Gimbal Ring/steering spline set screw

[jaybob275]

Hey Guys,

I was taking my boat ('77 Yarcraft w/ merc228) out for a shakedown cruise and about 5 minutes in the steering went loose and lost control. Luckily nobody was hurt and the boat is mainly fine but I have no steering. I took the outdrive out and the problem is the spline that goes into the gimbal ring to steer the boat is stripped. I know there is a kit to make it easy(ish) to replace and this could have been prevented had it been torqued right. But is it possible to put a set screw into the spline so it doesn't slip anymore. I was thinking drill it in, tap it and put a machine screw in. This would prevent it from rotating.

Does anyone have any input as to this idea? I know it isn't the correct way to do, but I think it would work.

Thanks

JBI

 

[boatguya1]

Re: Gimbal Ring/steering spline set screw

I'd not "rig" this part of the boat up shade tree style. As you said "Luckily no one was hurt", how about when your patchjob comes apart, will you be lucky again?

James

 

[45Auto]

Re: Gimbal Ring/steering spline set screw

Might want to try to figure out why Mercury didn't use a setscrew there in the first place .....

Quick load calc:

Flat plate load = Coefficient of drag x (.5 x fluid density x velocity squared x cross sectional area)

Cd of flat plate = 1.2
water density = 62 lb / cubic foot
30 mph = 45 ft / sec
Underwater area of Alpha drive equals about 1 foot x 1 foot. Call it half that area exposed to flow when it's turning hard, equals 1/2 square foot.

Load = 1.2 x (.5 x 62 x 45 x 45 x .5) = 37,665 pounds acting on center of pressure (Cp).

Looking at an Alpha and assuming the Cp is about in the middle of the exposed surface, that makes it about 8 inches from the pin when it's turning.

If the pin is 2 inches in diameter (1 inch radius), that means the load on a single point on the outside of the pin (what a setscrew or bolt will see) would be (37,665 x 8) / 1 = 301,320 pounds.

Better use a STRONG screw to take 301,320 pounds!

My 3/4 ton Suburban weighs about 5000 pounds. Would you turn your boat on it's side, then hang a stack of 8 Suburbans from your rigged up drive and walk under it? That's the kind of side load (37,665 lb) you're going to be putting on it. Probably the reason Mercruiser uses splines or a square drive to take the load. If you rig it up, hopefully you won't ever take anyone on the boat with you or be boating anywhere near anyone I care about!

 

[boatguya1]

Re: Gimbal Ring/steering spline set screw

So now picture the approx 8inch steering arm sticking off the top of the shaft. How much force must be applied to it to turn the 37,665 pound calculated side load to steer the boat. I'm guessing there's a slight problem in the calculation somewhere or I'm missunderstanding how it applies.

You could bolt a 77 merc drive to an unmoveable object and I'm not standing under it with 8 3/4 ton suburbans hanging off it.

To the OP don't do it it won't be safe.

James

 

[stonyloam]

Re: Gimbal Ring/steering spline set screw

Sounds like you are going to have to pull the gimbal ring and replace the upper pivot pin (splined) and steering arm. You can get the square/ splined shaft from Mercruiser, check out ebay for a splined arm (no longer available from Merc.). good luck.

 

[stonyloam]

Re: Gimbal Ring/steering spline set screw

Quick load calc:

Flat plate load = Coefficient of drag x (.5 x fluid density x velocity squared x cross sectional area)

Cd of flat plate = 1.2
water density = 62 lb / cubic foot
30 mph = 45 ft / sec
Underwater area of Alpha drive equals about 1 foot x 1 foot. Call it half that area exposed to flow when it's turning hard, equals 1/2 square foot.

Load = 1.2 x (.5 x 62 x 45 x 45 x .5) = 37,665 pounds acting on center of pressure (Cp).

That seemed a bit high, so I went over the calculation, and by inserting the proper units, and doing the calculation with unit cancellation you wind up with (assuming that cd is unitless) a value of 37665 ft lb/sec squared, which are a unit of force called a poundal. 1 ft lb is approx 1/32 poundal, so the force would be approx 37665/32= 1177ft lb. Still a considerable force to contend with. The setscrew would probably shear the first time you made a hard turn. Don't take a chance it right.

 

[bklein]

Re: Gimbal Ring/steering spline set screw

I have a 78 Cobalt where steering went out like this after unloading the boat and the outdrive dragged. Would the pin/shaft be now stripped or shift lever stripped or gimbal stripped? Does the engine need to be pulled to fix this? I got a quote of 14hrs labor....