Educate me again on prop efficiency

[achris]

Re: Educate me again on prop efficiency

The slip results come from the BAM slip calculator.

Just did a search on that calculator... It raises a few questions..

1. What unit of speed it to be used? (miles per hours, kilometres per hour, knots)
2. Where is the allowance for cupping?
3. Where is the input for prop diameter?

The fact that you consider my boat's excellent performance laughable is no concern to me.

I didn't say the boat's performance was laughable, I said the resulting figures were laughable, as in 'just not possible'. That would require the props to be running at less than a 2 degree angle of attack... As I said, just not possible...

For the record, I did not suggest that Solas props were better than Mercury props. My testing indicates they are as good; and a better value since they cost on average $50 less in the aluminum category.

The fact is they equaled the Merc prop in efficiency and every one runs smoother than the original Merc prop.

My boat has a CAT hull, which might help account for it's pretty good numbers.

Nope.... It has nothing to do with the hull, it's all about the prop. It still needs to be 4 degrees (+/-1) to be 'efficient'. Less produces less thrust, thus being unable to efficiently use the available power, and more means power lost in overcoming to steep an angle of attack....

As I asked. Post up your pitch, diameter, boat speed, engine revs and drive ratio and I'll run them through my calculator....

Chris........

 

[jestor68]

Re: Educate me again on prop efficiency

Bear with me; working on my first cup.

Here goes: 1.81 gear ratio. Alpha 1 gen II. 4.3L(220) Merc prop that came on the boat is a 15.5 X 18 Black Max. 4600 rpm @39.3 mph. Bam says 9% slip. Ran this prop for years, satisfied with performance.

First Solas prop(for ski pulling) 15.5 X 17 Amita; does 4750 rpm @ 38.1 mph. Bam says 9% slip. Prop gives better acceleration and less ventilation in turns.

Tried a 14.5 X 19 Solas and got results I predicted; 4550 rpm @ 40.3 mph. Bam says 11% slip. Smaller diameter not suited to my gear ratio.

The 4 blade actually proved less satisfying(except for the smoothness) than the larger diameter 17 for ski pulling.

Presently have a Solas 15.3 X 19 that I have not run yet. Hoping to use this as my light load cruising/fishing prop provided it operates within the rpm range(plan to test it this week).

All props are cupped. The Mercury Racing calculator gives me the same result. It asks for the same info as the Bam calculator.

Your remarks, like"it has nothing to do with the hull" bothers me. That's like saying all 22 foot boats will perform the same with this particular prop, regardless of the hull design.

 

[achris]

Re: Educate me again on prop efficiency

Just ran the numbers...

Your first prop. (18" black max). Slip 25.6% (AoA = 6.6?)
17" Amita. Slip 26.2% (AoA = 6.6?)
19" Solas. Slip 26.5% (AoA = 7.3?)

And I worked out why you are getting such low figures. BAM calculator wants knots, not mph ... and it doesn't take into account the cupping on the blade trailing edge. :facepalm: (If you look on their explanation page it actually tells you to add 1" for cupped props)

If you'd like to test what I'm saying on BAM calc, use the figures from my boat.
Ratio - 1.62:1
Pitch - 19"
Diameter - 14"
Engine revs - 4600 (checked on a calibrated tacho)
Boat speed - 49.7 (by GPS)

My calculations show that as a 20% slip, 5.6? angle of attack.

If you put in knots and add an inch for the cupping (I know that those props are all cupped), you end up with the same figures I get.... Try it....

Ok... you need to read up on props. What angle of attack is, how diameter effects propeller performance etc etc etc...

Have a look at this site. Read through all the clickable pages.. About Propellers

The comment 'it has nothing to do with the hull' refers only to the propeller figures, not how the hull will perform with any given prop...
This is the second time you have manipulated my words in an attempt to discredit what I'm saying, please stop doing it.

Chris.......

