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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Amazing. I would love to see hard data on the differences between these and trim tabs in terms of better fuel economy and increased speed. Conceptually, it seems to be a fantastic product! There are boats out there that even trim tabs can't correct the bad design of the hull efficiency. But maybe, these will. From some of the other details and packaging, if the price were comparable, they seem to be a better alternative to tabs.

Not to mention, I'm guessing they are not subject to corrosion. I had to add anodes to my new tabs. Fortunately, the anode makers make one specifically for Bennett tabs, simple bolt on.

I just replaced the tabs on my 47 commander with new tabs and came across this system in doing my research. What concerned me was that although they may be superior on plane, I just didn’t see how they would have much effect at cocktail speed +/- 8 knots when I need to correct for a list as people move around the boat. Giant tabs start at least minimally working at anything over about 4 knots. 

To the the other member who has the 47 with these, what is the performance like at slower speeds? Also what is your cruise and top end now? If there is really an improvement and they still work like tabs at low speed I may do it.

I see another advantage that has not been discussed in this thread.  We had overkill Bennett's on our 31" Commander (42" each if I recall correctly) and they made a following sea a handful even in the fully retracted position.  I vividly remember getting caught in seven footers right off our stern.  I think these fully retracted would have been much better in that kind of circumstance.   

Chris,

We’ve only had the interceptors on our 47 the entire time we have had it, so it is hard for me to compare between the two.  Load speed operation was never a problem.    I just assumed it was the weight of the 47 keeping her from rolling.  The Volvo tabs can be set for automatic trim.

Our top speed with the interceptors so far is 26.5 MPH, confirmed with 2 GPSs.  8.1 L gas engines.

I do worry about the longevity of the interceptors.  I am replacing 3 of the 4 tabs this summer.  Approx 8 years old.  Installed by previous owner.  I suspect the yard sprayed high pressure water up into the tabs during my last haul out.   3 of 4 went down at the same time.  They also appear to have been installed without shims, so they may have been bowed slightly to match the transom, shortening their life.   

Darin


Chris Wickersham said:

I just replaced the tabs on my 47 commander with new tabs and came across this system in doing my research. What concerned me was that although they may be superior on plane, I just didn’t see how they would have much effect at cocktail speed +/- 8 knots when I need to correct for a list as people move around the boat. Giant tabs start at least minimally working at anything over about 4 knots. 

To the the other member who has the 47 with these, what is the performance like at slower speeds? Also what is your cruise and top end now? If there is really an improvement and they still work like tabs at low speed I may do it.

No you're right, it's the hull design that it really doesn't roll. It takes a significant wake (usually from a-holes in giant sportfishes) to roll the boat. That's the design not the tabs. If I'm putting along at 7-8 knots, you know one of those days where everybody wants to go for a boat ride but you don't have any particular place to go, you wind up with 5 people on one side of the boat looking at something and get a degree or two of list that you can correct with regular tabs at low speed. Do the interceptors have the same effect or do you have to be going faster for them to work, is what I'm saying?


Darin H said:

Chris,

We’ve only had the interceptors on our 47 the entire time we have had it, so it is hard for me to compare between the two.  Load speed operation was never a problem.    I just assumed it was the weight of the 47 keeping her from rolling.  The Volvo tabs can be set for automatic trim.

Our top speed with the interceptors so far is 26.5 MPH, confirmed with 2 GPSs.  8.1 L gas engines.

I do worry about the longevity of the interceptors.  I am replacing 3 of the 4 tabs this summer.  Approx 8 years old.  Installed by previous owner.  I suspect the yard sprayed high pressure water up into the tabs during my last haul out.   3 of 4 went down at the same time.  They also appear to have been installed without shims, so they may have been bowed slightly to match the transom, shortening their life.   

Darin


Chris Wickersham said:

I just replaced the tabs on my 47 commander with new tabs and came across this system in doing my research. What concerned me was that although they may be superior on plane, I just didn’t see how they would have much effect at cocktail speed +/- 8 knots when I need to correct for a list as people move around the boat. Giant tabs start at least minimally working at anything over about 4 knots. 

To the the other member who has the 47 with these, what is the performance like at slower speeds? Also what is your cruise and top end now? If there is really an improvement and they still work like tabs at low speed I may do it.

Jim,

I have a 74 47 Commander---do you know how the trim tabs are actuated?  My dash toggles although seem to actuate a motor on each side, nothing is changing the angle of the tabs.  Port is down, Stbd is partially up--leaving the running attitude as you might expect.  Second year with boat, long time 41 & 42 commander owner. This one has the 370's Cummins engines.  A bit baffled and wondering if there is a reservoir to fill   or other hydraulic mechs which need service. 
 
Jim Rivas said:

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

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