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Old 12 May 2012, 17:52   #1
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How can I get my rib to plane?

Hi there

I am new to boating, recently bought a rib which is 2.7m with a 6hp short shaft 2 stroke fitted with dolefins.

It goes quite well on the plane with just me in it but with two and even three passengers it just chugs along trying to plane.

The rib is rated to a 10hp but was wondering if a slightly bigger engine or a change of prop will do the trick?

Any advice appreciated.

Thanks
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Old 12 May 2012, 18:33   #2
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Stick 10 horses on it!
It still may not plane with 3 though.
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Old 12 May 2012, 23:11   #3
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You probably need on average about 7hp per 100kg to plane, so with 3 large people you would probably need more like 20hp...
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Old 13 May 2012, 04:28   #4
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First question: Have you a RIB or a SIB? If it doesn't have a hard hull, it's a SIB. I'm guessing you have a SIB? If so, is it a Slatted deck or Airdeck? Answers on a postcard pls and we'll see what we can do for ya.
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Old 13 May 2012, 05:03   #5
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Removing the doelfins will gain you a couple of knots
They may give the stern lift but you also cause drag especially with a small engine.
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Old 13 May 2012, 08:41   #6
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Thanks all for your replies.

Its a brig falcon 275 with a hard hull and rigid deck. I have a 1997 merc 2 stroke 6hp.

I have read mixed review about the dolesfins if anyone has good advice whether to keep the on or take off.

Thanks again
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Old 13 May 2012, 11:38   #7
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Even though they can help on some craft What you have to think is doelfins were designed for much larger engines and only seem to be one standard size,
the surface area drag on say a 40 hp engine is nothing compared to a much lesser powered engine ,
It's only a 2min job removing or putting them back on to see any difference ,
I have known a few small engined doel finners to cut them down in size a little .
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Old 13 May 2012, 11:43   #8
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Get a 9.8 twatsoo 2 stroker nailed on the back end and then it will bat along nicely.
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Old 13 May 2012, 11:49   #9
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OK. Yeah, lose the doelfins. You might try playing around with the engine trim position - start in the lowest position and experiment. Move all the weight forward - you'll benefit from a tiller extension - £20 well spent. That said - I don't see you planing with three adults, maybe an adult and two kids.

It would go well with 10hp but I still think you'd be doing well to plane with three adults...
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Old 13 May 2012, 12:32   #10
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>>>with two and even three passengers it just chugs along trying to plane.

Is that you plus two/three?

Anyway as some have hinted... simple fact is a 6hp is too small to shift that many folks on the plane. My experience with a 3m SIB and 9.8hp is very lively with one adult, still planes with two adults or one adult and two children. Go above that and it struggles.

Ideally if you want to carry that many folks you need a 3.2m-3.5m with 15-20hp. If you have to keep the 275 then this is the ideal combination...

http://www.looking4aboat.co.uk/%5Cdo...0-15-56-27.jpg

For me doelfins are mostly used to try and correct an outboard/craft/weight issue. Get the outfit right and they are not needed just adding drag as said above.
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Old 13 May 2012, 17:05   #11
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Quote:
Originally Posted by eyespy999
Hi there

I am new to boating, recently bought a rib which is 2.7m with a 6hp short shaft 2 stroke fitted with dolefins.

It goes quite well on the plane with just me in it but with two and even three passengers it just chugs along trying to plane.

The rib is rated to a 10hp but was wondering if a slightly bigger engine or a change of prop will do the trick?

Any advice appreciated.

Thanks
I had a similar problem with three up in a small rigid hull inflatable. I fitted a fin to the front of the boat and the extra lift got it to plane quite easily, it acts like a hydrofoil. The only problem was steerage as extra leverage was required because the boat had more straight line momentum. I corrected this by adding a tiller extension to the outboard engine's handle. The fin was bought online from a chandlers in USA - something like East Marine....
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Old 14 May 2012, 07:03   #12
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Thanks all

I have just taken the dolefins off so I am going to give that a go some time this week if the sun comes out!

I have a short shaft engine, should I stick with that size if I was to change or would a standard do the trick? I see from the photo added Brig is almost identical to mine with a 9.8 tohatsu, the shaft looks longer though. Glad I found this website!

Cheers
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Old 14 May 2012, 09:46   #13
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As Fenlander says really, to do the maths:

Circa 2m SIB + 9.8 2 stroke + 2 adults (not fatties) + 2 small kids (not fatties) = plane*

*with careful weight distribution.

Note: use fatty adults or kids or substitute 2 kids for one more adult = chug city. Ideally a lower powered 3m SIB is better with one adult + 1 kid (unless you are happy river-pootling)...

