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Old 17 September 2006, 14:57   #1
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How fast is your auxilary

Going to buy suzuki 6hp 4 stroke as an aux this week, it will probably be 3 weeks before i can actuarly try it on the rib.Has anyone got this engine on 6 metre rib and would i get 6 knots at full throttle say with a slack tide no wind.This might be a fun way to keep to the speed limit and keep my main engine hrs down.Main reason for the 6hp is its own inboard tank size and weight.My boat is shallow v by the way.Posibly should have asked in the thread how fasts you boat.Or when someone gets really bored should start how fasts you auxilary then?Should be quite interesting In fact i,ll title this thread not exspecting any seriuos replies though
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Old 17 September 2006, 15:11   #2
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Quote:
Originally Posted by no fear
would i get 6 knots at full throttle say with a slack tide no wind.
Nope- I couldn't get above 3 knots in a 5m boat using a 5hp at full throttle .You're unlikely to make 6 knots-it's probably above your boat's hullspeed when used as a displacement craft.
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Old 17 September 2006, 15:53   #3
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i get 5 knots max from my 8hp mariner 2-stoke (my boat is 6.5m)
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Old 17 September 2006, 16:02   #4
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max speed is square root of waterline lenght x 1.34. So maybe 5 knots.
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Old 17 September 2006, 16:54   #5
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Quote:
Originally Posted by Limey Linda
max speed is square root of waterline lenght x 1.34. So maybe 5 knots.
I'm assuming you are using feet and knots in the calculation?

Interestingly I've seen some variation on the 1.34 factor...
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Old 17 September 2006, 17:18   #6
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Quote:
Originally Posted by Richard B
I'm assuming you are using feet and knots in the calculation?

Interestingly I've seen some variation on the 1.34 factor...
Yes. feet and knots. I have also seen variation in the 1.34 factor and I assume it is due to water salinity/density, temp. etc. Also this factor is only applicable to true displacement hulls that have no capability of planing. However, it is close enough for all practical purposes, I believe. The primary point being that it does not matter how big an auxilary you put on the max. speed is the hull speed unless the auxilary is big enough to allow the vessel to plane. BUT,and it's a big but, if you have an adverse tide of say 3 knots then a 5 knot hull speed is only going to give you 2 knots over the ground.
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Old 17 September 2006, 18:13   #7
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On my 6m rib, I get 4.3knots at WOT on my little Suzuki 4hp 4/s engine around the dock which has no tide. It's not fast by any means, but it'll get you home as long as you aren't going against 5knots of tide

-Alex
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Old 17 September 2006, 18:46   #8
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yep me too

I intend having this engine fitted by a builder because the 4/5/6 are all the same weight but the 6 has the option of a fuel connection direct from the main tank. I was told by some one who new 1 hp for every meter..it wll be two weeks until I have to make the final decision so let us all no how you got on.
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Old 17 September 2006, 18:47   #9
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I have only had mine out once since I had the aux on but I have the Johnson 6hp 4 stroke which is the same engine except painted white and with a different name on the outside - they are all built by Suzuki.

In a very short trial I clocked 5.5 knots on the GPS with a quick burst on full throttle (about 30 sec) but this was with a bit of a tailwind (about 15 knots on the rear quarter, not a direct tailwind) and I'm not sure what the water movement might have been. I didn't give it any longer than that because the engine isn't yet run in but it was a bit choppy which probably countered the tailwind, on flat water it may get to 6, not sure. This is on a 5.8m Humber Destroyer, one fat person (me) and not much else, main engine out of water.

Using the formula above I ought to get about 5.8 as the max. I can say one thing for sure; you don't realise how much wind a RIB catches until you're trying to control it with a little tiller steer aux engine and suddenly find you need 45 degrees on the tiller to keep the boat going in a straight line into wind

I wouldn't like to have a main engine failure offshore with 20 knots of wind blowing, but it still beats swimming

I also bought mine on the 1hp per metre rule, and also because of the light weight per HP and the external fuel input.
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Old 17 September 2006, 18:54   #10
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I think the message so far is that " bigger is not always better"

Regards, to all.
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