Tohatsu 9.8 speed

Tohat9.8

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I bought a used tohatsu 9.8. My boat, 1person and gear etc is about 450 lbs. Boat is aluminum v hull 12 ft. I don't seem to have any power/ speed. Not sure if I am hitting 10 mph. How fast should this boat and motor go?
 

SeaDooSam

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That boat should go at least 20 in my opinion. Is the prop damaged?
 

Tohat9.8

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Thats what I think as well. No prop damage. I do not run gas out of motor after each use. Never did on my 40 hp evinrude. Might be the problem with this motor though.
 

pvanv

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What is your actual model? Is it a 2 or 4 stroke? What prop are you running? Have you checked for a spun prop hub? Did you put a tach on it to see if you are near the correct WOT RPM? What is your transom height? What is your trim setting? Does the motor run well otherwise?
 
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Tohat9.8

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Paul. Its a 2013 4 stroke model 9.8A3 EF. 22 shaft. Anti ventilation plate about 2 inches below hull. Trim is fine. Previous owner installed large fins. Engine runs fine otherwise. Have not done any tests. Will take engine in I guess. I rebuilt a 40 HP lower unit once. But now have not time nor tools. It just seems at full throttle engine should be more revved if that makes sense. Seems low rpm, sluggish. Even for 4 stroke.
 

Tohat9.8

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I did slightly overfill when I changed oil. However engine was sluggish before that. When oil level was perfect.
 

Sea Rider

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What is your actual model? Is it a 2 or 4 stroke? What prop are you running? Have you checked for a spun prop hub? Did you put a tach on it to see if you are near the correct WOT RPM? What is your transom height? What is your trim setting? Does the motor run well otherwise?

Yep, spot on answer as usual. Without those parameters don't expect to seee the light at the end of the tunnel. Will have my doubts if that 9.8 being a 170 CC powerhead can make 20 knots at wot, 2 stroke 18 HP being 300 CC with same load does around 22 knots, so expect much less than that.

All Tohatsu OB's comes with medium pitches, going one less pitch will benefit hole shot and better top speed. An induction tach is a must install if wanting to maximize a less pitch to pull wot revs at its max..

Happy Boating
 

Tohat9.8

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Sea rider. I have 209 cc per specs. But actual could be lower? Your point well taken. I thought 20 knots perhaps a bit high as well. Will check out props. Install the tiny tach. Learned some things about my engine. Thanks guys. Will give an updste soon.
 

Sea Rider

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Yep you're right 209 CC, stated 170 CC thinking it was a 2 strokes OB. Install the Tiny Tach and go for a wot run as usually loaded, distribute load well on deck, combo must run on flat calm no wind water cond, report back max achieved wot rpm, can go from there maximizing a less pitch if in need to pull wot revs towards max 6 K range.

Bear in mind that if doel fin installed does not run parallel to water level when combo is on plane, doel fin will achieve more water drag which will slow combo down. Remove and test without, check if gets better....

Are there water splashes up or over transom, prop aerates at close turns or on choppy water at speed ?

Happy Boating
 
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pvanv

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Odds are you need to raise the transom height up to where the antiventilation plate is closer to the bottom of the boat. Older boats had transom heights of 15 and 20 inches. Modern Japanese motors are rigged at about 17 and 22 inches. Likewise, odds are you need less prop pitch. The tach will help a lot.
 

Tohat9.8

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Thanks guys. Ok first, if I remove the large fin, should I put bolts back on, or leave cavitation plate with the 4 holes? Also no water splashes, just water gurgling back there. Boat does not get on plane either. Paul, 22" shaft is correct. To raise cavitation plate closer to hull bottom, was thinking a 2" x 2" resting on transom aluminum cap?? I only use clamps so a little concerned doing that. Any other ideas? One other thing. I called my local boat guy, to see what pitch props he had for my motor. He has 7.5 and 8 pitch. I asked if had had lower pitch since I wanted more speed. He said higher pitch gives more speed, but puts more load on motor. Reading your guys suggestions to go to lower pitch has me a little confused. I think you are saying to get motor closer to max RPM lower pitch is way to go? But does that make top speed less?? I guess once I install tach much will be answered in that regard. I am taking boat out this weekend. Will do test runs and get back. thanks again.
Ed
 

Sea Rider

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Ed,

Remove all bolts from leg once fin is removed, can place duct, electric tape covering holes by the moment. First thing need to do is know if OB sits at the sweet height on transom. Go for a wot spin, OB trimmed to 90? for AV plate, prop shaft to ride parallel to water level, once on plane, being a tiller OB, pull head out transom and visually check at which lower leg is water flow passing at speed. Ideal is to pass right under small upper water defector plate. Forget all about AV plate height, you'll be more confused than ever.

