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Old 08 July 2013, 12:37   #1
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Choice of two props Tohatsu 9.8

My Tohatsu 9.8 2-stroke will achieve just under 20mph on my 2.85m SIB.

With two of us on board it manages 18.5mph.

This is with the standard 8.5 pitch prop (short shaft engine) However l bought last year a 9.5 pitch prop and with this fitted the boat reaches the same speeds as above, but at 500rpm less. It also takes a bit longer to "get out of the hole" although it does achieve it OK.

So at WOT the rpm is 5,000 with the 9.5 and 5500 with the 8.5. WOT range for this engine is given as 5-6,000rpm.

Which prop should l be using? My guess is the 8.5 although at displacement speeds the engine is quieter with the 9.5.
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Old 08 July 2013, 14:31   #2
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8.5 mun.
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Old 08 July 2013, 15:37   #3
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Would it do any harm to use the 9.5 only the engine is quieter for a given speed? For instance at a cruising speed of 10mph the engine is doing 3,000 rpm with the 9.5 and about 3,400 rpm with the 8.5.
For a cruising speed of 15-16mph the difference is about 500rpm
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Old 08 July 2013, 16:09   #4
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Go for the one that gives the max rpm at wot, that's with a light sib and you alone, assume you are using a induction tach right ? if so, which brand is it ? Ideal is to count with a prop pitch that deliveres near it's stated max 6 K.

Bear in mind that a with 8.5 with extra weight, mates, rpm will be reduced accordingly, but engine will be much happier and stress free reving through all the throttle range than with 9.5. Next 7.5 next less pitch should deliver near or full 6 K. BTW, are you and friend light guys to plane a 2.85 sib with a 9.8 ?

In your particular case, a 9.5 would be used if engine is over reving 6K at wot to drop rpm into safe working parameters. A higher pitch will lower and a lower pitch will increase rpm accordingly.

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Old 08 July 2013, 16:32   #5
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OK sounds like l should stick with the 8.5 then.

It gives 5500rpm at WOT one up or two up....it is the same.
Our combined weight is about 20 stones. The boat hardly pauses getting on the plane with two of us on board.
Even with the 9.5 pitch prop it still planes easily, although it does take a few seconds with the higher pitch prop.

With the 9.5 it tops out at 5,000 rpm which is at the minimum of the WOT range which is given as 5-6000 rpm
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Old 08 July 2013, 16:52   #6
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Place your friend as a bow mask and go full throttle. With a 9.5 and 2 boaters at wot you're lugging the engine at 5K, much worst if adding extra weight or more mates. It''s like going up hill with a loaded car up hill on third gear.

You have a tech size/HP limitation. A 2.75 Mtr sib has too much hull drag to overcome fast with a small portable engine, that same 9.8 will plane next 300/320 size sib much faster. Inflate tubes and keel to it's recommended working pressure to achieve a better hole shot, hull rigidity issues.

Stick with the 8.5, your engine will thank you forever. Probably your combo will benefit installing trim tabs ?

Happy Boating
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Old 08 July 2013, 16:56   #7
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Have you not contemplated with the idea of dropping down to the 7.5. Got mine of eBay for 40 tested with the tiny tach and within range of WOT maxed at 5600. Got two of us on the plane quicker and felt quicker although I didn't compare the top end with any Gps.

Edit post...... should have said maxed at 5700 with the 7.5 with just me in flat water
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Old 08 July 2013, 17:06   #8
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How do you guys measure RPM on these?

Interested in the prop thing too, AFAIK I have the 'standard' factory prop whatever that is?! but as per my '3 kts down, where's it gone?' topic I'm not seeing 17 kts like I used to.
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Old 08 July 2013, 17:16   #9
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I have a TinyTach on the engine to measure RPM.

I will stay with the 8.5 pitch prop. The boat achieves nearly 20mph and l don't think it would go any faster!
It was just that using the 9.5 it still managed the same speeds, at lower RPM.

The boat does have a V-keel which is probably helping. I am possibly expecting too much from what is in essence a tender.....not a RIB.
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Old 08 July 2013, 20:21   #10
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AJ,

Are you running a 2.75 Mt sib with a 9.8 as Lightning ? A 7.5 pitch prop would be worst to plane a 2.75 sib with 2 mates, although will have a better top speed with just one.

Lightning you have just written the right answer, you're expecting too much out of that small sib, it's just for tender use and potting around, unless both are skinny boaters to plane fast that combo. See if possible to sell the 2.75 and go for a larger 320-340 size sib, will perform much better with a 9.8 HP

Happy Boating
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