Prop size and gearbox ratios

[Tranona]

Ianj99 and Tranona interesting conversation. I can't really add a balanced opinion but I can add my observations from my own boat.

She has a Perkins 4236 with 1.91 : 1 gearbox reduction. I don't have my prop size at hand but it is a large diameter, quite large pitch and is 3 bladed.

The yacht was apparently set up for long distance cruising by a previous owner who selected the current arrangement to speed the boat along at low rpm. At about 1500 rpm she is doing 6 kts. At 1800 rpm she was doing about 7.5 kts but cavitating very loudly (1500 rpm is the sweet spot) with her stern well dug in. I also experienced having to motor into short 1 m chop, from a F7 and she still cut 6 kts at 1500 rpm.

I am not so sure what the maximum power rpms are for this engine but it will be around 2500 rpm.

I have since discovered that the original design had a 2 bladed prop and was not so pitched. At tick over I will get up to 3 kts so close quarters slow manoeuvring requires a lot of neutral to keep the speed down below 2 kts.

The big pitch on the current prop provides a lot of kick in reverse to the point of irritation. At slow speed bringing her to a stop still produces a lot of prop walk.

So I would tend to agree with Tranona that there is much more to be taken into account. I did a few searches on Googol when looking into re propping my own boat and there are lots of folks moaning about performance after changing propellors!

All the symptoms a prop that is too big - and maybe an engine that is too powerful for the boat.

If you can get the power curve to get the max HP (not what it says on the brochure), revs, waterline length and displacement you can get a good estimation of the optimum size from the Propcalc programme. Also useful to know the maximum diameter of prop you can swing.

 

[Tranona]

We'll have to agree to disagree.

Prop power absorption curves tend to show that dropping the prop rpm by 30% means a 50% drop in power requirement. Which means at 2500rpm the engine is only lightly loaded. I'd rather make it work a bit harder by overpropping to achieve the same speed at a quieter and more economical 2200 or so rpm and sacrifice a bit of the top speed.

As I said, it just depends on how often and for how long you want to run the thing flat out.

Dont understand this argument. Dropping the revs by 300 is only 12%. I don't have the power curve to hand, but I would think the drop in power is a similar amount. Why do you think it is lightly loaded at 2500? The prop on my boat is the original Volvo/Bavaria fitment and I guess they know a thing or two about choosing the right compromise.

 

[KREW2]

Yes, I joined the Westerly assocation in January. My Storm is fitted with a 16x15 2 blade prop, I say fitted it's on the ironing board at the moment. If I can get 5 knts at 2000rpm I will be happy. I will be interested to hear what sort of performance you get out of your new prop. I may want to take up water skiing.

 

[ianj99]

Observations after kiwi prop fitted

Boat: 37' steel ketch, 10t displ, long keel, Perkins 4108, 1.79:1 ZF10M g'box.

Original prop was a 17*12" LH and the gearbox was run in the normal reverse position for ahead (1:1.86). Cruising rpm was about 1800rpm at 5.5kts and max rpm was 2400 giving about 6.5kts - ie it was over propped. However it was relatively quiet and economical (.5gph) when cruising.

The Kiwi prop is 17"*21degree pitch (NB degrees not inches) RH.
Switching to a RH prop has cured the loud high pitched whine which I took to be gear whine and in calm conditions yesterday, I measured (average of runs in both directions using GPS):

1000rpm 3.7kts
1250 4.6
1500 5.4
1750 6 (.5kt more than with the old prop)
1900 6.3 no smoke
2000 6.6 gets upto 2000rpm quickly, then some smoke on full throttle and labours trying to rev higher
2150 6.8 (max rpm achieved, with the tide) some smoke on full throttle.

Clearly the pitch needs adjusting but I am not inclined to alter it by much - perhaps just to let the engine rev to 2500rpm.
I disagree with the idea of pitching the prop to allow maximum rpm to be reached where the engine produces more power than is ever likely to be required under most circumstances.

From Perkins' 4108 power chart, at 2000rpm about 28shp is available and as the engine revs easily to this but labours trying to exceed it, then all 28bhp is required to do 6.6kts. So the 40shp @3 000rpm continuous power rating is not accessible with the pitch as it is currently.

I could adjust the pitch for 3000rpm max and a bit over 7kts , but this would result in the 6kts cruising rpm being noisier and potentially less economical at about 2400rpm. The only advantage of this approach is in adverse conditions where the extra power can be used to maintain the cruising speed.

Whereas if the rpm is restricted by using a bigger pitch to about 2500rpm, the cruising rpm will be about 2000 for 6kts, with less noise (diesels run more smoothly and quietly when under a reasonable load) and better fuel consumption.
OTHER OBSERVATIONS:

The Kiwi prop performs very well astern, with noticeable retardation which was a major reason for fitting it, although there is more pronounced prop walk.

