Feathering prop pitch adjustment?

[Tim Good]

I have a 4 bladed feathering Variprop which works great. In the instructions it mentions how to easily alter the pitch. However it doesn't recommended what pitch is ideal for a specific type of boat. When greasing mine today it did occur to me that the amount of pitch didn't seem like much. This photo is in the forward drive position. What do you think?



Please excuse the excess grease and old anode.

 

[knuterikt]

What is your max rpm compared to engin manufacturers max rpm?

 

[Tim Good]

What is your max rpm compared to engin manufacturers max rpm?

Ummm. I don't know. My RPM counters is bust. Need to fix it and hasn't need a top priority. It's a 20 year old Perkins 80HP (M90).

 

[Tranona]

If you can cruise at around 70% maximum revs and achieve around 6-6.5 knots and then achieve maximum hull speed (8+ knots) at rated maximum revs then the pitch is about right.

You need hard data to calculate optimum propeller size by feeding it into a programme such as Propcalc on www.castlemarine.co.uk The purpose of being able to vary pitch on a prop such as yours is that you don't need to change blades if you need to change pitch. That gives more flexibility to match the prop to your boat. If you ask Varifold for a setting they will ask for essentially the same data as Propcalc and do the calculations for you.

The key variables are LWL (which gives a limit to displacement speed) displacement, engine output and revs and gearbox reduction ratio. This will have been provided when the boat was first fitted with the prop, So if you have not changed anything the existing setting should be OK. The only thing that is likely to change over time is actual rather than design displacement. Extra weight of gear if significant could limit the ability to achieve maximum speed or maximum revs and a reduction in pitch may correct this.

Without knowing revs it is difficult to assess empirically as you need to do a series of runs plotting speed through the water against revs at, say 200 rpm intervals from about 1500rpm to maximum rated rpm.

 

[knuterikt]

Ummm. I don't know. My RPM counters is bust. Need to fix it and hasn't need a top priority. It's a 20 year old Perkins 80HP (M90).

Without knowing that it’s little sense in starting to adjust the pitch.
you can get laser RPM measuring “gun”

 

[Daydream believer]

Water conditions will also make a difference. Whilst a fine pitch may drive the vessel at max speed in flat water it may not do so in choppy water. In that case you may need a greater pitch. This may not give extra speed but it could give the prop more bite in the chop so that you do not loose speed so much
So you should think back as to how your boat performed in rougher water. Was it getting nearly full speed in rough water without having to rev really hard. If not then a bit more pitch may help. The trade off may be greater fuel consumption, although some diesels might operate better at a more optimal rev range