Why not cruisers?

Greetings all. I have been pondering the idea of surface drives for my project thingey based on my Fjord 27 which currently runs a very heavy, tired, low power, inconveniently positioned Volvo inboard.
I like the idea of surface drives. Nice and simple. Flexibility (some)in where the engine is placed. One I am looking at is made by/for Lancing Marine. (See link if I have done it right)

http://www.lancingmarine.com/LM_Transom_drives.pdf

A few questions.

1.Why are surface drives not suitable for cruisers?
2. I know that they come semi up out of the water when on the plane but does it matter if most of the time they are submerged?
3. Why is this any different to a standard inboard submerged prop?
4. I understand that reversing seems to be a problem. I have asked Lancing how this issue is addressed by these drives but has anyone any idea how they might be in reverse judging by their appearance?
5. I like the idea of exploring creeks and upriver so the less dangling down in the water the better. Would I be best served by leaving the current keel on and what effect, adverse or otherwise, would that have on using a pair of surface drives? I was thinking perhaps reduce it rather than bin it altogether.

Some odd questions perhaps but I am exploring every possibility and option.

regards

arthor

 

If they are submerged, they are not surface drives. The upper blades, and sometime portions of the lower ones are off the water. That unloads the high revving prop blades to prevent cavitation.

 

Mike Bellamy from Lancing marine wrote me several years ago that he uses LM surface drives on his boat and was quite happy with them.
So he should be able to tell you all about reversing. I do not see the problem because reversing is a slow process with submerged props and there is enough distance between the transom and the props for the water to escape. Of course the efficiency is poor, but unless you think about starting a ferry service on a narrow part of a river, it is not a real problem.

From my own experience, the noise and vibration due to the changing blade load are far more disturbing. That would be a valid reason not to use surface drives on a cruiser.

 

See inserted notes.

In high speed applications, using small diameter high rpm submerged props, the blades are working in cavitation and are not very efficient. There is no great disadvantage in using a surface prop here and there is less drag from the normal appendages to support a submerged prop. The only significant disadvantage is the cyclic loading on the blades but this is a design issue that has been overcome.

Some more reading:
http://www.icmrt07.unina.it/Proceedings/Papers/c/44.pdf
http://www.boatdesign.net/forums/at...ller-low-speed-displacement-boat-img_0070.jpg

Rick W

 

It all depends upon your application. For example, if you are in need of a shallow draft propulsion system, then you can use a surfacing style of drive even on a displacement hull.

As for surfacing propellers characteristics, if the boat is a displacement hull one would not use a surfacing propeller on this application, rather one would use an appropriate propeller design.

If your hull is a plaining one, you have a design that is compatible with surfacing propellers, then you could use a surfacing drive system. The term "cruiser" might be misleading as Sunseekers, Baia's, Magnum's, etc are cruisers also and they use a surfacing drive system.

 

You have misread what I wrote.

It might be clearer if I stated - no appendage drag compared with the normal appendages required for a submerged prop.

Rick W

 

Regards, Brunello