Choosing outboard propeller pitch

[macd]

sandro:
I still doubt about the rpm meter.
Being a cheap Chinese one, I checked it against a friend's motor bike's rpm meter: A reading difference of 5% - 15% was found, but the bike meter display was a coarse bar row of variable

Was the rpm meter the type that clipped onto a spark plug lead?
If so, was your friend's bike 2-stroke or 4-stroke? If it was 2-stroke, like your outboard, you can rely on the readings within the variabiity limits you discovered. If the bike was a four-stroke, it is almost certainly giving false readings for your outboard.

 

[Seajet]

But surely that's only going to increase the rpm at expense of prop efficiency and will not give the increase in thrust that the Op requires in adverse conditions etc.

If the engine is not achieving full revs, it's not producing full power.

Once the engine is performing to its max, there will be more thrust; I have used this technique on a Yamaha 4 & Mariner 5 ( both 2 strokes but that doesn't matter ) and it worked well.

With the original props the engines struggled to push my Anderson 22 at anything like the speed I'd expect as they weren't building up revs; with a tiny bit of balanced filing, more whizz in every way and the engines are able to push her into quite strong wind & waves.

 

[macd]

If the engine is not achieving full revs, it's not producing full power.

Not quite so. The stated operating range is 4500 to 5500rpm. Peak power is delivered at 5000rpm.
Unfortunately no-one's been able to unearth a power curve, so we can't put any more figures on those.

 

[Seajet]

Good morning,

for my Drascombe Dabber I bought a Yamaha Malta 3hp outboard. As the motor, by eyeball (or earball), did not turn fast enough, I made a few tests with an RPM meter.

At throttle 3/4 open engine turns at about 1800 RPM and boat makes 5 kt. Further opening throttle until WOT, engine does not exceed 2000 RPM and boat speed increases only to 5.2 kt.
As motor can not reach its maximum speed, rated 5000 RPM, It seems to me that it is overloaded.

In neutral at 2/3 throttle it easily reached 4000 RPM; I did not open further.
In a bollard pull test at WOT it turned at 1600 RPM.

The hull speed of the boat is about 5 kt but, even if not increasing the boat speed, the motor should reach more RPM, with much water mess.

IMHO a propeller with a shorter pitch should allow the motor, with this boat, to reach its maximum speed and to output its maximum power.

I don't seek more speed - the hull is already at its maximum possible - but more thrust would be very useful and also safer in case of strong wind and waves from ahead, or for towing another boat in need.
I also wonder if the motor with this propeller is not overloaded and could be damaged.

The actual propeller is 7 1/4" diameter x 6" pitch. The other only suitable propeller available from Yamaha is 7 1/4" x 5".

Which is the advisable cruising RPM that shall be reached? 85% of maximum rated RPM?
Shall I change the propeller?

Since no prop can be borrowed for a test on water, and Yamaha parts are priced very high, I would be grateful to know the opinion of the experts of the panel before making an expenditure that could prove useless.

Thanks and fair winds

Sandro

P.S. I can give all the details of the tests. I didn't yet because the post is long enough as it is.

In this post #1, the OP reckons it's not revving anywhere near full potential.

Propeller thingie going round faster = more water pushed away.

 

[macd]

In this post #1, the OP reckons it's not revving anywhere near full potential.

Propeller thingie going round faster = more water pushed away.

I've read all that, thanks. None of it makes sustainable the claim that an engine not achieving full revs won't make full power. It may be the case; often it isn't.
Certainly, the OP's belief that his engine is only making around 2000rpm under load, if true, would mean he's getting a lot less than peak power: I've suggested he may be getting perhaps half the rated power. I've also suggested that half the rated power might not be enough to shove his boat along as quickly as he reports.

However, and resisting the temptation to be as patronising as you, there have been several suggestions that his rpm reading may be inaccurate, possibly wildly so. Till that's cleared up (see my post #21), there's little he can realistically do.

 

[Seajet]

I realise you are thinking of max torque, but by the OP's description the engine is way over-propped for the application - assuming there's nothing else restricting revs, and there doesn't seem to be by description.

When I tried the Yam 4 & Mariner 5 with standard props, it was VERY noticeable that the engines weren't revving properly.

