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Old 26 June 2019, 17:09   #1
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Prop for Yamaha 40hp ?

I have a 4m Avon rib and an old (90's) Yamaha 2-Stroke 40hp engine. Myself and a friend had it at 31mph and with a lighter person and calmer weather we have had 36mph from it.
I am needing a new prop. Is there anything out there a bit more modern or am I already at the speed limits of this set up ?

At one end of the bushing there is a 25 stamped at (12 o'clock) and the letter A at (6 o'clock). What does this mean ? Reason asking is I have seen a similar prop but it is stamped with 20 and A.

On the other side I have 48-73136-13 and printed on the side is 48 73136 A40 13P


If I find a prop for sale do all of the above numbers need to match ?


Sorry for the Novice Questions, I just want to buy the correct part or even a more modern prop, Thanks.

Is this the prop I need to buy ?

https://www.ebay.co.uk/itm/Mercury-4...s/273901485325
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Old 27 June 2019, 03:06   #2
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That's actually a mercury prop you've got on there, 13p. 10.5" diameter aluminium
Pretty much any 13 spline prop will fit that Yamaha, yamaha mercury and tohatsu all use the same splines and gearbox size for 40/50hp.

Depending on how the boat is performing you can change pitch, higher will give more top end, lower less top speed but quicker acceleration. If you're after better all round performance you should upgrade to a stainless prop, some of the best ones are made by Yamaha themselves, the "G" series semi cleavers. You can get them in 13-16p but the 15 and 16 are £400 odd, the 13G is £180 and 14G is £140. I've got a couple 14G for sale if you're interested, or can get new 13G's still
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Old 27 June 2019, 13:13   #3
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Bad music and dangerous for motor to play with pitches without counting with a installed tachometer which seems you don't have, right ? Safe wot rpm range is all, MPH is relative.

To prop right your motor there's 2 choices, running middle wot rpm for top end speed whichever the max MPH was achieved, max wot revs for top hole shot, both scenariost as usually loaded. Will need to check what's the min-max wot rpm range factory stated for your motor and play safely with prop pitches when doing recreational boating.

Happy Boating
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Old 27 June 2019, 15:07   #4
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Not neccesary here, this is a well known and tested setup, no need to reinvent the wheel
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Old 27 June 2019, 15:54   #5
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I put a 40V on a 4m searider and it struggled to get max revs with a 13p, went much better with a standard 12” Yamaha 6H5 alloy cupped blade type (actually quite similar to the Mercury design)
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Old 28 June 2019, 07:56   #6
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I have a Yamaha f40 on a ABNautilus 12 ft that weighs around 900 total. To get into the 5 to 6 k RPM sweet spot I have to use a 12 or 13 pitch prop. That gets me to around 34 miles per hour. I've tried a 14 pitch stainless steel prop but that was too heavy and too tall. Oddly enough there is not enough torque for that engine to pull that prop properly. I would not recommend the stainless steel for an engine that's small.
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Old 28 June 2019, 09:20   #7
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Quote:
Originally Posted by Roflhat View Post
Not neccesary here, this is a well known and tested setup, no need to reinvent the wheel
Who's post were you referring to, mine or yours ? Sorry mate, each motor/boat combo it's a unique instalation doesn't matter if using same type and HP motor. Hull shape, deck load, weight distribution, lower leg/transom height, trim angle, prop shape could be different from one installation to any other one, lots of variables accounts for more or less wot rpm difference. A 12-13 pitch prop will power the boat, only a tach will tell which one is better suited for motor to run safely inside its wot rpm range factory stated.

Like playing with prop pitches myself, but doing so with an induction tach it's a night day difference than by ear tach, doesn't matter iif having a musician ear. With a tach will be able to fine tune any prop in a half pitch size to slightly increase/decrease wot rpm if needed.

If you guys like boating with any HP motor while olympically ignore at which rpm the motor is revving up at full speed simply ignore this post and keep boating as you've been doing so far...

