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Old 11 October 2011, 17:22   #1
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Sib, 4hp and low speed cruising

For many people it will be strange question because many ones likes huge engines However, I have an idea about cruising (not only) in sib. I am begginer. I've just ordered my first boat. It will be 3,2m chinese sib with airmat floor.

Now I have a problem with choosing an engine. Of course, I want to have a bigger 9.8hp or 15hp. For example two stroke Tohatsu 9.8hp, 26kg. But for me it is still a bit too heavy. I have back problems and I have to take an engine to/from a car. So I can choose 4hp engine which can be 18kg in weight.

But I read that bigger boats (for example sail boats) which are heavy, have small engines like 4hp. A difference is a propeller and it causes the heavy boat to swim slowly at displacement speed.

I know that it is not a good idea, but at least is this a reasonable idea to give my sib an engine like 4hp and change a different propeller for low speed? Would be such solution good for a small wind and small waves on sea?

I want to make small cruises, up to 30Nm in range, near sea coast. I chose a sib because I have no possibility to buy something bigger or even rib. I will cruise with a tent, rest of camping gear and one or two friends. Speed is not important. Or rather I know that with about 250-300kg of loads I would need a big and heavy engine to plane on my sib.
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Old 12 October 2011, 03:15   #2
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Most small outboards only have 1 or maybe 2 propellors available. Changing it really isn't considered important and probably won't change performance much anyway.

30nm is a LONG way in a 3.2m SIB. Maybe if you stay quite close to sheltered places along the coast in case the wind or waves pick up. Because with a 250kg load and 4hp you are going to be out there puttering along at ~6 knots for the whole day and more.
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Old 12 October 2011, 03:48   #3
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I think you need to find a way to move a heavier engine if you plan to do 30 NM cruises. 30NM in good weather in a 3.2m boat close to shore is not stupid but it does need some planning. With 4 HP I think you will be lucky to do 6 knots with that size of boat loaded with all the stuff you would want for that sort of journey - and 4-5 knots is probably more likely. Even in areas with relatively low tidal stream you could find the sea running at a couple of knots so for some of your journey you may be going at half the speed. So a 30NM journey could take 6-10 hrs (more if you get the tides wrong). That is a long time on a small tiller steered boat. 10 hrs at full throttle means you will need something like a 5 gallon fuel tank with you - which will weigh a similar amount to the engine you don't want to lift (you could split across multiple containers - but you don't want to be spilling fuel about on the deck of a sib when trying to change over). Also I'd have thought the boat is going to weigh more than the engine so you will need to be able to lift that. Have you looked at good transom wheels as a way of getting it all from car to water? then its just a case of getting the heavy stuff out the car.
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Old 12 October 2011, 07:55   #4
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Originally Posted by Polwart View Post
Even in areas with relatively low tidal stream
No tide in the Baltic, P.

The water isn't as salty as ours either, and I suspect that they don't have much in the way of sea swell, the Polish lads on the shore here seem unfamiliar with the concept...
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Old 12 October 2011, 15:24   #5
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> Polwart and captnjack - thanks for your answers!

I am not sure, will 8hp or 9.8hp Tohatsu be enough to move 3,2 sib much better than 4hp? I am begginer - I don't know. Will it plane at weight of 200kg (two people + gear) + 27kg (engine) + 30-60kg (additional gas)? If yes, what in case of calm wind and waves. Will not it stop a sib from planning?

It is true that my sib weights 42kg (a hull only 30kg) but I have a luggage trolley. And I even modified it to be possible to use it on stairs. However I have to make some additional test. And note that I lift only one side of package to place it on a trolley. So I lift only half of weight. I also have some idea how to get it to the car - I try to use a line and blocks. But I haven't tested the idea yet. Transom wheels are also good idea.

6 knots is not very bad. This year I made short trip on river and estuary waters. One day we swim over 10h on small sib and without any engine. Only our hands. And we made almost 24km. The average speed was about 2,4km/h. So about 1,3knots! However I realize that a river and a lake is not a sea. A small wind is enough to make a trip impossible.

> willk
A sens of humour is very good . Yes, there are no tides on south Baltic but it has an opinion of sea which has short waves. So, we know what waves are

I live on south of Poland so I have similar way both to Baltic and Adriatic. A bit further are Black Sea, Aegean Sea, North Sea and others.
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Old 13 October 2011, 12:50   #6
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With two adults, and two small kids on board, I found it really hard to get on the plane in my 3.2 with an 8hp engine - however, with only one adult and one kid, I got about 20mph easily, and it hopped right up on to the plane
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Old 13 October 2011, 13:21   #7
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And it is the problem. For sure I will not buy 15hp - the weight. So if I have to choose between non-planning 4hp and non-planning 8hp or 9.8hp what to choose? The lighter, I suppose. Even, if stronger engine has other pros. For example, I can use it at less throttle than 4hp.
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Old 13 October 2011, 15:05   #8
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SIB and 4 hp.

I've got an Avon 311 Rib, and it has a tohatsu 9.8 2 stroke on the back. I tend to carry lots of bits and pieces, like anchor; chain and rope; oars; full tank and spare container; food and picnic (!); pump, other bits of gear and spares, and so on.

With my lady and myself ( total weight about 20 stone ) it hops onto the plane and stays there all day if we want.

So, go for the T9.8. Sure you won't be disappointed.
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Old 14 October 2011, 05:35   #9
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I'll throw this in as a tangent thought - two 4s?

It sounds like you are not too fussed about actual speed, but if the winds gets up in a SIB, a bit more power can be useful. Have a search - there was a conversation recently about twin small engines.

It would also mean if the weather was guaranteed(!) good, you could lave one at home, and mix & match to suit your journey / weather / load etc.

OTher option (probably better if you are mechanically minded) is to look at the old Evenrude / Johnson "mini twins" from the 1970s. They are ridiculously light for the horsepower. A bit basic, but my 2Hp clocks in at 13 Kg - fully fuelled! The longshaft 4 (aux on the rib) is 15Kg.... www.old-omc.de has all the old brochures scanned so you can see if there's anything that might work. Memory says the 6 was a bored out version of the 4, but I might be thinking the 6/8....
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Old 19 October 2011, 17:01   #10
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Thanks for the idea of light Johnson/Envinrude engines and the link. Unfortunatelly, the lightest (12-13kg) ones are without possibility to add external tank. However, these a bit heavier (18kg) are interesting.

However, I won't go for two 4s. I know, it gives flexibility but also some problems.
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