How to stop or minimize porpoising on a 86 242LS
 

The boat is a 86 242 LS with a 260 hp SBC/ Alpha and bennett trim tabs. If I lift the tabs all the way up and try to give the boat positive trim it porpoises and is uncomfortable to me and the passengers. I dont know what brand of prop is on it (no markings on it), but it is a round ear 3 blade stainless 21" pitch. If it run with the tabs down it helps a little but scrubs about 3 MPH. If I run with negative trim that has the biggest help but that scrubs speed too and I can feel more vibration in the whole hull. I have checked the hull for rocker on both port and starboard and found none. Someone please help me out and give me a suggestion on what may help.

not sure with the 350 combo, but my 88 had 454/bravo and it would porpose w/o tabs under 3400 rpm, truthfully boat wasn't happy unless running 3600-400 rpm's

Not sure on the porpoising but a 21 is a lot of pitch for a 260 horse 350 in a 242. What rpms are you at at wide open throttle? Shoot for around 4500 or so.

If you can get your hands on a mirage 17 or 19 or laser II 19 it would be worth a try and may help with porpoising.

Usually at slower speeds you can trim the drive down until the porpoising stops.

now that you mention it I ran a 21 mir plus on my 242 454/B1

I think it is a 21. The boat is in the water to not so easy to look at now. The engine turns about 4900 at full throttle. My other boat (F-18 with a 350 EFI Mag/ alpha) has a 21 Vengance prop on it and that turns about 5000 rpm.

bring the tabs down a bit. No reason for them to be all the way up. Althought it's not much, they give you more surface area in the water if they are parallel with the hull. Also, if your porpoising at crusing speed, give it just a little more throttle. That might help as well.

+1

My 255 does it with neutral trim and tabs. Just enough tab to stop the bounce is all you need. Is this a new condition or has it always been this way?

I agree that a 21 is too much pitch for that combo with much load on it. That boat came from the factory with a 19" pitch and that is pretty much spot on. I had a '86 242 SS which is the same hull with a larger cockpit. Laugh if you will but if all else fails try putting 2: 40 lb bags of sand in the nose and see if that helps. My 255 has a 20 gallon water tank in the nose and when it's half full or more the boat never porpoises.

My 242 ls when it had the alpha, and sbc would do something similar. With eveything down it would ride great and I would get the vibration in the hull. I would lift the drive a little to solve this problem. I could run with the tabs all the up and not have any porpoising. With that combo I ran either a 19p mirage plus or 21p mirage plus. With the 19p cruising was around 28 mph @ 3000 and top speed was 52 mph at 4800 rpm, It would come on plane fast and have great midrange. Now, with the 21p It would take a little longer to plane out; but, had great cruising speed, and top end speed was 56mph, cruising around 30mph at 3000.

Currently, I am having this "porpoising" problem with the bbc and bravo swap that I did, Im guessing that I upset the hull with the extra weight of the big block and bravo. If i lift everything up it gets a little wild. So, I just sent my 23p mirage plus prop out to have it balanced and labbed to see if this will help the problem. This is my first time having any prop work done so we will see if this makes any difference. What I currently do is leave the tabs down about half way at cruising speed and bring the drive up a little to solve this issue.

BTW, Patrick did you get the swim platform installed?

Steven, the swim platform in in GREAT shape, but I have not had a chance to mount it yet. The holes in the hull for the brackets are off by 3/4" so Ill have to fill in the old ones and drill 2 new ones

I don't think 21 is too much. We used to have a 1987 wellcraft nova 23 with a 350 magnum and ran a 19 or a 21. The 21 was best for medium load all around cruise. Wed go the 19 with more people and skiing or tubing. This boat had no tabs and never had any problems. I know it isn't a 242 ls but very similar size and weight I'm sure