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Old 07 July 2011, 08:43   #1
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SIB Advice

Afternoon Chap's.
This is my second thread in as many days, as you could write my knowledge of boating on a pin head i thought i'd start again.
A while ago on Boats&outboards, i bought myself a 2.7m Plastimo air-deck inflatable boat.
It was a steal, it cost me 50, it was being sold on behalf of church by a boat broker.
It turned up & it's perfect, if a little dirty.
I then went & bought a Johnson Seahorse 6HP outboard from a local marine engineer. Now this is old, i'm talking 1970's style old, although it does what it was bought for, to push the boat along faster than my Seagull did, we were going to use it to get to coves to do a bit of Spearfishing.
Now as great as that is, i'm bored of how slow it is. Call it the biker in me but i like something with a little "go".
I'm after advise really, i'd imagine the boat isn't ideal for any sort of high speed & as such i don't want to be going lightening fast, just quick enough that it blows your hair about a bit.
I don't really want to sell the boat as my money pit isn't very deep, what i'm after is the best sollution, i want a motor thats 2S, short shaft & with a bit of poke.
Life Jacket etc will be worn & i'm a very compitent swimmer.
Could anyone advise me on in real terms i could get away with on this size boat that would be effective? It says on the panel it's rated to about 5-6HP.
The guy i bought the outboard off said i could probably go fairly bigger, but over time, constant use i might damage the boat, it's going to be getting used 1-2 x a week so it's not going to happen over night & i'll probably be bored of boats in about 12 months time!

Any advise for this high on enthusiasm, low on money & knowledge is much appreciated!
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Old 07 July 2011, 10:59   #2
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A friend of mine has this same boat, his is about 10 years old and is actually a 2.65m, they now make the 2.7m in it's place. His is rated for 8hp.

In many ways it is similar to my QS 2.7 airdeck, which is rated to 10hp. So I'd say your safe to go to 8hp anyway. Yamaha made a nice 8hp but price may be an issue. And the heel of the hunt, the weight on the transom is important and if you're buying a vintage 2 smoke, that will be lowish. The biker in you may be more important - planting her in the holeshot will put huge strain on the boat, so a more measured approach may be needed if you overpower.

FWIW, my rig has shown a max GPS logged speed of 18.5kts (bare boat), with 17.5kts being easily achieved with safety equipment etc o/b.

Speeds like this are only possible in very flat water. Planing speeds (10kts) can be maintained in an average F3 and life gets worrisome in a F4, so a very fair weather outfit indeed...
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Old 07 July 2011, 11:06   #3
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In all fairness, the SIB is only going to get used on either mirror flat water, or a very small / light wind condition, for that reason firstly but also as i'm learning to Kitesurf so i'll be kiting on the windy days anyway!
I've been offered a 8hp Yamaha for circa 450 by someone i know.
My question is though, is the 6hp i have (obviously isn't going to be as powerful as a modern day 8hp) but what is the power difference going to be like?
I'd like it go go a bit quicker than it does at the moment, which isn't very fast, i'd say it's probably about 8knots at best. (i presume that's just down to it being old and having lost power).
Is 10hp going to be too much for it?
I'm not going to sit there planting it all day...
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Old 07 July 2011, 14:37   #4
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I don't think I can say whether or not you should fit a 10hp on a transom rated for 6hp - I'm sure you're big and bold enough to work out the pros and cons of that one yourself - it's a calculated risk...

As to the speed of your rig as is - the 6hp might be losing a touch of compression and lacking power. You don't say if it actually planes the boat? If it doesn't, then it's probably not up to snuff. The other thing I should have mentioned was tube pressure. These little things rely on pressure to create the rigidity needed to perform well so tube pressure should be spot on, only manageable with a gauge. I have no idea what the pressures in the Plastimo are but the QS is Tubes 3.6psi - Keel 6.4 psi and Airdeck 11psi.

Lastly, a small bit of weight or power can make a big difference to a boat like this, so the difference between a clapped out 6hp and a decent 8hp might be more useful than you'd think. Would he let you have it on appro for a day?
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Old 07 July 2011, 15:10   #5
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Originally Posted by willk View Post
I don't think I can say whether or not you should fit a 10hp on a transom rated for 6hp - I'm sure you're big and bold enough to work out the pros and cons of that one yourself - it's a calculated risk...

As to the speed of your rig as is - the 6hp might be losing a touch of compression and lacking power. You don't say if it actually planes the boat? If it doesn't, then it's probably not up to snuff. The other thing I should have mentioned was tube pressure. These little things rely on pressure to create the rigidity needed to perform well so tube pressure should be spot on, only manageable with a gauge. I have no idea what the pressures in the Plastimo are but the QS is Tubes 3.6psi - Keel 6.4 psi and Airdeck 11psi.

Lastly, a small bit of weight or power can make a big difference to a boat like this, so the difference between a clapped out 6hp and a decent 8hp might be more useful than you'd think. Would he let you have it on appro for a day?

I doubt it' he's ooop north & i'm down south!
I don't have a pump with a gauge, can you recommend a reasonably priced one?
I've only used it once & that was 2 up.
It did'nt feel fast with 2 of us on at all, i doubt it would plane tbh as its a 35 year old engine!
Ill see what i can come up with regards to a new engine.
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Old 07 July 2011, 15:46   #6
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Pressure gauge idea from Max. If you can fit one of these into whatever pump you are using it would be very handy...

The difference between one and two up on one of these things is huge. One up (20 stone load) mine is a firecracker, two up (35 stone) it is decidedly sluggish and will plane at about 14kts, but a F3 will stop it in it's tracks.

As I say, weight/power is everything...
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