outboard prop slip measurement on a 1990 Macgregor 26S

[walt]

A good definition of prop slip is here http://www.go-fast.com/prop_slip.htm

At some rpm/gear ratio/prop pitch, the turning of the prop "screws" over some distance in time and this is the "theoretical speed" the prop travels. The theoretical speed the prop travels must be higher than the actual boat speed in order for the prop to accelerate water backwards and produce thrust. The term "slip" describes the relationship between actual speed and theoretical speed and this relationship is

Slip = (Theoretical speed - actual speed )/ theoretical speed

So.. I have a displacement hull sailboat (1990 Macregor 26S) with a 9.8 Nissan four stroke and have two very different props for this setup. I was curious about what RPM I was getting wide open throttle (WOT) particularly with the three blade 8 pitch prop in the picture below. So I recently measured the boat speed vs. rpm for the 8 pitch prop. I put this into a spread sheet with the theoretical speed and slip calculated. The results are below. (edit - all speeds are in miles per hour)

First notable thing about this is that the "standard 8 pitch 8.5 prop that isnt supposed to work very well because this is a displacement hull - works just fine as the WOT rpm with this prop is 5172. That is within the specified 5K to 6K peak rpm range. Im getting close enough to full rated HP with this prop.

Second notable thing is that the slip numbers are overall higher than what I expected. I had expected that the slip would be lower for the low speed and then increase as the boat pushed up against the hull speed wall.

Today after I took the measurements on the 8 pitch three blade prop, I replaced it with the 5 pitch four blade prop. In a couple days I will repeat all the measurements above with the 5 pitch prop. We will then see what the "slip" numbers over speed compares for the two props..

Some background..

First, here is the 8 pitch 8.5 inch diameter prop that the measurements above were taken on

This is the five pitch four blade prop I just put on and have not yet measured.. This is the prop that came with the outboard and I replaced it with the 8 pitch shortly after I bought the outboard.

Details on measuring RPM
I bought a little unit that you wrap a wire around the spark plug wire but I didn't trust the results so brought an oscilloscope to the boat and tapped into the wire that drives the spark plug coil. This is a two cylinder four stroke and there is only one drive wire for both spark plug coils so the drive wire gets pulsed once each revolution. Both spark plugs fire each revolution but only one of the sparks fires the gas mixture (its a four stroke). The measurement I took from the scope was "cycles per sec" and you simply multiple by 60 to convert this to "revolutions per minute:

Below is what the coil drive signal looks like. Its likely cleaner than what the picture shows as I didnt bother to get a very good ground.

 

[Joe]

Lots of data .... my only question is your referring to your Mac 26s as a displacement hull. It has a retractable keel and is water ballasted.... did you do your speed test with the swing keel up or down? I noticed two of your recorded speeds were above the theoretical hull speed based on a water line of 23.5 feet.... source It may not make any difference .. since you're just trying to test props for your boat... but it may be something to think about.

 

[centerline]

Joe is correct that the numbers that is obtained wont absolutely reflect the advertised specs for the props, but when comparing the props on the exact same test equipment, a reasonable comparison can be made between the props tested.. especially when the "test equipment" is the boat they are being used on.
the ring around the prop can make a difference also, depending on how close it is to the blades, not just in hull drag, but directly affecting the water flow and how it enters and exits the prop...

 

[Sumner]

Thanks Walt for doing this and don't leave us hanging too long. I think the little tach I bought as the result of the other thread on here is the same one you did. How did it compare to the scope readings?
Sumner
=======================================
1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...http://purplesagetradingpost.com/sumner/endeavour-main/endeavour-index.html
Mac Trips to Utah, Idaho, Canada, Florida, Bahamas

 

[walt]

A little more info on the boat for that data.. Its an old 1990 Mac water ballast (not the new style with the large outboards) with a centerboard. The test was done at Lake Havasu which is just under 500 ft elevation, temp in the mid 50's F and no wind, no current. Speed was measured with GPS. The water ballast was full (always is on the old Mac classics), centerboard down, rudder down, sails all down. This is a very light 26 foot boat and the outboards tend to range from 6 hp to 10 hp. The 9.8 hp outboard is a little more than this boat needs but its no doubt a nice motor for this boat. You can sail these boats where you shouldn't with the light weight swing keel and retractable rudder and the ring on the prop keeps the prop from munching the rudder after a grounding (my rudder has some gouges from pre-ring days). When I test the other prop, I will keep the conditions identical.

Regarding the rpm reading with that tach instrument that has a wire that picks up the signal off the spark plug, the picture below shows how mine was set up.

