More propellors

[lumphammer]

I need to replace my two bladed folding prop this winter, and I will probably re-engine in the next two years. Currently I have an 23 year old, 18hp Sole diesel, and I would like to change to something like the Beta 13.5.

My question is, if I size the prop for the new engine what is the effect when running on the old, and also the same question the other way round if I size for the current engine what is the effect on the new?

The boat is an Achilles 840, 28ft loa, 24 lwl displacement approx 6,000 lbs.
Any advice gratefully received.

Richard

<hr width=100% size=1>

 

[snowleopard]

if the prop is too big for the engine, you won't get full revs and so won't get full power. if you try to push it you'll get a lot of black smoke. if the prop is too small for the engine it will rev freely up to the maximum revs but won't deliver the full power of the engine.

<hr width=100% size=1>

 

[alahol2]

I've just done several calculations, from different books and websites, (plus got a couple of manufacturers to make suggestions) for a new prop.
I'm in the process of fitting a Beta 13.5 into my 28ft Stag. All the calculations and suggestions come to roughly 13 x 9. How does that compare to your current prop?
More diameter/more pitch would run the Beta more slowly, Less diameter/pitch would let the Beta over rev (if my calculations are correct).

<hr width=100% size=1><A target="_blank" HREF=http://www.troppo.co.uk> Follow the Tightwad Sailor</A>

 

[lumphammer]

The answer to your question is - I don't know. The propellor came with the boat with no information, and every time it is out of the water I forget to measure it. (which raises a suplementary question - how do you measure the pitch?)

13 inch diameter is about right. I suspect it is not particularly well specified, I seem to get up to hull speed well before max revs on the engine, that is when the prop is not covered with barnacles, weed etc and both blades have opened out properly.



<hr width=100% size=1>

 

[gordon029]

Hi Sorry I don't know anything about propellers and PLEASE excuse me for cutting in on this technical conversation but you have a boat which I am trying to find out about.An achilles 840 has been suggested to me as a boat I should look at for a first boat as the triple keel would be ideal for tidal moorings.What I would like to know is how does this keel effect the performance.I got a copy of the report from the copy shop but this told me very little.If you have any info on this or anything else you think might be useful I would appreciate hearing about it if it is not too much trouble.Lastly I have not found a boat to buy yet so if you know of any I would appreciate hearing about them also.
Thank you

 

[oldharry]

13X9 sounds about right for the engine/boat. I suspect the Sabb being a more powerful slow runner will have a bigger prop - both diameter and pitch, and may not allow the Beta to come up to full revs/power.

measuring a prop's pitch is very difficult - so nearly all are marked with the diameter and pitch, either on the boss (hub) between the blades, or on the end flats. The normal marking convention is Diameter x Pitch, so the 13"x 9" should be marked 13X9.

This can be quite difficult to locate on an old fouled or painted propellor, and you may ned to spend some time scraping and cleaning back to bright metal before you find it. A sheet of 150 grit or thereabouts waterproof sandpaper is often necessary. Many makers put other numbers on as well, to do with their own ID, just to confuse issues. If there are several possible numbers marked, measure the diameter in inches, and look for that number followed by an X.

 

[arvin555]

I am not an expert but I just repowered my boat with a yanmar 3YM20 22hp max engine. I asked the dealer to bundle a prop and i got a gori folding prop. I went to their website and found a nice feature called prop calculator.

Basically you enter your boat and engine data and they will give you suggestion on what is best gori prop to use.

http://www.gori-propeller.dk/

My suggestion first try it with your current engine, and then try it with the engine that you plan on repowering with. Hopefully you will get the same propeller size suggestion. If not maybe it would be better to drop them an email and ask them for help. You of course don't have to buy from them at the end of the day, just get their idea. But I think Gori is one of the most popular props so if price is right, etc. you might want to go with them (I am not in any way connected to them).

TTFN
Arvin