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

[Sailpower]

Hi this is a question about the benefits of a 9.5" x 10" prop over a 9.25" x8", both three blade.

I've converted a '94 long shaft motor to short shaft so it will go on our 3.3m soft floor inflatable. The std prop is a 9.25 x 8 - at least that was what was on the longshaft motor.

My question is if it is worth swapping the prop for a 9.5" x 10" - what benefits might I expect? Maybe go for an 11" pitch?

I guess the 15 will be fast enough on the inflatable so top speed isn't likely to be a problem, but maybe speed getting onto the plane? We might have 300/350 lb in the boat travelling in an estuary with some small waves and swell with tide, so some power is useful.

I don't have anything sophisticated like a tacho and it will be a month or more before |I get on the water with the new combination.

Any help or insight into the difference the prop would make will be gratefully received.

 

[steelespike]

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

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[steelespike]

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

The 15 is capable of some serious rpm originally rated at 5500-7000.More recent numbers are 5000-6000.Without some performance details from you we can't give much detail from our side.Check out the induction tachs and some cells have a gps app.
The 8" prop should give strong performance good hole shot respond well to a load and may not exceed wot rpm with a light load.
Depending on your actual numbers a 10" would produce slower hole shot and be more sensitive to heavier loads.
It may produce higher speeds.An 11" would further reduce hole shot etc.
Ideally we need the wot (wide open throttle)rpm and gps speed lightly loaded.
Without these numbers there is no way to tell if the setup is working well or which way to go with prop selection.
At least an accurate gps speed lightly loaded reminding us of the prop size.
While your inflatable is light its shape doesn't lend itself to speed very well and usually suffers with added weight.
When figuring weight don't forget the motor,gas tank and gear.

 

[Sailpower]

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

The 15 is capable of some serious rpm originally rated at 5500-7000.More recent numbers are 5000-6000.Without some performance details from you we can't give much detail from our side.Check out the induction tachs and some cells have a gps app.
The 8" prop should give strong performance good hole shot respond well to a load and may not exceed wot rpm with a light load.
Depending on your actual numbers a 10" would produce slower hole shot and be more sensitive to heavier loads.
It may produce higher speeds.An 11" would further reduce hole shot etc.
Ideally we need the wot (wide open throttle)rpm and gps speed lightly loaded.
Without these numbers there is no way to tell if the setup is working well or which way to go with prop selection.
At least an accurate gps speed lightly loaded reminding us of the prop size.
While your inflatable is light its shape doesn't lend itself to speed very well and usually suffers with added weight.
When figuring weight don't forget the motor,gas tank and gear.

Thanks very much for the reply.
I was struggling to relate the pitch and blade diameter to power or speed. Would I be correct in thinking that in theory a larger diameter prop could deliver more thrust, but would need a more powerful motor as the diameter increased? Likewise, with pitch I'm thinking that a low pitch (say 5") would have the motor spinning twice as fast to cover the same distance forward as a 10". Does that relate to say 1st gear in a car and 4th for example (probably not a good comparison, but hope you see what I mean!)
I'll try and find an induction tach to try, and should be able to get some speed readings from a gps but I guess one isn't so useful without the other.

I guess I'm looking for a combination to get on the plane quickly without having the bow pointing straight up in the air - I'll try the prop I've got and look out for a s/h 9.5 x 10 to try.

 

[steelespike]

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

Very generally a 8" prop should act like a 8" prop assuming the same basic design within reason regardless of its diameter.
Diameter is a secondary consideration as far as thrust is concerned.Yes an increase in diameter can reduce rpm
Big heavy hard to push boats will generally do better with a larger diameter with an appropriate pitch.
If you look up props usually as pitch goes up you'll find diameter goes down and vice versa.
Lowering pitch does increase power, speed result would depend on the specific application.
The idea is to select the pitch that will have the motor rev freely within its rated rpm.
It is kind of like selecting a single gear in a car to do everything.
Too high it will be sluggish and lug the motor(forcing it to operate below its rated rpm).
Too low it has lots of power but may over rev and lack good top end.
Notice I didn't mention diameter.
While diameter can have a secondary affect on rpm It usually is the result of what drive the prop is on.
The final prop size will depend on boat size, weight,design and typical load and activities.
and of course the horse power and type of drive.Outboard,I/O or straight drive.
Phew! enough for now.

