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Old 25 May 2010, 04:58   #1
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SIB query from marine mammal strandings project - Your help please

Was wondering if the RIBnet masses (or should that be Massive! for real, aiyee) might be able to assist with a query for the Marine Mammal Strandings project (a good cause)....

I was speaking with them yesterday and they are struggling with their disentaglement boat and getting it to plane...fine when working with a cruising whale at 4 knotts, but a problem catching up with it if they fall behind!

Info on the disentanglement boat is:

Boat: Avon light inflatable CRRC 450 MK2 no HP system.

Engine: Yamaha 30H 2 stroke.

Prop: Standard fitment 3 blade 9 7/8 * 10 ½ - F.

The outboard cavitation plate was 25mm below the bottom line of the boat but the prop would readily cavitate with great difficulty getting up onto plane (all tubes inflated until blow off valves operate).
The engine was dropped down further by taking an additional 25mm off the transom but appeared to have little effect.
All trials with 2 persons on board, full tank of fuel, in calm conditions.

They are currently of the opinion they need a different prop....


So, can anyone make any informed suggestions on what else to try, what prop, the transom mounting etc etc....

Any and all help would be appreciated!

Dan
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Old 25 May 2010, 07:01   #2
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What speed do they get as it is, Avon list approx 20 kts and max hp is listed as 40hp.
Do they have prop guards etc fitted?
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Old 25 May 2010, 07:34   #3
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Quote:
Originally Posted by chewy View Post
What speed do they get as it is, Avon list approx 20 kts and max hp is listed as 40hp.
Do they have prop guards etc fitted?
Hey Chewy...won't plane and around 5 to 6 knotts....AFAIK no prop guard...

Thx D.
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Old 25 May 2010, 07:53   #4
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Same old questions.... what rpm does the engine reach, does it sound like its making max rpm, what gear is on the boat, can you try the engine on another boat to make sure that its running as it should?
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Old 25 May 2010, 08:18   #5
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Quote:
Originally Posted by Dan Worth View Post
The outboard cavitation plate was 25mm below the bottom line of the boat but the prop would readily cavitate with great difficulty getting up onto plane (all tubes inflated until blow off valves operate).
The engine was dropped down further by taking an additional 25mm off the transom but appeared to have little effect.
All trials with 2 persons on board, full tank of fuel, in calm conditions.
Dan - thats not a dissimilar size of boat to mine and a bigger engine so I'm surprised they are struggling to break through 5 knots. Off the top of my head the prop is the same (or very similar) size to mine too - and I get 20 knots.

There are some silly questions first... Does the bow go sky high? Does it help if they move weight (crew + fuel) forward. Is the engine trim set right? its not stuck on shallow drive or anything silly is it (thats the only time I have failed to get on the plane)? Is the prop damaged at all.

What do they mean by cavitate? do they mean ventilate (which is what most people mean) which is odd of the prop is 2" deeper than "normal" and not turning hard - or do they really mean cavitate (breaking down water). Both seem unlikely if everything is set up right. So could the prop bush be slipping (very difficult to test) - but high revs and low speed would be a sign?
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Old 25 May 2010, 11:10   #6
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Simple (and relatively inexpensive) thing to try would be to add a hydrofoil (or similar device) to try and isolate the prop from the surface of the water.

Are they sure they aren't spinning the hub of the prop? Shouldn't be ventilating with the prop that deep.

jky
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Old 25 May 2010, 15:39   #7
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Thanks all...will fwd on your queries and get back to you....your continued assistance really appreciated!
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Old 25 May 2010, 16:08   #8
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Is it trimmed all the way in on the pin?

The 5-6 knots does sound like the hub may be spun. Tell them to mark the hub and nut and see if the marks are still aligned after whacking the throttle open.
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Old 25 May 2010, 16:13   #9
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as chewy and jyasaki said ,think i would be looking at the engine perhaps not getting full revs ,,eg ,thottle out of adjustment,,or a jet in the carb blocked up and the prop hub spinning before i did anything else ,
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Old 26 May 2010, 05:51   #10
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Okay guys....sent MMS a link to the thread so far and they responded with their thanks and the following....

"No tacho so no idea of revs but revs go sky high when cavitates/ventilates. Tried various trim settings and obviously moving weight forward to get it on the plane. (plane can only be achieved on glass like conditions and even then it might cavitate/ventilate for no apparent reason). All kit is new but I’ll mark the prop and nut and check the hub is not spinning next time we drop it in the water."

Strange one isn't it....they had spoken to Yamaha who advised they go to a local dealer and try out various props.....we don't have any local dealers, and the local boatyard wouldn't lend any props (helpful)....

Not sure when they will have chance to test again....main worry is that it wouldn't work if they needed to use for real....any other feedback /suggestions appreciated. General consensus here is that the prop as detailed should be okay?

Thx. D.
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