Strange Hydrodynamic handiing..HELP!

[slick560]

Hi All,

I just got my hands on a 1998 Doal 250SC in beautiful shape with only 300 hours on the motor (a Merc 5.7L 260HP Bravo III). It has trim tabs as well as outdrive trim and an aftermarket wing and a dual stainless prop.

It runs like a top but I am experiencing some scary handling qualities. When advancing the throttle the boat will roll fairly violently to one side or the other with just me onboard. I always start with full bow up on the tabs and the outdrive trim all the way down. The trim tabs appear to function because I can force the bow up or down on both sides. However, I have to make constant adjustments to keep the thing upright.

Something is horribly wrong. Any ideas? Perhaps the props are installed incorrectly or the wing is causing some problems. I am a pilot by trade so am familiar with how these thing are supposed to function but am new to boating.

Where do I start guys? I appreciate any and all help. Thanks.

Matt

 

[QC]

Re: Strange Hydrodynamic handiing..HELP!

Hey Matt, welcome aboard!! Sounds like a great boat.

Get rid of the wing, and use the trim tabs bow down, then let us know. A Bravo 3, with trim tabs on a cruiser is perfect. I can think of very few combos that need a stupid hydrofoil less . . .

 

[Blk-n-Blu]

Re: Strange Hydrodynamic handiing..HELP!

IMOP I would start by losing the aftermarket wing and trying just the trim tabs. Check that the tabs are working correctly.

 

[smokeonthewater]

Re: Strange Hydrodynamic handiing..HELP!

yep ditch the wing.... then learn to use the tabs and trim correctly

trim down for holeshot.... you can also use tabs down but as soon as you are on plane those tabs need to be up...... you then trim the engine to best attitude and performance and last use as little tab as possible to level boat and possibly prevent porposing..... Again tho VERY little to no tab in the water at speed....

 

[26aftcab454]

Re: Strange Hydrodynamic handiing..HELP!

+1 to ditch the wing-
sounds to me the out-drive is trimmed too far out-
make very small adj on the trim tabs to level out-

start off trim in and tabs DOWN-

as you get up on plane raise the tabs and drive to your liking.

Sounds like you got a sweet boat- the more you use her and learn to gain control the more comfortable you'll be.

 

[slick560]

Re: Strange Hydrodynamic handiing..HELP!

Thanks a million guys......really appreciate the input....I will be back after this trip to fill you in on what happens....the wing will be history and I will try bow down then trim as necessary. thanks so much....can't wait to get home so I can try out these suggestions. I will advise......

Regards,

Matt

 

[Mischief Managed]

Re: Strange Hydrodynamic handiing..HELP!

I agree about ditching the wing. I also think you should check for slop in the steering. Bravos are notorious for having loose gimbal rings that allow side to side to side play at the outdrive.

Also, check to be sure the "celery stalk" is installed correctly in the oblong trim ram hole to the aft end of the driveshaft housing. It can be installed ahead of the anchor pin or behind it. The typical method is to have the "celery stalk" aft of the anchor pin. If it is installed ahead of the anchor pin, it can cause stability issues when trimmed all the way down.

 

[QC]

Re: Strange Hydrodynamic handiing..HELP!

Bravos are notorious for having loose gimbal rings that allow side to side to side play at the outdrive

Extremely good point . . . No matter what you find, all Bravo owners (Alphas too) need to maintain their Gimbal Ring torque and inspect often.

 

[slick560]

Re: Strange Hydrodynamic handiing..HELP!

Thanks again guys. I will be sure to haul it out of the water and check that stuff while ensuring the trim tabs are functioning properly.

Can someone explain why I should start all the way bow down on the tabs? What sort of travel do those have? I will be finding that out visually when I pull it next week I suppose. I kind of need a step by step guide on exactly how to test this stuff out while underway. The more specific the better. Also how do the outdrive trim and the trim tabs work together. Sorry for all the questions but I am trying to get a good handle on this stuff so I know what to expect. I understand with cruisers that weight and balance can be a big deal as well with the deep V-hull also so stability is somewhat compromised. I have only driven ski boats my whole life so this is very different. Again, it is a 1998 Doral 250SC. Thanks guys.

I will keep you posted.

With Thanks,

Matt

 

[Bondo]

Re: Strange Hydrodynamic handiing..HELP!

Something is horribly wrong. Any ideas?

Ayuh,... Like everybody says, Ditch the whaletail 1st off....
Then go out and Try It...
You shouldn't need to tab it up, or down with the B.III drive...
Just trim in the drive,+ leave the tabs Up for starters...

