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jasonkennedy
05-06-2011, 08:56 PM
hello ,
My question is about a neighbors boat that he acquired last year . Its not a donzi but i was thinking there are some pretty experienced folks in here that may have the answer. (thanks for your input in advance)

The boat : 1990 21.5 avanti (not the bay liner kind) made in Nj i believe.

Supposedly a 24 degree dead rise deep hull has a 502 with a bravo out drive .

He acquired this boat with a blown motor(454) and a bravo drive on the back.

The Problem(s) if there are 4 people or more on the boat it will not even plane even with tabs, if he tests the boat with only 2 people he can plane it but it will (as he described it ) fall over on its side at 50 mph .and he has to off set the trim tabs to keep the boat even . if he comes off the throttle real fast the boat will dart to the one side real hard ...

Possible solutions ? he had a nose cone he installed on the drive and i thought that that might have something to do with the darting to the left (i thought maybe it wasn't installed symmetrically)?

He thinks the drive is too high and wants to lower it 2 inches ?

A Friend of mine that owns a local marina says raising or lowering the drive would not cause this and it is more that likely caused by hull flexing ?

and ideas ?

gcarter
05-06-2011, 09:19 PM
What drive ratio and what prop?

jasonkennedy
05-06-2011, 09:42 PM
1.5 ratio and 14/25 prop

MOP
05-07-2011, 07:19 AM
The veering when you chop the throttle is normal in any single screw application, what is happening is the sudden reverse of the propeller torque. Under power the boat leans one way when you chop the throttle it will lean the other way, it is far more magnified on deep V's. Now a wild guess he has a cleaver or a cleaver style prop, I tested two different cleavers on my 22 and found them unstable. I could make the boat lurch to one side at high speed just running the trim in and out, switching to a round ear prop eliminated that. George had the same issue with his Minx, he gave his misses and friends a near heart attack one day. He like me dumped the cleaver! Many others have had success running them, note the switch from a same pitch cleaver to a round ear gave me 2mph. I am willing to bet drive height is not you guys issue, I have done quite a bit of fooling around with height on O/B's. A deep set lower contributes to chine walk, raising it helps to eliminate it, don't lower the drive! Borrow some props for testing, his current pitch sounds about right I think he just needs a different configuration prop. It takes some time to get a boat setup up with the right prop, and no two boats are alike.

jasonkennedy
05-07-2011, 10:48 AM
"The Problem(s) if there are 4 people or more on the boat it will not even plane even with tabs, if he tests the boat with only 2 people he can plane it but it will (as he described it ) fall over on its side at 50 mph .and he has to off set the trim tabs to keep the boat even . if he comes off the throttle real fast the boat will dart to the one side real hard ..."

Thanks for your reply , I personally have never seen a boat with this much power and only 21 LOA not be able to plane because it has 4 people in it , and also "fall over on it side @ 50" or any speed if there is a weight shift or somebody moves around in the boat ?

are you really thinking prop only is causing this ? I guess i should have added , to get the boat to run straight he has to put one tab slightly down . this does not seem normal to me at all .

gcarter
05-07-2011, 10:55 AM
I agree w/Phil, your friend needs to try a bunch of different props.

Offset
05-07-2011, 01:39 PM
Ok I will be a smart a$$ and just say take out the back seat!! I had a "performance" outboard at one time that had very similar problems eventually found that the hull had a twist in it and the more power you gave it the more it would twist (not overpowered). It was frightening to drive, there was no consistency with the handling good or bad. I just moved it on and counted my losses.

Some boats are just not very good.

MOP
05-07-2011, 05:44 PM
Ok I will be a smart a$$ and just say take out the back seat!! I had a "performance" outboard at one time that had very similar problems eventually found that the hull had a twist in it and the more power you gave it the more it would twist (not overpowered). It was frightening to drive, there was no consistency with the handling good or bad. I just moved it on and counted my losses.

Some boats are just not very good.

Yup me too! A boat the has been improperly blocked can get out of shape, he should take some basic measurements and do some eye balling to see if it looks Ok.

Phil

mrfixxall
05-07-2011, 06:04 PM
Put a 21 pitch prop on it and all the problems will go away..25p prop and the torque of the 502 will twist the boat all out of shape..the prop has too much bite for the boat,,it needs a little slip to correct the boat lein.

jasonkennedy
05-07-2011, 06:27 PM
thanks for all the input guys , seems to make sense

mrfixxall
05-07-2011, 06:33 PM
thanks for all the input guys , seems to make sense

what is your friends maximum rpm's?

thescooter
05-18-2011, 08:21 AM
i would go with the simplest cheapest solution first. (KISS) method
wrong prop like has been stated. od
try a larger diameter and less pitch prop, most of the time this gives you low end power and better handling but less top end speed.
sounds like the boat is setup with a speed prop.
with 4 people in the boat with the boat not performing correctly, you might need a power prop.
the props diameter vrs pitch is something like a gear ratio for a simple way to look at the problem.

thescooter
05-18-2011, 04:30 PM
does the outdrive seem to have any play or loose?
if there is this could be one of the problems.