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Old 11-09-2020, 02:37   #1
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What size motor ?

HI everyone,

I am looking for advice. We are buying our first boat.and we need to know if a 6 hp motor is bjg enough for it ? It was built as a sailboat in 1981, but we are only using it with a motor. It is a Jeanneau Pouldu, length = 4.1 metres width = 1.85 metres, weight =130 kilograms.

We will be really grateful for any advice.

Thanks !
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Old 11-09-2020, 03:43   #2
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Re: What size motor ?

My opinion is you need a bigger motor like a 9.9hp. More importantly, you need a high thrust motor.

https://yamahaoutboards.com/en-us/ho...st/high-thrust

Will you be going out in extreme ocean conditions or are you on a lake or other more calm body of water?

If the conditions are extreme, it may even be possible 9.9 isn’t enough, given your use of the boat without sails.
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Old 11-09-2020, 04:02   #3
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Re: What size motor ?

Hi chotu,

Thanks very much. The truth is that we live in France and whereas I would definitely prefer a bigger motor if I have anything bigger than 6 hp I am obliged to do a course to get a permit. I have no particular objection to that either, my only problem is that I am not sure that my French language skills will get me through the course and it costs between 300 and 500 Euro. That is not money I can afford to throw away and so we were hoping that a 6 hp would do the job.

We intend doing inshore fishing off the coast, within sight of land and would not be venturing out in bad weather. I have attached a picture.

Thanks again for your advice.
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Old 11-09-2020, 04:24   #4
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Re: What size motor ?

Chotu is correct - the ideal would be a high-thrust motor. Be sure to double check that the shaft length will work on your boat. The Yamaha only comes in a 20" shaft.
-Greg
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Old 11-09-2020, 05:11   #5
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Re: What size motor ?

Working within the 6 HP limit, a four stroke such as the Nisson/Tohatsu will give you more low end torque than a two stroke. Then, your choice of propeller is critical for non-planing heavy thrust operation. Your hull speed is 5.0 knots, and I'm inclined to believe that the six will do that, pending how much weight you put in the boat. Any left over HP at that point is just more money for very little gain in speed.
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Old 11-09-2020, 05:36   #6
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Re: What size motor ?

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Originally Posted by tkeithlu View Post
Working within the 6 HP limit, a four stroke such as the Nisson/Tohatsu will give you more low end torque than a two stroke. Then, your choice of propeller is critical for non-planing heavy thrust operation. Your hull speed is 5.0 knots, and I'm inclined to believe that the six will do that, pending how much weight you put in the boat. Any left over HP at that point is just more money for very little gain in speed.
+1 for this post.
With that weight boat 6hp will be ok. We have a 26ft boat weighing at a guess 1.5 tonne & an 8hp 2stroke tohatsu outboard drove it pretty well when its diesel engine was inoperable.
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Old 11-09-2020, 06:03   #7
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Re: What size motor ?

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+1 for this post.
With that weight boat 6hp will be ok. We have a 26ft boat weighing at a guess 1.5 tonne & an 8hp 2stroke tohatsu outboard drove it pretty well when its diesel engine was inoperable.
Thanks very much compass, that is a little reassuring. I basically know nothing, yet, and so just was not sure if a 6 hp would be man enough or not. So, basically you think it would be fine for that boat and just pottering around inshore ? Would you suggest 4 stroke or 2 ?
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Old 11-09-2020, 07:32   #8
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Re: What size motor ?

Make certain it’s a high thrust motor, a regular motor won’t do well at all. They are made to make small boats go fast, planing speeds.
A high thrust motor is made to push heavier boats at hull speed, think of it as gearing.
A regular outboard is in high gear, and a high thrust is in low gear.

A 6 HP high thrust will absolutely push a heavier boat much better than a 9.9 regular motor.

What happens is the small prop on a regular motor if you use too much throttle cavitates, slips if you will and the motor just revs up but isn’t pushing the boat, it’s the same as a tire spinning on a car.

Personally I like a four stroke, they burn much less fuel and pollute much less too, but they are heavier and often cost more and are more complex.

