Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

[Sea Rider]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

You'll need to go 2 pitch up from current installed prop at least assuming there's 200-300 rpm loss per pitch....

Happy Boating

 

[Sailpower]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

Hi,
thanks for the reply. I'll now try and get some props to try out - I may try the test again to be certain, but if the 6600 is repeated, would going up to an 11" (from current 8") be over doing it? it would to somewhere between the 5700 minimum test revs and the 6000 top limit.
The rev counter turned out to be a great asset in setting the tick over to a decent level - got it to 750 revs with only the occasional stall after a minute or two - its not a new motor after all.
thanks again,

 

[jestor68]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

150-200 rpm per inch of pitch is the universal rule of thumb concerning rpm change.

 

[Sea Rider]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

150-200 rpm per inch of pitch is the universal rule of thumb concerning rpm change.

Jestor,

After trial experimentation, somebody stated that this rule of thumb is not applicable at least on light inflatables, personally don't plan investing on a +,- pitch prop to find out as could turn out into a costly nonsense experiment as I'm happy with my current factorry delivered prop max wot rpm perforrmance. So theoretically poster would need to go at least 3 pitch up to overcome + 600 over revving rpm to be at max 6K so to be inside max 6 K wot numbers factory stated for that engine and assuming he will be carrying same stated load.

Happy Boating

 

[Sea Rider]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

Hi,
thanks for the reply. I'll now try and get some props to try out - I may try the test again to be certain, but if the 6600 is repeated, would going up to an 11" (from current 8") be over doing it? it would to somewhere between the 5700 minimum test revs and the 6000 top limit.
The rev counter turned out to be a great asset in setting the tick over to a decent level - got it to 750 revs with only the occasional stall after a minute or two - its not a new motor after all.
thanks again,

Sailpower,

Before you test other pitch, test that sib with you alone and gear empty, max 2 gal of fuel, tubes, keel must be inflated to 3.0 psi minimum, engine well height/transom seated, well trimmed, engine must be perpendicular to water level, on a wot run on flat no windy waters. With you alone probably rpm will go tad + 200 higher to 6800, but that we currently don't know yet...

That engine is over revving on that small light inflatable, if you pass that same engine to a larger and heavier + 380 will lower its max rpm accordingly. Being a used engine, probably previus owner changed pitch to match other boating application which you didn't know about ?

Try going for a 11 pitch if currently running a 8 as to lover 6600 rpm to nearest 6K, Once pitch is correctly dialed, adding a extra boater will drop rpm accordingly. Post results for the record to check if dropping 150-200 rpm per less pitch on a inflatable is a rule of thumb or a myth ?

A good idle rpm adjustment is the one that produces a dry clak while shifting forward at load and engine remains on, if dies, adjust idle carb screw a tad CW to remain on.

Hasppy Boating

 

[Sailpower]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

Sailpower,

Before you test other pitch, test that sib with you alone and gear empty, max 2 gal of fuel, tubes, keel must be inflated to 3.0 psi minimum, engine well height/transom seated, well trimmed, engine must be perpendicular to water level, on a wot run on flat no windy waters. With you alone probably rpm will go tad + 200 higher to 6800, but that we currently don't know yet...

That engine is over revving on that small light inflatable, if you pass that same engine to a larger and heavier + 380 will lower its max rpm accordingly. Being a used engine, probably previus owner changed pitch to match other boating application which you didn't about ?

Try going for a 11 pitch if currently running a 8 as to lover 6600 rpm to nearest 6K, Once pitch is correctly dialed, adding a extra boater will drop rpm accordingly. Post results for the record to check if dropping 150-200 rpm per less pitch on a inflatable is a rule of thumb or a myth ?

A good idle rpm adjustment is the one that produces a dry clak while shifting forward at load and engine remains on, if dies, adjust idle carb screw a tad CW to remain on.

Hasppy Boating

Hi, thanks for the advice. I'll check the set up for depth and trim angle before doing as you suggest.
The engine was a long shaft (now a short shaft) on a bigger cabin boat so the 8" pitch was probably useful for a displacement hull. I think an 11" pitch is the largest they do for the 9.9 /15hp range. I have a 9.9 carb for the engine which i could swap - it would limit the revs and produce less power at the top of the range, but maybe that would be enough. Some runs with a speedo would be useful.
I'll report back about the rev drop. Thanks again.

