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Old 01 August 2010, 16:58   #1
Country: UK - England
Town: Solent
Make: Phantom 21
Length: 5m +
Engine: Mariner 200hp V6
Join Date: Feb 2008
Posts: 138
Honda 20hp 4 stroke - what's the largest prop I can fit?

Hi all,

I've got a 20hp 4-stroke Honda outboard currently sporting a 4 blade 9.25x10" prop.

I'm looking to replace this with a 3 blade higher pitched prop in order to improve it's top speed.*

Obviously I'm limited by the physical size of the prop I can get on the spindle but I'm questioning how the performance of increased prop diameter (that can be associated with increased pitch) will be affected by the reduced distance from the blade tips to the cavitation plate upon operation?

If anyone has any experience/advice in this area it would be much appreciated.


J. *

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Old 02 August 2010, 17:00   #2
spartacus's Avatar
Country: UK - Scotland
Town: Aberdeenshire
Boat name: Sula
Make: Ribcraft 4.8m
Length: 4m +
Engine: Tohatsu 60hp + aux
MMSI: 235087213
Join Date: Jun 2007
Posts: 3,055
Could be one of these situations where you look at the engine, rather than simply changing the propeller. Your engine will have come with a standard factory propeller. In your case, 9.25" diameter x 10" pitch, 4-blade aluminium.

This is fine if the parameters of the boat and engine remain static. Your boat should achieve it's maximum speed at WOT (wide open throttle) at 5,500rpm. However a lot is dependent on the boat, the hull, number of people on board, sea conditions, etc.

Most engine manufacturers will have a chart that identify the propellers available for their outboards. For example - Tohatsu identify possible propellers for their 20hp, 4-stroke, from anything between 6" - 11.5" pitch (the distance travelled by one full revolution of the propeller). Engines from 50hp and above can benefit from switching to stainless steel by giving the blades rigidity, effectively optimising the job of cutting through the water, and another 2-3 mph top-end speed.

As a rule of thumb - a 1 inch reduction in propeller pitch will result in an increase of 150 to 200 RPM at full throttle.

First job will be to see what RPM you're achieving at WOT, then see what propellers are available, and the price to switch. A local Honda dealer would be my first port of call.

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Old 04 August 2010, 09:28   #3
Country: USA
Town: Oakland CA
Length: 3m +
Join Date: Feb 2005
Posts: 6,618
Another option is to have your current prop re-pitched. You can usually go an inch or so in aluminum. A good shop may be able to tweak the diameter as well.

Sort of depends on where your performance is now, and where you want to go.

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