Props

[Bloggsy]

I have a 6.7 mtr aluminium boat (Barcrusher) fitted with a DF175APX. The motor came fitted with a 3X16X18.5R prop and I have a spare prop which is a 3X15X21R. Can someone explain what the effect of changing props would have on performance or fuel economy etc.and which would be best fitted to my boat which is only used for fishing off shore  

[noelm]

Your motor needs to be able to achieve close to the manufacturers max RPM with a normal load, your prop is what allows this to be "adjusted", what's your max RPM now, flat out?

[kingie chaser]

@zmk1962 is someone on here that has a very good understanding of props but there are some old threads on here you could search for to get info from.

Comes down to a few factors which you would need to know the info for your vessel & would be hard for someone to give you an exact answer without that info

 

But this is video is a good summary of determining your corret pitch.

 

 

Edited January 4, 2021 by kingie chaser

[zmk1962]

Thanks for the mention KC (@kingie chaser) ... the prop topic was covered pretty well in the post you referenced... but a chat often helps to to get the head around some of the concepts and sift through the jargon.

@Bloggsy send me a PM with your mobile and we can tee up a time that suits if a chat will help.

As @noelm said, it would help if you have some idea if you are hitting the MAX RPM on the motor with the current prop 3x16x18.5R prop and also maybe several data points of what speed you achieve at different RPM settings. 

With that in hand we can start to guesstimate what effect the prop change you propose in the OP may have -

On 1/4/2021 at 12:28 PM, Bloggsy said:

The motor came fitted with a 3X16X18.5R prop and I have a spare prop which is a 3X15X21R. Can someone explain what the effect of changing props would have on performance or fuel economy etc.and which would be best fitted to my boat which is only used for fishing off shore 

Basically the spare prop has a 1" reduced diameter but a more aggressive pitch (and rake).  So whats the effect?

Generally the bigger the diameter to the more water is displaced and the more thrust the prop creates - but it also adds load on the engine and hence its important that you can hit WOT. If you can't you are overloading the engine. So with the current 16" can you hit WOT?

A high rake also adds load to the engine, so dropping from 16" to 15" dia with a higher rake may actually end up with the same amount of load on engine and be ok.

What's the effect of going from 18.5p to 21p?  Theoretically you will move forward an extra 1.5" per RPM . So if you do the maths with theoretical zero slip you will get about 9% additional speed at the same RPM. Which equates to better fuel efficiency etc. 

However, higher pitch props have less bite at lower RPM so your hole shot may be compromised - if you don't get enough bite you also may introduce cavitation issues which is damaging to the engine (this is different to ventilation !).

The above comparison is a theoretical scenario - you also need to consider different prop dynamics to calculate the slip and see if you are actually getting the full 9% benefit, or even how efficient your current prop is. I have provided links to the calculators in the post KC has quoted.

I've also written in other posts that as an offshore fisherman, the main considerations I have for my motor/prop combination is to deliver most efficient performance in the following RPM ranges:

1. Hole shot - high thrust to get up and onto plane, or to climb a wave - so this equates to bigger diameter and lower pitch to maximise prop bite at low RPM.

2. Ride the Wave - hold position on the wave - sometimes you just have to ride the back of the wave, pick your time and accelerate to the next one etc - this equates to prop performance in the 2500-3500RPM range.

3. Long Cruising - stay in the most efficient RPM range of the motor at YOUR BOAT HULL's ideal cruising speed. My Haines 6.35m hull is most comfortable around the 40-45km/h mark in the typical 1m conditions I go out in. I want to cruise at these speeds when my motor is in the ideal 3900-4500RPM range where it delivers optimum torque and economy. No point over propping my rig to say 19p where in this range it may try and drive me at 50-60+km/h because my Hull can't handle it comfortably and I'll be constantly riding the throttle, dropping to a non ideal RPM to reduce speed, then revving up again to recover etc. But don't get me wrong -  I can and often do punch up past 55km/h - its just not the most economical or comfortable range.

BTW, the Suz prop sizing guide has a pretty decent explanation as well. 

http://www.suzukimarine.com/~/media/marine/accessories/suzuki propellers/propellers_pdf.ashx  

Anyway, having said all that, and given you already have the spare prop I suggest you take some measurements and readings of your current set up performance: time to holeshot RPM and Speed at plane, Speed at different RPMs - say 2000, 3000, 4000, 5000, WOT.

Then change the prop to the spare one and repeat the above. Compare which prop better suits your intended purpose. 

As @wrxhoon1said in several posts, prop selection is trial and error !

Cheers Zoran

PS - anything to do with props is never a short answer !!!!!!

[kingie chaser]

On 1/5/2021 at 6:31 PM, zmk1962 said:

Thanks for the mention KC (@kingie chaser) ... the prop topic was covered pretty well in the post you referenced... but a chat often helps to to get the head around some of the concepts and sift through the jargon.

Exactly why I tagged you in on this.

I dont thing there is another person on FR who can explain these technical things so eloquently & well detailed 😉

Edited January 7, 2021 by kingie chaser

[Bloggsy]

Looks like first thing I need to do is see what current WOT revs are so when I get a flat day I will go for a run. Thanks for all the input.

[noelm]

That's the way to do it, while your at it, check where the cav plate on the motor is in relation to the water surface when on the plane, everything needs to be set for optimum performance.