Beta 38 vs 35 RPM

[Suijin]

I think the best answer to this question would probably come from Stanley at Beta’s distributor in Arapahoe NC.

[Clivevon]

Decisions, decisions.
I re-engined my (say 18000 lb fully loaded) 36 ft Westerly Conway 3 years ago with a Beta 35 replacing the old 36hp Volvo, which had always seemed powerful enough.
I decided against the 38hp version as I preferred a less stressed engine, & against the 43hp on a combination of cost & preferring a diesel to work harder rather than being underemployed.
Having said that, some Conways have a 42 hp Mercedes as original equipment.
I have been very happy with my Beta 35 but it sometimes struggles to get to 2800 revs. and we do get strong tides in the English Channel so I sometimes think a bit of extra power in reserve to punch against headwinds & foul tides (when I had obviously screwed up) might be no bad thing. I noticed that the Halberg Rassy 36, a similar weight to my boat, has a 50hp fitted as standard - which would seem to be way over the top & shorten the engine life as it would normally be working well below capacity. Still, I guess they know what they are doing? Decisions, decisions....

[Clivevon]

and I normally cruise at around 80% of WOT to give 85% of hull speed.

[Kokanee]

Originally Posted by foojin

Why would I choose the 35? I would guess the 35 would need a larger prop to drive the same vessel at the same speed. Perhaps the slower RPM would mean less wear and tear.

Any ideas?

Cheers.

Either engine will power the boat at maximum hull speed most of the time. The extra 3 hp on the 38 may give you an an extra 0.1 knot when motoring into a strong wind. That would be the only advantage. The cost would be a lot more wear & tear and noise.

Extra hp will not help against current. A full displacement boat is limited by hull speed through the water.

[Captain Puget]

I went with the 35 due to lower rev's/ less wear philosophy ... also quieter at lower revs.

Coming up on 3 years and happy.

[a64pilot]

Originally Posted by River Cruiser

Hp is is how fast you hit the wall. Torque is how far you push the wall.

Research your answer and I think you will come to understand.
What you say is the common misunderstanding.
Simply put torque is force applied, not necessarily any work done as movement isn’t required.
Horsepower is force applied and a weight moved a set distance over a set time, work is done.
Horsepower is basically torque x RPM

If you will look at every dyno chart there is, you will notice that the HP and the torque curve cross at 5252 RPM.
http://www.epi-eng.com/piston_engine...and_torque.htm

[a64pilot]

Originally Posted by keithw88

Re Alchemy’s comment about the fuel-efficient point (75% of WOT).

I have long wondered and occasionally researched where is the most efficient point on the speed curve, without getting much info except that operating conditions and particular boat characteristics make it difficult to predict which is the most efficient engine speed.

However, Alchemy refers to WOT - is this founded on science or just a good rule of thumb? And given the characteristics of a diesel engine, if the prop is properly sized (so as not to overload the engine), is there much or any difference in engine speed for any specific “throttle” position with varying load? I thought diesels pump more fuel in to maintain the set engine speed - and that a drop in engine speed (usually accompanied by black exhaust smoke) with more load indicates over-loading?

So would it be correct to re-write Alchemy’s statement as 75% of max engine speed, and would that be a solid guideline or just a rule of thumb?

The fuel efficiency sweet RPM is easy to find on an engine, just look at the BSFC chart and it’s the bottom of the curve.
However that does not correlate to the best MPG of the boat, not even close, in fact it’s actually an irrelevant number for us.
Reason is that the hull drag etc are way more relevant and are what drive fuel efficiency, not the sweet spot of an engine.

Attached is for my 4JHE, you have to see the whole page to see the RPM, in this case the bottom is roughly 2500 to 2800 RPM