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View Full Version : 1978 with a serious Porpoising problem


Theomedic
07-24-2012, 03:32 PM
I just bought a 1978 Stars and Stripes that has a serious porpoising problem. It even does it with 3 adults int the forward seats and no one in the back. Any suggestions on how to get rid of this problem? It does it at speeds over 28mph and you can't go over 35mph because it feels like it will flip over backwards its so bad. I heard trim tabs may help? If so anyone know what ones would work. Please help. Thanks!

old blue
07-24-2012, 03:39 PM
Stock prop size? RPM close to MPH 30mph@3000rpm?
Motor mounts solid?. Stringers solid?
Stock Velvet Drive Transmission.

Love the 78's. My first was a 79. Pictures help.

Theomedic
07-24-2012, 03:42 PM
I'll check those items. I know he said the trans was replaced. 3 years ago and it has a new prop, not sure on size or pitch, I'll check the rpm at 30.

old blue
07-24-2012, 03:55 PM
Prop should be 13X13 LH I believe. 78 boat should have a 1:1 velvet drive tranny. Take a picture of the motor and Tranny and people here will ID it for you. Prop should be labeled on the end.

good luck
kent

MIskier
07-24-2012, 04:15 PM
While it is unlikely that the prop or tranny is the issue, if the engine was switched to opposite rotation at some point that may be a factor. What is the condition of the hull bottom and stringers like? If you have structual defects causing the hull to distort it can be a cause of porposing. Another possible culperate is forward loading causing the center of gravity to be shifted proportionaly farther forward than normal, while this seems counter intuitive, it is the most common cause of porpoising in boat designs. If it is just 3 guys than that should not be an issue, but if it is 3 guys, coolers, tower, speakers, empty fuel tank, etc. that may be enough of a forward shift to cause the instablity.

old blue
07-24-2012, 04:31 PM
A lot of 78's have stringer issues that's for sure. I would want to eliminate that possibility first.
I have no idea about engine rotation. Your probably right about the prop, even if it provided too much lift, that's a lot of boat to lift.

Sounds like hull distortion...ouch!

Theomedic
07-24-2012, 05:30 PM
Prop is 13x13. Tranny is velvet drive 1:1. I can't upload pics today as I only have an iPhone. Our of isnt hooked up & I can't load pics from the phone. I noticed the motor mounts on the back only have bolts in the top & none on the sides.

Cloaked
07-24-2012, 08:30 PM
I just bought a 1978 Stars and Stripes that has a serious porpoising problem. It even does it with 3 adults int the forward seats and no one in the back. Any suggestions on how to get rid of this problem? It does it at speeds over 28mph and you can't go over 35mph because it feels like it will flip over backwards its so bad. I heard trim tabs may help? If so anyone know what ones would work. Please help. Thanks!Therer's a post here somewhere that describes my same issue with a new 79 model. It's there to stay. I tried everything.

Sold it with the knowledge that it was subject to that issue and the guy bought it anyway. It's still out there somewhere.

Mine got uncontrollable at about 37 - 38 MPH.

.

ctjahn
07-24-2012, 08:35 PM
I have a 78 and dont have that issue.... 351, 1:1, 13x14 prop.... (2 -3 -4 people in the boat)

but...

I do recall someone talking about adding a lip to the tail (and I have that).

cj

thatsmrmastercraft
07-24-2012, 08:45 PM
I have a 78 and dont have that issue.... 351, 1:1, 13x14 prop.... (2 -3 -4 people in the boat)

but...

I do recall someone talking about adding a lip to the tail (and I have that).

cj

Would like to see that. My '77 will porpoise a little when not loaded enough. Never an issue with a slalom skier or a footer behind the boat.

kyfooter
07-24-2012, 09:48 PM
What are the chances the previous owner hung the boat for long periods of time from the lift rings? I had a friend who did that to his boats years ago, and after a while, both of his boats porpoised. I was somewhat common to hang boats from the lift rings in boat houses in the 70s and 80s. End result...stringers/hull lose their original shape and the boat does things it shouldn't.

