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Porpoise, porpoise !!!


bajablue's Avatar
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09-05-10, 08:19 PM   #1  
Porpoise, porpoise !!!

HELP ! I have a 1983 Baja 184 HLS, with a Mercury 150 hp Black Max with a 20" shaft, a Bob's Machine Shop power jack plate with a 6" set back and 6" lift capability. The boat porpoises whenever I try to trim the motor up after going on plane. I installed the Bob's Machine Shop "True Straight" stabilizer plate in a attempt to stop the porpoiseing. No joy. The boat still porpoises. It leaps on plane, but as soon as I adjust the trim, bouncy, bouncy. I have to keep the motor trimmed ALL the way under to stop the bounce. Bounce occurs regardless of the jack plate position.

If you are familiar with this hull, or this problem, please help.

This boat is unbelieveably clean and in excellent condition. Motor is strong and has been well cared for. I want to fix this problem.

Thanks
Bajablue

 
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Rick Johnston's Avatar
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09-06-10, 04:03 AM   #2  
Welcome to the forums.

Are you getting the recommended RPMs at WOT? If not, you may have the wrong prop. Too much pitch can cause the propoising, and it's also the reason the motor can't get to full RPMs. The motor may have had the correct prop at one time, and a previous owner changed it thinking he'd get more top speed.

Another cause is the motor being mounted too low, but you've already tried raising it with the jack plate, right?

Some people say to add weight in the bow, but to me that is the same thing as trimming all the way down. (Causing the boat to never fully plane and bow plowing instead of coming up out of the water.) Won't hurt to put a body up there temporarily to see if it helps.

Because of the age of the boat, you may have a problem that's very difficult to correct: A hook in the hull. This is caused by trailer supports which eventually cause the hull to deform from bow to stern. Use a string line to check for deviation in the bottom between the transom and the point at which the hull levels off from the bow. Check it at several locations between the chines and keel.

 
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09-06-10, 09:23 PM   #3  
Welcome to the forum.

The porpoise is the nature of the beast in performance boats with a lot of weight hanging off the transom. What is happening is the prop is trying to lift the boat but isn't holding grip and the bow drops, then it grips and picks it back up.

There are some things you can do the help but with some and motor combination's it's more just trying to minimalize it.

As Rick said put a straight edge on the bottom of the hull to see if it's straight. A roller trailer can cause a "dent" called a hook in the bottom that will cause ill handling.

If it's straight and you are looking for all out speed get a water pressure gauge and start to raise the motor until you start to loose water pressure then go down 1/2". This will help handling all around.

Next also as stated try to move weight around in the boat, get a couple sand bags and try putting them up front and then in the back and see what that does.

My 15' viper with a 200 merc does it from 25 mph up to 50, I just don't go under 50 with it and it's all good.

 
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09-07-10, 03:59 AM   #4  
Thanks for your prompt responses. This boat is a 18 ft "bass boat". It is on a trailer with carpeted "bunks?". The trailer is the original trailer purchased with the boat. The transom does not have the "inset" at the bottom like modern bass boats. The transom goes straight down to the keel. With the Bob's jack plate at it's lowest setting, the anti-ventilation plate is 5 inches above the keel. Is this part of the problem ?. The prop is a three bladed, 21 pitch stainless steel item. The previous owner told me that it had been "cupped".

When the water is "right" the motor will reach it's max recommended rpm's (5500), and will even go to 6000, if I raise the jack plate higher. I've only run it that high once and very briefly saw 6 grand (I normally operate with 4500 to 5000 as my max operating rpm).

Water pressure runs 20/25 psi, at these rpms. GPS reads 55/59 mph, at these rpm's. However as soon as I trim up even 2 bumps, she bobs like crazy and gets skittish.

A friend suggested moving weight around, but I feel that is just avoiding the issue. (The boat is very lightly loaded with only five rods and ten tackle boxes, which are stored in the front compartments). I am the only person on board and I weigh 220 lbs. I keep both 12 gallon gas tanks at 3/4 full because I hate having gas come out of the vents and run down the side of the boat and trailer. This boat weighs approximately 1200 lbs, according the the meager info I have found on the WEB.

