SIB query from marine mammal strandings project - Your help please

Razorbill RIBs · 25 May 2010, 08:58

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

chewy · 25 May 2010, 11:01

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?

Razorbill RIBs · 25 May 2010, 11:34

Quote:

Originally Posted by chewy

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.

chewy · 25 May 2010, 11:53

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?

Poly · 25 May 2010, 12:18

Quote:

Originally Posted by Dan Worth

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?

jyasaki · 25 May 2010, 15:10

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

Razorbill RIBs · 25 May 2010, 19:39

Thanks all...will fwd on your queries and get back to you....your continued assistance really appreciated!

Nos4r2 · 25 May 2010, 20:08

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.

m chappelow · 25 May 2010, 20:13

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 ,

Razorbill RIBs · 26 May 2010, 09:51

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.

chewy · 26 May 2010, 12:31

From what has been said it does sound like the bush has had it.

9D280 · 26 May 2010, 12:33

If it's a large SIB, is the keel properly inflated (and holding air)?

if not the rear few inches could be acting as a good downwards deflector for the passing water as the dynamic pressure builds and forces the "hull" into a concave shape.

Slaphead · 26 May 2010, 12:50

Does it stay in the water for long periods of time?....if so any slime build-up on the hull will kill performance.

jyasaki · 26 May 2010, 15:06

Quote:

Originally Posted by Dan Worth

we don't have any local dealers, and the local boatyard wouldn't lend any props (helpful)....

Not that unusual. Most businesses shy away from having a bunch of loaner inventory on hand, even if it does help in the decision making process of purchasing.

I know I have no shops nearby which do loaner props for testing. Wish I did though.

Quote:

General consensus here is that the prop as detailed should be okay?

I think the general consensus was that you can't really tell until a) the prop works as it's supposed to, and b) you get an rpm reading.

But it does sound to me like it should be close, at least.

jky

phillnjack · 29 May 2010, 13:49

if it has launching wheels are they pulled back up into the boat ?

stupid question, but ive done it so i bet others have too !

I have 30hp suzuki on s400 and getting 26 mph no problem with 10 inch pitch prop.

i need a bigger prop to stop engine over revving, so a prop like their one should blast the boat onto the plane.

from standstill i have to hold on real hard to stop myself being launched out the back of the boat.

5 to 6 knotts is real y crazy speed for this type of boat.

maybe they dont have a inflatable keel to get it up ?

phill