That being said, you could try and add hook to the rear of the hull to keep the nose down. Shouldn't take much to make a difference.

snork
07-24-2012, 09:54 PM
isn't one of the causes of porpoising too much weight in the aft

ricford
07-24-2012, 10:03 PM
I would bet my bottom dollar on rotten stringers. I had a J craft, which doesn't use stringer, but the plywood floor acts as the stiffener, and when the floor rotted out it porpoised really bad. Once that was fixed the problem went away.

ctjahn
07-25-2012, 09:55 AM
Would like to see that. My '77 will porpoise a little when not loaded enough. Never an issue with a slalom skier or a footer behind the boat.

I will see if I can snap a pic this weekend... (and btw: My description may be wrong... but I dont know how to best describe it. - The photo should tell)

thatsmrmastercraft
07-25-2012, 11:41 AM
I will see if I can snap a pic this weekend... (and btw: My description may be wrong... but I dont know how to best describe it. - The photo should tell)

I know what you are talking about. No worries.

ahhudgins
07-25-2012, 08:18 PM
My neighbor had the same problem with is late 70s S&S. It only takes a very small amount of hull distortion or bad stringers to cause this to happen. He had the hook added to his hull and it's been fine for many years now.

ctjahn
07-25-2012, 09:59 PM
I know what you are talking about. No worries.

Not sure if this is a "hook" or just a lip but this is what I have.... (Sorry - bad pics via Iphone and yes its dirty.... too busy USING it)

Cj

psychobilly
07-25-2012, 10:41 PM
There's things you can do to the wheel that will solve the "porpoising" issue. I don't know the voodoo but I'm sure that Eric would know what to do. The wedge will work if you don't mind attaching it to your hull. It actually don't have to run the while width of the transom. It's also best to do with wood so that you can tweak it. Too much will shove your bow down. I hand out in a wood boat forum and this has been a topic of discussion over there. I looked last night for the post but couldn't find it. Look up Chelsea coach works on the net and email Mr Hot Rod. I'm sure he will tell you about the wheel voodoo as he has tweaked that hot rod that he built. You gotta see it, it's a work of art in the making and he's a real nice guy.

If you're just wanting to go the wedge route, go to the glen l forum and search out Dave lott's post othe Midnight Cry build. He added wedges to it as it was porpoising BAD as he LOTS of power in her. As you will read it was trial and error or tweaking to get her just right before he sealed the wedges.

IMHO on a MC like you have, tweak the wheel.

SkiDog
07-25-2012, 11:06 PM
My 03-X30 did the same thing. Thats one reason I sold the damn thing.

barefoot
07-25-2012, 11:17 PM
My guess is rotten stringers or hanging from lift rings. Both would probably create a slight U shape to the hull causing it to porpoise.

Another thought would be to add some weight to the bow. Throw some sandbags up front and see if it solves the problem. might be a quick and inexpensive fix for the time being...

Theomedic
07-28-2012, 09:37 PM
It has never been hung

Theomedic
08-19-2012, 09:48 PM
We were out tubing today with the kids and the boat doesn't plane out at any speed. Even at 20mph the bow is way up in the air with 4 adults in the front seats. I' beginning to think it may be the prop? I found a used prop from a 1979 Ski Supreme, does anyone know if it will fit my 1978 Mastercraft?

Cloaked
08-19-2012, 10:02 PM
We were out tubing today with the kids and the boat doesn't plane out at any speed. Even at 20mph the bow is way up in the air with 4 adults in the front seats. I' beginning to think it may be the prop? I found a used prop from a 1979 Ski Supreme, does anyone know if it will fit my 1978 Mastercraft?It should as long a it is a 1" dia shaft with a keyed mounting configuration. The exception would be a 1-1/8" dia shaft (either the prop or your shaft) or if it is a splined configuration from the Supreme. I think they had the 1" dia keyed confiuration. Also it needs to be LH rotation, and I think the Supremes used the LH Ford engines.

I doubt the prop is the culprit here. You'd likely just not get low end or top end performance but I do not think a prop would cause porpoising.

I had a 79 that porpoised at 36 + MPH. Never got it to lay down. From the sound of your issue, this is an increasingly noticeable condition as time goes on with speed becoming less of a variable. Is it possible that you're gaining weight from a waterlogged basement in the foam that was installed underfloor? It happens. Even if you pull the rear-centered transom plug, there are still areas in the floor that can hold water and not have an accessable drain point. If you have a leak (maybe along where the deck and the hull are married), it can be deceiving unless you have experienced this issue in the past. I'm just throwing out a bone here, and nothing makes me think this other than I have seen it happen in one way or the other. You may have a combination of issues leading to this particular one.

