propeller choice - request for further technical advice

[Hazymoonshine]

Hello,

Further to a previous thread, in which a lot of good advice was given, I have a couple of last questions which I hope someone can answer;

My engine is held back by an overpitched prop. I have now taken the boat out, and established I have a 12x9 3 bladed prop. Before I commit on the change, I am deliberating between the following options; swap for a 12x8; 12x7; 14x8.

Any guidance would be hugely appreciated.

The technical detail for those who might be interested is as follows:

BETA 13.5 3600rpm - max cont assumed at 3200/11hp
25mm shaft, 1:10 taper
Gearbox ratio: 2:1

Hull:
Bilge keel
19'10" waterline
2' draft
6' beam
4920lbs displacement

With the current 12x9 prop I can get 2400rpm at WOT, about 4.5/5kts. I should be able to get 3200, and achieve 6kts. I understand that reducing pitch by 1" will increase rpm by approx 450 - I am not sure about the effect of increasing diameter whilst holding pitch constant.

Having worked through various prop calculation programmesI have arrived at the shortlist - but having never done this before I am just trying to judge which is the best compromise.

At the end of the day, I would like to be confident that with a clean hull, and at peak power I can get 6kts through the water.

Thank you for your help.

H

 

[oldharry]

I dont personally find these Prop calc programmes much help - my Bukh 20 is overpropped at 16x14, but stil gives near waterlne speed, which according to the calclations it should not!

Different programmes give anythng between 14x13 down to 12x9, as being 'correct'. The Bukh agent says 'somewhere around 16x12, or 14x14. so who is right?

Incidentally, a useful way of comparing a prop is to add the 2 figures together Doing that you will seethat if you fitted the 14x8 (22) it would make matters worse than the 12x9 (21). Working backwards from that figure the Pitch is optimally 2/3rds the diameter.

FWIW the calculator I find most accurate gives 12x8 for your set up, which leaves me a little puzzled wh your set up is giving such poor results. If 12x8 gives full load/power, then 12x9 should not have such a dramatic effect. Suggests to me there may be something else amiss - such as engine timing, fouled hull, throttle lever not set up correctly, or choked exhaust.

Witht the price of props, its worth a trip to the Chandlery Barge at Hamble bridge, who have a container full of second hand props at a huge price saving when you are experimenting. /forums/images/graemlins/cool.gif

 

[marcusgillies]

instead of buying a new prop why not get 1/2 inch taken off the pitch of your existing one?

 

[Bejasus]

actually not bad advice to have the pitch altered at least you will know if you are heading in the right direction at a reasonable cost. When I bought my new prop, I specced it as 12 x 11, but the nearest they had was 12 x 10 and they just altered the pitch, which they can do by up to 1 inch. On the Broads, that was by Brimbelow Engineering. There are others out there.

 

[Pye_End]

The pitch should not have a big effect on RPM - and I do not think a fouled hull will (although a fouled prop might). Load on the prop will be more influenced by surface area, which is generally a factor of the diameter, as they are almost all 50% or 55% (3 bladed). It is possible to get a lower sufrace area but it is not to be recommended for most installations.

 

[Hazymoonshine]

Thank you for some really helpful replies. I sense I should investigate the engine set-up first, before moving on to the prop. If I get no joy there, then I will get the pitch reduced. What a good idea - it had never occured to me such a thing was possible.

Thanks again

H

 

[Rick]

Agree with Oldharry that something is amiss with current setup - calcs I ran show you need either 12x8 or 12x7, but unlikely to be a long way from what you have now - is your hull / prop fouled at present? .. ie when did you last slip and antifoul?
Having said above, I'd be inclined to go 12x7 as by generalisation there are far more operpitched than underpitched boats in most harbours - I'd much prefer to be a little "short" than "long" in the pitch.

 

[SwedishLass]

I have the beta 13.5, 2:1 reduction box 12X9 prop fitted to 26ft atalanta pushes along at up to 6 knots and achieves close to 3300 rpm. The prop is the suggested size based on the optional extras catalogue, more likely to be other issues,sooting on transom would suggest excessive load eg fouling etc or air restrictions. Alternatively filters or cable setting, does it rev without load?