RPM question?

[mnm99]

Should you be able to achieve the same RPM in both of these conditions at WOT? Does this sound about normal?
1 - 5am FLAT CALM...2 people I was able to achieve 4400rpm.
2 - 1pm CHOPPY, 3 people, and a filled 20gallon water tank I was only able to achieve 4000rpm.

 

[alldodge]

Should you be able to achieve the same RPM in both of these conditions at WOT? Does this sound about normal?
1 - 5am FLAT CALM...2 people I was able to achieve 4400rpm.
2 - 1pm CHOPPY, 3 people, and a filled 20gallon water tank I was only able to achieve 4000rpm.

Not as I can see it, while light chop helps reduce suction on a 50+ mph boat it has resistance IMO. You also add 1 person and 20 gallons of water, added at least 300 pounds (160 water, 140 small person).

Also don't know what boat, hp and engine

 

[steelespike]

We need to know a lot more. Is the boat 16ft or 26 ft.? I'll assume an I/O; is it 3.0, 4.3, 5.0, 5.7.?What year?
What is the gps speed at 4400? What is the speed at 4000.Was the weight evenly distributed?
Did you fine tune the trim?What size prop?

 

[mnm99]

Not as I can see it, while light chop helps reduce suction on a 50+ mph boat it has resistance IMO. You also add 1 person and 20 gallons of water, added at least 300 pounds (160 water, 140 small person).

Also don't know what boat, hp and engine

Yea weird. I'm in the process of dialing in props. I was just wondering if the different conditions, weight and outside temp difference would make a result of 400rpm less. Does it really matter what the boat , HP and props are ( not being a wize ***. lol )? They all stayed the same in both conditions anyway. Here they are if you want them, 2001 Monterey 262(26ft), 260hp, 5.7 I/O. I know the operating range is 4400-4800. GPS speed at 4400 was around 35mph, GPS speed at 4000 got up to 31mph?? Weight was even, trimmed the same as I always do. Dual B3 4 blade 24p.

 

[alldodge]

As a guy hear says, HP is how fast you hit the wall, torque is how far you move it (Tailgunner). You have a semi heavy boat, I'm thinking around 5500 to 6000 pounds and a 5.7 can move the boat. It takes most of what the engine has to get it up clip on plane. My thought is when you load the boat heavy it has a harder time coming up, or at least struggles more. If it had a big block the extra 300 pounds on plane would not be enough to notice, but the small block has given most all it has in torque to get it up in the 2800 to 3500 rpm. It has a bit more HP to give but no more torque

 

[H20Rat]

Does it really matter what the boat , HP and props are ( not being a wize ***. lol )?

Yes, it makes a huge difference! The larger the boat, the less it will be affected by extra weight. Had you done your test with a bayliner 16' cuddy with a 3.0l, your results would be drastically worse!

 

[steelespike]

Yea weird. I'm in the process of dialing in props. I was just wondering if the different conditions, weight and outside temp difference would make a result of 400rpm less. Does it really matter what the boat , HP and props are ( not being a wize ***. lol )? They all stayed the same in both conditions anyway. Here they are if you want them, 2001 Monterey 262(26ft), 260hp, 5.7 I/O. I know the operating range is 4400-4800. GPS speed at 4400 was around 31mph, GPS speed at 4000 got up to 35mph?? Weight was even, trimmed the same as I always do. Dual B3 4 blade 24p.

Your saying that it made 31 mph at 4400 rpm and 35 at 4000 rpm.And 4400 was a light load and 4000 was a heavy load?
I'm surely no Bravo 3 expert but your numbers don't make any sense. Physically impossible to make more speed at less rpm.And still at 36% slip.
A bravo should have amazing grip with solid slip numbers.Using a 1.65 ratio your slip at 4400 and 31 mph is 48% Your big boat would tend to have higher slip than say a 18 footer (12%) but 48% just doesn't make sense.especially a bravo.
It does Appear a 24" may be a little too much prop, also may account for the 400 rpm drop with the load.

 

[mnm99]

Your saying that it made 31 mph at 4400 rpm and 35 at 4000 rpm.And 4400 was a light load and 4000 was a heavy load?
I'm surely no Bravo 3 expert but your numbers don't make any sense. Physically impossible to make more speed at less rpm.And still at 36% slip.
A bravo should have amazing grip with solid slip numbers.Using a 1.65 ratio your slip at 4400 and 31 mph is 48% Your big boat would tend to have higher slip than say a 18 footer (12%) but 48% just doesn't make sense.especially a bravo.
It does Appear a 24" may be a little too much prop, also may account for the 400 rpm drop with the load.

Sorry I updated it. 35mph at 4400 and 31 at 4000rpm

 

[steelespike]

Those figures still indicate with a 1.65 gear ratio a very high slip.42% at 35 mph and 43% at 31.
One or more of the other numbers are wrong.GPS should be right unless we have a strong current or tide.
Gear ratio may be different than a 1.65 could be 1.47 that would make the slip numbers even worse.
Is the gps set for mph?Perhaps the tach is reading higher than the actual number.
Tachs are always suspect especially when numbers don't add up.

 

[Sea Rider]

(1) 35 mph at 4400 lightly loaded on flat waters, (2) 31 at 4000 with 3 up and chop.

That's normal. If you want higher rpm say 4400 while moving more load as in 2 achieving better hole shot but with slight less top speed, will need to drop at least one less in pitch.

