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8hp vs 9.9 specs

E

eric80

Regular Contributor
Hey,
I'm thinking of replacing my Yami 8 hp high thrust, xl shaft, remote, power tilt, electric start with a 9.9
I'm not finding the specs on the web so asking here.
what's the weight difference?
what's the difference between high thrust and not high thrust?

My boat on a trailer, full of fuel and no gear weights 5600 pounds. My 8hp is ok if it's calm and no wind but soon as the wind comes up or it's wavy the 8hp doesn't effectively move the boat, hence me thinking of going to a 9.9. will that make the difference or should i be looking at just changing the prop? the prop i have now says
Y-1 969 5 3/4
anyone have any ideas?
 
A high thrust extra long shaft is designed for sailboats, or other non planing hulls with a high transom.
With those boats, (heavy & slow), a regular motor will never be able to achieve its rated HP, because it will never rev up to its full rated rpm.
The solution is to use a finer pitched prop, less “bite” allows it to reach full rated rpm, which results in rated HP.
I would attach a tach, & see if your current motor is reaching full rpm. If so, changing props won’t help. If you can find the specs, see if your prop is a finer pitched than the “regular” iteration of your motor. If you go to a 9.9, you will also need a finer pitched prop than normal.
I have a Yammy 8hp manual for a mid ‘80s motor I picked up for a cousins cottage. It says 5000rpm. This page is the only intel I have on the prop nomenclature. Often the prop specs are also on the hub, maybe they re painted over?
It leads me to believe you have a pitch of 5 3/4”,which I believe is the finest you can get.
I’m not sure about diameter. The 6 & 8 hp evinrudes I have all run 7-9” pitch. There were a few of the 6hp, listed as saildrives that ran a 6.5” prop.
Evinrude 9.9s (mid 80’s) run a 10” pitch. They make a 7” pitch, listed as a sail model. I’m not familiar with Yammys, but this a guide as to the pitch you’ll need for a heavy boat. The weight difference is about 20 ish lbs.
These numbers are for 2-strokes, I’m not familiar with newer 4-strokes. The 2-strokes (9.9 hp) are about 76lb in long shaft electric start. A 4 stroke will be about 100lb
 

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The T versus the F which precedes the power rating denotes a different gear case. Having a higher gear reduction ratio.

The T9.9 having a 2.92/1 gear ratio versus the F9.9 having a 2.08/1 gear ratio.

The T versus the F models use a completely different propeller.

There is no T8 model now available from Yamaha USA.
 
eric80 said:
Hey,
I'm thinking of replacing my Yami 8 hp high thrust, xl shaft, remote, power tilt, electric start with a 9.9
I'm not finding the specs on the web so asking here.
what's the weight difference?
what's the difference between high thrust and not high thrust?

My boat on a trailer, full of fuel and no gear weights 5600 pounds. My 8hp is ok if it's calm and no wind but soon as the wind comes up or it's wavy the 8hp doesn't effectively move the boat, hence me thinking of going to a 9.9. will that make the difference or should i be looking at just changing the prop? the prop i have now says
Y-1 969 5 3/4
anyone have any ideas?
Click to expand...
How about 69G?

 
boscoe said:
How about 69G?

Click to expand...
I think that’s what he has now. I’ve read some posts that report the prop you’ve hilighted has been misrepresented as other ID numbers.
I believe he has a 5 3/4 pitch, all the props I’ve been able to look up, with that pitch, are around 11” diameter.
 
Diameters in the first column. Pitch in the second column. Part number in the last column.

The part number highlighted is the current Yamaha part number. There are other Yamaha part numbers for propellers having the same diameter and pitch. There is more to a propeller than just diameter and pitch.
 
thanks for your comments. here's the prop i have. the boats a 2006 Grady 208. 5600 pounds, on the trailer full fuel no gear.
some comments I'm getting elsewhere are saying a 9.9's not going to make much difference would changing to a different prop do the job?
1774818157738.jpeg
1774818187463.jpeg
1774818200718.jpeg
 
I owned a 1984 Skipjack 20 with 350 V8 that weighed a little more than your Grady. I was running a Yamaha 9.9 HiThrust and the most speed it would make in calm conditions was about 5.5 knots. I don't think that a prop change would've made much of a difference in performance.
 
No, it won’t. The prop you have is, as I suspected and posted earlier, the finest pitch prop you can buy for that motor.
That fine pitch with the larger diameter allows your engine to spin up to its full RPM, it’s exactly the same as when I used to fly airplanes with variable pitch propellers. We use the finer pitch for takeoff to allow the engine to spin a full RPM and generate the full power to get us off the ground.
Moving to a 9.9 engine, it’s going to give you roughly 25% more power, but only if you also equip it with the sailboat prop that will allow it to turn its full RPM.
For reference, many years ago, I used to keep a boat up on Great Slave Lake in the northwest territories Canada. I had a little six horse mounted on the transom of a MerCruiser IO. The fact that the motor is mounted off the centre line meant that as soon as the wind picked up the bow of the boat and shoved it around, I would have to go to full throttle and full angled steering, and even then it would take a fairly wide arc to get it back in line of where I was trying to go.
That’s the reason a lot of the fishing boats put a bow mounted motor on. It drags the bow around exactly where you want to go. On a boat with the weight of yours I would think a 15 would be far more effective, but you’re still going to see the wind to grab the bow and swing you around..
 
eric80 said:
Hey,
I'm thinking of replacing my Yami 8 hp high thrust, xl shaft, remote, power tilt, electric start with a 9.9
I'm not finding the specs on the web so asking here.
what's the weight difference?
what's the difference between high thrust and not high thrust?

My boat on a trailer, full of fuel and no gear weights 5600 pounds. My 8hp is ok if it's calm and no wind but soon as the wind comes up or it's wavy the 8hp doesn't effectively move the boat, hence me thinking of going to a 9.9. will that make the difference or should i be looking at just changing the prop? the prop i have now says
Y-1 969 5 3/4
anyone have any ideas?
Click to expand...
69G. As per the Yamaha data.

 
eric80 said:
thanks for your comments. here's the prop i have. the boats a 2006 Grady 208. 5600 pounds, on the trailer full fuel no gear.
some comments I'm getting elsewhere are saying a 9.9's not going to make much difference would changing to a different prop do the job?
Click to expand...
If the T8 is getting most of want you want, a T9.9 might get you all the way there.

Assuming the motor can turn it to above 5500 RPM when at wide open throttle, if you get the T9.9 model you might want to try an aftermarket propeller, if the Yamaha propeller that comes with the motor is not doing what you want.

Here is but one example. https://www.nettleprops.com/store/p/224-Solas-High-Thrust-Four-Blade-Yamaha-Propeller.aspx
 
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