A lot of the rotary advantage would be high RPM's. Which isnt all that helpful in a boat application. I mean you can only spin the prop so fast, then it cavitates and is useless. I know pettit has a rotary powered boat as well.

I have pics somewhere of a 5 bladed prop from a Bertram that cavatated so bad it got holes in it, at $20000 a prop the owner was not happy to say the least

Hull displacement has alot to do with torque issue's ! There are three basic type of hull and they're effect by what's driving them.

[quote name='GLC13B' date='May 30 2005, 04:04 AM']Hull displacement has alot to do with torque issue's ! There are three basic type of hull and they're effect by what's driving them.


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Does the shape of the hull have any effect on torque requirements? My impression was that boats didn't need gobs of torque but now that I think about it, it makes sense.



Mark

[quote name='inanimate_object' date='May 30 2005, 05:58 PM']Does the shape of the hull have any effect on torque requirements? My impression was that boats didn't need gobs of torque but now that I think about it, it makes sense.



Mark


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you ever try to run your hand flat in the water? its hard, but if you knife the water its alot easier-



just like when you hold your hand out the window- you can think of it the same as aerodynamics.

[quote name='Rob x-7' date='May 31 2005, 02:19 AM']you ever try to run your hand flat in the water? its hard, but if you knife the water its alot easier-



just like when you hold your hand out the window- you can think of it the same as aerodynamics.


[/quote]

Ok, but what's getting me is a boat weighing x kilo's is always going to displace x volume of water regardless of hull shape -how much of a difference do 2 different hull designs experience considering this?



Mark

same thing as a fat heavy car cutting through the air as a light one, they can weight different but still have the same drag. Its hull design and weight

Simple! Thanks Rob



Mark

There are three types of hull ! Also, your props have alot to due with this issue as well ! Some are TORQUE and others are for to RPM'S !

1) Displacement hull

2) semi-displacement

3) planning

Have a Fichtel and Sachs rotary in my 6500 lb. 28' Ranger 28,which would be a displacement hull,doesn't have any problem pushing her along at displacement speed.Picture of the Ranger 28 Candyman crossing the Straits of Juan de Fuca,uses about a half gallon an hour.