Why not cruisers?

[arthor]

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

[gonzo]

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.

[CDK]

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.

[Guest625101138]

Originally Posted by arthor

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?
Surface props operate quite differently to a nicely loaded submerged prop. The blade loading goes from full load to no load and the lifting front face virtually does nothing. There have been large scale models of ships using surface drive. The upper efficiency is limited to about 70% whereas a submerged prop can get as high as 90% but this is rare for a pleasure craft.
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?
If you can drop the leg so the prop is submerged then it is more like a long-tail than a surface drive. In this mode the prop will likely be overpitched if it was designed as a surface drive.
3. Why is this any different to a standard inboard submerged prop?
If the prop can be set down so it works submerged then you could fit a conventional propeller and it would work as a submerged prop. It will likely need some "cavitation" plate to stop it from ventilating particularly accelerating from low speed when the induced drag is high. It will form a vortex that feeds air down into the blades so it is no longer working as a 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?
If it works as a submerged prop it will go in reverse OK.
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.
If you can make it like a long-tail so it raises and lowers with a small change in angle it will work better than tilting the leg of an outboard or stern drive.

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

regards

arthor

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/att...t-img_0070.jpg

Rick W

[Rik]

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.

[Frosty]

[quote=Rick Willoughby; 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. [/QUOTE]

What? how can there be less drag from the normal appendages? you've got that mixed up hav'nt you.

Surface drives are unbelievably smooth, no vibration at all, up on the plane they are as smooth as jets. Reversing is an issue and varies from system to system but not really a prob when you get used to it.

[Guest625101138]

Originally Posted by Frosty

What? how can there be less drag from the normal appendages? you've got that mixed up hav'nt you.

....

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

[brunello]

Originally Posted by arthor

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?
Any drive system which places a propeller at about 50% disk submergence, could (or should) be defined as a surface drive. Other design parameters (prop design, reduction gear ratio, transom distance, transitional flow control devices, ventilation, steering means, power trim etc.) can make some surface drive systems much different from others. Surface drives potential has been mainly exploited on high to very high speed applications; this is simply because, in this case, the system is so much more efficient that - in practice - there in no other viable option. Also, grown from raceboat experience, some hands-on knowledge was available in this field, so it was relatively easy to get it right. There is a lot of prejudice about low speed applications of semi-submerged propellers (surface) to low speed vessels, mainly due to lack of knowledge. We (at Flexitab) have tank tested a displacement ship in the naples tank, in self propulsion mode, and recorded an overall increase of efficiency of up to 13%. Of course you need to custom design the application. So, in our opinion, there is not a single reason in the world why cruisers should not use an appropriate surface drive.
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?
Not sure I got the question, but the prop submergence is the key point for getting the efficiency. So you should design/select your drive system so that, when you use it (design speed) the prop is located at about 50% disk submergence.
3. Why is this any different to a standard inboard submerged prop?
Following point (2) above, the drive shaft comes out of the transom and the prop (at working condition) is about half out of the water. Unlike a standard 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?
Proper propeller and drive design does not, normally, give any big problem in reverse
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.
If you use twin drives, port and starboard, and keep a centreline keel, you should not have any problem.

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

regards

arthor

Regards, Brunello