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Author Topic:   Prop calculator, prop and actual results.
JohnJ80 posted 07-18-2003 05:05 PM ET (US)   Profile for JohnJ80   Send Email to JohnJ80  
I have a 75HP ficht Ram Evinrude 2003 motor on my D15. I installed a stilletto prop 13.25x15, the motor is all the way and its got a fin. Per the Evinrude spec, this motor has a lower unit ration of 2.03:1.

The prop calculator with no slip says I should hit 35Mph at about 5K - if I recall.

Took it out with the GPS today to just get some numbers on this. There were a lot of boat wakes so I could only get the trim up to about 50% before I started getting launched. Tach was at 5K give or take 100 and the GPS said that we hit about 41.9mph. If the water had been flat and i could have trimmed up higher, I think I could have easily gotten it up to 43-45mph.

If all is right, what that is telling me is that I probably have a different gear ration in the lower unit? Anyone know if Evinrude has different ratios available?

Anyhow, this is fun - lots of fast. But something seems to not make sense here....

I'm also wondering if were to put on trim tabs, if thisn't wouldn't help me to get even more speed in more varied conditions.

comments?
j

jimh posted 07-18-2003 06:14 PM ET (US)     Profile for jimh  Send Email to jimh     
In order to produce 42-MPH at 5,000 RPM, assuming 5-percent slip and your 2.03 gear ratio, you would need a 19-inch propeller. Perhaps you misread the markings on the propeller.

Another source of error could be the GPS readings. Maybe the GPS was having a bad day.

It is strongly encourage that when posting articles that the model name of the boat be clearly identified. The nomenclature "D15" is not used by Boston Whaler to identify any of their models in literature available to the customer.

JohnJ80 posted 07-18-2003 07:30 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
It is definitely a 13.25x15 prop on my 'Dauntless 15' (thanks Jim, good point).

The Al prop I had on there before was a 13.75x15. If I had put on a 19 pitch prop, the revs would have dropped a ton, and they didn't.

The GPS showed excellent signal strength, 4 or 5 satellites.

The GPS also correlated nicely with my fishfinder and pitot static speedo at all speeds until we were going 35Mph or faster. Then the FF started to deviate by about 20% (higher) and the speedo by about 3mph. So, I am pretty confident in the GPS.

If your prop calc is correct then the only thing remaining is the lower unit gear ratio.

j

David T posted 07-18-2003 08:35 PM ET (US)     Profile for David T    
Here is the math for calculating theoretical speed:

Engine RPM / Gear Ratio * Pitch / 1056 = MPH with no slip

In your case:

5000 / 2.03 = 2463 (prop RPM)

2463 * 19" Pitch = 46798 inch per minute / 1056 = 44.3 MPH

5% slip would put you just over 42 MPH

So your results seem in the ballpark to me...

David T posted 07-18-2003 08:37 PM ET (US)     Profile for David T    
Sorry - I was reading the posts wrong.

No way a 15" prop at that RPM is going 42 mph at 5,000. Your gear ratio would have to be more like 1.62

Marlin posted 07-18-2003 09:23 PM ET (US)     Profile for Marlin  Send Email to Marlin     
John,

Just so you don't feel all alone out there, the math doesn't work for me, either.

My 160 Dauntless with a 115 4-stroke runs 44-45 at 6000 RPM with a 13.25 x 16 stainless prop. Speed measured by GPS, 12 strong satellites plus WAAS, averaged on opposite legs. With 5% slip, the calculator (and my own math) predicts 41. I've verified the GPS in the car, checked the gear ratio, and verified the tach to be within 1-2%. All that's left is the prop, even though it's clearly stamped with the part number and pitch. Go figure.

Jarhead has the same boat and engine, and gets essentially the same results ( http://continuouswave.com/ubb/Forum4/HTML/001796.html ).

-Bob

jimh posted 07-18-2003 10:43 PM ET (US)     Profile for jimh  Send Email to jimh     
The math behind the propeller calculator is straightforward, and the forumulas used are explained in detail. A number of people have independently checked and all obtain the same forumula and results.

In my own experience, any time I have recorded field data and checked it with the propeller calculator it has produced very good agreement. The one figure you can't measure in the field, slip, usually comes out to be 5-8 percent.

Another possible source of error may be the Tachometer calibration. Often there are multiple settings on the tachometer (to adapt it to other engine brands and models) and perhaps this is part of the error in this case.

Bigshot posted 07-18-2003 11:29 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
HOLD UP PEOPLE!

