Prop Selection - Bayliner 175

[Scott06]

same to you

You need a nap.

 

[cyclops222]

There is a simple reason for a prop with more blades.
The torque at the prop shaft is more than a 2 blade can EFFICIENTLY use. The prop......Blows out...... starts cavitating with bubbles forming on the prop blades. A major reason why most 5 bladed props fail with high speed, high horsepower motors. Most race boats use 2 blade props only.

 

[Scott Danforth]

Most race boats use 2 blade props only.

Not true. Most race boats have CNC machined props with 6 or more blades

 

[cyclops222]

Mercury Marine has a report explaining the advantages number of blades offers.
The 3 blade has the highest number of advantages. The single blade has the highest efficiency. But vibration is destructive. They state the advantages of THEIR 5 bladed props. IF IF you will give up some top speed. Strange that those turbine powered unlimited race boat class is MANDATED to only use a 3 bladed prop up to 16" in diameter. At about $ 15,000 each.

 

[jlh3rd]

You need a nap.

naw....I don't think so.

 

[gsbarry]

I finally got out on the water yesterday and was able to test out my new prop (among other repairs/upgrades). A big thanks to all for the advice and knowledge sharing. I’m very happy with the new prop which was night and day vs the old (wrong) one. The prop i went with is the Black Diamond 3 blade 19P. I had a light load with only 2 people and no extra gear. Hole shot was quite good. Top speed at WOT was 38 MPH and rpm’s were 5000 (which is 200 higher than the service manual range) which calculates to 16% slip with my 2.0 gear ratio. I also have a Black Diamond 3 blade 15P that just arrived and haven’t tested yet (this is intended for heavier loads and water sports). Should i be concerned about the rpm’s being a little high?

 

[Ptr.Torch]

It's certainly possible my gauge is not accurate. My procedure for performing the compression test (cold motor) was to disconnect the distributor wire, remove 1 plug at a time, thread in the hose/gauge into the cylinder, then crank the starter for ~5 seconds until compression stops rising on the gauge, record result and move to next cylinder. My understanding w/ compression tests is that it's less about the absolute value but rather that each cylinder reads within ~10% of each other.

Sounds right. But it is uncommon to have such high numbers. Your scale may be off, uncalibrated. Gas burns in the 50 to 90 psi range (typical) higher readings on a racing engine or a diesel. I like to 75 to 80 psi when I'm testing

 

[Pmt133]

I finally got out on the water yesterday and was able to test out my new prop (among other repairs/upgrades). A big thanks to all for the advice and knowledge sharing. I’m very happy with the new prop which was night and day vs the old (wrong) one. The prop i went with is the Black Diamond 3 blade 19P. I had a light load with only 2 people and no extra gear. Hole shot was quite good. Top speed at WOT was 38 MPH and rpm’s were 5000 (which is 200 higher than the service manual range) which calculates to 16% slip with my 2.0 gear ratio. I also have a Black Diamond 3 blade 15P that just arrived and haven’t tested yet (this is intended for heavier loads and water sports). Should i be concerned about the rpm’s being a little high?

Don't push the throttle that hard and you should be fine. If you have a cooler drier air temp when you tested, this will change your WOT RPM as well. At 60F I get 200 rpm more than at 80 and also get 1.5 more MPH. That is enough for me to kiss the rev limiter in the cold (well... cool. 60 is perfect boating weather for me) but be perfect for the rest of the season. As long as your tachometer is accurate, I'd say the 19 is the correct prop. You ideally should be near the limiter with no load.

 

[gsbarry]

Don't push the throttle that hard and you should be fine. If you have a cooler drier air temp when you tested, this will change your WOT RPM as well. At 60F I get 200 rpm more than at 80 and also get 1.5 more MPH. That is enough for me to kiss the rev limiter in the cold (well... cool. 60 is perfect boating weather for me) but be perfect for the rest of the season. As long as your tachometer is accurate, I'd say the 19 is the correct prop. You ideally should be near the limiter with no load.

It was about 70 out, so sort of in line with your comments.

 

[Scott06]

I finally got out on the water yesterday and was able to test out my new prop (among other repairs/upgrades). A big thanks to all for the advice and knowledge sharing. I’m very happy with the new prop which was night and day vs the old (wrong) one. The prop i went with is the Black Diamond 3 blade 19P. I had a light load with only 2 people and no extra gear. Hole shot was quite good. Top speed at WOT was 38 MPH and rpm’s were 5000 (which is 200 higher than the service manual range) which calculates to 16% slip with my 2.0 gear ratio. I also have a Black Diamond 3 blade 15P that just arrived and haven’t tested yet (this is intended for heavier loads and water sports). Should i be concerned about the rpm’s being a little high?

That is pretty much right on what I did with my 3.0 boat. If your tach is accurate you are better off being slightly under propped like this. A tad above wot range isn’t gonna hurt.

i suspect for water sports 15” is to low in pitch , if you need more holeshot try a 17” and watch the over rev when lightly loaded. Would expect a black diamond 17” to pull about 5400….

 

[jlh3rd]

if it was my boat, I'd be happy.