Using Interceptors for Steering

Sounds like the interceptors used for steering on the Harley 40x10 deploy vertically which is different than what BMcF has used. From the Harley website http://www.harleyboats.com/home_page_006.htm

The Harley 40 with its standard Fixed Surface Drives has Interceptors for Steering. Interceptors are hydraulic vertical drag plates, mounted between the trim tabs and the transom, that come down about 4” on each side, of course one at a time. These have been tested on our boats as large as our 55’. They turn very sharp, much sharper than conventional inboard rudders, have by far the least drag at high speed, much less than rudders, offer excellent turning at all speeds, and even offer good turning manueverability in reverse.​

I assume that deployment on one side only of an interceptor located between the trim tabs and transom would cause a moment tending to roll the boat to the outside of the turn.

 

How they get a turning force and moment from a device (interceptor) that has a fairly high lift/drag ratio and is producing its lift in the vertical direction..eludes me. I guess if there was some sort of picture or figure to examine I might make better sense out of what they are claiming. Considering they are a v monohull, you could be right..that all the turning is simply from an induced roll angle.

 

I suppose I could ask him via email or phone call. I think I personally would like the boat to turn without excessive rolling. But that's just me.

 

http://miami.craigslist.org/pbc/bod/3531820628.html

Looks like he is trying to off load some SP propellers.

 

LOL!...I'm not surprised. If anything, I'm surprised they don't have more of them to sell.

 

Using interseptors for steering

Interceptors have a specific use, not to steer with. Dipping rudders work great and have no drag except when turning. Easy and simple to build especially fitted to a Pivotal drive Propulsion using Surface propulsion.Great to maneuver in confined space and great to reverse with Surface Drive.

The first interceptor plates were plates or a plank of wood bolted on the transom but about one inch below the transom to divert the water flow to lift the transom. Much better to design the hull and balance it correctly.

 

Tom,
Interceptors can and have been used to steer boats. Unlike those used for trim they are mounted on the sides of the transom not the bottom.

Since the first interceptor plates, there have been advancements.

I understand the concept of dipping rudders from your first post. Thank you.

 

In fact, that is often not the case. Extensive testing conducted by USN at the David Taylor basin duplicated the similar tests conducted in Germany a decade earlier and proved that for certain hull types, significant improvements in the overall hull L/D ratio can be achieved by adding transom lifting devices (intereceptors, trim tabs, wedges and combinations thereof).

We are in the middle of conducting sea trails with a 63-meter high-speed naval patrol craft that benefited to the tune of about 1.5% reduction in total drag (at the maximum speeds) with the addition of 6-degree wedge/tab combination at the stern.

We've seen total drag reductions of as great as 3%, compared to "bare hull" drag, on some high-speed catamarans where transom lift was incorporated in the overal design and LCG balance. In one particular example, the addition of appropriately sized and designed transom trim tabs and shifting some weight aft actually "saved" a high-speed catamaran yacht from being considered a failure and rejection by the owner.

 

Using interseptors for steering

I would be interested in any info on using interceptor plates for steering if anyone has web site etc., Hickman`s sea sledge had plates fitted on the sides of the hull to steer a long way back.

 

I'll see what I can dig up from our archives. This is one app that I am familiar with and described in an earlier post in this thread..


http://www.lamemarine.com/interceptor/steering.html


Nice clean and concise diagram of the differences between the interceptor and trim tab pressure profiles, that you posted, btw.

 

One thing that struck me as a potential disadvantage with an interceptor is their use on a "bracketed" outboard boat. Maybe I don't understand it correctly, but it would seem that the lift further aft with conventional tabs on a boat that has had its center of mass pushed too far aft by the bracket and outboard would be an advantage. A boat with a proper center of mass wouldn't need the extra lift aft of the main hull.

Is that right?

 

Not quite. Again..on many hulls, the overall efficiency of the boat is improved by having lifting devices at the transom and moving the CG back to use those lifting devices. For some reason, there is a persistent myth and misconception that lifting devices (tabs, wedges, interceptors, foils, and combinations thereof) are a band-aid to "correct" a deficient hull design. The reality is quite the opposite for very many designs.

 

While searching for something completely different on my computer I came across a paper that might be of interest here (see attached PDF): Widmark, C.: "Interceptor Steering - An Effective Means of Providing Directional Control of Waterjet Propelled Craft", RINA International Conference, Waterjet Propulsion III, 20-21 February 2001, Gothenburg, Sweden

 

It seems to me that interceptors are used mainly for stability control and perhaps to assist to turn a craft
and not as the principal turning force. For extra comfort in bigger and more sophisticated craft such additions would have benefits
.Water jets certainly need some lateral control and an alternative means of controlling a craft when the water jets stop operating you loose any steering at somtimes critical times.

 

That is an interesting information, and very understandable. Got to be investigated further. Thanks.