Prop advice to reach 6000RPM - Negative Slip?

[hwsiii]

Re: Prop advice to reach 6000RPM - Negative Slip?

Trend, I got your message so I ran some charts on your two props and they are enclosed. I really love the Solas Amita 4 blade props, but unless you are having a problem with being able to hold a constant speed in a head sea you appear to have a very good prop for your boat in the BRP 15" pitch, you are a little light in RPM but well within the upper motor limits of recommended RPM between 5,000 and 6,000 RPM, and even if you lose 200 RPM with a regular load you are still at 5,500 and if I were you I would look at raising the motor one more notch up, that will probably gain you 150 to 200 RPM at WOT and put you right at maximum RPM, if it doesn't start ventilating.

Speed and Slip

The next chart shows the Effective prop pitch, which is how much pitch is required to make the boat attain the speed it is attaining if there were ZERO prop slip from both props. If you will notice the 16" prop is actually producing 15.22" of pitch and the 18" is only producing 15.68" of pitch and it is a full 2" of theoretical pitch more than the other prop. This is a prime example of how much difference is attributed to the differences in blade geometry between props.

Speed and Effective Prop Pith

H

 

[trendsetter240]

Re: Prop advice to reach 6000RPM - Negative Slip?

H, thank you very much for taking the time to run the numbers for my props. It is helpful to see the graphical representation.

The BRP prop is a good "all-around" solution for my boat. However, I still wonder if I can get better performance with a 4-blade alum. or a SS prop.

You can see by the numbers that the boat doesn't plane until 4000RPM. What you can't see is that it will drop off of plane at 3900RPM! I may be asking too much from a prop but would a 4-blade solas amita or 4-blade solas HR Titan give me a better cruise without taking much off the top end?

The motor is as high as I can put it now, with the current setup the BRP prop will blow-out and start ventilating if I don't ease off the throttle as soon as the boat starts planing. I can then reach top speed and RPM by slowly increasing the throttle until it has settled.

 

[hwsiii]

Re: Prop advice to reach 6000RPM - Negative Slip?

Trend, if you will buy a Solas Amita aluminum 4 blade prop in their 15" pitch you will find that you will be able to get on plane at about 3,400 RPM and hold that plane down to about 3,400 RPM. You will probably lose a couple of hundred RPM, but you should lose almost no speed and when you load it up with people the ride and ability to stay on plane at lower RPM will be very nice, you will also notice many more improvements that you will like very much. If you go to a 13" pitch in that same prop you will find your RPM will rise to right at 6,000 RPM, but you will probably lose a couple of MPH at WOT


H

 

[v12mac]

Re: Prop advice to reach 6000RPM - Negative Slip?

H,
The motor is as high as I can put it now, with the current setup the BRP prop will blow-out and start ventilating if I don't ease off the throttle as soon as the boat starts planing. I can then reach top speed and RPM by slowly increasing the throttle until it has settled.

Why would you want your boat set up like this? You should be able to drop the hammer and go!
Are you sure your motor isn't too high?

 

[trendsetter240]

Re: Prop advice to reach 6000RPM - Negative Slip?

Why would you want your boat set up like this? You should be able to drop the hammer and go!
Are you sure your motor isn't too high?

My engine has manual tilt so if I want to trim the motor out to the optimum angle it behaves as I described. I tilt it like this when I am going to be running a long cruise to get the best speed and mpg.

For normal use I tilt the motor in two pins and the prop doesn't blow out.

 

[v12mac]

Re: Prop advice to reach 6000RPM - Negative Slip?

My engine has manual tilt so if I want to trim the motor out to the optimum angle it behaves as I described. I tilt it like this when I am going to be running a long cruise to get the best speed and mpg.

For normal use I tilt the motor in two pins and the prop doesn't blow out.

Ahhhhhaa! Good answer.

 

[trendsetter240]

Re: Prop advice to reach 6000RPM - Negative Slip?

Well I got my new 4 blade 13.25 X 13P Solas Amita in the mail today and took it for a test run.

Max speed was 33.7MPH @ 6600 RPM. Overall I did not like the performance of the prop but let me explain why.

Since I posted the above numbers I have discovered my bilge was holding about 450lbs of water. I cut out an access port in the floor and installed a bilge pump. Now my boat is 450lbs lighter and all my prop numbers have changed.

I have tested all 4 of my props and will likely start a new thread posting the results of these tests for further comment.

Thanks to all for the advice.

Edit: I should mention that the test today was with a very light load and I do plan on keeping this prop for use later in the summer. When the temp goes up another 20 degrees, humidity is up and I have a boat full of passengers and fishing/camping gear this prop should put right about 6k rpm wot.

 

[hwsiii]

Re: Prop advice to reach 6000RPM - Negative Slip?

Trend, I am very sorry to hear about the out come with your new prop, when you run some new numbers we can look at where you are now, but I believe the 15" pitch BRP is like you said a good all around prop.




H

 

[trendsetter240]

Re: Prop advice to reach 6000RPM - Negative Slip?

Yeah I was a bit disappointed about the result too but that's what I get for running my first test numbers with a bilge full of water. At least now I know how the boat will perform with a very heavy load.

What I didn't like about the solas prop was the lack of response. It felt like I was having to give way more throttle to get the boat to move as I expected it to. This was at all speeds. I guess I was expecting the 4-blade prop to really grip the water compared to the 3-blades I already had. Even gunning the throttle didn't have the same acceleration the other props did.

I have all my new numbers now and will make a new post later tonight if I get a chance. I would love to hear your thoughts on the new numbers.

Cheers~!