3 blade vs 4 blade prop selection

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Trolling, trolling speeds are often limited by hull length as in diplacement hull speed. Approx 1 knot per meter of hull length i think.
It takesminimal power to achieve hull speed, as soon as the bow lifts, the power requirements skyrocket. This is the worst economical rev range on a 2 stroke.
Keeping the nose down while trolling helps.

 Well put..
If one has a prop (pitch and slip factors) that has you sitting 1/2 way up on the bow wave, rather than behind it, the fuel bill goes right up up... If you are able to get that pitch right you should be able to troll without having to drop the bow for best effect.
BUT as said before.. its sort of a compromise...the ideal trolling prop may not suit the ideal cruise, or ski towing...and neither will get close to idea race prop.

This is the worst economical rev range on a 2 stroke.
Dosent matter if 2S 4S LPG diesel or hamsters in a wheel....its about the inefficient position the boat is on the bow wave... like slipping a clutch in a car with a big load on a steep hill.
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Post Options Post Options   Likes (0) Likes(0)   Quote MikeAqua Quote  Post ReplyReply Direct Link To This Post Posted: 25 Sep 2016 at 1:20pm
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I don't think I can increase propeller width (engine spacing).
you mean diameter.. the V4 omc hub?

Yes I meant prop diameter (currently 14").  The hubs are OMC 13 spline.

It's a two engine rig (3 cylinder 70HP Johnsons, made in 2001).  Engines bought for a song from OMC receivers complete with props.  

So at 4000 rpms what mph are you doing?

4000 RPM will give 23 knots in calm conditions with engines trimmed up.  That's a comfy cruising speed for the hull in moderate chop. 

RPMs are from the OMC tachometers.

GPS speed (SOG) from a Lowrance chart plotter, a Kus GPS-Gauge and a Samsung phone.  I have the most trust for the Lowrance but all three agree on speed.
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Yes I meant prop diameter (currently 14").  The hubs are OMC 13 spline.

Answered above post
"
In theory that could take upto approx a 15" diameter BUT they only make them to 14" diameter for the 13 spline V4 hub"

It's a two engine rig (3 cylinder 70HP Johnsons, made in 2001).  Engines bought for a song from OMC receivers complete with props.  
So at 4000 rpms what mph are you doing?
4000 RPM will give 23 knots in calm conditions with engines trimmed up.
Please confirm if the gearbox ratio is 2:1
if so slip IF 15 pitch is around 8%  and if 17 19% which is high

 but before these things are worked out.. things like total gross weight needed to be known.

Going back to the original post/ Poster
...
5.5 stabi, 90 hp  fully loaded 1500/ 1600 kg
Initial suggestion was slip was the problem, subject to power to weight ratios etc
As it turns out, slip is not the issue, but rather very under powered and too high a pitch.
Or put another way, a little 1960s mini going up the Bombays with 4 huge ppl inside, over loaded and the clutch sliping.
So reaches the correct speed for the power at the wheels that rpms.. but rpms/ power higher due to clutch slipping.

So the pitch  (final gear ratio at the road) needs to be reduced substantially....and then the slip sorted...at a lower pitch/ higher rpm/ higher power range.
Even so putting the rpm/ hp data together, could be looking at min 13 possible going as low as 11 pitch.

Going back over old sea trials of this stabi with a 90 hp on with 14x17" prop... 1 person up, basic std rig, same engine the slip from 3000 rpms thru to 5500 .. indicate even then the 90 hp is min powered.
Yet the recommended manufactures power range is 75 hp thru to 130 hp.   crunching numbers...ideally 120 to 140 with 3 ppl, fishing gear would be far better / sensibly powered.

hull ,  V4 motor, fuel, 1 person basic gear weighs in around the 750/ 810 kg.


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Originally posted by Steps Steps wrote:


"
In theory that could take up to approx a 15" diameter BUT they only make them to 14" diameter for the 13 spline V4 hub"

Please confirm if the gearbox ratio is 2:1
if so slip IF 15 pitch is around 8%  and if 17 19% which is high



Gearbox is 2.23:1

Pitch is 17" (stamped on prop)

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Would anyone have a 13×13 either 3 or 4 blade prop i could borrow on a calm weekend day to test rpms vs sog in the Auckland area. Must suit Johnson V4 13spline hub .
Any assistance appreciated.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 2016 at 10:32am
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Please confirm if the gearbox ratio is 2:1
if so slip IF 15 pitch is around 8%  and if 17 19% which is high

Gearbox is 2.23:1

Pitch is 17" (stamped on prop)

 Are you 100% sure its 2.31? Listings in my data base say 3 cylinder 70 hp '74 to 2001 is 2:42 the only 2.23 are 2S Johnsons under 8 hp and small 4S evinrude ????

 anyway

"4000 RPM will give 23 knots@ 2.23 17"   slip 9%  2 motors right?   getting down below 10% at cruise assuming a good power to weight ratio  is pretty good
 4000 rpm 23 knots @ 2:42 slip  1%  2 motors right? that lowers slip factors significantly but 1% at a cruise speed is a bit low.. 1% unrealistic
All depends on speed/ rpm accuracy and power to weight ratio and  correct gear box ratio 
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Originally posted by Steps Steps wrote:


Are you 100% sure its 2.31? Listings in my data base say 3 cylinder 70 hp '74 to 2001 is 2:42 the only 2.23 are 2S Johnsons under 8 hp and small 4S evinrude ????


