Revs for Beta 25

[Yealm]

I’ve got a 2 year old engine.

I’ve a vague recollection of the engineer saying to use up to about 2500 revs, but that’s only giving me about 4.5 knots (32 foot)- wondering if I mis-remembered.

What would be a sensible upper limit of revs? Maybe 3000?

 

[Supertramp]

From looking at the data sheet for a Beta 25 it makes 25hp flat out at 3600rpm. Depending on your boat, but assuming a normal shape and about 5 tonnes displacement you should get 5knts+ at well under 3600 rpm. Some people argue diesels like to run flat out but I have always found the best running to be around the peak of the torque curve at around 2400rpm for your engine. There is little point pushing beyond hull speed. Check your log - cross check with a GPS on Navionics or similar. If the speed still looks low but the engine revs fully you will need to start considering propellor sizing etc.

 

[Sandy]

Speed really depends on hull shape, but 4.5 knots with a 25 hp engine pushing a 10 metre vessel to me sounds low unless you have the Hanging Gardens of Babylon suspended from your keel. Your theoretical hull speed is about 7 knots. What is your average STW when sailing?

My own boat, 10.1 metres, with a 35 year old 18 hp Volvo Penta 2002 at c2,500 RPM will chug along at 5.5 knots STW for hours.

I'd look at how clean the hull/prop is, then go out on a calm day find a couple of transits/buoys and do some runs between the markers at different RPM settings and see where the engine sounds happiest.

You may find this document interesting.

Beta 25 Specification Sheet

 

[Yealm]

Speed really depends on hull shape, but 4.5 knots with a 25 hp engine pushing a 10 metre vessel to me sounds low unless you have the Hanging Gardens of Babylon suspended from your keel. Your theoretical hull speed is about 7 knots. What is your average STW when sailing?

My own boat, 10.1 metres, with a 35 year old 18 hp Volvo Penta 2002 at c2,500 RPM will chug along at 5.5 knots STW for hours.

I'd look at how clean the hull/prop is, then go out on a calm day find a couple of transits/buoys and do some runs between the markers at different RPM settings and see where the engine sounds happiest.

You may find this document interesting.

Beta 25 Specification Sheet

Thanks- 3 blade folding Featherstream, all fairly clean below I think! Heavy-ish boat- C32. I’ll do some more testing when I’m in more flat, tideless water..

 

[Bobc]

Thanks- 3 blade folding Featherstream, all fairly clean below I think! Heavy-ish boat- C32. I’ll do some more testing when I’m in more flat, tideless water..

Put the hammer down and see if it gets to 3600rpm easily. If it does, you can pitch the prop up a bit more.

 

[doug748]

I’ve got a 2 year old engine.

I’ve a vague recollection of the engineer saying to use up to about 2500 revs, but that’s only giving me about 4.5 knots (32 foot)- wondering if I mis-remembered.

What would be a sensible upper limit of revs? Maybe 3000?

I think there may be something amiss there Yealm, perhaps the prop is not opening fully, or is fouled?

I'm not a greatly fond of motoring but find that early season, flat water, I can make 5kts ok; similar boat, 2 bladed prop and Beta 20hp. I never use more than 2200 - 2400 revs above that it gets too noisy for me, I doubt I have ever tried anything like 3000 revs let alone the theoretical limit. I doubt you could steer a fin and skeg boat at those revs, it starts to get scary, certainly with the two blade job.

.

 

[Concerto]

On my 32ft Fulmar I have a Beta 25 with a 15x10 2 bladed Gori folding popeller. I normally cruise with 2200 to 2400 rpm and am doing about 5 to 5½ knots easily in sloppy water, a bit more in smooth water. Flat out I can get 7 knots, but drinks diesel a lot faster. It sound more like a propeller pitch problem. Best to test it out and if not happy have a word with Featherstream supplier.

 

[Tranona]

Sounds like the pitch is too low. 2500 is about right for cruising, perhaps a bit high, but you should get 5.5 knots in flat water. Suggest you do a series of runs in flat water, at 200rpm increments from 2000 up to as fat as it will go (expect it will hit 3600 easily) and plot speed against revs. You should get at least 6.5 knots at full power, but suspect you might be shy of this based on getting only 4.5 knots at 2500. Then talk to Chris at Darglow about whether you need to up the pitch.

 

[Yealm]

Thanks to all, will do some testing as suggested and review the prop pitch
I’ve noticed most other boats around are faster (but then they are sailing too!)

 

[Minchsailor]

I had a Nicholson 32 with the 2 cylinder Beta which I think is about 13-14hp. No problem with motoring at 5kns at around 2300/2400 rpm.

Can't remember the prop, but have you checked the gearbox ratio.? Think 2:1 is recommended.

Feed the numbers into something like Propcalc, which I have found is pretty reliable

 

[NDG]

Thanks- 3 blade folding Featherstream, all fairly clean below I think! Heavy-ish boat- C32. I’ll do some more testing when I’m in more flat, tideless water..

We’ve just fitted a beta 25 in our CO32 last year, replacing an old yanmar 2GM. We had the featherstream re-pitched by Darglow at the time and we are getting about 5 knots at 2000 rpm with a clean hull. It sounds like the prop pitch would almost certainly be the issue.

 

[johnalison]

Volvo give maximum revs minus 400/min as the cruising speed. For me this would mean motoring at 3000, which I have only done for short periods (Entering Dover!). With a similar 27hp engine and a 10m boat I normally motor at 2500 for extended passages, giving about 6.5 kn in still water with a 2-blade folding prop, when clean, though I have done 2700 for 24hr on one occasion. My policy would be to use the speed that sounds and feels best, and if the speed doesn’t match expectations give some attention to the prop.

 

[LittleSister]

I’ve got a 2 year old engine.

I’ve a vague recollection of the engineer saying to use up to about 2500 revs, but that’s only giving me about 4.5 knots (32 foot)- wondering if I mis-remembered.

What would be a sensible upper limit of revs? Maybe 3000?

There is obviously something amiss, in that you should be able to cruise significantly faster at 2,500 revs, but while you're identifying/sorting that your engine will be quite happy running at 3,000 revs, so don't worry at all about using those revs if it gives you the speed you want or need.

If the problem is your prop is undersized, underpitched or not fully opening, until you get that fixed your engine will never be able to put out its max 25hp if needed, and at 3,000 revs it will neither be putting out anything like the 22hp the Beta graph indicates is available at that engine speed, nor drawing the amount of diesel that would be required to achieve that.

On the other hand, if the problem is you've forgotten to retrieve your anchor . . .

 

[Yealm]

There is obviously something amiss, in that you should be able to cruise significantly faster at 2,500 revs, but while you're identifying/sorting that your engine will be quite happy running at 3,000 revs, so don't worry at all about using those revs if it gives you the speed you want or need.

If the problem is your prop is undersized, underpitched or not fully opening, until you get that fixed your engine will never be able to put out its max 25hp if needed, and at 3,000 revs it will neither be putting out anything like the 22hp the Beta graph indicates is available at that engine speed, nor drawing the amount of diesel that would be required to achieve that.

On the other hand, if the problem is you've forgotten to retrieve your anchor . . .

Darglow have given me a sea trial testing form to fill in - will report back!

 

[neilf39]

On my Beta 25 I get 5.5kn at 2200 revs with a clean bottom. 15x8 3 bladed feathering prop on a Konsort. 2400 is about 6kn. Extra revs are to push into strong headwinds/chop.