Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

[simplystressed]

took the 9.8 out for a run the other day on my 1500lb boat.

Looks like I'm getting about 4000-4200rpm at full throttle. The prop says F8.5 so I'm guessing its the stock 8.5D 8Pitch prop on there.

Based on my readings above and weight of the boat, what prop would best suit my setup? I am hoping to gain a slight amount of speed (won't get much over hull at 6mph) and make the most of the motor power/throttle range.

On a small motor like that, does dropping a pitch increase RPM's by 400?

thank you

 

[Sea Rider]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

That engine revs at 5000-6000 rpm at wot, so very under propped, will need to drop at least 2 pitch difference to gain higher revs, check Tohatsu props availability for that model. Are you using a induction tach for testing or you are assuming having 4000-42000 rpm at wot ? Iboats must have props for your engine, check

Happy Boating

 

[simplystressed]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

Thanks for the reply. I'm using a tiny tach to get the readings.

 

[pvanv]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

You can get a whole range of props for that motor: 5, 6, 6.5, 7, 8, 9, and 9.5, as well as the HT 4-blade "sail" props in 5 and 7 pitch. See http://www.tohatsu.com/accessories/prop.html
Make sure you have the tilt set so the motor is vertical under power (not plowing the bow down), and confirm that your jack height places the antiventilation plate at the water surface when running. If all that is correct, try the 6.5 or 7 pitch prop.

 

[simplystressed]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

You can get a whole range of props for that motor: 5, 6, 6.5, 7, 8, 9, and 9.5, as well as the HT 4-blade "sail" props in 5 and 7 pitch. See http://www.tohatsu.com/accessories/prop.html
Make sure you have the tilt set so the motor is vertical under power (not plowing the bow down), and confirm that your jack height places the antiventilation plate at the water surface when running. If all that is correct, try the 6.5 or 7 pitch prop.

Thanks Paul I played with a couple trims and it seemed similar. The AV plate is right at the bottom of the boat level I can't do much more with jacking it up much.

My testing was in the lightest configuration as it was just me in the boat; so I'm thinking of trying the 6.5P or 5P 3 Blade.

I think the 5P HT might be too much for my lighter setup and will overrev? Also don't want to lose any top end.

 

[JB]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

Let's put this in the Prop Forum. There others can learn from it.

 

[Sea Rider]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

You cannot have best hole shot with best top speed, one opposes the other, ideal would be to have different pitch props to test and choose the best for your setting, you could have 2, one that gives middle 5500 rpm for general use and other for top end speed if boating solo as to reach 6000 rpm. If you place a 5 prop with you alone giving near 6000 rpm engine will be ok, rpm will fall down with extra weight accordingly.

Happy Boating

 

[simplystressed]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

Hole shot isn't really a concern as the 9.8 will never plane the boat, but I want to get as much top speed as possible and have to take into account it could be 1-3 persons in the boat. So aiming for 6000 solo would be the best bet.

Right now at 4000rpm its not making full power, so I figure dropping pitch will both increase my speed and holeshot a bit?

Attached are my options from http://www.tohatsu.com/accessories/prop.html

 

[steelespike]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

I assume the boat is not intended to plane even with more power?
Based on a wot of 4000 and a max wot of 6000 I wouldn't hesitate to go
down at least 2".Can't be exact on a rpm gain but probably at least 1,000 higher.
As you said speed will be close to 6 mph and not likely to increase much.
So your looking to gain rpm not speed not being able to plane will stunt the rpm increase some. I wouldn't worry about losing speed with a prop change.
Assuming a planing hull a 5" prop with a 2.08 gear ratio at 6000 is estimated to reach 12 mph.
if the hull restricts it to 5000 = 10 mph.

 

[simplystressed]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

I assume the boat is not intended to plane even with more power?
Based on a wot of 4000 and a max wot of 6000 I wouldn't hesitate to go
down at least 2".Can't be exact on a rpm gain but probably at least 1,000 higher.
As you said speed will be close to 6 mph and not likely to increase much.
So your looking to gain rpm not speed not being able to plane will stunt the rpm increase some. I wouldn't worry about losing speed with a prop change.
Assuming a planing hull a 5" prop with a 2.08 gear ratio at 6000 is estimated to reach 12 mph.
if the hull restricts it to 5000 = 10 mph.

Correct, I can't see the 9.8 being able to plane this boat due to the weight. Currently hitting 6.2mph at 4200rpm. So I figure dropping down a couple pitches should get me into the 5500-6000 range solo and should increase speed slightly.

 

[Sea Rider]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

Try to match a pitch that gives near 6000 rpm at wot with you alone so engine is inside safe parameters and you are achieving near full 9.8 HP, besides nobody runs wot all the time, just 1/2 to 3/4 throttle.

Happy Boating

 

[simplystressed]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

thanks all, I'll try around 6P and go from there.

 

[simplystressed]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

I haven't picked up a prop yet, but I'm considering two of them:

the 6.5 pitch 3 blade
the 5 pitch 4 blade

I'm thinking the 6.5 is a good starting point, but wondered about the 5. Because the boat is only around 1500lbs, would the 5 pitch 4 blade possibly overrev the engine, as its designed for much heavier boats?

 

[pvanv]

Re: Prop for 9.8 Tohatsu 4 stroke based on my setup and RPM readings

I haven't picked up a prop yet, but I'm considering two of them:

the 6.5 pitch 3 blade
the 5 pitch 4 blade

I'm thinking the 6.5 is a good starting point, but wondered about the 5. Because the boat is only around 1500lbs, would the 5 pitch 4 blade possibly overrev the engine, as its designed for much heavier boats?

The 4-blade EE prop is for sailboats, barges, and other heavy, displacement boats. It might over-rev at a boat speed over 6 or 7 mph. For higher boat speeds, try the 3-blade prop.