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Old 29 April 2008, 00:37   #1
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Prop size; tunnel hull inflatable

Hello everyone,

This is my first post. I would like to ask recommendations on which prop size I should use on my 11' tunnel hull inflatable AMI Sportcat with a 2005 Mercury 15HP outboard. Currently with my setup and the stock prop on my 15hp; I think I am over revving the motor at full throttle as the rev limiter starts to kick in only when I open it all the way up.
I think the current stock prop is a 8" or 9" pitch. I am thinking going over to a 13" pitch. Can someone please share with me their knowledge and experience in pairing prop size with boat and motor. This is my first boat and I'm trying to tune it in to optimal performance in top end speed with out over revving it.

Thank you,

Jeff
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Old 29 April 2008, 11:33   #2
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Hi, Jeff;

Prop selection is a lot of trial and error in most cases.

General rule of thumb (with "general" being the operative word) is 100 to 200 rpm per inch of pitch, 100 rpm per quarter inch of diameter. Those figures tend to go out the window if your prop is way off to begin with.

If you are over-revving, you want a higher pitch; how much higher is sort of up for grabs. See if you can borrow props from a local shop, club, or friend. Once you get close to the desired range, the "rules" should apply a little more accurately.

Luck;

jky
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Old 29 April 2008, 16:15   #3
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Really, you can't just guess whats going on, you need to plug a Tacho (rev gauge) in and see what RPM its pulling. 15hp 4 stroke (i guess) needs to rev to 5500RPM, i was proping one today. Find out what RPM you doing and you can go from there.
Its about 200-300 RPM per pitch of prop, so you can work out easily what you are going to need.
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Old 29 April 2008, 16:43   #4
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Quote:
Originally Posted by Jeff Salinas View Post
Hello everyone,

This is my first post. I would like to ask recommendations on which prop size I should use on my 11' tunnel hull inflatable AMI Sportcat with a 2005 Mercury 15HP outboard. Currently with my setup and the stock prop on my 15hp; I think I am over revving the motor at full throttle as the rev limiter starts to kick in only when I open it all the way up.
I think the current stock prop is a 8" or 9" pitch. I am thinking going over to a 13" pitch. Can someone please share with me their knowledge and experience in pairing prop size with boat and motor. This is my first boat and I'm trying to tune it in to optimal performance in top end speed with out over reving it.

Thank you,

Jeff
If it's a 2 stroke 15 HP engine : Min/Max. RPM Range 5,200-5,800

If it's a 4 stroke 15 HP engine : Min/Max. RPM Range 5,000-6,000

Must place a tacho to read max wot RPM, then play with different prop pitches until the one that reaches 6,000 rpm or closest will be the correct prop for top end max speed performance. Do not pass max rpm range, will over rev your engine, and it's no good.

Not sure is portable engines comes factory delivered with rev limiter, at least 2 strokes do not, maybe 4 strokes ? Maybe are experimenting prop cavitation water issues and confusing with engine rev limiter kiking in ?

Happy Boating
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Old 30 April 2008, 13:08   #5
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Hi guys,

thank you for all the responses. I guess my next step would be to order the tiny tach. and go from there. but, do you guys think that the root of my problems would be the Doel-fin that I have installed. I have since removed it but have not had a chance to run it yet till this weekend. I'll soon find out. does it sound possible of a hydro foil over reving a motor on WOT?

thanks,

Jeff
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Old 01 May 2008, 11:45   #6
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Quote:
Originally Posted by Locozodiac View Post
If it's a 2 stroke 15 HP engine : Min/Max. RPM Range 5,200-5,800

If it's a 4 stroke 15 HP engine : Min/Max. RPM Range 5,000-6,000

Must place a tacho to read max wot RPM, then play with different prop pitches until the one that reaches 6,000 rpm or closest will be the correct prop for top end max speed performance. Do not pass max rpm range, will over rev your engine, and it's no good.

Not sure is portable engines comes factory delivered with rev limiter, at least 2 strokes do not, maybe 4 strokes ? Maybe are experimenting prop cavitation water issues and confusing with engine rev limiter kiking in ?

Happy Boating
The 4 stroke 15hp max revs is 5500rpm not 6000!!
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Old 01 May 2008, 20:16   #7
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Originally Posted by Turbodiesel View Post
The 4 stroke 15hp max revs is 5500rpm not 6000!!
According to Tohatsu tech specs the same 15 HP 4 strokes disguissed 15 Mercury version is 5000-6000 rpm, in theory should be the same factory delivered.

Happy Boating
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Old 01 May 2008, 20:24   #8
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Quote:
Originally Posted by Jeff Salinas View Post
Hi guys,

thank you for all the responses. I guess my next step would be to order the tiny tach. and go from there. but, do you guys think that the root of my problems would be the Doel-fin that I have installed. I have since removed it but have not had a chance to run it yet till this weekend. I'll soon find out. does it sound possible of a hydro foil over reving a motor on WOT?

thanks,

Jeff
A tiny tach will fit nicely, there's a double action model, rpm when running, hour meter counter when at stop, very useful for scheduled maintenances & easy to wire being a induction tach, When ordering specy number of cylinders, strokes, order with short cable wires (induction & ground cables)

Happy Boating
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Old 02 May 2008, 16:30   #9
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The Mercury 15hp is not a Tohatsu based powerhead, its completely different, i did double check with the Factory in the states, shown below.

15 FourStroke Specifications
HP @ Prop 15
kW @ Prop 11.1
Max RPM (WOT) 4500-5500
Cylinders 2 (in-line)
Displacement (CID/cc) 19.7/323
Bore & Stroke (in) 2.32 x 2.32
Bore & Stroke (mm) 59 x 59
Cooling System Water-cooled w/thermostat
Ignition System CDI w/ electric spark advance
Starting Manual or electric
Gear Ratio 2.00:1
Gear Shift F-N-R
Steering Tiller or remote
Alternator Amp 6 (manual)/10 (electric)
Alternator Watt 76 (manual)/126 (electric)
Trim Positions 5
Shallow Water Drive Standard
Exhaust System Through prop
Lubrication System Wet sump
Recommended Oil Mercury 4-Stroke Outboard Oil
Oil Capacity 1.0 L / 1.1 Qt.
Fuel Induction System 2-valve single overhead cam
Remote Fuel Tank Standard
Remote Fuel Tank (US Gal) 3.2
Remote Fuel Tank (L) 12
Shaft Length (inches) 15/20
Shaft Length (mm) 381/508
Dry Weight (lbs.) 111
Dry Weight (kg.) 50
Operator Warning System Low oil pressure
Available Propellers Standard equipment
CARB Star Rating 2
Warranty 3 years
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Old 04 May 2008, 03:56   #10
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catrib and prop size

There are a lot of possible reasons to explain your problem !

Questions :
- is it a new boat and motor, or used ?
- did YOU install the motor, and how is it installed (height ref hull) ?
- what is the hull shape ? - pictures from transom side showing the tunnel and motor are necessary
- transom angle ref hull ?
- did you check the speed (GPS) ?
- did you try to add weight at bow (one person) ? Changes something ?

The reasons :
- prop hub slippage if an old prop
- too high the motor
- positive trim if transom angle = 90 or close
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