# Outboard size for smaller inflatable, speed?



## MarkSF (Feb 21, 2011)

I have an Achilles LS4, which is 8ft6. I weight 280 lbs. I'm wondering about what size outboard to get for it. The manual says 6hp is the maximum.

I'm thinking of either going for the maximum, and hoping it will plane, or for a 2.5 or so and forget about planing. There's a 2.5 Suzuki that catches my eye.

So will a 6hp make it plane? If not I might as well just go for the small light o/b. I've heard stories of this size dinghy still not planing with 5 or 6 hp.

Could some people tell me what speed they get with what size engine?

TIA


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## dabnis (Jul 29, 2007)

I had a raft about the same size as yours with a soft floor and a 2HP
Honda outboard. With 2 of us it wouldn't plane but was more than 
adequate going against the wind and chop. Nice thing is that the motor
is light to carry and easy to mount, about 27 lbs. With your weight and another person on board I think a 6HP motor might not get it to plane unless
you have a rigid bottom boat?

Dabnis


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## MarkSF (Feb 21, 2011)

dabnis said:


> I had a raft about the same size as yours with a soft floor and a 2HP
> Honda outboard. With 2 of us it wouldn't plane but was more than
> adequate going against the wind and chop. Nice thing is that the motor
> is light to carry and easy to mount, about 27 lbs. With your weight and another person on board I think a 6HP motor might not get it to plane unless
> ...


So do you know what speed you got? I would like 10 knots. I want to use the boat to explore some of the south bay creeks and the extra speed would be useful.


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## sailortjk1 (Dec 20, 2005)

> Could some people tell me what speed they get with what size engine?


10'6" Zodiac Air Floor
15HP 2-Stroke Mercury
20Knots with me and one dog by the hand held GPS


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## Mark1948 (Jun 19, 2007)

I have a similar Achilles with a 5hp. I have not had the pleasure of it plaining. I suspect the dynamics of the craft are not conducive to plaining plus you will generally be in a mooring area and should be under controlled speed. I find the 5 perfectly workable not excessively heavy then I am not hauling and storing it on board, another consideration.


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## MarkSF (Feb 21, 2011)

Mark1948 said:


> I have a similar Achilles with a 5hp. I have not had the pleasure of it plaining. I suspect the dynamics of the craft are not conducive to plaining plus you will generally be in a mooring area and should be under controlled speed. I find the 5 perfectly workable not excessively heavy then I am not hauling and storing it on board, another consideration.


Thanks - so any idea of your top speed?


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## pdqaltair (Nov 14, 2008)

9-foot inflatable sportboat with 3.5hp and 170-pound crew will just plane at about 12-14 knots. Add ANY wieght, and we just throtle down and go slow.

I'm afraid 2.5 with 280 pounds might be a bit light in some wind conditions. I'd figure either 3.5 for going slow, or 10 to plane.


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## Faster (Sep 13, 2005)

It's quite unlikely you'll plane with 6 HP - esp with any other extra gear or people. I wonder how much more speed 5 or 6 HP will really give you over a 2.5 given you're not going to plane anyway.

But I'd go for the 3.5 just to get the neutral gear... nice light weight still for handling up onto the mothership, and our Nissan 3.5 has proven extremely reliable on our 8 footer.


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## JimsCAL (May 23, 2007)

I agree that a 9.9 would be needed to get up on a plane reliably with any load. A 2.5 or 3.5 would be adequate to push the boat at non-planing speeds - say 4-5 knots. I would go with the smaller engine. I have a smaller hard floor Achilles with a Yamaha 2HP 2-stroke. Engine is only 22 lbs and makes hauling it in and out of the inflatable a snap. I never tow the inflatable with the motor attached.


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## dabnis (Jul 29, 2007)

MarkSF said:


> So do you know what speed you got? I would like 10 knots. I want to use the boat to explore some of the south bay creeks and the extra speed would be useful.


I would guess about 5 knots on a calm flat water day with 2 people and gear in the boat. Not sure what your carrying or mounting situation is but for a little guy like me the light weight is really appreciated. I have a whole lot of full throttle hours on the 2HP Honda over a lot of years and it still runs perfectly.

Dabnis


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## fallard (Nov 30, 2009)

If the max power is 6, you don't want to go for more. Aside from the structural loads, the hydrodynamics might provide for squirrely and unsafe handling at higher power. 

My 9'6" Avon roll-up could get up on plane with a single 190# person aboard and if the wind and waves were cooperating when I had a 6 hp on it. Add another person or let the wind pick up (against you) and forget about planing. I eventually went to 4 hp and gave up on planing all together, but this motor (2 cyl Johnson) was half the weight of the 6 hp and I could lift it to my stern pulpit with one hand. You could downsize to a 2 hp motor, but if you have to go against wind and current, it might take a while.


