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Old 17 June 2010, 09:32   #1
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Motor for Gemini

Hey guy's, i'm a new member and this is my first post. I am active member of a few other forums, however this is the first boating related forum i have joined.

- I have recently purchased a Gemini 2.7 Air Deck. Purpose of this boat is basically a fresh water lake run-about/toy. Emphasis on "toy".

I'm considering a few different motor options and was wondering what people's thoughts were. Basically it will come down to power vs weight, and to what point the gains from additonal power are exceeded by that of the extra weight associated.

Before i go any further, without sounding harsh, i don't need any one's replies about unsafe practices, voiding warranties ect. I have owned small boats, with larger capacity engines before, so i am well aware of they they handle when paired with motor's above that of the manafactuer's reccomendation.

So, the hull is rated to a max of 10 HP. The transom itself is said to be rated to 45 kg's. As we know, 6 -15 are cast from the same block, so i could exceed the max reccomended by 5hp, upto a 15hp, without adding any extra weight to the transom. I have been in the same boat with a 15 hp Merc 2 stroke, one up, on full trim (top pin), and i fell it is capable of a fair bit more, in ideal conditions.

What i want to know is, could i fit a 25hp, weighing 46kg's? Obviously i wouldn't bother looking at 20 HP's due to them being the same weight) I know the transom itself will hold up with the extra weight, but how would it be once under way, and dealing with the extra pressure. I'm familiar with the re-enforcement of allminuim hulls and transom's, but this is my first RIB. Can RIB transom's also be re-enforced in a similiar way?

I know it wouldn't be capable of the same full bore take off's as a smaller lighter engine'd hull, and that i will sacrifice handling, manouverability ect.

But once up and running, could i expect much more top speed out of it? Or is the shape/design ect of the hull, limited before you even reach a certain HP point? Much like that of your typical displacement hull? Would the extra weight ( approx 10-15kg's) be enough to affect it the point where the additional HP is not of any gain?

My goal is top speed on a fresh water lake. I'd just like to have a bit of fun, and show up a few larger boats for a bit of a grin!



Any thought's on the choice of outboard?
thanks to any replies
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Old 17 June 2010, 11:28   #2
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Quote:
Originally Posted by Bus_Boy View Post

Before i go any further, without sounding harsh, i don't need any one's replies about unsafe practices, voiding warranties ect. I have owned small boats, with larger capacity engines before, so i am well aware of they they handle when paired with motor's above that of the manafactuer's reccomendation.

So, the hull is rated to a max of 10 HP. The transom itself is said to be rated to 45 kg's. As we know, 6 -15 are cast from the same block, so i could exceed the max reccomended by 5hp, upto a 15hp, without adding any extra weight to the transom. I have been in the same boat with a 15 hp Merc 2 stroke, one up, on full trim (top pin), and i fell it is capable of a fair bit more, in ideal conditions.

What i want to know is, could i fit a 25hp, weighing 46kg's?

Sounds like what you are seeking are some outboard specification charts, rather than advice from other sibbers on this board.

If you really want to "show up" the larger boats on the lake, why not look up the maximum payload capacity of the boat and subtract the weight of you, your fuel & your gear. With that number, just find the most powerful outboard that weighs less than that & slap it on. Also be sure to post the video.


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Old 17 June 2010, 12:00   #3
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It's a sib, not a rib and no, you can't feasably reinforce the transom so the idea is pretty much a non starter. You'll lose the transom eventually and the extra torque at takeoff is likely to flip you over backwards.
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Old 17 June 2010, 12:25   #4
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It's a sib, not a rib and no, you can't feasably reinforce the transom so the idea is pretty much a non starter. You'll lose the transom eventually and the extra torque at takeoff is likely to flip you over backwards.
That's what I meant to say.
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Old 17 June 2010, 19:30   #5
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Quote:
Originally Posted by prairie tuber View Post
Sounds like what you are seeking are some outboard specification charts, rather than advice from other sibbers on this board.

If you really want to "show up" the larger boats on the lake, why not look up the maximum payload capacity of the boat and subtract the weight of you, your fuel & your gear. With that number, just find the most powerful outboard that weighs less than that & slap it on. Also be sure to post the video.


Welcome to rib.net
Thanks for the reply so far.
I have been comparing the weight's of the HP options
15hp - aprrox 35kg's
25hp - approx 46-48kg's
Obviously give or take a few kg's for different manafactuer's.

The max payload capactiry for the boat is stated at 500kg's.
Myself combined with the 25 hp, plus fuel, would come to approx 171 kg's.
This is assuming though your post wasn't sacrastic ..........?

Like i said, i have a fair bit of exp with smaller boats using larger cap outbaords. My main concern is is the transom holding up to the extra force, and whether the extra weight associated with the hp increase, is enough to cancel out any gains.
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Old 18 June 2010, 06:25   #6
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Like i said, i have a fair bit of exp with smaller boats using larger cap outbaords. My main concern is is the transom holding up to the extra force, and whether the extra weight associated with the hp increase, is enough to cancel out any gains.
Was that experience gained with a 150% "over power" on a 2.7 m blow-up boat?

I would expect you could put the 25HP on it and have no problems at all provided you had the self control and the skill to drive it sensibly (it doesn't sound like it). That might make sense if it you had a spare 25 sitting in the garage; but there's no point on buying one specifically when 95% of the time you'll only be able to use 1/2 its power. You may also struggle to find a prop that would live up to your expectations / the available power: given that its a very light boat.

I don't think the question is how the transom will survive - its which dies first you or the transom.
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Old 18 June 2010, 09:22   #7
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Well, my prev exp comes from owning a Seal Super Skua (11.6') with 40 hp . I owned that for 3 years.

I have driven several T cat's, a 3.1 Ceasar with a 40 Enduro , and a 4.1 Gemini with a 50 Tohatsu, running modified Yamaha 70 carb, prop, plugs, timing, 100/130 ect..

Like i said, this is a toy. All i want to do is get the highest top speed i can, that is it. But anyway's, i might keep the weight down and go with the 15, and look into doing a bit of work to it before it goes on
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Old 25 August 2010, 08:36   #8
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30 hp Tohatsu.First trial this weekend, should be fun!!
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Old 25 August 2010, 08:56   #9
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30 hp Tohatsu.First trial this weekend, should be fun!!
Sweet rig! Going fishing?

Get some good pics for us
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Old 25 August 2010, 09:02   #10
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Don't think i'll be going fishing. More hoping to convince the missus to come with me so she can weigh the bow down to minimise my chances of winding up upside down facing the other way.

If all goes well, and will start building a new thicker floor, raised transom, and adding some hi-jackers, then i'll hit the dam with the GPS and tacho and see what this baby will do !!
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