Prop selection

[Mikered3500]

I put a 14.5x19 prop on my boat this week. Took it out today and was disappointed on the performance. It's a 21 foot Sea Ray center console with a 91 or 92 Johnson 225. The average speed was 10-12 mph at 4500ish rpms depending on waves. What direction should I go in order to get a prop to get me in the 25-30mph range? Thanks .

 

[interalian]

What pitch was the old prop? You're not going to double your speed going from 4500 rpm to ~5500. Have you weighed your boat lately?

 

[Mikered3500]

The other motor the boat had on it when purchased had a busted lower unit. The 225 I bought didn't have a prop on it. I was told on here to start with a 14-15x19 prop and go from there. So here I am. No I have not weighed the boat but I know its not a goose feather....

 

[dingbat]

That boat should run 40 mph with a 225 on the back.

15x19 prop is close.

have you checked the compression on the motor?

 

[Mikered3500]

Yup...just did....and it's 60-70 across the board....time for another motor I guess .I should have checked the compression before I bought it.

 

[dingbat]

Yup...just did....and it's 60-70 across the board....time for another motor I guess .I should have checked the compression before I bought it.

Never seen an outboard with low, “even” compression. Most deaths are single hole failures.

Find a new compression gauge and test again

 

[Mikered3500]

Just used my friends gauge which is a different brand, and it was similar .but from high 60 to 75 ish...so what do you think could be the reason for that? Any ideas?
​​

 

[dingbat]

Just used my friends gauge which is a different brand, and it was similar .but from high 60 to 75 ish...so what do you think could be the reason for that? Any ideas?
​​

all plugs pulled? Carbs wide open?

if these numbers hold true I’d decarb the motor and test again.

At 4500 rpm you should be running @ 30 mph waves or no waves.

 

[interalian]

Yeah, 4500rpm and 12mph is massive slippage. Something doesn't add up. Tach not accurate?

 

[Mikered3500]

I'm just going off of what I got .I thought it might be a slipping hub...but I've been wrong before. The port side plugs were golden tan color with hints of oil in them . Starboard side top was slightly black, middle plug looked like it was clean as they come in the package and bottom was oil bathed. But somehow kept even but low cimpresscom

 

[Ol Sarge]

I'm just going off of what I got .I thought it might be a slipping hub...but I've been wrong before. The port side plugs were golden tan color with hints of oil in them . Starboard side top was slightly black, middle plug looked like it was clean as they come in the package and bottom was oil bathed. But somehow kept even but low cimpresscom

I would guess from the one clean new looking plug that you're getting water in the cylinder somehow. Might not hurt to check and replace the head gaskets and try a new set of plugs.

 

[dingbat]

I'm just going off of what I got .I thought it might be a slipping hub...but I've been wrong before

Very well could be a slipping hub.

Take a permanent black magic marker and drawing a line across hub to prop and take it for a spin. If marks don’t line up, you have a spun prop.

Wiil the engine rev to 5500 rpm with the prop in the water?

 

[Mikered3500]

Well...I would try to run it again but when I compression tested the motor the second time it wiped out the starboard top cylinder plug threads some how...made me want to lose it but to answer your question about 5500 rpm it would not go over 47-4800. At all .but it sounded like it wasn't running right. Especially with the lack of compression...I'm considering finding a new motor all together and calling it a day.