Prop pitch vs Diesel WOT revs.

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oldgit

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Since purchasing the boat last year port engine will only to 2200 rpm at WOT.
The other will instantly go to recommended max 2600 rpm and stay there all day.

For one reason or another the fuel tank has been drained/cleaned and new filters fitted throughout.
Pretty certain the air supply is more than adequate.
All cables have been checked for adjustment.
Fuel pump has been checked for adjustment.
No smoke(honest) from either.
Both go to 2600 under no load.
Engines now total 350 hours from new.

Pretty certain it is prop pitch.
The existing props were supplied brand new by Vetus as per the original spec supplied by the builder of my boat regards engine rpm,gearbox ratios,weight etc.
But somebody got something wrong.

As it would appear we need to "find " 400" rpm,I think that the port prop needs about 2" on the pitch,to say 22" x 22 ?
What thinks you ?

prOpPITCh-1.jpg

Oops Starboard prop pitch is now correct. :(
The starboard gearbox is running in "reverse" and thats the standard ratio for 10 18 106 reduction version.
 
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1" less pitch will raise rpm by 200, hence 2" will raise it 400.

2600 rpm and a gear ratio of 2.1 makes 1238.1 prop shaft rpm which in it's turn on a 22" pitch produces 22.41 knots.

2600 rpm and a gear ratio of 1.91 makes 1362.26 prop shaft rpm which in it's turn on a 20" pitch produces 22.40 knots.

So you're perfectly right about the change of 2", provided it will reach 2600 rpm with the 20".

All in theory, ofcourse :)

Edit: You may have your pitch altered - cheaper than getting a new.
 
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Just a quick bash on the calculator using Crouches method as insuffcient data to do 'slip' calculation, however jumps at you straight away.

Port engine with BW 1.88:1 requires 22X17.5 (small difference but 1.91:1 is not correct ratio)

Stb engine with BW 2.10:1 requires 22X20.5.

Wee bit of pitch needs to be graunched OFF not ON port prop and if you wanted to be spend the money Stbd would take a widdy bit more.

Perkins are a little bit naughty with their spec sheets as they use 25 C test fuel and some weasel words about production tolerance, so if you do prop calcs safer to use 120 Hp input data in order not to overload engines.
 
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Ooops minor cockup on starboard prop pitch it is actually 22" x 18 ....now corrected.


The port gearbox ratio is the one given in the workshop manual for that model number.
SERNUB.jpg

Thanks for your input everyone,next time she is out the water will get the pitch altered a tad.
Can always alter a fraction more if not enough the first time round .
Not the least bit bothered trying to squeeze out last half knot,just for longevity of engine.
 
Following your changed numbers (your 20.17 edit) the history of this boat build looks interesting. The port engine is running a 10% less reduction in gear ratio (210/191) yet the boat builder fitted 10% HIGHER pitch. It should have been 10% LOWER, approx. Seems like someone got mixed up when they placed that order with Vetus!

You need something like 16 or 16.5p on the port side. That's possibly too much to tweak out of the existing 20p prop, so it might be new prop time, erk

Good luck
 
Thanks again all.Will see if prop can be tweaked if not not plenty of time to search round for correct replacement.
As for top speed we did once get up to 16 knots.Which is the sort of speed expected with this horse power in what is basically a Broom 37 hull in all but name..
 
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I took a back of a fag packet shot at 31ft LWL and guestidisplacement of 10 Tonnes am I light? .

Sounds like builder was in one legged man mode!! Take a look at http://www.velvetdrive.com/techinfo/V7172/V7172pg1.pdf BW only ever made one ratio of opposite rotation in either 71/72CR models, even the long defunct 73CR was the 1.88:1 marked with asterisk. As I said no big deal 1.91:1 Vs 1.88:1. Just indicates a hint of sloppyness.

Anyway with your amendment proof of pudding is in the fact that your stb appears to be correct. I do have a little concern regarding your on the nail 2,600 high idle, as well as 2,600 stb WOT, I would have expected high idle to be 2,600 plus almost 10% governor droop and WOT clean to be 2,600 plus 100 rpm as overload buffer to allow for weed growth.

One bit I cannot get my head around is why builder did his own calcs and went to Vetus??

Normal practice is to give the job to professionals Clements, CJR, Teignbridge, ZF Faster etc, etc, guys that actually design, cast and machine the stuff they sell. If they screw their calcs up THEY fix it at their dime. Vetus are just resellers of other folks gear backed up by guy out the back with simple prop calculator. I have come across this re-seller stuff, claims of being Nibral, Class I, prop scanned, all the B.S, turned out to be out of some low cost Phillippine gear meeting none of the foregoing.