There are some settings for that instrument that are confusing as to what they do.. I tried 01 and 02..

01 says 2P1R which I think means two pulses per revolution
02 says 1P1R which I think means one pulse per revolution

There is an 03, (1P2R) that I never tried. This may however be the one that gives the correct reading (see below)

The outboard is a 2010 Nissan 9.8 hp four stroke and there is a single set of wires (one orange, one black) going to the coil assembly then two spark wire to the plugs from that assembly. I am making the assumption that the orange wire pulses once per revolution sparking both plugs each revolution but I think that is accurate. This outboard runs well and a data point for this is that in neutral, I revved the outboard up to where I was getting 100 hz = 6000 RPM. I was very cautious doing this and only kept if there for a second but at slightly higher rev level the waveform on the drive wire to the coil started to have out of whack timing and I think this is how they implement a rev limiter. The electronic ignition control simply starts screwing up the spark timing when it want to limit the RPM. Simple and cheap, possibly just software in a microcontroller.

So anyhow.. I think my scope readings where I got one pulse per revolution is accurate (multiple hz by 60 to get rev per minute).

I had that tach instrument (cant remember the name.. it was about $12 plus shipping on Amazon) on while I was taking the O scope readings

Scope RPM / Tach Meter (set on 02)
1884 / 925
2670 / 1300
3486 / 1710
4836 / 2410
5172 / 2520

So over a fairly large range, the Tach meter set on "02" was close to 1/2 of what I got with the scope. Earlier I had compared 01 and 02 settings and 01 was 1/2 of what 02 displayed.. I dont know whats inside the instrument but Im fairly certain that tach meter is showing about 1/2 of actual RPM (02 setting) at least for this particular outboard. There was thread recently where Brian had measured with the same meter (he didnt list what setting he used - Brian, do you remember?) and it also "looked" like his RPM readings were about 1/2 of what they should be. But he thought his outboard wasn't running good. I think mine runs just fine. I may try this later as the 03 setting looks like it may double the reading again which would then match about what the scope is getting and also make more sense.

Note.. any cylinder on a four stroke will normally have one pulse per two revolutions (the 03 setting says 1P2R). This would seem to be the correct setting for a four stroke.. but not everything makes sense. I will give the 03 setting a try next time out (tomorrow)

 

[twalker H260]

Are you measuring your boat speed in miles per hour or knots? If in miles per hour it would correspond better to theoretical hull speed.

 

[walt]

A different thread would be good for the theoretical hull speed discussion, its WAY off topic here. . I noted in the first post that all the measurements are in mph as that is the speed unit the equations related to prop slip use.

FYI, I think the speeds I posted are accurate.

 

[twalker H260]

Sorry - I missed your edit to the original post that it was in MPH. I didn't go back to reread the op after my initial read on the thread. Didn't want to get into a discussion of theoretical hull speed at all but the numbers do match closely after converting from MPH to Knots. I'm sure that your speeds are accurate. I would not expect much in the way of current or tides in your location.

Looking forward to part II with the second prop.

Have a Happy New Year!

 

[walt]

Sorry also.. people are used to seeing speeds in knots on sailing forums. The equations in that link in the first post are in mph so all the measured speeds I post in this thread will be in mph to be consistent. FYI, I think the second prop will go slightly faster than the one I posted data on .. but I have already played with both of these props, just didnt measure rpm.

 

[walt]

I took the measurements on the second four blade five pitch prop today.
First, here are the two props. On the left is the three blade D= 8.5 inch, P = 8. On the right is the four blade D=8.7, P=5

The table below is the three blade 8 pitch prop (also posted above). All speeds are in MPH. Theoretical speed and slip were calculated in a spread sheet.

The table below is the four blade 5 pitch prop. All speeds are in MPH. Theoretical speed and slip were calculated in a spread sheet.

The image below is the speed vs rpm of the two different props plotted. Speed is in MPH

Some discussion to start..
I was surprised that the "standard" three blade 8 pitch prop had a HIGHER top speed than the four blade prop however its only about 4% faster. The three blade prop got the boat up to 7.7 mph, the four blade prop only got the boat up to 7.4 mph. Interesting that if your trying to get every bit of speed possible out of a prop because of current, the standard prop is better than the high thrust prop.

In the past I had measured the speed of the two props at about 5100 foot elevation and the three blade prop got the boat up to about 7.1 mph and the four blade up to 7.4 mph. Interesting that the four blade prop gets about the same speed at either elevation and I think this was because the outboard rev limiter wont let the rpm go much over 6K so probably had about the same rpm at both elevations. At 450 ft elevation, the outboard has more power and got the three blade prop up over 5K rpm. I didn't measure at 5100 ft elevation but the rpm was likely around 4800 peak.

