40hp on SIB

chuddster · 14 September 2016, 10:46

hi All

anyone running a 40hp 2 or 4 stroke on their SIB? what max speed are you returning? what pitch prop are you running?

thanks

_monkey · 14 September 2016, 11:09

I've got a Merc 50 on my Futurua (4.1M) and I am not getting over 30MPH - it is the standard prop; 12 x 22

chuddster · 14 September 2016, 19:52

Thanks monkey i have the 40hp on a quicksilver 4.3m and only returning 20knots with a 15 pitch prop i was expecting at least 28knots. I think my prop is too much maybe i should be running a 12 pitch.

berat · 15 September 2016, 08:45

Do you think 20knots is too slow for this combination? There must be some mistake. Check for your motor spect rpm it must be about 5500. If it is higher than 5500 you can get a 17 pitch to get higer speed. I will post a youtube link when i find that.
But i think motor high and the cavitation plate must be checked.

berat · 15 September 2016, 08:49

https://m.youtube.com/watch?v=R_szPh6z8Gk

That is a good or best example

chuddster · 15 September 2016, 10:11

that boat flies mine is no where near as fast as that i definiately have some problem. maybe ignition timing?

Nos4r2 · 15 September 2016, 11:19

Quote:

Originally Posted by chuddster

that boat flies mine is no where near as fast as that i definiately have some problem. maybe ignition timing?

The boat in the vid has a 50 on it, and it's been modified as well. There's no comparison.

Yours should be faster but its's more likely prop pitch or trim.

Don't go trying to play with timing, it's a recipe for holed pistons.

Fenlander · 15 September 2016, 11:38

Chuddster I'd agree not to go fiddling with the engine when it is so likely as Nos says it's a prop or trim issue.

I'm not sure you have ever said how many folks and what load you carry?

Have you checked that the prop hub hasn't failed and is slipping?

What make and year is your OB... 2 or 4 stroke? Has it ever performed as you would hope on this or any other boat? Apart from top speed does it start and run nicely as you'd expect?

Do you have a pressure gauge to know your tubes and keel are up to pressure... do you recheck on the water.

Are you using a mid trim posn on the OB? Is the OB the right shaft length for the transom?

As well as giving us this info to see if it sparks any clues getting out with a £10 Ebay rev counter and posting the results is really the next stage.

Assuming there is no glaring issue with the engine my gut feeling is a prop pitch around 11 or 12" would be a good starting point. If you post your OB details someone local to you on here might just have a prop you could try for one outing.

Northern Pike · 15 September 2016, 17:52

I had a Johnson 2 stroke 40 on my Futura Mk 3 and could only get about 25 mph.
Upping to a 50 didn't help much. I can get 29-32 depending on the prop.

_monkey · 15 September 2016, 17:56

Quote:

Originally Posted by Northern Pike

I had a Johnson 2 stroke 40 on my Futura Mk 3 and could only get about 25 mph.
Upping to a 50 didn't help much. I can get 29-32 depending on the prop.

which prop did you change to?

I put the details into Mercury's prop selector for a lightweight RIB with a 50HP and it recommends a 10.x" with a 14 or 15 pitch

Northern Pike · 15 September 2016, 18:01

I'm using a 50 E TEC. Its has a much different gear ratio than Merc.
I get 29 with a 15 pitch BRP stainless prop. When I load up the boat with weight, I get the same. With a 17 p I get more speed, but rev.s drop a bit, to slightly below optimal.

chuddster · 15 September 2016, 20:24

Gonna let the guys at marine shop give it a quick once over. Bought a tiny tachometer which ill put into use next week sometime.

Will post results soon.

Thanks for your input guys