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K. Kergan
K. Kergan, Marine Mechanic
Category: Boat
Satisfied Customers: 1031
Experience:  24 yrs exp. Inboard/Outboard mechanic Mercury/ Mercruiser, Volvo Penta and OMC, Johnson/Evin.
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I just took a trade on a 1987 34 Grand Sport. When I pulled

Customer Question

Hi I just took a trade on a 1987 34 Grand Sport. When I pulled the boat for bottom service I noticed two different props 1 15x17 and 1 21x17. My question is what is the factory prop size for this boat? 34' 13,800lbs dry with 330hp 454's. The previous owner hit a log and the marina put a spare on to get him back home. Thanks Rod
JA: Sometimes boat issues seem like they'll be really complicated, but end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Is there anything else the Marine Mechanic should be aware of?
Customer: none
Submitted: 9 months ago.
Category: Boat
Expert:  K. Kergan replied 9 months ago.

Hi, my name is Kerry

A 15x17" prop is a typical size for a large outboard motor or a small block inboard.

The larger is probably the right size.

The correct size is the size that allows the engines to rev up to their rated maximum RPM at wide open throttle.

If the engine revs past it's rated maximum RPM, the prop is too small.

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