Yamaha 9.9 High Thrust - Change Prop for Faster Small Boat

Troll n Toll

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May 21, 2014
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Fist Time, Long Time... Thank you to all the starcraft renovators on here for ideas with my Holiday project!

BTW, An "Error" window keeps popping up, if I hit backspace at the same time as the window pops up it sends me back to the main forum page and I have to start my post over again. 3rd time is the charm!



I have a 2006 Yamaha 9.9hp High Thrust that I use as kicker on a Starcraft Holiday V5 22ft. WOT is 4-6mph
I use the same engine to power my duck boat, a Starcraft Jetstar 15ft. WOT is 6-8mph. (I built a transom bracket to compensate for long shaft motor). The boat is just shy of planning off at WOT.

Prop: 3 blade aluminum, large diameter high thrust model, I haven't pulled the prop to read pitch. It appears to be a very high pitch.

Questions:
#1) Is it damaging to the motor to run WOT on the small boat. Maybe there isn't enough weight resistance to keep the rpm's in check. (I don't have an rpm reading device)

#2)I think I can use one motor to meet the performance needs of both boats by having two different props. Should I decrease pitch and keep same diameter to gain speed on the smaller boat?

My goal is to have one motor for two boat uses. Just change props as need be.

There seems to be plenty of power to push the small boat, but the throttle limits the prop from spinning fast enough to get on plane. I think that decreasing pitch will push the boat faster and up onto plane.

I think a small diameter prop would have higher slip and less resistance, causing even higher rpms.

I'd rather not spend money on multiple props to find out later that it was the wrong move. It doesn't need to be a perfect match, just "good enough"

Any help is greatly appreciated.

Thanks
 

steelespike

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Apr 26, 2002
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Usually if set back; if you go to your thread a yellow band will appear with a restore tab.
Is this a T9.9 if so it appears the ratio is 2.92.and prop diameter is about 12"Referred to as large diameter X low pitch
Is the speed by GPS? Without your prop size we can't suggest a prop size and can't tell roughly where the rpm is.
An induction tach will get you set up for accurate rpm.These work in almost any application.
Do you know the gear ratio?
 
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Troll n Toll

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Tach came in the mail Thursday. Had a break in the weather today, so took her out for a spin....

To recover all important data lost last week:

engine: T9.9 high thrust 4-stroke
gear: 2.92
prop: 11-3/4" x 9-1/4P
Speed WOT: 11.5mph
RPM WOT: 5220
calculated slip = 26%

Induction tach set to one spark/revolution (Is this correct for 2 cylinder, 4-stroke with tach wire wrapped around one spark plug wire?)

I got this speed and rpm reading with no gear in the boat, just me (215#s) I locked the tiller straight then walked up the the dashboard to level out the boat.

If I sit back at the tiller I just plow through the water:
speed WOT: 8mph
rpm WOT: 4700

Someone mentioned being under powered, but there are plenty of 14' boats with 10hp engines and 2 fat people cruising on youtube. My boat is 14.5' with a 1/4" plywood floor and closed bow, no bench seats.

Think the 12 pitch would help?
I havent seen any 11-3/4 diameter options, how does the prop diameter effect the whole thing? Small diameter = higher slip = keeping high rpms with higher pitch?

Thanks
 

Troll n Toll

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BTW - I am very aware of all the variables of boating performance. I got my OUPV "6-pack" license at 19yrs old, had it for 10 years now, working off and on as a captain on cape cod. I'm just looking for some guidance before investing in a new prop.
 

steelespike

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Messages
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My thoughts are at 11.5 if your planing your just barely planing. I think your high thrust is intended to handle the boat best and won't
be close to a regalar 9.9 at wot.
Prop rpm at 5250 is only about 1797 with the 2.92 ratio. Prop rpm with as 2.08 at 5250 is 2554.
I really think the high slip is a result of the low prop rpm. I think if you lower rpm on a F9.9 to a prop rpm of 1797 slip may be even higher.
Your indicated rpm is already a little low and a 12" will kill your performance.
You might try dropping to a 9" prop. Get your rpm up a little more. Lower slip perhaps, maybe planed a little better.
It does appear that at 5500 assuming less slip your speed may be closer to the 14 I came up at 13%slip.
I think the 9" prop will do better on your 22 footer as well.
Keep in mind a F9.9 at 2.08 ratio makes about 19 at 5250.
 
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