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  Merc 90 Montauk combo squirrelly coming off plane.

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Author Topic:   Merc 90 Montauk combo squirrelly coming off plane.
Landlocked posted 09-15-2003 01:48 PM ET (US)   Profile for Landlocked   Send Email to Landlocked  
Excuse the use of the word squirrelly but not sure how else to describe it. I just returned from vacation where I finally got a chance to put some significant hours on the boat and 2002 Merc 90 2-stroke I installed last year. I had noticed the problem I'm about to describe before but it seemed more pronounced in the light chop that was common in Perdido bay last week. It could also be the heavy load I was typically hauling that made the problem more noticable.

When coming down off of plane, the motor seems to catch making the boat really unstable for a few seconds. I can avoid this by first reducing throttle just a little, then trimming in and then reducing more until the boat gently settles down. If I have to come off plane quickly (as I did when a jet ski cut in front of me wednesday) its a little scary. If you don't have a very tight grip on the wheel, it could easily be torn out of your hand and the boat would make a sharp turn potentially flipping or throwing out passengers. There is no pull one direction or the other while on plane, its only when coming off.

I'm wondering if the motor is mounted too high. The cavitation plate is 1.25 or 1.5 inches above the bottom of the boat (can't remember, will check) The situation really feels dangerous. I can handle it but I'm worried that if I let my wife or someone else operate they wouldn't know how to deal with it when it happens.

Any thoughts?


Bigshot posted 09-15-2003 01:52 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Do you have a lot of water in the hull? I did in an old 13' and she would do weird things coming off plane due to a sudden shift in water weight. Do you have a swim platform?
Landlocked posted 09-15-2003 02:48 PM ET (US)     Profile for Landlocked  Send Email to Landlocked     
I really don't think there is water in the hull, I haven't weighed it though. I keep the boat on a trailer and covered. The previous owner kept it in a covered slip on a lift. I guess what makes me doubt its water is that I didn't have the problem with the old Johnson 85 I replaced. Since the old motor was mounted all the way down on the transom, I'm wondering if could be the motor height or height prop combination (3 blade 21pitch black max)

I'm getting about 42.5 mph with a light load at 5,250rpm wot with my current setup.


Landlocked posted 09-15-2003 02:50 PM ET (US)     Profile for Landlocked  Send Email to Landlocked     
Oh yea, no swim platform. I do have a "skimmer" transponder on the Port side and a speedometer bubbler on the starboard side.
Knot at Work posted 09-15-2003 02:56 PM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     

it sounds like your motor is mounted to high or to off center, left or right.

What year is your hull?

Did you buy your boat from killinger? I recommend you call Todd at Service and ask him.


Bigshot posted 09-15-2003 03:30 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Speed sounds right.....just narrowing it down. It may be as simple as the trim tab above the prop being out of adjustment. Height should not affect it being I am a lot higher than you and no problems coming off plane.
jameso posted 09-15-2003 03:57 PM ET (US)     Profile for jameso  Send Email to jameso     
I agree with the Bigone on this.
What direction does the boat try to turn? I mean is it engine torque or a hull or trim problem? Also does it do this with just you onboard? You mentioned small chop, is this a following sea or are you into the wind? I have chopped power in a following sea and it can get a little 'squirley' but have never felt the wheel would be torn out of my hand or the boat flip.
Landlocked posted 09-15-2003 05:39 PM ET (US)     Profile for Landlocked  Send Email to Landlocked     
Sorry for the delayed response - server was down.

The chop was wind generated - I was in the bay. I did experience some following seas while on the outside but I never was going fast enough to see if there was a problem or not.

When coming down, the boat first tries to turn right hard. When I correct to the left no matter how lightly it feels as if it oversteers and wants to go to hard in that direction. Once the boat settles down about halfway off plane the problem goes away.

I bought the boat used here in Nashville about 5 years ago and then repowered last year. There was never any problem until I repowered. Not positive about the year of the hull. The serial number indicates it is a 1977. The title indicates it is a 78 and the original owner advertised it as a 78. Also, the original motor was a 78. I assume it was manufactured in late 77 and not rigged and sold until 78. Serial numbers and title match so haven't worried about it.

If the trim tab was out of adjustment, wouldn't it pull hard one way or the other when up on plane not just when coming down?


