prop size

kriszimm

Cadet
Joined
Aug 16, 2014
Messages
9
The previous owner put an 18 on cause they didn't have a 15 he blew head gasket I bought it ran it blew head gasket he blew it right after putting the 18 on but someone said prop maybe to big
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
Messages
12,345
Have not seen nothing similar, unless big prop runs engine extremely under powered at wot, that's way, way lower min wot rpm factory stated for that engine. Extreme load on pistons which eventually will produce excessive heat build-up that does not dissipates well and blows head gasket. Need to check wot rpm with a tach and lightly loaded boat to check if inside/outside factory wot parameters. Then if needed select a proper size/pitch prop for that combo. BTW, which engine, boat is it ?

Could it be a insufficient HP engine along wrong prop for that boat ? You can eventually blow a head gasket too if having a poor cooling system not performing its homework right. Impeller, water passages obstructions, thermo, timing advance issues....

Happy Boating
 
Last edited:

Bondo

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Staff member
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Apr 17, 2002
Messages
70,465
Will a prop to big cause head gasket keep blowing

Ayuh,.... It's entirely possible, for the reasons Sea Rider says,...
Luggin' the motor induces detonation, which errodes the pistons, 'n can cause head gasket failure,....

What motor We talkin' 'bout,..??
 

kriszimm

Cadet
Joined
Aug 16, 2014
Messages
9
Mercruiser 3.0 and it ran fine for previous owner until he put an 18 prop instead of the 15 that was on it after he put the 18 on the head gasket blew didn't think about it I purchased it ran for three days blew again my mechanic said possibly to big a prop for that engine thought I'd ask get second opinion
 

samt

Petty Officer 2nd Class
Joined
Jun 5, 2013
Messages
113
I had an old chevy in highschool that kept blowing headgaskets, finally i took motor to a machinist and he found warped head.
 

Starcraft5834

Lieutenant Commander
Joined
Jun 2, 2013
Messages
1,677
To much prop for weight of boat..likely ran hard historically and put away tired.....get engine squared away and drop down to 15p.. in a 4 blade..
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Assuming a sound motor in the first place a 3.0 in a 18 footer a 15 seems unusually light and would normally over rev.
A 18" pitch seems definitely within a normal prop range.If the motor was so weak that it struggles with a 18" prop I think it would be a candidate for various mechanical; failures. Replacing a head gasket isn't just replacing the gasket. The block deck and head need to be checked for warpimg.
If the timing is off advanced too far or retarded might cause a gasket failure.If the 15 was indeed too light it may have been routinely over revving
which surely further weakened the motor.
 

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
The boat calls for a 21 pitch prop. How is a 18 pitch going to pop a head gasket?

The guy that suggested a warped head is on the right track.

Take off the head and take it to a machine shop.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
The boat calls for a 21 pitch prop. How is a 18 pitch going to pop a head gasket?

The guy that suggested a warped head is on the right track.

Take off the head and take it to a machine shop.
With a warped head or motor deck, gasket failure would be likely.
If the boat required a 15" prop to perform reasonably then a 18 would be too much prop.
And very likely would lug the motor.
 
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