IO Low RPM "Thumping"

I bought a 19 foot seasprite boat last summer. It is a 1986. Last summer I noticed that "on occasion" that boat would seem to "thump" after putting it in gear. If I stayed at low RPM while leaving the boat ramp, it would "thump" every few seconds. I couldn't really tell where the thump was coming from. As soon as I gave it some speed, it worked fine.

I've had it out three times this year so far and today, the thumping was especially bad. An almost continuous thump at low RPM's, and even some at middle RPM's. When I have it full open, it seems to be working just fine.

Anybody ever had this, or heard of it, or have any idea what it might be? I know you can't diagnose over the net, but maybe the symptoms sound familiar.

Thanks!

Couple things...I am certainly not a boat expert so there are lots of things I didn't know one shouldn't do. One of them is run the boat with the outboard in the up position. Many times we lift it up in the shallow water to where the prop is just below the waterline to give it some thrust. I now see thats a no no, and it seems I could have damaged my u joints right there.

As far as the "thumping", thats the best I could describe it. It's certainly not a continuous vibration. It almost sounds as if someone were below the boat and pounded their fist into the hull. When we were running yesterday at half speed, it almost seemed like I could predict when a thump would occur. Just about every five seconds...thump...five more seconds...thump. Not exactly, but pretty close. The thumping starts immediately upon shifting into forward gear, (don't recall if it happens in reverse) and is almost continuous now at low rpm's. It's way worse than last year when it only occurred a few times at low rps's. Obviously, it's a progressive problem that is getting worse.

I'm taking it into a marina tomorrow. They're about a week out on the service calls, or so they say. I'm hoping they'll just put it in the water, hear the thumps, tell me I needed some new transmission fluid, and off I go. I know it's a far out chance, but that's what I'm hoping for. <G>

Yes, it's an OMC Cobra. I assume it's a 1986 because that is the year of the boat. I paid $5000 for the boat and trailer last summer (probably a tad too much) but it's given us some fun. If the repair bill is going to cost to much, I'm not sure what my options are. I really don't want to put a lot of money into such an old boat. I wonder if I'm better off taking my medicine and buying another used boat with an outboard. The kids are into tubing and skiing. It's simply used for recreational water sport.

..Larry

Tacoma area Washington state. On Wed, 14 Jun 2006 08:39:54 -0700, "Mr Wizzard" <wiz@muffy-mail.com> wrote:

I think I have an electrical (new boat to me) as the cut out is kicking in when I hit the trim. It also will let me trim it into trailer mode while underway -- I know; off topic thread.....

Rob