Help me find correct prop

[mrmrva]

Hello everyone, I have a damaged propeller (one blade is partially damaged) and as it turned out it is also inadequate, so it should be replaced. Boat is aluminium 6.5mx1.5m (22x5 feet) with flat bottom and raised stern, not a factory boat but DIY and it is used for fishing on a river. Hull looks something like this

Outboard is Yamaha 4-stroke F9.9A mfg 1985, gear ratio 2.92:1 and optimum WOT RPM is 4500. The current propeller is a composite 3-blade 11 3/4x12 1/4. WOT RPM that I measured is about 3500, so not optimum. I've read all I could find online about propellers (1'' pitch change = 200RPM , 1" diameter is approximately 2.5" pitch, change from 3 to 4 blades ~ 150RPM, etc.) so I've calculated that "ideal" propeller would be 9 1/4 x 13 3/4 3-blade or 9 1/4 x 12 3/4 but could not find such a prop for this engine. My budget is limited and I don't have an option to try and test different props here where I live. I was looking at this as possible option http://boatpropellers.iboats.com/Yamaha/9.9_HP_(2000-2007)_11.00-Pitch_Amita_3113-093-11_Propeller/809/8527

Any thoughts and recommendations are appreciated

 

[craze1cars]

Re: Help me find correct prop

If I'm understanding all of this correctly, you have a 9.9 HP engine on a 22 foot boat?

If yes, then I will assume this is a non-planing boat (essentially it is a "displacement" hull). It is important to realize that for non-planing boats I believe 1" = 200 RPM is NOT a valid guideline and cannot be followed.

I do not have experience with propping non-planing boats, but I wanted to point this out for others who may have more experience with such boats and can hopefully help you out better.

 

[mrmrva]

Re: Help me find correct prop

Yes you are correct this is not a planning boat. If I understand correctly, and I am very new to all of this, since it has flat bottom it is sort of semi-displacement hull very often used here on river boats called The Danube hull.
Unfortunatelly I don't have any pics of my boat yet but here is something similar only without raised stern and a lot bigger and heavier

It is important to realize that for non-planing boats I believe 1" = 200 RPM is NOT a valid guideline and cannot be followed.

Wasn't aware of this so good tip.

 

[steelespike]

Re: Help me find correct prop

200 rpm per inch doesn't apply to small motors. Prop size is allways written diameter X pitch.
I don't think Yamaha made 4 strokes in 85.I couldn't find an 85 4 stroke in the prop section.Are you mixing oil in the gas? Also diameter should be around 9" not 12 1/4.
Are you getting the size off numbers on the prop?Near the base of the blades or under the prop nut.Your max rated rpm is 5,500 you should prop to that rpm especially in your setup.
An educated guess a 9 1/4 diameter X 7" pitch in a 4 blade.

 

[mrmrva]

Re: Help me find correct prop

Well we called official Yamaha dealer here and based on serial they said it was 1985 (although I wouldn't trust them completely) and I think this model was made from 1984 (it is definitelly 4 stroke I know that much ). Anyway I have a manual for 1987 model, F9.9AM and that is were I got specs. If it helps engine code is 6G8 and here are specs (it shows max WOT rpm = 4500)

I read prop dimensions from blade and was surprised by numbers so started this topic.

P.S. Out of curiosity, is there any rule of thumb for small engines and what are small engines (up to what HP)?

 

[steelespike]

Re: Help me find correct prop

So your dealing with a comepletely different animal.
Different than both the 2 stroke and 4 stroke we have here.
With your low rpm and high numerical gear ratio I think the variious rules of thumb
may not apply.Do you have any manual info on the spicific sizes available for that motor?
Also do you know the spline count? I have a feeling that motor is designed spicifically
for applications like yours. Your going to need a prop with a diameter similar to yours.
Perhaps the dealer being in that area may understand your needs and be acquainted with boats like yours.Also may be able to tell you what props are available.

 

[mrmrva]

Re: Help me find correct prop

Damn, just my luck. Spline count is 8 but there is nothing about prop size in manual. I will try giving dealer a call but I'm sceptic about their expertise especially with engine as old as this. Anyway, will post back here.

thx

 

[steelespike]

Re: Help me find correct prop

The props for the 1985 2 stroke Yamaha is an 8 spline. there is a 10" X 7"
iboats part number 3113-100-07. called a high thrust. But I feel you need something closer to a 12" diameter. imo an 85 isn't that old when dealing with boats and motors.
Hopefully if the dealer deals in setups like yours they will be understand about your rpm & gear ratio and know about that particular Yamaha.
If all else fails you could have the prop repaired and repitched.At a prop shop that knows what they are doing.I would start with a 1" reduction.Most props they can repitch up to 2"
but I feel when they go too far it can comprimise the results.If you go to a 10" pitch and it turns out too much I don't think you can go back.
That 7" 3113-100-07 prop I suggested could be repitched if necessary as well.

 

[mrmrva]

Re: Help me find correct prop

I've done aditional research and found on Piranha website OEM prop list for Yamaha 9.9 and there is my current prop in the list.

This one is called Dual Thrust since as it appears I have a High Thrust outboard designed for heavy loads and lower speed. I will call dealer in the morning to hear his opinion on the matter.

 

[steelespike]

Re: Help me find correct prop

Well done;If you want to gamble on the safe side try the 11 3/4 X 9 1/4 aluminum.
If it turns out rpm runs a little high you can just throttle back.
With aluminum a prop shop may be able to tweak it a little if you want to fine tune it.
Also if you hit something you can have it repaired.If hit something with a plastic it will break a blade off and on rare occasions plastic props do break spontaniously.

 

[mrmrva]

Re: Help me find correct prop

Well I've called the only dealer over here and explained him the situation. Without much thinking he said 11 3/4 X 9 1/4, and the price ~320$ Same thing in the US costs 91$. So I guess my friend from CA will have to make extra room in his luggage and bring it to me since he is comming here end september. Anyhow, steelespike thanks for your advices, you've steered me in the right direction

 

[steelespike]

Re: Help me find correct prop

Good luck, please let us know how you make out.
Spike

 

[mrmrva]

Re: Help me find correct prop

Will do, and thanks again Spike

Petar

 

[mrmrva]

Re: Help me find correct prop

Well to report back as promised, prop arrived from US 10 days ago and I am getting around 4400 rpm@WOT which is ideal for this engine. Don't know about speed, didn't have GPS to test but all in all I'm satisfied.

 

[steelespike]

Re: Help me find correct prop

Sounds just about perfect.Would be interesting to see your wot speed.
I bet the slip numbers are pretty high.If you give me the prop size wot rpm, speed and gear ratio.You can figure the ratio by rotating flywheel by hand and counting revs for one rotation of the prop.We can figure slip.

 

[mrmrva]

Re: Help me find correct prop

Gear ratio is 2.92:1 I know that from manual. I will find someone with GPS phone and make speed measurements with old and new prop to see what the difference is and post it here.

 

[steelespike]

Re: Help me find correct prop

Did a little calculation using your prop size,gear ratio, rpm, 20% slip to arrive at about 10 mph Of course your unusual boat shape and boat/motor combination might produce considerably higher slip.and less speed. Probably the most efficient speed would be 6-8.
It would be fun to see some pictures.

 

[mrmrva]

Re: Help me find correct prop

My friend took these this weekend, don't have better ones yet so here you go

I suppose you don't have anything similar in US