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Old 30-07-2012, 13:34   #1
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Ideal HP outboard for 10' RIB?

I have a 10' Caribe rib (new to me) and an older Johnson 7.5hp 2 stroke outboard probably mid '80s engine. The outboard starts easily and runs well but doesnt seem to have enough power to get the 4 of us up on a plane.

Me: 130lbs
Wife: 125lbs
Kids: 50lbs ea

It will just barely get up on a plane with just me and the kids and Im not sure if its just a case that the Johnson is simply old and tired and just no longer delivering 7.5hp or if I should be looking for something bigger like a 9.9hp. Ive gone over the motor and am pretty sure I have the right mix gas and fresh plugs. Like I said it starts right up and doesnt smoke.

I really like the old johnson cause its a simple motor and its only about 65lbs where as the 9.9s are all at or above 100lbs and are 4 strokes which means Ethanol problems while Im cruising in the US anyway.

Curious to know how other 10" RIB owners are powering their dinks.
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Old 30-07-2012, 13:56   #2
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Re: Ideal HP outboard for 10' RIB?

I've used them with both 9.9 and 15hp two strokes. 15 was significantly faster, but 9.9 got the job done very well.
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Old 30-07-2012, 13:58   #3
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Re: Ideal HP outboard for 10' RIB?

15 hp Yamaha 2 stroke.
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Old 30-07-2012, 14:04   #4
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Re: Ideal HP outboard for 10' RIB?

Quote:
Originally Posted by ohdrinkboy View Post
I've used them with both 9.9 and 15hp two strokes. 15 was significantly faster, but 9.9 got the job done very well.
I seem to remember hearing a long time ago that the 9.9 was really equivalent to a 15 and that the hp was rated at the prop so you just needed to switch the props to make a 9.9 deliver 15. There was some regulatory reason for the 9.9 as certain lakes would not allow anything over 10hp or something and so the OB industry responded by making the 9.9. Does any of this make sense or is it just urban legand?

Also curious on what sort of weight a 9.9 or 15 for that matter are as 2 strokes? Im pretty sure they are lighter than the 4 strokes but not sure how much.

Is the consensus then that my 7.5 is probably just not powerful enough to plane the 4 of us?
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Old 30-07-2012, 14:24   #5
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Re: Ideal HP outboard for 10' RIB?

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Originally Posted by SV Demeter View Post
I seem to remember hearing a long time ago that the 9.9 was really equivalent to a 15 and that the hp was rated at the prop so you just needed to switch the props to make a 9.9 deliver 15. There was some regulatory reason for the 9.9 as certain lakes would not allow anything over 10hp or something and so the OB industry responded by making the 9.9. Does any of this make sense or is it just urban legand?
The 2-stroke Tohatsu=Nissan had something like this: 6, 8, 9.8HP where very similar, but not just the prop, the carb needed different jets. (I bought a 8HP, made it a 9.8 later).
The 9.9 and 15HP 2-strokes also were very similar, but I didn't spend time to go through the partslist to find differences.


Quote:
Also curious on what sort of weight a 9.9 or 15 for that matter are as 2 strokes? Im pretty sure they are lighter than the 4 strokes but not sure how much.
Tohatsu 4 stroke 9.8HP: 81.5lbs
Tohatsu 4 stroke 15HP: 114lbs
Yamaha 2 stroke 15HP (not available in the US, I think): 36kg ~80lbs

Quote:
Is the consensus then that my 7.5 is probably just not powerful enough to plane the 4 of us?
yep. get a lightweight 2 stroke 15HP when you get to destinations which sells them. Until then, take your time.

Dirk
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Old 30-07-2012, 14:48   #6
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Re: Ideal HP outboard for 10' RIB?

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Originally Posted by LeaseOnLife View Post
The 2-stroke Tohatsu=Nissan had something like this: 6, 8, 9.8HP where very similar, but not just the prop, the carb needed different jets. (I bought a 8HP, made it a 9.8 later).
The 9.9 and 15HP 2-strokes also were very similar, but I didn't spend time to go through the partslist to find differences.


Tohatsu 4 stroke 9.8HP: 81.5lbs
Tohatsu 4 stroke 15HP: 114lbs
Yamaha 2 stroke 15HP (not available in the US, I think): 36kg ~80lbs

yep. get a lightweight 2 stroke 15HP when you get to destinations which sells them. Until then, take your time.

Dirk

Thanks will keep my eye out for the 2 stoke yamaha 15, maybe used in the US? Im not cut from the brand new outboard cloth...
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Old 30-07-2012, 14:53   #7
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Re: Ideal HP outboard for 10' RIB?

If i was buying, I would get the biggest motor the dink will take and you can afford. Usually people don't complain about too much power.
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Old 30-07-2012, 15:00   #8
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Re: Ideal HP outboard for 10' RIB?

I have for few years a Yamy 9.9 4 stroke, love it, with a full tank i can run the tender for 10 days so far, very economic, great torque under load, with 2 or 3 onboard can be in a plane quick, dont burn oil, dont foul plugs, start always at first pull, very quiet, to me a great engine until some idiots stole it in St Lucia, the only problem to me is the weight, is heavy.

