Prop size and gearbox ratios

[samwise]

I am trying to come to terms with the relationship between prop size, gearbox ratio and engine size.

I am replacing the two blade prop fitted to the Volvo 2002 in our Westerly Storm and in the process of supplying the required specifications to the makers of the new prop ( Darglow), have discovered that we may have been over-propped.

All this is a bit of a steep learning curve for me, having assumed that there was a standard set of ratios and prop sizes. Darglow tell me that the 18hp 2002 came with either a 2.4 or 3:1 ratio gearbox and I am setting out this weekend to work out what we have by counting the crankshaft rotations against prop shaft rotations ( The information plate on the gearbox is inaccessible and unreadable).

I was interested to read the current posts on engine revs on the forum. We usually run at around 1800 rpm which gives us around 5-5.5 kts knots in a flattish sea. We have rarely run the motor close to peak revs or indeed much above 2500, simply for noise and vibration reasons. The 17in diameter prop we inherited also has a fairly steep pitch.

The engineers to whom I have spoken say that this set up has limited our ability to transmit the full power of the engine should we ever need it. Thus far, we haven't, but the new three blade feathering prop of the correct (16in) diameter should lift that restriction.

I'm not sure I have got this right. Overpropping and ( I assume) underpropping) is new territory to me. Any engineers out there who can clear away the clouds?

 

[Bilgediver]

I'm not sure I have got this right. Overpropping and ( I assume) underpropping) is new territory to me. Any engineers out there who can clear away the clouds?

If you think of over propping in a similar way to a car trying to drive up a hill in 5th gear. It struggles and slows down till the engine torque can cope. However if you change down gear then the engine can cope better and you will actually go faster up the hill

The propeller should be sized in diameter and pitch so that the engine is delivering around hull speed at cruising revs . If over propped then as in your case this is happening at a lower speed. This has two disadvantages. One is that as the hull gets fouled the boat will slow down more quickly than if there was a correct match where you could increase engine revs and the other is that the engine is actually being run at loads OVER the torque line which means running conditions are exceeding the makers limits and so increase the chances of problems such as increased ring and liner wear and possibility of burnt exhaust valves.

This situation can be rectified by reducing the prop diameter , tweeking the pitch or both. Quite substantial changes can be made without changing props.

Reducing the diameter is actually increasing the slip !

If you look at a propeller blade you will see how by tinkering with the leading or trailing edge it is possible to tweek the effective pitch.

This is a much bigger problem on ships where normally there is very litttle slip and it is easy to overload the engine if the builders calcs are wrong.

Under propping is the opposite where you do not achieve the required speed even when the engine is at full revs. The engine is then not loaded up nor developing available power.

To select the correct prop you need to tak into account the gearbox ratio as your rpm gauge is recording the engine revs and not the propeller revs and it is the propeller revs which are required.

Castle marine do a calculation page that many use and you will find it here.

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

 

[saltwater_gypsy]

Unfortunately I get a message that the Castlemarine programme is incompatible with Windows7. !@%$****^ or words to that effect.

 

[Tranona]

Not much to add, except that you should not need to count the number of prop turns etc. There should be a plate on the gearbox with the forward and reverse ratios stamped on it.

It is critical that Darglow know this so that they can set the pitch stops at the right position for the appropriate pitch.

I think your engine peaks at 3000 rpm, so a 2.4 box gives a shaft speed of 1250 whereas a 3.1 gives 967. This is a big difference.

Ideally the greater reduction will allow you to swing a larger diameter prop which can be useful on some boats, but will make probably little difference in your case provided the pitch is correct.

Just as an example I had a Yanmar 1GM with a 3.3:1 box so that we could swing a 15 inch prop because it was behind a wide long keel. Just replaced the engine with a Nanni 14 with 50% more power but a 2.6:1 box. Using the same prop (like the one you are having) as spinning it faster will allow it to absorb the extra power.

 

[samwise]

Many thanks for all the responses so far, I think the picture is becoming clearer! The data that we could read off the gearbox plate didn't seem to relate to anything to do with ratios and was simply a serial number as far as could be seen. This week-end I intend to send one of my smaller and more supple children into the tunnel to try and have a closer look. We will also try a mirror on a stick. Also tried the mobile phone camera trick but that didn't work either. So the "count the turns" process would provide back up information. One day I'll have a boat with one of those walk-in engine rooms!

