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Old 20 May 2012, 03:20   #1
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sib to rib????

Hi guys!!!! Say....I have a 11ft. Air floor sib with a 15 Hp. 4 stroke Suzuki.outboard on it. It runs at about 21 mph with one or two people. In it. I would like to get up to 25-26 mph or so with one or two people. I was thinking. On getting a rib instead of the sib. The air floor, and extra drag maybe being a sib, hinders speed? Would getting a rib make the 15 Hp /302cc go any faster??? What if I added a solid aluminum floor?? More weight, but more rigid..., and not as much "flex " in the boat. I have the floor aired up to 10-11 psi...so it should be up to pressure. Any thoughts???? What about the Walker Bay Genisis 310 rib?? Thanks for your. Thoughts and help. Joe
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Old 20 May 2012, 03:42   #2
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would you like rib that you can store ?
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Old 20 May 2012, 03:44   #3
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in your garage ? i have rib that built in 3 sections and removal tubs i cant spell !
i did like my quicksilver 3.80 but everone sweares about bombard a380
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Old 20 May 2012, 04:29   #4
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in your garage ? i have rib that built in 3 sections and removal tubs i cant spell !
i did like my quicksilver 3.80 but everone sweares about bombard a380
complicated

factors affecting speed

down force
mass in boat
mass of boat
mass of engine (2 stroke lighter 4 stroke)

forward thrust
engiine power ( if you have 15hp is it putting out 15hp ie tuning)
Prop condition/size pitch
trim of engine

Drag
hull pressure/shape
rigid hulls shape shallow v deep v
dole fins

so if you go from a sib to a rib you will double your boat weight.

I think your easiest cheapest solution is check your prop/pressure of boat.
youll need a tiny tach ( or similar easy cheap to set up,see you tube) check manual for boat/prop for pressure and working rev range of engine.

pump up boat to correct if not already

engine with tiny tach fitted go out in boat on your own if you have correct prop you should be 200rpm under max at wot (wide open throttle) (my engine range is 4500 to 5500 rpm so with correct prop i should be 5300 rpm with me in alone)

if not change prop pitch ( each change of pitch = 150 to 200 rpm

so going from a 12 pitch to a 11= reduce by 150/200 rpm

going from 12 to 13 = increase 150/200rpm.

if you have the wrong prop this will have a big affect on speed

i have a 4m rib with a 30hp two stroke top speed 28 mph

hope this helps!!
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Old 20 May 2012, 04:43   #5
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Good morning

As well as your air floor being at the correct pressure, make sure your engine is trimmed correctly and you have the correct prop pitch. In simple terms, try the boat with the trim to different positions, and consider that a lower pitch prop will accelerate faster, a higher pitch prop will give better top end.

As long as your boat is configured correctly, you should have no trouble getting the speed you want. I had a Suzumar 360 aluminium floor, with a Suzuki 15 4-stroke, and at around 80kg, it easily achieved speeds of 25mph with one person, so the engine shouldn't be a problem.

The Genesis rib you are considering weighs around 100kg, so this would not increase your speed. An aluminium floor would also increase the boat weight by about 30kg, so the added rigidity would not necessarily improve speed. It is also unlikely that you could add an aluminium floor to your existing boat due to tube position, etc.

If all else fails, you might want to consider an aluminium hulled RIB. Xpro have a 325 that only weighs 42kg. Check it out at www.seaswift.co.uk.
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Old 20 May 2012, 05:07   #6
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I have my engine manual...and I can get prop pitch from 7-11. I have a pitch of 7 and 9 already...would an 11 inch make that much difference. In speed?? The mechanic at the marine shop says no...I
Would not see hardly any difference. In speed at all. Thanks. Joe
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Old 20 May 2012, 05:26   #7
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I would say a 9 degree pitch is about right. If you switch to an 11, it might take a bit longer to get on the plane.

What is the make/model of your airfloor SIB??
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Old 20 May 2012, 06:27   #8
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Delphinus California.
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Old 20 May 2012, 08:57   #9
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I have my engine manual...and I can get prop pitch from 7-11. I have a pitch of 7 and 9 already...would an 11 inch make that much difference. In speed?? The mechanic at the marine shop says no...I
Would not see hardly any difference. In speed at all. Thanks. Joe
Hi i think thats my point on props.

unless the mechanic or someone has experience of your identical hull and the same engine its difiicult to tell .....only one way, rev counter.

can it make that much difference............yes

if wot with the 9 on is up to 200 under max no more speed can be gained, if its over or under speed could be gained.

note make sure your width of prop is correct ie

mine is 9 7/8 x 12 f .......9 7/8 width......12 pitch ......f is the type
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