1979 Allison SST Handling Problems - New Restoration and Build

[CarolinaBurt]

Hi Guys, a friend has a real problem with porpoising in his 1979 SST. This is a complete hull repair/restore with a fresh rebuilt 1988 XR4 2.4L Mercury. The boat suffers porpoising so bad that he cannot get it to speed. Even with negative trim, the boat cannot be driven past the porpoising problem.
Things I know -
1) The hull and pad are perfectly straight - Nothing convex that would normally make porpoising worse and nothing concave (hooked).
2) I mounted a 4" jack plate and have the propshaft 2.5" below the pad (both set at level like we're supposed to do)
3) I mounted a water pressure gauge so he could raise the outboard - the higher propshaft should have reduced porpoising, give easier steering, and reduce gearcase drag. The 2.5" is as high as the stock "small" XR4 case will go up.
4) 2 batteries - 1 each in the lockers just behind the driver/passenger seats (where the factory put them).
5) 80 lbs trolling motor on the bow
6) 10 gallon tank in the transom well 1/2 full (50 lbs approx)

What am I missing? Help/input needed. I've been driving my own Allison and others for 20 years, this one has me confused.....

Thanks, Gordon

 

[CarolinaBurt]

I should have added that the XR4 is set-up with a new, Laser II 24" pitch prop.

For those not familiar with this vintage SST, there is zero (0) set-back built into the transom. The boat's pad is directly below the transom. Oddly, the transom also lacks the normal 12 degrees negative angle - this one's mounting is near vertical and 90 degrees to the pad surface. I've measured and we can still achieve 5 degrees of "negative trim" relative to a level pad, but the porpoising is still there.

I'm stumped....Gordon

 

[SLOmofo]

First, I know nothing about this boat. But I know when I had my lower unit on my XB 2003 buried it porpoised. "propshaft 2.5" below the pad" sounds excessive to me butt I know nothing. Hopefully someone here or at the factory will speak up.

 

[THutch]

First, I know nothing about this boat. But I know when I had my lower unit on my XB 2003 buried it porpoised. "propshaft 2.5" below the pad" sounds excessive to me butt I know nothing. Hopefully someone here or at the factory will speak up.

Gordon
I had a 1979 SST with a Johnson 110. Ran a Raker 22 on it. It ran great but was hard to get the porpoise out. What I found was the boat liked to be close to WOT. Running it slow was little fun due to porpoise. I ran 0 setback. Can’t remember how far below pad for the prop shaft. I did have 2 batteries on board and trolled on bow so I’d say setup should work.
This vintage runs very different than the XBs so forget what you know about those. I’d say stab the gas and trim to keep bow high. I know I run my XB much lower trim.

Hope that helps
Travis

 

[Jessie]

What is the water pressure? Can you try a different prop, like a 23" Tempest?

 

[CarolinaBurt]

Guys, I just finished up a call with the Allison plant this morning and learned that these hulls would porpoise if over-weighted. With the unknown weight of the fiberglass repairs, addition of the XR4, running 2 batteries, and the fuel/oil injection tanks - we're asking this 16.5' hull to lift and carry more weight that it was ever designed for. At some speed, the hull will carry the weight as the hydrodynamic force increases, but the ride there would always be difficult and take a skillful hand. That is not what my friend had in mind, so we'll be adding some trim-tabs onto the transom between the pad surface and the lifting strake on each side. Adding more planing surface at the rear of the boat will help to offset the additional weight and give a flatter ride.

Jessie, we're at approx 12 psi, but higher settings resulted in loss of pressure and excessive ventilation during holeshots. 2.5" is where we can best maintain our 12 psi and still get a decent holeshot without the prop ventilating wildly while trying to get a bite. Keep in mind that this boat has no built-in setback, and almost no negative transom angle so the gearcase is just a few inches behind the pad.

It is really like no Allison transom I've seen before.

Thanks for all of the inputs and time you've all given to this. This Allison group is one of the Best on the internet!!! Gordon

What is the water pressure? Can you try a different prop, like a 23" Tempest?

 

[CarolinaBurt]

Here are a few photos -

 

[SLOmofo]

Here are a few photos -

Overweight=AllisonBoatAbuse....you should be