Over-propped?

[sv_telemachus]

Thanks all, lots to think about. I’ve owned the boat one year, two previous owners since 2005 (and owners of past 10 years and were quite relaxed with 1800rpm).

I never took it above 1800rpm until recently entering the Duoro in Porto at full ebb and noticing the steam/smoke.

It sounds like the calculations for the prop are broadly correct and the lack of black smoke or full rpm in neutral renders over-propping unlikely.

To summarise the helpful inputs, I will try and explore these as next steps:

• Prop and/or hull fouled (photos are from July when boat was lifted and new antifoul applied, but will recheck)
• Bad fuel
• Fuel injectors need servicing
• Governor lever not hitting full ahead (I was referring to the cockpit control hitting max in earlier posts)
• Air filter blocked (just replaced this so unlikely)
• Partly-blocked / undersized exhaust (can’t speak to the transom exit but the hose is 50mm throughout)
• Alternator calibration
• Prop over-pitched (but unlikely)

100% agreed on the merits of a folding prop but the budget was at max (unlike the rpm!).

 

[Tranona]

Just to add a bit about running at 1800rpm. Not a problem as it is running under load. You hear a lot about avoiding running at low rpm, but the concern is about low load rather than the actual rpm. For example using the engine to charge batteries as the alternator puts minimal load on the engine. Long periods of using the engine to complement sails (motor sailing) where sails are doing most of the work. again the prop puts low load on the engine. This is where the Bruntons prop is useful as it adjusts the pitch of the prop to put more load on the engine..

Won't be anything to do with the exhaust size. 50mm is fine and common for that size engine - indeed many 50hp engines use the same. Fuel supply, injectors and controls are most likely. Probably worth having the compression checked while the injectors are out.

 

[oldmanofthehills]

I have mildly overpropping on my Beta25 as our shiny brandnew prop was bought for a Bukh 25 with 3:1 gearbox not its current 2.6:1. So possibly 15% overpropped. Its an LM27 and does 6.5kt under full power but only reaches about 3200rpm under load not the 3600 expected.

Thus theoretical over propping roughly matches 12% reduction in max rpm.

A/ I never expect to use max power except in extreme circumstances against desparate head seas (with the resulting really unpleasant resulting passage)

B/ The problem is no worse than having a really fouled prop, which clearly the geabox must be able to survive. Beta commented on it when discussing other issue but eventually said I might get slight reduction in max speed.

C/ I could have prop bite adjusted by place in London from 16/13 to 16/11, but so far not bothered. It does hull speed

The OPs problem seem worse than that and would suggest 100% over propped which seem very unlikely. So I expect engine simply not giving full power. There should be no reason why it does not do 3600 or there abouts in nuetral. Restricted airflow so fuel not burning? Problem with one of injectors giving vastly over rich mixture? Something wrong

 

[sv_telemachus]

UPDATE: Called Beta who were helpful as always and advised that this particular era of Beta 38 had a max tested idle rpm of 3,190 not 3,600. They provided the test data sheet from 2005 (attached)

So they were happy that given the age of the engine, 3100rpm in neutral is close enough to 3190rpm. They felt that while perhaps a little overpropped, it is likely to be steam and not smoke and not critical.

Nevertheless, this thread has provided a long list of basic engine checks that I will do - would be glad to reduce the steam/smoke and get up to hull speed under load.

 

[pandos]

I had similar a few years ago.
Turned out to be pump timing. Loosened the pump fixing bolts and forced it a tiny amount, then re-tightened, huge improvement. (Old diesel fitter had suggested this)