Happy Boating

 

[mnm99]

Those figures still indicate with a 1.65 gear ratio a very high slip.42% at 35 mph and 43% at 31.
One or more of the other numbers are wrong.GPS should be right unless we have a strong current or tide.
Gear ratio may be different than a 1.65 could be 1.47 that would make the slip numbers even worse.
Is the gps set for mph?Perhaps the tach is reading higher than the actual number.
Tachs are always suspect especially when numbers don't add up.

How is slip calculated? Can you explain to me? Thanks

 

[mnm99]

(1) 35 mph at 4400 lightly loaded on flat waters, (2) 31 at 4000 with 3 up and chop.

That's normal. If you want higher rpm say 4400 while moving more load as in 2 achieving better hole shot but with slight less top speed, will need to drop at least one less in pitch.

Happy Boating

Currently I have a 24P and am going to a 22P to see what happens.

 

[alldodge]

How is slip calculated? Can you explain to me? Thanks

As a prop turns thru the water it moves forward. A 24 pitch prop with no slip should travel 24 inches per revolution. If you put your numbers in a prop calculator like bblades it will tell you where your at.
http://bblades.com/propeller-slip-calculator/

In the above calculator to find out something else just fill in which one you know and leave on blank, then press calculate in that column. My guess with a boat as heavy like yours and with only a 5.7 I would bet you have a 2.20 ratio drive and would still put your slip at 23. My guess is because you have it trimmed high and your starting to ventilate. Just guessing

 

[Blind Date]

My '97 SeaRay Sundancer 250 w/5.7 EFI B3 is spinning 22's. WOT RPM is right on @ 4800 lightly loaded. I'd say it is over-propped with a 24P and that's your problem. That 262 is a nice boat. On my short list when I was looking.

 

[mnm99]

I have an update.. I don't know how accurate it is....I changed to the 22's. While I had the boat out I filled it up at the gas station, 40 gallons. So 40 gallons weighs around 240lbs. Anyway..There was a slight chop out today. I was only able to get it to 4300rpm at around 36mph. Wind was blowing around 10mph. Cruise at 3400 at around 27mph. I tried to trim as high and low as I can go. Also played with trim tabs. I went into the wind and against. I did gain 300rpm with the 22's though. The part that confuses me is I went out last week with the 24's and was able to reach 4400. It was flat clam with no wind at the time though. I verified my tach is correct already. If I drop to 20's I'm loosing top end and cruise big time from the original 26's I had. With the 26's I was able to get to 4400 at around 39mph and cruise at 30. The problem was it was a dog out of the hole. Now it jumps out of the hole. I have to really play with it when it's flat calm I guess. I cant see how a boat can reach max RPM in all kinds of conditions. It's so hard to tune when the conditions are changing so much.

 

[alldodge]

I have an update.. I don't know how accurate it is....I changed to the 22's. While I had the boat out I filled it up at the gas station, 40 gallons. So 40 gallons weighs around 240lbs. Anyway..There was a slight chop out today. I was only able to get it to 4300rpm at around 36mph. Wind was blowing around 10mph. Cruise at 3400 at around 27mph. I tried to trim as high and low as I can go. Also played with trim tabs. I went into the wind and against. I did gain 300rpm with the 22's though. The part that confuses me is I went out last week with the 24's and was able to reach 4400. It was flat clam with no wind at the time though. I verified my tach is correct already. If I drop to 20's I'm loosing top end and cruise big time from the original 26's I had. With the 26's I was able to get to 4400 at around 39mph and cruise at 30. The problem was it was a dog out of the hole. Now it jumps out of the hole. I have to really play with it when it's flat calm I guess. I cant see how a boat can reach max RPM in all kinds of conditions. It's so hard to tune when the conditions are changing so much.

Your tach is off, there is no way you can go from 22 pitch at 4300 WOT, then with 26 pitch you can get 4400 WOT. The numbers on the 22's should be higher then the 26's. While wind is an issue another facture is current. I would suggest getting a handheld tach,
http://www.tooltopia.com/electronic...scplp6811579&gclid=CLvY1orqo8YCFQmUaQodT1UAlQ

 

[mnm99]

Your tach is off, there is no way you can go from 22 pitch at 4300 WOT, then with 26 pitch you can get 4400 WOT. The numbers on the 22's should be higher then the 26's. While wind is an issue another facture is current. I would suggest getting a handheld tach,
http://www.tooltopia.com/electronic-...FQmUaQodT1UAlQ

Well they were 2 different props all together. The original were reconditioned dual 26p 3 blade B3 props. The second 24P DUAL 4 blade from Hill marine with a cup on the second. The 3rd 22P DUAL 4 blade form Hill marine with a cup.
I looked into that tester. It won't do a peak hold. There's no way for me to see it since it will be down below. Can't leave the hatch open at WOT.

 

[steelespike]

How is slip calculated? Can you explain to me? Thanks

Using a calculator site you take the rpm,gear ratio,speed, pitch and calculate for slip.
the 5 parameters are,rpm,ratio,pitch,speed, and slip.you usually calculate for slip to evaluate the
how well a set up is working. Slip in a typical runabout will be in the low double digits.

 

[alldodge]

Well they were 2 different props all together. The original were reconditioned dual 26p 3 blade B3 props. The second 24P DUAL 4 blade from Hill marine with a cup on the second. The 3rd 22P DUAL 4 blade form Hill marine with a cup.
I looked into that tester. It won't do a peak hold. There's no way for me to see it since it will be down below. Can't leave the hatch open at WOT.

That makes a lot more since. Kind of helps to keep adding those bits of info so we don't forget or have to go back and re-read the whole thread again

 

[Texasmark]

Did anybody factor in the air density/temp aka bang for the buck at dawn vs later in the day. Does matter!