Propeller for 9.9-HP Auxiliary

Optimizing the performance of Boston Whaler boats
whalerpaul
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Propeller for 9.9-HP Auxiliary

Postby whalerpaul » Sat Dec 22, 2018 2:23 pm

Boat: 1987 OUTRAGE 18 with 2005 Honda BF150, used in the San Francisco Bay area primarily ocean fishing.

On my 1999 Yamaha T9.9ELRX four-stroke-power-cycle auxiliary engine, I'm replacing a damaged aluminum propeller with a stainless steel propeller.

Give me a suggestion on pitch.

Should the propeller be three-blade or four-blade?

I expect to always be limited to hull speed of 5 to 7 nautical-miles-per-hour on the OUTRAGE 18, and I plan to go low on the pitch in case I have to tow in someone in; I may use a propeller with pitch of 7 to 8-inches.

I boat in saltwater. I would use the auxiliary engine for trolling at 3 to 4-nautical-miles-per-hour and as a backup in case of a failure of the main engine, Honda BF150.

The SOLAS propeller chart suggests a 9.25-diameter 9-inch pitch, but I can see that propeller has much less blade than the damaged propeller I have on there.

The prior prop was an aluminum three-blade apparently close to 12-inch diameter and much bigger blades than the Solas. The old propeller has a quarter-inch clearance to the [anti-ventilation] plate. From pictures I see on line of the T9.9, the propeller comes pretty close to the cavitation[anti-ventilation] plate. The 9.25-inch-diamter propeller has quite a bit of gap to the A-V-plate and much smaller blades and the old propeller, thus I'm concerned that this suggested propeller may not have enough push.

The old propeller came with the Yamaha T9.9ELRX engine when I purchased it used, and that propeller seemed to work okay--although I rarely used it.

Any comments would be appreciated. Thanks--Paul

Acseatsri
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Re: Outrage 18 kicker prop advice needed

Postby Acseatsri » Sat Dec 22, 2018 5:41 pm

I've always found that increasing diameter is always more efficient than increasing pitch. I would stick with a large diameter four-blade aluminum propeller. You could use it more safely in the shallows, and not have to worry about breaking gears if you hit something--unlike the unforgiving stainless prop.

PS- my Honda 9.9 "Thruster", used primarily for sailboats, came standard with a large four-blade propeller.

jimh
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Re: Propeller for 9.9-HP Auxiliary

Postby jimh » Mon Dec 24, 2018 10:41 am

I don't see much need for going to a stainless steel propeller. At 10-HP an aluminum propeller has sufficient strength that the blades can easily handle the power involved.

Why not get the Yamaha OEM propeller for the Yamaha auxiliary engine?

Also, a 9.9-HP auxiliary is going to be more than sufficient for an OUTRAGE 18. It should have plenty of power--with the right propeller.

As for pitch, using these parameters:
    RPM = 4000
    RATIO = 2.5
    PITCH = 6
    SLIP = 25
the boat speed calculates to
    MPH = 7

whalerpaul
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Re: Propeller for 9.9-HP Auxiliary

Postby whalerpaul » Mon Dec 24, 2018 6:56 pm

I am just seeing these responses now. Thanks for the information.

I had been leaning towards a stainless steel propeller because my previous aluminum prop got damaged when pushed over an incline while trailered and traveling in reverse. I felt that a stainless steel propeller would be easier to repair if that happened --first time in 25 years.

I had not been considering the potential damage to other driveline parts like gears, if contacting solid objects with the propeller running. I'll assume from what is said here that it would not be unusual for a stainless steel propeller to strip internal gears if it got motored into a solid object. So now I am leaning back towards an aluminum prop.

The Yamaha T9.9 can be used on a wide variety of craft and I expect I am not anywhere near its limits with my 18 Outrage. I need it for slow trolling and getting me back if the main fails.

How far should the tip of the prop be from the anti-cavitation plate (minimum gap)?

I can understand that getting too close to the plate (the surface of the water) could induce ventilation. Is ventilation a concern?

