# Looking for prop explanation, not just answer



## shooter5635 (Nov 7, 2016)

Alright. I picked up a 2017 Alumacraft MV1648 NCS and I LOVE it. It's incredible the difference in this boat vs my previous flat bottom. 

Anyway, I threw my 1989 2 stroke evinrude 15hp tiller on the boat. 2 guys plus hunt load trimmed out correctly and I'm traveling 18.9 mph at 6200 rpms. I have a Solas 9.25 x 10 prop on the motor now. Prop slip calculated out to be 22. I heard that you want prop slip around mid teens for your average boat. I read up on prop slip, and understand what it is but I read on this forum that lowering prop slip will not help performance which just doesn't really make sense to me. Can someone offer an explanation? 

I'm also looking at getting a new prop if it'll help the motor. I found an evinrude spec chart (https://www.rubexprops.com/boat-propellers/rpm-range/?make=Evinrude) and I'm 800 rpms under max rev (7000 per the chart). I used the prop calculator in the sticky and it suggested I go with a 9 x 9. 

Thoughts? 



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## wmk0002 (Nov 7, 2016)

I have 2016 1648 NCS flat bottom with a 1992 15 hp Johnson. I run about 20 mph with a light load and 2 guys at 6250 rpms with an old factory 10" pitch prop. Slip calculates out close to yours. With the same load and a newer factory 8" prop it runs 6800 rpms and does about 18-19 mph depending on load distribution. Prop slip on that one is closer to 11%. IMO both props have good enough holeshot. The boats draft so shallow, it doesn't take much to get on plane.

Right now, I like the 10" prop as I feel I get better mpg's. I run either prop wide open unless I am idling using my sonar so the lower rpms seem to help.

Lower prop slip is always better but, for my hull at least, in order to achieve it it requires a smaller pitch prop which still drops top speed despite being slightly more efficient. I'm sure it gets better as you go up closer to the max hull rating of 35hp.


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## DaleH (Nov 7, 2016)

I for one would NEVER put a prop on the boat that didn't allow the OB to turn screws_ to be no more than 200 RPMs_ off the started maximum (which may be higher RPMs than where it was 'rated'). So in your case, with that 7K max motor, I'd pitch her to run up 6,800 with normal gear, me alone and 1/3 to 1/2 fuel. 

One can chase their tail and spend good $$ optimizing prop slip, but to me, if you invest in a premium aftermarket prop, then you'll be way ahead. In the smaller motors I've had great success with the new pressed aluminum 4-blade props by Turning Point, while in my old tuna boats (now all sold off ...) I ran 4-blade PowerTechs on the big Suzi 4-strokes. Merc _Inertia_ props were always held in high regard for V6 motors, but the Suzi 250s and up run a 16" wheel, otherwise those Mercs props were just too small in diameter.

Prop slip cannot be eliminated ... but it can be minimized!


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## shooter5635 (Nov 7, 2016)

So, if I do reprop to help minimize slip, it seems I will lose some speed. What will I gain? 


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## DaleH (Nov 8, 2016)

shooter5635 said:


> So, if I do reprop to help minimize slip, it seems I will lose some speed. What will I gain?


Doubtful you will 'lose' speed ... 

What you will gain is a longer running (longevity) motor, improved efficiency at cruise and WOT speeds, and better overall handling, if indeed the right prop for the motor and boat as YOU have rigged it. 

Have you tried the prop calculator posted in the OB Forums? 

Note you are not chasing either 'slip' or 'speed' per se, but you are trying to *optimize *(all-around ... miles per gallon, take off thrust, torque, load-carrying ability, handling, efficiency, etc.) ... and by not over-loading the motor by adding too steep a pitch prop that the boat wears. That optimization is a net balance of all of those factors.


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## shooter5635 (Nov 8, 2016)

DaleH said:


> shooter5635 said:
> 
> 
> > So, if I do reprop to help minimize slip, it seems I will lose some speed. What will I gain?
> ...



This is more so the information I was looking for. Thank you very much! I just didn't know how to phrase the question. 

I did use the prop calculator. One issue is I don't know how specifically much my gear weighs, but I estimated it out and it recommended a 9X9 prop. I asked that they contact me so I'm going to bounce some ideas off them and see what they say. 


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## richg99 (Nov 8, 2016)

Just trying to learn something here. 

The prop chart that I looked at for a 15 hp 4 stroke 1989 Evinrude said 5500 / 7000 rpms
https://www.rubexprops.com/boat-propellers/rpm-range/?make=Evinrude

So, when sizing props, does one always try to max out at the 7000 figure?
If you were "in the range" would that be sufficient?