I think 99% of your Problems are the Whaletail, myself...

 

[QC]

Re: Strange Hydrodynamic handiing..HELP!

I/Os and OBs have the thrust pushing below the boat from behind the transom and below the CG. That force drives the bow up and the stern down and you begin to dig a deep hole in the water as well as have difficulty seeing over the bow when you try to get on plane. Cruisers in particular are heavy and while bow up is good for top speed, it is generally bad for trying to plane the boat.

There are a few solutions, one is negative drive trim which helps offset the propeller forcing the bow up, another is to help the stern stay up so that the bow pushes down and you end up with more surface to plane the boat on. That is the tabs, maybe a better way to visualize the bow down force is call it stern up . . . The other is more power which drives the bow up and over the bow wave quicker and minimizes the whole dern issue.

Trim and Tab combos are endless, and there are different combos for different water, wind, depth etc. However, with a relatively heavy boat you will probably want close to neutral trim on both once you are up and cruising, so that would be tabs even with the bottom surface, and propeller direction straight aft as well. The proof will be once you are up and plaining say at 25 MPH, mess with all of it to see which increases or decreases speed. Higher speed for the same throttle would be the most efficient setting for that particular speed.

Personally, I micro adjust tabs and trim almost constantly. One reason is I like to play with stuff, and the other is I can find small adjustments that can make a difference for the water directly in front of me, or passengers moving, or wind gusts etc.

 

[pinellas50]

Re: Strange Hydrodynamic handiing..HELP!

I'm assuming by wing you mean hydra foil. If that's the case get that off of there.

My boat had one on it when I bought it used. On plane, the boat would start to list very badly to one side or the other at times. It would travel straight but be over on one side as if you were turning the thing for all it was worth. Scared me pretty bad a few times.

I took the hydra foil off and never had another problem with it.

 

[26aftcab454]

Re: Strange Hydrodynamic handiing..HELP!

This is what I do- your boat is not too diff from mine..my boat may be heavier.

I start with trim tabs & out drive all the way down- push throttle over to gain RPMs- When I feel the boat lift out of the water on to plane-I raise the tabs and trim up. my boat pops up quickly. I pull the throttle back never going over 3600/4000rpm. I adj the tabs & trim up until I feel the boat running smooth-rough waves I trim down some-lower tabs. On flat conditions see where you get most speed/rpm with out pushing throttle .

I may be totally wacko but this is what I do. Like anything else-the more experiance the better you are.
Joe

 

[smokeonthewater]

Re: Strange Hydrodynamic handiing..HELP!

yup that works but as Bondo said put the tabs all the way up and leave em alone to start..... I doubt you will have any trouble getting on plane with the drive all the way down.... Learn your boat and how to trim the drive first... deal with the tabs later... fwiw all the way up will be level and neautral.... you can not lift the bow with tabs but rather only lift the stern.....

 

[JustJason]

Re: Strange Hydrodynamic handiing..HELP!

It's a dang shame that somebody drilled through the B3 anti-vent plate to mount that pigeon wing.

 

[QC]

Re: Strange Hydrodynamic handiing..HELP!

That's pretty offensive to pigeons . . .

 

[seabob4]

Re: Strange Hydrodynamic handiing..HELP!

A Sting Ray/Doelfin/whatever on a B3 leg...

Now I've seen everything...

That's what the former owner was doing when he did that install...

 

[JustJason]

Re: Strange Hydrodynamic handiing..HELP!

That's pretty offensive to pigeons . . .

Har har Kev

 

[seldomseen]

Re: Strange Hydrodynamic handiing..HELP!

The trim tabs are best used by boaters new to them only after arriving at cruise speed and only to keep the boat level. Passenger loading and wind from either side may cause a boat to list at speed and then the trim tabs are used to level the boat. Starting off, the Trim tabs should be in the up position so that they are not used. At speed they may not need to be used at all. Just use the tabs at speed to level the boat if needed. The fin removal likely will result in a great improvement.

 

[QC]

Re: Strange Hydrodynamic handiing..HELP!

Well, I want to be on record as saying use your tabs whenever, and for whatever, they are of benefit . . .

Cruisers squat pretty good and Bravo 3 or not, I like using less power and feeling the boat lift onto plane in a more single motion (as opposed to squat, climb over, plane out). With that said I agree that this combo should not struggle much even with neutral trim tab application. But the guy is a flippin' pilot, you'd think he could get the hang of flaps for takeoff, and angle of attack (drive trim) and tab adjustments for level flight . . .