Tohatsu and Yamaha 25HP Outboard

I have alway had a soft spot for Tohatsu and have had 3 of their new outboards. They have all given good service but reluctantly i have chosen a new Yamaha 25 over a 25 or 30 hp Tohatsu. I wanted to switch from a 20hp Tohatsu to a 30 hp Tohatsu as pushing an old catamaran into headwind and tide becomes too much for the 20hp but on looking into the specs I realised that the bigger engine still uses the same 9.25 inch diameter propellor as the 20hp. So I looked at the high thrust Yamaha 25 and realised that it has a heavy duty gearbox and swings a 12.25 inch diameter propellor and has a gear ratio of 2.42 compared to the 2.17 of the Tohatsu. Only downside of the Yamaha is that they do not do an Ultra(25") shaft so I will have to do a Bay Manufacturing extension kit if necessary. Quite a difference in weight also, the Yam is about 15kg heavier-I wonder what that means , is it heavier built overall?
I hope the much hyped Dual Thrust props of the Yamaha high thrust engines is not just sales pitch!

Diameter is of little issue. There is a whole range of props for the MFS25/30, so pick the one that gets you closest to max WOT RPM. "Heavy Duty" is a marketing term, and has no relevance in engineering. Gear ratio is irrelevant, since you will select the prop pitch to match the application. Yamaha HT props (not made by Yahaha, btw) are good in reverse, but the Tohatsu/Solas HT prop for the 30 is also intended for high loads at slow speeds.

Hi Thanks for comments
With respect, what you are saying flies in the face of a lot of generally perceived ideas in the sailing community on best props for slow moving yachts. A larger diameter prop on the Yamaha is enabled by the better gear ratio than the Tohatsu. Similarly the very highly rated Yamaha 9.9 is believed to punch well above its weight I think by having an unusually high gear ratio and so swinging the same size diameter (but lower pitch) prop as a 20hp. A big prop relative to the size of engine. Yamaha designed the 9.9 specifically for moving heavy slow boats, are you suggesting they do not know what they are doing?
The term heavy duty with respect to the gearbox on the high thrust Yamaha motors refers to the size of the gearbox to accomodate a 12.25" prop.
I would agree that the main attribute of the Tohatsu high thrust 4 blader (in my case 4 10"x7" replacing 3 9.25"x9") was much better thrust going astern and not much difference going forwards.
Please do not interpret my comments about Tohatsu engines as detrimental as I think they are great engines in every way except that their spec in this engine size may not be as suitable as the Yamaha spec for my particular application, apart from the Ultra long shaft options which is better. The Mercury Bigfoot Engines also had the same concept i.e. higher gear ratio and bigger props. A pity they do not make those any more.

Large diameters are for very slow rpms, say in the hundreds. Outboards turn in the thousands. Gear ratio has nothing to do with the diameter, but a larger gear case can allow a bigger diameter. Yes, the Yam 9.9 HT is a good motor. But it is heavy and costly. 4 blades will shovel slow-speed water more than a larger diameter 3-blade. Not to deter you from the Yam, just to suggest that the Tohatsu is less expensive, lighter, and has very similar thrust. Combined with the Ultra Long 25" shaft, and the Tohatsu is the preferred motor for the application.

Yamaha build their motors quite a bit more solid than Tohatsu does, that is also why it is slightly heavier. I use to race both Tohatsu and Yamaha - just to put it into perspective, on a Yamaha you can leave the gearbox and engine mountings stock standard as they are heavy duty, on the Tohatsu it needed to be strengthened.

deejaycee_2000 said:

Yamaha build their motors quite a bit more solid than Tohatsu does, that is also why it is slightly heavier. I use to race both Tohatsu and Yamaha - just to put it into perspective, on a Yamaha you can leave the gearbox and engine mountings stock standard as they are heavy duty, on the Tohatsu it needed to be strengthened.

Click to expand...

Thanks dee... it was my suspicion that the Yam was heavier built. Do you have a view about the merits of a 12.25" prop rather than a 9.25" prop for pushing a heavy catamaran? Putting my boat/engine details through the VicProp calculator shows as 12.3" x 7.2" prop for a 25Hp engine. They also note that if I have to use a smaller prop with a bigger pitch that will work but will be less efficient. the 9.25" prop for the Tohatsu 30hp with say 11-12" pitch also has a pitch to diameter ratio of 1.18 which they say is for fast boats, which mine is certainly not.
VicProp take on things
[FONT=Times New Roman, Times, serif]Choosing the Right Prop[/FONT]

[FONT=Times New Roman, Times, serif]A propeller converts the turning power of a marine engine at the shaft (called torque) to usable power which moves the boat (called thrust). It works like a pump, sucking water from ahead, accelerating it and then discharging it astern. This produces thrust which pushes the boat forward. [/FONT]

• [FONT=Times New Roman, Times, serif]Different boats require varying amounts of thrust (and consequently varying amounts of slip) to propel them efficiently. [/FONT]
• [FONT=Times New Roman, Times, serif]Slow moving, heavy boats require high thrust and operate most efficiently at high slip.[/FONT]
• [FONT=Times New Roman, Times, serif] Fast moving, light boats require relatively low thrust and, therefore, operate most efficiently at low slip. [/FONT]
• [FONT=Times New Roman, Times, serif]Thrust is developed by accelerating a mass of water. It might seem that the same thrust could be attained by turning a small diameter propeller rapidly (giving a high acceleration to a small mass of water), or by turning a large diameter propeller slowly (giving a low acceleration to a large mass of water), but this is not necessarily so. Energy is lost in friction between the accelerated water and the surrounding water. Therefore, in selecting a propeller shaft speed, it is important to strike a favorable balance between shaft speed and boat speed. The resulting propeller size should have a pitch ratio (pitch divided by diameter) according to the following table for best efficiencies:
  
  [/FONT]

  [FONT=Times New Roman, Times, serif]Type of Boat [/FONT]	[FONT=Times New Roman, Times, serif]Pitch Ratio Range[/FONT]
  [FONT=Times New Roman, Times, serif]very heavy cruisers[/FONT]	[FONT=Times New Roman, Times, serif].55 to 0.8[/FONT]
  [FONT=Times New Roman, Times, serif]average cruisers [/FONT]	[FONT=Times New Roman, Times, serif].65 to 1.0[/FONT]
  [FONT=Times New Roman, Times, serif]medium and fast cruisers [/FONT]	[FONT=Times New Roman, Times, serif].80 to 1.2[/FONT]
  [FONT=Times New Roman, Times, serif]high speed cruisers, & runabouts[/FONT]	[FONT=Times New Roman, Times, serif].90 to 1.5[/FONT]

  
  [FONT=Times New Roman, Times, serif] [/FONT]
• [FONT=Times New Roman, Times, serif]Victoria Propeller uses a computerized propeller sizing program to help you choose the correct propeller for your boating needs. Based on your vessel data, we will provide a suggestion to help optimize your boat’s performance.[/FONT]

Well put it this way, you have a high trust motor so it was built with a bigger prop in mind pushing a heaver load. You definitely have the right motor for your setup. I never really believe in these calculators but rather in physical performance being the ex boat racer that I am. I have a pro-pulse adjustable pitch 4 blade prop (they are very cheap) that I used for setting up my rigs, this way you can test what works best. We use to race 50hp motors pushing anything from a 14 pitch bunny ear prop running at 6500rpm in the rough water to a 18 pitch cleaver fan for speed on the flat water. The lower the pitch, the more hole shot and power you have, the higher the pitch the slower the hole shot but faster top end. The standard prop the motor comes with will be more than sufficient for your setup. I think you will be surprised by the power of the high thrust range out the box as it comes. My neighbour (who is a very big guy) recently replaced his 2 x standard 30hp Yamaha 3cyl 2-strokes with 1 x 60hp Yamaha four stroke high thrust series, props all stock standard as out the box. What a difference! The 2 motors took strain getting him on plane, this motor jumps him right out.

Of course, the Tohatsu (being a 3-cylinder) will be a lot smoother, and is available in Ultra-long, and is lighter and less expensive, even in the 30 hp version. If you really want the Yam, go for it. It's a very good motor in its own right. I can't knock the engineering; only the cost, weight, and shaft length.

The tires on your pick up truck use solid ground to move the vehicle.----The outboard propeller drives the boat in a non compressible fluid.-----The propeller on an airplane drives in a gaseous / compressible fluid.-----They all behave different with speed.---------Large bulb on the nose of a freighter and submarine at slow speed.-----Pointy gearcase on high speed gearcases for outboard racing.------The nose on a jet liner is not pointy like that on a supersonic fighter !-------All for a reason.-------The transfer of power by an outboard propeller has a lot to do with the SPEED of the vessel.-----No such thing as a set-up that is perfect for everything.