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My boat is so cool it has hydraulics! (It bounces)

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My boat is so cool it has hydraulics! (It bounces)

Old 10-19-2020, 07:04 PM
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Default My boat is so cool it has hydraulics! (It bounces)

My first post! It's a long one.

I just bought my first boat. I wanted one I could flat pay for and not have a note. So I found a 1999 cape horn 17cc 7.5ft beam. It has a ttop added that is a little big, but it is the kind with the tarp top. Has 115 johns 2 stroke. 80lb thrust riptide 24v with everything mounted in the bow(added after I bought and didn't help my issue) everything on the engine mounting is stock. No plates or setback.

The issue:

I first got the boat on the water it bounced. Anytime I would get on plane it would start bouncing and increase in severity. If I am trying to go WOT the bounce gets scary and I pull back. Its starts slight and on glass calm water is the worst, it builds and builds until I just pull back. The trim is down full, if I trim up it gets worse faster. I have to ride in the full down trim position.

When I first tries fixing it i tried a different prop. The first prop was 14 inch 17 pitch. The hole shot was terrible. Not good at all, top end was 42ish. I went to a 13.5 with 15 pitch. Hole shot was great, but top end 30ish. But the bounce was the same both props. I ended up with a 13.25 with 17 pitch and it was a decent hole shot with 36ish top end. But the bounce was the same.

Then we tried height adjustment on the outboard. Went all the way up with it.. the motor would blow out too easy. Couldn't turn without blow out. Let it down one hole and it would maintain without blow out unless i turned kinda hard.. the bouncing was still there, couldn't tell the difference. So I let it back down to the original hole.

I dunno what else to do. I cant believe they produced this boat with a bounce like this.. some bounce would be ok.. but i have to constantly pull back and then speed back up..

Next attempt will be to add more negative trim with a jack plate or neg trim spacers.

Any ideas? Anyone else deal with this?
Old 10-19-2020, 07:10 PM
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What is the model number of the Johnson 115?

When the boat hit 42 MPH, what was the engine speed (RPM)?
Old 10-19-2020, 07:11 PM
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FYI, what you describe as “bouncing” is more commonly known as porpoising.
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Old 10-19-2020, 07:21 PM
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You need a hydrofoil AKA whale tail. And / or trim tabs. They make a tab that doesn't require power - maybe "smart tabs"? Not sure. But they are just spring loaded.

The hyrdofoil on your engine will really help. Make sure the engine is all the way tucked. The tail will make it more stable on the drift as well.

I put one on my old boat and it made a HUGE difference.

FYI, what you are experiencing is called "porpoising" search for that and you'll find other recommendations. But for a few hundred $$ you can make some major progress.
Old 10-19-2020, 07:26 PM
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Have you tried a hydrofoil, self leveling trim tans or smart trim tabs and a 4 blade prop? Have you tried moving weight forward. Move more weight to the bow. Porpoising is caused by too much weight aft with a wide beam to length ratio. Moving the outboard up one hole or two will also help.
Archie
Old 10-19-2020, 07:26 PM
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Originally Posted by Tom W Clark View Post
FYI, what you describe as “bouncing” is more commonly known as porpoising.
I'm sorry on the verbiage. I said bouncing instead of porpoising because people say "that is normal just trim down." My friends boat porpoises, but is tolerable. My boat bounces. Hits hard and lifts right back out the water and does it again.

But yes porpoise is the proper term.

On the first prop 42 mph was 4400 rpm. Wot should have been 5500. Now i am 36 mph at 5300 rpm with my current prop. I will get the number on the engine. Gimme a sec to find it.
Old 10-19-2020, 07:33 PM
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Originally Posted by Tom W Clark View Post
What is the model number of the Johnson 115?

When the boat hit 42 MPH, what was the engine speed (RPM)?
I'm sorry. All I can find is the serial number ATM. I will keep looking.
Old 10-19-2020, 07:35 PM
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Did you check for water in the hull?
Maybe some pics might help.
Old 10-19-2020, 07:41 PM
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Originally Posted by Tom W Clark View Post
What is the model number of the Johnson 115?

When the boat hit 42 MPH, what was the engine speed (RPM)?
All I can find ATM is the serial number. I will get that number as soon as possible, but i wont see my boat again until saturday.
Old 10-19-2020, 07:43 PM
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Originally Posted by Birdmayne View Post
You need a hydrofoil AKA whale tail. And / or trim tabs. They make a tab that doesn't require power - maybe "smart tabs"? Not sure. But they are just spring loaded.