No way in hell can a 15" prop at 5k with a 2.03:1 ratio do 41mph. My 17" with a 1.83:1 at 5500 on a Montauk did the best of MAYBE 42-43mph(full tilt boogey). Trust me I run the numbers until I get the best. I have tried 6 different props on my Montauk to do better, etc.....sorry.

Sal DiMercurio posted 07-19-2003 01:08 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Ditto on big shots post.
Keep in mind, gps will not give you the correct reading if there is current, as it measures your speed over the bottom not the water.
If your running against a 5 mph current your gps will tell you your going 5 mph less then you really are aver the top of the water, because your actually running 5 mph faster then the gps is reading over the bottom.
The same applies if your going with a 5 mph current, the gps will tell you your moving 5 mph faster over the top of the water, then you are really going.
Unless your on a lake where theres no current or horrizontal water movement, the gps aint gonna tell you the truth.
Yes, in a car it will be correct,[ because it's measuring your speed over land ] but i'v seen an awful lot of gps's that lie when it comes to speed over water.
In order to get the best gps reading you must hold your speed for about 1 mile, not 200 - 300 yds.
Sal
JohnJ80 posted 07-19-2003 10:49 AM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I don't disagree with any of this. I'm an engineer (electrical with 25yrs in practice) so I understand in detail the mechanics of the whole thing, how GPS works, measurement error etc..

I am going to do further testing with the GPS. However the fact that the rest of the instruments agree fairly well is troubling - lending credibility to the GPS numbers.

This was done on a river - a very large river (St. Croix)- the portion of which where a section of about 1/4 mile wide empties out into a lake like area nearly 1.5 miles wide (Lake St. Croix) and average depth of over 50'. So, the current is neglible. Also the measurements were taken cross current - or across the vector of the current. Also cross winds (neglible 0-5 variable). I also was able to show essentially the same numbers both up and down river (varying only in the decimal by about a 0.25 mph - about the max the current would be).

The GPS would show speed over ground and would include the vector component of the current. The fishfinder and the pitot static tube speedo would not show the contribution of the current but only show the speed through the water.

Both the pitot tube and the fishfinder seemed to be much less in agreement with the GPS at the last 10% of throttle speed. However, their agreement with the GPS at all speeds below that is remarkably stable and varied by the same percentage regardless of direction of travel (up, down, or across the neglible current). Both actual read faster than the GPS at the upper ranges (last 10%).

The motor, I believe, has a rev limiter capability in its software. So I doubt that there is much of a variance in the tach. In addition, the numbers appear about right at WOT and were consistent with the Al and SS propellers. Qualitatively, the engine sounds wide open at WOT and does not sound like it is straining or overreved. It just sounds, well,... wide open. So both quantitatively and qualitatively, I think this instrument is also correct - or at least, correct enough.

Adding the 13.25x15 SS prop dropped the revs about 150 rpm from the Al 13.75x15 prop - a combination the smaller diameter (increasing) and the more aggressive rake and cupping (decreasing). I doubt I would be able to turn a 19. If I recall, the expectation is that this would drop ~800+ rpm. That is alot of a WOT rpm of 5K. That also didn't happen.

So, it is a puzzle. I will do more testing, I had many of the same questions posed here. I think concentrating on the lower end gearing is/will be interesting - for one. Looking for any info that anyone may have on the gearing in this motor.

I think that I will also do some testing at other speeds - noting the rpm and seeing what comes of that.

Certainly, I am not trying to point out that the prop calculator is wrong or flawed. I am just confronted with this puzzle that I would like to solve.

Regards,
John

Perry posted 07-19-2003 01:20 PM ET (US)     Profile for Perry  Send Email to Perry     
I have a 2002 Honda 90 on my Whaler and with the 15 pitch Solas stainless prop that I use for pulling water skiers I get 2.1% slip at wot and with my Honda 16 pitch performance stainless prop I get -.2% slip at wot. I had my tach tested and Honda says my gear ratio is 2.3. I used 2 different gps with runs in 2 directions over several days. I just figured my props are very efficient.
Sal DiMercurio posted 07-19-2003 03:07 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
2% slip is almost un-heard of.
Not that I'm saying it's not true, just very, very hard to swallow.
Sal
Jerry Townsend posted 07-19-2003 04:25 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Sal - with all due respect - a GPS doesn't care if you are on water or running upstream or downstream, even with a significant current - or even flying in an airplane. The GPS velocity calculation is based on the triangularization from at least three satelites and your GPS unit and assumes that you are traveling in a straight line. Also realize that the sampling rate is relatively fast - in the range of 1 to 10 seconds - and if I recall correctly, 1 second. As such, a run of 1/3 or 1/2 of a mile would give reasonable accuracy for the calculation.

