Tohatsu 9.8hp 2 stroke

[slate1234]

Hi all after a bit of advice I have the above outboard on a 10ft sib me 180lbs (13stone) and light tackel and it flys reaching around 5800rpm useing aF8, 8.9X8.3 prop, if I add another person around 160lbs (12stone) it stuggles to get on the plane and will not go over 4100-4400 rpm I would like to buy another prop but dont know whitch one to order on the tohatsu web site the prop chart is

M8 & M9.8 (2-Stroke)	Light	8.5	9	3	ü(short shaft)	3B2B645170
Moderate	8.5	8	3	ü(long shaft)	3B2B645151
	8.9	7	3		3B2B645140
Heavy	8.7	7	4	(high thrust)	3B2B645120
	8.3	6	3		3B2B645126
	8	5	3		3B2B645125
	8.7	5	4	(high thrust)	3B2B645110

But in my manual I have a diffrent range of props 1: 8.5x7.5
2: 8.9x7.0
3: 8.5x6.5
I plan to swap props according to weight any help would be welcome

 

[steelespike]

I'm not sure what the chart is telling us Is moderate 8.5" or 8" ? 3 blade? Or 8.5" X 8" 3 blade?
A 1" reduction in pitch could net a 300 to 400 rpm increase.
Before changing props have you tried different trim settings? Usually you would trim down (prop closer to the boat)
Is the motor very deep in the water at speed?Usually the less motor in the water the better.
when you add weight.
Small Inflatables usually struggle as you add weight.Is it a ridged keel and at maximum inflation?
What is the max rated rpm?Is your present prop a 8.3 or 8.5?

 

[Sea Rider]

Slate,

There are some important tech issues to consider :

-Inflate tubes, keel to min 3.0 PSI with gauge, you need boat rigidity.
-Test on flat calm, no wind waters with evenly ballanced sib.
-Engine must be well height transom matched & well trimmed *
-Engine must be 100% healthy, prop included.

If you don't meet those conditions,
Useless buying a new prop to test. Would start working to meet those conditions and trying same prop again with boating mate, then plan going for a lower pitch prop if combo doesn't want to plane.

If you meet all those condittions.
You need to go tag lower in pitch to rev up lost max wot rpm.
1-2-3 pitch lower ? This is not a exact rocket science. Wot testing, trial and error is a must do.
Down ssue : With a 9.8 you are underpowered, 10 footer needs minimum a 9.9 HP engine. A 9.8 is 170 CC, a 9.9 is a 250 CC, great difference in power.

As you have said, a 9,8 planes well with you alone, not with 2. So need a prop maximization, by how much ? Ideal is to have a box full of different prop pitches to trial them with boating mate , but you have to buy a new one to wot test.If wild guessing, probably could dial best X prop pitch in a single purchase, but probably not.

Happy Boating

 

[slate1234]

The current prop is 8.9x8.3, it is a deep v inflatable keel all pressuers are correct Steelespike you are correct the chart reads as you stated, I have tried trimming in out and raiseing the engine on the transom with shims, it will get on the plane it just struggels, once on the plane it's 14mph on the gps with two and around 19mph on my own, I know I will be loseing a little top speed but I dont mind that

 

[Sea Rider]

Try going at least 2 less in pitch from actual standard prop to rev rpm up, no one will blindly recommend going for a spot on prop pitch to achieve top rpm stated for that engine. 2 less prop pitch, loaded as stated and tached under wot trial will be a good starting point.

A 10 footer medium loaded sib with a deeep V inflatable deck/keel has a tremendous hull drag to overcome fast with just a 9.8 HP without prop top maximization. Good luck.

Happy Boating

 

[steelespike]

Try the 8.9 X 7. Do you know the wot rpm rating? It appears to me that 5800 is pretty high as it is.
The other 7" seems to be for high thrust.
You will be over revving with a light load for sure.

 

[Sea Rider]

2 or 4 strokes Tohatsu 9.8 HP engines WOT at 5000-6000 rpm, If having already a 8.9 x 8.3 which is a 7 prop according to upper chart you must go down minimum 1-2 sizes to a 5-6 to rev up and overcome moving/planing current heavy load as opposed going with just yourself. Do you plan to buy several props to try them out ?