 

[Sea Rider]

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

Go for a wot spin with you alone with well trimmed engine fully inflated and well ballanced sib, install a good quality Tiny Tach, Hardline tach) on engine. Try each prop at wot, read achieved max rpm for every single prop, and compare their reading to it's factory wot recommended working parameters. Stay with the one that delives the closest to max rpm, factory delivered props on light inflatables usually do that.

Happy Boating

 

[Sailpower]

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

Thanks both, all much clearer I'll get a small tachometer (just seen some on ebay from the far east for less than $10) and do some wot runs when its all together.
Props seem to be a little harder to come by in the UK - I see some Solas ones in the US on offer for $35 /$50 but its over $100 by the time its sent to the UK and duties paid. Locally they're around ?90 new. Probably too much to buy and try out as an experiment.

Thanks again for the advice.

 

[steelespike]

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

I don't know if a $10 tach is a good investment. The Tinytach is more but has a proven reputation.
Be sure to indicate that it is going on an outboard.

 

[Sailpower]

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



This is the sort of thing I was looking at - it wouldn't be a permanent fixture, just to diagnose the optimum prop choice. It logs hours too, but its not going to be such an issue for me. The wire simply wraps around the HT plug lead for the impulses,

 

[Sea Rider]

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

Personally would't buy a cheapo tach, going for reputed brands is well spent money, be aware that if not precise saving $ 20 could be a stupid saving that could ruin a costly engine.

If going for a hour tach you could make it a permanent install at front lower pan. Besides reading rpm throughout the whole throttle range, can target to dial best cruising rpm for fuel economy. The hour counter will be very usefull to keep reccorded engine worked hours, change oil and oil filter, gear oil, plugs, etc if being a 4 or 2 strokes engine. Some induction tachs must be ordered with programable firing sequence or won't read well or accurate. Check before you order.

Happy Boating

 

[Sailpower]

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

Thanks for the photo, I can see its a useful place to have one mounted. I agree in principle with the idea of always buying the best parts you can afford, especially when it comes to critical components.
At the moment my usage is around 30 hours, per year, mainly during vacations, and I change the oil, plugs and impeller each season so my thinking is I'm not going to get close to service being needed due to the use the motor gets. I do log the hours separately so I have a measure of the work done by other components, but this motor is new to me and I'm giving it a fairly thorough check over since I've converted from Long to Short shaft, new thermostat, plugs and putting it back to tiller control from cables.

I guess all of the tachos on the lower price range may vary a little, as they are only gauges and not precision instruments. To get a Tiny Tach from the USA would probably cost around ?80 - ?100 - around $120+ and I guess my budget is pretty tight.
The props I'm trying are what I would imagine are safe experiments as they are factory fit 'std' configurations so unlikely to encourage the motor to over rev wildly, just alter the performance characteristics slightly. If I were trying out more radical prop dimensions on a motor they weren't intended for I would be much more cautious about over revving, or conversely lugging the motor at low revs.

I'll keep looking for a mid range tacho, but all I see so far are the really cheap ones which do allow a choice of firing sequence, or much more expensive ones at 4100 and up. Sometimes the mid range ones are the same cheap ones in better packaging of course. I guess we don't have the boating scene in the UK that exists in the USA - must be the weather and lack of large bodies of water to use recreationally!

Thanks again,

 

[Sailpower]

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

I think I've found a suitable one here

New Motorcycle Motorbike engine RPM counter Tachometer Hour Meter 2 stroke UK | eBay



It looks pretty similar to the one posted by Sea Rider and at around $9 must be worth a try. I'll report back.

 

[Sea Rider]

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

I think I've found a suitable one here

New Motorcycle Motorbike engine RPM counter Tachometer Hour Meter 2 stroke UK | eBay

View attachment 216846

It looks pretty similar to the one posted by Sea Rider and at around $9 must be worth a try. I'll report back.

Would say it's Hardline exact same, the Hardline Hourtach costs $ 35.00 in USA, in UK is listed as advertized at 13.75 Pounds. The posted model is the old type, probably won't read well or read double, the new one has a small pushbotton on face, check before buying. The cool thing about being programmable is that could be used on any type of engine...

Happy Boating

 

[limitout]

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

Thanks both, all much clearer I'll get a small tachometer (just seen some on ebay from the far east for less than $10) and do some wot runs when its all together.
Props seem to be a little harder to come by in the UK - I see some Solas ones in the US on offer for $35 /$50 but its over $100 by the time its sent to the UK and duties paid. Locally they're around ?90 new. Probably too much to buy and try out as an experiment.

