Testing The Underwater Exhaust In Various States Of Trim In Preparation For The FPB 97: Motion Impact

One of the most difficult design aspects to get right is an underwater exhaust. Powerboat builders and designers have been wrestling with these issues for years, and nobody–let us repeat that, nobody–has a pat answer. With conventional motor yachts, there is so much horsepower involved in propulsion that exhaust noise is a major issue. Add in engine rooms that are almost always near the center of the vessel, and the need for–and difficulty with–an underwater exhaust multiplies.

In our case the options are easier. We have very small power requirements, so noise and vibration are minimal. The engine room is all the way aft, so the noise is isolated from the living quarters. The aft location coupled with small engines and big rudders makes it possible to place an underwater exhaust, if indeed it is warranted, behind the prop(s) and rudder(s). This eliminates the inefficiencies that occur when you are injecting exhaust, and the related turbulence, ahead of the propulsion/steering foils.

In the case of the FPB 97, we have a scale model in Wind Horse with which to experiment, which is what we’ve been doing of late. We have set her up with extra payload, so that hull immersion forward and aft is closer to the FPB 97. In the case of the underwater exhausts, this affects the imersion of the exhausts, and how they sound during various sea states.

At the same time, this gives us data on the behavior of the canoe body in unusual (for Wind Horse) load conditions. There are some factors we’ve been studying that point us in a certain direction, and we are testing to see how these work. Changing the trim, moving fuel/water forward or aft to immerse or raise the stern, allows us to get an idea of how the FPB 97 might react.

We are a ways from finalizing things…there are still some configurations to test, but we are zeroing in on a decision.

In the meantime, we’ve made a short video, taken going uphill against a 4-to-7 ft (1.2/2.1 m) sea.

 In this video Wind Horse is carrying full fuel and an extra 4 to 5 tons of water. She is right on 100,000 pounds, about 45 metric tons, about 10% over her normal maximum gross displacement. The breeze is in the low-to-mid 20 knot range, seas are right on the nose, and she is running at 1900 RPM, averaging her normal 11 knots through the water. (To see the video larger size, you can view it at our smugmug page.)

The camera microphone picks up a variety of background sounds, and to our surprise, you can hear the clothes dryer going, and Linda clinking dishes as she works in the gally. Engine sound level is quite subdued, and you can listen for tonal variations on the underwater exhaust that might be occurring as a result of the heading, seas, immersion of the exhausts, etc.

The video is one long shot, no cuts or edits, so that you get an accurate sense of motion, sound, and ambiance, without a series of cherry picked sequences.

Occasionally you will here a thump as the fluke of the oversized Rocna anchor drops into the sea. In these conditions we doubt the anchor on the FPB 97 would ever touch, as she has significantly more freeboard forward. The deadrise angle–the cross sectional shape–of the FPB 97’s hull is more like the FPB 64: i.e. softer riding than is the case with Wind Horse.

We have a bit of surfing footage as well, which we’ll post in a few days.

Posted by Steve Dashew  (August 13, 2012)

4 Responses to “Testing The Underwater Exhaust In Various States Of Trim In Preparation For The FPB 97: Motion Impact”

  1. Patrick S Lasswell Says:

    Reducing low frequency noise preponderance from video recordings is probably going to be a pain. A somewhat less expensive method than establishing a calibrated reference and applying a filter might be to install an external microphone to your camera. If you’re using the EOS-1D X, you can attach a small external mic, shotgun or cardioid as you prefer, and see what results you get from that. (I must admit a certain amount of concern regarding adding additional input to cameras that powerful, they’re so close to sentience that adding a microphone might just push it into full conscience. If your EOS-1D X starts asking you for manipulator arms, reflash the BIOS immediately.) My friend the sound guy waxes rhapsodic about the importance of sound in recordings and is quite pointed about the importance of using a good microphone. Bad lighting and good sound is emotional, good lighting and bad sound is a lot harder to sell. B&H Photo lists a range of microphones for that camera ranging from $150-750. Although expensive, the Sennheiser ME66 is quite strongly recommended as a shotgun mic. Sound is a rabbit hole time and money disappear into, but if you’re not happy…

  2. Steve Dashew Says:

    The camera used in this video is the Canon S95, which actually as pretty good filters, better in fact than the 1DX. We have shotgun mics, throat mics, etc., but have not used them in years.

  3. Rod Manser Says:


    Pretty impressive to be over loaded beyond max gross and still do 11+ kts head into that sea. When you mentioned that the 97 has a softer dead rise than the 83, does that mean less hobby horsing or less pitching moment?

    Finally, is the surfing video available?



  4. Steve Dashew Says:

    The 97 will pitch a bit less in the same sized seas as a result of its longer length, finer entry down low, and significantly increased mass. It will also have a softer ride because of the deadrise angle differences. You can see all of the Wind Horse and FPB 64 videos at https://setsail.com/videos-slideshows/ where it set has its own link.