High Thrust Outboard question

[fourlyons]

Hi there,

Does anyone have any personal experience changing from two Yamaha 9.9 High Thrust outboards to one Yamaha 25 High Thrust?

Specifically, do you think you got the same amount of thrust?

Obviously I'm more than doubling the horsepower, but the prop on the 25 is not twice the blade area of a 9.9.

I think it's time to replace my outboards and if I go to the 25 I can save 60-70 pounds and some complications.

[rbk]

Biggest issue you’ll have is ensuring you’re working the 25 hard enough at your cruising speed. If you do go with it I would get the lowest pitched prop you can for the engine to help you get into your proper rom range. I have a 25 trolling motor on my lake/river boat (22’ aluminium) and can get a decent rpm range with a smaller low pitch prop on it trolling around or using it for downriver control.

[rslifkin]

What kind of speed do you get with just one of the 9.9s running? If it's a usable slow cruise, the pair of small outboards may be more efficient as you can shut one down at lower speeds. You lose that ability with the single bigger outboard.

[fourlyons]

Thanks for the replies.

I get 5 knots with one engine at about 3/4 throttle. I assume, maybe wrong, that I can just throttle the 25 down and get the same speed. Maybe I misunderstood your statement?

My fear is the 25 won't give me the high end I get with the two 9.9 engines, about 7 knots, back when the engines were newer.

[fourlyons]

I reread your comment about the low speed, I think the concern is about fuel consumption. Yeah, I thought about that, but as infrequently as I use the engines, I may be willing to live with what I expect would be a pretty small difference, especially as the 25 is fuel injected.

[rslifkin]

Originally Posted by fourlyons

Thanks for the replies.

I get 5 knots with one engine at about 3/4 throttle. I assume, maybe wrong, that I can just throttle the 25 down and get the same speed. Maybe I misunderstood your statement?

My fear is the 25 won't give me the high end I get with the two 9.9 engines, about 7 knots, back when the engines were newer.

The 25 throttled back will do 5 kts just fine. My thought was more that when running at a lower speed like that, a single 9.9 might be more efficient than a lightly loaded 25.

[Knot Safety]

It’s always good to have redundancy of two engines. If I were in your shoes, I wouldn’t switch to a single engine for any reason.

[fourlyons]

I get it, but it's a sailboat, the engine is the redundancy, and vice versa. And it's a mono hull.

Originally Posted by Knot Safety

It’s always good to have redundancy of two engines. If I were in your shoes, I wouldn’t switch to a single engine for any reason.

[rbrentp]

I went the other way but it was a cat. The biggest difference was cavitation with the 25 because they aren't as long The 25 pushed the boat about 6 and the pair of 9.9s were at 8.

[fourlyons]

Thank you, that addresses my question exactly, and maybe verifies my suspicion, that the propeller on the 25 is not big enough to deliver the horsepower to the water.

Actual cavitation, as I understand it, is due to a propeller being too highly loaded. While the leg of the outboard being too short, would lead to ventilation of the prop. Just a pedantic point.

Originally Posted by rbrentp

I went the other way but it was a cat. The biggest difference was cavitation with the 25 because they aren't as long The 25 pushed the boat about 6 and the pair of 9.9s were at 8.

[rbrentp]

To be fair, the issues is both. The 25 above 3200 rpm at any thing but glass like water sucked air. Waves were worse.

[rbk]

This is where prop selection comes in. If you are running a standard or high pitch prop you are more likely to get cavitation as they are designed to move a boat fast, when the hull limits the speed like a sailboat no amount of power will push it faster and creates too low of pressure on the backside of the blades so it cavitates (actually exploding bubbles on your prop usually a high speed issue)



Ventilation (often mistakenly referred to as cavitation) is caused by inducing air from the trailing edge of the boat (whirlpools, bubbles etc) often caused by the boat squatting and dragging the transom.



If you switch to a lower pitched prop it will spin (rpm) faster while moving the boat slower getting a better bite while creating less low pressure on the blades.



My above comment has nothing to do with fuel consumption and more that engines are designed to run in a specific rpm band, operating below or above that band for long periods will shorten the engines life and reliability. While you wont blow the head of it running it low there are a list of issues that will arise from low ending it.

[fourlyons]

All great points.

To be clear, was the 25 that you tried a Yamaha T25 High Thrust, specifically?

Originally Posted by rbk

This is where prop selection comes in. If you are running a standard or high pitch prop you are more likely to get cavitation as they are designed to move a boat fast, when the hull limits the speed like a sailboat no amount of power will push it faster and creates too low of pressure on the backside of the blades so it cavitates (actually exploding bubbles on your prop usually a high speed issue)



Ventilation (often mistakenly referred to as cavitation) is caused by inducing air from the trailing edge of the boat (whirlpools, bubbles etc) often caused by the boat squatting and dragging the transom.



If you switch to a lower pitched prop it will spin (rpm) faster while moving the boat slower getting a better bite while creating less low pressure on the blades.



My above comment has nothing to do with fuel consumption and more that engines are designed to run in a specific rpm band, operating below or above that band for long periods will shorten the engines life and reliability. While you wont blow the head of it running it low there are a list of issues that will arise from low ending it.

[rbk]

Originally Posted by fourlyons

All great points.

To be clear, was the 25 that you tried a Yamaha T25 High Thrust, specifically?

Doesn't matter really. A high thrust OB is just geared lower. This along with prop selection gives you more options on the lower end of the spectrum. High thrust OB with Lowest available pitch prop = low top end speed / greatest amount of thrust. You could have a high thrust out board with a high pitch prop and it will behave like a regular OB with a standard pitch prop for example.

[fourlyons]

Thank you, but I do know all of that.

And the question does matter.

The Yamaha 25 High Thrust does not come with a particularly high, low end gearing (2:1) and a very limited selection of props. I believe the lowest pitch available is 12". That's why I was asking how that engine specifically performed compared to Yamaha 9.9 High thrusts with high thrust props performed.

If you did not use a Yamaha 25 High Thrust and compare it to two 9.9 Yamaha High Thrusts, then it doesn't necessarily answer my question.

Originally Posted by rbk

Doesn't matter really. A high thrust OB is just geared lower. This along with prop selection gives you more options on the lower end of the spectrum. High thrust OB with Lowest available pitch prop = low top end speed / greatest amount of thrust. You could have a high thrust out board with a high pitch prop and it will behave like a regular OB with a standard pitch prop for example.