Thanks again for the advice.

personally I think you are creating a problem that doesn't exist.

it is my understanding that small portable motors are best used only with the stock factory props and you ONLY need to change props if it is for a very specific and SPECIAL situations or extreme applications.

leave it with the prop that came on it and it will work fine but if you start trying to reprop it just for the heck of it or because you think it should be done, then I have a feeling you will put the motor under more strain then it should be and might make it perform worse after you spend all that money.

if it doesn't work right when you finally get out there and test it with the factory prop, then and ONLY then should you start thinking you might need to change the prop.

keep the prop that came on the lower unit you are using, it matches the gears in your lower unit and I would be shocked if you find a better prop for it.

just my 2 1/2 cents

 

[Sailpower]

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

Hi Sea Rider,
They had two listed, this 2 stroke one which I bought and I assume will record 1 pulse for 1 revolution, or a 4 stroke one which (I assume) will record 1 pulse for two revolutions. I hope they sent the right one!

 

[Sailpower]

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

Hi Limitout,
Thanks for the advice

I agree about using a std prop, trouble is the factory fit for long shaft is 10x7" pitch and for short shaft 9.9hp its 9.5x10" and for the 15hp 9.5x12" - gearbox and lower unit identical on all models. (Taken from the parts lists on line) I've converted long to short so the prop will be for heavier boats rather than for a lighter one that I have.

Older engines specified the 9.25x8" prop for the long shaft as I found fitted to mine.

I guess I assumed it was better to go for the 10" (the 12" is listed obsolete on the parts list) but as my original question, I wondered what the difference between 8" and 10" pitch might make on a 3.3m inflatable using the 15hp Johnson 2 stroke.

From the replies I'm getting the answer is 'try it and see', which I guess I will, and check the revs with my new tacho.

 

[Sea Rider]

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

Wanted you to be aware of the programmable thing. Threw money into the blue by buying 2 standard hour tach reputed brands to find out when tachs were installed that one read double readings, the other just half, both at wot, so useless on my 2 strokes15 Evinrude 96 and Tohatsu 18-30 HP engines.

On 2 strokes must be set to 2 strokes per revolution, if being factory set to 1 strroke per rev won't read properly. So, yours is a 2 or 4 strokes 15 HP engine ? Sometimes standard non programmable tachs don't work as expected, post when installed and water tested.

Happy Boating

 

[limitout]

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

well to answer your "pitch" question the pitch = theoretical distance traveled by one revolution of the prop so a 7=7", 10=10" and 12=12" and each time you go higher its more strain on the engine so your using up more of the power that's there.

as to which is the best choice the tach will tell you that because it tells you when the engine is most efficient use of engine power vs load

 

[Sailpower]

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

Wanted you to be aware of the programmable thing. Threw money into the blue by buying 2 standard hour tach reputed brands to find out when tachs were installed that one read double readings, the other just half, both at wot, so useless on my 2 strokes15 Evinrude 96 and Tohatsu 18-30 HP engines.

On 2 strokes must be set to 2 strokes per revolution, if being factory set to 1 stroke per rev won't read properly. So, yours is a 2 or 4 strokes 15 HP engine ? Sometimes standard non programmable tachs don't work as expected, post when installed and water tested.

Happy Boating

Hi, the one I bought was listed for two stroke engines rather than 4 stroke. I assume that means its pre set to display revs based on 1 spark for each revolution. My understanding (maybe wrong) was that a normal 2 stroke engine fires once for each revolution (One up stroke and one down stroke) A four stroke fires once every two revolutions, induction down, compression up, power stroke down, exhaust stroke up.

So I bought the one for 2 strokes, and it should work for a 2 cylinder in the same way as a single cylinder, assuming both cylinders fire simultaneously on completion of each compression stroke. I will post back when its assembled and running - may be a week or more though.
Thanks for the input,

 

[Sailpower]

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

Hi, I'm running a 15hp Johnson 2 stroke on a 11' (3.3m) inflatable (soft floor rather then RIB) and wondered what prop I ought to be using? I started this last year but got held up getting on to the water.

I took it out today with the rev counter attached and clocked it at 6600 revs at WOT with the two of us in and some gear. I didn't get a gps speed but the boat was twisting quite a lot across the waves and I really didn't ant to go any faster haha.

I believe the max operating revs are supposed to be 5000 -6000 - with the minimum test revs being 5700, so I'm thinking to change the 8 pitch prop to a 9 pitch? is that the right way to go, or is it from 8 to 7?

Advice appreciated!