I never really tested the (now broken) used propeller that came with the used T9.9 motor, but it has only a 1/4-inch-gap to the plate. It seemed to work okay, but I never could see the area of the prop while under power.


------------------

I like the suggestion of going to a bigger prop (eg 11" - 12") vs the 9.25" listed from the Solas Prop selection online tool, and potentially going to 4 blades. Is there any downside to going with a 11" - 12" diameter prop with 7-8 pitch in my situation? ... how much flexibility do I have? what specs should I stay within? I was hoping to hear of some actual setups that seemed to work well for folks. I dont think I will be able to test multiple solutions ( running different props) so I need to be in a reasonable ballpark with my first choice.
I would like to be trolling at 2-3 knots typical, 5-6 max at as low an rpm as reasonable.

JimH--what is the diameter of the propeller for the example calculatoin you did? Did the model also provide for the 18 Outrage as the boat?
Also, are there guidelines for targeted maximum slip?
Can I get access to the model you are using?
-------
Acseatsri - do you know the diameter and pitch on your prop? Does it come very close to the plate? I would expect that a T9.9 pushing a sailboat is a lot more effort than pushing an 18 outrage.


------

whalerpaul
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Re: Propeller for 9.9-HP Auxiliary

Postby whalerpaul » Mon Dec 24, 2018 7:08 pm

Oh, and as to:

Why not get the Yamaha OEM propeller for the Yamaha auxiliary engine?


I presumed that there was no 'standard' OEM prop for the t9.9 because it could be used on such a wide variety of applications. Although, JimH may be implying that there is a standard large blade that comes with it.... maybe that was what was on mine when I bought it used.

thanks,
paul

jimh
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Re: Propeller for 9.9-HP Auxiliary

Postby jimh » Mon Dec 24, 2018 11:03 pm

whalerpaul wrote:...what is the diameter of the prop for the example calc you did?


There is no parameter for diameter in the calculation, so I did not use any diameter information.

whalerpaul wrote:...did the model also provide for the 18 outrage as the boat?


There is nothing in the calculator that includes input for any sort of data about the boat, so I did not input any data.

whalerpaul wrote:....are there guidelines for targeted maximum slip?


Generally if a propeller is working effectively the SLIP will be under 10 (percent). The value I used, 25-percent or 0.25, would be for a situation where the propeller was not working at maximum effectiveness, as might occur when trying to propel a hull that was a very heavy load.

whalerpaul wrote:Can I get access to the model you are using?


There is no model, just algebra.

Crankshaft speed x gear ratio = propeller shaft speed

Propeller shaft speed x pitch = speed of advance

Speed of advance x (1 - SLIP) = boat speed

Throw in the proper unit conversions.

For expediency, you can build the relationship into an on-line calculator. See

http://continuouswave.com/forum/viewtopic.php?f=7&t=41

Here is an example to demonstrate the method. It uses the parameters I gave initially:

    RPM = 4000
    RATIO = 2.5
    PITCH = 6
    SLIP = 25 (0.25)

(4000-crankshaft-evolution/1-minute) x (1-shaft-revoluition/2.5-crankshaft-revolution) =
1,600-shaft-revolution/1-minute

(1,600-shaft-revolution/1-minute) x (6-inch-advance/1-shaft-revolution) =
9600-inch-advance/1-minute

(9600-inch-advance/1-minute) x (1 - 0.25) =
7200-inch-advance/1-minute

Now some unit conversions: 60-minutes/1-hour and 1-mile/63360-inches

(72000-inch-advance/1-minute) x (60-minutes/1-hour) x
(1-mile/63360-inches) =
6.8-miles/1-hour =
6.8-MPH boat speed


Nota Bene: I had to edit this about 30 times to get it right. If you read it while I will still editing, you may have seen some wrong numbers.

jimh
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Re: Propeller for 9.9-HP Auxiliary

Postby jimh » Tue Dec 25, 2018 12:40 am

The plate above the propeller is the anti-ventilation plate. It is there to stop ventilation of the propeller when the engine mounting height puts the propeller upper blades near the surface of the water--that is the theory.