 

[Sea Rider]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

Trail engine at wot with 15 HP carb and 8 pitch as suggested, then test with 10-11 pitch and try again. Stay with prop pitch that gives the nearest 6 K rpm reading. To convert a Evi or Jhony 9.9 to a 15 must also change complete reed valve assembly and carb. 15 HP RV assembly has slight larger diam intakes than 9.9 diam intakes to receive more fuel from larger carb. A tech birdy at the OMC facrory told me so many moons ago.

Happy Boating

 

[dazk14]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

Hi, thanks for the advice. I'll check the set up for depth and trim angle before doing as you suggest.
The engine was a long shaft (now a short shaft) on a bigger cabin boat so the 8" pitch was probably useful for a displacement hull. I think an 11" pitch is the largest they do for the 9.9 /15hp range. I have a 9.9 carb for the engine which i could swap - it would limit the revs and produce less power at the top of the range, but maybe that would be enough. Some runs with a speedo would be useful.
I'll report back about the rev drop. Thanks again.

Throwing away HP is - well - nutty. :faint2: You'll make me fall off my rocker.

You'll have no problem getting the rev's down. Being way out of the power band, it should shed RPM's quickly to ~6K at which point it will start resisting.

Just going from an 8" pitch to a 10" is a 25% increase in pitch. While it won't respond linearly, you get my point.

You have a 2.42 gear ratio for those with slip calculators. I estimated at 6600 rpm you ran about 20mph with 3% slip. 21mph would create a negative slip number.

Is it an air floor? Some of the Asian knockoffs have a protruding front keel that is supposed to act like a V, but tend to limit hull speed.

They also can becomes performance dogs when you begin loading them, which tends to happen with most small inflatables.

The 15 make peak HP at 5500, so if you'll be generally lightly loaded, try the 11. It should be the fastest, with a range of 5000-6000, it gives you still some carrying capacity.

Sea Rider is giving you good sound prop advice trying to run it at the top of the range for great all around performance, but you may need to try both props for ideal.

20 lashes for even considering to convert a 15HP to 9.9HP Sailpower.:whip:

 

[Sailpower]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

Wow, wasn't expecting a whipping! Thanks for the input though, I'll work on the prop first of all.
if my first figures were correct, and the revs drop by 300 per pitch, an 11 would still run at 5700 but if a pitch only drops by 150 I would still be running over 6000. I think the only thing to do now is to try the props and see what happens.

The inflatable is soft floor and has an inflatable keel which is mostly out of the way as the bows rise, but would present greater resistance at low displacement speeds. I think its a generic design although it is branded Excel.

I can get a 10" prop at a more reasonable price to try out, but its a 2 blade - haven't done it so far as I thought it will it give more slip and therefore be less of a true comparison. Will try and get a three blade 11" to try as well.

Thanks for the advice, the whip marks are fading already.....

 

[Sea Rider]

Re: Best prop for a 15hp Johnson on an 11' inflatable 'SIB'?

Sailpower,

For best prop pitch trial performance that 11 footer must have tubes and keel inflate to at least 3.0 psi and air deck if being a soft inflatable to at least 12 psi so to have a rigid inflatable which will ride much better on water, will have less drag than one being under inflated and will have a faster planing response. It's good to have a lightly loaded inflatable deliverring it's max or near it's max wot rpm factory stated for that engine, speciall if liking racing against other inflatables or a storm is chasing you.

That way you'll know that engine will rev happily inside safe wot parameters. On the other hand, as soon you start adding weight, rpm will drop acccordingly. That 11 footer well top inflated delivering max wot rpm will be good to transport 2 adults up and probably a third one at a nice plane speed. Used to have a OMC 305 sib with a EVI 9.9 running full 6 K and with 2, driver and boating pal got on plane really fast with superb cruising speed.

Happy Boating

 

[devilfishMN]

Until you get over 1000 lbs total weight, you will be fine with a 10 inch pitch. If you want to go any high in pitch or number of blades, you will need a tachometer to know if you are still within a safe operating RPM.

That 8 inch pitch prop was probably the original prop that would prevent the engine from failure if the motor was stuck onto a 5,000 lbs fishing boat as a kicker motor for trolling.

 

[southkogs]

Please read the rules about diggin' up old topics. Closed.