The second surprising thing is that the 5 pitch prop has lower slip numbers than the 8 pitch prop. 8 pitch prop is in the .5 plus range, the 5 pitch prop is in the .4 plus range.

Also, this is copied from the first link

5% on a light cat to 25% on a heavy workboat. Most hi performance applications will be between 7 and 12 percent.

Interesting that with the under 10 hp outboard on the displacement hull, the slip numbers for either prop are way higher than the range given in that link..

FYI, I still dont like the 5 pitch prop, you have to have the RPM's up for any speed and its more vibration and noise on this boat.. cant wait to get the 8 pitch prop back on.

 

[walt]

FYI, for the tach instrument, the "03" setting and connected to the spark plug wire like I showed a couple posts back looks like it gives the correct RPM for a Nissan or Tohatsu 9.8 four stroke.

 

[walt]

A couple more notes about the two props.. The 9.8 Nissan in the extra long shaft comes standard with that four blade 5 pitch prop. I think its too low of pitch for at least the boat I have. But it will also run the boat significantly slower at idle than the other prop. I think this is a better fishing trolling speed, maybe this outboard gets sold often as a kicker for fishing boats. Second, the four blade prop that allows the exhaust to exit either end of the hub does have a significantly better reverse. So good that you have to be careful as outboard mounts are good at handling the thrust when going forward but the mechanism that holds the outboard down in reverse is a LOT weaker. Just testing this yesterday I think I put a lot of wear on the reverse lock down mechanism.. even had it pop out once from all that force (NOT good..).

Finally, this is a picture of a rough day at the office yesterday morning (I recently sort of retired). My partner is checking to make sure there is no wind and also looking out for pirate ducks.

 

[Johann]

Nice data, thanks for sharing! It seems that the 5" is way under pitched, and the 8" is just slightly over pitched. I wonder how the 7" 4 blade high thrust would perform...

 

[walt]

that sounds about right to me.

If you look at the equation for theoretical speed, it is based only on the prop pitch. Theoretical speed and then slip have no variable for prop area. Theoretical speed indicates how fast you are "throwing" water out the back end compared to your actual boat speed and you can get a force out of this by either accelerating the water faster OR by moving more mass (F=MA). So a prop with more blade area would in theory push out more water and could get the same thrust (force) with a lower acceleration. My guess is that the four blade prop has higher blade area and that is why it shows a lower slip number. (equation from the link in the first post)

Sumner has that 7 pitch four blade prop and a new tach.. maybe sometime he can anwer that question with some data. I dont have any problem at all with the four blade prop other than it has to operated at about 1K RPM higher than the three blade prop for most speeds I motor at and its a lot nicer (less vibration, less noise) at the lower RPM.

My opinion is that the 7 pitch four blade prop is likely very nice for a sailboat about this size. The three blade just looks like it probably is more efficient in forward with the more streamlined hub and swept back blades optimized for one direction only - and has slightly better gas mileage but its likely just not that much to matter for how far most of us motor. The big hub for the 7 pitch would still be very good in reverse.. just be careful as that outboard reverse hold down isnt built for real high thrust in reverse (as I found out yesterday)

 

[Sumner]

Nice data, thanks for sharing! It seems that the 5" is way under pitched, and the 8" is just slightly over pitched. I wonder how the 7" 4 blade high thrust would perform...

Note: Walt answered before I got this up.

I have the same outboard and did replace the 5 with the 7 four blade and like it but have no comparison between it and the 5 or the 8 Walt is running. I've also never tried to run at hull speed and don't have a tach for rpm comparisons (did just buy one). Noise wise I like to run between 5-6 kts and sometimes less if I'm in no hurry and want even less noise and have to motor.

Walt thanks for the tack info and also the prop info. I feel that the 5 is for those sail boats that are a lot heavier than our Macs. I believe there are some that are in the 5000-6000 lb. range that also have more freeboard than ours and the 5 probably works well for them, especially getting in and out of a slip or off a dock. Something like a Catalina 27 that could go over 7000 lbs. with crew and supplies.

One last note is that running around 5 I seem to get right at 10 mpg with a boat that is way heavier than stock and I moved the water line up about 1 1/2 inches and it sits on that at the beginning of a trip. Also like Walt said you do have to limit the throttle in reverse but I feel the 4 blade might not require you to rev the motor as much to get the thrust you might need at times in reverse. I have no plans on changing the prop. I do carry the 5 as a spare along with spare mounting gear but haven't had to use it.