 

[Tail_Gunner]

Re: Educate me again on prop efficiency

achris are you trying to redefine how slip should be measured? I did take a close look at BAM"S site every reference used to measure speed is mph...no where did i find knot's.The reference comparing a 23 to a 17 and the same result's.....Think about that a bit. Merc has great prop's but they now where near dominate offshore racing not even close..

 

[emilsr]

Re: Educate me again on prop efficiency

My name is Ray, and I'm a propaholic.

Now that we've got that out of the way and before this gets anymore out of hand here's a few observations.....

I've never really found prop calculators to be that useful. Sorry, that's just my opinion. Better to go by the experience of others who have actually done the testing.

Offshore racing isn't "dominated" by any of the big prop manufacturers; Mercury, Solas, Michigan, etc. Companies like Herring make many (most) of them, and they cost in the thousands of dollars.....more than several boats I've owned. My guess is that none of us on here will ever run them, so that's a moot point.

Prop slip on a v-hull boat will ALWAYS be more than 10%. Sometimes way more. 15% is actually very good at WOT, and at cruise speeds 20% is a decent number. Racing cats with 5-7 blade custom props can sometimes get below 10%, but again that isn't the type of boat we're talking about here.

Why do some people report lower slip numbers? They aren't necessarily lying (unless you're a fisherman or a racer then all bets are off )....the problem is in the data aquisition:

Concerning speed, speedometers lie. Everyone knows that. GPS usually registers a few mph faster than radar....so which is correct? There's lots of debate on that subject, but the bottom line is that measuring speed isn't an exact science no matter which method you use. There is always a margin of error....a range if you will.

The same goes for measuring rpm. Tachometers lie too, but most folks seem to take them for granted and assume the indicated numbers are correct. Usually they aren't, so again we have a margin of error and a range (not an exact number).

Finally, measuring prop pitch is far from an exact science as well. If you were measuring a board flat blade at a certain angle to the hub then the pitch can be accurately measured....but that isn't how props are made. Some have more cup than others, some have a more graduated pitch throughout the blade (both towards the tip and fore/aft), and some are just guestimates by the prop's manufacturer. Once again (as with every data point but gearing), we have a margin of error. Just because the prop says it's 17" of pitch (or whatever) doesn't necessarily mean that it is.....just that the manufacturer says it is.

Add all those margins up and you can come up with quite a variance when you're doing the math (or letting a calculator do the math for you).

Back to the OP and his original questions/comments:

There is a reason someone swapped out the drive for a higher numerical ratio. That's a better option for your boat and the speeds it's capable of. You're better off spinning a slightly larger prop at a lower speed (which is what you get when you go from a 1.5 to a 1.84). If it was me I'd stick with that drive ratio. I'd also get your rpm up into the recommended range (preferably on the high side) but verify the accuracy of your tach before collecting a pile of props.

Finally, as for brands, it honestly doesn't matter. Blade size, shape, rake, cup....there are a lot of more important things than what brand is on the box. They're all pretty good, but some are better in certain applications than others.

PS: When you're testing, try to do so at the speeds/throttle settings that you'll actually be running. It does no good to tune for WOT if you run at a slow cruise all the time. The prop that performs the best at that speed is the "best prop". It may not be the fastest OR have the lowest slip numbers at WOT which is generally where the testing takes place.

Whew!!!! I'm done. Hope this was helpful.

 

[achris]

Re: Educate me again on prop efficiency

Thanks Ray, nicely put....

 

[Tail_Gunner]

Re: Educate me again on prop efficiency

Opinion's...soo much information so little time. This thread is all over the place .....small go fast boat's....offshore gofast boat's fishing boat's and aside from Jestor's post a lot of generalization's...Slip calc's are quite valuable in determining many thing's other than actual slip. And i do agree with some of the statement's..Try this to the op state the actual size weight and type of boat..actual gearing and condition age of the prop's...Fishing boat...deep v Id be willing to bet she's a 5000lb fishing boat.

As to low slip number's it Would be nice for 45 Auto to chime in...Now he has some low slip...and then there is duoprop's..ohh stepped hulls's etc etc etc.

