Merc 2.5 tunnel boat question

[d&smach]

I have a Seebold tunnel with a 2.5 merc and a Lee tunnel that I have never run (needs a few days of rigging). Not official racing just serious playing. The Seebold tunnel has a terrible porpoise or bounce from about 40 to 75, feels like about a three foot high power bounce. I'm just getting it going again after several years and if I recall turning it in a slight arc would eliminate the bounce and trimming it under doesn't seem to help. I guess it is called 'transition'. If you have the nerve you can straight line, full throttle, and drive thru it. That's all fine but if you just want to cruise several miles on the river it is impractical to stay below 40 or stay above 80. Is this just a Seebold issue or are all tunnels like this? If I go to the effort to transfer alot of parts over to the Lee tunnel will it be the same problem?

[largecar91]

I've had a few Seebold Tunnels that I raced. One was a glass hull. If the water was calm, It would bounce. It was good rough water boat. This was an SST120 boat. My wood hulls never bounced

[derekfl]

This is where polygamy would be nice. Put one wife on the front of each sponson…

[WaterZebra]

Yeah...........we called it the Seebold gallop or lope. My glass Seebold SST140 always did it. My Hoffman never did. Funny thing was the glass hulls that Tim Seebold and Billy Joule had never did this. Maybe it was driver technique or propeller selection? I thought the boat was always trying to find its balance point. You might try changing the fuel tank and battery locations but I took note of where Tim Seebold's stuff was located and mine wasn't much different? I never could solve the problem. I tried a engine set back spacer that Terry Rinker made up for me. It helped a little. The other problem with the glass Seebolds hulls was once the top deck and hull pieces were bonded together (shoe box) that was it. They had a problem with frames that were put in to stiffen the sponsons. I always wondered if the hull would "warp" under power and slightly change lift and airflow? Once I took delivery of my Hoffman the Seebold never left the garage.

[d&smach]

Here is a picture of something I am going to try today. I'm not trying to defeat the design of the boat at high speed. These tabs won't be used at speeds where the boat naturally smooths out, just in the so called transition range. I'll try disconnecting them, increase speed and note where the bounce begans. Then make another run with them slightly down and see if the speed where the bounce starts is higher. If I can get to a point where I can reach 80 mph without a bounce, then I will be at a speed that they are no longer needed. Then I guess I'll use hydraulic cylinders to raise and lower them to make it smoothly through transition. I'll post results. Also here is a picture of the Lee tunnel that I might transfer several parts over to from the Seebold tunnel. Incidently, the Seebold is wood and the Lee is glass.
520989520990

[WaterZebra]

Here is a picture of something I am going to try today. I'm not trying to defeat the design of the boat at high speed. These tabs won't be used at speeds where the boat naturally smooths out, just in the so called transition range. I'll try disconnecting them, increase speed and note where the bounce begans. Then make another run with them slightly down and see if the speed where the bounce starts is higher. If I can get to a point where I can reach 80 mph without a bounce, then I will be at a speed that they are no longer needed. Then I guess I'll use hydraulic cylinders to raise and lower them to make it smoothly through transition. I'll post results. Also here is a picture of the Lee tunnel that I might transfer several parts over to from the Seebold tunnel. Incidently, the Seebold is wood and the Lee is glass.
520989520990

I never had a wood Seebold that galloped or loped. I think it's your weight distribution?

[KIRCHNER]

I have a skater18, w/ 2.4,220. still trying to stop the mid range bounce. tryed a 4bld stern lifting prop, no good, reduced setback to 4'' a little better. if i go into a turn, it will stop bouncing . i cant drive thru it, too much for me. i was told to try MORE + trim, not less. im going to try my o/h super bowlifting prop, with 0 trim. its great if you go 36mph, & dont fall asleep. boating mag says 80 w/ 200. im think 70 is accurate. its 1000 lbs. ill post if i find a cure for the bounce. also have a 16' magnolia cat, 225, no bouncing, 2 mid-rear tanks!

[d&smach]

Without the tabs the bounce starts at 49-50. With the tabs set very conservatively I hit about thirty and the boat wildly pulled to one side, just like only one tab was on the boat. As I tried again the front end was forced down so hard it was spraying water in my face as I tried to accelerate! Apparently a little tab goes a long way. I should make them shorter and narrower and barely have the tips of them in the water. I don't think the tabs were more the 1/4" to 3/8" into the water at the tips. I think I'm giving up on the Seebold and try the Lee and hope for the best.

[WaterZebra]

I have a skater18, w/ 2.4,220. still trying to stop the mid range bounce. tryed a 4bld stern lifting prop, no good, reduced setback to 4'' a little better. if i go into a turn, it will stop bouncing . i cant drive thru it, too much for me. i was told to try MORE + trim, not less. im going to try my o/h super bowlifting prop, with 0 trim. its great if you go 36mph, & dont fall asleep. boating mag says 80 w/ 200. im think 70 is accurate. its 1000 lbs. ill post if i find a cure for the bounce. also have a 16' magnolia cat, 225, no bouncing, 2 mid-rear tanks!

When you turn, the straight flow thru of air is reduced therefore the amount of lift is reduced. It sounds to me like both boats and situations require more payload weight to be located forward. An easy temp fix to try is with sand bags. Put a couple some where forward and see what happens with the bounce. You might find that the extra weight in the right place actually makes the boat faster? If the result works in your favor, then start moving fuel and batteries fwd. If you are using props with no bow lift (rake) chances are likely its a weight / balance problem? If you really want to go after the problem, put the boat on the floor with a log in the presumed center and then get in and see which way it tips(CG). Myself, I'd try sandbags.........easy to do!

Sometimes more weight produces more speed. A Sport J racer set a kilo record of 105 mph by putting a toolbox in the nose of his Seebold.

[Jimboat]

Pretty normal for your setup.

Here's an article on Porpoising (https://www.screamandfly.com/content.php?325-Porpoising-Why-It-Happens-and-How-To-Fix-It) that might be of interest.


Here is a picture of something I am going to try today. I'm not trying to defeat the design of the boat at high speed. These tabs won't be used at speeds where the boat naturally smooths out, just in the so called transition range.

Tabs will definately reduce the porpoising at lower velocities, but they can be very dangerous at speed. Don't recommend.