Front of the boat bounces up and down

[dolens]

I was wondering if this happens to anyone else or if this is just a normal characteristic of the Classic 18. While moving along at about 40 or so, when you hit wakes just right, the front of the boat starts to bounce up and down pretty good. It doesn't ever really calm down unless I back off the throttle. I even went with it a few times and throttled back and forth just to make the boat pop out of the water even more.

Also Is a top speed of about 65 with 2 people and a full tank of gas about right for a 300HP Merc? Thats with the trim about half way up, and trim tabs in the up position

Thanks

[Last Tango]

Top speed sounds about right for your boat, but we need a lot more info to tell you. Did you measure with a GPS or speedo? What kind of Speedo? What drive and ratio? Which prop? That info would be helpful to the techies who will try to help you resolve this here.

Trim halfway up at 40MPH sounds like it is way too high. I rarely went above 1/4 up on the drive trim on my 18 Classic with MX6.2MPI, Bravo One w/1.5 ratio, and Mirage 23P prop.

Over trimmed, or the wrong prop could explain the porpoising at lower speeds.

Others will also suggest checking to see if you have any hook at the stern of the bottom. I'll let them amplify on hook.

[BlownCrewCab]

Well since you have trim tabs, controling the porposing is easy, when ever cruising at Part throttle just add some tab. The porposing is just the nature of the boat, it could be a high X dim, Or a Low X dim, but since you have tabs just consider it part of the boating experience. when you want to crack it open just raise the tabs a bit. They will also help getting on plane & and staying on plane at slower speeds.

[dolens]

Thanks. I will get that info and add it to the post. It was measured on a Gaffrig Speedo. I'm not sure of the prop or the ratio(what is the ratio?). The drive is an Alpha One.

The trim tabs I am still getting used to. They were a little difficult to get used to at first. It seems that I only have to use them when the are more than 2 people in the boat, or if the 2 people in the front are much different weights. I had 5 people in the boat a few days ago, and what a bear to try and turn. It seemed like when I tried to turn, the boat just wanted to stand on its side, and not actually turn. Maybe this is also a trim tab issue.

[DickB]

You want to trim the drive down for turns. At slower to moderate speeds, the boat will suddenly roll over on its side – quite startling for first-time drivers and passengers. It's the nature of the hull.

Look on the drive for label to indicate gear ratio and the side of the prop for specs and/or part number. A Mercury Mirage Plus 23P is standard equipment. I have the identical boat, engine, and drive. 65+ on the speedo and 62-64 actual on GPS is typical for me. Best ever on the standard prop was 64.9, and a buddy’s labbed prop added about a mile and a half. (A “labbed” prop is tuned and altered by a lab, usually by balancing, thinning the blades, and making each blade true to the others.)

A little bit of tabs down, especially in wakes, will all but cure the porpoising. Tabs up full for full-speed runs as was mentioned. Your trim at full speed sounds about right to me. What RPMs are you making at WOT?

A GPS is useful to find ideal drive trim and tab settings at various speeds. With one, you can see the results of trim and tab settings in minor speed changes that are missed on the speedo.

To have fun with the boat, trim the drive down and tabs down a bit to plant the bow, then take some high-speed turns, working your way up in speed and sharpness of turn as far as you dare. I gave coworkers rides last summer at a department picnic. Our company also often has a company picnic at a local amusement park. One comment after a ride was “No need to go to the amusement park – just take a ride in the Donzi!”.

[Lenny]

To have fun with the boat, trim the drive down and tabs down a bit to plant the bow, then take some high-speed turns, working your way up in speed and sharpness of turn as far as you dare.

I do not like the "Tabs Down" thingy going into turns ... maybe it is just me, but a quick "skip" and 180 instantaniously kinda fills my pants up. :)

[dolens]

http://www.donzi.net/forums/showthread.php?t=56145

DICKB, I can see what looks like the same number on my prop that you have in your picture. Its hard to make out, but I think their the same.

Its very startling for first timers, like myself. I feel like if I keep going the boat will just roll. I am starting to get the feel for it, but it definitely takes time. I cant believe how agile this boat is. I jumped an oncoming boat wake, that wasn't even that big, and slammed the throttle down while I did it. It felt like I must have gone about 2-3 feet off the water, and come down slightly tilted. Definitely an OH CRAP moment! while I caught my breath for a few minutes. My neighbor has a Chris Craft Lancer 20, and its a pretty quick boat, but when I take him for a ride in mine, his face lights up and all he says is DAMN this things fast. He calls his a Bentley and mine a Ferrari.

As far as the Tabs go, for now Im with Lenny. I am slowly starting to mess with them, but they are pretty damn touchy. Ive noticed you have to keep them pressed for 1/4 seconds at a time vs holding them down.

Ill have to check my RPM.s next time I go out, but I think around 4-5k??

