Convert 15HP outboard to High Thrust

[RonaldJJames]

I have a catamaran with 2 x 9.9 High Thrust engines. Love the engines reliability, but when the wind blows hard in a tight docking situation they're a little under powered. 15 HP High Thrusts would be ideal, however the next size up on high thrust engines that Yamaha offer is 25HP, which would be overkill, apart from not fitting in my engine wells.

So, would it be possible to convert a 15hp 'regular' engine to High Thrust. I presume it's at least gearing changes (and other changes as well?) and maybe the different ratio gears are not available, but would be interested in any comments.

Thanks in advance

Ron James

[99yam40]

I always thought it was only the lower unit gears and prop that made it high trust, but I do not know for sure.

Might just look for what you need to convert your 9.9s to 15hp is my thoughts

[RonaldJJames]

I forgot to mention my engines are 4 stroke. AFAIK, there is no simple upgrade of 9.9 4 strokes to 15. I've only seen upgrade options for 2 strokes.

[99yam40]

See how posting you motors model # would have prevented this post of mine.

[rodbolt17]

will the engines actually turn the rated RPM's?
most blow boat kickers wont cause it has a factory prop.
those engines should hit 5500 rpm.
if they cant your not making the rated power.

[99yam40]

Still would be helpful if you posted you complete model #

along with the prop you have now and the RPM you are able to hit at WOT

[RonaldJJames]

Model numbers are T9.9LE
Serial number on one engine is G(6?)AVK L 1008822 D
Not sure of the prop details. It's the standard prop that comes with the High Thrust engine.

[RonaldJJames]

I don't have a rev counter so I don't know what maximum revs I'm getting. Are you suggesting I might get more power from a non-standard prop?

I'm not interested in speed. More interesting in torque at low speeds when trying to get into a windy berth.

[99yam40]

Find out what props you are using and what RPM you get when Running at WOT.
Motors do not have one prop to fit all applications/loads.
they sell lots of different props to fit the applications even high thrust

they also sell duel thrust props that thrust well in both directions from I have read.
in case you need reverse to work better

My guess is this motor is not a US model as I did not see a T9.9LE

[boscoe99]

I don't have a rev counter so I don't know what maximum revs I'm getting. Are you suggesting I might get more power from a non-standard prop?

I'm not interested in speed. More interesting in torque at low speeds when trying to get into a windy berth.

What he is suggesting is that you will get more power and thrust from being propped correctly. The standard propeller that came with your motor may not be the best one for your particular usage. A high thrust type is preferred, you might just need a different one.

The motor needs to be able to turn 5500 RPM when you are at wide open throttle. So, a tachometer is needed to see what you are getting. Changing propellers can be like shifting gears. You may be in third or fourth gear with the propeller you have when you need to be in first gear.

Standard pitch propeller on a T9.9 is a seven pitch. Yam offers a 5 3/4 pitch which might be more suitable.

[zenoahphobic]

prop selection

What he is suggesting is that you will get more power and thrust from being propped correctly. The standard propeller that came with your motor may not be the best one for your particular usage. A high thrust type is preferred, you might just need a different one.

The motor needs to be able to turn 5500 RPM when you are at wide open throttle. So, a tachometer is needed to see what you are getting. Changing propellers can be like shifting gears. You may be in third or fourth gear with the propeller you have when you need to be in first gear.

Standard pitch propeller on a T9.9 is a seven pitch. Yam offers a 5 3/4 pitch which might be more suitable.

I've looked at this problem myself with 9.9 high thrust. The prop should be large diameter (11 inch?) as well as large blade area. I simply did not find enough choice out there.

I think the 9.9 is probably to small and outdated, that should be updated/replaced with a slightly higher output model for these more modern times where we expect greater performance.

[RonaldJJames]

I think the 9.9 is probably to small and outdated, that should be updated/replaced with a slightly higher output model for these more modern times where we expect greater performance.

Hence my original question. Is it crazy to think about converting a 15HP engine to high thrust. If not how would one go about it?

[boscoe99]

There is no "yamaha" way to convert an F15 to a T15. The T series of motors typically use a larger gear case, turning a larger diameter propeller, with higher ratio gears.

The T9.9 is fine for what it was intended for. Heck, in some applications it might be more than is needed. There is no one Yamaha that fits all, does all. Maybe the T9.9 is simply not the motor for your boat. Even with two of them. On the other hand maybe it is, but it is not propped as it might be.

Will one T25 fit where your two T9.9's fit?

Best that you can do with your present motors is to make sure that you are propped correctly. Try the Yamaha propeller that will fit your motor with the lowest possible pitch. You might look around and find another brand of propeller that will fit your T9.9 with even less pitch. Less pitch is like shifting to a lower gear in your car.

All the horsepower in the world won't do any good if you are not geared (propped) optimally. An F350 can be a dog of a motor if it is over propped.

[99yam40]

As I said before they make duel thrust props, I believe they has better trust in reverse.
being a Cat with 2 motors reversing one may give you better control in some situations in tight spots if you have more thrust in reverse.

Test yours at WOT and find out what props you have to start out with

[zenoahphobic]

prop

What he is suggesting is that you will get more power and thrust from being propped correctly. The standard propeller that came with your motor may not be the best one for your particular usage. A high thrust type is preferred, you might just need a different one.

The motor needs to be able to turn 5500 RPM when you are at wide open throttle. So, a tachometer is needed to see what you are getting. Changing propellers can be like shifting gears. You may be in third or fourth gear with the propeller you have when you need to be in first gear.

Standard pitch propeller on a T9.9 is a seven pitch. Yam offers a 5 3/4 pitch which might be more suitable.

I get somewhat annoyed when it is suggested the "right" prop is one that allows the engine to reach max revs! This is merely a convenient mantra.
The OP has stated he is not interested in revving these at such a high speed. Neither would I want to rev these motors at full 5500 rpm for docking manouvers - certainly would attract onlookers who would probably worry and question the operators skill.
There is a reasonable revs to do this at and the prop must be maximised for this task! The negative is that the engines may be overrevved because of the subsequent lack of resistance.
My comments a based on the assumption that these engines are not the prime source of propulsion and are used for slow speed taxiing etc.