Propeller pitch question. How fine should I go? New Honda BF6 SHU

[graham2burton65@gmail.com]

Hi folks,
thanks to everyone for you opinions on newer outboards, I'm going with Sea Simon on this one.
I have just committed to the purchase of a new 6hp Honda to use in my BC20.

I know I want a finer pitch 'saildrive' propeller but I am not sure high fine to go.

the standard is     7- 7/8 diameter  x 6-3/4
finer is                7 - 7/8 diameter x 5 7/8
finest is               7 - 1/4 diameter x 5 1/2

So I need to decide between the last two, although I have negotiated to have the standard one thrown in as a spare (with an eye to later resale value etc)  have you direct experience of which one I should use? Thanks.

I do not know if the pitch options are big steps.
I do know that too fine and the engine will rev out too early and easily.
Too coarse and it will labour and not get up to full revs.

Why did I choose the Honda?
I know there is always interest in which engine to buy.

The power head is rubber mounted so it should stop some vibration reaching the boat.
It has a decompressor valve that makes it particularly easy starting and I am hoping Jane can start it.
It was nice to take up the 6amp charging option all though I suspect I will rarely use it (motoring home with the autopilot on?)
I have always been impressed with Honda engineering.
It is more likely to work with modern fuel and I will be relieved to have the new fuel line and tank I have ordered as I believe fuel lines degrade and are not always up to harsh 2021 petrol ( I am replacing a ten year old Mariner)

negatives ?
I loose out on the front mounted gearchange of the Tohatsu (that was second choice)
Its more expensive
I hope nothing else is negative!

I have followed the lengthy and very useful discussion on E power.  Sadly not for me at the moment.  I want to be away from marina electricity and/or taking it home every night to charge. 

Hope you are keeping well in these difficult times
Graham B

[Graham W]

It's probably no help at all but when I had my Mariner 6HP outboard, its high thrust/saildrive propeller was an aluminium three-bladed 8.4 × 6 RH. Probably closest to your "finer" option. Like you, I kept the standard propeller as a spare.  The improvement in performance from the saildrive was extremely impressive and it was ideally suited to a displacement trailer sailer.  Fewer revs and lower fuel consumption, more responsive and (I might have been imagining this) much less noise and vibration.

[Paul Beardsell]

The diameter of the prop is to dissipate (i.e. transmit) the power - a more powerful engine needs more diameter. Or blades.

Pitch is to do with matching boat speed vs engine speed. You choose the pitch to match the max power RPM of the engine with your required speed, usually this is the hull speed where, rule of thumb:

(hull speed in knots) = 1.34 * sqrt(waterline length in feet)

In between the engine and the prop is usually a gear ratio, often 2:1

For more relaxed engine use often people overpower and under rev. I.e. if you have surplus power go for a higher pitch prop to reduce engine revs. A 6hp engine on a BC20 probably only requires 3hp actually generated so you run it at reduced RPM - bear that in mind when using this ready caclulator:

https://goodcalculators.com/boat-prop-calculator/

I believe 10% slip is correct. If you get enough power for anticipated conditions at 3000 RPM (do you?) and the gearbox ratio is 2:1 then it's a 5.4 inch pitch you need for 6kts.