3 to 4 bladed prop advice?

adamf

adamf

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Hi, first post here - after some advice please?

I run a circa 2000kg offshore fishing boat - deep V hull. Volvo D3 190HP with SX Leg.

The leg came with a 14.25x21" ali 3 bladed prop - it works OK - I get near WOT (but never use it) and get a decent cruise speed BUT! Ive never questioned if I could do better and after speaking with a few friends I see that a 4 bladed prop could give me better hold shots and more economical cruising. I spend most of my time cruising in The Channel heavily loaded at moderate speeds.

I spoke to my local prop shop (in Poole) who agreed on the upgrade and suggested a 15x19" prop - however it's £600 and no going back it it's wrong or doesnt work and they couldnt guarentee it.

I then spoke to another prop shop up country who again also agreed.... but suggested a 14.25x21" prop - same as what I have..... but with 4 blades not 3.

The looking on the net, most advice seems to say stick with same dia and drop down in pitch for the extra blade??

So I'm confused and dont want to commit the cash unless I'm pretty sure what I need?

Any advice?
 
If I was the owner of a prop shop and could sell props with no come backs everyone would be running around with new props four blades or otherwise. If it ain't broke don't fix it!!
 
The problem with props-they are all different. You could buy 3 different brand 14.25 x 21 props and they would all have different performance characteristics. I'm with the others, if its not broke dont fix it.

But if you really want to have a play, £600 is far too much for a single aluminium prop. Try speaking to Steel Developments, they have 4 bladed props to suit the SX for £120, at that price I might have a mess around, you can always keep it as a spare.

I do have a bit of a prop 'fetish' :o, and currently have 9 different props for my 2500kg sportscruiser. I have a mix of 3, 4 and 5 bladed props in Ali and stainless and the truth is while a bad prop will turn your boat into a dog, good props will all perform within about 3kts of each other on a leisure (rather than high performance race) boat.

Have you tried the mercury prop calculator ? Its a good way of getting some idea of what prop will work.
 
omega2 said:
Been there done got the t shirt, the "need for speed" is the road to ruin.
Click to expand...

99% of secondhand props for sale will be as a result of extra top speed/faster out of the hole pipe dreams .
Most of the worlds shortage of aluminium has probably been caused by the above.
Somebody spent a lot of time and money calculating which propeller would give best fuel economy and which propeller was least likely to destroy engine/gearbox on your boat.
Add the cost of lift in/outs and the money soon begins to mount up and all for a knot or two which could probably just as easily won by dumping a few bits back ashore.:)
 
Thanks for the comments.

The boat is dry stacked so no issues with regards lift outs. I've since found that I'm 400rpm short of WOT and that the boat builder now fits 17-19 pitch props... I've got 21"

I've picked up a secondhand Ali prop, same dia that I'm currently running but extra blade and 2" less pitch.

It's not top speed I'm after. I couldn't give a hoot about that, it's a good hole shot and the best cruising revs I can find.

I'll let you know the outcome when I try it at the weekend.

Adam
 
adamf said:
thanks for the comments.

The boat is dry stacked so no issues with regards lift outs. I've since found that i'm 400rpm short of wot and that the boat builder now fits 17-19 pitch props... I've got 21"

i've picked up a secondhand ali prop, same dia that i'm currently running but extra blade and 2" less pitch.

It's not top speed i'm after. I couldn't give a hoot about that, it's a good hole shot and the best cruising revs i can find.

I'll let you know the outcome when i try it at the weekend.

Adam
Click to expand...

take an inch of pitch off the one you have.
 
Two boat owners where I am bought 4 blade stainless for the same reason you state. Both boats perform perfectly on existing props but they wanted to get on the plane quicker and be able to cruise at lower revs. Both took advice from a prop specialist. One boat performed terribly and has gone back to original 3 blade ali that came with boat and the other says it feels different but hasn't done what he wanted so both conclude a waste of money.

These were 135hp outboard engined 20/25 ft boats used for fishing.

Gary
 
No need to repeat others' replies, just wan't to add a bit of info regarding planing boats, in particular with sterndrives or outboards.

90% of a prop's performance properties comes from design, 10% from material. Means that 'upgrading' to Stainless or other high grade metals may be good for other reasons, but main speed/mpg results can be achieved by selecting right type/blades even of alu. Composite/plastic props are in a league of their own and normally are not seen to perform anywhere near alu.

If your aim is to reach recommended rpm at wot (which you should always do since this gives best blend of load on engine and performance), you begin by stating current rpm at wot carrying your normal load
On planing hulls one inch change in pitch equals a change of 200 rpm (higher pitch=lower rpm and vice versa). As manufacturers state rpm as a range, go for the upper limit!

Depending on the work to be performed by the prop (heavy boat, power to weight ratio) and the working conditions (engine trim/tilt, prop running height) and any functional issues (eg. dragging air to the prop when turning at plane) you may opt for various materials, blade designs, -areas and number.

Any good prop dealer will allow you to buy only one prop and to swap within a given period of time until you get the right type. Of course the props need to be as new when returned.

If you wan't to replace a prop, damaged beyond repair, options may differ but when you do the maths a £600 spend is hard to justify against fuel savings only. Also bear in mind, that hitting 'something' with the prop will cause an alu to break. Stainless is much stronger and commonly survives - instead the shock transmits to the driveline :rolleyes:. If damaged, £600 will only cover a fraction of the bill....

