Tohatsu 9.8 prop question

[gahunter706]

I have a Tohatsu 9.8hp that I bought used a few months ago that the stock 3 blade prop on it and the guy that I bought it from also gave me a 4 blade prop for it. I went to put the 4 blade on the other day to test it out and noticed that the prop sits outside of the gear case. I wanted to know if this is normal or if I have the wrong prop before putting it in the water. Thanks.

 

[Sea Rider]

Can you put spacer & screw nut fully in as to have enough space clearance for the split pin to be inserted across the prop shaft & prop nut ? Could be a prop that has a different rear shape compared to its OM.Anyway if wanting to play with props, pitches must install an induction tach to check if OB runs inside its factory wot range while boating with you alone on flat calm, no wind water cond. Coudl be lugging the OB which is bad and not knowing anything about it.

Happy Boating

 

[gahunter706]

I didnt even try to put the washer and prop nut on, but im pretty sure it would have went on fine with enough clearance for the split pin. This motor is used as a kicker on a 18ft boat so it doesnt get on plane, and really doesnt see full throttle very often. I think this prop is similar, if not the same as what comes on the mercury pro kickers.

 

[pvanv]

You have the optional High Thrust prop. They work great for very heavy loads, such as sailboats... especially in reverse. The larger hub is specifically to allow the exhaust to bubble back along the LU housing when in reverse, instead of getting on the blades. There are 2 pitches available, 5" and 7". They are OK for some RIBs, but generally not what you want for higher speeds. I've used them on a lot of kickers, usually 7 pitch. On my 8,000 pound sailboat, the 9.8 gets to 6,000 WOT RPM at 5+ knots. A 7 pitch would be down around 4,000, so on my heavy, displacement hull, the 5 pitch is the way to go.

 

[Sea Rider]

You can always maximixe OB props to better push heavy loads which are impossible to plane with a underpowered OB. Boaters takes for granted that any underpowered OB with standard delivered prop can be used ideally as a kicker. At an emergency while highly loaded and at full grip won't know if lugging the engine badly, that's why a porp maximization with an induction tach is a must go for to count with a proper prop for engine to rev at around or its max wot rpm range as in the example Paul is giving.

Happy Boating

 

[gahunter706]

You have the optional High Thrust prop. They work great for very heavy loads, such as sailboats... especially in reverse. The larger hub is specifically to allow the exhaust to bubble back along the LU housing when in reverse, instead of getting on the blades. There are 2 pitches available, 5" and 7". They are OK for some RIBs, but generally not what you want for higher speeds. I've used them on a lot of kickers, usually 7 pitch. On my 8,000 pound sailboat, the 9.8 gets to 6,000 WOT RPM at 5+ knots. A 7 pitch would be down around 4,000, so on my heavy, displacement hull, the 5 pitch is the way to go.

Ok thanks for the info, I was mainly worried about the hub sticking out because I haven't seen a prop like that before, but I can definitely see how it would help in reverse. This is no 8000lb sailboat, its an 18ft aluminum boat probably only 3000lb fully loaded with gear and people. The stock 3 blade has been performing just fine the past few weeks, but I wouldn't mind getting some more push with the 4 blade.

 

[Sea Rider]

Install an induction tach and check both props at current boating load at wot to be on the safe wot range, preferably towards the max wot revs for that underpowered OB used as a kicker. Don't know your boating condition, but one thing is pushing a combo on flat calm no wind waters when main OB fails, and other worst story on choppy, windy water conditions. You never know when max rpm demand for that OB will be needed. It's a precaution issue to have in mind.

Performing fine doesn't mean that OB is working at its correct wot rpm range, only a tach will tell...

Happy Boating