.

.

Cloaked
08-19-2012, 10:07 PM
I would bet my bottom dollar on rotten stringers. I had a J craft, which doesn't use stringer, but the plywood floor acts as the stiffener, and when the floor rotted out it porpoised really bad. Once that was fixed the problem went away. Along the same line that I was referring in the pervious post.

Theo, I'd say for all practical purposes of the boat, you'll need to pull the floor and look it over this winter, or sooner. It's a decent project but it seems like you need to at least eliminate that possibility, or fix anything you find along these lines. Once you're there, I'd plan on redoing the stringers and build it right all the way from the hull.

.

madcityskier
08-19-2012, 11:15 PM
Might talk to Devils Lake Watersports it Boaters Choice in Brooklyn. They're not a MasterCraft dealer, but Damon is knowledgeable and will likely work with you to try to fix the problem.

Theomedic
08-20-2012, 11:13 AM
A couple of requested pics, hope they load?

Here are direct links: I can't get images to load without errors?
http://i1066.photobucket.com/albums/u404/Theomedic/6917c771.jpg
http://i1066.photobucket.com/albums/u404/Theomedic/f9e1a713.jpg
http://i1066.photobucket.com/albums/u404/Theomedic/b12d3770.jpg
http://i1066.photobucket.com/albums/u404/Theomedic/8f589448.jpg
http://i1066.photobucket.com/albums/u404/Theomedic/b52b8227.jpg
http://i1066.photobucket.com/albums/u404/Theomedic/6cde6c5c.jpg

I will try to load them again.

Theomedic
08-20-2012, 11:19 AM
Looks like it does have a leak, I believe it's where the swim platform attaches as it looks like previous owner tried to calk it with clear silicone. The bilge area was full after leaving boat in water overnight.

Theomedic
08-20-2012, 11:22 AM
http://http://i1066.photobucket.com/albums/u404/Theomedic/6917c771.jpg

Theomedic
08-20-2012, 11:23 AM
http://http://i1066.photobucket.com/albums/u404/Theomedic/6917c771.jpg

Theomedic
08-20-2012, 11:27 AM
http://http://s1066.photobucket.com/albums/u404/Theomedic/?action=view&current=6917c771.jpg

Theomedic
08-20-2012, 11:28 AM
http://http://i1066.photobucket.com/albums/u404/Theomedic/th_f9e1a713.jpg (http://s1066.photobucket.com/albums/u404/Theomedic/?action=view&current=f9e1a713.jpg)

petermegan
08-20-2012, 07:33 PM
I agree with Cloaked. It sounds very possible you have water under your floor. It is a pity that there is not an accurate weight for your boat/hull combination so you can run it over the nearest weighbridge. You could have 50gallons in the underfloor area.Have fun

Cloaked
08-20-2012, 08:36 PM
Looks like it does have a leak, I believe it's where the swim platform attaches as it looks like previous owner tried to calk it with clear silicone. The bilge area was full after leaving boat in water overnight.A common issue with the older boats. The last platform I repaired due to this issue, I raised the trailer up and water dripped from the boat for five days. I eventually pulled the floor, removed the foam, and placed thru-bolts in place of lag screws.

Your packing gland could also be leaking but do not ignore the platform. Only one way to fix that. Been there too. Your mileage may vary.

.

.

liledgy
08-20-2012, 10:16 PM
The boat probably didn't weight more than a 80-85 mc (2250lbs.) most likely a little less.

Theomedic
08-21-2012, 02:04 AM
Did you have to pull up the entire floor or just the back where the fuel tank was? I may have to wait until winter to take on that project, not much season left in Michigan.

byronic
08-21-2012, 08:42 AM
There's plenty of season left in Michigan! Four weeks in a shorty,four weeks in a full suit,four weeks in a dry suit. Twelve more weeks,then it's time to get the DN out.

Theomedic
08-31-2012, 11:25 PM
Well I tried adding a temporary hook the the full width of the boat, that didn't work two different times using two different techniques. now I borrowed a prop to test that out. I'll let you know this weekend if it helped with the porpoising.