There is a little noise from the prop at low forward rpm but this disappears over 1000rpm so combined with the absence of gear whine, and low cruising rpm makes for very smooth, effortless and quiet progress.

 

[Red Admiral]

Yes it does look like that, except it is a different colour -- a sort of muddy grey. The problem we had reading the plate is that the hot water tank is fitted over it and makes it difficult to get to. However, this week-end a successful sortie by smaller son extracted the data and we now think that 3.02 is the ratio we have.
As I said in my earlier post, our potential spline failure problem had been sorted before we bought the boat, but there is lots of info on the Westerly Owners site. Have you joined WOC? They recently signed up their 3000th member.

With a 17" prop the gearbox will certainly be 3:1. My boat was fitted with the 2002 engine and 3:1 gearbox and had been deliberately overpropped with a 17x15 propellor. Overpropping is never clever! It can exacerbate any tendency to overheating but, more importantly, it will compromise your motoring performance into a nasty head sea and head wind when you need every pound thrust you can get - only available if your engine is propped correctly. I had Norris reduce the pitch on mine to 13 inches. The engine ran more smoothly, used less fuel and the boat was faster!

By the way, boat speed in calm water depends on waterline length and has little to do with engine max power or with correct propping.

 

[ianj99]

By the way, boat speed in calm water depends on waterline length and has little to do with engine max power or with correct propping.

That's ridiculous - its as much to do with hull shape, engine power AND prop as waterline length.

In my case, the hull speed is in the region 7 to 7.5Kts but I cannot achieve it with the curent prop pitch because about 35 to 40bhp is required which demands 2800 to 3000rpm on a 4108. So currently max rpm is 2100 - or about 30bhp which translates to 6.7 knots.

I used a couple of online calculators to check my estimates:

http://www.castlemarine.co.uk/pitch.htm and

http://www.psychosnail.com/boatspeedcalculator.aspx

The latter gives a good indication of power required for a particular speed, and the former, the prop size to achieve this at various engine power and prop rpms.

The results show that about 30bhp at 2250rpm is needed for 6.5kts, 36bhp @ 2700rpm for 7kts, & 45bhp @3600rpm for 7.6kts which seem quite valid for my boat. Since 7.6kts would be unrealistic and uneconomical, I'll aim for about 2500rpm and 7kts by turning the pitch adjusting screws about 1/3rd as advised by Alan Pollard of Vectamarine, the Kiwi importers.

 

[Tranona]

Pitch your prop to achive near to max rated revs as possible. If you do not use full revs you do not use all the power your engine produces - so you might just as well have a smaller engine. The max speed in flat water is limited by waterline length, but your ability to achieve that is a function of power and weight of the boat. If you set your prop correctly you will comfortably achieve max hull speed at near max revss, which allows you to cruise comfortably at between 60-70% max power, leaving plenty in reserve for adverse conditions.

 

[BlueSkyNick]

I know this is an old thread which has been resurrected, but everybody seems to be talking about the power curve across the rev scale which ignores where optimum torque is derived, and at what fuel consumption.

 

[Tranona]

I know this is an old thread which has been resurrected, but everybody seems to be talking about the power curve across the rev scale which ignores where optimum torque is derived, and at what fuel consumption.

That is because for the most part the shape of the torque curve is relatively unimportant. For most small diesels it is pretty flat in the operating range - say from 2-3000rpm and the specific fuel consumption curve is also flat. So fuel consumption is almost a direct function of revs/hp produced. Unlike automotive engines where torque is important for acceleration, marine engines operate at almost constant revs. Just as an example the Beta 35 has peak torque of 90nm at 2000 rpm - but 85nm at 1400, and at 3000, and is never less than 80nm.

Similarly fuel consumption is almost linear to power produced, so you will consume the same fuel drawing 20 hp out of a 30hp max engine as out of a 40hp engine.

This is why it is important to match your max engine power and prop so that you achieve the max hull speed at near maximum revs and cruise 65-70% max power. In the example of the Beta 35 that would mean cruising at 2000 rpm, using around 26 hp and that is the point of max torque and lowest specific fuel consumption. In terms of speed on (say) a 32ft WL that would be a max speed of around 8 knots and cruising just over 6 and a fuel consumption of about 2.5l hour.

 

[ianj99]

Pitch your prop to achive near to max rated revs as possible. If you do not use full revs you do not use all the power your engine produces - so you might just as well have a smaller engine. The max speed in flat water is limited by waterline length, but your ability to achieve that is a function of power and weight of the boat. If you set your prop correctly you will comfortably achieve max hull speed at near max revss, which allows you to cruise comfortably at between 60-70% max power, leaving plenty in reserve for adverse conditions.