Happy Boating
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Old 30 June 2019, 08:22   #8
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Thanks for the replies, still somewhat confused, can anybody share a link to the exact prop that came with the engine when it was built or a link with the prop that will best suit my needs ?, it's a bit of a minefield out there.

Thankyou in advance for the time taken to point me in the right direction !
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Old 30 June 2019, 09:22   #9
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Quote:
Originally Posted by nousername View Post
Thanks for the replies, still somewhat confused, can anybody share a link to the exact prop that came with the engine when it was built or a link with the prop that will best suit my needs ?, it's a bit of a minefield out there.

Thankyou in advance for the time taken to point me in the right direction !
As has been said there is no one size fits all, no such thing as a standard prop. To get the correct prop for your boat you need to know the rpm flat out with your average loading. Engine needs to reach as close as possible makers max rated rpm.
You can buy a small induction tach off ebay for around a tenner which will do to work out your required size.
A stainless prop will always be more efficient and you often need to use a lower pitch if going stainless as the extra efficiency can result in less wot rpm but a stainless prop probably speced will give a small performance advantage over ally, wether its worth the extra cost is a decision for you.
Take the boat out measure the wot rpm with your current prop and people can advise accordingly anything else is guesswork
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Old 30 June 2019, 14:14   #10
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Quote:
Originally Posted by beamishken View Post
As has been said there is no one size fits all, no such thing as a standard prop. To get the correct prop for your boat you need to know the rpm flat out with your average loading. Engine needs to reach as close as possible makers max rated rpm.
You can buy a small induction tach off ebay for around a tenner which will do to work out your required size.
A stainless prop will always be more efficient and you often need to use a lower pitch if going stainless as the extra efficiency can result in less wot rpm but a stainless prop probably speced will give a small performance advantage over ally, wether its worth the extra cost is a decision for you.
Take the boat out measure the wot rpm with your current prop and people can advise accordingly anything else is guesswork

My current prop has sustained damage, it has a crack on one blade and a £2 size it missing from another blade. I can buy the tach, I hope my damaged prop doesn't alter the rpm much but will reply in due course, Thanks
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Old 30 June 2019, 14:21   #11
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Can't you borrow a good prop from someone to get a baseline? Even slight damage can have quite an affect on performance
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Old 30 June 2019, 14:23   #12
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Quote:
Originally Posted by nousername View Post
My current prop has sustained damage, it has a crack on one blade and a £2 size it missing from another blade. I can buy the tach, I hope my damaged prop doesn't alter the rpm much but will reply in due course, Thanks


Steel developments will restore that prop to ďas newĒ condition without breaking the bank.
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Old 02 July 2019, 15:58   #13
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Prop Maximization.-

To have it clear, all new motors factory delivered with a prop are usually medium pitched, are ok for motor's break in period, but usually not good for speed purposes at wot (wide open throttle. Usually will need to go for less pitch props for motor to run at least middle to max wot rpm range factory stated for each motor brand and HP model.

It's an excellent idea to prop right any combo when buying second hand ones as your load could not be same if the previuos boater propped right his motor to run top, owner propped wrongly his combo or is boating with a wrong factory delived prop.

Buy an induction tach, install it, go for a wot run on flat calm water cond as will be loaded and report max wot revs achieved with current damaged prop or any other one to see the wot rpm difference between both props. Need to know which is the min-max wot revs factory stated for said motor to prop it spot on.

This is an excellent cheap induction hour/tach :

https://www.hardlineproducts.com/pro...o-8-cylinders/

Happy Boating
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Old 03 July 2019, 16:57   #14
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Ok Guys, Thanks for all the advice etc.

I have used the marine weld to temp. fix the prop, enough for testing and i've bought a tacho .
Not sure i'll find any pure flat water but I hope to get out somewhere over the weekend and test the WOT rev count.