 

[lw395]

Larrson and Elliason's book 'Principles of Yacht Design' has a good chapter and some graphs, you might get it from the public library.

You can get nice quiet cruising in flat water by overpropping, but you might oneday want your engine to develop max thrust whilst slightly aground.
Another bluddy compromise!

 

[Pye_End]

Run your engine (with clean prop) at fully welly and see what rpm you are getting and compare with the max rated rpm. If it is reasonably close then you are not over-propped.

 

[KREW2]

I have a Westerly Storm, and most were fitted with the VP 2002. I assume you must have the MS2 gearbox. I am doing the spline conversion on mine, so at this moment in time it is sitting next to me in our utility room. The only number I can see on it that may relate to the drive ratio is 3.02, I cannot see yours being any different.

 

[samwise]

I have a Westerly Storm, and most were fitted with the VP 2002. I assume you must have the MS2 gearbox. I am doing the spline conversion on mine, so at this moment in time it is sitting next to me in our utility room. The only number I can see on it that may relate to the drive ratio is 3.02, I cannot see yours being any different.

I have a Storm too and I assume it is the MS2 although the Westerly habit of using a wide range of suppliers and components means that nothing is guaranteed. Darglow tell me that the ratio was either 2.4 or 3, so the 3.02 you have relates to the latter.

What's your experience with this engine? What size prop do you swing and what would you reckon the hull speed to be? As I have said, we tend to run ours at around 1800 rpm when cruising, which gives us around 5-5.5 kts in flat water, although I have spoken to owners who seem happy to run at much higher revs.

Thankfully, someone had already done the spline conversion on ours!

 

[KREW2]

The VP 2002 is a good engine in my opinion, I had one on my old boat, and it never missed a beat. I have only just bought my Storm, so no nothing as to how it is going to perform. Before I went ahead with the purchase I asked on here if the VP 2002 was "grunty" enough to push her along, most said it was, and if your 5.5 knots at 1800 rpm was in slack water that is good enough. I used to run at between 2000 and 2500 on my old boat, and she was a lot lighter. Does your gearbox look like this, http://www.yachtworld.com/core/list...reham-(ashore)/United-Kingdom&boat_id=2054669
If so it is the MS2, I really don't think they would fit any other gearbox to the VP 2002, and I certainly don't think they would fit different ratios on the same class of boat. The weakest link on the whole setup is the gearbox coupling, the splines wear, and drive is lost without any warning. It's not a question of will this happen, but when. Both the Moody, and Westerly owners associations have good images, and information on this problem, its well worth looking at.

 

[Ubergeekian]

Unfortunately I get a message that the Castlemarine programme is incompatible with Windows7. !@%$****^ or words to that effect.

It's free and therefore you get what you pay for, but for all that it's a horribly written program. Unless you install it in c:\ it can't find its own data files and it has no uninstaller. I have managed to install it both through Wine and in an XP virtual machine, but the output is unreadable in both cases.

John Bunt of Exeter used to distribute, free, a spreadsheet which did very comprehensive prop calculations.

 

[pappaecho]

Have you tried "Propcalc" which is a freeware program which allows you to change two variables, the prop size and the input power, so that you can play around in a virtual manner, before coming to a conclusion.

I changed a Perkins 4108 to a Beta 1305, with an existing 15 x 15 prop, which was oversized, and held the engine back. It was recommended a 15 x9 prop which I fitted, which gave me excellent slow speeds, but meant that to get 6 knots I had to scream the engine, whereas with the 15 x15 ...2,300 rpm was fine. When I played with Propcalc, it was clear that a 15x 11 was the right balance, and this then proved to be right

 

[ianj99]

I think much depends upon whether you want the full engine power. If you do then you will therefore be using a lot more revs than you want to and also more fuel.

Therefore it is better to overprop (& over engine) to a certain degree so that a reasonable cruising speed can be achieved without excessive rpm. eg my 36' ketch does 5.5Kn at about 1800rpm, yet maximum rated continuous power is 37 at 3000rpm. (I haven't tried it flat out as the injectors and pump were due for a service so can't say what rpm it would actually reach)

Provided the rpm is not held back too much ( ie it will still get to within about 90% of the continuously rated rpm) and when needing max speed, you open the throttle and then ease it back a shade until the revs just start to drop there should not be an issue with overloading it.

