Bow bounce? not rise.

[lonemust]

Hey y'all finally got to take my boat out last sunday. My problem is after i get on plane the bow likes to bounce and bounce hard. If I trim in I can get it to settle down but loose speed. Trim out I get my speed back but the bounce too. WHat can or could I do to stop it from bouncing. Oh so y'all dont "yell" at me, I have a '70 starcraft v180 io(18 ft) with a merc 140. On plane trimmed in it cruises about 32 mph@3500. Trim out it got up to 38.9@ around 4000rpm. But it have more bow bounce the I can handle.

 

[bsktball55]

Re: Bow bounce? not rise.

If you are getting bow bounce or Porpoising, you are trimmed out too far. If you start porpoising, you have to trim back down a little or speed up. Your motor just isn't able to hold the bow up at that speed. If you have a lot of weight up front, you may try transferring some of it to the back of the boat.

 

[NSBCraig]

Re: Bow bounce? not rise.

or try smart tabs

 

[lonemust]

Re: Bow bounce? not rise.

Smart tabs?? Are those those "fins" mounted to the stern. If so I have them, mine are "fixed" Meaning I cannot adjust them on the fly.

 

[Borchik]

Re: Bow bounce? not rise.

If thats all the rpms you are getting you have too big of a prop, having too large a prop will actually cause your issue.

 

[metriccrescentwrench]

Re: Bow bounce? not rise.

If thats all the rpms you are getting you have too big of a prop, having too large a prop will actually cause your issue.

Original post said he CRUISES at that throttle setting:

On plane trimmed in it cruises about 32 mph@3500. Trim out it got up to 38.9@ around 4000rpm.

Smart tabs?? Are those those "fins" mounted to the stern. If so I have them, mine are "fixed" Meaning I cannot adjust them on the fly.

Smart tabs are "self adjusting". Many people here have them and like them alot. Search the iboats boating store.

 

[haulnazz15]

Re: Bow bounce? not rise.

Original post said he CRUISES at that throttle setting:

He also mentioned 4000rpms and 39mph which on a 17' boat with a 140 is probably his WOT. He may be overpropped, not letting his WOT rpms get high enough.

 

[lonemust]

Re: Bow bounce? not rise.

He also mentioned 4000rpms and 39mph which on a 17' boat with a 140 is probably his WOT. He may be overpropped, not letting his WOT rpms get high enough.

I have an 18' not 17' and no I have not reached wot which is 4600rpm. The 140 runs between 4200-4600 at wot. It was running with a 14 1/2-23 prop. Last weekend was the first time I had it out this year so I'm still working some bugs out of it.

 

[QC]

Re: Bow bounce? not rise.

If you have a lot of weight up front, you may try transferring some of it to the back of the boat.

Backwards for solving porpoise. You want to try shifting weight forward . . .

 

[lonemust]

Re: Bow bounce? not rise.

Backwards for solving porpoise. You want to try shifting weight forward . . .

I thought that moving weight back was odd. What about building a ballast tank for the bow. I have room to put one.

 

[Silvertip]

Re: Bow bounce? not rise.

why ADD weight when you can "redistribute" it. No sense dragging an extra person around (in the form of dead weight) when it is not necessary.

 

[gozierdt]

Re: Bow bounce? not rise.

Before you decide on a final solution, you can try two things. First, you may want to experiment with moving your tabs further down. That will create a down moment on the bow, which will be similar to adding weight there. The problem with this solution is that if the tabs are too far down, it'll be like having the motor fully trimmed in, and will limit top speed. Second, try adding some kind of temporary weight to the bow- a couple of old batteries, a spare tire, a friend that can crawl up in the bow when the porpoising starts, etc. I suspect a combination of the two will go a long way to mitigate the problem.

 

[lonemust]

Re: Bow bounce? not rise.

