[comship]

Hello to everybody I would like to ask if anybody has experience regarding Interceptors. We are currently building a new 32 Motor Yacht which shall feature this system (active model which acts as stabilizer as well). Humphree's Company carried out an initial survey by taking account the hydrodynamic data of the Yacht as submitted by the shipyard and they are talking for at least 3.5% reduction of the Hull's resistance.

[AMG]

I have used them on a 30-footer with a top speed of 40+ knots. Can´t say that they are working better than normal trimtabs on this kind of boat, but they are working well and are also less sensitive if you are using the boat where there is ice, as we do.

On stepped hulls we can not use them as they need pretty clear water.

Interesting to hear what your experience will be on such a large yacht!

[YachtForum]

Originally Posted by AMG

On stepped hulls we can not use them as they need pretty clear water.

Lars has provided good advice. Recently, I was contracted to design an entirely new trim system for a vessel previously using a derivative technology. I haven't tested the Interceptors on hulls with shaft & strut propulsion systems, but in water-jet applications, I suggest you consider the conventional alternative. I know of atleast one private vessel, water-jet driven, that is encountering hi-speed handling issues with the Humphree system. In speaking with the captain of the boat, I described in detail the phenomenon I expected he was encountering and he stated this is exactly what was happening. He is currently having the boat retro-fitted with conventional tabs.

[comship]

It is true that three years ago these kind of systems had many problems (both electically and mechanically) but in this Shipyard they have been used a year ago on a 24 m propellered yacht (they achieved 2.5 knots increase) and this year on two water jet 27m Yacht again with obvious increase on the final speed. My Yacht shall be launched on March 2008 (it has propellers), prior mine a 40m three water jet Yacht shall be launched (June 2007) equiped with Humphree Interceptors. Mr. Klarksson from Humphrees has assured me that in the worst case they are much better than the traditional trim tabs beause virtually transfer the LCG to fore (thus getting into plane more quickly and effectively), they are made of Carbon Fiber, they are simpler to install as they do not have hysraulic system

[BMcF]

Originally Posted by comship

Mr. Klarksson from Humphrees has assured me that in the worst case they are much better than the traditional trim tabs beause virtually transfer the LCG to fore (thus getting into plane more quickly and effectively), they are made of Carbon Fiber, they are simpler to install as they do not have hysraulic system

Interceptor a simpler, smaller and more 'compact' a transom lift device than a trim tab, yes. More effective..no. When comparing a properly designed trim tab system to a properly designed interceptor package of equivalent 'beam length', the L/D performance is very similar between the two for a moderate range of trim tab deflection. By 'moderate' I mean that more lift continues to accrue for a trim tab as the tab deflection angle is increased beyond about 8-10 degrees down..surpassing that of an interceptior at some point, but at increasingly unfavorable L/D numbers. However, both devices can, and do, produce the 'same' transom lift for a given case and thus, potentially, the same benefits (on a hull form where benefits can be had at all..not all hulls benefit from either device in hydrodynamic terms).

A drag improvement of 3.5% due to the addition of transom lifting devices (tabs or interceptors) is 'in the realm' of what I have seen before on certain hull types, so that part of the question could be realistic for your case..only your hydrodynamicist would know for sure.

[Innomare]

Another thing to consider with jet-powered boats is reversing. It depends of the configuration, but sometimes trimtabs can have a negative impact on the capability to go astern of the boat. Interceptors will hardly have an influence on that.

I know that many Italian and French built opens (Mangusta, Leopard, etc.) are waterjet-powered and still use interceptors, so I wouldn't say the combination is necessarily a wrong one.
Carl, could you describe that phenomenon?

For roll stabilization, trimtabs will be effective at lower speeds than interceptors.


Bruno

[YachtForum]

Hi Bruno,

The problem is best described as a phenomenon because it isn’t always constant. It also takes a particular combination of design errors to facilitate, i.e. intake gullet placement, deadrise, hull speed and placement of the plates. In any other app, it wouldn’t make a difference, but with flow disrupter plates, a pocket forms preceding the plate. Because of the vacuum created at the lip of an intake gullet, a laminar separation occurs just aft of the lip. These two areas can converge resulting in varying pressure differentials between the lip and the disrupter plates. The end result is a tendency to wallow at speed and a hull that won’t hold a true course. This problem becomes more pronounced with increased speed and aerated water under the hull.

As BMcF mentioned earlier, Interceptors don’t offer the leverage of a true trim tab. In addition, a trim tab with sufficient area will be more effective for achieving plane, especially under varying loads and CG’s.