prop size for 50hp johnson

[fishface14]

hi, the johnson engine (year 1999) on my 5mtr rib seems to be over reving quite badly when going at WOT, it shoots right up to 6500rpm as soon as you give it all the power even with 3 up. so i think it might need a prop change, atm it has a 12-1/4 x 15 what prop would you sugest to get the best performance out of it? or is there anything else i could try to keep the revs down?


just to add the top speed im getting out of it atm at 5500rpm (the maximum recommended rpm for the engine) is 19kn

any advise would be great!

thanks

[bosun]

If the prop is complete and undamaged then to reduce the revs you need to go up in pitch,my guess would be 13'' or maybe 14'' pitch.
As a comparison on a 5m humber with a new (fuel injected) honda we use a 13'' pitch prop.
on a slightly smaller avon with an older (carb) 40 honda we also use a 13'' prop
and on another 5m humber with a 70 evinrude two stroke we use a 17'' prop.

19 knots sounds really slow though,is the boat very heavy and is the bottom clean?

The above 5m humber with the 70 does 34 knots on gps with just me aboard
the other ribs with 40's on are doing mid to high 20's at a guess.

OOps I just re-read your post and you say that it already has a 15'' prop on it
are you sure? The figures don't seem to add up. It doesn't have prop damage does it??

[Downhilldai]

Sounds like the bush is spinning.

What's the lower unit ratio on the Johnson 50?

[fishface14]

the hull is clean, there is a few scuffs on the propeller but nothing i would consider serious.

im not sure what the gear ratio is but ill have a look in the manual later. would fixing/replacing the bush be an expensive or big job to do?


thanks for the help

[Downhilldai]

Around £40-£50 to have a prop re-bushed.

[bosun]

Been thinking about your problem and came to the same conclusion as Dai,just took me 24 hrs longer.('cos I'm not an expert whereas he is!) I was quite confused last night but a spinning bush is all it can be really,like a slipping clutch in a car.
Pitch wise based on what I posted previously 15'' should be about right??

[Downhilldai]

Originally Posted by bosun

Pitch wise based on what I posted previously 15'' should be about right??

Do you know the L/U ratio of the Johnny 50?

[bosun]

based on a quick google search 2.42-1 same as my evinrude

[bosun]

Hub will be smaller than mine though won't it ? (I have got a spare 15'' pitch stainless prop)

[Downhilldai]

Originally Posted by bosun

based on a quick google search 2.42-1 same as my evinrude

Ah ok, thanks - quite a low ratio.

A 15" prop at 5500rpm with zero slip should make around 32mph (28ish kts), so allowing for 12% slip, you'd be down to around 25 kts at 5500 RPM. As the boat is only making 19kts, then there's theoretical slip of 33%, which is a lot, but not as much as you would expect with a spinning bush.

Where's the cav plate relative to the underside of the keel and is it definitely a 15" pitch prop?

If the boat used is a 4m+ Zodiac Pro as in the OP's profile, then I'd expect the boat to make 33-35kts with 50hp, which suggests it needs a 19"-21" pitch prop (assuming 12% slip and a max 5500rpm)