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## chef2sail (Nov 27, 2007)

We have a 9.6 Walker Bay with a Tahatsu ( same as Nissan) 4 stroke 4 hp. Lowest hp engine with both internal and exteral tank coinnection in all the brands we looked at. External tanks lets us explore for a long period of time without worry about running out. Wife and I combined ar over 300 total weight. (me the majority) and it moves along at 10+

Get it with an extension tiller handle and enjoy hours of gunkholing please like we do. We keep it on davits so its easy to drop and go,

Dave


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## PaulinVictoria (Aug 23, 2009)

We have an 8ft of thereabouts Zodiac, rollup, soft floor with a Suzuki 2.5hp. With both of us onboard (well over 280lbs) and half throttle (we're still running the engine in), we are getting around 5-6knts I think. Might be a little less, have yet to also have the GPS on it. I don't think it will quite plane, although it does lift up pretty well so far. Will let you know how it goes once we can gun it


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## capecodda (Oct 6, 2009)

*2.5 and an 8*

I've got a 2.5 Suzuki and a 8 Yamaha.

The 8 will plane a 10 ft hard bottom. The 2.5 will not, even with just me onboard (150 lbs). But the 2.5 weighs about 30 lbs, and the 8 weighs 60 lbs.

One other consideration, for whatever engines you are looking for, check the weights. I think I remember that the Yamaha 8 was the same weight as the 6 (same block I think, different carb). Might want to buy an 8 if there is no weight penalty.

YMMV


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## sailak (Apr 15, 2007)

My Achilles is about 9', I have a Yamaha 2.5 4 stroke. With me (190) the wife (140) and the dog (20) we cruise along at 5 - 5.5 knots GPS speed and it's a wet ride to weather.


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## ahab211 (Jan 6, 2008)

I have a ten foot, hard-bottomed dinghy with a nine-nine merc. It goes like a bat out of hell but only planes well using an extension and sitting on the front seat! Light is right and I'd go with the lightest motor that'll do the job. A 3.5-5 H.P. Should do the trick.


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## Mark1948 (Jun 19, 2007)

Never look into speed. Just want to get out to the boat and get the sails up. Basic transportation to and from.


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## MarkSF (Feb 21, 2011)

Mark1948 said:


> Never look into speed. Just want to get out to the boat and get the sails up. Basic transportation to and from.


I'd use it for that too, but here in the South Bay there are some interesting places to go gunkholing with a shallow-draft boat. It's a 4 mile run down the creek to the bay from the launch point, so it's certainly nice to be able to do more than 5 knots. Even 7 or 8 knots turns an almost 2 hour round trip into almost 1.


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## lancelot9898 (Dec 30, 2008)

I have a 20 plus year old 10 foot Archilles(wooden floor)SPD Model which is rated at 10 hp and weighs in at 100lbs. With a 5 hp 2stroke outboard(around 45 lbs) it will plane me at 200lbs and my wife at 130lbs with some effort by moving our weight forward. Once on plane we can move more aft and back off the throtle somewhat. Not sure as to the pitch of the prop, but I remember asking for a different pitch than what came with the motor.


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## CaptKermie (Nov 24, 2006)

*Exploring?*



ahab211 said:


> I have a ten foot, hard-bottomed dinghy with a nine-nine merc. It goes like a bat out of hell but only planes well using an extension and sitting on the front seat! Light is right and I'd go with the lightest motor that'll do the job. A 3.5-5 H.P. Should do the trick.


Wow! I'd love to go like a bat out of hell! 
I have a 5hp Honda on my 9'9" Titan/Maxim, inflatable floor& keel and under the right conditions I get 10-12 mph on the gps. I too have to use an extension and be right up front and there needs to be enough ripple and wind on the water to get under the bottom of the dinghy to actually get it to lift onto plane. I weigh 220lbs. Most times I do not make it onto plane but when conditions are just right I do. I bet a 6hp would do it but if I could get a 9.9hp I would. I tow mine with the 60 lbs engine on the transom of the dink. If you are into exploring then you will need the higher HP. My next dink will be 8'6" with a 6-8 hp engine, I love exploring.


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## Freerider (May 1, 2008)

I just picked up a 1998 6hp evinrude 2 stroke today for my 10'6" Zodiac. It planned easily with 2 people and fishing gear. I used to have an older 8hp for it and I couldn't tell the difference between them.


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## Pub911 (Oct 25, 2007)

Glad your purchase worked out. I have an ancient Johnson 6hp that used to get my Zodiac air-floor 9'6'' up on plane with 2 adults, 1 child and a 90 lb yellow lab named Loosie.

I then bought a 10'2'' "AquaMax" with big rounded aft tube ends (vs. the conical ends on the Zodiac) and can't get it to plane with the same load. Just today, I was lighter by 90 lbs (Loosie stayed at home) and we did get it up on plane after shifting the load forward a bit. But it's very touchy.

I appreciate all the back and forth on this thread so I thought I'd share my $.02 especially since I appear to be right at the edge of planing with my set up.

Good luck.


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