prop size

[Crownie82]

I have a 1994 crownline bow rider with a 3.0 mercruiser and alpha 1 out drive. It has a 14.5 x 18 four blade prop and it blew my head gasket twice just bought from friend that installed new prop and have problem old prop 14.5x15 never had trouble what is the correct prop for my boat tjanks

 

[alldodge]

I have a 1994 crownline bow rider with a 3.0 mercruiser and alpha 1 out drive. It has a 14.5 x 18 four blade prop and it blew my head gasket twice just bought from friend that installed new prop and have problem old prop 14.5x15 never had trouble what is the correct prop for my boat tjanks

:welcome: to iboats

How about some more info on what's going on. I don't see how a prop can cause a head gasket to blow. The head gasket will below if it gets to hot, bolts not torqued properly, and a few others. Give us some info on how this happened both times and engine serial number. Also when was the last time the impeller was replaced?

 

[Crownie82]

My friend Owned this boat and it had a 14.5x15 prop the chipped it loading and bought s 14.5x18 soon after installing new prop head gasket blew he had repaired then I purchased boat drove it 4 times tubed some then head gasket blew again was told prop to big for the engine havnt tried it since this last head gasket repair was told again the prop to big for engine I do not have serial # boats an he away in storage

 

[Crownie82]

He had no trouble with it till he put bigger prop on

 

[Crownie82]

It was replaced in May this year soon after it blew

 

[alldodge]

Still doesn't make since to blow a head gasket. If a prop is to large the engine will just not reach optimum WOT and will be slower taking off. What is the temperature your seeing when running

Also is this an EFI or carb engine, and does it have closed cooling?

 

[Bondo]

I have a 1994 crownline bow rider with a 3.0 mercruiser and alpha 1 out drive. It has a 14.5 x 18 four blade prop and it blew my head gasket twice just bought from friend that installed new prop and have problem old prop 14.5x15 never had trouble what is the correct prop for my boat tjanks

Ayuh,... Welcome Aboard,.... The Proper prop is the one that allows to motor to rev to it's limit at Wot operation, with a normal load,....

In yer case, it oughta run 4800 rpms at Wot, normal load, at proper trim,.....

So,....... What rpms was it runnin' with each prop,..??..?

 

[Crownie82]

I'm running 3500 with the 14.5x18 four blade prop and I believe it's a carb it's a 4 cyl and it cools from raw water and out. The guy I bouht the boat from said he was running 3800 rpm after changing to new prop then head gasket blew fixed I ran it with 18 prop and it blew again mechanic said to big prop for that engine was stumped and I'm here looking for another direction repairs costly if not prop causing it thanks for help

 

[alldodge]

I'm still under the thought you don't have a prop problem, you have an engine problem. As Bondo mentioned you should be able to turn 4800 rpm and your only getting 3500. There is a possibility that your tachometer is off but not that much. As before I see blown head gaskets as to much heat or something wrong with the head, block or both. When the gasket blew what was done to get it running again? What kind of work was done other then replacing the gasket?

 

[Crownie82]

The mechanic had head machined and replaced gasket the first time he just replaced gasket but when I took in I told him I wanted head machined

 

[alldodge]

The mechanic had head machined and replaced gasket the first time he just replaced gasket but when I took in I told him I wanted head machined

The problem with mechanics is with anything there are good and bad. Not saying he doesn't know what he is doing but just replacing the gasket the second time and coming up with to large a prop is not instilling any confidence. Not sure if the head was machined the first time, but you can determine if it needs it. Use a steel ruler, straight edge and some feeler gauges. Measure what the clearance is under the edge. Or take the head to another machine shop and ask them to check it only and provide info.

 

[Crownie82]

He said he used filler guage and the gap was not really any but like you said he can say he did all that and didn't this second time he fixed it when it warms up will run again see if it blows gasket and if so will have take to another mechanic. Thanks for information

 

[alldodge]

By the way, I think we need to move this to the Merc engine side to get some more opinions. If your OK with that we can see if a Moderator can add to the title and move it for you

 

[Crownie82]

That would be great thank you

 

[UncleWillie]

Another piece of information needed, is the ratio of the drive. All ALPHAs are not the same.
A 3L engine will use a 2:1 Alpha drive.
If someone swapped the drive out for a 1.6:1, thinking it would go faster, that would be a big mistake.
You would be under-propped and over revving with a 14.5 x 15 prop on a 2:1 Drive. It should have a 19-21 normally.

Check on the Decal on the Right side of the drive.

 

[Crownie82]

I will have to look we have boat in storage about an he away. Also is a three blade 19 better than the 18 four blade.

 

[Bondo]

I will have to look we have boat in storage about an he away. Also is a three blade 19 better than the 18 four blade.

Ayuh,..... If it can't turn an 18" prop up to 4800 rpms, a bigger prop is gonna make it worse,......

I can see where detonation probably is causin' the head gasket problems, if the best it can do is 3500 rpms,....

You need to verify that it's got the 1.98:1 gear ratio in the drive, 'n I suggest a compression test to see where the motor is at,....

 

[steelespike]

If the boat originally had a 15" prop Going to a 18" would lower the rpm as much as 400 to 600.
Probably making the motor Lugg,struggle to plane and pull tubers. Lugging is bad for the motor.
Assuming the the boat is 18 or 19 ft a 3.0 would usually have a 19 or 21" pitch prop turning about 4500 to 4800 rpm. speeds approaching 40.
For heavy loads and water sports usually a 17" would be suggested with the stipulation to keep the rpm within the suggested wot range.
If the 15 inch was producing a normal rpm range 4500--4800 at wot there are probably motor issues.
Might be as simple as the timing set wrong and/or not advancing as it should.
Maybe verify the tach is correct and do a compression check, note the location of each plug and the condition.

 

[alldodge]

Were talking over a thousand rpm then it should have. IMO that's an engine problem no matter which way you look at it

 

[Crownie82]

Well I appreciate all the help and advise