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Old 05 March 2020, 07:12   #1
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Country: Greece
Make: Joker
Length: 5m +
Engine: Yamaha f100aet
Join Date: Mar 2020
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New prop

Hi. New member here

What I have is:
Joker coaster 5.8 Rib
Yamaha F100aet 4stroke carb & 5hp for emergency
2x 100ah batteries
Usually carry 2-4 persons
80L tank
Yamaha prop Diameter: 12.625, Pitch: 21
Gear ratio 2.31
Manufacturer wot rpm 5000-6000

I bought it used so after some tests and calculations I realized that I need a new prop with less pitch

I am between these two turning point hustler: 13.25 x 17 or 13.25x19

According to my research the boat can achieve 42mph with 100hp Motor, I have achieved 34-37 but with lots of slip around 25% on WOT and 33% at 4000rpm cruising 23 mph

So what I think is that the correct prop Is 17 pitch but if for some reason the high slip continue I will end up lowering my top speed and increase my mpg

If I accept that the high slips will continue I will probably need 19 pitch


Ps: the engines wot at 5000-5500 depending the loads

Sorry for being too extensive, thanks for your help
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Old 06 March 2020, 01:35   #2
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Country: Greece
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Just to ask simpler:
Should I pick the 17 based on the fact that the ribs will slip 4-8%
Or should I pick the 19 based on the fact that it will increase +400rpm over my current 21 ?
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Old 06 March 2020, 05:13   #3
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Country: UK - England
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Make: Ribcraft
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Quote:
Originally Posted by nick93 View Post
Just to ask simpler:

Should I pick the 17 based on the fact that the ribs will slip 4-8%

Or should I pick the 19 based on the fact that it will increase +400rpm over my current 21 ?


Before you start chucking money at props, have you checked the engine height on the transom. Youíre within the recommended WOT range (just) & you might gain a few RPM. The slip figures sound odd, but theoretical prop calcs are notoriously inaccurate.
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Old 06 March 2020, 09:27   #4
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I just measured it. Itís 0.3inch below standard
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Old 06 March 2020, 13:44   #5
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Country: UK - England
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Quote:
Originally Posted by nick93 View Post
I just measured it. Itís 0.3inch below standard


Not sure what your ďstandardĒ is, normally you start with the AV plate in-line with the bottom of the hull & work from there. All hull/engine/prop combos are slightly different & itís trial & error to find the sweet spot.
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Old 06 March 2020, 15:11   #6
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According this measurement


On my next trip I will defiantly make some WOT tests and post detailed results here.

Another consideration maybe to justify the excessive slip is that small prop diameters tend to slip more.
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Old 06 March 2020, 17:03   #7
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Country: UK - Scotland
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Quote:
Originally Posted by nick93 View Post
I just measured it. Itís 0.3inch below standard
Id start with the av plate 1" above keel line see what happens
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Old 19 March 2020, 11:44   #8
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Country: Other
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Make: Sea Rider 450 Rib
Length: 4m +
Engine: Tohatsu 5/18/30 HP
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Quote:
Originally Posted by nick93 View Post
According this measurement


On my next trip I will defiantly make some WOT tests and post detailed results here.

Another consideration maybe to justify the excessive slip is that small prop diameters tend to slip more.
Those posted parameters are for Merc motors, the distance between both lower leg plates are only 2 inches, the Yam is much taller. Anyway that motor should rev to 6 K at wot, varying deck load is a wot rpm serial killer, ideal is to prop the motor right with a fixed load but in your particular case that's mission impossible.

The best transon/lower leg height for Yam motors is for water flow to skim right under the upper splash plate once the combo is riding on plane.

With actual prop, play with different motor heights till that cond is met or as close as possible.To dial best transom/motor height match will need to have a visual inspection out transom to know what each height dials. Report your findings...

Happy Boating
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