This boat has been garaged kept for 25 years, and has always been stored and transported on this trailer. I will check for hooks, or have it done by a pro.

What would you suggest I do with the prop, more or less pitch ? Four blades ?

Thanks again for your replies, and suggestions.

Bajablue

 
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09-07-10, 05:29 PM   #5  
It sounds like you are getting great speed out of it and might just be over trimming. With your setup you probably don't need a lot of trim once at speed. If you have a rooster tail taller than your motor you are over trimming and wasting forward thrust to push water up.

What speed does it start to porpoise at and when does it stop.

On my 20' boat I run 12" setback but because of the weight of the hull I can run full trim at speed. On my light 15' with 2.5" of setback I run negative trim once it's on plane and more negative the faster I go.

 
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09-20-10, 07:24 AM   #6  
Posted By: samuari It sounds like you are getting great speed out of it and might just be over trimming. With your setup you probably don't need a lot of trim once at speed. If you have a rooster tail taller than your motor you are over trimming and wasting forward thrust to push water up.

What speed does it start to porpoise at and when does it stop.

On my 20' boat I run 12" setback but because of the weight of the hull I can run full trim at speed. On my light 15' with 2.5" of setback I run negative trim once it's on plane and more negative the faster I go.
Thanks for your response. Anything less than full bottom trim, (motor tucked all the way under) produces porpoising. Speed is good, but I believe this motor position is probably working the motor and lower unit very hard. When the river is rough, or I cross wakes, it gets brutal !!

The previous owner tells me that he did not have this issue. I have strung the hull, and it is true. The only thing I have changed is the jac plate. Could the height of the motor, and increased setback cause this problem ? If i remove theis jackplate, should I just put a non-power plate on, or should I invest in a different model which is lower, and has shorter setback. To refresh your memory, I am running a Bob's Machine Shop, 6" setback Bass Jac. This jackplate leaves my cavitation plate 5" above the bottom of the transom. The transom on the Baja 184 HLS is a 90 degree, it is straight from the top of the transom to keel. it is approx 23" long.

Thanks,
Bajablue

 
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09-20-10, 12:38 PM   #7  
If the previous owner didn't have a problem, and the only thing that changed was your addition of the jack plate, why not put it back the way it was when you bought it and see if it corrects the problem?

Out of curiousity, why did you add the jack plate?

 
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09-20-10, 07:34 PM   #8  
When you put the jack plate on it moved the center of balance back further causing the porpoise. A thinner manual plate might take care of it. Bob's makes a convertible plate that can be used at 2.5 or 5".

I'm guessing you are looking for more speed. If you didn't notice that much difference in speed you could just bolt it back on the transom at it's highest setting.

 
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09-28-10, 04:57 AM   #9  
My first boat was a 1999 Procraft DC 200, with a Suzuki DT 225, and a CMC jackplate. The CMC died, and I replaced it with the Bob's Machine Shop "bass jac". When the Suzy blew up I purchased the Baja, and because the BMS bass jac was so nice, and quick, compared to the Interstate jack platewhich was on the Baja at purchase, I had a marine shop r/r the jack plates and mount the BMS unit on the Baja. I no longer have the Interstate jackplate, so I'll probably purchase a manual jackplate with a 2" setback, and lower positioning of the motor. Initially I will mount the merc 150 directly to the transom, and see if that cures the porpoising.

If that is successful, I will then try to find a manual plate that keeps the motor low on the transom. I suspect the 6" setback and 5" higher position of the motor is the cause of the problem. Because the Baja came with the jackplate, I am worried wether mounting the motor directly to the transom might cause some other problem; i.e. stress on the transom?

I apologize for the delayed response, and thanks for your comments.

has07

 
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09-28-10, 06:54 PM   #10  
Bolting the motor on the transom will be less stress on it. I would bet you will gain a little low end but lose a little top end.

You could also look at changing props or having yours worked by a good prop shop, adding some cup might be enough to keep it lifted up.

BMS sells a convertible manual JP with 2.5" of setback and 5" of lift with 6* negative tuck to help with take off.
I have one on my viper and it has held up fine with a 200 v-6 merc and a lot of nitrous.

 
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