How fast is your auxilary

[PeterR]

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

[Nos4r2]

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.

[Colin F]

i get 5 knots max from my 8hp mariner 2-stoke (my boat is 6.5m)

[Limey Linda]

max speed is square root of waterline lenght x 1.34. So maybe 5 knots.

[Richard B]

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...

[Limey Linda]

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.

[Alex Brown]

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

[IBWET]

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.

[BogMonster]

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.

[Limey Linda]

I think the message so far is that " bigger is not always better"

Regards, to all.

[badbaws]

My yacht only 5 knots flat out anyway.. hence I bought another RIB!!
I used to have a 2hp honda on my 4m SR and got 4 knots out of it.. I was impressed by that...

[BogMonster]

Originally Posted by Limey Linda

I think the message so far is that " bigger is not always better"

Regards, to all.

I reckon so. I'd hate to try and control a 9 or 10 metre rib with a little aux engine in a decent breeze. I'm gonna do some more practicing on mine before I need to use it in anger (which I hope I don't of course) and also to put a few hours on the engine in case I suddenly need to run the poor thing at WOT for a sustained period, not a good idea on a brand new motor...

The cool thing about the aux is that because you can get 90 deg of lock in one direction, you can spin the boat on the spot. When you open the 6hp to WOT doing this, it fairly whizzes around. I got some odd looks doing rapid pirouettes on the seafront the other weekend, but for some reason it appealed to my puerile mind

[Nos4r2]

Originally Posted by Limey Linda

I think the message so far is that " bigger is not always better"

Regards, to all.

I'd always err on the side of bigger is better.
It makes no odds what speed your aux will push your boat at in flat calm-It's far more likely you'll need to use it in crap weather (sod's law) and a toy engine won't be a lot of use if it can't push you over the waves or against the wind-which you can garuantee is going to be right on the nose(more sod's law).

I played with auxilary sizes on my old boat (a 5m microplus<spit>hardboat) with a full weekend's worth of gear aboard a few weeks before I bought the SR4.
Over 2 weekends I used a 9.8 Mercury, a Seagull 40+(3hp) , a 4hp Johnnyrude and a 5hp Crescent on it in both flat calm, and various conditions including wind against tide with a bit of chop:-

The 9.8 (which was too heavy really and was the engine that was on it when I got it)made 8.5 knots in flat calm but would do it into f4-5 and still made 5 knots in a medium sea with the odd small whitecap. It was a pain having it on a drop-down bracket though and it made the boat too stern-heavy.
The Seagull made 3 knots but managed better than the Crescent and the Evinrude in a slight sea, wind against tide.
The Evinrude and the Crescent both made 3-4 knots in flat calm and were just about usable in a slight sea but were bloody useless in wind against tide.

Bear in mind the Microplus is a heavy boat-but after trying those out I wouldn't assume small is OK. If there's any option get the biggest PRACTICAL.

[PeterR]

Boy i wish i,d asked earlier.I guess as normal it boils down to compromise.weight cost and convience and the unknown.Any aux.is better than none if your main engine breaks down.I have actually ended up today with a suzuki 5 hp done apparently 20 minute work 2003 model although at the time of perchase was told it was 1 yr old(theres one born every day)However it seems almost brand new not a bargain like Codders got but I,m content.I will endevour to try it out soon and post results thankyou all for your input and hope others will also post as i think this info is very helpfull to be on record.

[Alex Brown]

I am aware the 4hp would struggle in a bad sea, but at the end of the day it's an emergency measure. If I'm on my own, and it's rough as anything, I certainly wouldn't be close up to shore, if the main engine dies there are two options available, 1 is the aux engine, and 2 is the anchor with 50metres of rope, and plenty of chain. Both of these together should hold you in some position, which would give time to get hold of the emergency services should the situation be that dire.

Going through some of the smaller channels here, the tide runs faster than 6knots, so all I could expect the small engine to do is steer me through - putting a 6hp engine on wouldn't give much more benefit.

If you want complete peace of mind, get a dual setup like benc which would get you home on the plane on one engine.

-Alex

[Downhilldai]

I use a 6hp 2-stroke Soozook. Never measured the speed, but it's ample to push us along at a decent pace. I'll see what it actually does on the GPS next time out.

[Nos4r2]

Originally Posted by Alex Brown

Going through some of the smaller channels here, the tide runs faster than 6knots, so all I could expect the small engine to do is steer me through - putting a 6hp engine on wouldn't give much more benefit.


-Alex

True-but in that situation you only need it to get you somewhere safe til the tide is in the direction you want. Not exactly desirable though.

I once spent 9 hours beached in Shell Bay after my gearbox self-destructed and the tide was dropping (springs ). I came back in the next day on the auxiliary as soon as I could refloat it. Luckily the POS boat it was on had a small cabin and the dog made a good hot water bottle

[BogMonster]

Originally Posted by Alex Brown

I am aware the 4hp would struggle in a bad sea, but at the end of the day it's an emergency measure. If I'm on my own, and it's rough as anything, I certainly wouldn't be close up to shore, if the main engine dies there are two options available, 1 is the aux engine, and 2 is the anchor with 50metres of rope, and plenty of chain. Both of these together should hold you in some position, which would give time to get hold of the emergency services should the situation be that dire.

I agree with that. I've only just got my aux and when I have been out with a bit of a breeze and only one engine, I have always tried to keep awareness of the wind direction versus any nasty sticky up bits, so if the engine died and I didn't manage to get it going again in less than a minute, I had plenty of room to sling the anchor over the sharp end and yell for help! Not much use doing that if you are only 15 or 20 metres from the shoreline...