Propeller advice needed

[adrungil]

Our Linjett 30 weighting 3,6 tons lacking thrust. Engine is YSB8 8HP Yanmar, however I do not know gearbox ratio nor propeller pitch. As I can put engine on full revs without a problem and boat still makes max 4 knots in calm conditions, assumption is the propeller pitch is too small?

- Can somebody help identifying pitch of propeller in picture?



- As there is no space for larger prop (see other picture), assume the only option with minimal drag increase and not going into expensive feathering options is to change the prop to different pitch or larger (wider) blades? Any suggestions what pitch I should look for?



Thanks in advance!

 

[Sailfree]

One solution is to find out exactly the correct size prop for your boat - I can't help you there.

Another is to go for a feathering 3 blade which should firstly shorted the blade length but secondly pick one with a pitch adjustable from the outside - then you can play with the pitch to your hearts content. The supplier will be able to advise on an initial pitch to start with.

 

[fisherman]

http://www.candoengineering.co.uk/ should be able to tell you what you need, given HP, boat length and type, and in your case, maximum diameter. They also supply, refurb or repitch.

 

[cmedsailor]

Your current propeller does not mention 12LH9 by any chance right? Difficult to read looking at the first photo.

 

[Tranona]

You cannot fit a larger diameter prop. The one you have is arguably too big as there is minimal clearance to the hull. The size should be stamped on the hub. If you are getting maximum revs then you could try an inch mre pitch or change to a 3 blade prop. However with the meagre amount of power you have for the size and displacement of the boat you are unlikely to get much more usable speed. Really need double the power to get hull speed.

 

[seumask]

For the size of Boat and Engine power available, your performance sounds reasonable, not far off what I would expect. Not sure you will get much improvement with a fancy prop.

 

[Topcat47]

I drive a 4.6 tonne Nic 26 with a 1GM10 at ~ 6 knots which is virtually hull speed. I can't see a 3.6 toner needing more if the engine is in good nick. You will need around an inch clearance around the prop, so the diameter looks about right, I'd not go bigger myself but the pitch is an area you should consider before going to the expense of re-engining.

There are prop selection algorithms on the web that'll give you a guide and propellor specialists on the Hamble and probably local to you, too, who can advise. I'm not that keen on re-pitching myself, beyond a little tuning, but I had mine balanced and the pitch "restored" last year to good effect. It wasn't much cheaper than a new prop, though.

It's currently "fashionable" to fit engines far larger than necessary and I was "advised" to fit a 2GM when I replied my engine, fifteen years ago. Diesels need to run hard if they are to remain in good health and at 18hp, the 2GM would've been overkill by around 80% on anything ever required to drive the hull. I replaced an old Volvo for a lighter engine of similar power and not regretted it. Your hull doesn't look particularly hard to drive, so get an appropriate engine if you do decide to go down that route.

Check your waterline length, this is a limiting factor on hull speed. You'd be surprised how much additional waterline length you get when heeled over and this usually means you can sail a bit over nominal hull speed.

 

[Tranona]

But the old YS 8 actually produces around 6hp, even though it is called an 8. Those smal differences in actual hp are massive in %age terms.

 

[adrungil]

Thanks for all the replies. I do understand, that 8HP (or even 6 as Tranona mentioned?) on a small side and to get full 6,5 knots of hull speed of this boat could require new engine. Expensive feathering option not practical with present engine, also 3 blades not an option due to extra drag while sailing (my sailing to motoring ratio is 10 to 1... Anyhow, since presently engine does not "feel" the load, assume could try to get 0,5-1 knot of extra speed by re-pitching or installing new prop of same size/different pitch...

Have contacted C&O Engineering - thanks, Fisherman, for the link! - will see what they propose.

 

[Tranona]

That prop has a very small blade area and is what is known as a "sailing" propeller or more unkindly as a "skinny" and is not designed for optimum propulsion. You can get 2 blade propellers with larger blade areas which may give you more speed.

Assuming the size is really 12*9 (and that sounds about right) you can check it out yourself using the calculator of www,castlemarine.co.uk although you need details of your engine power, revs and gearbox reduction ratio. Any propeller supplier will need this information together with dimensions of the boat to make a recommendation. Also worth checking the actual revs you are getting from the engine as the prop should allow it to reach full revs in flat water.