To have a gap of 1/4-inch between a propeller blade tip and the anti-ventilation plate is fine. I would not get much closer. Once I tried a propeller that was a very close fit, perhaps 1/8th-inch gap or less. The pressure of the water coming off the blade tips started to remove the paint off the underside of the anti-ventilation plate and, if run long enough, I think it would have started to cut away some aluminum, too.

Acseatsri
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Re: Propeller for 9.9-HP Auxiliary

Postby Acseatsri » Thu Dec 27, 2018 4:59 pm

The propeller on my 9.9-HP Honda "thruster" is a 10-inch-diameter x 6.5-inch-pitch four-blade propeller, spinning through a 2.33:1 gear ratio. The propeller is specially designed for pushing heavy loads, not uncommon on 6,000-lbs sailboats. The propeller is claimed to output 15-percent more thrust in forward and up to 40-percent more in reverse. I believe the only difference between a standard 9.9 and the "Thruster" is the propeller and also an exhaust bypass from thru-the-prop hub when in reverse.

jimh
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Re: Propeller for 9.9-HP Auxiliary

Postby jimh » Wed Jan 02, 2019 1:43 pm

Let me add two anecdotal observations about small auxiliary engines and propellers:

--I used to race on a small sailboat, with auxiliary power provided by a small outboard. The owner observed while shopping for an outboard engine that the 15-HP model cost no more than the 9.9-HP model, so he bought the 15-HP. I suspect that engine was set up with a propeller intended for use on speedy little power boats, and it was miserable at pushing around this 25-foot sailboat hull;

--I recall that John F. remarked after installing an 8-HP Hi-Thrust Yamaha T8 auxiliary engine on his OUTRAGE 22 that the little engine produced better handling in reverse than the 225-HP main engine.

whalerpaul
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Re: Propeller for 9.9-HP Auxiliary

Postby whalerpaul » Wed Jan 09, 2019 3:15 pm

Acseatsri wrote:The propeller on my 9.9-HP Honda "thruster" is a 10-inch-diameter x 6.5-inch-pitch four-blade propeller, spinning through a 2.33:1 gear ratio. The propeller is specially designed for pushing heavy loads, not uncommon on 6,000-lbs sailboats. The propeller is claimed to output 15-percent more thrust in forward and up to 40-percent more in reverse. I believe the only difference between a standard 9.9 and the "Thruster" is the propeller and also an exhaust bypass from thru-the-prop hub when in reverse.


ACSEATSRI--thank you for the information. Do you use this [engine and propeller] ONLY on your sailboat application?

Or, do you use [the engine and propeller] on a Boston Whaler boat, too?

I [am] leaning toward the Solas 10-inch-diameter and 7-inch-pitch four-blade propeller (at a price of) $72. I need to make sure that [for] use on OUTRAGE 18--much lighter than a sailboat--I won't damage the 9.9-HP engine or its gears by using this high-torque propeller. It should come in handy when I have something heavier to tow--not very often--like a sailboat.

:-)

--paul

jimh
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Re: Propeller for 9.9-HP Auxiliary

Postby jimh » Wed Jan 09, 2019 5:13 pm

To determine if a propeller is suitable for use with a particular engine and boat combination, the simplest method to assess the suitability is to measure the engine speed at full-throttle. If the engine is able to accelerate to its rated engine speed for full-throttle operation, then you can assume the propeller is properly sized with regard to the power available to turn the propeller from the engine and the load the propeller imposes on the engine.

Acseatsri
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Re: Propeller for 9.9-HP Auxiliary

Postby Acseatsri » Sun Jan 13, 2019 4:51 pm

Re my Honda 9.9-HP Thruster engine: I use it as an auxiliary engine on my 23 Walkaround. These HONDA 9.9-HP engines are frequently used as auxiliary engines on sailboats and larger boats. I only hang the Honda 9.9-HP on the transom during tuna season when I'm making offshore runs [longer than 100-miles] to use if my Honda BF250 fails. So far, I've never had to fire it up.