Upsizing the inboard outboard

[Mikedefieslife]

just checked the Yamaha and it has a 7 1/2x8 B prop on a 5CMH engine. It doesn't have the black (dual thrust) collar thats for sure. I'm now doing some googling to see what that prop size equates to.

Surely you're 5hp motor cannot reach maximum RPM with a 7 1/2 x 8 prop on a sail boat.

My 8hp Yamaha has a 8 1/2 x 6 1/2 prop can can't reach maximum RPM in a 30kg dinghy with two people in it.

Obviously the sail boat is much heaver too, so I would assume that you are over working the engine if using the standard prop.

 

[fredrussell]

...The standard prop and sail drive type are interchangeable so you could swap one for the other if you want to keep the same outboard.

Coleman, It’s not just a question of changing the prop. A saildrive (AKA high thrust) outboard has a different gearbox to turn the prop slower but with more ‘grunt’. If you just stick a sail drive prop on a non sail drive motor it’ll be far from satisfactory.

 

[coveman]

Coleman, It’s not just a question of changing the prop. A saildrive (AKA high thrust) outboard has a different gearbox to turn the prop slower but with more ‘grunt’. If you just stick a sail drive prop on a non sail drive motor it’ll be far from satisfactory.

Incorrect - Yes on more modern engines the gearbox ratio is different but on an elderly Yamaha 8hp 2 stroke it is purely the prop that is larger and different pitch and they are interchangeable.

 

[langstonelayabout]

Incorrect - Yes on more modern engines the gearbox ratio is different but on an elderly Yamaha 8hp 2 stroke it is purely the prop that is larger and different pitch and they are interchangeable.

Going back to the 2006 sailpower outboards that I have owned, both had different (larger) lower legs in front of the larger propeller. This was to accommodate the larger and lower ration gearboxes. The newer Yamahas were the same.

Best contact the outboard manufacturer to double check before spending any money on a new propeller.

 

[LONG_KEELER]

It would be nice if the OP could borrow one of those Tiny Tac type RPM meters
to make sure that he is getting somewhere near the max RPM in the handbook before
making any changes.

If the gear box ratio is known, there a number of online propeller calculators that could also help.

 

[seumask]

We changed from a Yamaha 8 hp 2 stoke to Tohatsu 5hp 4 stroke on our 25 ft Hunter delta. Both were standard engines with standard pitch props. We went far with it on the 8 hp, but it was a chore to carry enough petrol to cross the channel, not to mention mixing up the 2 stroke. The 5 hp 4 stoke was in my opinion better than the 2 stoke as it was considerably quieter and much more economic. We found that the engine was only really used if the wind was light and maneuvering in and out of Harbour, Marina's and moorings. If the conditions were challenging the boat performed much better under sail than under engine, as I've commented previously on some quite challenging conditions. So if I was you I would seriously consider the 4-6hp four stoke from Tohatsu- Mercury- Mariner and not the 8hp stroke, which is great but noisy and smelly! IIRC they were similar weights around 28 kg. Fourstoke 8 hps were another leap up in weight and difficult to manhandle.
I was always tempted by the 9.9 hp 2 stoke Tohatsu which was same weight as the 8hp Yamaha.

 

[coveman]

Going back to the 2006 sailpower outboards that I have owned, both had different (larger) lower legs in front of the larger propeller. This was to accommodate the larger and lower ration gearboxes. The newer Yamahas were the same.

Best contact the outboard manufacturer to double check before spending any money on a new propeller.

I would agree - best to check - but as "fredrussel" queried what I said, I rang Robin Curnow Outboards, Penryn ,Falmouth ( Main Yamaha dealers) this morning to check and they told me it is purely the prop that is different - if it has the black collar on the prop then it is the "high thrust/saildrive" version and the gearboxes were all standard. As you say, best to check the specific engine.

 

[fredrussell]

Well Robin Curnow is wrong Coveman. This from the Yamaha.eu website here:

FT9.9L - marine-engines - Yamaha Motor

• Special gear ratios for higher thrust
• Dual-Thrust prop – high-thrust in Forward/Reverse

I’ve got a Yamaha FT8 high thrust outboard and I can tell you it has a different gear box to it’s non high-thrust counterpart : the F8. It’s not just a question of changing the prop.
Do you honestly think outboard manufacturers would give outboards a different model code if they had just put a different prop on it? A Yam F8 has an 8” prop. The F8 has a 12” prop. That’s a huge difference- the gearbox ratios are different to suit.

 

[Boathook]

I would agree - best to check - but as "fredrussel" queried what I said, I rang Robin Curnow Outboards, Penryn ,Falmouth ( Main Yamaha dealers) this morning to check and they told me it is purely the prop that is different - if it has the black collar on the prop then it is the "high thrust/saildrive" version and the gearboxes were all standard. As you say, best to check the specific engine.

My prop has the black collar on it and it is a yamaha dual thrust with a fine pitch. It was originally used to convert a Yamaha 30 2 stroke to be suitable on a displacement boat and allowed the engine to reach full revs (approx 5500) giving a top boat speed of around 6.5 knots on a standard gearbox. The high thrust outboards have a much larger diameter prop, coarser pitch and a gearbox to suit but still allow the engine to reach full revs.

