim in center. I have trim tabs 24 wide by 12 deep on each side and with the tabs can stay on plane down to 9 to 11 MPH. Makes the ride much safer and more comforable. With time and experience you will get better in rough water.

One thing not to do with a following sea is go over the top of a swell as it can be deadly. Instead ride the back of the swell but high enough you can see over the swell. This allows you to see other boats coming out or rocks sticking out of the water. You will be able to steer to port or starboard to avoid boats rocks or hit you entrance channel.

 

[tashasdaddy]

Re: Boat Porpoising...HELP!

weight distribution in the boat also has a lot to do with how it rides. what was your load, and where was it placed.

 

[Hashi]

Re: Boat Porpoising...HELP!

WOW!....I really appreciate all your expert advice. Thanks, Boatist and Tashasdaddy. I'll play with the trim a little more and see if this helps. Otherwise, I'll look into trim tabs or stay out of the water if its too choppy. I just ordered a 21p Enertia and hope this will help mitigate my problem, as well.
If you guys ever need some advice on architectural concrete, let me know.

 

[gss036]

Re: Boat Porpoising...HELP!

["look into trim tabs or stay out of the water if its too choppy"].
As much as I like to go out, I always look at the NOOA report and heed it. I have been caught out in some rough water and it is not much fun and very scary.
Proverbs says, one that listens to advise and accepts instruction will be wise in the end. Reread Boatist's reply several times and heed his advise for your own safety as well as your familys.

 

[QC]

Re: Boat Porpoising...HELP!

Hashi,

All of the above is good advice. I would work on the weight balance first. In smooth water you should be able to trim all of the way out at WOT and just start to porpoise. If the porpoise is too severe, shift weight forward (this is what I think your main issue is). If you cannot get the trim out enough before the propeller starts to blow out, move weight aft. Anyway get the weight balance right first and then I strongly recommend trim tabs on all V-hulls. Your 20 degrees of deadrise should be able to handle some pretty nasty water comfortably. Get the weight balance right and the tabs and you will be a happy camper (boater). Good luck!!

 

[stevens]

Re: Boat Porpoising...HELP!

Just to nitpick on a point - the above described problem of bouncing hard in the choppy waves is not the same as "porpoising" as such.

Getting the boat trimmed right to handle the chop requires some hands-on experimentation with weight distribution, speed, trim angle of the engine/outdrive, best use of trim tabs under some conditions, and learning how to best attack the waves. Props & trim tabs & other won't help if you don't take the time to learn how to handle the boat properly.

Getting rid of "porpoising", i.e. boat bouncing up & down at high speed even on flat water, is a matter of getting the boat balanced right, i.e. weight distribution, prop bow/stern lift, engine trim and trim tabs. Here, it is more an issue of getting the right set-up.

 

[QC]

Re: Boat Porpoising...HELP!

Yeah, I reread his initial post and agree, that may not have been porpoising at all. I guess I was thinking he needs to be sure he has it right, and needs to know where the boat's balance is best, to begin with. Then he needs to figure out the right combination of trim etc. in different water conditions. I agree stevens, as you pointed out, both are extremely important.

FWIW, I had a 20 footer with only 16 degree deadrise, and in big, white cap, chop she was just hopeless. The only thing I could do was trim out, and use the tabs just to keep her barely on plane with bow as high as I could get it. About 12 MPH to insure I wouldn't get any water over the bow. Back then I boated with a friend's 20 foot Maxum with 19 degrees that could do 20+ without tabs, same day, same water and frankly that guy was absolutely clueless. I can now run over 35 in the same water with 20 degrees and 23 ft. so very different issues with different bottoms and different operators . . .

I am pretty confident if other boats were running away from you that with a few tricks etc., you can run on top with that boat in some pretty nasty chop, you'll need to trim out a little and maybe control her with tabs, but I think that she'll do it.

 

[Nandy]

Re: Boat Porpoising...HELP!

define porpoising?

 

[chrisg]

Re: Boat Porpoising...HELP!

Getting rid of "porpoising", i.e. boat bouncing up & down at high speed even on flat water, is a matter of getting the boat balanced right, i.e. weight distribution, prop bow/stern lift, engine trim and trim tabs. Here, it is more an issue of getting the right set-up.

as he said

 

[Hashi]

Re: Boat Porpoising...HELP!

Hi Stevens and QC,

Thanks for "load/weight distribution" response. On that particular day, I only had my wife and a barely half loaded cooler aft. But, I recently added two troll motor batteries in the engine compartment that could've added to the problem.

On smooth days, the boat planes out nicely with no "up and down" motion of the bow at WOT. I'll try bringing some weight forward next time and play with the trim before I invest in trim tabs. The idea of drilling holes in my hull makes me a bit uncomfortable, not to mention voiding the warranty.

You guys are GREAT!

Rich