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Prop size and compatibility


Blade

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Hi Raiders

Now that I have had a major service and tune up on my motor. The mechanic did a water test to sign off the motor he advised me that the prop that that I have is too small (it's a 16p vengeance)

He said the boat gets on the plane quickly but lacks top end and that the motor revs too high at WOT (6200 - 6300 rpm) he suggested an 18p Prop. But 18p seems to be an uncommon size (I am looking for a 2nd hand prop) 

I have found a 2nd hand 19p prop would this work on my motor. 

Motor and hull specs

Hull 2001 Stessl 5.5m Blue Water 

Motor 2001 Mercury Saltwater 90hp 2 Stroke 

Thanks in advance 

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It will work but like the 16p the question is how well?

As a rule of thumb each increase in pitch raises your top end speed about 6% and because it’s a more aggressive pitch it will reduce the wot rpm (motor working harder). But it lowers your hole shot performance. The 16p would have a lot of push at hole shot rpm (was the boat set up that way coz it carried a lot of load) so moving to 19p you’d lose some of that.

Ask your mechanic for his opinion on the 19p it all depends on what problem you are trying to solve.

PM me if you want to discuss over phone

cheers Zoran  

 

 

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18" or 19" pitch depending on the prop you choose . Vengeance is a very basic prop . If you go 3 blade Enertia is best. Vensura in you go 4 blade . It is trial and error with props.

If you close to Ermington boat ramp I have 17"  18" and 19" Enertia and 19" Vensura. Happy for you to try either of them and then you can decide what suits you best. You need to use your own hub kit. I only have hub kits for V6.

Not interested in selling 17" or 18" Enertia but I may consider selling 19" Vensura .

Edited by wrxhoon1
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Interesting subject that I have never had to think about just having a tinny but I like learning about these things for future endevours so doing some reading.

Sounds like there is a bit to it in the hull shape & weight balance etc.

 

Still I have to wonder why in a 2001 boat/motor combination its now coming to light 19 years later that the prop is the wrong pitch?

 

Just my curiuosity kicking into gear 🤔

 

Edited by kingie chaser
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Original owner may have regularly carried more load (more people onboard) or towed skier(s)/tube(s) Or been very stern heavy with his deployment- so needed better hole shot performance at the sacrifice of top end speed. 
Or this may not be the original prop - just the one the boat was sold with. 
Cheers Zoran 
 

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11 minutes ago, zmk1962 said:

Original owner may have regularly carried more load (more people onboard) or towed skier(s)/tube(s) Or been very stern heavy with his deployment- so needed better hole shot performance at the sacrifice of top end speed. 
Or this may not be the original prop - just the one the boat was sold with. 
Cheers Zoran 
 

So if what your saying is the mechanic is not nessacarily correct in his findings?

 

I can see a boat with 3 more passangers & full of fishing gear is going to perform a lot differently but will it still get to max WOT??

I know when I just have 1 other person on the tinny makes a lot of difference in my overall speed & as they are up front my hole shot is normally a bit quicker.

Doesnt mean I need a new prop though right!

I guess thats where you need to find the balance??

 

Of course I onlyhave manual trim so all things are not equal I guess?

Edited by kingie chaser
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I was just responding to your statement that it was the "wrong pitch" since 2001. It's not that black and white...right and wrong. There is a lot of ok grey!!!! Hence I mentioned there could have been factors which made that 16p work for the original owner.... the mechanic advised that it was too small - it's implied that advice resulted under the conditions that he tested it - I would additionally ask him if there was sign of cavitation? - if yes, then the prop is definitely too small. If not, then whether its the wrong prop depends on how @Blade intends to use it. 

Ideally you want to spin the biggest prop and pitch your motor can for hole shot (low RPM Torque) and for max speed (WOT)  - under your typical boating conditions and typical boat load and typical boating use ......

As @wrxhoon1 said choosing the right (optimum) prop can be trial and error.  Factors to consider:

  1. motor max recommended RPM (WOT),
  2. motor torque output at different RPM (the turning force applied to the prop at different RPM) -
  3. the propeller pitch and diameter: smaller pitch has more bite, bigger diameter has more bite - but spinning a too small pitch too small diameter too fast will cause cavitation (under propped) etc 
  4. the propeller design and material of construction( ally, s/steel, 3 blade, 4 blade): s/steel has less distortion than ally so will have more bite in the water requiring more torque, thinner blades will have less resistance so spin faster with same torque, etc.
  5. the boat hull design (deep V, flat bottom): V's sit deeper in water - need more thrust/prop bite/torque esp for hole shot etc 
  6. the boat hull balance (weight distribution)
  7. the intended use of the boat (typical hull payload, towing - skiers etc, racing)
  8. the water typical water conditions (calm water- estuary, lake; broken water-offshore)
  9. etc

Cheers Zoran

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5 minutes ago, zmk1962 said:

I was just responding to your statement that it was the "wrong pitch" since 2001. It's not that black and white...right and wrong. There is a lot of ok grey!!!! Hence I mentioned there could have been factors which made that 16p work for the original owner.... the mechanic advised that it was too small - it's implied that advice resulted under the conditions that he tested it - I would additionally ask him if there was sign of cavitation? - if yes, then the prop is definitely too small. If not, then whether its the wrong prop depends on how @Blade intends to use it. 

Ideally you want to spin the biggest prop and pitch your motor can for hole shot (low RPM Torque) and for max speed (WOT)  - under your typical boating conditions and typical boat load and typical boating use ......

