whats with this prop estimate black magic????

[bob johnson]

how can you figure how fast a boat should be going just based on the prop and the rpms?????

on the omc forum someone asked about a 90 hp on his 16 ft boat and was only getting 5000 rpms at 50 mph..( didnt want to hi-jack that thread!!)

what if he mounted that 90 on the back of a 250 foot coal barge and ran the rpms up to 5000 rpms???? does anyone think he can go more than 15 mph???

or is that were slippage comes in????

I mean i can run my kicker motor at 5500 rpms with a 10 pitch prop...but i wont be doing mote than 10 mph max?

how does anyone "figure" what a prop-motor combo will do?????

I had always thought my boat should be going faster than it does with my big motor

I wondered how guys are getting high speeds with high pitched props..18 , 19 and 20 or more... and still turning the right rpms for the motor????

do they have really light boats???

seems like most guys with 115 hp are going faster than my boat no matter what size boat they have...

and i have a slippery Mod V... aluminum boat

bob

 

[Bondo]

Re: whats with this prop estimate black magic????

or is that were slippage comes in????

Yes...

 

[Silvertip]

Re: whats with this prop estimate black magic????

It's not black magic. You cannot expect a 200 MPH Corvette to run that fast pulling a 5000# boat. It's all about engine rpm, prop speed, prop pitch, slip, and load. In your example, a 90 HP engine on a barge will simply NOT rev to its recommended WOT rpm because it can't. Put the right prop on and it will rev were it should, but speed will be whatever that prop provides.

 

[hwsiii]

Re: whats with this prop estimate black magic????

Bob, if you will fill this form out I may can do some White Magic for you.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

If you would prefer to not go to this much trouble there are many other people who will come along and gladly help you.

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the maximum recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make. manufacturer and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a board under the keel and sticking out to the anti ventilation plate for a reference
Motor Transom Height

10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a digital tachometer and hour meter here: http://cgi.ebay.com/ebaymotors/Tacho...Q5fAccessories $ 35
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. Gas ________
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by just pushing the throttle at the helm and not at the carburetor, checked compression, and looked at the plugs and checked spark as well as timing and advance, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.



H

 

[bob johnson]

Re: whats with this prop estimate black magic????

It's not black magic. You cannot expect a 200 MPH Corvette to run that fast pulling a 5000# boat. It's all about engine rpm, prop speed, prop pitch, slip, and load. In your example, a 90 HP engine on a barge will simply NOT rev to its recommended WOT rpm because it can't. Put the right prop on and it will rev were it should, but speed will be whatever that prop provides.

is not the 90 hp on the barge the same thing as a 6 hp on my 20 foot mod v?

the motors rpms wind through the roof... but the boat goes only 6 mph....


as someone said, it is probably attributed to slip

why wont the 90 hp rev to its limit????

the barge isnt getting on plane....

bob

 

[bob johnson]

Re: whats with this prop estimate black magic????

Bob, if you will fill this form out I may can do some White Magic for you.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

If you would prefer to not go to this much trouble there are many other people who will come along and gladly help you.

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the maximum recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make. manufacturer and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a board under the keel and sticking out to the anti ventilation plate for a reference
Motor Transom Height

10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a digital tachometer and hour meter here: http://cgi.ebay.com/ebaymotors/Tacho...Q5fAccessories $ 35
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. Gas ________
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by just pushing the throttle at the helm and not at the carburetor, checked compression, and looked at the plugs and checked spark as well as timing and advance, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.



H

thanks.... I may end up doing that, but the question is more for understanding, than it is to figure my set up.

I sometimes wonder why some boats have a 23 pitch prop...and mine can only use between maybe 13 and 17...if i need to stay in the right rpm band

but they dont seem much different in many ways than my boat or set up

while id love to go fast once in a while it isnt critical for my usage...

bob

 

[Silvertip]

Re: whats with this prop estimate black magic????

It is exactly the same thing as you describe. Only thing is the 6 HP is propped with about 6 - 8 inches of pitch where the 90 is propped with about 19 - 21 inches of pitch. Rummage around your house and find two boards of equal length. One should be about an inch wide and another about six inches wide. When you get to the water next time, use the boards like a paddle. The wide one (high pitch) moves through the water much harder than the small one does it not? So the small board would be no more difficult to move on a barge than on the smaller boat. The big one would not even move the barge but on the smaller boat it would. Simple physics. The prop is the transmission for your boat. Need more power you gear down (less pitch). In other words, the 90 HP engine can't rev to its recommended WOT because it has too much prop pitch for the size boat it is pushing.

 

[bob johnson]

Re: whats with this prop estimate black magic????

ok, to continue real life scenarios...if i backed my trailer down on a steep ramp and kept the boat tied to the trailer and started my motor... and reved it in gear....if i am far enough from shore i wont lose my water supply....will the engine not rev very high?

are you saying that i wont be able to reach the WOT that the combo normally would produce?

the boat wont go anywhere...

I was going to fill out the form whsiii supplied, but I dont have my manual at work..and on the net it is impossible to nail down what the gear ratio is for my motor..

the problem i have looking up anything on my motor, is it is a unique model..

the ocean pro( S models) is a lot different than the M models

I know the intruder 115 is the same basic motor as my 115 60 degree block looper.

but its ratio is listed as 2.25:1 and WOT is 5000-6000

all other info I find matches the special 115( not 112!) at 2.0:1 and 4500-5500 , which is the same as the 112, and m series 115...

I think i found out that my lower unit does not interchange with the cross flow motors...

if anyone knows what the 95-97 ocean runner and ocean pro lower unit gear ratios are , let me know...

i have one of each,...and I actually just put the 97 on my 95, because i used a S/S prop during hunting season and hit something..and bent my prop shaft bad.

bob

 

[smokeonthewater]

Re: whats with this prop estimate black magic????

differentt engines have different gear ratios which also affects prop pitch selection but YES you are exactly right.... if the load is increased the prop pitch must be lowered or the rpms will go down..... if you were able to reach the proper wot rpm with your boat on the trailer that would be a STRONG indicator that you were under propped....
your 6 on the back of your 20' boat is propped for the job or it would lug... put the same engine on the back of a square stern canoe and it will over rev.... re prop it to bring rpm's down at wot and it will gain alot of speed.... return it with the new prop to the 20' boat and it will lug and make less speed than it currently makes

 

[QC]

Re: whats with this prop estimate black magic????

if i backed my trailer down on a steep ramp and kept the boat tied to the trailer and started my motor... and reved it in gear....if i am far enough from shore i wont lose my water supply....will the engine not rev very high?

OK, I'll bite. Yes, the engine will not rev very high assuming your trailer and truck stay put

are you saying that i wont be able to reach the WOT that the combo normally would produce?.

Yes . . . it won't reach WOT RPM unless it started to ventilate or it pumped all of the water out of the lake. You kinda covered this with the deep enough in the water thing. This is what matching an engine, gear ratio and propeller to the boat is all about.

ok, to continue real life scenarios....

If that's what you call this exercise, then OK . . .

 

[trendsetter240]

Re: whats with this prop estimate black magic????

Bob, definitely fill out hwsiii's form. You will learn a lot about your setup and get the best possible prop as an outcome.

p.s. Just remember to wave a dead chicken over the outboard after you install the new prop.