Another mistake on 9.9 high thrust 4 stroke Tohatsu?

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vicrider

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I just bought a 4 stroke 9.9 Tohatsu and thanks to old age and not reading thoroughly I got sent the High Thrust with 4 blade 7" prop. This is a total nightmare to me since it is terrible on my 16' light aluminum v-hull. I have a 10" pitch 3 blade coming and hope this cures my problem since this is my last motor for me and for health reasons and 10hp limits I got the 9.9 with electric start and power trim. Is the 10" pitch 3 blade going to make this the motor I wanted or is the High Thrust internally different with crank and valve timing? I'm an old guy who has lived 2 stroke all my life and the 4 strokes are a new experience to me. I did some search on site but only found 2 stroke info on my question.
 
Not knowing the exact answer but after lots of research before buying a new high thrust type outboard for my 26 ft yacht, i think you will be delighted with new propeller you have coming.
If your boat really is lightweight and you too, I would expect you to plane quite fast. Too much weight will kill that of course, but you probably knew that. Really depends on the boat. Here we have some 16 ft boats that are just large lightweight dinghies and 10 HP will shove them along. Other aluminium boats of same length are designed to be driven by 75 -120 HP and are fuller and heavier, being designed for fishing and so on in less than flat calm.
Please report back.
 
Thank you John. Prop should be here tomorrow if PO is on the ball and a change will in order. I am 77 years old and once my kid mounted this rather heavy motor on I'm happy with electric start and power trim. The 9.9 Tohatsu is the same powerhead for 15 and 20 horse so the powerband should get me and the kid on plane with a light rig. An older 9.9 two stroke did the trick. I'm not looking for speed but every boater who's been around for awhile knows that big jump when you quit pushing water and start riding on top of it. Though it's not a cheap route to go a person can always upgrade the ECU and make necessary changes to move up in power and still keep all stickers and serial # a 9.9.
 
I had a high thrust Yamaha 9.9hp 4 stroke. Those engines are designed for displacement hulls (ie sailing boats) aren’t they? That’s what I used mine on - 26ft sailing boat. They’re not really designed for getting a small boat on the plane.
 
High thrust = Lower geared.

On my Mariner 5hp 2T there is no mechanical difference to the engine between the various props - only the prop changes. I'm pretty certain yours will be the same.
 
Mark-1 said:
High thrust = Lower geared.

On my Mariner 5hp 2T there is no mechanical difference to the engine between the various props - only the prop changes. I'm pretty certain yours will be the same.
Click to expand...
That’s true of the smaller outboards, but on some bigger high thrust models there is a different ratio gearbox and a huge (12” vs 8” on my Yam) prop.
Not sure if the OP’s Tohatsu is the same though.
Edit: looks like Tohatsu high thrust is just the prop that’s different - apols to OP, ignore my posts!
 
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fredrussell, I understand the prop change difference and that my present 4 blade 7" prop is designed to push at limited speed and maximum power. I also double checked and gear ratio is the same for both motors (2:15) and the rpm range is the same so I'm really hoping John is right and prop change will make a big difference. Actually, since many of the small lakes around here are NO WAKE lakes with no planing allowed (supposedly) the 4 blade could make a great trolling prop, especially when pulling backward against the transom. Should be able to troll as slow as an electric is capable of.
 
vicrider said:
fredrussell, I understand the prop change difference and that my present 4 blade 7" prop is designed to push at limited speed and maximum power. I also double checked and gear ratio is the same for both motors (2:15) and the rpm range is the same so I'm really hoping John is right and prop change will make a big difference. Actually, since many of the small lakes around here are NO WAKE lakes with no planing allowed (supposedly) the 4 blade could make a great trolling prop, especially when pulling backward against the transom. Should be able to troll as slow as an electric is capable of.
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In choosing a 10" prop you have gone from almost one extreme to the other. You may find 10" too coarse giving you too high a minimum speed for trolling and low speed manoeuvring
Did you consider an 8.3 " prop or were you advised to opt for 10"

Why did you choose a 9.9 rather than the lighter 9.8 ? ....... Power tilt ?
 
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VicS, the reason for 9.9 over 9.8 was powerhead displacement. I'm not portaging it. It's going to stay on the boat. I wanted all the motor I could get under 10 and the Tohatsu has same cc as the 20 hp. Being an old bike rider, car racer, and one around before the technology of today we always said you can't beat cubic inches and the 9.9 over 9.8 is a big jump in that. We'll see how 10" does and can always change that if I have to. Never hurts to have an extra prop anyway in the boat. As far as trolling, I seldom do it and if I do it's only to move around structure or dam faces with the trolling motor while casting. As you mentioned power tilt was an important choice for me as was electric start. My heart will thank me for that choice.
 
So it's been awhile since I've responded to this thread and things move through the pages fast on this site. I needed time to get my 9 x10" prop in and on the water. Well, the difference is amazing. I also opened up the restrictor plate a step up on the basis of the 15hp and 9.9 using same ECU. The 10" planes quickly and smoothly and though I need to get out there with my GPS I would guess we're easily around the 20mph. The extra cubes of the Tohatsu, the opening of the restrictor plate, and the 3 blade 10" prop replacing the 4 blade 7" turned the high thrust heavy load pusher into a light load flyer (for what it is...I know it's not like my 70 mph bassboats were).
 
And a probably last update on the rig. Not only is it better than I'd hoped after running the 4 blade 7" prop my worries about it being a detuned high thrust 9.9 are definitely unfounded. I'm a big guy like two lightweights in the boat by myself. Went out with a tach and GPS today on flat water with a very light chop. With motor about an inch deeper than it should be, trimmed pretty straight, it topped at 26mph @ 5500rpm. Couldn't ask for more since I was just hoping getting on plane at 20mph or so. Add another 200# to boat and it still did 23-24 @ 5300 rpm. The only thing besides prop was a small opening of the restrictor plate so if I get checked by DNR I am still a 9.9 and the electric start and power trim sure makes this old guy glad he made this choice.
 
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