I also have the prop guard which I think might of actually resulted in more grass clogging up the prop a couple times in Florida but that grass was floating in big mats and not live grass on the bottom. I have it primarily to also keep the prop out of the rudder that does look like 'jaws' attacked it with one big hunk out of it,
Sumner
=====================================================
1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...
Mac Trips to Utah, Idaho, Canada, Florida, Bahamas

 

[walt]

FYI, I did this measurement because of two pieces of advice you see posted on the internet.

1. You need a high thrust prop for current.. (for my measurements, the three blade conventional prop is actually better than the high thrust prop in current)
2. You need a high thrust prop for a displacement hull.. Not always true. My conventional three blade prop works fine getting the outboard up into the specified RPM range of 5K to 6K RPM.

FYI, that three blade prop is the one sold stock on the short shaft Nissan/Tohatsu 9.8.

 

[centerline]

its not surprising to me that the numbers are as close as they are... due to the pitch of the blades, and the number of them on the 2 different props
if you were to keep everything about the props the same, EXCEPT swap the blade numbers respectfully, you would see much difference in the final results between the 5 vs 8 pitch blades... OR if one were to increase the size/area of the blades on the 3 blade prop, and decrease the size/area on the 4 blade prop.... because the surface area of the blade(s) pushing against the water makes a huge difference in numbers...
there are so many variables that can be changed when attempting to "prop" a boat, that even when at opposite ends of the spectrum of the process, with different porpellors, the final results can be close, due to one or more of the other variables being too far the other way...

 

[Joe]

If you had a displacement hull... the boat wouldn't attain the top speeds you're showing... your research only applies to the vessel being tested.... while the title of your piece suggests otherwise..... so I recommend you simply change the title of your research project to "outboard prop slip measurement on a Mac 26s" and leave hull type out of the discussion.

I would also suggest converting results to knots.... mph/1.2 = knots per hour.. because this is a sailing forum... for that matter you can use any standard you want.. the article you use as a guideline expresses results as a percentage.. rather than absolute numbers.

According to your data... top measured speed is 7.7 mph or 6.4 kts ..... my 27 footer, with a fixed lead keel and a 25 foot water line will not attain that speed under power.... the prop will start cavitating around 6 kts...

 

[walt]

I would also suggest converting results to knots.... mph/1.2 = knots per hour.

If you want to convert the numbers to knots, I would suggest dividing mph by 1.15. Your number would introduce an immediate error of over 4%.. (actual number is 1.150779)

Ok.. this thread can turn to theoretical hull speed.

I measured a peak speed of 7.7 mph or 6.69 knots (your number of 6.4 knots is inaccurate) under the conditions described previously. This boat has a water line of 23.5 feet. The equation for theoretical hull speed (knots = 1.34*(LWL)**.5 says this boat has a theoretical hull speed of 6.495 knots.

Did I violate the laws of physics...

. the prop will start cavitating around 6 kts

I dont think this is even remotely true unless you have a damaged prop. Can you provide some more background on this (I would much prefer some reference link). If you make up the answer, fine also, just let us know.

FYI, any hull has a drag vs speed curve and the wave making causes the drag curve to become steep in the range of the theoretical hull speed calculated number. If you are pushing the boat with a prop up near the t hull speed and increase rpm, the prop will push out more and more water - which is to say the theoretical prop speed increases (see the slip equation) but the boat speed increases very little because of the steep drag curve. Boat speed stays about constant but theoretical prop speed goes up = increase prop slip. If you look at the prop slip numbers I measured, they are going up when I get near the peak speeds. If I could have applied more HP, the slip numbers would have kept climbing.

 

[walt]

From this link.. https://en.wikipedia.org/wiki/Hull_speed

Wave making resistance depends dramatically on the general proportions and shape of the hull: many modern displacement designs can easily exceed their 'hull speed' without planing.

These include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. Such hull forms are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferriesand other commercial, fishing and military vessels based on such concepts.

Vessel weight is also a critical consideration: it affects wave amplitude, and therefore the energy transferred to the wave for a given hull length.

Heavy boats with hulls designed for planing generally cannot exceed hull speed without planing.

Light, narrow boats with hulls not designed for planing can easily exceed hull speed without planing; indeed, once above hull speed, the unfavorable amplification of wave height due to constructive interference diminishes as speed increases. For example, world-class racing kayaks can exceed hull speed by more than 100%,[1] even though they do not plane. Semi-displacement hulls are usually intermediate between these two extremes.

Ultra light displacement boats are designed to plane and thereby circumvent the limitations of hull speed.