However, running with or against a current does affect the performance of the boat by changing the drag of the hull and the performance of the prop. As such, a current affects the prop calculations being discussed. Therefore, try to make these test on a lake or a large, slow river.

I have never played the prop performance calculation game - but will run a few tests and see what comes out of it. ----- Jerry/Idaho

Sal DiMercurio posted 07-19-2003 08:37 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Jerry, in Alaska I must travel up & down rivers with 8 kts of current.
One of my boats [ 32 ft ] only runs at 17 mph at wot, when i'm bucking the tide [ current or horrizontal water movement ] my gps reads 8 or 9 mph when I'm throwing a 5 ft wake & blowing spray 20 ft in the air & acually moving over the top of the water at 17 mph.
When i'm running down stream with a big outgoing tide with the current my gps reads 25 mph, yet I know i'm only moving 17 mph over the top of the water, yet 25 over the bottom of the river, so gps is "NOT" the correct way to see how fast your boat is running, because it's just not true when theres current.
As i'v said, the only time a gps is acurate on a boat to measure speed, is on a land locked lake where no current is present.
My gps on my boat can't give me a correct reading until i'v run at least a 1/2 mile minimum.
Theres times when I hit the throttle & know i'm well over 55 mph yet the gps is telling me 28 mph until it really trangulates & the shortest distance i'v found is right around 1/2 mile.
When you run 1/2 mile at 60 mph it only takes 30 seconds but 30 seconds at that speed is a loooong time if the water isn't just perfect.
Sal
Sal
Sal DiMercurio posted 07-19-2003 08:39 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Jerry, in Alaska I must travel up & down rivers with 7 - 9 mph of current.
One of my boats [ 32 ft ] only runs at 17 mph at wot, when i'm "bucking" [ running against ] the tide [ current or horrizontal water movement ] my gps reads 8 or 9 mph when I'm throwing a 5 ft wake & blowing spray 20 ft in the air & acually moving over the top of the water at 17 mph.
When i'm running down stream with a big outgoing tide with the current my gps reads 25 - 26 mph, yet I know i'm only moving 17 mph over the top of the water, yet 25 over the bottom of the river, so gps is "NOT" the correct way to see how fast your boat is running, because it's just not true when theres current.
As i'v said, the only time a gps is acurate on a boat to measure speed, is on a land locked lake where no current is present.
My gps on my boat can't give me a correct reading until i'v run at least a 1/2 mile minimum.
Theres times when I hit the throttle & know i'm well over 55 mph yet the gps is telling me 28 mph until it really trangulates & the shortest distance i'v found is right around 1/2 mile.
When you run 1/2 mile at 60 mph it only takes 30 seconds but 30 seconds at that speed is a loooong time if the water isn't just perfect.
Sal
JohnJ80 posted 07-19-2003 08:56 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
GPS measures speed over ground (or bottom).

Speedos measure speed through the water.

Therefore, if you are going head into a 8 kn current and your speedo says 17kn, you are going only 9kn over the bottom.

If you want to measure speed accurately in current with a GPS you either need to run perpendicular to the current (assuming the current isn't huge) or better yet, make known distance run with and against the current and average the two.

The same practice can be used for wind - up and down wind or across. If you average the two you pretty much get rid of the effect of the wind.

j

Jerry Townsend posted 07-19-2003 10:58 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Sal - the difference in our thinking is that you are basing speed relative to the surface of the water - and I think of speed as relative to a stationary surface. As such, the numbers you reference (17 mph wot, 8 - 9 mph and 25 - 26 mph) are correct.

GPS units give location/velocity information relative to a stationary surface at that altitude.

Speedometers (used on boats) are typically of a spinning wheel device or a pitot tube. A pitot tube is a tube designed to permit measuring two relative pressures which are indicative of speed. Both of these are measuring the speed of the boat RELATIVE to the water. Each of these techniques have pros and cons regarding cost, accuracy, maintenance, et.al.

Maybe we should call the reading from a boat speedometer as "water speed" - akin to the "air speed" common in the aircraft related areas. That is - the speed of the vehicle relative to the medium that it is in. All aircraft use pitot tubes to measure the speed of the aircraft RELATIVE to the speed of the air. In the case of an aircraft, the "air speed" is of utmost importance because the performance of the plane is dependent on that RELATIVE velocity. But, a pilot does not base navigational factors on the relative speed - nor should the operator of a boat.

