What is a High Thrust outboard?

Rivers & creeks

Rivers & creeks

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We need to replace the outboard on Muddy Paws this year and the current model is a 9.9hp Yamaha "High Thrust". We are going to replace it with a 15HP as she needs more grunt, but do we have to go for an "High Thrust" model or not?

Boat is a 27 foot Cat.
 
You will probably find that the 15 hp is the same engine with different jets. The 9.9 is designed with a different power curve to normal outboards, which makes a big difference, but the biggest difference is normally the propellor. The high thrust propellor has much larger blade area, and the stern power is boosted by a clever exhaust gas gizmo on the back end of the prop.
 
'High Thurst' refers to the propeller.

If you use a standard outboard on a large boat (eg a yacht), it will tend to whisk the water rather than provide useful drive.

A High Thrust propellor is larger diameter and smaller pitch, so whilst it won't have the top speed of a small dinghy, it will have plenty of 'grunt' to manoevure a heavier boat around.
 
The thrust is like the gearing.

Imagine low thrust like a low gear. Can move heavy objects as slow speed.

High thrust, is like a high gear, can move light objects at high speed.


If you attached a high thrust engine to a heavy boat, the engine wouldnt be able to reach high revs and maximum power output, itd be like trying to pull away from the traffic lights in 5th gear. It would put a lot of load on the gearbox.

If you tried to use a lower thrust prop on a dinghy youd easily be maxing out on revs at low speed. Like trying to go along the motorway in 2nd...

Your boat is kind of a bit inbetween a heavy and a light boat so id go for a mid pitch prop on a 15 hp. Best thing to do is just ask the manufacturer (NOT the chandlers as they tend to just say anything so they dont look stupid!!). The boat manufacturer is also sure to know what would be best.

Mat
 
[ QUOTE ]
Imagine low thrust like a low gear. Can move heavy objects as slow speed.

High thrust, is like a high gear, can move light objects at high speed.

[/ QUOTE ]

Good anology, but the wrong way around.

High Thrust is like low gear.

Low Thrust (normal type) is like high gear.
 
[ QUOTE ]
'High Thurst' refers to the propeller.

If you use a standard outboard on a large boat (eg a yacht), it will tend to whisk the water rather than provide useful drive.

A High Thrust propellor is larger diameter and smaller pitch, so whilst it won't have the top speed of a small dinghy, it will have plenty of 'grunt' to manoevure a heavier boat around.

[/ QUOTE ]

The high thrust propellor is marginally larger in diameter, but the biggest difference is the blade area, it is larger, thus pushing more water volume.

You can get high thrust propellors for a number of different sized outboards, but there are also some outboards that have been designed specifically for the purpose of pushing larger craft, and the Yam 9.9 is one of those.
 
High Thrust 9.9 Yammys rock. I've got one on a 32fter and can do 7.5kts in flat water, nice /forums/images/graemlins/smile.gif

I also have a 15hp std Yammy which I very much doubt would do the same.

The HT9.9 gives good bottom end push to get you moving. I think my 15 would be OK once I'm up to speed but would take a lot longer to get there from a stand still and be hopeless in any waves.

I'm all for 4 strokes now as well.
 
Power is meaningless on its own - whats important is how this power is delivered to the propeller.

Relatively heavy boats like your cat need an engine that can deliver serious grunt at relatively low speeds. This calls for a 'large' prop diameter, 'large' blade area, and 'large' reduction ratio in the gearbox.
Note that these 'larges' are all relative.

A 'high thrust' outboard does all of the above much more effectively than an 'ordinary' outboard.

Hence it is quite possible that a 9.9 hp 'high thrust' might have the same effective power as a 15 hp 'ordinary'.

Here is an extreme example. Imagine a heavy displacement (say 12 tonnes) Cygnus type fishing boat. She might typically have a slow speed Gardner diesel engine which might deliver (say) 115 hp. This would probably drive her along very happily, and also give her a fair bit of extra grunt when required for (eg) towing trawls, or punching into head seas.

Now imagine putting a 115 hp two stroke outboard engine on the transom of this big heavy fishing boat. You open up the throttle. What happens? Not much. You might be doing a few knots with the engine screaming away at full revs. And as for punching into a head sea, forget it!

'But it is the same amount of horse power!'

Yes, but the diesel might have a 3' diameter prop with a 4 : 1 reduction gearbox, while the outboard might have a 10" diameter prop with a 1.5 : 1 reduction gearbox.
Both handling the same amount of power, but in different ways.

The Gardner in this example is a 'high thrust' engine, while the outboard is a 'high speed' engine.
 
Thanks everyone - it makes perfect sense to me now. I suppose in the ideal world a 15hp High Thrust engine would be best - but Yam don't make one!

Time to hunt out a new 9.9HT.

Thanks again,

Simon
 
I had a high thrust prop on a 30 hp Yam.

However, you may find that the difference between the 9.9 and the 15 is not as big as you expect.

Personally I would be looking at all the engines that weigh around the same as the 9.9 + a very small amount, and making my decision on those.
 
[ QUOTE ]
I suppose in the ideal world a 15hp High Thrust engine would be best - but Yam don't make one!

[/ QUOTE ]

No but Honda do. I have just purchased a Honda BF20(identical to the BF15) and specified the HT prop at the same time. This is quite a heavy unit but it is 4 stroke and was also specified with electric start and power tilt.
 
I had a Yam 9.9 High Thrust on my last boat.

I understand (from the dealer) that it had the same powerhead as a 15 hp, but a different gearbox and propellor.

It is the gearbox and prop combination that delivers the high thrust.
 
Im sure its just the prop but the engin must have a suitable power curve.If it makes its power at 5000rpm then the prop would have 2500 rpm in general and it might not be able to turn a large prop that fast??

I dream of a "cruising" prop just to have a reverse gear!Entering a lock at any speed with no breaks is bad for the nerves not to mention any other boat in the way!

Entering at a speed i know i can stop by digging my fingernails into the lock sides befor hitting something is so slow the lock keeper might think im waiting for the next group!!

Where can i buy a "large" prop for a Marina 8 HP (2 stroke) what size should i look for??
 
I think we are all in need of another Trou story.... I am just imagining you and your folkboat, digging fingernails into the mossy sides of the lock while steaming in..... 'tally ho monsieurs!'
Much more fun than stories about gearbox ratios and blade areas and thrusts versus torques!
 
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