Those interested can get the free Quicksilver book "All you need to know about propellers" here:
http://www.spidybot.com/prop_/Quicksilver_Propellers.pdf
 
adamf said:
I've since found that I'm 400rpm short of WOT and that the boat builder now fits 17-19 pitch props... I've got 21"
I've picked up a secondhand Ali prop, same dia that I'm currently running but extra blade and 2" less pitch.
Click to expand...
After reading your OP, I refrained from throwing in other comments along the lines of "don't waste your money", because you already got that sort of feedback.
But you didn't tell that you're 400rpm short of rated max rpm with your current prop.
I which case, fitting exactly the same type of prop, just 2" shorter, is usually the best bet.
You might be fine also with a 19" 4 blades screw, but aside from the additional blade itself, whether or not you'll get those 400rpm COULD depend also on other characteristics of that prop, compared to the previous one.

Anyway, in the light of the fact that your current prop makes the engine struggle to reach its max rpm, the suggestion of the second of the two prop shops you previously mentioned does not make any sense, whilst I could agree with the first one - assuming that the leg can handle the slightly larger diameter.
Keep us posted on the results, when you will have tried the 19" four blade.
My guess is that you will get some more RPMs, but not all of the 400 you're looking for.

PS: welcome to the asylum!
 
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When I advertised my boat for sale I put that it would do 25 knots. Someone who had recently bought an identical boat contacted me asking what engine I had as his boat would not do what mine would (and mine was the heavier hardtop version) We both had the same engines but mine has a 4 blade propeller but his has a 3 blade propeller.

--------------------------------------------------------------------------------------------------------------
Skibsplast 7m for sale
 
So I'm confused and dont want to commit the cash unless I'm pretty sure what I need?

Any advice?[/QUOTE]

You should be! .In real life " propping" is done by trail / error @ sea trails on hull no 1 .
It's the prop slip that the maths fail to calculate
And- the distortion of the material , flexing under load
.Sure we know the displacement , can cal drag, friction, surface area, - match rpm ( looking at various engine " curves" on graphs ) , throw in air temp - effects engine power , toss in windage - can be + or - on the day, sprinkle in sea temp - effects density , Plus loads of other variables ( foulling.) ,which some are more important than others like full tanks + cruising stores ,1m waves or 3m waves to cut through ( changes hull profile as you go ) Hence dispite computer aids ( remember rubish in = rubish out) .
It's really down to best guess+ previous experiance based on past similar vessels .
I know Sunseeker when trialling hull no 1 say for example a new ( I dunno I,ll make up a new one ) erh let's say a Pred 44 , with a pair of MAN 800,s
As I said the boffins will have 1/2 dozen prop sets on the quay next to the lift - to trail on hull1 .All of them will be trailled before the ( none boffins ) come up with the " factory" offering on hulls 2 - to end of production .
So really a lot of effort/ man hours has gone into or should have gone into the original builders propping choise .
I suppose with a smaller lighter boat the " band width" will be greater , because you can move the variables ( well the important one ,s ) - like all up wieght as a % easily .
As MapishM says a smaller Dia or/ and smaller pitch - will spin faster ie accelerate out of hole faster, but then reduced top speed , or if at lower cruise , engine is over its sweet spot - boomy + inc fuel usage ?
Propping is a black art - unfortunately not an exact science that why there is no real answer to OP,s Q,s
 
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[QUOTE
Propping is a black art - unfortunately not an exact science that why there is no real answer to OP,s Q,s[/QUOTE]

For example
We run 2, 8 metre ribs
Both with Evinrude e tec 225hp
Fitted a new engine to one last year which came with a four bladed stainless prop 22 pitch
Great, really suited our purpose --- cruising at 25 kts at 3000 to 3250 revs
Tother boat had to maintain 3600 to 3850 to perform the same
So thought I would shove the same on
However I didn't need a stainless prop
Couldn't get an ally one to the same pitch and diameter
So got an ally one that was 2 pitches out and 'bent' it and 'cupped' it to suit!
Deffo better than the three blader
Not quite as good, the boats are about 150 revs apart now
Hull shape and seating positions (weight distribution) might be a factor plus I might have needed a bigger hammer!
 
Finally got this sorted.

The supplier sent me out the wrong prop last week, right size but was too short in the hub. After a lot of to and fro, they offered to do send me a new Volvo prop at a good price.

So for the record I had a 3 bladed Ali prop 14.25x21" I was getting 3600 rpm. Max rpm for the D3 Volvo is 4000 rpm.

I now have a 4 bladed Ali prop 14.25x19". I went out today for a short blast up the harbour, a full tank of fuel, Little Reub and a huge bucket of tools onboard.

In summary it is a better set up but it will take a few hours of use in varying conditions to really get a feel for it. The old prop wasn't that far off, but with this new prop I feel I'm getting the most out of the engine, and the right pitching and extra blade seem to do what everyone said they would: hole shot seems faster, I'm now planning at much lower speed, just over 10 knots, I get lower revs at cruising sped, for example I used to cruise at 18 knots for 2800rpm, I now get the same speed at 2600rpm. I did give it a WOT run.... That was scary! I've never wound the engine up to 4000 before! It felt like it was screaming and I was concerned that I'd go over.... But nope, at the throttle stop I was bang on 4000 so that was good news - this gave me 34 knots! I've also got the benefit if having a spare prop to carry just in case.

So really chuffed and I look forward to trying it out a bit more this spring.
 
Well worth doing, especially since high revving diesels like the D3 must be propped to reach max rpm other wise the exhaust gas temperatures get too high and they melt valves, pistons etc.

400 rpm low as you were before would be considered in the danger zone
 
Scarron said:
Well worth doing, especially since high revving diesels like the D3 must be propped to reach max rpm other wise the exhaust gas temperatures get too high and they melt valves, pistons etc.

400 rpm low as you were before would be considered in the danger zone
Click to expand...

Thanks for that. Yes, it's strange, as at first thought, I'd imagined that not achieving max revs was a good thing as it meant the engine was not having to rev as hard, but in fact due to the gearing aspect that the prop creates the opposite is true as you say.
 
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