tohatsu 9.8 prop

[AJ.]

Just bought a new prop for the above on ebay. When I put the old and new side by side it is now clear that the previous owner has filed the prop big time assume to get rid of some dinks.

Going to be interesting if I notice any difference in performance. I assume with a thinner edge it will bite more and the bigger surface area will get my 2.9 Sib on the plane quicker.

[Locozodiac]

Buying spare props without installing a induction tach on engine for wot testing can turn into a very bad & costly wrong purchase. Must test both props with you alone, preferably on flat cal no wind waters and check max rpm achieved at wot with each prop.

Then compare both readings to factory stated min-max wot rpm parameters to know if engine is running in the safe side. Which are the pitch numbers stamped on the old & new prop ?

Happy Boating

[AJ.]

Hi Loco,

Thanks for the reply, the prop that I have purchased (white one) is as per the Tohatsu 9.8 manual. I have the long shaft so purchased 12 spline 8.5"x7.5 and is a genuine Tohatsu prop (also came in Tohatsu packaging) PART NO. 3B2W64515-0.

I have not got a tacho but feel I would be safe with selecting the manufacturers recommendation.

I assumed the original prop was standard but it wasn't until I compared them side by side that I noticed the difference. The stamp on the new prop clearly states 7.5 the one that came with the engine has H7 and on closer inspection has been seriously filed down as the leading edge is quite thick and you can see the filing marks where the paint has come off following a respray. The old prop will be used as an emergency spare and now I know should I ever use it again I wont be taking this prop to WOT.

[Locozodiac]

Hi AJ,

If you are going for an induction tach, better go for a hour/tach meter which is a must install. Test your white prop, specs for both 9.8, 2,4 strokes are 5000-6000 rpm. If near 6000 rpm or slight less with you alone you'll be fine, bear in mind that if placing extra mates wot rpm will decrease accordingly, but still inside safe wot parameters.

Hardline induction hour/tachs are very simple, reliable and small to install outside engine. Will need to set firing sequence to read well. 7.5 is the standard factory delivered prop, if over reving will need a 8.5 pitch to lower max rpm if boating full throttle with you alone, but that can only be determined with a tach.

Happy Boating

[Nos4r2]

AJ, isn't that motor an auxiliary as well?

[PeterM]

Originally Posted by Nos4r2

AJ, isn't that motor an auxiliary as well?

I thought that..but I wonder if that was just a justification...let's be real..it's never going on the back of his boat...

Comment was passed this afternoon he should just sell it and get a ss for some sib action.

[AJ.]

Originally Posted by Locozodiac

Hi AJ,

If you are going for an induction tach, better go for a hour/tach meter which is a must install. Test your white prop, specs for both 9.8, 2,4 strokes are 5000-6000 rpm. If near 6000 rpm or slight less with you alone you'll be fine, bear in mind that if placing extra mates wot rpm will decrease accordingly, but still inside safe wot parameters.

Hardline induction hour/tachs are very simple, reliable and small to install outside engine. Will need to set firing sequence to read well. 7.5 is the standard factory delivered prop, if over reving will need a 8.5 pitch to lower max rpm if boating full throttle with you alone, but that can only be determined with a tach.

Happy Boating

Hi Loco,

This engine is just for playing around on with a little SIB, the main purpose of it was as NOS points out below as an Aux.

Having spent quite a bit of money on it in repairs as it looks like it wasn't looked after that well by previous owners I don't want to spend a great deal more on it.

How much are these hour/tacho things?

[samt]

AJ, I've got a Quicksilver tiny tach your welcome to borrow if you want to test you props.

[Nos4r2]

Thing is, IF you're going to use it as an aux, you're probably better off propping it so it overrevs a bit on the sib and don't use full throttle. If you need to actually use it as an aux, it could be an issue if you don't make full power.

If realistically you aren't, just prop it right-but then, you might as well chop it in for a short shaft.

[AJ.]

Cheers Sam, are you around this Sat?

[AJ.]

Originally Posted by PeterM

I thought that..but I wonder if that was just a justification...let's be real..it's never going on the back of his boat...

Comment was passed this afternoon he should just sell it and get a ss for some sib action.

It's virtually new with all the replacement parts! Actually I am sticking it on the RC not this but next weekend so shove that up your tailpipe with the potato!!

