Beta 38 vs 35 RPM

[foojin]

Re-engining. Considering a Beta 38 with 3600 RPM. The Beta 35 is built on the same Kubota engine, but with 2800 RPM.

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.

[maine50]

I believe that the goal is to have shaft RPM 1500 or less. So I think the question might be are the different transmission ratios available that gets you to that goal.

[a64pilot]

Originally Posted by foojin

Re-engining. Considering a Beta 38 with 3600 RPM. The Beta 35 is built on the same Kubota engine, but with 2800 RPM.



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.

I would tell you that the difference between the two is the engine RPM, except that is a lot of RPM to be only 3 HP.
Assuming they are the same engine I can’t explain how 800 more RPM only gets you 3 HP.
However if those numbers are true, I’d go with the 35 and enjoy the slower turning motor myself.

Engine RPM does not necessarily correlate to prop RPM, transmission ratios do, it’s possible that the 35 could spin the prop faster even with lower engine RPM if the transmission were set up that way.

Surely a Beta Rep could explain what is going on, there must be more to the story, even having two engine choices with the HP being that close just doesn’t sound right, there have to be other considerations.

[foojin]

Originally Posted by a64pilot

I would tell you that the difference between the two is the engine RPM, except that is a lot of RPM to be only 3 HP.
Assuming they are the same engine I can’t explain how 800 more RPM only gets you 3 HP.
However if those numbers are true, I’d go with the 35 and enjoy the slower turning motor myself.

Engine RPM does not necessarily correlate to prop RPM, transmission ratios do, it’s possible that the 35 could spin the prop faster even with lower engine RPM if the transmission were set up that way.

Surely a Beta Rep could explain what is going on, there must be more to the story, even having two engine choices with the HP being that close just doesn’t sound right, there have to be other considerations.

Talked to someone today at the local boat show who is a dealer and has installed many Betas. He shrugged at the question, admitted he wasn't sùre why Beta would offer the two so close in HP.

[Jon Neely]

Research de-tuning, that's what the 35 is, Nigel Calder talks about the process. After lots of research we went with the beta 35. It's all about torque. We have been very happy. Hc33 25000lbs laden, beta 35 with the TMC 60 2.45:1

[foojin]

Originally Posted by Jon Neely

Research de-tuning, that's what the 35 is, Nigel Calder talks about the process. After lots of research we went with the beta 35. It's all about torque. We have been very happy. Hc33 25000lbs laden, beta 35 with the TMC 60 2.45:1

Thanks Jon, will look it that when I get back to the boat.. Is that in one of his books or should I be googling?

[Stu Jackson]

Originally Posted by foojin

Talked to someone today at the local boat show who is a dealer and has installed many Betas. He shrugged at the question, admitted he wasn't sùre why Beta would offer the two so close in HP.

There's a trustworthy source.

[shakey doug]

Just a thought, but could the 3,600 rpm unit be for fitment to a 2 pole 60hz Generator?

[NCboatrx]

The Beta 35 and 38 are basically the same engine but the fueling is different. On the low speed version (Beta 35) more fuel is injected to provide a little more torque and the RPM is restricted to 2,800.
Propeller speed can be adjusted by using transmissions with different ratios. Thus, the Beta 35 with 2:1 transmission will have almost the same propeller speed as the Beta 38 with 2.45:1 transmission.

[S/V Alchemy]

Originally Posted by maine50

I believe that the goal is to have shaft RPM 1500 or less. So I think the question might be are the different transmission ratios available that gets you to that goal.

Or different prop pitching. The general idea is related to the engine plus the transmission plus the prop pitch forming the fuel map that, again generally, is most efficient at 75% of WOT.

For me, with a steel full keeler and 2700 RPM maximum with a Beta 60, that's around 1800-1900 RPM and about 6.2 knots in a flat sea. Your mileage will almost certainly vary.

[a64pilot]

Originally Posted by Jon Neely

Research de-tuning, that's what the 35 is, Nigel Calder talks about the process. After lots of research we went with the beta 35. It's all about torque. We have been very happy. Hc33 25000lbs laden, beta 35 with the TMC 60 2.45:1

Yes, it’s real common in everything from outboards to airplane motors.
However 800 RPM should be more than 3 HP though, it may be fueling as another poster said, and I’d suspect a different cam as well.
Something is recovering a lot of HP in that 800 RPM drop.

Torque and HP are inseparable, there is no difference, it’s different ways of measuring power is all.
What we consider a high torque motor is one that makes good HP at lower RPM

[TrentePieds]

Since you are in the Salish Sea, go here:

Beta Marine Canada - Contacts and Dealer Network

Any one of these dealers, let along the distributor, should be able to give you an answer far better than we can. Any one of them will make a firm recommendation based on your particular boat. I'll bet that there are enuff Tayana 37s with Betas already in them that the dealers can give you a reliable answer right off the cuff.

If you need propeller work, Osborn Propellers in North Vancouver are the "go to people" for most of the commercial and pleasure fleets in the Salish Sea.


Cheers

TrentePIeds

[Scout 30]

The 38 costs $300 more than the 35 which is attributed to the addition of an oil cooler. I'm no expert but if the 35 does not need the oil cooler wouldn't it follow that it's not working as hard to achieve similar HP?

[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?

[River Cruiser]

Originally Posted by a64pilot

Yes, it’s real common in everything from outboards to airplane motors.
However 800 RPM should be more than 3 HP though, it may be fueling as another poster said, and I’d suspect a different cam as well.
Something is recovering a lot of HP in that 800 RPM drop.

Torque and HP are inseparable, there is no difference, it’s different ways of measuring power is all.
What we consider a high torque motor is one that makes good HP at lower RPM

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