 

[achris]

Re: Educate me again on prop efficiency

Opinion's...soo much information so little time. This thread is all over the place .....small go fast boat's....offshore gofast boat's fishing boat's and aside from Jestor's post a lot of generalization's...Slip calc's are quite valuable in determining many thing's other than actual slip. And i do agree with some of the statement's..Try this to the op state the actual size weight and type of boat..actual gearing and condition age of the prop's...Fishing boat...deep v Id be willing to bet she's a 5000lb fishing boat.

As to low slip number's it Would be nice for 45 Auto to chime in...Now he has some low slip...and then there is duoprop's..ohh stepped hulls's etc etc etc.

And you forgot variable pitch props... And choppers and cleavers..

 

[jestor68]

Re: Educate me again on prop efficiency

According to the Mercury slip formula, your boat's performance shows 3% slip. They use MPH; not knots.

Your method results in slip numbers comparable to a tub boat. No sale. I'll keep using Mercury's formula.

Earlier you stated you had not seen a Solas prop with a angle of attack of less than 10 degrees.

I'm not going to add an inch of pitch for the small amount of cup in the aluminum prop. I can see it with a SS prop; not aluminum.

All your techno babble smoke screen has done is make the Mercury prop appear as bad as the Solas.

 

[jestor68]

Re: Educate me again on prop efficiency

ROFLMAO... really!

So why does it take a 23" Solas prop to keep the revs down to the same as a 17" Merc prop, that actually pushes the boat faster? Yes I saw the results first hand..... And why do all the high performance racing and endurance boats use Merc props? If that doesn't prove, beyond all reasonable doubt, that the Solas is so much more inefficient, then what do I have to do to convince you?

Since Solas does not make "high performance" props(except one bass boat prop), how does what racing teams use have anything to do with it.

 

[jestor68]

Re: Educate me again on prop efficiency

I'll test the other Solas prop today. When I'm done, I'll plug the numbers into The MERCURY slip calculation formula and see how it does.

Based upon what the others are doing, I figure it'll score something between 10 and 13 percent; using MERCURY's U.S.A. method of figuring slip.

 

[jestor68]

Re: Educate me again on prop efficiency

I adjusted my numbers to the next higher whole number(as Mercury uses whole numbers in their calculator) because I realized that the values were very close to the tipping point of rolling over to the next higher value.

So the new scores for the Merc is 10%. The 15.5 X 17 Solas gets 10%, while 14.5 X 19 scores 12%.

As a side note, I was reading one of the many articles on proptology today, and the "expert" author mentioned that an inch of diameter was the equivalent of 2-3 inches of pitch. I always thought it was more like a 1 to 1 relationship. So, if you buy his theory, that could explain your experience with the larger diameter 17/ small diameter 23 performing in a similar fashion. After all, it's not uncommon to see as much as a 2 inch difference in diameter in props 6 inches apart in pitch. I cannot recall seeing a 23 pitch and a 17 pitch of the same(or even close) diameter.

 

[WSKIPPER]

Re: Educate me again on prop efficiency

WOW...what a boatload of useful prop info...Thanks guys...it's been a bit since the strife of life has allowed me the time to get back and finish my testing...the sweet used deals fell through on the Merc props...sooo...I bit the bullet and went ahead and ordered the Solas 4 blade 23 al...a quick trip to the pond and the RPM's were most perfect...4750 wot...42.5 to 43 mph/GPS...sorry...forgot to switch to kts..oop's...what I did notice was a reduction in hole-shot with 4 people onboard(2 over 250lbs)...30gl of water...100gl of fuel...avg gear...6000#of Sea Ray 250da...5.7 magnum 4bbl repower...1.84 alpha...no over-rev's at all with this one...I'll get back and do some hole-shot/cruising comparisons with the Solas 4 x 21 and 23...same load...prop is so very smooth I must say...and no over-rev!!...cruising rpm's around 30mph were lower as well, by a few hundred, but my time was short on this trip...I'll post more accurate numbers with the next run...Thanks again guys...fantastic info for the less experienced as myself...William