Silvertip, The only way I could shift weight forward anymore is to move th seats forward more then I wouldn't be able to drive it. I would be right up against the steering wheel. I had everyone as far forward as I could without putting them ON the bow. My boat is NOT a bowrider, it is a closed bow.The bow deck is about 5-6' long( havent measured it just a guess). That is why I asked about a ballast tank in the bow. Between my wife and I we weigh about 450 lbs. My kids together weigh a little under 400lbs. The daughters boyfreind weighs 125. So most of the weight was in the front seats. and with that it still would porpoise bad above 25 mph. It would get so bad my wife almost bent the windshield support and freaked her out. It even spooked me a couple of times. The only thing that would get it to stop was to trim in almost all the way. One line up from the bottom on the guage.

 

[QC]

Re: Bow bounce? not rise.

Are you carrying any tools? Anything heavy that you can move under the bow? Is there room under the bow? Shifting is like a double dip, you lighten the stern and get some weight in the bow.

Also, one notch up on the trim at 25 MPH may be the deal. I also agree about trying to adjust your tabs down some too.

 

[bsktball55]

Re: Bow bounce? not rise.

Backwards for solving porpoise. You want to try shifting weight forward . . .

Sorry for the wrong info. I was going on personal experience that I've had when I've had people riding in the front of my bass boat, it has trouble keeping the nose up and likes to porpoise. It may just be the extra weight in the boat that is doing it though. Thanks for clearing up my mistake.

Can you explain the reasoning? My thinking was that porpoising was caused by the motor not being able to hold the bow up so it kept falling down, that's why I though less weight would make it easier. Thanks for you help.

 

[gmc1962]

Re: Bow bounce? not rise.

Have you tried a Whale Tail/Cavitation Plate........I use them to increase the effectiveness of the trim. It may give you more control with less trim. If it doesn't work it will be a good conversation piece.......

 

[gozierdt]

Re: Bow bounce? not rise.

Sorry for the wrong info. I was going on personal experience that I've had when I've had people riding in the front of my bass boat, it has trouble keeping the nose up and likes to porpoise. It may just be the extra weight in the boat that is doing it though. Thanks for clearing up my mistake.

Can you explain the reasoning? My thinking was that porpoising was caused by the motor not being able to hold the bow up so it kept falling down, that's why I though less weight would make it easier. Thanks for you help.

Basktball55,
The usual problem in porpoising is that as you trim the motor out, less and less of the hull is in the water. Eventually you get to a state where the trim is so far out and the speed so fast, it pushes the hull up too far for the water to support, sorta runs too fast for itself. So the hull falls back down until the water supports it again. This "up too far-fall back down" cycle just keeps repeating itself until you slow down or trim back in. By putting weight forward, you're helping keep the bow down, preventing the cycle from getting going. Trimming tabs down has the same effect, helps keep the bow down.

 

[RWilson2526]

Re: Bow bounce? not rise.

how much speed do you lose when you trim in.....I mean cant you just keep it trimmed in so you dont poroise?

 

[lonemust]

Re: Bow bounce? not rise.

When it is popoising I'm running about 38.9mph. If I trim in I drop down to about 28.6 mph. Now I don't have a GPS, These speeds are off my Eagle Magna II fish finder. When I trim in while porpoising I also have to back down the throttle too, in order to get it to quit. I tried just trimming in and it settled down a little but did not stop. the only way to get it to stop slamming is to back down the RPM's that's why I loose so much speed. The fast I go the worse it gets. Even trimmed all the way in it still does it. Wife is worried that the hull will break in half from slamming down like it does.

 

[tschmidty]

Re: Bow bounce? not rise.

The hull won't break in half. The tabs you have look like just L shaped plates which essentially just extend off the back parallel to the hull running surface.

Other recommendations to put weight forward will help to a certain extent.

I think Smart Tabs would be your best bet though, replacing the "tabs" you have now. The smart tabs will act almost like a shock absorber, which is pretty much what they are, a shock absorber attached to a trim tab. When the bow starts to rise, the tabs will bite the water more, helping to keep the front down. When you are on a good plane, the pressure from the water will help keep the tabs compressed so they won't affect top speed much.