Proper Propping

[Wedger]

We have recently replaced the prop on Otter after a brief encounter with a gravel anomaly near Mudeford. Since then we have failed to achieve rev out, and rarely more than 16 knots as apposed to the 19-20 knots on calculation.

 

History.

 

1. With no kit on board and only myself and very slightly built wifey along for the ride, I achieved 19.8 knots and as I remember pretty much revved out to 3900 rpmwith the old prop.

2. Since engine rebuild, the first loaded trip out we, (

Edited July 21, 2007 by Wedger

[Paul D]

Not an expert on props but from what I have learnt.

 

1) You need your engine to reach full revs under WOT as if it is not reaching higher revs it is over loaded and not good for the engine.

 

2) A smaller pitched propellor will achieve this and not necessarily at the expense of top speed as the engine will be developing more revs and hence more power.

 

 

[Paul J]

revved out to 3900 rpmwit

you need to be getting at least 5000 i think

 

PJ

[Wedger]

you need to be getting at least 5000 i think

 

Sorry Paul, I didn't make it clear.

 

It's a Volvo TD130 3,900 is top revs

 

T

 

[Coddy]

Hi trev

 

Have a word with Prop revolutions at Cobbs Quay.

 

I am sure they will be able to give you all the best info you need.

 

or here http://www.steeldevelopments.co.uk/

 

Coddy

[toerag]

You're only getting 20 knots out of your MI with a 130? That seems a bit slow to me, I can get 31-32 with my 150 version of the same engine. I have a set of A8 duoprops on - what are yours? I reckon on losing 2-4 konts of top end speed if the leg is dirty and the hull a bit slimy, but don't really notice much effect until I get to 23 knots +.

[Wedger]

A 270/280 hybrid leg, single 15 x 17 long hub prop. plus wing fins. Leg & hull clean.

[duncan]

Trevor

 

Always an interesting one when apparently identical props produce different results!

 

You certainly need to run other checks before you should do anything with the prop.

 

Most importantly would be checking that the engine revs to 3900 in neutral - that rules out linkage issues.

 

Your prop set up isn't standard with a relatively large diameter but the general principles still apply - 1" of pitch = 200 rpm

 

However you aren't going to come to any harm simply trying a smaller prop as the engine will be governed (unlike a petrol esp OBs) and you may well get a quick feel for the issue going this route. certainly need to be able to get 3900 with the boat in it's normal load configuration.

 

Your comments (and others) about overloading are correct although you won't be wasting fuel with a diesel in the same way as a petrol as you will only burn fuel appropriate to the HP being used to drive the boat at any given speed - ie throttle setting and revs are not a driver to fuel consumption they are a function of drag and power delivery curves...........

 

ps bet those wings are costing you 2 knots as well............

[Wedger]

Very interesting Duncan, I must learn more.

 

Much to try here then.

 

The smaller prop we have is marked 15 x 19, which matches up more closely with the 1" of pitch per 200 rpm ( if of course it hasn't been altered in the past and not re-stamped). I couldn't tell just looking at it but the blades of the smaller prop definately have less surface area but the same diameter.

 

I will begin by changing one thing at a time.

 

1. Rev test.

 

2. Small prop on + full trim variation.

 

3. Wing off next + full trim variation.

 

Will report back.

[duncan]

15x19 is a coarser prop than 15x17..........it will deliver even less revs providing it stays in contact with the water.

 

having smaller blade surfaces is another thing entirely as those with the ETECs will know from the comments last year! if you do get more engine revs I doubt they will be translated into hull speed and you may well find the prop breaking out/slipping.

 

problem is that you clearly have a non standard set up and I wouldn't want to even guess the gear ratio of the leg so it's tough to think in terms of a 'normal' pitch. however 15x15 or 15x16 would seem to be what you need based on 'relativity' rather than 'normality'.

[mw]

I have had similar problems down from 26knots to 14knots boats only been the water since may so bottom not to bad removed boat to copper coat in june not been out since i thought it may be the penant ? i have bought a four bladed prop smaller blades spin quicker but has more grip not been to fit yet as i have to get inthe water to change. i am also going to change fuel filter just in case

 

mark

[shytalk]

you said the engine had been rebuilt did that include the injector pump and have you had the full performance since rebuild if not if the fuel pump has been rebuilt or reset by anyone it may need tweeking as the factory settings are wrong

[Wedger]

A rebuild in the sense that it was dismantled and reassembled with some new parts. Injector pump not altered.

 

No load rev test 100%, easily achieved 3900 rpm.

[Wedger]

Out yesterday in relatively flatters conditions. Tried all variations of trim and not able to get over 13.5 knots fully loaded. Not revving out. Smaller prop next run and will try fins off as well. (fins and goggles required).

