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Old 18 May 2024, 09:51   #1
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Size of Aux

Looking for some opinions, I have a 5m Humber Destroyer. We mostly explore around Scotland, some of the more memorable trips have been round Mull, round Raasay and Rona and round Cape Wrath. I normally travel with wife and no other boats and I am thinking of an aux. I have a 60hp Suzuki with around 70 hours and not let me down yet so it's really just to get me to a safe place, keep me off rocks or from drifting out to sea if I lost engine.

I am torn between a 3.5hp aux or a 6hp. Reason I exclude 4 and 5 is that 6 is same engine and same weight so why not. I think I'm decided on tohatsu, internal tank, long shaft. 18.4kg/780 vs 25kg/1116. Help me decide, will the 3.5hp meet me needs?
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Old 18 May 2024, 12:18   #2
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Weve found the 3.5hp Mariner four stroke long shaft an ideal aux. on our 5m rib. Permanently stored safely onboard, light enough to be deployed when required and pushes us along at a reliable 5 knots max displacement speed irrespective of load.
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Old 18 May 2024, 12:55   #3
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General rule of thumb is 1hp/metre
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Old 18 May 2024, 13:52   #4
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I agree with Dave generally 5 metre rib 5 hp
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Old 18 May 2024, 20:25   #5
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Quote:
Originally Posted by Pikey Dave View Post
General rule of thumb is 1hp/metre
That is the guideline I understand the standard to be too.

Long shaft vs short shaft is a question for a tape measure, and location. You may have to custom build a mount, which can be a pretty big project or very simple depending on hull design. Mine was a royal pain getting angles right and everything else to fall into place.

Store in place or inside the tubes? For me I want instant access to the kicker. I start it up every time the boat goes in the water, so I know it runs. In rough waves I would not want to be trying to put a heavy 5-6hp outboard onto the transom.

Steer from the helm or the tiller? I prefer the helm as the tiller is not the most comfortable position on "my boat", having to reach back over the seat. With my narrow transom I have to tilt the main up pretty high to allow enough clearance for the motors. There is almost no clearance between them. We built a trial mount barely bolted and clamped, then built the official mount after getting the heights set correctly (Which got modified again as I felt the main needed to go lower during sea trials). It required buying a longer U-bolt for tying down to the trailer. Transom holes for said U-bolt had to be filled and new ones drilled along with other bolt holes. Many RIB's can have the kicker bolt straight to the transom and you are lucky if this is the case. My mount is a couple of inches higher which gets the kicker further away from taking a stern wave.

Make sure to use a dedicated fuel tank for the kicker (I built a shelf for the kicker fuel tank in the anchor locker to move weight forward). I set mine up so I can swap fuel lines either direction with a quick connect.
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Old 18 May 2024, 21:09   #6
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Youre right to consider an auxilliary. That part of the coast has intermittent radio contact, and at times, just a few boats.

Used to have Tohatsu 3.5hp 2-stroke on my Zodiac Pro. Bit agricultural and the saddle and tilt stop was weakest part. Had Ribcraft 4.8 since 2010 and Mariner 4hp has been trusty companion. I could have probably got away with standard shaft, but opted for long. Cracking little engine. Own fuel supply, but I carry 2-stroke oil if I need to refill.

The impellers, or rather the water pumps clog very easily with salt. I carry nylon strimmer wire if the tell tale is weak to shove up. If its hanging on the transom, consider a ratchet strap. As mentioned, the saddle is weakest. Service it annually.

Personally Id go a little larger and get 6hp. Youll probably never need it, but if you do, then the extra horses wont go a miss. Renew starter rope too every couple of years.
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Old 18 May 2024, 21:24   #7
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Thanks for all the advice. I am thinking of the 6hp now. Interesting what you all say about starting it up regularly. I was thinking that if I buy new, start when I get it and then stow it away that would have been better. Seems I am wrong about that one.

