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Old 11 May 2004, 15:31   #1
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Single outdrive, any differences from single o/b? Advice

Hello all,

I am new to this, it has taken me quite a while to work out how to post!

I have viewed the forums, with interest, for a while as a guest, but have finally decided to register (still don't trust this internet thingie).

I have a question that the varied operators of RIBs of all configurations may be able to assist with.

I have experience of single (115hp) and twin o/b (x2 50hp) set ups, a dory and rib respectively. During close quarter manoevouring these are different, but easily managable. If pushed, I prefer twins.

I am just about to make the switch to a alumiunium RBB with inboard 1.7 Mercruiser (which thankfully seems popular and good). WHOOPS pushed something and posted accidently there I'll continue....

diesel that will drive mercruiser Alpha leg. This is where I know nothing. How similar are outdrive to outboards. The application is for estuarine waters, beaching frequently, and with sustantially amounts of debris floating about on springs.

We run S/s props and generally are pretty good at dodging the big stuff, but clearing the prop mid channel is normal. So can I trim an outdrive out and fiddle about with it at anchor, like an out board?

Also, while I'm here is the slow speed handling likely to be the same with a diesel o/d, or is it gonna be props? Have had bad experience of big prop 21" with 115hp Mercury at idle speed once! Supplier knew nothing, we ended up on 17"!

thx for any comments.

t
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Old 11 May 2004, 16:12   #2
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Outboards & Outdrives

If you can handle a single outboard then you should be fine with a single outdrive.
The only problem I find with customers is that they are used to seeing which way the outboard is pointing, something you cannot do with an outdrive unless you have an indicator.
When I have teached on boats with the 1.7 diesel I have always found the slow speed tick over gives you abot 4-5 knots so be carefull when selecting forward as there is alot of torque and you move away quite quickly.

Best of luck with the new RIB.
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Old 11 May 2004, 17:13   #3
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Having had twin o/b on my last boat and a merc 1.7 on my current boat, I would say that getting to the prop was probably easier with the o/b. That's partly because the alpha 1 leg doesn't come as far up as an o/b (only just above the water.... but better than some legs I believe) but mainly due to the extra height of the transom in my diesel configuration. May be worth looking at that too! How long are your arms?


Mike C
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Old 11 May 2004, 17:32   #4
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Low speed handling won't be quite as easy because of the huge ammount of torque at tickover. Apart from that i think its a great choice. The leg does tilt far enough to dry the boat out but it won't be easy to reach the prop from in the boat.
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Old 11 May 2004, 17:50   #5
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Thanks

Thanks very much for that fellas,

It will have a 5hp o/b donkey engine if it all goes really pear shaped, so we won't be totally helpless. Plus there is going to be a protective cage/frame (bump guard, not sure what they are called?) around the o/d, above the waterline. I'll have a look at this when we take her out and see if we can clear/change props on the water. If not, may see about getting a step plate or something so that we can get to it. Drysuits are the norm, and normally we will be able to drop the hook and have a think before it gets too scary.

I am looking forward to the sea trials, manufacturer has been exceptionally good thus far and we have ended up with a bespoke vessel that has to meet v. specific and unusual requirement, inc Brown code for Cat 6. Not trying to be cryptic, but as it is a workboat (of sorts) and we haven't actually got our mitts on it yet, then I don't want to say too much yet. After trials, and with official sanction from work, I'll let you all know.

Thanks again.

t
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Old 11 May 2004, 18:02   #6
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T, think you will find several differences. Firstly althought the leg won't trim up as already mentioned you can lie on it to reach the rope round the prop or use the brush handle / boat hook which i find quite useful to carry.

At slow speed the turning circle may well be substantly smaller than an outboard rib because the prop is mounted quite a bit further aft giving better leverage or thrust angle on hard lock. Whiteshoes commented last weekend that he was suprised I was able to turn a Pacific 22 in her own length just with full lock and at slow speed.

Not being able to see the drive can be a problem but quicksilver do a tilt switch/guage which would be worth the extra couple of quid.

Pete
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Old 11 May 2004, 18:14   #7
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Quote:
Originally Posted by simmons0
Low speed handling won't be quite as easy because of the huge ammount of torque at tickover.
Why would that make it more difficult?

A diesel RIB with lots of torque is very easy to manouever at low speeds because you can get lots of turning moment with very little forward speed. With an outboard you need to use more revs and can easily end up with more forward speed than you really want!

John
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Old 11 May 2004, 18:16   #8
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Interesting

Pete,

Thank you. Hadn't thought of that, but it makes sense. We often set nets (bow on) from the foreshore and these require a fine degree of control with the engine generally trimmed up quite high (just below the surface and pulling) running astern so that we can pull the boat and the net away from, the shore. From what has been said these o/d's don't seem radically different from o/b's. Apart from potentially more rapid 'take up' due to torque, there aren't likely to be any major differnces running astern, other than higher turning circle (which could be a major plus!), are there?

Finally, whilst I have a pool of knowledge to tap into.....are there any major differences in the maintenance regimes etc of o/d's v o/b's? Other than cooling which I'm sure someone will mention ,but setting the nets only takes about 3 mins, so gearbox, engine etc should be ok for brief period (Has been with Yams for over 10 yrs!).

thx,

t
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Old 11 May 2004, 19:03   #9
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Yes a trim guage is a must because you cant see what the leg is doing behind the transom.

JK - well you can get a lot of turning movement but it doesn't come at a low speed because the moment the engine goes into gear you are on the move quite quickly.
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Old 11 May 2004, 19:16   #10
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Try getting the wheel hard across while you are in neutral then just engaging forward or astern at tickover for a couple of seconds. Works a treat!

John
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