Now i have a 15hp enduro, this one is a 2 stroke nice engine to, cons: burn oil, my tank full last few days , easy to repair,reliable, and is lighter compared to the 4 stroke , probably my next engine will be a 15 hp 4 stroke , mainly because i dont have much troubles to lift the heavy engine to my boat anyway .

Good Luck
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Old 30-07-2012, 15:01   #9
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Re: Ideal HP outboard for 10' RIB?

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Originally Posted by ohdrinkboy View Post
If i was buying, I would get the biggest motor the dink will take and you can afford. Usually people don't complain about too much power.
They complain when they have to haul said outboard aboard or have to buy gas for it though or have to drag the dinghy up a beach. I want the smallest 2 stroke I can find that will get my family on a plane.
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Old 30-07-2012, 15:07   #10
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Re: Ideal HP outboard for 10' RIB?

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They complain when they have to haul said outboard aboard or have to buy gas for it though or have to drag the dinghy up a beach. I want the smallest 2 stroke I can find that will get my family on a plane.

In this case at least you need 9 hp, we talking about 300 pounds, you dont see much diference to haul the boat in the beach between a 9 and a 15, is just a bit more of pull, unless your family dont tilt the engine up...
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Old 30-07-2012, 15:09   #11
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Re: Ideal HP outboard for 10' RIB?

Caribe 10x (heavy boat) with Merc 15 Sea Pro 2 stroke

gets on plane with big 3 adults, but not 4

Merc gets abused mainly from lack of maintenance, very simple to work on. Runs on any gas, even over a year old. I do keep the mix at 50:1, thought about trying less oil, but figured more oil makes up for my lack of maintenance.

AFAIK, these are still available new in Bahamas and Mexico.
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Old 30-07-2012, 15:16   #12
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Re: Ideal HP outboard for 10' RIB?

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Caribe 10x (heavy boat) with Merc 15 Sea Pro 2 stroke

gets on plane with big 3 adults, but not 4

Merc gets abused mainly from lack of maintenance, very simple to work on. Runs on any gas, even over a year old. I do keep the mix at 50:1, thought about trying less oil, but figured more oil makes up for my lack of maintenance.

AFAIK, these are still available new in Bahamas and Mexico.
Had a 2001 Yamaha 15 hp 2 stroke, used to use 100:1. Now I use 50:1 in my new Yamaha 15 hp 2 stroke. I think Yamaha recommended 100:1 in the US due to EPA. The same engine bought elsewhere 50:1 is recommended. By the way they are still available in the Bahamas.
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Old 30-07-2012, 15:40   #13
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Re: Ideal HP outboard for 10' RIB?

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Originally Posted by ohdrinkboy View Post
If i was buying, I would get the biggest motor the dink will take and you can afford. Usually people don't complain about too much power.
+1

A 15HP 2 stroke Yamaha was $2500 last year at Mins in Georgetown, Exumas last year. A few hundred less if you can stomach Nassau.
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Old 30-07-2012, 16:55   #14
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Re: Ideal HP outboard for 10' RIB?

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+1

A 15HP 2 stroke Yamaha was $2500 last year at Mins in Georgetown, Exumas last year. A few hundred less if you can stomach Nassau.

Minns Watersports in Georgetown sells the Yamaha 15 2 stroke for $2300. They include the first service (after the break-in) at this price. They change the gear oil and check everything else out, compression etc.. In addition, when you buy the engine, they will take your old engine off and mount the new one on your dink. In Nassau the same engine is $2100 but all you get is the engine in a box. No service at all. Although the engine is about 80 pounds it's still no lightweight and it sure helps to have some young, strong guy hump the engine on and off for the service.

If you want to save $100 and you're in GT you can buy the engine from the dealer in Nassau and he will put it on the mailboat. Freight is about $100. You'll have to change the oil yourself after the break-in.
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Old 30-07-2012, 17:31   #15
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Re: Ideal HP outboard for 10' RIB?

Demeter

We have an Achilles (Double Floor) 10'5" foot RIB with a Yamaha 15 hp Four Stroke Engine...

My Yamaha has been very reliable and starts everytime. It is incredibly thrifty on fuel; I even reduced the on board gas tank from 5 to to 3 gallons.

I have found Yamaha parts in every country we have visited, which could not be said for some of the other brands.

When we first bought the engine it was a little slow out of the hole and didn't plane right with 400 lbs of passengers/cargo or more.

I went to a new prop with 1" less pitch, which allows the raft to plane with the same weight. On the negative my top end went from 25 to 22 mph.

On the down side, the Yamaha 15 hp 4 Stroke Engine weights 114 lbs compared to the 15 hp 2-Stroke weight of 84 lbs. Because of that you will need a crane or lifting system of some kind lift it on and off the dinghy.

Also, do not use your RIB's recommended horsepower to select your engine.

You need to check your raft's recommended engine weight on the data plate... Older rafts were designed for two stroke motors and normally have a max transom weight of 90 lbs, but say "rated for 15 hp". New rafts are built for the heavier 4-Stroke motors and allow for more weight on the transom.

Good Luck!
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