But, getting back to the boat case - if I were in your boat, I would be basing everything on the GPS - say you were 34 miles from your destination, bucking an unknown but say a 10 mph current, and the speedometer was saying 17 mph. You would think that the estimated travel time would be 2 hours - whereas your actual speed is only 7 mph (as indicated by the GPS) and the real travel time is then about 4.9 hours. That difference of 2.9 hours might well alter your thoughts and plans.

Now of course, the relative water speed does have merit in some applications - such as some fishing endeavors, et.al.

But, as said previously, when making the prop performance calculations, make the test runs on as smooth and stationary water surface as possible. --- Jerry/Idaho

Sal DiMercurio posted 07-19-2003 11:26 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
When you sell a boat & advertize it goes 100 mph, that dosen't mean over land, it means speed over the water, "NOT" the ground under it.
My very best speed attempt was 196.56 mph on landlocked water, if I ran it in Alaska with the current, I could break the world record, but thats "NOT" how the speed of a boat is measured.
If I ran my boat from Pittsburg Ca to Sacramento Ca a distance of 60 miles at wot against the outgoing tide [ current ] it would take me about 1 hour & 15 minutes to travel that 60 miles even though i'm running "AT' 60 mph
"OVER THE WATER ", it would indicate I only ran approx 54 mph
Then when I got there & turned around & came right back catching the same outgoing current that I bucked all the way up, I would get back home in 45 minutes indicating my boat now runs about 66 mph instead of 54 which is "NOT" true.
To measure the speed of your boat, it "MUST" be over the "WATER", not the land under it & the only correct way your going to get that is with a speedo if theres current involved.
Or run it on water where current isn't present.
Believe me, I lived by the loran & gps in Alaska on the Bering sea & yes when "TRAVELING" a distance, then you must calculate the land speed on the gps or loran, but not when someone asks you how fast your boat is.
Sal

tully_mars posted 07-19-2003 11:50 PM ET (US)     Profile for tully_mars  Send Email to tully_mars     
This is interesting and it made me want to run the numbers on my Montauk with the 2002 75 Ficht. My top speed is 39.1 mph marked on a GPS (multiple times on different days, this is an average), with a 13.75 x 17 prop, 2:1 gear ratio. The calculator came up with like -3% slip!! How is that?

So I also ran the numbers on my 23 Conquest and came up with -2% slip or so, (can't remember exactly on this boat but it was neg. also). I am deadly sure about the prop sizes and gear ratios and speeds also.

Tully Mars

tully_mars posted 07-19-2003 11:52 PM ET (US)     Profile for tully_mars  Send Email to tully_mars     
An adder here, could the negative slip be the effect on cupping of the prop? From what I have understood, cupping can act as additional pitch on the prop, and the calculator doesn't seem to factor this in. Maybe this answers the original question?

Tully Mars

jimh posted 07-20-2003 07:59 AM ET (US)     Profile for jimh  Send Email to jimh     
It's not the calculator that is not factoring cup into the pitch, it's the guy who made the propeller!

Some of these anomalous results may be due to the actual pitch of the propeller being greater than the pitch marked. Most propeller designs contain blades that are not a simple, constant pitch. They often have progressive pitch and a cup. If the boat is really well set up, it may be that at high speeds the extra pitch of the propeller comes into play.

However, I think some measurement error is probably responsible for the results mentioned above.

JohnJ80 posted 07-20-2003 10:35 AM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I am certain that I don't have a measurement error but I will check again. The results were reasonably verified on 3 different speedos - pitot tube, fishfinder and GPS. Since the results agree well and the measurements over ground and water speed correlate, then the current is a neglibile factor.

I think what is more likely, since the results shown with two FICHT motors differ strongly from the prop calculator, that the actual gearing is NOT 2.03:1 but something that would yield faster results.

I checked my prop and it is definitely a 13.25x15 - that is what is marked on the hub. I just checked it.

I also think that Jim's conjecture about variable pitch might be right. I know my boat is set up right - Jeez, I spent a ton of time getting that right and taking the empirical data to figure it out.

So, this takes out a lot of the variables - set up, speed measurement etc... About all that is left is the lower unit gearing.

Anyone with non-FICHT motors show results that beat the calculator?

J.

JohnJ80 posted 07-20-2003 08:52 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
ok. its official.

I repeated the run in both directions and got 40.3 going one way and 40.9 going 180 degrees opposite. Speeds are from the GPS over a one mile run (approx). Average is 40.6mph. Results are roughly the same as listed before with the speedo and fishfinder. GPS had good correlation and was in 3D mode.

Prop is definitely a 13.25x15. That is the stamped part number on the prop.

So, what is the collective wisdom on this? I think we can now rule out measurement error.

J.

