Regular 3 Blade Prop on Yammy 9.9 HP High Thrust???

[Whole in the Water]

"I would also suggest re-thinking those “High Thrust” anything? If more power is needed consider going with more horsepower (e.g. 15HP over the 9.9 HP Bigfoot or anything else high trust. It is horsepower verses torque issue. That will also help get rid of some of those vibration issues mentioned."

This quote from Charlie on another post got me thinking. My high thrust Yammy 9.9 does vibrate on my welded aluminum boat. IMO there iare few things worse that an annoying vibration to ruin a peaceful days fishing. From Charlie post it sounds like the high thrust model, with their higher torque vibrate more than regular 9.9 hp models.

So my questions is this - can you take off the larger 4 bladed HT prop and replace it with a regular 3 bladed 9.9 prop?

Will it work? What kind of HP will you get - more or less than 9.9 hp? but most importantly will it vibrate less???

 

[Sharphooks]

I have a Hi-Thrust Honda 9.9 and it has zero vibration. It's so quiet sometimes I have to put my hand on it to make sure it's running....

At first blush, you could probably get away with going three blade on a hi-thrust but I'd be willing to bet that you should consider increasing the pitch of the 3-blade to more or less match the RPM's of the 4 blade (at an equal throttle setting).

The reverse is also true: if you're going from a three blade to a four blade, you would decrease the pitch in order to stay within a predetermined RPM bracket at WOT

 

[trendsetter]

So my questions is this - can you take off the larger 4 bladed HT prop and replace it with a regular 3 bladed 9.9 prop?

Will it work? What kind of HP will you get - more or less than 9.9 hp? but most importantly will it vibrate less???

Yes you can replace the 4-blade prop with a 3-blade prop. It will work just fine.

You will not get a HP difference; the engine will still produce it's rated HP as long as you get the correct pitch. The only possible HP change would be less HP if you choose a prop that doesn't allow the engine to reach it's full RPM.

Usually 4-blade props are smoother than 3-blade props because they have more blade surface area. Have you checked your current prop for dings or bent blades that might be causing the vibration?

Cheers

 

[Fisher Down]

I agree with sharphooks about my mid-90's 9.9 HT Yami using the stock 3-blade;
quiet, smooth, and no vibaration at all (on my plastic bote).

F D

 

[profisher]

You can't go to a regular prop on a HT model Yammi. You would have to change the lower gearcase as well to a standard non HT unit. The HT are geared really low and made to swing the larger diameter props. You would loose all your pushing performance by going to a regular prop and the gearcase would still limit rpms.

 

[cheech]

My Yammie HT has a stock 3 blade prop. Go figger...
Yes it is an HT model.

 

[AlK]

The prop on my 8 HT only has the 3 blades, but the prop is a lot bigger than what is on the non HT models. By the way my 8HT Yam runs very smoothly and very quietly. Based on my experience I am not convinced that the prop alone causes the viabration. Over the years I have had experience with 4 kickers, (Merc 2 st, Suzuki 2st, Evinrude 4st, and now Yam HT 4st.) The only one that I had a problem with viabrating was the Evinrude. What I found was that at trolling speed it viabrated very badly, but if slightly increased or decreased the throttle the viabration disappeared.

 

[profisher]

My interpretation of a regular prop is a non high thrust prop. My Ht 9.9 Yammi has a 3 blade on it as well but a HT prop. I thought the question was if he could switch to a non HT 3 blade prop. The Suzuki 9.9 Ht had some vibration issues right on the sweet spot trolling speed for some boat owners. They found they had to re-balance the crankshaft. This didn't eliminate the vibration but moved it away from the trolling speed sweet spot throttle setting. I think most engines will develop some amount of harmonic vibration at certain rpms. Some more than others and the root cause is in the rotating assembly of the engine, not the prop. If the prop is damaged that is another story.

