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Old 06 July 2006, 15:11   #1
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Country: UK - England
Town: Towcester
Boat name: Treliska
Make: Ribcraft
Length: 6m +
Engine: Outboard 225 Optimax
MMSI: 232009522
Join Date: Feb 2006
Posts: 187
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Prop Advice needed

OK, second post on this topic so i hope no one gets bored...

We have a Ribcraft 6.3 with a 150 Opti on, that regularly carries 10 diver plus kit plus 2 crew. Total load probably in the region of 1300 kgs.

At the moment it is turning a 3-blade vegance, 17P. Loaded it is VERY slow to plane, and tops out at 4800 rpm. Manual says WOT 5600-5750. At 5600 rpm (3 up plus a bit of kit) we get 38 knots.

Interesting part is....

One mariner dealer said "change to a 15p prop, 13 is WAY to low" he actually said the engine would explode, but i think he was joking.

Another dealer said "go to a 15p, 4 or 5 blade prop, to get better pick up"

I think the prop diameter is 13.5, so my logic is to get a 14.5 x 15p 4 or 5 blade prop as well....

Except, steel developements said, "there is no 13.5d vengance, it must be a 14.5, you need a 15.5 x 13p, and they only come in the 3 blade flavour, 15p won't give you enogh extra rpm" The trouble is, this is different to what they said the last time i rang them with the same numbers!

I then rang Ribcraft to ask their advice, and they said they couldn't comment on a specific boat, and to go and try it. When i said i had, and gave them the numbers they said i needed to get a different prop...(doh!)

So.... HELP! Does anyone have any opinions? I don't care if it's guesswork, calcualtion or came from your tea leaves.

andy
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Old 06 July 2006, 15:36   #2
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Country: UK - England
Town: Salisbury/Poole
Boat name: Blue C
Make: XS 600
Length: 6m +
Engine: 125hp Opti
MMSI: 235082826/235909566
Join Date: Jan 2004
Posts: 1,439
Check this out Prop slip
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Old 06 July 2006, 15:38   #3
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Country: UK - England
Town: Looe
Make: Delta
Length: 4m +
Engine: Mercury
Join Date: Apr 2004
Posts: 1,335
Yes i would suggest going for a four blade prop it will give you much better pick up. But the smallest pitch four blade prop mercury do is a 17" VenSura Prop. But you need to be carefull when propping your boat as you dont want it to over rev when your running light. Its a hard one to say as you dont want to higher RPM when light but you want performance when heavy. So its going to be a big compromise, or you carry two props!!. Or you make sure no one over revs it when light and you put a 15" on to get better performance when heavy. If all that makes any sense!!
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Old 07 July 2006, 05:15   #4
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Country: UK - England
Town: Towcester
Boat name: Treliska
Make: Ribcraft
Length: 6m +
Engine: Outboard 225 Optimax
MMSI: 232009522
Join Date: Feb 2006
Posts: 187
RIBase
Quote:
Originally Posted by ashbypower
Check this out Prop slip
thanks, but i've looked at a few prop calculators. The're all great, but they don't tell me how much to prop down the boat to get it on the plane. I was hoping someone out there has a similar boat in a similar application.
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Old 07 July 2006, 06:34   #5
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Country: UK - England
Town: Towcester
Boat name: Treliska
Make: Ribcraft
Length: 6m +
Engine: Outboard 225 Optimax
MMSI: 232009522
Join Date: Feb 2006
Posts: 187
RIBase
Well, problem on the way to being solved. Just spoken to Propellor Revolutions in Poole. Thomas who i spoke to really seemed to know his stuff, and was the first person to actually consider the power curve of the engine, and also suggest looking at the height of the engine on the boat.

He also offered me a ali prop to try first , and then decide on buying one. He's been the only person to do this so far. Thus, no more guesswork or tea-leaf reading required . Why don't all prop suppliers do this? I would'nt buy a boat without driving one, or a jacket without trying it for size
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