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PostPosted: Fri Jul 20, 2018 10:52 am 
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fusioneng wrote:
Try Iboats.com


I did, nothing fit.

The suzuki uses 10-tooth splines, that's the problem. Even suzuki doesn't have a replacement prop for 2.5hp.
They do for 4hp and up.

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PostPosted: Fri Jul 20, 2018 11:14 am 
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Location: Colorado
Quote:
The suzuki comes with 7" 1/2 x 5" 1/2 propeller.
I just want to get a 7" 1/2 x 7" prop. I look and look, leading to no where.


what problem are you trying to solve?


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PostPosted: Fri Jul 20, 2018 11:57 am 
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walt wrote:
Quote:
The suzuki comes with 7" 1/2 x 5" 1/2 propeller.
I just want to get a 7" 1/2 x 7" prop. I look and look, leading to no where.


what problem are you trying to solve?


I am trying to go faster. :lol:

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PostPosted: Fri Jul 20, 2018 12:13 pm 
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You know that Stringy measured about an ideal RPM with the Suzuki for as much speed as possible with the stock prop. I think he actually could have gone to slightly higher RPM but that would have required a lower pitch prop - opposite of what you are proposing. Peak speed comes when you achieve peak RPM at the highest torque the outboard can achieve at full throttle.

But OK.. please just do a good job measuring both your speed and rpm before and after you get the new prop.


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PostPosted: Fri Jul 20, 2018 3:16 pm 
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Location: Sarasota,Key West FL
Think of prop pitch as a nut traveling down a threaded rod. If you have a fine thread the nut travels slower than a course thread at a given rpm.
It takes exactly 1hp to propel a TI to 6mph.
On our boats we have no desire to ever run our engines at any higher rpm than 1/4 throttle because the outboards are way to loud.
The hull speed on a TI is 8.6mph which should take right around 2.5hp to propel the boat at that speed. (sqrt(18.5) x 2.0= 8.6mph), (the hull formula multiplier on this hull design is 2.0, (not 1.34)).
Our TI’s with the twin 2.3hp hondas running at low rpm, (putting out a little over 1hp ea.) propels our TI’s to 7-8mph with no sails on flat water. We prefer this because the engines are extremely quiet and we can easily talk over the engine noise, (about the same noise as a torqeedo 403). Better yet we get great fuel economy, around 2-3 hrs runtime per tank of fuel, (which works out to 80 mpg). Fuel economy of course goes higher with more wind, (obviously).
Keep in mind these boats are much lighter than a standard boat, and because the length to width ratio of the hull is 8/1, the boat glides thru the water up to hull speed with much less resistance than a standard boat, (just look at the swirling water behind the transom on a square backed johnboat). I think all the specs on these motors revolve around a standard 1000 lb boat.
There is an advantage to twin engines because they help each other overcome the power overhead.
With our hondas running at minimum rpm, (throttles all the way down) the boat travels 6mph, we cannot go any slower without killing one motor. Which is actually a problem in no wake zones.
We have custom 7” pitch props installed.
I don’t recall any top speeds from 5-6 yrs ago with the stock 4.5” pitch stock props, and never ran twins with stock props, ( no point because the boat would not have gone any faster with twin engines running the stock props, ( go back to the nut on a threaded rod example).
I couldn’t care less what the max hp or max rpm on the engines is. The boat never sees either. My purpose is to get up to hull speed as quietly as possible, using as little fuel as possible.
With my planing hull mod I moved the stern wave back 3ft behind the boat, which allows the boat to plane, ( faster than hull speed). Once clear of the harbor with the throttles locked to a constant rpm and throttle, once I raise the sails and start pedaling, the boat speeds up to ten mph, and the rpm’s on the engines increase quite a bit, ( increasing fuel economy and speed). That’s in 3-5 mph winds of course with more wind the boat goes faster of course, (but not from the engines).
I would be curious to know how fast a stock TI goes with the stock prop on a standard suzuki 2.5 running at idle, ( those numbers tell me everything I need to know).
I’m pretty sure none of this will be of any use to anyone, but those are my numbers, well documented pretty constant over 4 yrs of use.
I don’t know much about the suzuki, but the Honda’s were pretty bulletproof and reliable in over 5-6 yrs of troublefree operation in some pretty hostile offshore conditions.
FE

EDIT: Prior to adding the second engine, my 1/4 throttle, (locked throttle) speed was 5-6 mph, (on flat water before deploying sails), with one engine, adding the second engine increased the flat water (no sails) speed to 7-8mph, and I was able to decrease the throttle setting/rpm, (making the motors quieter, (which was my goal). There are probably 30 videos on youtube with the single engine. Back then, (with the single engine my normal sailing cruise speed in very low winds, (3-5mph) was 7-8 mph.


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