It seems that the OP has a 27-year-old motor that is running "OK"? Why fix what isn't' broken?

I readily admit that I know darn little about prop sizing. Never changed a prop in the many boats that I owned, other than to go to Stainless after ruining too many aluminum props on the oyster reefs here.

I was never concerned about maximum speed.

richg99


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## wmk0002 (Nov 8, 2016)

shooter5635 said:


> DaleH said:
> 
> 
> > shooter5635 said:
> ...



Sorry my post wasn't more helpful. 

I will add that a 4 blade prop like Dale recommended would likely have less prop slip than the same pitch prop in a 3 blade. Typically 4 blade props are associated with turning less rpms, having better holeshot, and being slightly slower than the same 3 blade prop but plenty of people claim otherwise. My theory is that a 4 blade prop on a small outboard on a non-performance jon boat hull can potentially give better speed than a 3 blade since typically the prop slip is so high. I'm thinking if you can drop 1 pitch and go to a 4 blade you could potentially still raise your rpms some over the 10" pitch and greatly reduce the prop slip. Sorry I have no science to back this. But I do plan on ordering a 9" Solas Amita 4 blade for my 15hp soon to test this out on. The hub is starting to slip on my OEM 10" prop so I need another anyways.


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## shooter5635 (Nov 8, 2016)

wmk0002 said:


> shooter5635 said:
> 
> 
> > DaleH said:
> ...



I didn't mean any offense by my reply! I've just always been one who likes understanding how things work. Please keep me posted on the 4 blade prop! I'm curious as to your results. 


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## DaleH (Nov 8, 2016)

richg99 said:


> Just trying to learn something here.
> 
> So, when sizing props, does one always try to max out at the 7000 figure? If you were "in the range" would that be sufficient?


We *really need Pappy here* to comment on loading of the smaller OBs, but with bigger motors, being "in the range" at the beginning of the season can mean you will be at or well down into the LOW end of the range during the season! Without a doubt I can tell you that from my years of experience with deep-V or offshore hulls or big/heavy boats (my 25' Parker was > 3-tons wet) and running 1 or 2 V6 OB motors mounted (that were all 2-strokes back then) ... that less not adding oil ... the *surest way to KILL the expected life of an OB motor was to run too steep a pitched prop and not achieve close to the max RPMs!* That's like running a truck with manual transmission around in 2nd gear and omitting 1st gear all together. It just 'lugs' the motor. 

Then again, the duty cycle of a 250hp motor is well beyond the demands of a small OB, IMHO. I recall Pappy once opining that it was the heat that kills the bigger motors, i.e., pistons scuffing, etc. 

And to me, the biggest point of all ... is that *boat's just get heavier as the season progresses*! You start to add more gear, the hull gets fouled more (saltwater boats) and in warmer Summer temps, the fuel is not as efficient, where normally aspirated motors can lose up to ~200 RPMs alone just from the hotter, less dense air. If you start out with a _marginally propped rig_ in the cooler Spring with a clean running bottom ... you are just going to lose RPMs as the season progresses. 

In the 80s and 90s I never saw so many blown OBs! Now my older bro had twin 150 Yams, 1987 vintage and we had his SS counter-rotating props tuned/tweaked by NE Propellers so that he was no more than 200 RPMs off the rated maximum RPMs whilst at WOT. He sold the boat 2-years ago and those motors (identical copies of the OMC patent [expired]/design) were still running flawlessly! My bros and I used to average running over a dozen OBs between us each year, 1/2 of them big V6 puppies. But with the loss of the fisheries, we all sold off our big boats with the big V6 motors. But none of our V6s were ever propped to run less than 200 off max RPMs when all typical gear aboard was carried, with 1 or 2 aboard and a 1/3rd fuel loading. We never had any issues ... but we also decarb'd them every 50-hours with Seafoam by the shock treatment.


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## CedarRiverScooter (Nov 8, 2016)

Back in the day, I drilled holes in the prop hub to let exhaust out so prop would slip some during holeshot. There was an Evinrude tech sheet on this, it had dimensions on where to position the holes. Worked good, the exhaust only affected holeshot, did not affect top end.

Took some guts to drill into a good prop hub!

Just random info . . .