The worst thing you can do is to let it belch black smoke by using too much throttle for long periods.

I

 

[Tranona]

Once you establish your gearbox ratio, let Darglow set the prop at the right pitch. They use angles rather than inches of pitch because the prop has adjustable stops. They have a huge amount of experience of setting up props correectly, and if it is not quite right (unlikely) it is possible to adjust it.

 

[ianj99]

Once you establish your gearbox ratio, let Darglow set the prop at the right pitch. They use angles rather than inches of pitch because the prop has adjustable stops. They have a huge amount of experience of setting up props correectly, and if it is not quite right (unlikely) it is possible to adjust it.

I think you need to tell them what you are trying to achieve speed / performance wise.

A prop for maximum speed - ie to absorb maximum power at max power engine rpm, won't be correct if you just want a nice quiet cruising speed using minimal rpm.

You can spend many happy (!) hours calculating size and pitch but you end up with a educated guess based on the range of sizes churned out by the calcs. Hopefully it will be close enough or tweakable by an expert prop maker.

I

 

[Tranona]

I think you need to tell them what you are trying to achieve speed / performance wise.

A prop for maximum speed - ie to absorb maximum power at max power engine rpm, won't be correct if you just want a nice quiet cruising speed using minimal rpm.

You can spend many happy (!) hours calculating size and pitch but you end up with a educated guess based on the range of sizes churned out by the calcs. Hopefully it will be close enough or tweakable by an expert prop maker.

I

That is precisely why you leave it to the supplier. The optimum is to achieve maximum hull speed at maximum revs. Messing about with "over" or "under" propping on a sailing auxiliary is a waste of time. A prop that achieves that will provide comfortable cruising speed at relaxed revs of about 70% maximum. For example my Volvo 2030 gives me 7.5 knots at 3600 and cruises at 5.7 at 2500. Overpropping might give cruising speed at lower revs but in my experience often makes manouevering tricky because tickover gives too high boat speed.

 

[ianj99]

That is precisely why you leave it to the supplier. The optimum is to achieve maximum hull speed at maximum revs. Messing about with "over" or "under" propping on a sailing auxiliary is a waste of time. A prop that achieves that will provide comfortable cruising speed at relaxed revs of about 70% maximum. For example my Volvo 2030 gives me 7.5 knots at 3600 and cruises at 5.7 at 2500. Overpropping might give cruising speed at lower revs but in my experience often makes manouevering tricky because tickover gives too high boat speed.

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.

 

[Deleted member 36384]

Low RPM High Boat Speed Propellor Combinations - An Observation From Such A set Up

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!

 

[cmedsailor]

For example my Volvo 2030 gives me 7.5 knots at 3600 and cruises at 5.7 at 2500.

Hmm, strangely that sounds a bit low. I think you have a Bavaria 37 (??) which is quite similar to my boat (Beneteau Oceanis 361) in weight and shape. I also have the MD2030 and my reverse gear has a ratio of 2,35:1. At 2500 which is the cruising rpm I get IN FLAT AND CALM sea a speed of around 6,5+ knots and at maximum 3400 (if I push it more it may reach 3500) a speed at around 7,8, no smoke, or anything nasty behaviour. My propeller is a 2-blade fixed (original from the factory) and size is 16X11. How then we have so much difference at 2500 rpm? Is the gear box OK?

 

[samwise]

The VP 2002 is a good engine in my opinion, I had one on my old boat, and it never missed a beat. I have only just bought my Storm, so no nothing as to how it is going to perform. Before I went ahead with the purchase I asked on here if the VP 2002 was "grunty" enough to push her along, most said it was, and if your 5.5 knots at 1800 rpm was in slack water that is good enough. I used to run at between 2000 and 2500 on my old boat, and she was a lot lighter. Does your gearbox look like this, http://www.yachtworld.com/core/list...reham-(ashore)/United-Kingdom&boat_id=2054669
If so it is the MS2, I really don't think they would fit any other gearbox to the VP 2002, and I certainly don't think they would fit different ratios on the same class of boat. The weakest link on the whole setup is the gearbox coupling, the splines wear, and drive is lost without any warning. It's not a question of will this happen, but when. Both the Moody, and Westerly owners associations have good images, and information on this problem, its well worth looking at.

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.