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Discussion Starter · #1 ·
Went out today to try the new prop, (baja 190 sport 205 hp, 4.3lx alpha, 21p SS avenger prop) previous prop was a 4 blade comprop 22p. Holeshots and top speed much better than previous prop, but now the thing porpoises like craze at cruising speed.. flat out no problem.. I can get it to settle down a bit if i trim the drive all the way in, same if i put my trim tabs down, but then it feels like the boat is "digging", loading the motor and the bow dosnt ride as "high" as it used to without a problem. still 10 times more fun then the old POS prop.. jumps on plane now with no hydrofoil installed..
What is the propper way to correct this to trim the drive in, or to set the drive where it "should" be and try and correct with the tabs? or????? Thanks
 

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sometimes weight distribution can help, tabs can correct, prop choice can get rid of sometimes...

in my experience with a similar boat, high rake props like a laser 2 would give the most porpoising in the midrange and fallten out on the top end - they would also air the hull out more and gave a better top end. trimming the drive in will get rid of it at a given point, but at the cost of driving the bow into the water and slowing you down. all fo your weight is in the *** end and the boat is riding very close to the center of balance so the bow bounces up and down easily...

trim tabs - even smart tabs (have shocks you set instead of helm adjustable) make a huge difference by pickoing the stern up a little essentially extended the running surface of the hull.

In your situation, two things worked for me. 1. trim tabs, 2 going with a prop that had less rake and bow lift. going to a prop that got rid of most of it and then using the tabs to get rid of the porposing that I had over a narrow range was the best solution overall giving the best top end and overall performance.

Just my experience...
 

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There are two prop approaches that you might consider since a prop swap has already proven to be a factor on your boat. A bow lifting prop like the Mercury Tempest may hold the bow up if thats what the hull wants. I stern lifting prop like a Bravo or Rev4 might help if thats what the hull wants. The problem is when the prop is doing the opposite of what the hull wants to do. That, combined with other factors like weight distribution, can cause a boat to porpoise. Sometimes its just a bad hull design but since you have already found that propping can effect it, I would try some more.
 

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Discussion Starter · #4 ·
Thanks for the replies.. yes i cant believe the difference a prop made on this boat. at least im heading in the right direction.. :)

1. i already have a set of Bennett trim tabs installed.. is it better to correct it with the tabs, or by trimming the drive.. I thought that trimming the drive all the way down would be harder on parts.. so want some advice on the better of the two.

2. The prop I have is a Mercury Avenger 21P, any way to tell if it is a Bow or Stern lifting prop? i bought it used and thats what the part # showed up as..

now i did have some weight in the boat ( wife and 4 small kids) and was much better when i went for a spin solo, not as bad but still there but need to find something for when they are in the boat since i dont get out alone often. Thanks!
 

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Porpoising is caused by the boat not knowing where it's cog should be. Unless it's just a piss poor design, it because of the prop. They will carry the hull in different positions.
 

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Discussion Starter · #7 ·
ok, so what do i need to look for in props to chose the correct one, When i look online or in a catalog, all it shows is the dia. and pitch.. Is it the rake that effects the bow or stern lift? Cant afford to keep buying props and I know of no place around that lets you try before you buy, so just trying to get pointed in the right direction.. at least i know now what causes this in the first place.. Thanks!
 

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Before you start buying different props try making adjustments with the tabs and drive.

As wtfo stated high rake props give the best bow lift and are many times faster but can porpoise at cruise. You can get rid of it with a combination of drive and tab adjustments.

Every boat is different so see what seems to work the best on yours. But in my experience it is a combination of both the drives and tabs. In other words, don't leave the tabs up and put the drive all the way down and don't keep the drive trimmed up and put the tabs all the way down. Use a combination, the drive up a little and the tabs down a little. Hopefully that makes sense it's tough to type!
 

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Discussion Starter · #13 ·
Fyhr Factor said:
Weight distribution... it will help, try putting the kids up front, bow rider, right???
Wish i could but its a closed bow..

blackhawk said:

Every boat is different so see what seems to work the best on yours. But in my experience it is a combination of both the drives and tabs. In other words, don't leave the tabs up and put the drive all the way down and don't keep the drive trimmed up and put the tabs all the way down. Use a combination, the drive up a little and the tabs down a little. Hopefully that makes sense it's tough to type!
Think I will try playing with this the next time out before i go ordering another prop..

wild mike said:
call Julie, she is an advertiser here


http://www.throttle-up.com/
If I cant get it to settle down with the tabs/drive I will be calling her.. Thanks for all the replies..
 

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I had the same boat/engine back in '89. I don't recall having any problems with porpoising except around 45 mph as the boat was trying to get up on the strakes. I only used the tabs for planning, rough water or to correct listing. I tried MANY props, the best was a Mercury 23 Lazer II.

Do you have the 18 gal fuel tank that's mounted in the engine compartment on the port side? After the 1st year I yanked it out and made a 28 gal fiberglass tank that went under the back seat and down between the stringers. Made a difference in handling. The 2000's and up were renamed 204's and had under floor tanks.

At cruising you should have your drive about 1/8-1/4 trim, drop tab(s) JUST ENOUGH to correct list and provide a comfortable ride with no propoising. I used to cruise at 3200-3300 RPM which was around 33 MPH. At about 45 the boat would propoise as it tried to get up on the strakes, once past this it would sail clean with the bow high. If your cruising at or near that speed there's nothing you can do but slow down or cruise WOT...
 

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Discussion Starter · #15 ·
275Boss said:
At cruising you should have your drive about 1/8-1/4 trim, drop tab(s) JUST ENOUGH to correct list and provide a comfortable ride with no propoising. I used to cruise at 3200-3300 RPM which was around 33 MPH. At about 45 the boat would propoise as it tried to get up on the strakes, once past this it would sail clean with the bow high. If your cruising at or near that speed there's nothing you can do but slow down or cruise WOT...
BINGO! just mine starts at about 40, possibly because of the 21p prop compared to the 23?? its just such a narrow area that it does this and it seems to be about the speed where i want to cruise it.. mid 30's just seems to be, well abit slower than i want to go ;) .. and dont like to run it all out all the time either.. I may try and get ahold of a 23p prop,.. if it can delay the porposing a few more MPH it may be perfect..

And yes it does have the 18gal tank that sits to the side.. thought about putting in a larger tank awhile ago but then again with the 21* hull, there is no trip that far I would want to take this boat on and beat myself to death..
 

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andysperformance said:
BINGO! just mine starts at about 40, possibly because of the 21p prop compared to the 23?? its just such a narrow area that it does this and it seems to be about the speed where i want to cruise it.. mid 30's just seems to be, well abit slower than i want to go ;) .. and dont like to run it all out all the time either.. I may try and get ahold of a 23p prop,.. if it can delay the porposing a few more MPH it may be perfect..

And yes it does have the 18gal tank that sits to the side.. thought about putting in a larger tank awhile ago but then again with the 21* hull, there is no trip that far I would want to take this boat on and beat myself to death..
What are yor RPM's with the 21" prop? With a 23" Laser I was at 4900-5000 lightly loaded, speed was about 53-54 GPS. I suspect you may be overturning the 21.

I added the larger tank as we used to run a river to the ocean almost every weekend, 35 miles one way. With the weight of fuel towards the center of the boat and the trim/tabs set properly, you'd be surprised how well this boat handled rough water. It been almost 20 years but I can clearly remember the fun I had with that boat.
 
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