Filing the blades in very small, balanced increments was first suggested by my father, who first learned about propellors ( airscrews ) when training as a Leading Air Mechanic with the Fleet Air Arm in 1942.

My next door neighbour also has an A22 and we bought the Mariner 5's together on a SIBS deal; he found the same lack of revs and did the same filing to his prop, with the same good results; he's a very good mechanic on racing and other high-end cars.

I'm not saying filing a prop will cure rabies or make one irresistible to supermodels, but it does sound like the answer for the OP.

 

[macd]

I realise you are thinking of max torque, but by the OP's description the engine is way over-propped for the application - assuming there's nothing else restricting revs, and there doesn't seem to be by description.

No, I'm thinking of max power. I do know the difference, and their relationship.
As to modifying the prop, I wouldn't be as wary as some of following your suggestion of paring a bit off. However, I do think the OP needs to be absolutely sure of the revs his engine is developing under load, i.e. whether his tacho reading is real. He does seem to be going about things carefully, but in a field that's not altogether familiar to him: I'm half deaf and don't need a tacho to tell the difference between 2000 and 4000rpm.

 

[VicS]

Good morning,
Thanks for all the answers.

VicS, why do you think that a 17% difference in pitch would not affect things very much? I was afraid instead that the change would be too big.

Sandro

I think it will make a proportional difference but not more than double the rpm to get into the 4500 plus range corresponding to max power

If the engine is not achieving full revs, it's not producing full power.

Once the engine is performing to its max, there will be more thrust; I have used this technique on a Yamaha 4 & Mariner 5 ( both 2 strokes but that doesn't matter ) and it worked well.

With the original props the engines struggled to push my Anderson 22 at anything like the speed I'd expect as they weren't building up revs; with a tiny bit of balanced filing, more whizz in every way and the engines are able to push her into quite strong wind & waves.

But by cutting part of the blades away you are increasing the rpm, at the same water speed , by reducing the efficiency of the prop. The power output from the engine will increase with the increase in rpm but the extra power developed will be lost due to the reduced prop efficiency.

Your case is a little different because you were not reaching the hull speed you expected. Filing bits off the prop may well have had the desired result cheaply but fitting a more suitable prop would have done so without the efficiency loss.

No, I'm thinking of max power. I do know the difference, and their relationship.
As to modifying the prop, I wouldn't be as wary as some of following your suggestion of paring a bit off. However, I do think the OP needs to be absolutely sure of the revs his engine is developing under load, i.e. whether his tacho reading is real. He does seem to be going about things carefully, but in a field that's not altogether familiar to him: I'm half deaf and don't need a tacho to tell the difference between 2000 and 4000rpm.

The OP seems fairly confident that his rpm figures are not out by a factor of 2 and I think , as far as is possible via the forum , he is right about that. The tach may be off calibration of course and reading a bit low but not by as much as half. Checking against a known accurate tach is the initial way forward I think.

 

[macd]

Vic, good sense, as always.
However, I'd suggest that checking against a known accurate tach on a 2-stroke is the best initial way forward. If it confirms the OP's earlier rpm readings, then he can move on with confidence.

 

[Sandro]

Macd, yes, plug lead type. As I said in my second post, the test bike is 4 stroke. The meter can be set up for 2 sparks per revolution, or 1 spark p. r. (2 stroke), or 1 spark every 2 revolutions (4 stroke) and I set it up accordingly (so I expect but...).
I agree that the surest check would be against a 2 stroke motor.
Please believe me, I am old but still can tell 2000 from 4000 rpm by ear. It's just because of that I started inquiring, bought the tachometer, experimented, posted, etc.

VicS, please correct me if it's the case. I heard a saying that, as a rule of thumb, a change of 1" in pitch woud give a change of 200 motor rpm. This was said speaking of 18" diameter propellers and gear ratios about 2:1. Fitting the Yamaha 7 1/4" x 6" would give 1" difference, that is only 200 rpm, unless the diameter is to be taken in account, or is it?
I don't think so but if yes the difference would be 600 rpm.

I didn't yet get an answer to the last practical question of my long post.

