Which prop to use

[Yanros]

I have a new Yam 70 4stroke on an Osprey Viper 5.2m rib.
The max Rev range in the book is 5300-6300rpm
With a 15 prop it revs to 5800rpm and 26kn.
My preferred cruising speed is 19 kn which is 4600rpm
With 13 prop I get 26kn at 6200rpm and 19kn at 5000rpm.
My main concern is fuel economy. The Rib is used for fishing so no towing anything or warp factor speeds.
My thinking is because the 15 is reving slower at my prefered cruising speed then this is the better prop for fuel economy.
Or is it???? Any thoughts would be most welcombe.

[jeffstevens763@g]

I’d use the 15 if your happy with the hole shot OMO

[Pikey Dave]

Your prop should allow the engine to achieve max WOT RPM. Your max rev range is 5300-6300, so at 5800 you’re bang on the money.

[jonp]

I had an F70 on a 5m zodiac, which I owned two props for. A 13 pitch and a 15 pitch. I used a prop lock instead of the usual nut and split pin which meant I could quickly and easily swap props depending on the activity I was doing on the day. Heavier loads I would use the 13 and light loads the 15 pitch. Yam aluminium props are pretty cheap and carrying the one I wasn't using onboard the boat meant I always had a spare as I do long range remote trips in my boats.


I tried various stainless solas and Yam props and found so little in performance difference that I decided to stick with aluminium. Personally Ive not seen any benefits in stainless until you get up above 115hp.


I have around 8 props to fit the 70-130 Yamaha and the frequency I change boats its not worth selling them as most of my boats fit into this hp range.

[jonp]

Originally Posted by Pikey Dave

Your prop should allow the engine to achieve max WOT RPM. Your max rev range is 5300-6300, so at 5800 you’re bang on the money.

Yamaha this side of the pond recommend propping to the higher end of max rpm. When we prop tested an F70 on three boats Ive owned with different dealerships they have gone for the 6000-6200 range.

The f70 isn't an overly powerful engine so propping to the upper range greatly helps in bar crossings and maintaining speed in rough conditions. Ive owned three and would happily have another on small light boats.