This will be a long post, but it will also be a step-by-step guide to replacing ordinary trim tabs with modern interceptors on a 31' Commander. The performance improvements are astounding so if you are thinking about replacing your old trim tabs this may be a useful post.

First, What’s an Interceptor?

Interceptors are not a totally new invention. The have been around on larger, commercial, vessels for some time. It is only recently the technology have been adapted for small pleasure crafts from 18-60 feet.

How Interceptors work is best described in this 1 minute video.

Planning for Interceptors on my 31 Commander

When I bought the boat in 1998 she came with Bennett trim tabs, approximately 3 feet wide on each side. I don't know when they were installed, but they were not exactly new looking even back in 1998 , so I reckon they had been in service for the better part of 25 years. Those of you who tried to run a 31' without the tabs know that the 31' model needs them. The Bennetts operated ok, but I would rather replace them before they would break down in mid-season.

At a boating fair I came across the company Zipwake and the product with the same name. I was very impressed by their live demonstration and immediately decided this was something for me and my boat. The automatic roll and pitch function and quick response was part of the equation, in addition to the advantage of the physical advantages when it comes to drag compared to ordinary trim tabs.

So last year I installed Zipwake Interceptors and I haven't regretted it for a moment. The installation is actually quite easy if you are the average handy man, so below are some photos from the installation. But let me start by sharing the result; Is this any good?!

My experience using the system in real life

I was told that effect of the interceptors is quite dramatic compared to trim tabs. That kind of claim can of course be the salesmen's standard lingo, but I must say I agree.

The first thing I noticed is how much faster the boat comes up in plane. Thanks to the automated and quick response it seems to almost magically do away with the planing threshold. The second thing I noticed was that the Zipwakes manages to keep the boat more level without sacrificing speed at equivalent RPMs. This comes with the added advantage of taking on sea and wakes much much better. In the past I often had to reduce speed in order not to hit incoming waves too hard. The third advantage is that the system itself handles any adjustments necessary caused by changing weight distribution onboard (people moving around), changing wind and sea conditions etc. The system does this so fast and so smooth that you hardly notice that it is adjusting, you feel like you are in optimum plane all the time.

At a cruising speed of 21-22 knots the interceptors only protrude 15-30% of their max length, depending on load and seas conditions. That means they are sticking out a meagre 4-9 millimeters below the hull. And still produces such an effect! On A side note: when thinking about which tools Dick Avery and his team had when they designed the 31 Commander in the early sixties it is amazing that they got it so right without using computer aided simulation and design support.

Installation

The old Bennett trim tabs had to go first of course. Here is the one on the starboard side.

Since the transom on the 31 Commander is slightly curved, trim tabs or interceptors cannot be mounted directly to the transom. You need some kind of mounting plate which is curved to fit the transom on one side and flat on the other to match the tab or interceptor. On my boat the "original" plate was made of oak (top two pieces in the photo). Since they survived 18 years I decided to make the new one for the Zipwakes in oak as well. You can of course make them in GRP instead but I did not have the boat in a temperature controlled place suitable for such work.

The oak mounting plate and the inner part of the Zipwakes are mounted to the hull using through-the-hull nuts and bolts.

In my case I opted for 2 interceptors per side. More about that later.

Since the Zipwakes will only protrude about 30 millimeters maximum, it is important to mount the Zipwakes as low as possible. 

The outer part of the Zipwake interceptors are now in place. The control cable to to each Interceptor goes through the hull above the water line.  (There is an option to feed the control cables behind the interceptors and through the mounting plates and hull , but I found that this was not possible on my boat since it would interfere with the longitudinal stringers on the inside of the hull.

Next up is the power distribution unit that operates the interceptors. The cable is for power supply. At this point is it worth mentioning that the Zipwake interceptor systems is entirely electrically operated. There is no hydraulics involvedAt the base there are connectors for up to 6 interceptors (3 per side) + connector for the control unit.

This unit needs to go somewhere fairly close to the Zipwakes but doesn't need to be accessible under normal operating conditions. 

So, in my case I decided to mount it on the inside of the transom, just below the cockpit deck.

In this photo I haven't yet connected the cables from the interceptors to the power distribution unit.

The final step was to mount the control unit at the helm.

Zipwake, by the way, had absolutely top class installation manuals and packaging making it a joy to work with.

In this photo you can actually see the interceptor fully ejected. It protrude about 30 millimeter maximum. Compare that with the "gigantic" flaps of normal trim tabs.

How many interceptors do you need?

Zipwake make the Interceptors in various lengths from 300 millimeters up to 700 millimeters. You can have one interceptor per side or you can have 2 or even 3 per side. I consulted with Zipwake support and they came to the conclusion that one 700 millimeter interceptor per side should do the job on a boat of my size. However, because it is rather heavy adding an additional 300 millimeter Zipwake on each side would be better. I could have started with just 2 x700 and adding 2x300 later if needed but that would mean a lot of extra work with mounting plates and so forth so I decided to go for 2x700 + 2x300.  Probably a bit of a overkill, but as they say at Zipwake; more interceptor length will give faster and better response!

