Selecting the right pitch prop

Gledhill · 03 July 2016, 18:41

Hiya lads I've just finished swapping my outboard from a yam 70 to a yam 90.
I use to run a 13x19 on the 70 which would take me to around 5300rpm at WOT the 90 didn't come with a prop so I put the 19 straight on onto my 90 today. I've just been out and at WOT it's running at 4700-4800rpm I would really like the engine to be at 5500rpm does anyone no what pitch I should be running please?? Also considering putting a stainless prop on just wondered about the correct pitch to use for a stainless and also does anyone have any recommendations for which prop to buy?? Thankyou 👍🏻

Nos4r2 · 03 July 2016, 18:49

I think you need to check the max RPM of the motor.

If memory serves me correctly, the target RPM at WOT (to get the rated 90hp) is somewhere between 4500 and 5500 on a Yam 90 (but don't take my word for it,check it) so 5500 RPM may be a little high. I'd be looking at maybe dropping to a 17" pitch, but no more.

Gledhill · 03 July 2016, 19:03

Hiya pal yer the max rpm is 5500 I've been told by a yammaha engineer to make sure the engine is running to that other wise the engine will not perform correctly. Just wondering weather to drop to a 15?? Don't want to loose the top end speed though

Last Tango · 03 July 2016, 19:10

Is the boat's performance what you expected ?

Might be an 18" stainless would be the solution....if the prop is what's limiting RPM and it's not down to something else.

Gledhill · 03 July 2016, 19:17

Yer the boats performance is great just would like a bit more top end speed if possible and the boat the Rev to 5500 or as close as possible

Gledhill · 03 July 2016, 19:20

I'm thinking 17 aluminium or 15 stainless due to the Flex of the alloy. I tried a 19 stainless on my 70 and it dropped the rpm by 500 at wot

Last Tango · 03 July 2016, 19:35

Best if you could borrow a couple of props to try.
I'd expect going from 19 alloy to an 18 stainless would give a marginal increase in RPM (not all the way to 5500 though) and a slight increase in top speed.

Gledhill · 03 July 2016, 20:34

A stainless looses around 400 to 500rpm in my opinion so I'm thinking a 13x15 stainless could be the answer it's a an expensive mistake to make though if it's wrong

Pikey Dave · 04 July 2016, 06:27

Quote:

Originally Posted by Gledhill

A stainless looses around 400 to 500rpm in my opinion so I'm thinking a 13x15 stainless could be the answer it's a an expensive mistake to make though if it's wrong

Are you sure about that? Normally you gain rpm when changing from ally to stainless for the same pitch.

Lions led by donkeys

Poly · 04 July 2016, 07:05

Its odd that you've gone up in HP but the same prop has gone down in RPM. If they have the same gearbox you've presumably upgraded the engine to end up slower?

jyasaki · 04 July 2016, 08:16

Quote:

Originally Posted by Pikey Dave

Are you sure about that? Normally you gain rpm when changing from ally to stainless for the same pitch.

Backwards. SS is stiffer, so it holds pitch under load. Aluminum sheds a bit under load, so revs higher. In addition, SS props usually have more cup, and weigh more, both of which lower rpm even more.

jky

matt h · 04 July 2016, 11:50

is is actually a 90 as its not just swapping the carbs.. my SR 54 ran 13x19 and would gain just touching 40 at 5300..(documented figures at the time)

Last Tango · 04 July 2016, 12:43

400-500rpm would be a lot on a like for like. I've found very little change in rpm and maybe a couple of MPH improvement at the top end. The blades don't flex as much but they're thinner and present less rotational resistance.

I did find the slip from a vented Laser 11 let me push the pitch a bit further without compromising hole shot. (compared with the Vengence I took off).

40mph+ with a 15" would have to have a unusual gear ratio at 5500.