DuoProp Help

[TwoStroke]

Am looking at a second hand 21' boat with a duoprop and inboard. Bearing in mind I don't have alot of MB experience - do you recommend these. From the marketing guys they seem to give you a smoother ride, or is it a load of marketing waffle and when it goes BANG you have to write out a bigger cheque????

Thanks for your help.

... I want to be back on the water!

 

[Divemaster1]

Depends a lot on age and mainteneance of the drive and engine.

In general sterndrives, including duoprops vs with shaft gives (in no particular order):
1) better top speed and thus crusing ... others have more experience, but this depends on type of boat....
2) ability to drying out
3) higher maintenance cost
4) reduced reverse thrust
5) Lower aqcuisition cost

Duoprop sterndrives vs. traditional sterndrive gives:
1) better top speed and thus crusing speed & reduced fuel consumption
2) boat faster up to planning speed
3) Increased maintenance cost and spares cost
4) Higher aqcuisition cost

Probably heaps more....stuff that can be said here..

Divemaster1

 

[duncan]

add the removal of trim (torque) steer as a distinct benefit to the (single) duoprop
nothing more annoying than triming in a drive/outboard to handle sea condiitons better and having to fight to hold the wheel over to the left all the time.

 

[DavidJ]

Just taking a lead from Duncan. Has the boat got trim tabs. In my view essential for a 21ft boat especially if you have 2 or more people on board for levelling out and to get over the planning 'hump'
On the duoprop yes, there are minor advantages but I think it more a marketing/fashion thing so the retained value of the boat will be more with duoprops.
David
PS Stay away from the rocks, duoprops are expensive things to replace.

 

[PGD]

some good points here - I've got a 24ft now with duo props - yes 400 quid for a new set if you ding big style. about 200 for recons sets. I've just re-engined and had an old sx drive - the duo props give much better take up when you push the throttle forward also the baot used to snake left and right - not not me pissed - it's seemed to have stopped doing that now - got tilt / trim and trim tabs - not tried the top end stuff yet - on the Thames you see - no fun alowwed above 4 ktns boo.

yes there is a bit more to go wrong at the bottom end - so the servicing and checking the oil / seals etc is essential

Peter

 

[whisper]

Just two queries.
Why does a duoprop have less reverse thrust than a prop on a shaft? I've always felt that the latter tends to have almost as much sideways thrust when in reverse, due to prop walk, until you get a lot of way on.
Similarly when asking a question about drying out on an earlier posting I got the impression that you can cause damage to the bellows by leaving the drive in the fully tilted position.
Any further thoughts on the latter in particular would be appreciated.

 

[TwoStroke]

Thanks for your help. Will ask about the trim tabs before I go to have a look at it and will get it checked out thoroughly if it has. Don't fancy a big expense after buying it.

Cheers!!

... I want to be back on the water!

 

[duncan]

last part first - this is normally an issue associated with long term habits - certainly when storing boat out of water, leaving on mooring etc it is recomended to leave the drive trimed down - the 'natural angle' for the bellows; rathert han having them stretched.
regarding reverse thrust teh duo prop will be more than the single outboard/outdrive prop but may well be less than the shaft driven prop because of their intrinsic properties. shaft props are (normally) set up to run and be efficent at lower revs - they have greater dia and pitch. When initially putting into reverse at low revs you may get more 'omph' - however - you normally need it 'cos the boat is heavier etc! They also have more blade working near the hub (smaller than outdrive) which effects their properties.

 

[KevL]

Might be urban myth but I was told that nasty little barnicles can attach themselves to the underside of the opened bellows if you leave the leg up. Then when you drop the leg to use the boat they get trapped and can saw their way through the rubber. In normal use the constant sideways flexing as you steer keeps the little buggers from growing too large before they drop off.

Like I said it could just be urban myth..

KevL

--
It may be the early bird that catches the worm but its the second mouse that gets the cheese.

 

[whisper]

Thanks for the info. It sounds as if I may be buying the wrong type of boat (single duoprop outdrive),as it will be left on a drying mooring.
It's only taken us four years to find something that meets our spec!!

 

[Divemaster1]

Cannot answer the "drying out" question, as most of my boating have been with shaft drive, and have never had an issue with drying out. But by the look of it, you already have had some replies on that one....

With respect to reverse thrust on sterndrives, I know that nearly all sterndrive propeller installations are designed with high speed (high speed is of course relative...), using "cleaver" style propellers. These propellers, due to the thick trailing edges, concave pressure face, and often heavy trailing edge cupping, are poor performers in reverse. With respect to the details of why, I propose you ask some experts...

You may find some more answers to your queries on propellers and performance issues on the site below...

http://www.castlemarine.co.uk/your.htm



AHM