Looking forward, fair winds to all.

Sandro

 

[VicS]

VicS, please correct me if it's the case. I heard a saying that, as a rule of thumb, a change of 1" in pitch woud give a change of 200 motor rpm. This was said speaking of 18" diameter propellers and gear ratios about 2:1. Fitting the Yamaha 7 1/4" x 6" would give 1" difference, that is only 200 rpm, unless the diameter is to be taken in account, or is it?
I don't think so but if yes the difference would be 600 rpm.

I didn't yet get an answer to the last practical question of my long post.

Looking forward, fair winds to all.

Sandro

Ive not heard that rule of thumb and I dont understand it. Maybe the cold is getting to my little grey cells

The logic I applied, purely theoretical I admit but mathematically sound I hope, is that if the engine rpm is 1800 at a particular water speed with a 6" prop and you replace that with a 5" prop the the rpm at the same speed will be 1800 x6/5, which equals 2160. An increase of 360 rpm but still a long way off the aim of getting into the 4500 to 5500 range


The answer to the last practical question in your long post, which I think is

Now, from a very practical point of view, forgetting the Chinese meter, only considering that gradually opening the throttle the motor rpm follow the throttle opening till 2/3 - 3/4 of the travel and then don't increase anymore till WOT, considering that what I am aiming to is not more speed, (I already reach critical hull speed with little throttle), but is more thrust, what can go wrong (apart from possible money wasting) if I fit the 7 1/4" x 5" Yamaha propeller?

Nothing will go wrong.
The engine will run somewhat faster than previously at all speeds below max hull speed. Because it is running faster it will be a little noisier, will use more fuel and potentially develop more power but once you reach the hull speed any extra power will be wasted in trying, but failing, to push the boat faster. You might just be able to get a very light boat up on the plane, and be able to utilise the potentially available extra power.

 

[Seajet]

VicS,

I suggest you stick to pills & potions and leave the nuts & bolts stuff to those who are qualified & know about it.

A higher revving engine - ie reaching normal design potential isn't ' just a bit noisier ', it's developing more power, if tuned and propped correctly, all it can offer.

An overpropped low revving engine may well be lovely and quiet, but progress will be questionable - the whole point of this thread - and an engineer's toes will be curling hearing the poor thing straining !

Reprofiling the prop allows the engine to transmit the max power, now attainable, to the water now the prop' drivetrain allows it.

A higher powered more torquey engine with a longer pitch prop' would be ideal but the OP doesn't have that and anyway such engines tend to be unreasonably heavy.

 

[Seajet]

Just to add as I don't think I'd made it clear, it's the trailing edge one should file, but as mentioned evenly balanced in very small increments, 1mm at most per time.

 

[VicS]

VicS,

I suggest you stick to pills & potions and leave the nuts & bolts stuff to those who are qualified & know about it.

.

Who the hell do you think you are
and
What the hell do you mean by "stick to pills and potions"? Any prescription medication I may be taking is none of your business

 

[macd]

Macd, yes, plug lead type...

Understood, Sandro, and apologies for slighting your hearing (which was especially rich considering the state of mine:ambivalence.
That seems to nail it on the rpm front, leaving you with the other options to consider. I'm impressed that the Malta can push your boat along at the speed it does at those revs.

 

[Seajet]

Who the hell do you think you are
and
What the hell do you mean by "stick to pills and potions"? Any prescription medication I may be taking is none of your business

VicS,

I'm sorry I seem to have touched on a nerve; I was thinking ' I'm an engineer and you're a chemist ', I didn't know or mean to infer you're on any medication and I wouldn't joke about such a thing, a dear friend of mine has to have serious medication to even function - after she was spiked with a huge o/d of LSD a long time ago - the bad guy was comprehensively sorted but the damage was done.

As to my original rebuttal, it is down to the fact that you seemed keen to put down my suggestion of filing the prop', and seemed to be suggesting more rev's just means more noise and no more power, which unless the prop' is cavitating I heartily refute.

 

[rogerthebodger]

May I point out that VicS is a Chemist not a Pharmacist. It a Pharmacist who deals with pills and potions

Over the many years VicS has been on the fora and the many years VicS has had a boat he has shown a high level of knowledge about Outboards motors.