Solution = 3.5m or larger SIB + 15 to 25 hp
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Old 16 May 2012, 11:02   #14
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Quote:
Originally Posted by eyespy999 View Post
Thanks all



I have a short shaft engine, should I stick with that size if I was to change or would a standard do the trick? I see from the photo added Brig is almost identical to mine with a 9.8 tohatsu, the shaft looks longer though. Glad I found this website!

Cheers
Short Shaft and Standard are the same size (15"), the other sizes are Long (20") and Ultralong (25").

If your boat is designed for a standard/short shaft you must stay with that size.
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Old 17 May 2012, 12:35   #15
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It's the old Archimedes thing. An 8ft 6 boat with 3 adults in will sit lower in the water than a 10ft 6 boat. The engine will have to work harder to lift the smaller boat the extra distance to get it up on the plane. Because it sits lower, it will push a bigger wall of water in front at displacement speeds than the longer boat. It's got to climb over this wall to get on the plane. I've got boats of both sizes, and with the same size (9.8) engine, the bigger one is always "on the the step" earlier and with less trauma than the littly.
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Old 18 May 2012, 04:45   #16
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I'm guessing a 2.5m grp hulled Avon with a 4hp won't plane with one adult and a two yr old then?

Would it with a 6.6hp long shaft 4stroke?
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Old 18 May 2012, 05:33   #17
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I had a 2.4m Avon rib with a (fairly knackered) 5hp Yamaha 2 stroke. I could get that to plane with one if not both my sons in when they were about 6 and 8. I would think that a 4hp might just manage to get a 2.5 m up on the plane with a light load.

That 2.4m was probably the most abused boat and engine I have ever owned. My eldest asked if he could take it out on his own at about the age of 6/7. I said he could, if he could start it, thinking no chance. Big mistake, he managed to start it after 2 or 3 attempts and I dont think the throttle came off the stops for the next 3 years and the hull was mostly airborne, how was I to know he would top out at 6'4", 16st of solid muscle and have trials for the GB rowing squad! (I'm 5'10" and built like a stick!)
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Old 18 May 2012, 06:15   #18
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Quote:
Originally Posted by Oscarguitar
I had a 2.4m Avon rib with a (fairly knackered) 5hp Yamaha 2 stroke. I could get that to plane with one if not both my sons in when they were about 6 and 8. I would think that a 4hp might just manage to get a 2.5 m up on the plane with a light load.

That 2.4m was probably the most abused boat and engine I have ever owned. My eldest asked if he could take it out on his own at about the age of 6/7. I said he could, if he could start it, thinking no chance. Big mistake, he managed to start it after 2 or 3 attempts and I dont think the throttle came off the stops for the next 3 years and the hull was mostly airborne, how was I to know he would top out at 6'4", 16st of solid muscle and have trials for the GB rowing squad! (I'm 5'10" and built like a stick!)
Was yours a hard hull?

Sounds great! I'm 5"6 14 stone, wife is 5"4 and 8 stone, daughter is negligible.

Just buying a 2.5m Avon rib (1995) and may buy a Johnson 4hp 2stroke with it. Think I'll river trial it and see if it pushes it.

Though I have access to use a 6.6hp long shaft 4stroke. May be too heavy, rated to 5hp. Ooh what to do.
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Old 18 May 2012, 07:48   #19
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Actually ours might have been a 2.5m now I think about it. It was a proper Rib, GRP, shallow v hull and solid transom that I used as a tender to the boat I had then. Great little rib but I needed something bigger as my lads got older and didnt want to come with their parents any more in the big boat! aAn 4m Avon with a 40hp on (had two, first one was stolen while being serviced by LS Marine??) soon sorted that out!

I don't think Avon sold that many 2.5s, I never saw another one. If you can get a 5hp you will have a cracking set up, the 4hp might just be a bit underpowered for the 3 of you. (maybe give the 6.6 a go, but the long shaft will be literally a "real drag" and could negate the effect of the extra couple of horses, should be entertaining finding out though!)

It's often said that you have the most fun in the smallest boats, and we had a great time in ours.
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Old 18 May 2012, 10:01   #20
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You have to consider that a 2.7m boat is really a tender, irrespective of design category, and is not intended to be used as a planing sports boat.

Also, many 6hp's are single cyl, and do not have the torque required to get on the plane quickly.

The displacement or effective weight in the water of this boat would cause too much drag to plane efficiently, irresepctive of engine size.

If you can get a SIB or RIB above about 3.2m, with at least a 9.9, go for it! Otherwise, boating isn't going to be much fun for more than 1 person.

As suggested, the idea of shifting weight and using a tiller extension is much more effective that Doel fins, and will reduce drag if you are not in a position to change.
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