Repeated prop hub failures.

[Pessemist]

I originally started this post under Mercruiser but maybe this is better place to ask a final question:

I have a 1988' Searay Dundancer 260 with a 7.4 liter Mercruiser engine and Alpha 1 drive.

With different props and new hardware I have spun 4 prop hubs. I have never hit anything. Usually occurs when plowing along (never on plane).

The input I got on the Mercruiser board was that the hub/prop combination was being over come by the torque of the big block 7.4 l Mercruiser engine.

I have replaced the sterndrive unit which was originally 1.32 gear ratio with an aftermarket one geared at 1.47 (can't get 1.32 anymore).

The original Searay spec called for a 15.25"x15 prop. Checking prop calculators I should get close to the same performance with a 15.25"x16 prop but they don't make this paticular size.

My question is: I can go to a 4 blade aluminum at 15"x16. The hub/prop combination is rated at 350hp (mine is 310hp).

Is my thinking OK here? I usually just cruise around on plane, seldom at WOT and top speed is not a concern. Getting on plane asap is my priority to limit 'plowing' which stresses the hub.

Here is the prop I am thinking about:

Michigan Wheel Vortex XHS Aluminum Boat Propeller #992202 for Mercruiser Sterndrive Bravo One 7.4L 300 - 330 HP - iboats

Thanks in advance for any feedback

 

[craze1cars]

Re: Repeated prop hub failures.

What brands were the 4 hubs you spun? Try something different from those lol...

You could just buy a prop with a solid hub instead of a rubber hub. This is quite common among high performance boats with engines that make 300 to 400 hp or more. Solid hubs simply can't spin.

On the downside if you have a solid hub, the new weak point in your driveline will now be the Alpha itself, which is already tasked to the max running behind a 7.4L. Most alphas are rated to handle 300 hp max.

You're absolutely positive you don't have a spun engine coupler and all your hubs have been fine? It acts the same way as a spun prop hub...

Honestly if it were mine, I'd buy a prop with interchangeable rubber hubs (hubs cost about $40 to $70 each depending on brand), and I'd buy two hubs with my prop and carry a spare on-board. Consider the prop hub a disposable/consumable, and change it/dispose of it as needed.

You'd be better selecting a pitch based on performance of props you have personally tried and used, instead of Searay Specs. How was performance/RPM/speed of the props you used before you spun the hubs, and what kind of props were they? Use those as a baseline and pitch up or down from there as needed.

 

[Faztbullet]

Re: Repeated prop hub failures.

The hubs you spun was they the newer plastic hubs orthe old style rubber hubs? If plastic usually they wasnt torqued to 100lbs on nut or if both rubber and plastic you could have a blockage that is restricting cooling water out prop exhaust will will cause hub to melt and spin.