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Very cool. Thanks for sharing. Will have to consider -

Alarik,

We have the Volvo Penta version of the Interceptors on our 47 Commander.  They work well.  The system can be set to automatic and even takes GPS speed into account to adjust themselves.  I am somewhat concerned about the longevity of the system vs. traditional tabs.  I am kind of a fan of keep it simple stupid.  

We will see how they hold up.  They do perform well.

-Darin

Darin; Wasn't aware Volvo even had interceptors in their lineup. They certainly do not in Sweden. There is very little info about it on their website. Since both Volvo and Zipwake have their HQs in Gothenburg Sweden, I would not be surprised if Volvos version resulted in a spin-off namned Zipwake. Or that Zipwake is the OEM for Volvos interceptors.

I forgot to mention that Zipwake is distributed in the US by Imtra Corporation.

Hello Alarik:

Excellent posting!  I too was aware of the Volvo Penta product, could be the same item.  These Interceptors provide superior performance over traditional trim tabs.  European made marine equipment usually get a name change when they arrive here in the states.  Another product would be SLEIPNER which is known here as SIDEPOWER.  Coincidentally, Imtra corporation handles both of these products, they are in New Bedford, MA.

As an aside, did anyone know that Twin Disc also produces trim tabs out of their Belgian facility.  They are far and away better than what I have seen produced here in the states.  Just a side note.

Best Regards,

Jim Rivas

Good to hear from you Jim! Looking at photos of the Volvo Interceptors and the Zipwake ones the sure have a very similar design. The control panel is very different though.

Alarik,

Wondering if they are the same.  What does the connector look like that goes on the tab?  Mine go through the hull and connect with a Deutsch connector.  There is a control box on the helm and a GPS antenna on the hardtop.

-Darin

Vovlo Interceptor Tabs on a 47

Volvo Interceptor Manual

von Hofsten Alarik said:

Darin; Wasn't aware Volvo even had interceptors in their lineup. They certainly do not in Sweden. There is very little info about it on their website. Since both Volvo and Zipwake have their HQs in Gothenburg Sweden, I would not be surprised if Volvos version resulted in a spin-off namned Zipwake. Or that Zipwake is the OEM for Volvos interceptors.

Excuse my ignorance, but I will put it on display anyway. On some boats that are not CC, trim taps are critical to get the boat to plane and to have the bow down enough to see, and bring the speed that they plane to a low enough number so they are not running 3500 rpm's just to stay on top.

They say that the fixed wedges or trim tabs provide a longer running surface, so the key is the running surface increase more so than the downward pressure of the adjustable tab.

How would this product address this issue?

BTW, how much do they cost?

I'm pretty sure that the added running surface of trim tabs is not a factor, and the downward force they create is the true factor. If it was just running surface, they wouldn't need to move up and down. As further evidence, I just removed a swim platform from my boat that added about 8 inches of running surface. This produced no additional lift on my boat. 

The Interceptor and others like it, create a cushion of water in front of them as the flow is interrupted, thus lifting the stern while running. The farther the 'dam' moves down, the bigger the cushion, the more the lift.

Hi Jim:

For the 1971 model CC 47, 55 and most probably 60 Commanders, the trim tabs were redesigned to being longer and wider which were a great improvement in their running angles translating into more speed.  Great redesign which was further introduced later on the 41’s and 45’s.  Tim Toth further enhanced these tabs by further refining their design for optimal performance.  So in other words, the increased sizing did have a marked better running angle as well as speed increase.  Fortunately I had two 47’s and two 55’s to observe the differences, one each with the smaller tabs and one each with the larger set.

Jim Rivas

Darin,

The looks of your interceptors are very similar to my Zipwakes. The system layout is also the same. The actual connectors are very different. Mine looks similar (but not equal) to NMEA2000 connectors.

Also my dashboard control unit has a built in to it. A separate GPS receiver/antenna is optionally available. In my case the system works fins without it. 

BTW, are you coming to the Rendezvous 27-29 July? I would love to see you there.

Darin H said:

Alarik,

Wondering if they are the same.  What does the connector look like that goes on the tab?  Mine go through the hull and connect with a Deutsch connector.  There is a control box on the helm and a GPS antenna on the hardtop.

-Darin

Jim Frens is right. Here is an illustration from Zipwakes site that illustrates Jims point.



Jim Frens said:

I'm pretty sure that the added running surface of trim tabs is not a factor, and the downward force they create is the true factor. If it was just running surface, they wouldn't need to move up and down. As further evidence, I just removed a swim platform from my boat that added about 8 inches of running surface. This produced no additional lift on my boat. 

The Interceptor and others like it, create a cushion of water in front of them as the flow is interrupted, thus lifting the stern while running. The farther the 'dam' moves down, the bigger the cushion, the more the lift.

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For your reading pleasure, the latest issue of the Chris-Craft Commander Club's newsletter, Styled In Fiberglass, along with past issues, can you accessed through the link below.

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