beta 25 replacement/ prop /shaft/?

[seascape]

Sorry to seem clueless but I'm struggling to get to grips with exactly what I need to fit with a new Beta 25 engine (in a sadler 29) in terms of the prop / shaft .
at present the Bukh DV20 has a 1" prop with a 16x 13 3 blade prop
does anyone know if these are suitable for the new engine ?
how exactly are all these details arrived at?
thank you
richard

 

[macd]

In your case I suspect they'd be arrived at in the first instance by asking Beta. They're really very helpful people.

 

[Topcat47]

You'll not find much in the way of guidance because there are just so many variables.

Your Sadler has a displacement hull speed of around 6.5knots, max.
Your shaft needs to transmit the maximum torque developed by your engine and the minimum diameter will be determinerd by the shear strength of whatever material your shaft is made of. Once you have those two pieces of information, a basic textbook on engineering science will give you the necessary equations. Don't forget to add a sensible safety factor, then choose the next size up in shaft diameter cos you'll not find, say 23.5mm shafts out there.

Prop selection is nightmare. The diameter of the prop coupled with the pitch determines the torque required to drive it and the thrust it develops. Ideally the pitch should be sufficient to drive you along at hull speed at around 80% throttle, although some people like as much as 30% power in hand. I looked into the calcs when I re-engined my own boat and decided it wasn't worth the effort of second guessing prop and stern gear manufacturers, applying the calcs for myself so I certainly wouldn't advise anyone else to do it.

My Nic 26 has a 1GM10 (one area where I did stick to my own counsel as I don't believe in overdriving a yacht). It's easily capable of driving the boat at 6kts with a clean bottom. The 1" shaft came with the boat and I installed the Yanmar so that it matched the existing sterngear. I still replaced the prop though with the one reccomeded by the engine supplier for my boat. Well worth the money, too. /forums/images/graemlins/smirk.gif

 

[srp]

Do a Google search for an Excel file called propcalc.xls - its a free calculator which should give you a start. Two friends have fitted new Betas and both have said the advice and help that BetaMarine provide is second to none. I have bought props, shafts and shaft gear from Lancing Marine and had a similar excellent service from them - I'm talking about real practical help and advice here from people who know what they're talking about.
My own limited experience suggests that the small 13hp Beta will be ok, along with your existing 1" shaft, but you will probably have to change the prop because the Beta (I think) will run at a higher rpm than the Bukh.

 

[tritonofnor]

try asking mike lucas yachting , they've re-engined several in the past with both Beta and Nanni, so should have some relelvant info.

 

[stevebirch2002]

Give Paul a ring at First Marine Propulsion at Shamrock Quay in Southampton. Great prices, knows what he is talking about and very helpful. He knows the Beta very well (they buy their sterngear from him!)

I have no attachment to the company.. etc etc

 

[maxi]

Paul at first Marine is an excellent direction to take.

If you have a 1" shaft and bearings etc. why change? it's more expense and complication.

If it's any help, I re-engined my 32' with a Beta 28 driving a 1" shaft and it's absolutely fine. So far as your prop is concerned, first determine whether your Bukh is right or left handed, the Beta is right handed so that may make your decision for you. Propsizing is an artform in itself but Mr Google will take you to several sizing programmes. I would suspect that you would be looking at soemthing closer to 14"x10"

 

[seascape]

Many thanks to you all for the excellent advice
Anyone any idea of the value of the 20+ yr old Bukh dv20
New pre heat
runs well
Only problem oil leak /(possibly front end)
Thanks