Help please

[bogwoppit]

Hello folks im new to this boating game and am really enjoying it, i have a 14ft mayland with a 5hp motor on it, everything is fine at low revs but as soon as i try to speed up the prop just seems to keep spinning in thin air i think this is called cavitating but im not sure. At standstill the prop is about 8" below the waterline is their anything i can do to prevent this,

 

[gordmac]

Is the prop slipping on the shaft rather than turning?

 

[RobWales]

Hello folks im new to this boating game and am really enjoying it, i have a 14ft mayland with a 5hp motor on it, everything is fine at low revs but as soon as i try to speed up the prop just seems to keep spinning in thin air i think this is called cavitating but im not sure. At standstill the prop is about 8" below the waterline is their anything i can do to prevent this,

Tilt engine up or remove boat from water, do not start engine and remove the spark plug cap and tie away from the spark plug (do not want any accidental starting here) and put in gear F or R grab hold of prop and try to turn it....if you are able to spin it without turning the engine over then the hub is very much the suspect as opposed to cavitation.

 

[Dave_Seager]

The reason that the propeller may be slipping on the shaft is because they are designed to do that to protect the engine.

Older and simpler engines use a shear pin. This is a rod that is fitted through the propeller shaft at right angles. It engages in a slot in the forward face of the propeller boss to drive it. If the propeller his an object that might stop it from turning the shear pin brakes (shears) and the dirve ot the propeller is lost.

More modern and larger engines usually have a rubber insert in the propeller. The inner core of the propeller is driven by the shaft, the rubber sleeve is fitted over the core and the actual propeller is fitted over the sleeve. The sleeve is bonded to the core and the propeller but the bonding is designed to fail if overloaded.

If the shear pin is brokenb then the propeller usually spins fairly freely on the shaft and it is obvious that it has broken. However, they can sometimes catch enough to transmit drive at low throttle openings. The rubber insert will always provide some friction and can easily continue to work at low power.

 

[kashurst]

check that the cavitation plate is about 1" - 1,1/2"below the keel of the boat

 

[bogwoppit]

Thanks for that, prop doesnt seem to spin independant to shaft, when under power prop seems to surge sometimes fine then not as if it were being lifted in and out of water

 

[kashurst]

the prop needs to be below the keel not the water line - other wise it will suck in air. Does the engine scream to full power when this happens? If so the prop is too high relative to the keel.

 

[bogwoppit]

Think you might have hit the nail on the head there, i will measure it at weekend, the transome is wooden woulld it be ok to take some out thus lowering the prop?

 

[kashurst]

measure the distance from the cavitation plate to the keel first. The cavitation plate needs to be lower than keel so the prop is completely in clear water. you need about one inch clearance. With that measurement you can work out how much lower your outboard needs to be.
if your happy with that - its your boat, your decision. but remember measure twice, cut once.





P.S.
If your not 100% sure get someone else to check it too before you get the saw out.

 

[RobWales]

measure the distance from the cavitation plate to the keel first. The cavitation plate needs to be lower than keel so the prop is completely in clear water. you need about one inch clearance. With that measurement you can work out how much lower your outboard needs to be.
if your happy with that - its your boat, your decision. but remember measure twice, cut once.





P.S.
If your not 100% sure get someone else to check it too before you get the saw out.

Whatever you do and ulness you are a very good d.i.y guy who is comfortable working with GRP? DO NOT go cutting down your transome! unless I'm very much mistaken this area on a Mayland 14 is a grp moulding inside and out hence sealed but with a wooden core, chop it at your peril as water ingress will ruin the whole integrity of the transome....seek professional advice first or buy a longshaft motor.

 

[sarabande]

err, Lakesailor has done just that (transom cutting) on his boat. Customary neat job

I can't find a pic, but it is a practical suggestion.


Measure twice, though

 

[RobWales]

err, Lakesailor has done just that (transom cutting) on his boat. Customary neat job

I can't find a pic, but it is a practical suggestion.


Measure twice, though

All well and good however I stand by my statement....if its a wood encapsulated in grp transome then it needs the waterproof integrity to be maintained.