Beta 35 or Yanmar 30 in a Moody 346?

[Tranona]

Replaced a 4108 with a Beta 35 three years ago.

Different type of engine. Perkins had a lot of torque, hence 15 x15 prop, which pushed the boats at a good 6 knots at 1800 rpm.

Beta was held back and had to be repropped at 15 x 9. Hence to get 6 knots needs to be run at 3000 rpm. so any fuel saving are negated by throttle settings difference.

On the other hand you are replacing a 35 with a 35 so the prop question should not be serious, unless the old engine was high torque and relatively low revving, when you might need to reduce the prop size by reducing the pitch slightly.

If you have the torque outputs for the old engine you could compare them with those on the Beta website.

This is very misleading. It is little to do with torque, but more to do with the shaft speed. You cannot expect to use the same prop with an engine that produces its power at a higher rpm and probably has a different reduction ratio. Prop sizing is a function of max bhp, shaft speed, weight of boat and LWL. Change any one of those and the prop size changes.

 

[sailorman]

This is very misleading. It is little to do with torque, but more to do with the shaft speed. You cannot expect to use the same prop with an engine that produces its power at a higher rpm and probably has a different reduction ratio. Prop sizing is a function of max bhp, shaft speed, weight of boat and LWL. Change any one of those and the prop size changes.

i have a MaxProp so changing the pitch was easy, i had to increase the pitch one notch. the info is O/b so not to hand.
15 x 9 seems small i have an 18" prop.
the Nanni 37.5 replaced a 4108 both rated @ 3000 RPM

 

[A1Sailor]

This is very misleading. It is little to do with torque, but more to do with the shaft speed. You cannot expect to use the same prop with an engine that produces its power at a higher rpm and probably has a different reduction ratio. Prop sizing is a function of max bhp, shaft speed, weight of boat and LWL. Change any one of those and the prop size changes.

I have never really understood this - but now I think I do! Is the other factor the gearbox then, and its reduction ratio? This is of course fixed, but you can get different boxes with different ratios - right? If a gearbox has a ratio of, say, 33:1 does that mean that if the engine is doing 3,300rpm the propshaft is doing 100rpm? If 1,650 then 50? Do different combinations of engine & gearbox require different propellors? Presumably yes - duh.
Sorry if these are daft questions - but I need to complete my learning objectives for today! I might even get a kertifikate...

 

[Tranona]

I have never really understood this - but now I think I do! Is the other factor the gearbox then, and its reduction ratio? This is of course fixed, but you can get different boxes with different ratios - right? If a gearbox has a ratio of, say, 33:1 does that mean that if the engine is doing 3,300rpm the propshaft is doing 100rpm? If 1,650 then 50? Do different combinations of engine & gearbox require different propellors? Presumably yes - duh.
Sorry if these are daft questions - but I need to complete my learning objectives for today! I might even get a kertifikate...

You are on the right lines. Just as an example, I have just changed the engine in my boat from a 1GM (9hp) with a 3.3:1 reduction approx 1100 shaft speed to a Nanni 14 with a 2.6:1 approx 1400 shaft speed but was able to keep the same size prop because of the extra power. If I had chosen the more common 2:1 I would not have been able to use such a large diameter prop because the shaft speed would have been too high.

If you want to do a bit of experimenting go to www.castlemarine.co.uk and look at the Propcalc programme. Put in dat afor a known boat/engine combination and then change gearbox ratios and number of blades to see the change in recommended props. The most common reduction ratios in small boat auxilliaries are between 2:1 and 2.6:1 with max engine revs between 3000 and 3600, although there are some boxes such as the Yanmar running greater reduction to allow the use of larger diameter low pitch props for boats like mone - heavy with long keels and wide deadwoods.

 

[A1Sailor]

Thanks for the info Tranona. I've bookmarked the link you suggested, for future use.

 

[pappaecho]

This is very misleading. It is little to do with torque, but more to do with the shaft speed. You cannot expect to use the same prop with an engine that produces its power at a higher rpm and probably has a different reduction ratio. Prop sizing is a function of max bhp, shaft speed, weight of boat and LWL. Change any one of those and the prop size changes.

When I posted my comments, I failed to mention that both engine and gearboxes has the same reduction 2:1, which means that the shaft speeds were exactly the same with the same prop.
The word power, is not accurate in this system, and more to the point is torque.