[BigGrizzly]

Dolans, Everything you are describing is soooo common to new Donzi owners. I don't use tabs unless absolutely necessary, I trim if you do not have trim then tabs. Get seat time, experiment. Now if you can find an experienced Donzi owner to rid with you this will help. Other boat jockeys do not have a clue they just guess and talk. Forget any bass boat guy they are useless in this area. Now Fill the tank, go out and drive it until you need gas then come back to the board. This is a mandatory exercise or you will curl up and wilt.:wink:

[Just Say N20]

BigGrizz,

Not doubting, just curious as to what about bass boat driving experience doesn't translate to the Donzi experience?

I realize there is no pad on a Donzi (except for the one Ed D made), so they won't pop out of the water and ride as freely, but wouldn't the concept of trimming, and controlling chine walk be the same?

[txtaz]

Yes the 18c is more nimble than those red 22c's out there and takes seat time to get used to. I always left my tabs neutral and powered through turns and waves. Set the tabs to level the boat and leave them unless the weight shifts or just slow cruising. Then drop them some. Also while idling around, dropping them all the way will reducing the wounding bow syndrome.

Start with your drive in and pick it up as you pick up speed. As you hit waves a little turn through the wave will smooth out the bounce and keep the bow level.

Have fun and get lots of seat time before pushing the boat.

[mattyboy]

they all do it but from what i have seen the prop can help alot in this area,and can change the speed range and duration of when they do it.
tabs help but you have to learn how the boat rides with them But no tabs in turns once you've buried one in a turn and the boat is riding on it's rub rail and you are going straight full pants as Lenny says
the classic roll in turns and a downward deflected tab can act as a rudder( with countersteer) once the boat rolls onto it's side.

as everyone said seat time the right prop and you'll find the tabs being used less and less and the porpoise is not as bad and can be controled

[Classic Doddy]

I just had my 2004 18' out the last few weeks and Biggrizz is right on. First time Donzi owner and there are so many little things to learn. Regarding the bouncing, I keep the same speed and played with the trim (keep the tabs up) to make it settle.

I still can't believe how sweet and smooth the ride is. I have the 6.2 with a 25p Mirage-P (1.65 ratio) and with the wifey and full tank, I hit 66 - 67mph (~5100 rpm). I did forget to snap in the back seat... found it and no problem.

Dodman

[DickB]

The Bennett web site has a good tutorial on trim tab use:
http://www.bennetttrimtabs.com/guidetotrimtabs5.php

As you have discovered, the short burst method of adjusting tabs is correct.

I may be in the minority here, but unlike a number who generally use little or no tab and trim primarily with the outdrive, I typically use tabs and outdrive trim in tandem as Bennett describes.

[dolens]

Thats a nice article on the trim tabs! Thanks

Thanks for all the advice! I will definitely get the seat time, that wont be an issue. I'm sure Ill get the hang of the Donzi rather quickly.

Its nice to know newcomers have a place like this to come and find out all good stuff before they get into too much trouble.. I just need to go to a lake or reservoir where I can experiment a little more. I live on a lake where people have nothing better to do than turn you in for what they think is reckless driving (anything over 35mph)...

Ill report back in the next few weeks...

[osur866]

I too have an c-18 and very rarely use tabs unless ideling or when trying to hold the boat on plane under 35 mph but once you get past 35-40 mph start to trim up and pull the tabs up and you wont go bouncing down the lake. As stated above burn a couple tanks of fuel through it and experiment with it and you will find yourself using the tabs less and less. Much can be controlled by throttle, and as far as the laying the boat over on its rub rail in a sharp turn well just one perk of owning a classic Donzi enjoy be safe and have fun. Steve

[chrisc2]

I am for no tabs/trim first,add tabs later. You really won't need tabs until you are comfortable with what you have for range and control with trim. Down the road you will use tabs and know when and how to do so. It won't take long, just try not to confuse things in the short term by messing with the tabs. This will cut down on those 'Sphincter' :doh: moments in the turns too.

[Donzi Vol]

What are trim tabs? What's power trim? :wink:

[Ghost]

What are trim tabs? What's power trim? :wink:

Well, first the Earth's crust cooled and dinosaurs roamed the planet...

[joseph m. hahnl]

Thats a nice article on the trim tabs! Thanks

Thanks for all the advice! I will definitely get the seat time, that wont be an issue. I'm sure Ill get the hang of the Donzi rather quickly.

Its nice to know newcomers have a place like this to come and find out all good stuff before they get into too much trouble.. I just need to go to a lake or reservoir where I can experiment a little more. I live on a lake where people have nothing better to do than turn you in for what they think is reckless driving (anything over 35mph)...

Ill report back in the next few weeks...

Oh Yah:yes:!!! The jealousy is pretty common:shocking:. Slow boaters don't get that classics plane out at there top speed:eek: . 45mph is pretty mild and average cruising speed for me. I want to point out that porpoising can also be caused by surface tension of the water.Sometimes coming into a shallow from the deep water will bounce the boat. Trim tabs are great for driving the bow through the water or driving through the waves not over them. my girl doesn't like it rough:frown::shocking:. So my tabs are mostly down in cruise and all ways up in a tight turn.

At lower speeds tuck in the drive.

Or Put the tabs down a little and trim up the drive and you will feel the RPM increase. 3500 Rpm for fuel economy:yes: