Power Comparison of 2 Outboard MODELS

[rasorinc]

Using the Tohatsu outboard site http://www.tohatsu.com/outboards/ I am trying to determine a comparison for equal power output or close to equal
for a high trust engine of say 60 HP compared to a regular 90 HP engine. The High Thrust engines are geared 2.66 to 1 and the regular engines around 2.03 to 1. Why buy a High Thrust engine if I can buy a higher HP model for close to the same money if it has equal or better power. Tohatsu offers a 60 HP High Thrust and a Regular 90 HP and both are geared 2.66 to 1. I would rather have the 90 HP model. I hope my question makes sense. The boat will be used the go up the major rivers of the US and back down.
THANKS MUCH.

 

[gonzo]

The power is exactly the same for two 60HP or 90HP. The high thrust will move a heavier load at a lower speed. You need to calculate the target speed and resistance of the boat at that speed.

 

[rasorinc]

Thanks Gonzo.
The boat will have only one engine and is a planning hull flat in the rear 8-10' an 28' to 30 long by 9' and quite heavy with provisions for 2 for 30 days or so. I cannot seem to grasp how a 90HP can be equal to a 60 HP (only one engine). There will be major currents to overcome then periods of flat water (especially the return trip) an then I would like speed without a low gear grunting forward at max RPMs. What am I missing with this old brain???
Maybe I should consider a larger kicker engine say a 30 HP high trust and a 60 HP regular cruising engine and run both when in strong currents.?? Thinking of fuel economy.

 

[gonzo]

The efficient range on modern engines is fairly wide. That is, they will provide similar economy through a large RPM range. There will be little difference in fuel per HP between probalby 60 to 100%. Considering the difference in weight of a 60 and 90 is less than the weight of a 30, the larger engine is the economic way to go. Two engines give you the safety of redundancy, so it is something to think about.

 

[Mr Efficiency]

The 90 Tohatsu has a 2.33 to 1 box so far as I can see.

 

[Easy Rider]

I think the Evinrude's have 2.67-1 gears … and don't lack for speed.

 

[gonzo]

Running the 90 at 2/3 throttle will be about the same as a 60 wide open. The difference in weight is about 100lbs. However, to get the same thrust at the same RPM, the propeller should be sized accordingly. Is this a heavy boat, or carries heavy loads?

 

[otseg]

The 60 is l liter displacement with the 2.66 gear case.
The 90 is 1.5 Liters displacement with the 2.66 gear case.
Near the same money?

Often the same engine displacement models are identical but the higher HP versions are mappped to rev higher. In this case you are getting 50% more cu inches. Go big.

 

[boatbuilder41]

I agree... go big ..... when the extra power is needed.... then it will be available....

 

[philSweet]

I disagree... go small. Because you'll get the same job done with less fuel burn; and because I like to be ornery. The big motor is designed to go faster. If you can't go faster, it is worthless. You can't utilize the torque the 90 makes. That requires a higher pitch prop than what you want.

Match the foot and prop to the thrust, and match the power head to the power required. Displacement boats want good thrust and modest power. Props are usually square or a little under square. Buy the motor that comes with the correct pitch prop stock from the factory. They actually know what they are doing there.

 

[otseg]

In theory I thought the smaller engines gave better economy but in practice with my 22' catamaran I put a pair of 175's on it and it will go 60 mph but throttled down to 3000 rpm I go 30 and get 4 mpg. About the same or better at that speed as a pair of 90's.

The bollard pull of the 2:5 to 1 gears and big props is phenomenal. I dragged a 60' sailboat up river through various mud drifts at 8 kts and hardly noticed the resistance, then boogied back to the ramp at 50. I smacked a sunken Civil War ironside in the river and bent a prop shaft. We used the boat on one engine for several weeks and without changing props would still run 40 mph.

 

[Steve W]

Normally I would be a proponent of less hp if it were a displacement boat but if both the motors have the same gearing and the prices are close I would go for the bigger one also, especially if it is a planning boat.

Steve.

 

[drmiller100]

>>>> I disagree... go small. Because you'll get the same job done with less fuel burn; and because I like to be ornery. The big motor is designed to go faster. If you can't go faster, it is worthless. You can't utilize the torque the 90 makes. That requires a higher pitch prop than what you want.


Fascinating.

A 30 foot long by 10 foot wide boat, and you are convinced a 60 horse motor will plane this beast comfortably?

My experience is 1/3 to 3/4 throttle fuel economy is about the same. Above 7/8 throttle it is really common to enrich the mixture to get more horsewpower at the expense of fuel economy.

In other words, a 90 horse at 2/3 throttle will get better mileage than a 60 horse at WOT.

I question whether a 60 horse will plane a 30 foot boat with an 8 foot beam with any kind of weight to it. I'd be skeptical of the 90 horse.

 

[philSweet]

From memory, I though the previous threads dealt with a slower boat. I see the OP did specify planing speed above, at least for light loads. So probably requires something like a hp for every 80 pounds of boat for a flat bottomed design. 60 hp would work up to 5000# or so, but might be a bit of a struggle at the top of that range. And it would require some attention to trim, so maynot fitting your definition of comfortable.

The question remains as to whether the 90 would be any better, or if he would simply need to get a bigger foot in order to use the power. I still think if you look at the stock props supplied with the motor, the unit that comes with the correct prop is the one you want to buy. If that hp is not enough, go to a bigger class of engine.

 

[drmiller100]

Again, I'm fascinated by your "logic." Propellers are cheap. Engines expensive.

Some of the assumptions you are making:

Bigger engine has a propeller to go faster from the factory? Perhaps gearing and pitch are different to give more thrust at same speed.
Propellers cannot be changed?
If the engine is too small, simply sell the now used engine for a substantial loss and go buy a bigger engine, but for golly sakes don't consider swapping propellers out.

Where I live we have snow, and we've been known to swap tires out in the winter for better traction. Do you go buy a new car when the tires on your automobile wear out? To me, this is a valid analogy.

They make different engines for a reason, and they make different propellers for a reason, and it is perfectly acceptable to swap propellers out for specific applications.