Stainless prop sizing

Rupert Bear · 20 June 2004, 19:00

I have been trying to get a prop for my RIB sorted for some time now but have ended up confused and mystified.
I have a 15" alloy which is perfect for the boat but decided to get a SS to see if I could get any more oomph and to get a prop that wasn't quite as easy to damage.
Easy I thought. After asking I bought a 15" SS and the boat went slower and lost 300-400 revs. So it went back and through a mistake it became a 16" rather than the 14" it was supposed to. Boat went even slower and lost another 200 revs.
It went back again and was altered to a 14", boat goes slower again and lost another 100 revs.
Now I am completely confused, I intend to put the alloy back on again and make sure she is still OK (was last week) before calling SD but does anybody have any idea what could be the cause?
I may as well put the alloy back on and save £250 on a prop

roohairy · 20 June 2004, 20:02

Are these props all the same diameter? The numbers don't make any sense at all! As you said put the original back on and see if it's ok.

As I read it you're now down 600 - 700 rpm with the 14" SS, is that right? If you mean you're down 100rpm with that prop then maybe your original prop is a 13", would sort of add up then.

If your original is 15" and the engine is ok, then the new props I think must be a different pitch from their markings. Can't explain it any other way, would be interested to hear what you find.

Rupert Bear · 20 June 2004, 20:24

My alloy is a 15" Yam prop. both props are 13.75" diameter.
I get 5400-5800 24.5-30knts empty/loaded resp with the alloy.
The SS tests have always been empty and got 5200 for the 15", 5000 for the 16" and 5100 for the 14" in that order. It is the same prop repitched. The boat goes 1-2 knts slower with the SS ( not suprising at these lost revs)
I am totally confused as I put the alloy back on between the goes of the SS and got my normal figures. I will put it back on and try again after trying the 14" SS today.
These figures have me foxed!

roohairy · 23 June 2004, 00:54

I'm no expert, but this is what I've been able to put together about props: if there's anything wrong here can someone please point it out, this has been pieced together from loads of different sources over the years and I use it from time to time, been ok so far, but....

As a rule of thumb, if you put on a prop with a smaller pitch you will gain about 300rpm per inch. Vice versa if you go larger, this only works around the optimum prop size for the engine as far as I know. But if you change to a stainless steel prop the same size then it's equivalent to going down one inch in pitch as far as an alloy prop is concerned.

I'd speak to the people you got the prop from, if it's who I think it is then I'm sure they'll want to sort it out as much as you do.

Lastly, can you post your gearbox ratios and I'll look at them too.