Duo Props

[bryan666]

What is the purpose of Duo props? Is it to reduce cavitation or to give more thrust for the same size of prop? I presume the props counter rotate?
I have asked several other boaters but nobody seems sure.

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[PGD]

yes prop counter rotate and I belive you get more thrust. Just re-engined to diesel and now have a duo proip oytdrive - boat goes like the wind - seem to pop up far quicker now. I'm sure there loads more technical advice to be had but I like em.

Cost more thou especially if you prang em.

400 quid for a new set - ow !

Peter

<hr width=100% size=1>cooking on gas - no way - running on diesel /forums/images/icons/wink.gif

 

[TRH]

400 quid thats cheap.Beter than Keyparts ? 500+

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[scottie]

one advantage is that the torque is balanced out and there is no steering bias
this is very noticeable in reversing of a beach
the idea was used with torpedoes where straight line running is distinct advantage


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[HowardB]

Also means that for a given amount of torque you can have smaller diameter props. ideal for outdrives, and low drought boats where not having a single wide diameter prop would prang the anti cavitation plate and increase draught. 2 props at 9 inch type diameter better than one 18 inch diameter! Mostly useful for bigger engines with loadsa torque / power.

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[terryw]

Stainless Volvo props are £500 EACH

<hr width=100% size=1>If your nose is running, and your feet smell, you are upside down.

 

[Alistairr]

I had the choice when ordering my new boat (Rinker 232 cuddy) an Alpha1 or B3? i picked the Bravo 3! Wise choice or not?

I'm still waiting for it, hoping the bloody war doesn't slow down production in America!!!!

<hr width=100% size=1>CHEERS.

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[HowardB]

I think a good choice and would have done so myself. Should lead to easier handling

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[Alistairr]

And better performance??

<hr width=100% size=1>CHEERS.

Help needed to name new boat, PLEASE.

 

[terryw]

Re: Horses for Courses

I think the choice is down to many factors. My last boat was a 25 foot with single engine (270HP) and Duoprop. I think this was a great setup. My current boat is 28 foot with twin engines (2x240HP). It started off with Bravo 3 legs, and although the performance was awesome, if not a little harsh when the turbo cut in, its close quartes manouvering was a pig, because as soon as it was put into gear it took offlike a scalded cat.
It now has Bravo 1 drives, and is a complete transformation, and will be the recommended setup for all future boats of this make and model.

<hr width=100% size=1>If your nose is running, and your feet smell, you are upside down.

 

[ari]

Re: Horses for Courses

Duoprops give more efficient runnning (ie more speed and same consumption or better consumption at same speed), smoother, cancal out torque effect (more useful on a single engined boat than a twin engined boat as twin should be counterrotating anyway, whereas a single tends to "twist" the boat against its rotational direction. Imagine gripping the bit of an electric drill and seeing the reaction of the drill) and better directioal stability, especially backwards.

I'd recommend them without doubt.

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[spannerman]

I gues you have ordered the 350 mpi Magnum motor, if so I would have gone for the Bravo 1 leg as this will give the best top end performance. To quote the official Mercruiser drive recommendation book which we use in the workshop

Bravo 1 -- for high performance, high speed boats.
Bravo2 -- for large family boats, which are driven for extended periods at economy cruise RPM's
Bravo3 -- for stable performance in turns and fast acceleration , and use with engines with high torque.

Interestingly none of the race boys use duoprops as they know two props produce more drag.

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[hlb]

Re: Translation

Bravo 1----Lasts five minutes.
Bravo 2----Lasts ten minutes.
Bravo 3----Lasts a bit longer if very carefull. Only using half throttle and preying to allah.

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Haydn

 

[oldgit]

Re: Translation

My old earlyMercruzer gear used to eat rams like there was no tomorrow.Volvo stuff is far better built and lasts/forums/images/icons/laugh.gifStill shafts is best

<hr width=100% size=1>Oooh look its still not dark and its nearly 5pm

 

[Alistairr]

Sorry you're wrong, i chose the 5.0 mpi 260php motor, i felt the 350 magnum would be just too heavy on the fuel, and the mpi should be a bit better but not so much top end speed, but who wants to cruise at 60mph?
My last boat had a heafty outboard so i dont have any experience with out legs, i had two options Alpha1 or bravo 3, was told that both were good but the B3 would plane faster and corner better than the Alpha1, hence my decision, time will tell if it was right.
If money not an issue the 350 magnum with corsa exhausts would be nice........

<hr width=100% size=1>CHEERS.

Help needed to name new boat, PLEASE.

 

[spannerman]

Hi Alistairr, I only mentioned the 350 as there are three Rinker 232's I look after and all have the 350 mpi, two have B3's and they top out at 52mph and the one with a B1 is a little faster. Interestingly the owners reckon they are not too expensive to run as the engine isn't working very hard in these boats. True the B3 will plane faster and handle better especially in reverse. I have to be honest we have never seen a B3 with a motor under 300hp as it takes quite a bit of hp to turn all of the gears.

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[Alistairr]

Time will tell wether the B3 will work with the mpi or not! I presume Rinker must think its okay as they offer it as an option in there brochure. Tell me again that the Rinker itself is a good boat! Please!!!!

<hr width=100% size=1>CHEERS.

Help needed to name new boat, PLEASE.

 

[spannerman]

I am sure you will be delighted with the Rinker, no matter what people say you can't beat american produced boats for value for money. And they understand the market and what people expect from their boats, where are the Brit small boat manufacturers now such as Driver,Phantom, Ring, Shakespeare,Fletcher etc? I can guarantee that you will just have to put in the gas and turn the key, have fun. Steve

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[Alistairr]

Cheers Steve.

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[duncan]

can't help feeling its the larger gear casing that provides the greater drag rather than 2 props - the props are supposed to deliver the drive to get over the drag!
Many race boats use DPs - pretty sure the batboat thingy has them, but the volvo casing has a lower profile as well as slightly more efficent props and many prop combinations.
I have also noticed with my b3 that it likes to be well down in the water which will again put off the racy crowd with their semi surface piercing props etc

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