#1 2020-09-29 18:34:24

John_Brooke-Smith
Member
Registered: 2020-09-15
Posts: 3

Matching prop to Beta B20 Engine and TMC60A 2:1 reduction gearbox

We have just fitted a Beta Marine B20 engine (20BHP at 3,600 rpm) matched to a TMC60A gearbox (2:1) to our V30.  The original prop (matched to the original (1988) Yanmar 2GM20  engine) was a 14x12.  The prop engineers did the maths for the Beta and replaced it with a 14x8, but that revved too high with very little propulsion.  They then provided a 15 x 7, but that was too big with virtually no tip clearance.  We are now waiting for a 14 x 10.  We had not realised this would be so complicated and were wondering if anyone had fitted the same engine/gearbox combo to a V30 and could offer a view?  Any help will be much appreciated!

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#2 2020-09-30 14:42:49

Colin_Reid
Member
Registered: 2003-11-02
Posts: 27

Re: Matching prop to Beta B20 Engine and TMC60A 2:1 reduction gearbox

well this doesn't answer your question, but the 3 blade prop on Tui (V30) was 14/7 with a Nanni D21 which is similar to Beta (same Kabuta engine, different marinising). That worked fine. The Nanni is recommended optimum revs at 2800 but I usually run it at 2500 which gives plenty of power and speed with extra in reserve. I replaced it with a Featherstream feathering prop last winter. Darglow the manufacturer looked at the original prop and engine specs and specified accordingly. The Featherstream seems fine. Much more thrust astern, no loss off power forward and a cleaner feel through the water under sail. I think it has helped with weatherhelm. That was what I was hoping as turbulence from a fixed prop makes a a big area of the rudder ineffective.

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#3 2020-09-30 19:06:33

John_Brooke-Smith
Member
Registered: 2020-09-15
Posts: 3

Re: Matching prop to Beta B20 Engine and TMC60A 2:1 reduction gearbox

Thank you Colin.  That is interesting and helpful.

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#4 2020-10-02 13:07:51

Jonathan_Hopper
Member
Registered: 2004-03-23
Posts: 101

Re: Matching prop to Beta B20 Engine and TMC60A 2:1 reduction gearbox

John -

This seems a bit odd.

Firstly, can I double check that you are talking 3 blade propeller.

Beta recommend a 13 x 9 for this engine.   Using say a 14 x 9 you would expect a max rpm that you achieve (underway) is perhaps around 3,200.   On paper, a 14 x 8 seems like a good compromise to me, and in theory you should be looking at a max rpm of maybe 3,300 or 3,400, but certainly not more than 3,600.

Can you confirm what rpm's you are getting with 14 x 8 at full chat and underway?

If your engine is apparently, let's say, over revving, particularly with little thrust, then it may well be worth looking to see if there is any prop slippage on the shaft (is the key in tact?   was it torqued up with a torque wrench?), or is there any clutch slippage - has the gear cable been correctly installed?   Is the gear lever travel correct?   Do you know anyone with a tacho who can check it?   Was the coupling torqued up (if this is the means of it not slipping)?

Edit: Is the prop clean?

Last edited by Jonathan_Hopper (2020-10-03 10:15:46)

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#5 2020-10-05 18:56:39

John_Brooke-Smith
Member
Registered: 2020-09-15
Posts: 3

Re: Matching prop to Beta B20 Engine and TMC60A 2:1 reduction gearbox

Thank you Jonathan, and sorry for delay in my reply.
Yes, it is a 3 blade prop and brand new out of its wrapper.  The 14 x 8 revved to 3,600 rpm (deafening!) and we were still only making about 5kts waterspeed in flat water. I am pretty sure the engineer who fitted the engine (who accompanied us on the sea trial and shared our dissatisfaction), has checked and rechecked all the workmanship on his side, but if the 14 x 10  does not provide the solution, we will have to go back to these issues.  I was just thinking that, given the popularity of these Betas, there is probably one which is fitted to a V30 out there, whose owner might be able to pass on their experience.

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