Moe posted 09-15-2003 06:23 PM ET (US)     Profile for Moe  Send Email to Moe     
When the AV plate is considerably higher than the transom, there isn't very much of the trim tab in the water, especially when the motor is trimmed out. So you might not notice an improperly set trim tab until the boat came off plane. However, that wouldn't give you the sudden reversal you're experiencing.

It sounds like you have a case of bow steer. That's usually on plane from the motor being trimmed in too far for the load. It sounds like your boat may be too heavy in the bow for the weight at the stern (is the new motor significantly lighter than the old?), and that you're holding a heavy bow up with out trim. When the throttle is chopped in this case, the bow would nose-dive without the thrust to hold it up, and the stern lift, especially in a following sea, creating the condition you describe.


Landlocked posted 09-15-2003 10:34 PM ET (US)     Profile for Landlocked  Send Email to Landlocked     
Interesting theory Moe.

Not too sure how my bow could be heavy, nothing up there compared to the rear 1/3 of the boat which holds a Cooler and ice, tanks, and 2-people. The front cooler typically only holds a tool box and lifejackets. The bow locker holds only a small anchor and 100' of rope.

The thought that the trim tab is out of the water at plane may be very valid though. I have noticed that my new depth finder does not work at high speed and the transducer is at about the same level as the trim tab on the motor - maby just a little higher. I had been considering moving it down towards the keel. Its located just inside the "step" on the hull now.

I'll look tomorrow and see where exactly it is in relation to the tab, may need to monkey with the tab setting a little.



Moe posted 09-15-2003 10:41 PM ET (US)     Profile for Moe  Send Email to Moe     
I was just basing it on your first post about the heavy load making it more noticable. That's why I was wondering if the motor was lighter and/or the bow heavier. The symptoms sure describe bow steering.


lhg posted 09-15-2003 11:47 PM ET (US)     Profile for lhg    
This problem sounds like a propeller torque situation to me. Are you running any kind of a high raked performance series prop? These tend to add to this problem, but it can happen with any prop on a high powered, light hull, which you have. If you're traveling at a fast speed, trimmed out, and pull the power fast, especially with an elevated engine, the transom can jump sideways quite alarmingly, as the prop torque is released from the boat and steering wheel. I had this problem with my 1973 Merc 150 in-line 6 on my Nauset.

On my 18 Outrage, with twin Mercury Laser II performance props, pulling the power fast REALLY scoots the transom sideways. So I always ease up the power slowly when going over 40-45. I have no such problem on my 25, since the props are counter rotating.

If this is your problem, ease up the power until the boat has slowed down to 35 or so. Then there should be no problem. Trimming in should not be necessary.

Landlocked posted 09-16-2003 10:32 AM ET (US)     Profile for Landlocked  Send Email to Landlocked     
Thanks for all the advice. I definately have some things to look into when I take the boat out on the river this weekend to run the saltwater out.

Moe, in my original post I was just trying to present some observations that might have been associated. I was in a lot more chop than I typically see here on the river and the boat was definately loaded heavier than normal, 2 adults, 2 small kids, 24 gallons of fuel, And a 48 quart cooler full of ice in the back. Not to mention a ton of drinks and kid accessories. Usually its just me and a sixpack out in the boat so there was a big difference. All of the extra weight was positioned in the rear though between the console and the stern.

I did look at the level of my transducer (which is obviously coming out of the water at high speed) and the trim tab this morning. The transducer is mounted about 5 inches inside the outer step in the hull. With the motor tilted about the same angle as it would be on plane, it appears that the trim tab is at the same level as the transducer so your theory may be valid.

LHG may have a good idea as well. The new motor definately has a lot more torque than the old 85. As I stated earlier, I've always noticed a little unstable feeling when coming down off plane but it was just worse with a full load. So, it could be prop torque. Question, If torque is the problem would it worsen or improve if I drop 2" in pitch size-say down to a 19"? I had considered it in the past to try and get WOT up closer to max 5,500 from its current 5,250 but figured it wasn't worth it since I rarely run wide open anyway. I also thought I might be getting some fuel efficiency advantage by running in "high gear" with the larger prop.

One other observation, with the full load last week, I was only able to get 5,000rpm and 39mph trimmed out at WOT. Anymore trim would break the prop free.


Bigshot posted 09-16-2003 10:37 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Prop size sounds good....keep it.

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