Cruisers Forum
 


Reply
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 18-01-2021, 07:23   #1
Registered User
 
fourlyons's Avatar

Join Date: Mar 2012
Location: Baltimore, MD
Boat: 39' Custom built junk rigged cat ketch
Posts: 132
High Thrust Outboard question

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.
fourlyons is offline   Reply With Quote
Old 18-01-2021, 08:03   #2
rbk
Registered User
 
rbk's Avatar

Join Date: Feb 2016
Location: Canada
Boat: T37
Posts: 2,037
Re: High Thrust Outboard question

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.
rbk is offline   Reply With Quote
Old 18-01-2021, 08:08   #3
Registered User

Join Date: Jun 2019
Location: Rochester, NY
Boat: Chris Craft Catalina 381
Posts: 1,670
Re: High Thrust Outboard question

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.
rslifkin is online now   Reply With Quote
Old 18-01-2021, 09:02   #4
Registered User
 
fourlyons's Avatar

Join Date: Mar 2012
Location: Baltimore, MD
Boat: 39' Custom built junk rigged cat ketch
Posts: 132
Re: High Thrust Outboard question

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 is offline   Reply With Quote
Old 18-01-2021, 09:07   #5
Registered User
 
fourlyons's Avatar

Join Date: Mar 2012
Location: Baltimore, MD
Boat: 39' Custom built junk rigged cat ketch
Posts: 132
Re: High Thrust Outboard question

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.
fourlyons is offline   Reply With Quote
Old 18-01-2021, 09:07   #6
Registered User

Join Date: Jun 2019
Location: Rochester, NY
Boat: Chris Craft Catalina 381
Posts: 1,670
Re: High Thrust Outboard question

Quote:
Originally Posted by fourlyons View Post
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.
rslifkin is online now   Reply With Quote
Old 18-01-2021, 09:14   #7
Registered User

Join Date: Jul 2019
Location: Canada
Boat: Lagoon 400
Posts: 146
Re: High Thrust Outboard question

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.
Knot Safety is online now   Reply With Quote
Old 18-01-2021, 09:28   #8
Registered User
 
fourlyons's Avatar

Join Date: Mar 2012
Location: Baltimore, MD
Boat: 39' Custom built junk rigged cat ketch
Posts: 132
Re: High Thrust Outboard question

I get it, but it's a sailboat, the engine is the redundancy, and vice versa. And it's a mono hull.
Quote:
Originally Posted by Knot Safety View Post
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 is offline   Reply With Quote
Old 18-01-2021, 09:36   #9
Registered User

Join Date: Jul 2012
Posts: 220
Re: High Thrust Outboard question

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.
__________________
Brent
Snowgoose
The two most important days, are the day you were born, and the day you figured out why. Mark Twain
rbrentp is offline   Reply With Quote
Old 18-01-2021, 09:56   #10
Registered User
 
fourlyons's Avatar

Join Date: Mar 2012
Location: Baltimore, MD
Boat: 39' Custom built junk rigged cat ketch
Posts: 132
Re: High Thrust Outboard question

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.
Quote:
Originally Posted by rbrentp View Post
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 is offline   Reply With Quote
Old 18-01-2021, 10:06   #11
Registered User

Join Date: Jul 2012
Posts: 220
Re: High Thrust Outboard question

To be fair, the issues is both. The 25 above 3200 rpm at any thing but glass like water sucked air. Waves were worse.
__________________
Brent
Snowgoose
The two most important days, are the day you were born, and the day you figured out why. Mark Twain
rbrentp is offline   Reply With Quote
Old 18-01-2021, 10:26   #12
rbk
Registered User
 
rbk's Avatar

Join Date: Feb 2016
Location: Canada
Boat: T37
Posts: 2,037
Re: High Thrust Outboard question

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 is offline   Reply With Quote
Old 18-01-2021, 10:33   #13
Registered User
 
fourlyons's Avatar

Join Date: Mar 2012
Location: Baltimore, MD
Boat: 39' Custom built junk rigged cat ketch
Posts: 132
Re: High Thrust Outboard question

All great points.

To be clear, was the 25 that you tried a Yamaha T25 High Thrust, specifically?
Quote:
Originally Posted by rbk View Post
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 is offline   Reply With Quote
Old 18-01-2021, 11:07   #14
rbk
Registered User
 
rbk's Avatar

Join Date: Feb 2016
Location: Canada
Boat: T37
Posts: 2,037
Re: High Thrust Outboard question

Quote:
Originally Posted by fourlyons View Post
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.
rbk is offline   Reply With Quote
Old 18-01-2021, 11:47   #15
Registered User
 
fourlyons's Avatar

Join Date: Mar 2012
Location: Baltimore, MD
Boat: 39' Custom built junk rigged cat ketch
Posts: 132
Re: High Thrust Outboard question

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.
Quote:
Originally Posted by rbk View Post
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 is offline   Reply With Quote
Reply

Tags
outboard

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Want To Buy: Carburetor for 2005 Yamaha 9.9 High Thrust Outboard mattyc Classifieds Archive 2 25-06-2016 10:01
20 to 25 HP Extra Long Shaft High Thrust Outboard? rgesner Engines and Propulsion Systems 1 17-02-2014 12:43
For Sale: Yamaha T8 High Thrust Kicker Motor Real World Classifieds Archive 2 04-08-2011 08:36
9.9hp vs 15hp High Thrust Honda Outboard & Gasoline in Caribbean ArmyChief Engines and Propulsion Systems 11 15-07-2010 15:28

Advertise Here


All times are GMT -7. The time now is 09:32.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2021, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.