And I say that as a BSc Mechanical Engineer with 40 od years in Engineering Design And holding C Eng MIMechE before retiring when I went in to computer hardware and software design.

 

[Seajet]

May I point out that VicS is a Chemist not a Pharmacist. It a Pharmacist who deals with pills and potions

Over the many years VicS has been on the fora and the many years VicS has had a boat he has shown a high level of knowledge about Outboards motors.

And I say that as a BSc Mechanical Engineer with 40 od years in Engineering Design And holding C Eng MIMechE before retiring when I went in to computer hardware and software design.

Chemist was the term I used, with the general idea pills and potions are more his thing than engines !

I am a trained & qualified engineer before and after I was a trained & qualified photographer.

Vic has formed a habit of being snippy with me, so when he tried to have a go at my perfectly reasonable suggestion I was snippy in return.

Playground stuff.

I'm sure Vic is able to speak for himself, and as I mentioned recently re a Chichester Harbour meet up, I'm sure we'd get along well face to face.

Now go back and read what I actually said.

 

[rogerthebodger]

Chemist was the term I used, with the general idea pills and potions are more his thing than engines !

I am a trained & qualified engineer before and after I was a trained & qualified photographer.

Vic has formed a habit of being snippy with me, so when he tried to have a go at my perfectly reasonable suggestion I was snippy in return.

Playground stuff.

I'm sure Vic is able to speak for himself, and as I mentioned recently re a Chichester Harbour meet up, I'm sure we'd get along well face to face.

Now go back and read what I actually said.

Having reread post #22 you are both correct.

I have been through the same design process for the main prop on my yacht and fabricated and modified one from stainless steel.

The best way to modify a prop that is over size is to change the pitch to keep the slip low and thus efficiency. I did that to my prop as I found mine was still over propped but you can only change the pitch by a smallish amount by bending the blades, to increase the change of pitch you need to remove the blades and re-weld the root at a different angle the twist the blades to maintain the pitch/diameter ratio so the effective pitch across the blade is constant. If this is not done you will loose efficiency or the prop in its ability to convert HP into trust.

Now if you simply reduce the diameter which is the simplest way to modify an over propped prop the pitch/dia ratio will remain constant but the slip will increase thus again reducing the efficiency of the prop.

Now which ever method is used the maximum speed of the engine will increase and providing you don't get to a point where the maximum of the power curve you will increase the HP available at the prop and thus the trust. So you will get an increase in usable trust an may be boat speed but due to the extra slip thus lower efficiency you will not get the maximum thrust you could get if the pitch was changed to allow the engine to reach the same maximum speed thus the same HP. The less slip would allow a greater thrust to be developed for the same engine HP.

In such a small engine this loss of efficiency by changing the prop diameter instead of pitch would not be to noticeable but the increase in the engine HP would and is what the OP wants.

I have had lots of time when I have been snipped of and my view is if it due to a difference in view I try to clarify my view rather than just snipe back of if I was wrong I just take it on-board without reply.

BTW Boots the chemist is a

I also know VicS can stand up for him self and what he does is up to him.

As said before the first thing that the OP needs to do is to measure the RPM of the engine can attain with the current prop as it gives a base point to determine whatever the OP does to the prop he can measure the effect of the mods for better or worse.

BTW Boots the chemist is a Pharmacist who deal with drugs, a Chemist studied the reactions between chemical compounds which Boots do not do. This day and age in the western world a Pharmacist is a drug policeman for the government and not a maker of drug compounds.

Off to the the sangoma to get my pills and potions

 

[Seajet]

Rogershaw,

I have to disagree with one small point, the assertion that more rev's = just more noise was never right.

Just before you nip off to the Dispensary / ( Sea ) Witch Doctor / Merlin...

I was never talking about filing the tips off the prop' blade diameter wise; I must not have explained it properly, what I mean is a small amount - say 1mm at a time - off the trailing edge of each blade.

If one wanted to be fancy about it one could make it more off at the root and less off towards the tip, but a simple bit off all the way along is fine for a crude thing like a small outboard, as long as the reduction is balanced among the - usually 3 - blades.