The hyrdofoil on your engine will really help. Make sure the engine is all the way tucked. The tail will make it more stable on the drift as well.

I put one on my old boat and it made a HUGE difference.

FYI, what you are experiencing is called "porpoising" search for that and you'll find other recommendations. But for a few hundred $$ you can make some major progress.
I have googled this issue to death.. the biggest thing i find is "just trim down"

I have considered that tail.. but I wasnt sure enough.. but i will get one and try it saturday maybe.
Old 10-19-2020, 07:50 PM
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A foil / tail is a band-aid that treats the symptom, but does not solve the problem. Most porpoising issues can be solved with the good prop, and improved motor mounting height (the two go hand in hand).
Old 10-19-2020, 08:08 PM
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Put a yard stick on the hull from transom forward and see if the bottom is flat. If it has hook or rocker it may do this:

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Old 10-20-2020, 04:01 AM
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I am curious. What excuse did the previous owner give you why he was unable to sea trial the boat with you?
Old 10-20-2020, 04:37 AM
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My bet is it has a rocker. Probably sat on a trailer not properly supporting the hull.
Old 10-20-2020, 04:50 AM
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I have to give OP the Benefit of the doubt. He has raised the engine up and down, he has changed propellers and issue still there. It might be as simple as a weight transfer issue.

Where are your batteries located? Do you have trim tabs? If not they might help and get as long as you can to act as a longer hull.

you are under propped as well, you should be seeing close to 6000 rpms, Your engine is not seeing full power backslash and torque and it could be affecting the ride negatively simply because there is not enough thrust to balance out the hull.
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Old 10-20-2020, 05:14 AM
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As Tom is pointing out, fixing the actual problem is ideal. Send pictures of motor mounting and where the cavitation plate is relative to the bottom of the boat. Send pictures of your motor and prop. Make sure there isn’t obvious damage to the hull. Make sure there is no water in the bilge. Move starting batteries from the stern to the console. Get weight farther forward. Is the motor a four stroke? I’m wondering id the motor is too heavy for the hull and a 90hp would have been better. Regardless, send pics and this forum can help.

All that said, Bennett or Lenco electric trim tabs will eliminate the bounce. They will also let you level the ride, and get on plane faster and easier. They also allow you to lean the boat away from a cross breeze, versus the boat’s natural tendency to lean into the wind...this will greatly improve ride comfort and reduce spray in a cross wind. Adding electric trim tabs is one of the best things you can do to improve a small vee bottom boat.
Old 10-20-2020, 06:15 AM
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slingblade, porpoising is caused by too much weight in the aft portion of the boat. before you do anything expensive or complicated just move some weight forward. Even having a passenger sit in the bow will usually do the trick. If moving weight makes a difference then move on to a fix. The cause can be serious or simple. I had a skiff that started to porpoise and the cause was a cracked hull that was putting water into the inner hull. When the boat ran the water would go to the stern and that would start the problem. Or it may as simple as the fuel tank and battery put too much weight in the stern. This may not be the cause of your problem but it is easy and cheap to find out if it is.
Old 10-20-2020, 07:11 AM
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Originally Posted by Slingblade81 View Post
I have considered that tail.. but I wasnt sure enough.. but i will get one and try it saturday maybe.
My buddy had the same problem. The foil immediately fixed it.
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Old 10-20-2020, 08:57 AM
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This is common with the 17OS. First things first, check your engine height, there is a good chance that it is way too low, and that will cause porpoising. Secondly, you will want a stern lifting prop for this boat. Semi-cleaver. Powertech SCD3 16 or 17p is common on this boat with a 115 engine.

lastly, the 17OS needs trim tabs. 9/10 17OS's will have a hydrofoil. Which is a band aid to the problem and increases drag. You need a set of electric trim tabs. Don't waste time and money with the self leveling tabs or a hydrofoil.

If you do all of the above and still have porpoising issues, I would be very surprised.

Old 10-20-2020, 09:39 AM
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Standard treatment:
1. Make sure there isn't extra weight in the back like water in the bilge.
2. Redistribute weight (batteries moved FWD etc).
3. Get a "stern lifting" prop (probably a 4 blade).
4. Raise the motor as high as you can without problems.
5. Trim tabs
6. Whale tail.
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