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## richg99 (Nov 8, 2016)

Sounds like Props are an art, not a science. richg99


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## Pappy (Nov 8, 2016)

I am following it for you now.
A couple general rules of thumb. 
Higher pitch props are more efficient in the mid-range to upper ranges of the boat/engine capability. 
Lower pitch props become less efficient as speed goes up. Diameter, rake, blade design and overall blade area plays a large part in this.
Remember the "speed prop" vs the "ski prop" era? 
Another given rule is that if you want to go fast then push a small column of water at a high rate (less diameter and more pitch)
If you need to push a heavy load you push a large column of water at a slower rate. (more diameter/blade area and less pitch)
This is where the diameter vs pitch comes in and where the questions of "efficiency" or slippage come in to play. 
Not every prop will do everything asked of it in any boat/engine combination but probably more so in a small boat like what we are talking about.
The engine should be propped out in the upper half or better of the recommended range. For this engine I would like to see 6500rpm or a bit better. 
I can go into the testing and using a base line and recording results and watching your slippage numbers change as you make your changes but every time I do all I get back is that cricket chirping sound in the background.


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## DaleH (Nov 8, 2016)

CedarRiverScooter said:


> Back in the day, I drilled holes in the prop hub to let exhaust out so prop would slip some during holeshot. There was an Evinrude tech sheet on this, it had dimensions on where to position the holes. Worked good, the exhaust only affected holeshot, did not affect top end.


That does work for some prop applications and the 4-blade prop I run on my 60hp has vented holes. They work like this ... at slower speeds they allow air to vent through the holes, so the prop slips and allows the wheel to spool up when applying throttle. At higher RPMs, the water pressure exceeds the air pressure, so the holes effectively close up.



richg99 said:


> Sounds like Props are an art, not a science. richg99


... and you would be quite right! My buddy and I ran the same motors, but had different hulls (his 22' deep-V, mine 25' heavy mod-V hull) ... using the same recommended 4-blade SS Renegade prop w/ high cup and rake, it lifted his stern and ran flat out 'de ballz' as the French would say, while on my hull it raised the bow sooooooo much I seriously had to deploy my trim tabs, which killed my fuel efficiency (as I ran an instant readout fuel computer hooked into my Northstar GPS).

I used to say_ 'Rigging the right prop on a boat isn't rocket science ... it's more complicated than that!'_


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## shooter5635 (Nov 8, 2016)

I guess my biggest question right now is how do I move forward without spending what I assume would be hundreds testing different props? 


I greatly appreciate all the input you guys have offered. I love learning how this stuff works and it's all making more sense with every post.


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## richg99 (Nov 8, 2016)

I am the least knowledgeable of the respondents, but.. I wouldn't switch Props unless I could "try it before I buy it". I believe that there are prop guys who will let you swap until you find the right one. 
richg99


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## Pappy (Nov 8, 2016)

CedarRiverScooter said:


> Back in the day, I drilled holes in the prop hub to let exhaust out so prop would slip some during holeshot. There was an Evinrude tech sheet on this, it had dimensions on where to position the holes. Worked good, the exhaust only affected holeshot, did not affect top end.
> 
> Took some guts to drill into a good prop hub!
> 
> Just random info . . .



That information came out of a need. 
Bass boats were in their infancy. They were short, all the weight was in the back of the boat and propellers were not advancing in design as fast as the boats were changing. Engines had trouble planing these boats. The hull bottoms were not very well designed and the bows pointed at the sky and the boat sat there and dug a very large hole in the water while slowly getting on plane. 
OMC had a 49ci. 75hp engine that really only developed power from about 3500-4000rpm on up....then it came on like a freight train. 
OMC marketing was touting the engine as a ski engine among other claims. Another marketing vs engineering story so never mind. 
We were tasked with making this engine plane out boats like the small, tail-heavy bass boats with a full load of persons and gear on-board. 
An engineering liason by the name of Dan Kostoff came down to our Stuart test center and started experimenting with adding exhaust gases into the driving side of the following propeller blade. Hole location and hole size were were all over the board during early testing and by the time we were done we had reams of test results and a mountain of brand new scrapped propellers but the data was solid. 
The basis for every ventilation hole you now see in high performance propellers or any propeller that needs to help get a heavier boat on plane originated here. What you have to appreciate here is that back then our test methods consisted of repeated runs on a measured course with a stop watch.....! If fuel data was needed we had to make precision runs at an exact RPM and trim angle and measure fuel usage over time through large burettes! In short, back then it was a long slow process. 
Dan Kostoff was brilliant and as solid as they come. He passed away a couple years ago and with him we lost an incredible amount of technical knowledge housed in a very good man. Sorry for the long winded ride down memory lane!


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## CedarRiverScooter (Nov 9, 2016)

Thanks, Good to know we have direct access to an industry insider here!


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