If you were having to lift the motor into the boat when it was in the water often, then the lighter weight of a two stroke really matters, but you won’t be doing that, so burning less fuel and polluting a whole lot less is good.

I’m fairly certain that a small two stroke outboard pollutes more than a full size automobile, much more.
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Old 11-09-2020, 07:59   #9
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Re: What size motor ?

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Make certain itís a high thrust motor, a regular motor wonít do well at all. They are made to make small boats go fast, planing speeds.
A high thrust motor is made to push heavier boats at hull speed, think of it as gearing.
A regular outboard is in high gear, and a high thrust is in low gear.

A 6 HP high thrust will absolutely push a heavier boat much better than a 9.9 regular motor.

What happens is the small prop on a regular motor if you use too much throttle cavitates, slips if you will and the motor just revs up but isnít pushing the boat, itís the same as a tire spinning on a car.

Personally I like a four stroke, they burn much less fuel and pollute much less too, but they are heavier and often cost more and are more complex.

If you were having to lift the motor into the boat when it was in the water often, then the lighter weight of a two stroke really matters, but you wonít be doing that, so burning less fuel and polluting a whole lot less is good.

Iím fairly certain that a small two stroke outboard pollutes more than a full size automobile, much more.
Hi a64. Thanks ! So, you think a high thrust 6 hp motor would be adequate for my flat bottom 286 lb boat ?
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Old 11-09-2020, 08:43   #10
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Re: What size motor ?

In a nutshell, yes.

A64Pilot has given you good guidance, as usual. However, his advice may run into some practical problems. Yes, you want a 4-stroke for more torque and less pollution. But, you may have difficulty finding a high thrust model under 9.9 HP, and it may cost more. Perhaps he knows of a good candidate. Then, you can at least partially mediate the "gear shift" problem by your prop choice. A low pitch propeller acts like low gear, while a coarse pitch propeller acts like high gear. At 5 knots or less you want low gear, as A64pilot suggests. The economical solution for you may be a very ordinary 6HP four stroke motor with a fine pitch prop. The right choice of prop is one that tops out (maximum RPM) at 5,500 to 6,000 RPM. Run it at 4,800 for a long life.
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Old 11-09-2020, 09:09   #11
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Re: What size motor ?

Hello Flyboy,

Video depicts a Yamaha 6hp with stock "high speed" propeller at less than half throttle (no idea what % of total power).

Boat weighs 15,000 lbs. and is moving at 3 kts.

I think your 286 lb. boat will be fine with a 6hp engine with fine pitch propeller.

Steve

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Old 11-09-2020, 10:35   #12
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Re: What size motor ?

It might be a good idea to consult with a motor manufacturer, such as Yamaha. You can get their tech phone number from their website. They can provide tech assistance over the phone and get you very close to what is ideal. I've done this with a power boat I own (secondhand) that I wasn't sure if it had the correct pitch. Good Luck!
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Old 11-09-2020, 10:48   #13
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Re: What size motor ?

Six HP should definitely be sufficient for your boat and usage. Compared to Honda, Mercury/Tohatsu, Evinrude (2.15:1) and Yamaha (2.08:1), the Suzuki offering has the lowest gearing with 1.92:1 and a correspondingly slightly lower wide open throttle (WOT) rpm of 4,750 to 5,750, making it arguably your best bet. They all weigh around 25kg so a good deal less than the 8 or 9.9HP engines ...
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Old 11-09-2020, 10:56   #14
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Re: What size motor ?

Thanks everyone for your sage advice to a beginner, really very helpful and I am very grateful !
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Old 11-09-2020, 13:01   #15
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Re: What size motor ?

6 hp is overkill. All you need is a 2 hp. The rule of thumb is 2 hp for each 1000 lbs . I've had 2 hp on a 2000 lb boat no issues got to hull speed. I've had a 5 hp on a 3500 lbs boat again more than enough. and a 4 hp on a 2200 lb boat. Motor manufacturers take the same motor and rate it at different speeds. ie a 4 hp at 4000 rpm is a 6 hp at 5500 rpm. They are identical motors.
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