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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10598
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I'm mounting a 25hp Mercury 4 stroke Bigfoot on a 14 foot Sylvan

Customer Question

I'm mounting a 25hp Mercury 4 stroke Bigfoot on a 14 foot Sylvan aluminum boat. Merc recommends having the anti-ventilation plate in line or within 1" below the bottom of the boat.
Currently, the plate is a good 4 inches above the bottom of the boat. I have the engine mounted as low as it can go.
Submitted: 6 months ago.
Category: Boat
Expert:  Tan replied 6 months ago.
Hello. My name is***** will have to cut the transom down or get an engine with a different shaft length. The other option will be to put a bracket on the transom of the boat to lower the engine.What is the transom height of the boat now?
Customer: replied 6 months ago.
Total transom height is about 19". I haven't had this issue on other boats.
Expert:  Tan replied 6 months ago.
It isn't the boat the engine is the wrong shaft length. You have a short shaft motor and need a long shaft motor. They make 15& 20 inch transom heights. These measurements can be with in 1" of the 15 or 20 inches.Your options are cut transom or get a engine with correct shaft length.You just haven't run into this situation yet it is very common for people to have incorrect shaft length for wrong transom height. You used to be able to get a extension kit to extend 15 to 20 inch but not anymore. Post back with questions.
Customer: replied 6 months ago.
Well it's a Bigfoot so it's already longer. I've never had the boat on the water with this engine on it, so not sure how low the transom will sit in the water.
Expert:  Tan replied 6 months ago.
Are you measuring the plate just above the prop? This is the plate that needs to be even or just below the bottom of the boat. Here is a link to transom heights.

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