I did actually have an 8HP 2 stroke from another boat that I recently sold, I did consider it as an aux but decided I didn't want to faff around with oil at sea and it didn't have an in-built tank either. With space at a premium on a 5m boat I didn't want to carry an extra tank which is why I sold the 8 and was thinking of a 4stroke replacement with inbuilt tank.
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Old 18 May 2024, 22:27   #8
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Originally Posted by Highlandy View Post
With space at a premium on a 5m boat I didn't want to carry an extra tank which is why I sold the 8 and was thinking of a 4stroke replacement with inbuilt tank.
I have an AB450 so roughly the same size boat, but my guess would be a narrower hull on mine.

Built in tank on the kicker is only good to go so far. At a slow non displacement speed what took you an hour travel with the main will take up to 4 hours to return on a kicker, which is why I rigged up my fuel connections to be transferable. You also want an independent fuel source as the main tank may have water in it from say a poor fuel fill.

My boat sits very butt heavy to the point I moved the battery forward into the console and the kicker tank is as far forward as possible. My Mantus anchor is overkill for the boat, let alone the 25 feet of chain, with 375 feet of line, by choice as I scuba dive off an unattended boat and need it to be there when I return...hopefully. For all else there is always a DSC radio in my drysuit pocket. Point being there is more room in a small boat IF you utilize it.

Yeah I know I haven't gel coated it and probably never will in what is an old boat, but it has plenty of epoxy to protect the wood.
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Old 18 May 2024, 22:40   #9
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Yes, I suppose the tank could become corrupt which is the result of the engine failure. However, being a smaller rib with only a 60hp there is no big tank under the seat. What I have is seperate 25l tanks. 2 came with boat but I bought another one after I had to do the walk of shame through a small village on Mull with a petrol tank as I was worried I wouldn't have got back to Oban. When we go on big trips we normally set off with 3 full tanks and use the third, third, third system. One tank out, one tank back, one tank reserve. The chances of all three tanks being tainted, although possible is less likely. Thoughts were, fill up internal tank on aux and refill from one of the 25l tanks where required. I will carry a funnel.
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Old 19 May 2024, 00:18   #10
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You are set then. No need for a funnel as you really don't want to spill fuel into the bilge on accident, just make sure you can connect into the mains connection off a different tank, although I have no clue with your setup IE: switching valves, plug/unplug, etc. All the small 5-6HP outboards have a fitting on them for external fuel tanks. Mine is the opposite, it has no fuel tank. I prefer that as I don't have to worry about either using the fuel up, or dumping it into a vehicle to refresh it. In California we have 10% Ethanol that doesn't last long. You can buy VERY expensive gas that lasts dang near forever in small cans.
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Old 19 May 2024, 15:53   #11
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i have a 5.5 meter humber and my main engine is the max rated for the transom so i could only go with the 3.5 hp or i would have gone over weight i also use a short shaft as i borrowed a long shaft 3.5 to see if it would work and it was dragging the skeg and prop in the water while under way the short shaft doesn't do that both versions push the rib along at 5 knots so iv not lost any speed going with the short leg
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Old 20 May 2024, 09:19   #12
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Interesting, I had always thought a short shaft would not be long enough.

Thanks for all the advice so far guys. The only thing nobody has said yet is what sort of speed difference I will see 3.5 vs 6. I don't mind the extra weight if there is a marked improvement

A few folk have said the 3.5 moves along at 5mn but how fast would a 6 push me I wonder.
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Old 20 May 2024, 09:35   #13
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i think 5 or 6 knots is all your ever going to get thats about the point you start building a big bow wave your never going to get over that with a small engine
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Old 20 May 2024, 14:44   #14
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Because you are sat down at displacement speed you can get away with a short shaft
I used to have suntrip with a 40 long but a 3.3 short shaft worked well as an auxiliary
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Old 20 May 2024, 16:59   #15
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I doubt there will be much difference in speed between the two what will probably be different is the rpm that speed is achieved at. My thoughts are if you want a get you home then go bigger so its not screaming its nuts off for hours on end, if you want a get you out of trouble engine IE something to get you clear of danger or keep you head to wind whilst you await assistance then go smaller. No real hard & fast rules re aux engines & its a difficult decision when your buying something you hope you'll never need to use.
As mentioned as long as the leg is in the water a short shaft will push the boat at displacement speed but reverse will be poor but generally not an issue in the circumstances you'll be using it
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