I'm not 100% sure.  I looked at 4 online databases. 3 said 2.23, 1 said 2.42.  I don't have the owners manual any more. 

Model number is BJ70VLSIS.  I do have two engines.

Using 2.2.3 or 2.42 gear ratio either way it sounds like I have minimal slip going on?


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As has said before.. its not just slip.. its about power to weight ratios at any given rpms having enough difference of required power HP at the prop and the potential....
Get the boat powered correct 1st.. then prop for what going to be used for...and those rpms.
If not powered right for the end use.. its a matter of making as best possible use of a bad situation....

Using 2.2.3 or 2.42 gear ratio either way it sounds like I have minimal slip going on?

 read what I said again


"4000 RPM will give 23 knots@ 2.23 17"   slip 9%  2 motors right?   getting down below 10% at cruise assuming a good power to weight ratio  is pretty good
 4000 rpm 23 knots @ 2:42 slip  1%  2 motors right? that lowers slip factors significantly but 1% at a cruise speed is a bit low.. 1% unrealistic


All depends on speed/ rpm accuracy and power to weight ratio and  correct gear box ratio
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Post Options Post Options   Likes (0) Likes(0)   Quote MikeAqua Quote  Post ReplyReply Direct Link To This Post Posted: 27 Sep 2016 at 9:25pm
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1,227 kg (including me)

2 x 70 HP


2.42

The speed readings I trust - 3 separate GPS receivers agree on speed and distance covered.

The tachs?  No idea.  They match at full throttle.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2016 at 10:37am
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 on :
 2x 70 hp 140 hp for a trailered plaining hull total weight 1250kg
 Propped for general use/ fishing 2 engines cruise around the 7% slip WoT 3%
At 5000 rpms 36/ 37 mph WoT
19 pitch.
4000 rpms  26/ 27 mph around a 7% slip
estimating the hp/ rpm graph  @ 4000 rpms you have a good 30% difference between actual hp to required on a 19 pitch... min 15 to 30%

If my  conservative estimate of the rpm/ hp curve is ball park u could up to a 21 pitch blade same slip numbers to 30/ 31 mph  which would give better economy...bit more fuel per hr but get there faster.. less time.
But  around 4500 thru to WoT  you will be 39/ 40 mph Max load loading on the power head but loads distributed between 2 legs gearboxes on the 21 pitch.

"19 pitch.
4000 rpms  26/ 27 mph around a 7% slip"
Thats around the "26 knotts " that you are doing are currently doing IF data readings are correct

 All the info is spread around several posts above.. quick look cant find your current prop pitch numbers.
 A 1% slip at a cruise of 4000 rpms is unrealistic  19 pitch would give 30 mph  17 pitch 26 mph
And at a WoT 5000 rpms 17"  33 mph   19"  37 mph

Subject to data accuracy 21 would be bit high  19 more practical  Ideally 19 1/2 or 20" ....If u actually have slip as low as they are, there is no room to fine tune between the pitches by increasing cupping and/ or rake.



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I bought a photo-tachometer today, so I can check actual RPMs vs tachometers. Speed I'll just have to trust the GPS.

If everything checks out I'll try increasing propeller pitch from 14 x 17 to 14 x 19

(I can't find 14 x 20 or 14 x 20.5 in the correct hub)

I have a fuel flow sensor on the way so I'll be able to see the effect of different props on fuel economy.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep 2016 at 9:16am
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I bought a photo-tachometer today, so I can check actual RPMs vs tachometers.
$30 induction tachs of trademe work very well.. as mentioned before
Speed I'll just have to trust the GPS.

GPS units are accurate..  older phone units refresh times tend to be slower but good for constant speed. Accuracy depends on clear space between satellites rather than the units

(I can't find 14 x 20 or 14 x 20.5 in the correct hub)

 Correct same as trying to find a diameter over 14 for lower pitches.. as said before you have 2 motors and that gives huge grip.. far more If your data is correct or close than need....so going slightly small diameter should not be an issue.
 You will also.. as mentioned before not find 20 or20.5 pitches.
Pitches tend to go up in 2" increments in odd numbers.. fine tuning between is one with rake and cupping
As mentioned before, several very expensive after market and Merc do have them.. with interchangeable brand hubs

I have a fuel flow sensor on the way so
 As mentioned before.. cars do distance per fuel volume... tractors  bulldozers do volume fuel per hr.
Cars and general use boats stop start  vary speed, loads etc so .
 Bulldozers, big gin palaces tractors pulling a bailer or plugh all day are constant load
 Even L/ hr info for cruise on a car or boat will not be efficient.
This is why.
If I cruise @ 4150 rpms @ 68 km/hr I use .6 nm/L
If I cruise at 61 km/hr  I use .78 nm/L
cruise at 54 km  I use  .84 nm/l
 The faster I go the more fuel per hr I use but there is a balance  where getting there faster for the overall trip.. stop start, moving place to place, water chopps up , less trim etc my general best cruise speed  is around the 60 km/hr
Now throw in a curely.. with 115 V4 hp best overall was 53 km @ 3900 @ .75 nm/l
 The former numbers are 150hp V6
 the bigger engine uses more fuel per hr but faster cruises with butterflys far more closed results in significantly improved economys with similar slip numbers.


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Post Options Post Options   Likes (0) Likes(0)   Quote MikeAqua Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep 2016 at 5:56pm
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Yep nm/l is the parameter I'll be using.  

The fuel flow sensor will network with the GPS so it can calculate nm/l, display instantaneous reading and calculate a running average.

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