Assume OB is running with factory delivered prop which is ? check number on prop body. To understand how pitches works will need to install a induction tach once correct OB height is matched. Check max wot rpm as usually loaded on flat glass water. Ideal is for OB to run at near or max of it's factory wot rpm range which is 6 K.

If say OB achieves just 5.5 K wot revs with an 8 pitch, will need to drop one less pitch to a 7 pitch to pull wot revs to around 6 K. IF OB over revs say 6.5 K with too much less in pitch will need to go one pitch higher. Under powered OB's on large boats must be prop maximized in order to get the full HP out of them to perform decently..

Bear in mind, that if OB runs say 5700 wot revs with X prop, will achieve slight better top end speed than if running say full 6 K, but, as there's always a but, with 6 K combo will achieve better hole shot than with 5700 rpm, at slight less top speed. Can't have best of both worlds in just one prop, for that will need a expensive variable pitch prop...

Report your findings about OB height and wot runs with tach...

Happy Boating
 

Tohat9.8

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Sea Rider, thanks. A couple of things. I did get a number on prop the other day, forgot to put in my last post. Prop has an F7 stamped on blade. I researched that. Another forum said that meant pitch was 7? Diameter right at 9". Understand the trade off regarding pitch/hole shot/top speed and the RPM relation to all that. Understanding that better now thanks. I do know AV plate is about 2.5" - 3" below hull. Tohatsu recommends .2" - 1" below, so I am too low. You suggested I disregard that for now so I will. I will do a WOT this weekend. I hope by taking off fin it planes out because the water is not flowing like you say it should, not even close. I remember how my 40 HP flowed like you are saying, this is not anything close. Just a lot of churning at WOT. I will check for the parallel, 90 degrees etc when testing. Will adjust thrust rod as needed to get to that 90 degree. and, I was later thinking about using duct tape as well to cover the holes for fin.. Great minds think alike lol.
 

Sea Rider

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Try to plane combo with trim set to 90?, test on flat calm no wind water cond as early in the morning. If prop aerates lightly on a strraight water course and awfully at close turns definitely OB sits way high on transom, will need to modify transom height for water flow to skim right under upper water deflector plate and prop aeration stops.

Why is important for OB to sit at the sweet spot, will have least lower leg water drag, excelent prop water bed height to achieve best thrust. This is the sweet transom/lower leg height you should aim for for that 9.8 to work decently.

https://www.youtube.com/watch?v=oBeZjaA76VM

Boater had a different issue, water splashes over transom, which is not your particular case. Can you post a back side pic of rear transom without OB attached to have a look.

AV plate sitting 1-2 inches under hull/keel doesn't say much as you won't lnow if water flow at speed will pass under the sweet lower leg height, only checkable by visual inspection as in vid. This is the fastest way dialing it and not losing precious boating time on wild guesses, playing with OB height, trying different trim settings, adding doel fins. If you happen to have at hand your Owner's Manual read it fully to understand how trim, boat ballance works

Happy Boating
 

Tohat9.8

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Sea Rider. Excellent video. Will look for that flow when I test this weekend. I cannot take OB off at this point since boat is ready for weekend trip. I will video my test run for you to look at. And when I get back I can pull motor and take that pic for you. I did install tach. I ran OB in driveway. At choke RPM in idle speed/neutral was 940-950. Which matches my manual for the 9.8A3 EF. However once engine warmed up, no choke, idle speed/neutral was 1120 RPM. idle speed/forward was 1090-1100. idle speed/neutral should be 950 and idle speed/forward should be 900.
 

Sea Rider

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A pic or video is worth not writting 1 K words LOL!! The 3/4 inch wooden shim used needs to be chopped down 1/2 cm for OB to sit tad down and disappear small side splashes seen on video.

A video or pic will be enough to check water flow passing by, will know what needs to be corrected. Idle rpm is sometimes impossible to dial with induction tachs as numbers are quite erratical al low ide speeds.

With OB fully warmed up can adjust idle rpm a tad less to a point for OB not to die when geared forward as loaded. This idle adjustment needs to be done while combo floats on water not on a driveway...

Will be interesting to see your findings..

Happy Boating
 
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Tohat9.8

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Ok. No pic. Too windy. I could only get into river. So not calm. But not too rough. Speed was 4 mph into current. 7 with current. Most important I think.At WOT I HAD 3300 RPM. Tohatsu owner manual states 5000-6000 RPM.
 
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