Suggestions for propeller change to improve performance

[Keith Russell]

The Mariner 5hp four-stroke that came with my CC19 is wanting for thrust in my view. I believe it is fitted with the 'standard' prop. The top speed is OK but I often want more initial bite and 'umph'. Should I consider replacing the prop, to get more thrust for effective manuevering? If so, please suggest specs, and particular cost-effective source(s), if known. Also, any suggestions for effectively reducing the engine's noise level would be appreciated! Thanks, KR.

[zimp]

My CC19 has a 6HP Yamaha. I replaced the standaard propeller with lower pitch propeller (6.5 inch). This gives more thrust at low speeds. The dowside is more rpm gives more noise....

[Malcolm Sadler]

I inherited a Mariner 6hp when I bought Rondo. When I took it for its first service to a specialist outboard firm in Reading, he asked about the boat and immediately advised I change to a different prop.

It doesn’t get the boat moving from stationary as quickly as the one I had, but once under way it pushes with more power at lower revs and with a lot less cavitation. It is absolutely fine for manoeuvring (usual trick of short bursts of high revs with outboard turned as well as rudder if I need to turn on a sixpence).

The way the man explained was it to me was that the original prop was intended for light, planing boats, but I needed something for a displacement hull form.

Looking at the invoice I see it was part no 48-812951 A02. The cost in 2017 was £75.71 plus VAT.

Cheers

Malcolm

[chrisr]

I inherited a Mariner 6hp when I bought Rondo. When I took it for its first service to a specialist outboard firm in Reading, he asked about the boat and immediately advised I change to a different prop.

It doesn’t get the boat moving from stationary as quickly as the one I had, but once under way it pushes with more power at lower revs and with a lot less cavitation. It is absolutely fine for manoeuvring (usual trick of short bursts of high revs with outboard turned as well as rudder if I need to turn on a sixpence).

The way the man explained was it to me was that the original prop was intended for light, planing boats, but I needed something for a displacement hull form.

Looking at the invoice I see it was part no 48-812951 A02. The cost in 2017 was £75.71 plus VAT.

Cheers

Malcolm

Yes, this. A "sail drive" prop. Have one on my Honda 5. That was about £50.

[Justin Greig]

A useful discussion about propellors has been going on on the 2024 Rally WhatsApp group which contains some useful info worth preserving on the forum - so I have cut and pasted some of the exchanges below:

[Richard Woods] I have 6hp Yamaha. The factory fitted prop was no good for Cape Cutter as started to cavitate above half revs. I changed mine - made big difference, now usable over whole rev range, less speed more grunt.

[Malcolm Sadler] I had the different prop fitted to my mariner a few years ago. As Richard says, it’s sooo much better. I’ll go and check invoice for description
[Simon Head] It’s the saildrive prop. I transferred it from my old Mariner to my new one.
[Malcolm Sadler] Yes. My invoice says 48-812951 A02

[Richard Woods] Original prop was 7 1/4 × 8 replacement 7 1/2 × 7. First number is prop diameter in inches, second is pitch, again in inches. Therefore replacement is 1/4" digger diameter (cannot go bigger or will catch on anti cavitation plate) and pitch is 1" less. Diameter and pitch is stamped on one of prop blades and it is genuine Yamaha part no 6E0 - 45943-01-00.

Diameter and pitch are stamped into rear of prop blade. First picture is the replacement prop, now on engine. Other photos are of two propellers side by side. Photo does not show clearly, but putting one over the other you can see diameter of replacement is bigger. Pitch is all to do with blade angle, side view does show the difference in angle of the two blades. Working it out is a bit complex, pitch is how far one revolution of prop moves boat/engine forward with no friction, drag etc. Also interesting to note is the shape of propellers is different, replacement has a straight trailing edge, original has curved trailing edge - former will give more thrust.

Here is how to calculate propellor pitch - good luck if you want to try it!
https://naval-architect-jobs.blogspot.com/2011/10/marine-propellers-pitch-and-how-to.html?m=1
It is not easy and even more complex if the blade has a compound geometry - best to look for the marking on the propellor.

[Andrew Scullion] Most three blade props have one blade start aligned on the circumference where the previous blade finished, so you can estimate this quite easily by measuring the length of the prop cylinder shaft (forwards-backwards direction) from where the blade starts to where the blade finishes and multiply by three, one for each blade and this is approximately how far the pitch distance is.
Interestingly my Mariner engine is a 6 Hp, (again I think it is the same engine as the 4 or 5 Hp with an alternate carburettor) and mine appears to have a 205mm (8inch) diameter 150mm (6inch) pitch so even higher thrust.