As @wrxhoon1 said choosing the right (optimum) prop can be trial and error.  Factors to consider:

  1. motor max recommended RPM (WOT),
  2. motor torque output at different RPM (the turning force applied to the prop at different RPM) -
  3. the propeller pitch and diameter: smaller pitch has more bite, bigger diameter has more bite - but spinning a too small pitch too small diameter too fast will cause cavitation (under propped) etc 
  4. the propeller design and material of construction( ally, s/steel, 3 blade, 4 blade): s/steel has less distortion than ally so will have more bite in the water requiring more torque, thinner blades will have less resistance so spin faster with same torque, etc.
  5. the boat hull design (deep V, flat bottom): V's sit deeper in water - need more thrust/prop bite/torque esp for hole shot etc 
  6. the boat hull balance (weight distribution)
  7. the intended use of the boat (typical hull payload, towing - skiers etc, racing)
  8. the water typical water conditions (calm water- estuary, lake; broken water-offshore)
  9. etc

Cheers Zoran

Thans Zoran, like I said Im new to it so just tring to get an overall understanding.

I'll keep reading :biggrinthumb:

Edited by kingie chaser
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yes revs are to high . was this boat set up as a ski/tow boat.? before changing prop.the 1st thing to do is confirm motor is at correct height ,rule of thumb you should see top of cav plate level with water or slightly above  at wot once this confirmed then move on to prop selection .solas on the goldcoast is the go to for me you can use there prop caculator on line or give them a call if they still are operating in these strange times cheers dunc333

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Thanks all, only catching up on this thread now so to answer some questions. 

The boat was not used as a ski boat only for fishing with 2-3 people and gear.

Boat has a large live bait tank but not that large that you would need to go to a smaller prop. 

The mechanic reckons the previous owner must have damaged the prop that the boat came with and replaced it with this one because it was cost effective. 

I am looking to get a good balance of hole shot and top end wanting to try a 19p and see how I go. 

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So after a fair bit of searching I cannot find a 2nd hand 18p prop only 19p. Spoke with the mechanic and hw reckons 19p would be an over prop. 

Checked the mercury prop calculator online and they recomend an 18p in the vengeance and 19p in the aluminium. 

Is there any reason not to look at an aluminium prop in 19p? Much cheaper would be able to buy a new one.

I also came across a brand I haven't heard of Baeksan, any advise re this brand? 

Thanks 

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Odd number pitch is more common than even for some reason. If you are looking to buy new you can get anything you want. It is up to you if you want to buy ally prop, I wouldn't but I have a big V6 engine. The reason your mechanic suggested one" pitch bigger for ally is because ally tends to distort under load, S/S is much stronger. That's not to say ally won't do the job but S/S will do it better. 

Don't forget it is a bit of guesswork on the pitch that will suit your application best. A lot will depend on boat load and where the boat is used . Ideally you want a prop that will allow the motor to spin within the manufacturers recommended rev range when the boat has normal load when the engine is trimmed up just before ventilation starts. I prefer my engine to be close to the top specified rev range but at least midway .

If you know someone with the same boat as you have check what prop they have and what RPM they get at WOT

I recommend Enertia over Vengeance, a better all around prop in my opinion. Any S/S will perform better than ally.

https://www.mercurymarine.com/en-gb/au/propellers/sport-series/vengeance/

https://www.mercurymarine.com/en-gb/au/propellers/comp-series/enertia/

 

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I have a 2001 5.6 quinnie with a 90hp Honda 4 stroke and am running a 19.25 x 17 aloy prop as was on it when bought it and also recommend by Solas when I replaced 2 years ago.  She is no rocket with a top speed of about 25 knots, but probaly normal for a 4 stroke.  I have a 60 litre bait tank on back.  Ron

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1 hour ago, campr said:

I have a 2001 5.6 quinnie with a 90hp Honda 4 stroke and am running a 19.25 x 17 aloy prop as was on it when bought it and also recommend by Solas when I replaced 2 years ago.  She is no rocket with a top speed of about 25 knots, but probaly normal for a 4 stroke.  I have a 60 litre bait tank on back.  Ron

Hi Ron, could you please confirm its a 19.25 x 17 prop?  19.25 is a huge diameter for an outboard. Typical diameters that I have seen for larger outboards are 14, 14.25, 14.5in.  

The 60L on the back could be making your hull stern heavy - hence the 17p recommendation in your case. 

Not sure what year model your 90HP is, but I googled and found this:

https://www.webbemarine.com.au/listing/bf90-90hp-outboard/

Which shows a WOT 6300 RPM, and 2.33:1 gear ratio. 

You didn't say it, but if your top speed of 25kts was at WOT 6300 rpm then plugging the 25kts (28.77mph) and the above into this calculator: http://www.csgnetwork.com/marinepropcalc.html

Produces:

image.png.023c3a171b93e6ac0c7827d1102a8397.png

The conclusion in your case (and if you were at WOT RPM) is that you are getting a lot of prop slip (33.9%) with that particular alloy prop. There is potential for you to get much better traction/bite and hence performance (speed and fuel efficiency) with a stainless prop leveraging thinner stiffer blades and perhaps a more aggressive design or more blades.

As a comparison - on my current set up fitted with a 14.5 x 17p Enertia propeller I produce 5850RPM 70km/h implying I am getting around 3% prop slip. 

Could you please confirm the RPM you were running at for the 25mph data point, and if infact you can get to the WOT RPM with your current set up. ...  That would then be a 17p reference point for @Blade

Cheers Z

Edited by zmk1962
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