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Old 13-01-2007, 14:32   #1
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Bukh Propellor

Is anybody able to help with advice on getting more speed out of my propellor. I have an Allan Wright designed Marauder yacht. It is 8.4m LOA with a waterline length of 7.5m and a beam of 3.08m. It has a 10hp Bukh which is the standard engine from 1985. Motor runs well but I feel the prop is not correct. It is a shaft drive with a 2 blade folding prop of unknown dimensions. Unfortunately there is no rev counter so I can not tell the revs. At 1/4 throttle I get 3.5kts, 1/2 4.0 and full 4.7kts. I notice that it seems to rev but with nobody at home. I also find that under sail at around 5-5.5kts, if I turn the motor on I will get another .5kt. My expectation is to get 5.5kts under full throttle or is this expecting too much. These speeds are with no currents, wind and a clean hull. I did buy the boat for sailing but at times, around the Sounds, a motor comes in to play quite often

Andy
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Old 13-01-2007, 15:57   #2
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Propeller design is based on the engine and the transmission and the given purpose - a sail boat. It's a very well understood science and you can't make it better if you already have the correct propeller. The goal is to spin the engine to maximum rated rpms at full throttle in gear on open water and then measure the speed. This is a pure matter of engineering to the specs and there is nothing you can do to make it "better". If you took the engine specs and transmission specs to a prop shop they could tell you exactly the correct prop pitch and diameter. Your hull of course will determine your performance too. A longer water line is always a plus. Two extra meters of water line would do a lot for your performance.

Increasing the prop diameter or pitch will cause it to bite harder in the water but at the expense of rpms. That will add more stress on the engine and transmission than you want and really won't make you go faster just trash the transmission quicker. If the prop was too small then that would be a real god reason to change it. They can generally add or subtract about 3 inches of pitch and so you might not need a brand new one. The key is to take the numbers to the shop and let them crunch them for you. They can at least look at the prop and balance it and shine it all up for a small fee.
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Old 13-01-2007, 18:10   #3
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Hi Andy, your waterline length is 5kts. so 4.7knts is mostly spot on. The difference of .3kt could be inaccuracy of the log or that you are right on hull speed. When you sail and get that little extra with motor, it is often due to a compleatly different factor. When the boat heals, it can often present a little more WLL. So you get a little more speed out of her.
So you sail in the sounds?? where are you based??
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Old 13-01-2007, 18:18   #4
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Hi Alan

Have been sailing with friends on Seafox for last 3 years and purchased own boat 7 months ago. So far been around Queen Charlotte and Tory. Just got back from 2 day fleeting visit to Resolution and Hitau yesterday. Popped out of Tory and a good sail back to Mana in sea fog. Had 3 weeks down there over Christmas and moved all around the area to either where the wind was either blowing, or not. Thank you for the advise. maybe it is not all bad and maybe me just expecting too much from a 10hp
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Old 13-01-2007, 21:41   #5
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Oh you know Darryl?? sorry ;-):-)
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Old 13-01-2007, 23:02   #6
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Come on Alan, just cause I sit to pee!!!

He's trying to match his speed with a 10.6 Lotus.
His Marauder sails flat out for it's size. A boat always feels slow when you are being passed.
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Old 13-01-2007, 23:45   #7
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Hey Andy, when Darryl isn't looking, hang something off his stern to slow him down. Or tie a drogue off his keel. :-)
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Old 14-01-2007, 02:14   #8
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seafox:
When at sea; we all sit to pee.
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Old 14-01-2007, 02:51   #9
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Darryl's not a bad bugger as far as buggers go. Taught me alot about sailing with more to come. Think I will just polish the heck out of the prop and keep it clean
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Old 14-01-2007, 11:41   #10
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Yeah there ain't a lot you can do with 10hp. I would say it is doing just fine. Just remember how much fule you use against what Darryl uses. That'll put a smile on ya face.
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Old 14-01-2007, 14:05   #11
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bugger, I have been using diesel. What is this fule stuff I should be using Alan??
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Old 14-01-2007, 16:26   #12
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My secret formulae.:-)
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Old 14-01-2007, 22:49   #13
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7hp in 7 tonne...

I used to have an old 7hp single cylinder Yanmar in the ferro Hartley RORC 32' that I built.
7 hp powered a 7 tonne boat along very nicely.
I didn't notice any wealthy nobs in their overpowered plastic surging past.
10hp should be plenty for your boat. I would suspect a combination of folding prop in poor condition and incorrect pitch/diameter for your boat and sailing conditions.
It sounds like you want your power to be available in some pretty specific conditions.
I would suggest clearly defining those conditions, getting the gearbox ratio and power/torque curves (I found this on the internet - there are others) for your engine/gearbox (not to mention the model/serial numbers) and then discussing what is needed with your local (recommended) prop shop.
I note that the 10hp for the Bukh is a continuous rating so it may be possible to load it up.
If your boat is coming out of the water I would check that the folding prop is functioning correctly and measure the diameter and pitch.
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Old 14-01-2007, 23:17   #14
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When I last looked at his prop it had a fair amount of tooth ache (wobble). It is a two bladed folding prop. Does the toothache lose any performance? Should he get it re bushed or tightened?
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Old 15-01-2007, 01:42   #15
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Originally Posted by seafox
bugger, I have been using diesel. What is this fule stuff I should be using Alan??
Ground up Wombat mixed with a liberal dose of methane. Works reasonably well but drinks too much, plays good cricket but crap rugby.
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