Interceptor type hull trim devices

Discussion in 'Hydrodynamics and Aerodynamics' started by Boston Hatteras, Jan 2, 2025.

  1. Boston Hatteras
    Joined: Jan 2025
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    Location: Boston MA

    Boston Hatteras New Member

    Good evening. My name is Russell Robins. I'm a regular contributor over on the Sam's Marine Hatteras forum under my screen name BostonHatteras. I thought I would join you guys and see what you're up to over here. The topic I'm researching currently is the effectiveness of interceptor type trim devices on older planing hulls, specifically my 1970 45' Hatteras convertible. The boat was built with the typical hatteras pocketed molded fiberglass tabs however they were deployed by now long forgotten internally mounted right angle electromechanical actuators that gave up the ghost this past year. Not a bad run for the old system but its time for a new system. The boat weighs in the slings of a newish travelift 37500 wet and the overall length is 45'1. waterline length is around 42' 10 ish. she has about 375 hp per side on 871 DD engines and can hit 22 knots on the pins, cruise is 19.5/20. The boat also has smart rail lifting rails of 8' length each fore and aft. I'm finding it really difficult to find unbiased opinions on whether this is a suitable application of the vertical blade type trim systems. I'm primarily concerned with the performance of whatever system I choose, the money is a secondary, although not an unimportant concern. The way I see it now, I have three options and not in any particular order: cut the transom and create a recess to hold a single bennett actuator similar to the vast majority of hatteras boats, another option is to lengthen the existing pocketed tab and have a bennett actuator extend off transom at its design bracket angle of something like 40 degrees, and the third option is the interceptor trim and fill the existing trim tab pocket. Everything requires a fair amount of work but I'm no stranger to work and none of what I mentioned is any issue to complete, its just making an intelligent informed choice first. What does the group think about interceptors at 20 ish knots?
     
  2. fallguy
    Joined: Dec 2016
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    fallguy Boat Builder

    Based on my limited understanding, for boats running less than 60mph, Interceptors are to go to if the money is there.

    The reason is immediate response and immediate and automatic corrections. They are the ideal. Contact the manufacturer and let them give you their pitch and advice.
     
  3. Boston Hatteras
    Joined: Jan 2025
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    Boston Hatteras New Member

    Sounds like you already called them, that's right out of the brochure. Was looking for someone with experience
     
  4. fallguy
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    fallguy Boat Builder

    I haven’t seen the brochure. Just well read. But call them. Why not?
     
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Welcome to the forum BH,

    So, looking at the simple basics – hydrodynamically – of your hull, from the figures you posted you have:
    A length-displacement ratio of just over 5 at 5.1 – not bad.
    Running at a Fn of around 1.0 max speed, not bad
    And a simple Hp/tonne around mid 40s – not bad.

    There are several reason for fitting an interceptor, but the primary one is to assist a hull that finds itself in a less than optimum condition. Either by poor design, or simply to enhance the design – hydrodynamically.

    So with the case of your hull, I suspect it falls into the less than ideal for most of its loading conditions. By that I mean the LCG, is not in an ideal position to get the best performance of the hull. An interceptor will do that for you. But just like trim tabs, they work best when there is speed of flow under the hull.

    In other words, adjusting the amount of extension of the vertical tab (interceptor) should, I do stress should, assist in getting over the hump easier and maintaining a better attitude when planning too. A simple trial and error will allow you to work out the amount of extension for each loading condition and speed.

    The beauty of an interceptor is also it takes up much less space externally and internally.
     
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  6. Michael11
    Joined: Jan 2025
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    Michael11 Junior Member

    Hi Russell, welcome! For your 45' Hatteras, interceptors can work well at around 20 knots as they improve lift and reduce drag. They're more responsive than traditional tabs and can be more efficient, with less maintenance due to fewer moving parts. Since your boat is on the heavier side, ensure the interceptor system you choose can handle the weight and provide enough lift. Installing them could also simplify your transom layout compared to other options. Overall, interceptors are a solid choice, but it’s worth comparing all three options based on your specific needs and the boat’s performance.
     

  7. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    We've delivered quite a few interceptor systems since the late 1990s, for trim/list control as well as for active pitch/roll stabilization. Our application "rule of thumb" is to not offer interceptor systems for a vessel that expects much control below 25 knots...30 preferred. We've also delivered many stabilization systems that use transom trim tabs. From my read of your situation and options, I would suggest that longer trim tabs replacing those you have, with transom actuators would be the most effective option, giving you decent control well below even 20 knots.

    I've attached a picture of what the components of a trim tab stabilization look like for a 57' monohull yacht. The actuators are designed to keep as much as possible "inside" the hull and not outside in the water. Not directly applicable to your vessel..just put out there as food for thought.
     

    Attached Files:

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