Happy Boating

 

[slate1234]

It's a two stroke I plan on buying one prop so I will have two one for when fishing alone and one for when I have the exra weight, I will just swap accordingly

 

[steelespike]

His prop is 8.9 X 8.3 Using a 2.15 ratio at 6000 rpm 10% slip the calculator indicates 19.7 mph He indicates he gets about 19 at 5800.
Operating the motor no higher than 6000 with a 7" = 16.6 a 6" would be 14.3.
With the little motor a 1" pitch reduction would result in more rpm than the rule of thumb 150 to 200 rpm per inch.
If you were getting only 200 per inch how would he make up 1900 rpm with only 3" change to work with.
It's my opinion that a 1" reduction will take him way beyond 6000 rpm.
​ A 1" change on a 8" prop is about 1/8th of the pitch a 1" change on a 19" prop is 1/19th. of the pitch.
​ A 50 hp has about 9 props to pick from for a total of 11" starting at 8 and ending at 19.
If you include specialty props the is actually more.

 

[slate1234]

wow steelespike,,,,,,,eeerrrrm I rang the dealer he said I should get a 8.5x7.5 so I have ordered a 8.5x7.5 prop just to be clear I will be swaping props according to weather I am fishing alone or with a freind, I do have a tinytach so I can keep an eye on the rpm does this sound correct

 

[steelespike]

A 7.5 seems reasonable. Should help your hole shot and let you get more rpm with a load. Yet while likely to go over 6000
you may be able to stick with the 7.5 and just watch the rpm.Dealer should know best.I didn't even notice there was a 7.5 available.
Would be very interested to know the rpm and gps speed.

 

[slate1234]

So I took the day of work, it's hot calm and life is to short, I got the boat out, engine in the test tank for a warm up and the door bell ring's, it's the courier with my new prop (8.5X7.5), I fitted it with in ten minutes, then invited my uncle to come fishing for a few hours lol he bit, well I can say it topped out at 5900rpm and 14mph on the gps, 11mph was nice and I don't like running at WOT all the time, and it jump's straight on the plane, so I am a happy person today, steelespike I might do waht you said and leave it on for my next outing on my own and just keep an eye on the tacho,
Thank you for the replys

 

[steelespike]

So it appears you gained at least 1500 rpm in a little less that a 1" pitch change.Wow!
That dealer really knew his props. This is the first time I've been able to help prop a small hp motor
and see the results. In a way it was lucky we had a boat that planed yet had a pretty massive loss of rpm.
If it was just struggling to plane we would have had to guess more on a prop size.

 

[Sea Rider]

Slate,

Gland to read that finally combo planed with 2 with no problem. Just for the record which are the cast numbers on prop body on both old and new prop, don't give sizes, Is the new one a 3 or 4 blade prop ?

Portable engines on light applications will rev higher than on heavy ones. Somebody changed from 13 pitch to 11 pitch on a light combo and gained 1,000 + rpm more at wot, so 500 rpm per 1 pitch incrementt. So 150-200 rpm stated gain doesn't work on inflatables.

Happy Boating

 

[steelespike]

While this setup is a "light combination" with the additional weight in the inflatable it became a" heavy application".
Losing 1400 rpm at 4400 in a planed condition. at 14 mph.The 7.5 gave back 1500 rpm in that same heavy,hard to push application.
I think we have to agree a soft keel inflatable becomes hard to push with added weight.Certainly not going to have a light application reaction.
The 7.5 at 5900 and 14 mph gets about 28% slip the 8.3 on a light load 5800 gets about 10% at 19 mph.
Now you put that 9.8 on a 18 footer for a kicker your not going to gain 1500 rpm with less than an inch change but your much more likely to do a 2 or 3"
to help the rpm, but the boat isn't going to plane so rpm will be a struggle.

 

[Sea Rider]

A 10 footer inflatable is for 4. With extra 160 Lb boater will be medium loaded. the issue here is that he's way underpowered with a 9.8 for that size sib, has high water drag to get out of the hole fast, at least 9.9/15 HP is recommended by maker. He's just trying to take the best out of that engine as he already owns it. if using a larger engine with standard prop with same total weight would be a faster plane story.

You will achieve near or sometime exceed max wot rpm on "lightly loaded inflatables" provided you have the top HP engine transom stated for that inflatable.

Happy Boating