What size propellers??

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  • 10th April 2007 at 7:15 pm #10953

      I've just installed a central diesel (20 HP Beta) and a twin hydraulic system and I'm trying to identify the correct size prop's that I should be using.

      Currently 2 x 3 bladed 12″ x 10's have been installed and I'm finding that I'm way over pitched (both prop speeds and max RPM are well down on the manufacturers figures and I'm not attaining the cruising speed that I'm looking for. The manufacturers literature quoted a max speed of 6.75 knots (cruising speed 6knots?) but I'm not geeting anything close to that.

      Whilst I appreciate that Brunton folding prop's would probably be the answer…unfortunately I don't want to spend £800+ / prop.

      Can anyone provide any recommendations?

      14th April 2007 at 10:40 am #11463

        Hi, on Kala Kai which has a central Perkins Perama rated at 18HP the original fitment was two three blade 11X8 Radice props. These worked well and pulled about 3500 revs. Max speed was about 6.4. I changed these a few years ago for two Varifold props at 12×7 as 12 is their smallest size. Reverse performance and stopping suffered considerably and max speed fell to about 6 knots. Sailing performance and rudder response was improved however. If I was based in a marina or used them regularly I might consider going back to the three bladers, as it is I am happy to have the extra performance and 'feel' of the folders. Hope this is of some use, but past experience even on much bigger vessels shows that prop selection is something of an art rather than a science!
        Good Luck.
        Pete

        16th April 2007 at 12:23 pm #11464

          Thanks Pete, I'm inclined to agree that its something of an art rather than a science!

          I'm working with two companies at the moment and it's going to be interesting what they both come back with……probably two totaly different recommendations just to confuse things!

          Bruce

          19th April 2007 at 7:57 pm #11465

            Hi Bruce, Good luck with the two companies. I would love to hear of the results. If you are able you can email me on mail@petedymond.co.uk Regards Pete.

            14th May 2007 at 10:34 pm #11466

              Hi Pete,

              Well I'm pleased to say that both companies agree and were now looking at 2 x 3 bladed 12″ x 8's props as the solution (1 LH and 1 RH rotation). As it happens I've also spoken to another close friend who also owns a HT27 with the same engine configuration, and they used the same prior to moving to folding props.

              Keeping my fingers crossed!!!

              Will keep you posted once their fitted and we've done some sea trails etc.

              Bruce

              15th May 2007 at 4:34 pm #11467

                Hi Bruce, I hope they work OK. You have a couple of BHP more than me and a new installation so the 12x 8 sound good. Something I did not mention in my earlier post came to light when I had terrible black smoking and lack of power recently. After some exhaustiive( pun intended) tests and new injectors I found that the exhaust elbow near the water injection point was almost completely blocked with concretion. A new elbow cured the problem and gave more thrust to enable the folders to work a bit better in astern. Had I still had three bladers on it is likely I would have revved higher or indeed pulled the larger size you are looking at.

                Let me know how you get on,
                Regards,
                Pete

                5th March 2008 at 2:05 pm #11469

                  Hi there,

                  Sorry for the lengthy delay….forgot to respond as promised last year.

                  In summary the 3 bladed fixed 12″ x 8 props are working a treat. The engine almost reaches its max rpm …at a speed of approx 6.8 knots and at 1900rpm we’re cruising @ 5.5….that’s pretty dam good from where I'm sitting!

                  If there is a criticism of the installation….it’s that the props do affect the sailing performance – probably in the order of -0.5 knots when left spinning or -0.8 knots if locked (both controls in reverse). Currently I'm seeking clarification from the hydraulic specialists as to long term affects on the motor if left spinning so I’ll keep you posted.

                  Bruce

                  6th March 2008 at 5:58 pm #11468

                    Bruce,

                    It seems strange that the sailing performance is worse with the blades locked. Race boats routinely lock the blades (if no folders fitted) and it's supposed to change the loss of speed from 0.5knt to about 0.3knt. Of course that's with just 1 prop. Excuse my complete ignorance, but with the gearboxes in reverse, is the water trying (and suceeding) to slowly turn the props in reverse? Racers use a gearbox lock and one I saw used a mole wrench on the coupling – a bit scarry if you forget it's clamped on there and start the engine!!! 🙂

                    9th March 2008 at 8:43 am #11470

                      Hi DD
                      Our Ht27 has two varioprops 12×7. our Nanni's produce 11 hp each. We made a trip from Marseille to Holland over Rhone and Rhine on motor.We passed the Rhone to Rhine canal with some 130 locks of around 5 meters wide.A lot of manouvering.
                      The average speed through the water was 5,5knts at 2500rpm,s
                      Only once one of the propellors failed to open. there were 7 freighters around us coming from all directions, with high waves I simply had to reduce the enigines to idle, because of the rocking horse effect of the HT . When the waves passed I wanted to continue through th small opening between to approaching vessels. When one of the props failed to open the other steered the boat almost immidiately 75 degrees to port. we were heading fo a speeding freighter. After a few foreward backward revs the prop opened and we continued the trip over the Rhine at 12 knots over ground!!
                      I'm very happy with the folding props now, as we hardly use the engines anyway. But for a river-trip I would have preferred regular props, but who is crazy enough to do that?
                      regards Juup

                      10th March 2008 at 10:48 pm #11471

                        Hi pastasimon,

                        I agree with you that race boats routinely lock their blades (typically 2 bladers)….but from what I recall they mark the shaft such that the blades are aligned with the back of the keel thus minimising turbulence/resistence to a minimum?

                        The reason why I'm placing the motors in reverse (remember – I'm running with hydraulics so I'm actualy changing the direction of flow of the hydraulic oil at the proportional valve) is because if left in forward the shafts just spin at silly speeds …as they do if left in neutral. In reverse there appears to be enough resistence in the system to stop the spinning…..but her speed is affected as outlined earlier.

                        Bruce

                        10th March 2008 at 10:55 pm #11472

                          Hi Juup,

                          If I had a £ for every person who's recommended Varifolds I'd be a rich man!!!

                          Whilst I appreciate that sometimes they can let you down/stick as you have described…I'm quite sure that this is the right way to go when funds permit.

                          Bruce

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