Recommended propeller pitch

[irishjim]

I have an old Mercury 85hp Blueband, which did have a 17" pitch prop which will be replaced. I have been advised to fit a smaller pitch (14") to stop possible cavitation. It is propelling a 16' fibreglasss cuddy cabin.

Any advice would be appreciated

Thanks - Irishjim

[iank]

Irishjim,
If you had no problems with the 17 I can see no reason to change down to a 14, (normaly do this to increase the revs, each inch droped = add 200 rpm) to the best of my knowledge cavitation is caused by haveing the motor trimmed out to far and/or turning to tight a corner, there are probly a few other factors that cause cavitation but I dont know them.
Cheers Ian

[fishingnottake]

does changing pitch vary top speed as well? like if you go from a 17" to a 19 your revs will drop but your speed increase or have i got it all wrong?

[Cheech]

Not necessarily. I had a 19 and got a spare 21 with the boat. The 21 dropped the revs, but also dropped the top speed because it was just way wrong for the engine. I bought a 17 which is perfect for the boat. I get a little less top speed than with the 19, but all other areas are much better. Cruising is better, getting out of the hole is better, and economy is better.

If I went to a 15 I would probably kill the engine as it would be the other extreme from the 21.

If you want to know for sure what is best, ring solars and arrange a test on the river. They are very good. I did this with my last prop. We went for a run and took 3 props, switched them over and then we knew which one was the right one.

Cheech

[familyman]

Going to a 14 will increase your cavitation/ventilation problem.I doubt it is cavitating ,does it play up in a straight line?or only in turns ?I would be looking at motor height before spending $200-350 on a new prop.Do a search on here for motor height .I would say your motor is too high as a 85hp blue band should not be able to generate cavitation bubbles driving a glass 16'er.
cheers jon

[Cheech]

One thing I forgot to mention, Jim, be careful to not go too small because old motors are often not suited to reving higher compared to newer motors. Make sure you stay around the rev range recommended.

[ssab1]

really need a tacho to confirm revs are in ideal range for boat weight all boats will vary according to hull design,load and conditions.look at prop pitch as try to do 100 in ist gear of your car or trying to climb a hill in top gear motor over revs or labours .some where inbetween is your prop .over or under reving your motor aint good.

[rajawolf]

irishjim,

What is your tacho reading @ WOT (full throttle) with the 17" pitch...?