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Diameter Or Pitch For New Prop.


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Afternoon. 

Quick question (which I suspect will require lots of technical answers)

Bourne 35 with 4.107 Perkins. 

Tickover - 850rpm. 

4 mph - 1250rpm

5 mph - 1800rpm

6 mph - 2200rpm

Does this sound about right? Am I revving a bit high for these speeds?

I think I may be a bit under propped. What do you reckon? 

If I am looking for a new prop my thinking is....

Bigger pitch = more forward push which means same speed for fewer revs

Bigger diameter = Bigger blade area which means potentially more stress on the drive line but a corresponding drop in rpm. 

What do you all think? Pitch or diameter change?

Or both?

Or are my revs in line with what you have experienced with similar sized boat and engine?

Thanks

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For a start, the 4107 ticks over at 700 rpm.  This can be adjusted on the injector pump.  Too high a tickover could cause a violent gear change and wear on the flywheel thrust plate.

The rest of the revs seem too high to me.

Do you know what size the present prop is?

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Next project was to work out how to reduce the tickover. 700 it is then. 

I cleverly forgot/didn't think about measuring the existing prop size when I had it out of the water. Too excited to get it floating to think about that. 

So would it be pitch or diameter that would get the revs down in the best way? 

Limited Internet research has thrown up an approximate reduction of 400rpm per 1 inch of pitch change. 

That would mean that 4mph would be about 800 with a tickover of 700 which seems a bit close to me. 

 

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My folks have a Bourne hulled AF35 which has a Perkins 4108 with 2:1 gear box. The original prop was 16.5x11 and the plaque on the dash gives the following revs/speeds......

1000 = 3mph.

1500= 5mph.

2300= 7mph.

The boat now has a 17 x 16 prop fitted and it is really over propped IMO, it will only rev to about 1800 or so flat out and is quite slow to respond to the throttle when mooring up etc. It even stalled the engine when going astern on one occasion(not helped by the throttle linkage reducing revs on initial movement).

On the plus side, once underway above 4 mph the engine is nice and quiet but below that it tends to grumble along and you often need to pop in and out of gear when following very slow stuff etc which I find to be a pain.

I've never noted exact revs to speeds as I use GPS and it's always different depending on the tide but revs are greatly reduced.

Hope that adds some colour to your conundrum:default_biggrin:

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From a very hazy memory my father, who designed the Griffin/Bourne 35,  put on 15" propellers with the 4108 but not sure of pitch.  I will try and check this in a few days if I can find some old papers in my office. There is a website which will do the calculations for you https://www.vicprop.com/free-propeller-sizing-calculators or you could pay Andrew Wolstenhome to do the calculations.

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I'd check the prop you have on to start with, on mine the original prop was 17x12 never had a problem until changing the engine from a BMC 1.5 to a slightly larger Nanni. Started to seem to struggle against some tides, on checking the prop we found it had been 'refurbished' several times in its lifetime including once when i first got the boat, and it had lost an inch in the diameter. (just remeasured it now as it's a garden ornament) went with the standard size for a new one and what a difference it made.

 

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Slightly over propping imo is a good thing. Every boat engineer I’ve discussed it with has stated the same

BA’s Beta 50hp is rated at 2600rpm max. She is slightly over propped. 
 

With a 16” four blade, she manages 2500rpm-ish in deep water which is about 9/10mph

1000rpm is about 4mph whilst 1200 is about 5mph

So the only time the Beta & box are under max load stress is at 2500rpm which is rare but does happen on Breydon every crossing but not for all of it

That’s  a Bourne 40 hull at around eight tons

Hope this helps 

Griff

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Hi Andy If a ex hire boat they were generally over propped to stop over revving a starting point would be a 17"x13 i have a Axiom propellers prop on mine it's a new generation type prop these resist cavitation and weed pick up i am very pleased with the performance of mine, I suggest you contact them ph no 01832730457 they are very helpful. John

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Hi Andy

On my bench i have a prop off a Bourne 35 it measures ( after a lot of head scratching)  17" x 9" however this doesn't help as its not the one on your boat'

i would suggest you get the engine revs set as per Vaughans advice, then when you are next on the southern broads pop along and i will lift your boat, leave it in the slings for a couple of hours so you can measure the prop and obviously the tapper on the shaft. then you can make an informed decision. and as you have been a good customer in the recent past it is free of charge.

