1978 with a serious Porpoising problem

[Theomedic]

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]

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]

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]

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]

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]

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]

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]

Originally Posted by Theomedic

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]

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]

Originally Posted by ctjahn

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.