 

[Whole in the Water]

Thanks for all the input guys. After I checked I realize now I have a 3 bladed prop, but it is a wide HT version. After doing some more research I think Profisher is right - I would need to change my gearing as well as the prop to really see about improving things.

 

[Charlie]

"I would also suggest re-thinking those “High Thrust” anything? If more power is needed consider going with more horsepower (e.g. 15HP over the 9.9 HP Bigfoot or anything else high trust. It is horsepower verses torque issue. That will also help get rid of some of those vibration issues mentioned."

I may need to clarify that comment for you?

Every outboard I have ever heard, touched, or felt – vibrates to some degree. Even at an idle. If you don’t feel or hear it, just touch the kicker and you will feel the vibration. The higher the RPM the more vibration is usually felt. Running a 15HP at a lower RPM will produce less of that “normal” vibration than running any smaller 9.9 at higher RPM.

This quote from Charlie on another post got me thinking. My high thrust Yammy 9.9 does vibrate on my welded aluminum boat. IMO there are few things worse that an annoying vibration to ruin a peaceful days fishing.

From Charlie post it sounds like the high thrust model, with their higher torque vibrate more than regular 9.9 hp models.

So my questions is this - can you take off the larger 4 bladed HT prop and replace it with a regular 3 bladed 9.9 prop?

Changing the prop will probably NOT help, unless the prop itself is damaged.

Will it work? What kind of HP will you get - more or less than 9.9 hp? but most importantly will it vibrate less???

As previously stated the High Thrust has a different lower unit. If you go to Yamaha website you will find several different props available; however, you will also find those props are interchangeable with the other non-High Thrust engines. Just changing a prop will NOT change the gear ratio; plus, you still need to run the correct prop to obtain the correct RPM range, which is based on YOUR application and boat.

Yamaha’s are not really known for excessive vibration issues. Now with that… the higher the RPM, the more vibration apt to be felt, especially on welded aluminum boats. Fiberglass hulls – not so much.

Does reducing RPM also reduce vibration? Does increasing RPM decrease vibration? If you do have more vibration at a certain RPM range and it goes away at higher RPM, I agree that would most likely be a harmonic vibration. Sometimes adjusted out - sometimes not.

Other than that if it really is an excessive vibration, it would most likely (really only be) a driveshaft or prop out of balance. That can fixed. If it is not one of these, you are most likely feeling and dealing with a normal vibration through the mounting bracket and that welded aluminum hull, it is mounted to.

If I wanted to reduce any normal vibration, the first thing checked and/or changed would be that mounting bracket and/or any existing damper. If there is no damper, might also try finding and installing some type of damper between the engine and where it mounts to the hull?

 

[Whole in the Water]

Thanks Charlie. I will follow through on some of the things you mention. I think the best and simpliest thing I should do is just put on a rubber damper between the transom and the motor bracket. The engine runs normally, nothing is out of balance and the prop is in good shape. It is probably just be me being over sensitive to the normal kicker vibrations that seem to be magnified on a aluminum boat.

 

[GLG]

You didn't say what year or how many hours so just incase.
Have you pulled the plug to see what condition they were in?
Done a comresion test?
Air leak in the fuel line?
Spark test?
Way cheaper to check the basics first.
I've heard that they don't run right with old plugs.
I would look there first.
GLG

 

[ReelSlim]

Thanks Charlie. I will follow through on some of the things you mention. I think the best and simpliest thing I should do is just put on a rubber damper between the transom and the motor bracket. The engine runs normally, nothing is out of balance and the prop is in good shape. It is probably just be me being over sensitive to the normal kicker vibrations that seem to be magnified on a aluminum boat.

I think you are right on the money there WITW. I bought a Silverstreak tiller model for running around the lakes and put a 25Hp Yamaha on it. I had originally mounted it on white Starboard and the vibration was brutal! I finally mounted it on thick rubber and it knocked out about 80% of it. I just figure thats as good as it will get on that small boat. I know your rig is a lot bigger but I think depending on how it is mounted etc is more sensitive on some welded boats. IMO