PS Don't tell everyone

Robin

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1 hour ago, Robin said:

i would suggest you get the engine revs set as per Vaughans advice,

The speed control cable is attached to the control lever on the injector pump and there is a small threaded setscrew on either side of it, with lock nuts.  These act as limiters, for slow and fast speed.

The Perkins pushes the throttle closed (as I remember) whereas the Nanni pulls it, so with the Morse lever in the neutral position just undo the setscrew which is touching the lever, until you get the right revs and then tighten the locknut.

The spanner is the same size as for the fuel bleed nuts.  3/16 AF, as I remember.

Do this with the engine running of course, but keep your fingers well away from the alternator belt!

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Thank you all so blimmin much for your advice. 

I am working away for a week or so but will play with tickover as per your advice and see how that all goes. 

Just another quick question 

Can I trust the rev counter or would anyone advise another way of checking? 

Surely someone will have invented a thing that checks it more accurately than a 50 year old cable driven rev counter?

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If it is the old cable type it is probably accurate!

Personally, I find a Perkins "sounds right" when it is ticking over at the right speed.  Imagine the sound a black cab taxi makes, when it pulls up at the kerb, and that is about it.  The Perkins 4107 was a taxi engine!

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I have found my fathers calculations for a Bourne/Griffin 35 which was for the B.A. wash report. Lady Laurel boat no. 13 in the report, displacement 5.3 tons, wetted surface 360 sq ft. It says 16 x 10.5 at 1500 r.p.m. will give 9 m.p.h. There are various other sizes calculated and h.p. calculations. The wash report was a public document so it might be possible to look up all the calculations.

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And it can weaken and break engine mounts. 

Really?  If a power unit is for instance rated at say 50hp along with the gearbox, engine mounts etc.  Surely the max thrust is rated at a max revs.  If the drivetrain is over propped way too much thereby 'strangling' the engine with decreased revs, then max thrust will never be achieved so I don't 'Get' how that puts excess stain on the engine mounts

Griff

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20 hours ago, annv said:

Hi Andy If a ex hire boat they were generally over propped to stop over revving . . . . . . . . . . . . 

Hmmm.  I’ve been told the opposite, that is most hire boats are under propped to ensure that even with minimal daily running, over revving the engine to maintain a set speed will spin the alternator faster to help recharge the batteries more quickly.

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56 minutes ago, Mouldy said:

Hmmm.  I’ve been told the opposite, that is most hire boats are under propped to ensure that even with minimal daily running, over revving the engine to maintain a set speed will spin the alternator faster to help recharge the batteries more quickly.

I've heard of some private boats with large engines being under propped for inland use to avoid running at tick over and help the alternator, mainly outboards and outdrives as it's easier to change props.  Downside is if you did go out to sea you loose a lot of performance unless you change the props back.

We always propped the boat correctly in the first place and then restricted the speed by other means, if done correctly you can leave a few more revs for reverse to maintain a good stopping power.

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Mouldy - That make sense, from what I see and more realistically hear when out on the rivers the hire boats seem to be revving their nuts off without much progress compared to most private craft, it is certainly the case for us.

Hydraulic drive seems to be the worst too

Griff

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2 hours ago, BroadAmbition said:

And it can weaken and break engine mounts. 

Really?  If a power unit is for instance rated at say 50hp along with the gearbox, engine mounts etc.  Surely the max thrust is rated at a max revs.  If the drivetrain is over propped way too much thereby 'strangling' the engine with decreased revs, then max thrust will never be achieved so I don't 'Get' how that puts excess stain on the engine mounts

Griff

I was thinking that but it depends on where the peak torque is produced maybe, although a full revving engine is still going through peak torque anyway, an over propped engine will never reach it's rated hp due to lack of rpm.

Over revving an engine with little load is no better than running at idle at the same load, only benefit is the alternator rpm for charging, the bores are still likely to glaze eventually and lose compression/burn oil.

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Two comments on the debate, from my own experience running hire boats, but not necessarily based on "the science" :

1/.

I have always known hire boats to have a coarse pitched prop, to give power at low revs, especially when going astern. What we want is manoeuvrability, not speed.  How the big prop affects high speed doesn't concern us as the revs of the engine are governed by the boatyard anyway.

2/.

Once the engine has been revved up on starting, to "cut in" the alternator, it will charge, even on tick-over.  You may see a small difference in amperage when revving up, if you have just started and the batteries are low, but not during a days cruising. It makes no difference.  I have checked this often with an ammeter.

In a Bourne 35 with a reduction gearbox, at 2000 revs I would expect to be doing over 7MPH.

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