Propeller Size Recommendations

[lakelad]

I've got the option of two propellers and would like recommendations on what one to use. The boat is a 4.75m Quintrex Freedom Sport (Aluminium Bowrider), with a 60hp Mercury 4 stroke (2004 hull and motor).

The two props are both stainless 3 blades:
1. Apollo (10 1/2 inch x 13) Michigan Wheel XHS - https://www.propellerdepot.com/michigan-wheel-apollo-10-12-x-13-rh-993406-boat-prop
2. Solas (11 3/8 inch x 12)  - https://www.propellerdepot.com/solas-new-saturn-11-38-x-12-rh-3331-114-12-prop

The smaller prop is on the Mercury 4 Stroke 4 cyl already from previous owner, and the second prop is off a 2 stroke 50hp 3cly Mercury.

I'm looking to use the boat for fishing, and pulling kids in a tube.. a quick Google tells me a larger prop (Solas) may be best to pull a family worth of weight out of the hole and tube etc. I don't mind losing a little top speed. Am I on the money?

 

(Note I found a user manual, which had a range of props but not these sizes.. http://www.dunnva.com/images/Mercury_Mariner_Download_2001_Service_Manual.pdf)

Edited November 28, 2022 by lakelad

[noelm]

You need to be able to achieve close to manufacturers max RPM with a "normal" load, it doesn't mean you are going to run at max RPM all the time, but, for best performance, fuel economy and motor longevity, your motor must be able to get there.

[zmk1962]

Just a qualifying question ... the Solas is nearly an inch (7/8in) larger in diameter ... are you sure your motor has the clearance to accept this prop?

Having said that, its difficult to say which will be better as you are also comparing two different manufacturers propellers, not just of different dimensions but also with different blade thickness, profiles, cup shapes etc etc.

The basic theory is

1) a smaller pitch will move you forward less per revolution so theoretically it will give a lower max speed at the same RPM but it will grip more at lower rpm ... from memory each increment in pitch represents about 6% speed difference (so the 12 should be slower than the 13 but with better hole shot)

2) a larger diameter will move more water so generate more thrust

then onto the theory you need to add the real life considerations .... the design of your hull - friction?, the efficiency of the prop - slip?, the engineering of the prop - blade flex etc...

.... so you may actually end up faster and have better hole shot with the Solas, if all the other real life dynamics are in your favour.

The only way to know is to run a test with each prop ... and as Noel say ... you need to be able to reach Max RPM other wise you will be doing engine damage in addition to using more fuel.

The other considerations get somewhat more technical. A smaller pitch prop (the solas) may be easier for your motor to spin up, but because you have also increased its diameter, the outer tip of the prop will be travelling significantly faster than the blade of the Apollo 13.  That extra blade speed may cause cavitation - basically water vaporisation at the low pressure points on the prop blade - the tips, the back of the blade. Cavitation is like millions of tiny hammers banging away on the prop surface, with each impact going up through the lower leg gears - NOT A GOOD THING.  Cavitation will damage both the prop and the engine.

So ... the net is, you know how it performs with the Apollo 13 .... run a test with the Solas 12 ( and compare the hole shot performance - time to hole shot, RPM and speed at hole shot, and max RPM, Max speed). Inspect your prop to ensure no cavitation.  (hole shot = getting on plane)

Best of luck with it ... and please don't forget to post back!

Here's some background reading ...

https://www.pbo.co.uk/gear/choose-right-propeller-boat-62058

Cheers Zoran

[noelm]

Just to add, before you even think about props, make 100% sure your motor is at the right height, this is vital before anything else.

[lakelad]

Thanks @Noelm and @zmk1962, I appreciate your logical advice and time taken to share, that all sounds fair and makes sense to me.

Will run some tests assuming it fits, both motors are Mercury of similar ages and hp if that helps.. otherwise it might be a short experiment! Will post back after I have had a chance to try them both.

[lakelad]

Hi @zmk1962 & @noelm I've run some tests. The larger prop gets the boat out of the hole better, and still allows revs to max out (WOT 6000rpm). Top speed is perhaps a little slower, but not much in it (1-2 knots maybe).

Hopefully the info from this thread will help others, there is also some great info in the user manual with a 'Propeller Selection' section which pretty much matches the advice in this thread (RTFM hey..). 

[zmk1962]

Thanks for posting back @lakelad.  Yup... RTFM ... you can't re-invent this stuff - just re-quote it.  BTW, don't forget to check for any signs of cavitation on the prop after a few more runs on the water. Hopefully there won't be any.

Cheers Zoran.

[dunc333]

hi lake lad so easy to change props on mercs my opinion would be get the big prop in stainless and get the smaller one in ali and swap when doing different activities cheeers dunc33