I will report back with the info and with a bit of luck one of you kind gents can advise me the best way forward
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Old 03 July 2019, 17:18   #15
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>>> with a bit of luck one of you kind gents can advise me the best way forward

Ahh remember this is a forum... we will all advise on different best way forward.
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Old 05 August 2019, 16:23   #16
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I bought the rev counter (total waste of money) and took the boat out to test. Unfortunately I had excess weight on board (fishing trip x 3 guys(in a boat that couldn't swing a cat))before the engine cut out...Dirty plugs and excess weight causing fuel to run out off the carb.

My point is if I got 36mph, (36.8) to be exact with full (25l) petrol, loads of gear, battery, anchor and 2 up... Why didn't someone pick up on me already being in the boats comfort zone ? With less weight I could be pretty darn close to 40mph, think the propeller it was running was pretty much perfect for my set up...

Thanks all the same to everyone that made an effort to advise, not all advice was correct but I decided to buy the same prop as I had for the above reasons
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Old 06 August 2019, 17:52   #17
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The issue here is that all motors needs to run at "wot" inside their safe wot rpm range factory stated for each HP and motor brand. Need to know which are the min-max safe wot rpm range for yours. Check sticker if having one or the Owner's Manual if available.

Load is a wot rpm serial killer, would be interesting to know which wot rpm was achieved lightly loaded, say solo boating and with 2-3 up. Speed is relative, could be over revving the motor lightly loaded and extremely lugging the motor heavily loaded, but that you don't know till the induction tach is correctly installed and working properly on proper flat water conditions.

Test same new prop as suggested and report your wot rpm numbers with 1-2-3 on board to determine if new prop its doing its homework right..

Happy Boating
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Old 07 August 2019, 09:18   #18
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Quote:
Originally Posted by nousername View Post
My point is if I got 36mph, (36.8) to be exact with full (25l) petrol, loads of gear, battery, anchor and 2 up... Why didn't someone pick up on me already being in the boats comfort zone ? With less weight I could be pretty darn close to 40mph, think the propeller it was running was pretty much perfect for my set up...

Maybe because thatís not what you said in your OP.
Maybe Rolfhat essentially did, but you paid more attention to the noisier posters.
Maybe because you are still hoping to get another 10% out of it, and those sort of things need the right data and knowledge.
Or maybe itís because free advise on an Internet forum is worth exactly what you paid for it.

That Tacho may actually prove useful long term too - helping you spot a gradual reduction in performance that comes with your badly maintained carbs or realising that your bodged prop repair is failing.
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Old 07 August 2019, 15:50   #19
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Originally Posted by Poly View Post
Maybe because thatís not what you said in your OP.
Maybe Rolfhat essentially did, but you paid more attention to the noisier posters.
Maybe because you are still hoping to get another 10% out of it, and those sort of things need the right data and knowledge.
Or maybe itís because free advise on an Internet forum is worth exactly what you paid for it.

That Tacho may actually prove useful long term too - helping you spot a gradual reduction in performance that comes with your badly maintained carbs or realising that your bodged prop repair is failing.
I have to agree with poly on this it certainly seemed from your initial post you wanted to eek out the last few available kts. In which case finding your wot rpm with a known prop is the starting point to selecting a "tuned" prop which if going stainless for best performance you dont want to get it wrong or your 100s of £ out of pocket
If you were happy with your current prop why bother to ask why not just go and buy the same replacement.
Those small tachs are around the £5-£10 mark & may be of use in the future
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Old 11 August 2019, 15:12   #20
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Yeah, Sorry Guys!!!, Posted that after a few beers.
What I meant was that the tach was a waste of money due to me being over weight before the engine cutting out, not that it was recommended and useless, i'm sure it will come in handy one day.
You are both right, I didn't explain myself in the OP and I am a total novice, ie' have no idea what I am actually wanting or even asking, so my apologies for that.

I have decided just to fix the boat up and either modify the seating and centre console to allow more space or buy a bigger, more modern set up. The latter will be happening either soon as or over winter time I hope.

Issue is pulling weight and actual size of the boat due to pulling capabilities of car and storage space, thanks for the replies, they did help and sorry for my embarrassing reply, I really should stay away from the internet if under the influence.

All the best
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