Theomedic
09-02-2012, 10:00 AM
I swapped out the OJ 4-blade 13x13 prop for a stock 3-blade 12 3/4 prop yesterday. That didn't seem to help the porpoising problem.
I'm going to try adding the hook again during the week but not go the full width of the boat as I think the prop wash prevented me from getting it to work. The temporary hook I added just blew off the bottom as soon as I started. Will keep you posted.

Cloaked
09-02-2012, 01:22 PM
Well I tried adding a temporary hook the the full width of the boat, that didn't work two different times using two different techniques.

I swapped out the OJ 4-blade 13x13 prop for a stock 3-blade 12 3/4 prop yesterday. That didn't seem to help the porpoising problem.


same issue with a new 79 model. It's there to stay. I tried everything.



..........................

Theomedic
09-02-2012, 02:16 PM
How about a trim tab? I'm willing to try it.

Cloaked
09-02-2012, 02:28 PM
How about a trim tab? I'm willing to try it.
That is the one thing I did not test. You're own your own there.

The reality is this (from my point of view and experience): Anything you do will only disguise the consequences of the problem. The root cause will not go away.

Sorry that you are experienceing this issue.

Short of a long story for me... I talked to a lot of people about my 79 (back in 79 - 80) and I ran into a guy here that used to work glass at Mastercraft. His bottom line was that the mold that the hull came from created this problem. Not much one can do about it, I was told.

Hopefully you can find a resolution from further research and discovery, but I conceded the 79 to another buyer, said buyer fully knowing the issue. He said I'll take the boat and I can fix it... I was on that (nearby) lake for another while and he never did fix it, doing all the things you and I have both tried (less the trim tab). Maybe a genius from Texas, or someone with knowledge of a fix beyond my efforts, can suggest something to to resolve.

.

ahhudgins
09-02-2012, 06:03 PM
How about a trim tab? I'm willing to try it.

You mentioned that your temporary hook came off so I’m guessing that you really didn’t get much of a test? Adding a trim tab does the same thing as a hook, so you might want to give the hook another chance before spending the time and money on the tab. My Maristar 200 used to ride bow high and only porpoised when I had more than 3 people in the boat, and it got worse when I added more weight (even the bow).

My hook is a piece of nylon that I cut like a wedge and it’s only 3/16 inch at the trailing edge. I used “Goop” to glue it to the hull. I put Goop on the strip and held it in place with painter’s tape and shoved some wooden dowel between the driveway and the strip to keep pressure on it. I let the Goop set up for at least 24 hours and then smeared the Goop over the leading edge of the strip. ¼ wedge was too much and took 2 MPH off my top end. I filed it down to 3/16 and it worked perfectly. The boat planes quicker and I lost no top end speed. One winter I will have mine fixed if I can find a gel coat guy who knows what he is doing.

I had 9 people in my boat this weekend, no porpoising and I could still hit 41 MPH with a few “Big Girls” in the boat. You may have a bigger problem with your hull or stringers, but my neighbor added the hook to his older S&S and it fixed his problem.

Theomedic
09-04-2012, 11:42 PM
I'll give it a try, can you give me some more info?
Where can I get the nylon?
How wide is the nylon 2-3 inches?
How did you cut the taper?

madcityskier
11-27-2012, 10:48 PM
There's a boat that just showed up on facebook on the mastercraft project page that had this home repair for porpoising. Thought you would like it. He's planning to remove all this during the restoration and go with something more subtle like what ahhudgins did.

Theomedic
03-17-2014, 12:26 PM
Well I have an update on this: I installed a Bennett Wake Tab on it last summer and I'm currently writing a detailed post with pics about my install. How do I link this thread to the new post, just copy and paste or is there a way to link the two threads?

JohnE
03-17-2014, 01:41 PM
Well I have an update on this: I installed a Bennett Wake Tab on it last summer and I'm currently writing a detailed post with pics about my install. How do I link this thread to the new post, just copy and paste or is there a way to link the two threads?

Post the link to the other thread and I will merge them.

Cloaked
03-17-2014, 07:22 PM
Well I have an update on this: I installed a Bennett Wake Tab on it last summer and I'm currently writing a detailed post with pics about my install. How do I link this thread to the new post, just copy and paste or is there a way to link the two threads?
Go to the first page of this thread. Then in the URL address box of your browser, highlight the address and copy. Then paste into the other thread.

Done deal.

.