 

[coveman]

Well Robin Curnow is wrong Coveman. This from the Yamaha.eu website here:

FT9.9L - marine-engines - Yamaha Motor

• Special gear ratios for higher thrust
• Dual-Thrust prop – high-thrust in Forward/Reverse

I’ve got a Yamaha FT8 high thrust outboard and I can tell you it has a different gear box to it’s non high-thrust counterpart : the F8. It’s not just a question of changing the prop.
Do you honestly think outboard manufacturers would give outboards a different model code if they had just put a different prop on it? A Yam F8 has an 8” prop. The F8 has a 12” prop. That’s a huge difference- the gearbox ratios are different to suit.

I'm sure you are right about that engine - if you read my posts you will see I was referring to an old Yamaha 8hp 2 stroke not the modern equivalent.

 

[VicS]

Well Robin Curnow is wrong Coveman. This from the Yamaha.eu website here:

FT9.9L - marine-engines - Yamaha Motor

• Special gear ratios for higher thrust
• Dual-Thrust prop – high-thrust in Forward/Reverse

I’ve got a Yamaha FT8 high thrust outboard and I can tell you it has a different gear box to it’s non high-thrust counterpart : the F8. It’s not just a question of changing the prop.
Do you honestly think outboard manufacturers would give outboards a different model code if they had just put a different prop on it? A Yam F8 has an 8” prop. The F8 has a 12” prop. That’s a huge difference- the gearbox ratios are different to suit.

My prop has the black collar on it and it is a yamaha dual thrust with a fine pitch. It was originally used to convert a Yamaha 30 2 stroke to be suitable on a displacement boat and allowed the engine to reach full revs (approx 5500) giving a top boat speed of around 6.5 knots on a standard gearbox. The high thrust outboards have a much larger diameter prop, coarser pitch and a gearbox to suit but still allow the engine to reach full revs.

My 6 hp Evinrude Yachtwin has a 9¼ " x 6½" prop with big" Mickey Mouse ear" blades compared with the standard 6hp prop which is 8½" x 9" but the gearbox ratio is the same.

There are other difference between the standard and Yachtwin engines
The Yachtwin came with a 20" shaft length as standard it also has a modified exhaust system with the outlet between twin anti-ventilation plates rather than behind the prop. These two features allow it to mounted with the prop deeper without increasing exhaust back pressure and without exhaust gas being drawn into the prop when going astern.
In addition it has a battery charging coil and rectifier as standard.

There would seem to be no need to produce alternative gearboxes with different ratios when the overall difference in ratio can be achieved simply with different props . Maybe there is some reason why Yamaha have done this

 

[fredrussell]

I’ve always assumed it’s to give a similar power delivery to a diesel inboard engine. ie, a large prop that turns slower with more torque at lower rpm. I’ve never used a two stroke saildrive outboard so cant compare but the 8 & 9.9 high thrust Yamahas are highly rated outboards, especially in the States where they are often found hanging off the transoms of sub-30 foot sailboats. When you put one in reverse the stopping power on a boat has to be seen to be believed!

 

[oldharry]

I believe it is true that 2 strokes will accelerate more quickly . That is one reason why they are still popular for sports boats. They will get the boat planing y or a water-skier up more quickly

It remains total nonsense to claim that " for the same horsepower the 2 stroke is actually more powerful. " The power is determined by the ICOMIA 28/83 test procedure and is measured at the prop shaft

Confusion here between power and thrust. Its a complex area, but briefly an 8hp engine turning a small prop at high speed is designed to travel through the water quite fast. The same engine with higher ratio gearbox (therefore turning the prop more slowly) will not be able to produce so much speed through the water, but will be able to push a much heavier hull more slowly. Its a bit like changing gear in the car. Top gear is good for motorway speeds, but useless at climbing steep hills with a heavy load, needing a low gear.

An 8hp o/b engine with a 'standard' gearbox and propeller is best suited to a reasonably light boat travelling quite fast (7-8 kts) probably on the plane, at which point the propeller is working at optimum efficiency. A 'high thrust' version with a higher gear ratio giving a lower propeller speed , will have a larger area propeller with finer pitch for the same engine speed. The propeller will not travel so far through the water per single revolution (finer pitch on the prop) so the top speed is reduced from 8 knots to around 5.5 - 6knots. But The extra power is then available to move the heavier load at a lower speed (like the lower gear on the car). It goes on a long way from there in abstruse mathematical calculations because prop efficiency is determined by diameter, blade pitch, designed speed through the water, shaft speed (revs) and pwer available to turn it. Its further complicated by propeller slip - the fact it is pushing against water, unlike the car which is (hopefully!) not slipping on solid ground.

Most outboards are a compromise between power and thrust, as the manufacturer has no control over the size and weight of the boat it will be attached to. So they produce 'standard' or 'high thrust' versions. The standard 8hp will get your small rib happily on the plane, but be unable to push a heavy hull against wind and waves effectively. (Climbing a hill in too high a gear). The 'high thrust' version will never get up on the plane, but will be working efficiently pushing a heavy displacement boat through rough conditions (like driving uphill in lower gear with a heavy boat trailer and all the gear!)

On an inboard engine installation the exact prop diameter and pitch can be calculated quite closely to match the boat, engine and hull speed which is why they are generally more efficient.