[Afishionado]

Quote ..i have bought a four bladed prop smaller blades spin quicker but has more grip

 

Unfortunately this is not nesessarily true. If you consider the total arc the prop spins is 360 degrees it would be obvious that a 360 deg disc would not propel in any way. If you cut the disc into 4 segments and angled those segments you would create a sort of propeller. However the gaps presented by vectoring the segments would not allow much power as they are too small to allow sufficient water past. To rectify this one could make the gaps larger and the blade area smaller or reduce the number of blades to three or two keeping the blade area high.

To go any further with this post would require a thesis on propeller function and efficiency and I for one don't have the time. But suffice it to say that the NEED for a 4 bladed prop is very specialised and that 3 bladed props over all have the best effficiency.

I would be interested to find out what the performance change will be by going to a 4 bladed prop (Of the same diameter and pitch as the 3 bladed one?)

 

Mad Mike

[Wedger]

Duncan

 

Spot on chap. Prop stamped 15 x 19 with smaller blades produced less revs and less speed.

 

Back to Prop Revs for a proper calc.

 

T

[Wedger]

Interestingly I have the following calculation. Castle Marine (calc program)

 

Equipment spec

Heavy planing cruiser/workboat

1 engine

130 hp

3900 rpm

1.89 ratio leg

boat weight 2.5 tons ........that's pessimistic

waterline taken as full hull length 21' (makes no odds with planing type)

 

Conclusion

 

130hp at shaft

2063 shaft RPM

speed length ratio 3.06

Max speed 19.99 knots 22.99mph

Calc slip ratio 0.17

pitch ratio 0.83

 

Recommended prop

 

dia 17.19"

pitch14.19

 

As this cannot be achieved Use 4 blade 15

[great white]

Hi Trevor

 

We used to work on 1" of pitch altering the WOT RPM by about 200 revs

 

Therfore 2" over pitch could reduce the Max revs achieved by about 400RPM

 

These were for an outboards 3 bladed prop.

 

Not only will you not get the WOT that you should, you would also be overloading the engine across the whole speed/power range.

 

Prior to my time looking after OBM's I was unaware of this, I ran a new Yamaha on a standard prop. after a few years this overloading caused a big end bearing to go. A costly repair was needed.

 

Charlie

[toerag]

Do you not have a volvo leg on it? I can't believe you can only get 14 knots out of her. Crikey, even with 6 aboard and full fuel and fishing crap I can get 26 out of mine.

[Wedger]

270 with 280 bottom end

[mw]

Hi Trevor

 

What work did you have done on the engine?

Have you put more weight in the cabin is the boats not gettinig on the plane.

I Fitted the four bladded prop on my boat it is so much quicker onto the plane and on top end speed i can't get it to 26 knot quite but i have alot of weight on it .

 

 

mark

[Wedger]

The engine was dismantled, had corrective therapy and reassembled. Testing afterwards gave excellent results - high and very even compression. Very responsive throttle, revving out with no great effort with the old prop. Smokey when cold, as with these Volvo

[great white]

Hi Trevor

 

I am sure you will have done much research but have you taken the old prop in to a prop company to see if they can ascertain what it was changed to? they can only change the pitch by a couple of inches at the most.

 

Or failing that have you spoken to Volvo to see if they know the boat/engine and what prop size they calculate it to be

 

When I built Great White I used Tiegnmouth Props, they calculatted what type,size pitch to fit from the boat and engine specs.

On launching I was very pleased to see that they had it 100% correct.

I am not sure if they still trade but I am sure there are many companies around [Lake engineering 01202 621631, Prop revelutions 01202 681662] that may be able to assist

 

Good luck

 

Charlie

 

 

[Wedger]

Yep Charlie, I kept the old prop and a re-measure is precisely my next move. I will take the knackered one in to compare with the new one and take it from there.

 

Cheers

T

[toerag]

Have you had the fins off yet? I still think you're suffering some kind of drag issue. What sort of trim angle does she run at at full speed? I have trimtabs, but always leave them set fully up - the drag they create when lowered is horrendous. I find it best to keep the stern as light as possible, as weight in the stern really hurts performance. I've found that running at 0 or +1 on the engine trim gauge to be the most efficient.

This might be a silly question, but is the drive fitted at the correct height on the tramsom?

PS. to find the best engine trim angle set the throttle at certain revs then adjust the leg in and out whilst watching your speed ( I use slack water and the GPS speed). At maximum efficiency the boat will be most easily driven, and thus the speed and revs will rise.

PS. unless you've had the boat weighed and know the weight for sure you could well be over 2.5 tonnes, a friend of mine with a 150 AD31P like mine said the crane drive told him his was 2.6 tonnes.

PPS. Sorry for not seeing your post saying what sort of leg you had and asking you twice!!