At 1800 rpm, the Perkins produced about 6 knots. The Beta 35, only just over 5 knots with everything being the same, that is reduction ratio, and prop. The engine was being held back, because it did not have enough torque to turn the prop.
AS posted, the change of prop sorted that problem, but the engine has to be run at 2800 rpm to get 6 knots in normal conditions, such as punching the tide in Portsmouth harbour during springs. Running a new and more efficient engine at 2800 was cancelled out by the relatively slow Perkins, even though it was old and less efficient

 

[sailorman]

When I posted my comments, I failed to mention that both engine and gearboxes has the same reduction 2:1, which means that the shaft speeds were exactly the same with the same prop.
The word power, is not accurate in this system, and more to the point is torque.

At 1800 rpm, the Perkins produced about 6 knots. The Beta 35, only just over 5 knots with everything being the same, that is reduction ratio, and prop. The engine was being held back, because it did not have enough torque to turn the prop.
AS posted, the change of prop sorted that problem, but the engine has to be run at 2800 rpm to get 6 knots in normal conditions, such as punching the tide in Portsmouth harbour during springs. Running a new and more efficient engine at 2800 was cancelled out by the relatively slow Perkins, even though it was old and less efficient

The perkins 4108 would be rated @ around 38hp @ 3000 RPM. it could generate up to 48hp at higher revs

 

[Tranona]

When I posted my comments, I failed to mention that both engine and gearboxes has the same reduction 2:1, which means that the shaft speeds were exactly the same with the same prop.
The word power, is not accurate in this system, and more to the point is torque.

At 1800 rpm, the Perkins produced about 6 knots. The Beta 35, only just over 5 knots with everything being the same, that is reduction ratio, and prop. The engine was being held back, because it did not have enough torque to turn the prop.
AS posted, the change of prop sorted that problem, but the engine has to be run at 2800 rpm to get 6 knots in normal conditions, such as punching the tide in Portsmouth harbour during springs. Running a new and more efficient engine at 2800 was cancelled out by the relatively slow Perkins, even though it was old and less efficient

There is something odd about the figures you are quoting. The Beta 35 has max revs at 2800, not 3000. Although I do not have the Perkins power curve to hand, I cannot believe that it produces significantly more torque (or BHP) at 1800 than the Beta which produces 24bhp and 90nm @1800 (max 93 @ 2000) and over 85 from about 1350 onwards.

The prop sizes you quote do not make sense. For the boat in question the calculated prop size for the Beta is 16*13 and 16*12 for the Perkins - both very close to the Beta recommendation of 16*11 as a starting point. Both of these would give a max speed of around 7 knots.

You don't say what boat the engine(s) were fitted to, so although you would expect a small change in pitch to allow for the higher revs/greater power of the Perkins, the difference you quote seems wrong - as does the maximum speed you were getting, unless it was a very heavy boat with a short waterline.

 

[pappaecho]

I have a Jag which is 240 bhp and generates 450 ft lbs of torque, and cruises at 80 mph at 1800 rpm.
My previous E Class had 250 bhp and 220 ft lbs torque. At 80 mph it needed 3000 rpm.
There is a fundamental difference between torque and bhp, usually associated with the stroke of the engine, the longer the stroke, the greater the torque, but not necessarily the rpm

 

[Tranona]

I have a Jag which is 240 bhp and generates 450 ft lbs of torque, and cruises at 80 mph at 1800 rpm.
My previous E Class had 250 bhp and 220 ft lbs torque. At 80 mph it needed 3000 rpm.
There is a fundamental difference between torque and bhp, usually associated with the stroke of the engine, the longer the stroke, the greater the torque, but not necessarily the rpm

I know all about that, but I bet if you superimposed the power and torque curves of the two engines in question you would be hard pressed to determine which is which. Small high speed diesel engines (unlike high performance petrol car engines) have very flat torque curves. This is because they are virtually constant rpm engines - you determine the power you need to achieve the speed you want and leave it there. Acceleration (which is where torque counts) is largely irrelevant. You are only teling half the story with your two car examples - you need to look at the shape of the power curves - not just at the maximum power and torque. It is not just about the bore/stroke ratio, it is about the capacity of the engine and its max revs. I expect you will find the bhp of the two engines at the revs you quote is fairly close - but of course there are other issues such as the weight of the car and its drag coefficient to take into account. You will find the smaller capacity MB produces its power at higher revs and has shorter gearing so that it revs higher at a given road speed.

However, little of that is relevant to displacement boat powering. On most displacement boats you have a fixed top speed and the power needed to achieve that is largely a function of weight - heavier the boat, more thrust required. You choose the prop so that you can achieve maximum speed at close to maximum revs. On your example - if hull speed was 6knots and you are achieving this with 1800rpm in the Perkins - the boat only needs about 24hp - which is what the Perkins is producing at those revs - just as the Beta will.

That is why I was suggesting there is something odd about the data you are quoting as it does not "fit".