[samt]

Originally Posted by AJ.

Cheers Sam, are you around this Sat?

I'm not Saturday. I am Sunday and next week.

[Locozodiac]

Originally Posted by AJ.

How much are these hour/tacho things?

AJ, big difference between pottering around and aux use. Both Hardline hour/tach or tach alone costs $35.00 US. What like much better than same Tiny Tach equivalent is that Hardline uses just one pick up standard cable, can be trimmed to size when engine has been finally cable routed as needed, does not use a ground wire and tach is smaller in size.

Tiny Tach uses 2 cables, pick up & ground, both joins at a soldered connector, the solid red pick up cable tends to break with engine vibration at connector's end. Other down issue is that will need to wrapp several rounds of left over cable inside engine once tach & pick up cable is routed. If trimmed to size to avoid wrapping, will need to redo the whole soldering process to match same as when factory delivered.

Happy Boating

[AJ.]

Originally Posted by Locozodiac

AJ, big difference between pottering around and aux use. Both Hardline hour/tach or tach alone costs $35.00 US. What like much better than same Tiny Tach equivalent is that Hardline uses just one pick up standard cable, can be trimmed to size when engine has been finally cable routed as needed, does not use a ground wire and tach is smaller in size.

Tiny Tach uses 2 cables, pick up & ground, both joins at a soldered connector, the solid red pick up cable tends to break with engine vibration at connector's end. Other down issue is that will need to wrapp several rounds of left over cable inside engine once tach & pick up cable is routed. If trimmed to size to avoid wrapping, will need to redo the whole soldering process to match same as when factory delivered.

Happy Boating

Hi Loco,

Thanks for the explanation i have borrowed a Tiny Tach of Samt (thanks) and will be testing this weekend on flat water with me on my own and then with a friend. Will publish results for those that are interested

[Locozodiac]

AJ,

Post wot results, I'm a firm believer every portable motor should have one installed as standard delivered props used in light inflatables could be slight over reving at wot and boater not knowing anything about it. It's perfect for optimizing boat engine needs to perform inside safe wot parameters.

My 18 horse with me alone on flat no wind waters is achieving top 5800 from max stated 5800, confidently screaming safely with standard prop inside 5300-5800 rpm.

Happy Boating

[AJ.]

Hi Loco et al.,

Took the Sib out today with the recently bought 7.5 prop measured the below:

1 Man:
Tickover 1480
In gear 1280
WOT 5990

2 Man:
Tickover 1540
In gear 1330
WOT 4720

No matter high hard we tried we could not get it to plane with 2 people, SIB (Airdeck) was inflated to correct pressures.

With one of us no problems but it wasn't as fast with this prop, so clearly the old prop that had been filed down was over revving the engine. Although not as much fun for the engines sake I will stick with this one

[Locozodiac]

The 1-2 man test was using the new prop, right ? Just for the record, did you tried same test with old prop and 1 man to check wot numbers ? So this test confirms that standard delivered props on light weight inflatables achieves near 100% of it's max factory stated wot parameters.

The issue about not achieving full plane with 2, probably just fast displacement speeds with lower rpm it's due to the fact that sib is too short, hull has too much water drag for engine to overcome fast. Same engine will perform much better on a 330 & larger size sib. That's why you should never buy a sib less than a 320 size..

Happy Boating

[whisper]

Would the following statements be correct?

1, If it's a long shaft then the sib bow will want to go up instead of plane so the sib won't be moving fast enough for the outboard to pick up the revs and its gripping better than the smaller prop.

2, If you use the smaller prop it is slipping more in the water and picking up more revs

3, If you had the same engine but a short shaft then you would find that it would probably get up on the plane with the larger prop.

4, If you did buy a short shaft and trim it out when using it as an aux you would probably find that you would circulate enough water around the transom for it to be used as an aux however it would be less efficient than the long shaft... That's if it was long enough so that enough of the leg was in the water?

[PeterM]

To get across whispers well made points:

Loose some weight tubbs, get a decent sib, stop faffing with a long a shaft and get decent engine. Clear? Good....

[whisper]

Originally Posted by PeterM

To get across whispers well made points:

Loose some weight tubbs, get a decent sib, stop faffing with a long a shaft and get decent engine. Clear? Good....

Don't sit on the fence Peter