Rib prop

paul127

Have a six meter rib has a 90 yamaha with a aluminium prop when I got it , I purchased a stainless steel prop the same as the aluminium (13/19 r) . With the aluminium it will rev to 6000 rpm but with the stainless it will only go to 5000 rpm , when they are the same pitch and degree why is this . Is it just trill and error or is there a way to work out why size stainless steel prop of the aluminium prop

vinnie13

was the old prop in good condition?

fergal.b

Hi Paul127, aluminium props can flex and flatten out a bit when under load where as a stainless steel one will hold it's shape, are you getting the same speed at 5000 rpm as 6000 rpm.



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breghall

paul127 wrote: »

Have a six meter rib has a 90 yamaha with a aluminium prop when I got it , I purchased a stainless steel prop the same as the aluminium (13/19 r) . With the aluminium it will rev to 6000 rpm but with the stainless it will only go to 5000 rpm , when they are the same pitch and degree why is this . Is it just trill and error or is there a way to work out why size stainless steel prop of the aluminium prop

I find it hard, the that two same pitched props vary by 1000 rpm. general rule of thumb is that a 1 degree pitch drop when using s/s props would lower your revs by 200 rpm. Generally a drop in pitch is needed when changing to a S/S prop to keep the engine at it's recommended RPM range. As Fergal has already pointed out, aluminimum props twist more and get distorted , so the strength from s/s props blades will cause the engine to lug a bit more with a s/s prop so you should get a drop in rpm. But to have such a large decrease is strange.

Not all props are the same, different manufactuers will have different blade thickness, different blade rake, and so on. You make have the same size prop by 2 different manufactuers but difference in performance can be miles apart.

Correct propping of a boat is trial and error, over here we can't 'rent' out props (you can in places in the U.S. for example) and see how they go, we pretty much have to buy blind and hope that the performance is ok.

Was the Aluminium prop ever repaired ?

Daibheid

You've gotten some sound advice there already. Experience says a stainless prop tends to equate to a full step up in pitch so if you were running a 19" pitch a SS prop same pitch is like putting on a 21" - hence the drop in revs and I'd expect a drop in top end and acceleration too.

I take it we're talking about a 2 stroke here. I've had two ribs (5.3m and 5.5 both heavy UK built) with that particular engine and never ran bigger than a 17" (alu) so I'm almost surprised you could rev out with a 19" on a 6m rib. I'm guessing we're talking something fairly shallow V/ narrow bodied/ light construction.

I'd suggest dropping to a 17" pitch if you must have a SS prop. Hopefully you dind't shell out 4/500 for a shiny new example but if you did the shop may exchange it for you.

If we're talking about a 90Hp 4 stroke Yamaha or the Mercury/Mariner same engine DO NOT run it over propped - they respond by contaminating the crankcase oil with unburnt petrol!

I'd also generally caution against SS props because if you should hit something with one it won't give like an allow prop so will probably break your gearbox. I do like them as they are more efficient but I have one in the garage the last year or two and been meaning to repalce the current alu one but look at the chips on the alu one and bottle out every time! If you are in and out of shallows or tidal estuaries that have lots of junk floating about......

BTW the increased efficiency with SS props isn't so much about less flex than alloy, it's the greater strength of SS allows the blades be made thinner which reduces resistance through the water. If alu blades were flexing that much cyclic stress would see a lot of blades flying though the air!

One last thought.... props are sometimes fettled to change their characteristics so someone may have reduced the pitch on your nominal 19" alu prop which might explain a lot of what you describe.

Hope that lot helps some.

For any of you with a shed load of boat bits like me I'm looking for a Honda 13.25x19R if anyone has one. A 14.25x17R might also do the job....