Boat with bouncing bow

[dan (or)]

Hi! Thanks for your help in advance. I just purchased a 50 merc 1993 tt ls elect tiller. Nice motor. On the maiden run on my 16 Lund, it porpoised like crazy! So I filled the live well and it still bounced especially when I trimmed the motor up. (Water was bouncing out of the live well!)
Any suggestions? The boat lay out is custom; I remodeled the interior last year. 15 gal gas tank behind the driver's seat, live well where the front bench used to be and two batteries at mid ship in the rod lockers. I think I have too much weight in back?

What would a Doel fin or the like do for the boat? Should I move the batteries forward? I would hate to move the gas tank but I can't have it run this way....

dan

Out in Oregon land where the walleyes are bigger and nobody fishes for them

[cisco]

How extensively have you experimented with the trim? It sounds like a trim problem, but could be coupled with a mount problem. If too much of the shaft extends into the water beyond your transom, you'll have porpoising problems, too.
I had one like that (with a 90 Merc) and my friendly marine man raised the motor a couple notches -- ran fine after that.

[REW]

Obtain the services of a "200 lb buddy" and have him help you evaluate your problem.

While running your boat at various throttle settings and trim positions -- have him seated at different points in the boat.

Find, if his movement of 200+ lbs to different spots - have any effect on porpoising.

If his movement has little effect on porpoising - you won't be gaining much by moving gas tanks or batteries.

In any eveny I would look to
1. Your motor position on the transom - is it on the 3rd hole up?
2. If this is a lund -- with the riveted portion of about 1/2 inch long extending past the transom on the back of the boat -- often -- just bending down this 1/2 inch extension about 1/16 -1/8 of an inch , can completely solve a porpoising problem.

This was the factory fix, for many lund hulls of past years which exhibited the problem. The tech would take the boat out for a test drive, and base on the severity, and his experience, would simply use a hammer and leveling bar - to slightly "tweak" this back extension down just a bit. "worked every time"

Think about porpoising --

When a boat is porpoising -- the boat is actually balancing in the transom. At the height of the "bow bounce" the only thing in the water is the transom and the motor lower unit and prop. All the rest of the boat is out of the water.

So the fix -- of bending down a tiny bit of the aft protion of the hull introduces "hook" into the hull. As the bow rises -- this "hook" causes more and more drag - which tends to counteract the force that is causing the bow to rise, and the "hook" tends to force the nose back down to a point of balance. When this balance occurs, the bounce stops. The trick is to put in just enough "hook" so that the bounce stops -- without causing so much force, that it tends to drive the bow deeper into the water.

Take care

REW

[George Eh]

Nobody's asked you yet but what is the horsepower rating for your boat? It seems to me if your boat had bench seats it started out as a utility boat and most 16' utilities are rated for 40 hp. I have a 16' Starcraft which also had a front bench and was a utility, but I built in a casting deck and storage boxes for battery etc. The rating on my boat is 40hp. and a friend has a 16' Lund (utility) which is rated the same. Do you have too much motor for your boat? Good boating from Rainbow Country!!!

[Polski]

I had the same problem with a 14' deep-v that I put a new 25hp Merc on. I went through alot of what your talking about trying. The fix was putting on a Dole fin. My boat planes out right away with no porposing. It's well worth the $25, and I know other people who had the same results. The above post also mention some important basic things you need to check first, like over power or a motor thats to long for your transom. The cavitation plate on the motor should be right at the bottom of your boat. Good Luck.

[Frank_Walsh]

Another thing you can do that worked for me was to install aluminum wedge plates between the motor and transom. These allow more downward trim when you are lightly loaded, and reduces porposing markedly. Since you said that this was a tiller, and the fuel tank was behind the drivers seat, you may have to relocate this forward to achieve better weight distribution.

[dan (OR)]

Thanks for the responses! A little more information for all who have replied.

1. The motor is mounted with the cavitation plate dead even with the bottom of the boat.
2. There isn’t a riveted edge that extends past the transom. (I read your informative early post from the archives REW, thanks and I always appreciate your insights into any problem posted on the site!)
3. Horsepower rating on the boat is 50hp. It was a ws16 1983.

I will be purchasing a “doel fin” or the like. Any thoughts on which model is superior the doel fin/ stingray style or the EDGE / SE sport style that sticks a little further past the end of the motor?

I threw in 80 lbs. of dog food in the front deck’s access hatches (still in the bag) this greatly helped propoising but I am trying to maximize the motor’s performance on the boat without adding additional weight which reduces top end speed some…

So my questions are:

If I move the batteries to the bow will that reduce their life based on more abuse by the beating they will take under normal conditions?

Which doel fin or the like is recommended?

Thanks again for your thoughts and ideas.

Dan

Out in Oregon land where the walleyes are bigger and nobody fishes for them 

[Tracker]

i would recommend going with the aluminum wedges first(Bass Pro or Cabelas). As i too had the same problem with a 16' Lund. The wedges took care of my problem. The nice thing about them is it is NOT permenant. if it does not work,take them out. Drilling holes in your cavitation plate for a fin is scary. It weakens it and i have seen the results. hope this helps

[Kevin in CO]

Ditto on the dole fine. My 16 ft tracker had that problem in '89 when I bought it. After two months of experiments with moving weighted objects around in the boat, I bought a dole fin. Problem solved immediately.

[SteveP]

Dan, As you have indicated that your motor's anti-ventilation plate is level with the bottom of the hull you're most likely close to an optimum stable running height.
You do not want to add any unnecassary weight anywhere in the boat. I personnaly would not sacrifice any deck space or good useable storage space if not absolutely the last resort. Relocating batteries is fine and you should not loose any life by being up front as long as they are secured firmly. Again , don't do this yet until you've tried some other easy fixes.
The trim angle is of course the easiest to check and if you are tucked all the way in and still bouncing the wedges may help but you will sacrifice some performance.
It sounds like your rig is an ideal candidate for a fin. It will add lift and usually robs very little in performance. You want to be able to use your trim to maximize your boat/motor/prop performance.
Are you propped within the motors ideal RPM range. The bite of your prop can also have an impact. If you don't have a spare prop you may want to experiment with a few different props to see if there is a difference. If the motor came off of a heavier boat you might be under-propped and over-revving and not getting a good bite.

[REW]

Dan,
Have you had a chance to do anything with the fin that you were going to mount?
Take care

REW