No big deal about the Far East, Rolls Royce, Stone Manganese Marine, ZF Faster all have their props made there, however I am less sure of quality control in say a Manilla sweat shop.

Well if 22X18 works, sounds like Pt needs to be at least 22X16 as suggested by JFM.

As you are outside the scope of the 'graunch' zone with your 22X20 no harm in getting one of the above propeller professionals to re-run the calcs for you if you are purchasing a new prop. Life is hard out there. I would get the calcs done by the 'Slip' method nail down a good price for new prop, then if they suggest your 22x18 is not ideal, ambush them with suggestion that they propscan and adjust your Stb wheel as required all for the same deal.....It is their reputation that is on the line.
 
Solitaire said:
I'm with you in this. Speak to a real expert!
Click to expand...

I can tell you, knowing Paul Williams from both sides of the fence he is an OK guy and done some good stuff, however he has had his fair share of screw ups. Paul is a bugger for taking his calculations real close to the knuckle resulting in engines being overloaded a few years down the line.

If you want to pick any guys brains there has to be a cup of tea in it somewhere!

OK looks like a new prop is on the cards so fair to ask away, but remember all the players are professionals, until the £ went wobbly the guys from ZF Faster made the rest look like Fred Flintstones.
 
"I took a back of a fag packet shot at 31ft LWL and guestidisplacement of 10 Tonnes am I light? "


Suspect estimates are nor far off regards weight and WL.
The pitch cockup was probably due to fact that the boat was home built.Took the original builder and then his son just over 30 years to complete the job.Not sure when engines or stern gear were added into the brew .
Perhaps a change of plan to smaller engines at some point,as the gearboxes appear to be over specced for this power unit.

transoceanthemove.jpg
transoceantanktest.jpg
 
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Looks to me like the props are the right sizes ish, but maybe the order was mucked up between the specs for the left hand one and the right hand one? I.e. if port was 22x18 and starboard 22x20 it should achieve near 2600rpm on both?

So specs for left handed and right handed have been switched in translation?

Edit - Just noticed JFM also noticed this!
 
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Swap props?

alb40 said:
Looks to me like the props are the right sizes ish, but maybe the order was mucked up between the specs for the left hand one and the right hand one? I.e. if port was 22x18 and starboard 22x20 it should achieve near 2600rpm on both?

So specs for left handed and right handed have been switched in translation?

Edit - Just noticed JFM also noticed this!
Click to expand...

Just an idea? Swap Props?
 
" Swap Props"

Props are contra rotating, poss me being a bit thick ,if I swap props then surely need to also reverse direction of gearbox at other end of shaft.This then puts the two gearbox ratios back where we started.
Gearbox ratios are fixed.If it goes one way its 2.10.1 if its set up to go the other its 1.91.1
.......and you cannot just turn props round when swapping due to the shaft taper...
Arghh.... am I missing something.
 
oldgit said:
Arghh.... am I missing something.
Click to expand...

No you're not missing anything. I thought of swapping the props but the result is unfortunately you get back exactly where you are now, exactly as you say (only you'd then have outward turners, not inward turners, and on this boat you want inward turners imho)

If you could magically remachine the tapers (insert a liner?) you could put the props on backwards but that means lots of expensive machining

Bottom line is you need to get that prop down to 16.5p or 16, or maybe 17 would be acceptable. From 20p inch. Thats a lot of bending on a 22inch dia prop and might not be doable - you have to ask a prop expert about that. Otherwise it really is new prop time, yuk
 
Ok,so the starboard prop is more or less OK purely because it works roughly at the proper revs.
The Port prop needs tweaking to get the required pitch but if not possible we need to find something around 22" x 16-17 ish to achieve the required rpm.
Thanks once again every one who spent anytime pondering this rather arcane subject. :)
 
Fred, as you spend 99% of the time just off idle and get the vapours if the throttles are advanced more than that I'm surprised you noticed.
Next time you're doing "speed" runs please let me know as the other Medway members will not believe and require photographic evidence, and I'll have some medical assistance ready.
 
Your just jealous I can now keep up with you !

New Commodores Sail Past week after next ,suggest you put your valuable collection of Ming vases and Lava lamps on the floor of the boat as we go past.
 
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