That's the accepted wisdom, but my point is, where the engine is larger than strictly needed it can be run at lower rpm and hence be less noisey. (I really do not like engine noise and why is it that marine engines and their installations suffer from far worse noise, vibration and harshness than automotive engines. My ears are a metre from my campers 2.4L 5cyl 80bhp diesel and even at 3000rpm engine noise is not obtrusive....)

Surely, the only requirement is to decide on how much of the available (excessive) power you want to have in reserve to be able to maintain cruising speed in adverse conditions.

If a quiet and smooth 2000rpm & 6kts cruising speed requires approx 22bhp, then pitching the prop to let the engine reach say 2500rpm where 36shp is available gives a 14bhp reserve.
So no, the remaining 14bhp isn't accessible, but nor is it likely to be needed and a smaller engine could have been fitted. But then you are back to square one - having to run at much higher rpm and suffer the noise....

4108 power curves attached.
Ian

 

[Tranona]

The 4.108 has very low specific power output compared with modern diesels and may tolerate running at lower revs without the well known problems such as bore glazing.

The basic issue is whether it is sensible to install an engine rated at 51hp when by your own calculations it only needs 22hp to achieve a sensible cruising speed. Most yachts today are just not so grossly overpowered.

As to noise, most of this does not come from the basic engine, but (in comparison to an automotive application) the difficulty of isolating the noise from the rest of the structure. If you ran an automotive diesel bare it would not be significantly quieter than a marine unit. It is the shrouding and insulation plus the "escape" route underneath the vehicle that makes the difference.

 

[ianj99]

At the risk of labouring the point, referring to the power curve, if I was to size and pitch the prop to follow curve 3 and lets assume this resulted in 8kts at 4000rpm, then 6 kts would require about 3000rpm and 6.6kts 3400rpm.
(From speed trials, about 22bhp is needed for 6kts and 30 for 6.6)

The result would be disappointing and the feeling that the engine was having to be revved too hard would exist, apart from the noise and potential fuel consumption issues.

 

[ianj99]

The 4.108 has very low specific power output compared with modern diesels and may tolerate running at lower revs without the well known problems such as bore glazing.

The basic issue is whether it is sensible to install an engine rated at 51hp when by your own calculations it only needs 22hp to achieve a sensible cruising speed. Most yachts today are just not so grossly overpowered.

As to noise, most of this does not come from the basic engine, but (in comparison to an automotive application) the difficulty of isolating the noise from the rest of the structure. If you ran an automotive diesel bare it would not be significantly quieter than a marine unit. It is the shrouding and insulation plus the "escape" route underneath the vehicle that makes the difference.

The engine has been in the boat since it was built but if I was re-engining it, I would still opt for a larger than strictly necessary engine.

 

[BlueSkyNick]

The engine has been in the boat since it was built but if I was re-engining it, I would still opt for a larger than strictly necessary engine.

I am taking out the same engine as yours, 58hp, and going for 35hp on the basis it is just right (Tranona's description of the sums is exactly as TS Marina explained it to me)

I overpowered a previous boat and regretted it, apart from the additional cost and weight, it simply didnt get used at optimum revs around 70% of maximum.

 

[ianj99]

I am taking out the same engine as yours, 58hp, and going for 35hp on the basis it is just right (Tranona's description of the sums is exactly as TS Marina explained it to me)

I overpowered a previous boat and regretted it, apart from the additional cost and weight, it simply didnt get used at optimum revs around 70% of maximum.

Well I'm afraid you have fallen for the theorists and spent a lot of money needlessly.

You have the 35bhp which you require available from the 58bhp engine by simply running at the rpm at which it produces it

Thus, if you had ignored the experts and overpropped it so the prop absorb the 35bhp at whatever speed and rpm you desired, you'd have had a low revving quiet, economical cruising speed with plenty in reserve.

I suspect you will end up having to rev the smaller engine a lot harder, with possibly more noise and greater fuel consumption and may live to regret your decision unless there are other reasons to change the engine.

I don't disagree with Tranona and the experts, I just believe that sometimes you need to think outside the box than religiously follow the theory, which really only applies where the engine is more closely matched to the boat.
In other words, look at what shp the boat needs to drive it to the speed you want, then choose a prop to give you this regardless of what engine is fitted.

I can after all speak with personal experience - the Kiwi prop I have just fitted amply demonstrates the principle. I can now cruise at 6knts - 1/2 a knot more than with the previous prop, at the same 1800rpm, with the engine working harder and more efficiently since the prop power absoprtion curve is closer to the engines power output curve. Ie 22bhp is needed for 6kts and from the power chart, 25 is available at 1800rpm.

Would you change your car for one with a smaller engine because you never used the full performance - no you just use the surplus torque to avoid gearchanges and hence use fewer revs....

Ian

 

[BlueSkyNick]

You are missing a point: I wanted to replace the engine anyway so it is not a waste of money. In fact I have saved some by not buying another 58hp unnecessarily.