Prop Size Question: Recently bought Shetland 468 / family four

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driscos

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Hi,

We've recently bought a family four sized (Shetland 498) boat with an ex RNLI Mariner 40hp motor.

We're worried that the prop size is too small for the boat.

Does anyone know what prop size will be good for this size (18ft) boat with the 40 hp engine? Is there a mathmatical calculation?

Thanks
Simon
 
I would start with the question - why do you think that the prop size is too small?

Maths and prop calculators will only be able to work from assumptions whilst practical experience on your particular boat will enable very accurate 'propping'
 
Do you have a tacho? That would be your best indication. You'd need to know the right RPM for max power from your engine. As an example my 60hp merc (on a similar sized boat to yours) gives max power between 5000-5500 rpm. In the boat with 2 people I could only hit 5000 so I changed from a 13" to a 12" which gave me just over 200rpm. So now sitting slap bang in the middle of the power band and have 250rpm either side to allow for extra\less load.
 
Thanks all,

We don't have a tach fitted, so that's probably the next thing to do. It seems to us that the engine prop is creating a lot of froth for not a lot of forward motion. It's just a feeling we have.

Simon
 
Thats got little to do with your prop size and more to do with your shaft length :D. Alot of ex RNLI 40's were short shaft (17" transom, I had one), your boat will need a long shaft (22" transom).

Your prop will be cavitating (catching air at the surface). The plate just above the prop should be between level to about 2" below the lowest part of your transom.
 
The water does look very white behind the boat.

If it is the prop length are we better off trading the engine in for a long shaft one? Or can you just swap in a different shaft?
 
Does the engine seem to rev up once the boat (tries to ) climbs onto the plane? Look at the boat out of the water, where does the plate above the prop (anti-cavitation plate) sit?

Measure your transom, anything over 17" needs a long shaft.

If this is the problem you need a replacement engine as it wouldnt be worth converting it.

BTW, your boat isnt 18ft, a shetland 498 is so called because it is 498cm long, making it 16' 4". My boat is an Oldenburg fast fisher 17, it came with an ex-rnli 40hp mariner. I knew when I bought it it wouldn't be right but wanted a bit more power anyway. For your boat you ideally would want a 50-60HP anyway. 40hp is a tad underpowered.
 
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