Over the LINE posted 07-21-2003 11:27 AM ET (US)     Profile for Over the LINE  Send Email to Over the LINE     
J-80, you are not alone. I do not have the numbers with me , but my 13' runs faster than is possible, according to the prop calculator. The only thing that I can figure is Yamaha calls my prop a 15 pitch but that is not actual. The shop tells me the tach is accurate and I have GPS numbers with upstream and downstream averages. Thought about bringing up the same question you have, but did not feel like being "beaten up" over it.

Good Luck

Tom2697 posted 07-21-2003 02:25 PM ET (US)     Profile for Tom2697  Send Email to Tom2697     
I think jimh said it correctly. Most props are progressively pitched. The better (ie - more expensive) props will be better tuned by the manufacturer. If you don't believe this, try an identically sized, but cheaper, prop on your boat and see how fast your boat goes. There is even a company out there (can't remember the name though) that will tune a prop specifically for a boat. They even boast better efficiency and higher top end speed in their ads. Probably, this is what is happening...

As for speed...this is why speed records (land, air, or water) are ALWAYS taken over a one mile strech (aka - "between the lights") AND in two directions. Drag racing is another animal. These people care only about uni-directional speed and more importantly, acceleration.

For those who want to get really technical...speed over water IS speed...speed over land is velocity.

JohnJ80 posted 07-21-2003 03:32 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Actually, if you want to get technical about it - velocity is a vector and speed is the magnitude of the vector. ;-)

Anyhow, this whole thing is very interesting to me. I don't necessarily agree with the variable pitch arguement since that would also argue that the revs would drop more - higher pitched variable part should drop the engine revs. That doesn't seem to be happening.

j

Jerry Townsend posted 07-21-2003 04:53 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
John - technically speaking, you are correct. For those unfamiliar with the word vector - a vector has a magnitude and a direction.

But when we use the word speed, we are also talking about velocity - but not in the strict sense of the word. That is, frequently we use speed and velocity somewhat interchangeably and meaning some distance per unit time.

I have not followed this entire thread but will take time this evening to do that. However, I agree that a more progressive or larger pitch will, by definition, decrease the engine rpm.

Several days ago there was discussion regarding accuracy of the measurements. There is one measurement that I suspect everyone is neglecting and that is the tach. I would guess that the allowable error on a typical tach might be on the order of perhaps +/- 5%. These instruments are typically not high accuracy items and are intended to give a reasonably accurate indication or rpm. Of course, if one had an accurate and calibrated digital tach - that would be a different animal. ------- Jerry/Idaho

Tom2697 posted 07-21-2003 05:40 PM ET (US)     Profile for Tom2697  Send Email to Tom2697     
A vector is purely direction and magnitude. Velocity is direction and distance over time (aka - speed). Hence why "over land" usually has a direction associated with it....although, it is usually dropped off in daily conversation.

Progressive pitched props have a lower pitch at the leading edge which increases progressively across the blade face. This is generally to provide for better hole shot. However, marketing gurus can "work wonders" with designs. Just look at fishing line....20# class line marketed as "the strongest 15# line"...Many of us are guilty of buying it.

Could it be that we also buy a "smaller" prop for better holeshot but in reality, we buy the correct prop for our boat. Otherwise, why does one prop perform differently from another?

One last thing, variable pitch is different from progressive pitch.

JohnJ80 posted 07-21-2003 10:39 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
So to extend this variable pitch discussion (I'm still not sure I understand the difference between variable and progressive pitch). Does this imply that the prop calculator is not accounting for this characteristic. Would that be then only for a vanilla prop - maybe more like the standard Aluminum ones?

The prop calculator assumes a 'screw' through the water - a relatively straightforward calculation - and is specific. how would the variable/progressive nature of a higher tech prop relate to this?

J

Tom2697 posted 07-22-2003 10:59 AM ET (US)     Profile for Tom2697  Send Email to Tom2697     
The easiest way to understand a variable pitch propeller is to think about a light plane. Pilots adjust the pitch of the propeller for best performance while maintaining a constant engine rpm. A variable pitch boat prop works in the same manner. I believe a few manufacturers tried this in the past but nothing great came out of it...yet. I heard mercruiser is working on this for I/O's but it is still far from coming to market. Progressively pitched props have fixed blades that have a nonconstant pitch over the blade area. Gives sort of a "best of both worlds" performance.

My thoughts on the discrepancies that people are finding with the prop calculator is that this tool is simply a guide to aid in the buying process of a new propeller. One still needs to test run their boat with the chosen prop to determine if it is the best choice.

For a better understanding of how the tune of a prop can alter boat performance, check out this site:
www.propscanusa.com

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