Cause of slamming on Gemini 105MC

Seeking input. I've been trying to figure out a way to reduce slamming on my Gemini anytime I'm out in average ocean conditions or even when there is a good chop in bay waters. My thought was that the slamming that occurs on Geminis (and other cats that have low bridgedecks) was due to the force of a wave hitting the area that angles back and down that connects to the lowest section of bridgedeck (see attached photo showing the area where the slamming occurs delineated in red). But now I'm questioning that and am wondering if what's going on is what I've been thinking of as a Thunderhole Effect, i.e., channeling of water and air into a confined space that has too little room for it to escape, causing compression and a subsequent mini-explosion when the compressed water/air is released. On my Gemini, the sound of the slamming is eerily similar to the sound when water rushes into the cave at Thunderhole, channels in to an increasingly narrow space from which there is no escape, then explodes back out due to the compression and subsequent, fairly violent release.

I'm preparing to drive to Cape Breton, Nova Scotia on Tuesday to work on my Gemini and, among other things, install 4X4 ribs running fore and aft in the small section (roughly 6'X6') where the pounding occurs (ground zero?). My thought is that these ribs will diffuse the force of the waves hitting that section and reduce the slamming. But, if the slamming is caused by compression and release of channeled water and air that has no place to go, my rib idea probably won't reduce the slamming, unless the ribs somehow reduce the compression, e.g., by permitting easier venting or otherwise lessening the compression. Thoughts?

Pete222. Yes, you can put on ribs, it may help. Other cats (eg older Prout designs) put a central underwing rib of larger dimensions to "split" a wave to reduce pounding a bit. There is also another way which may help. Reduce the weight of the cat. Get rid of anything you don't need. I have seen some cat of this size lose 500 lbs of extra rope, anchor chain, old barely used sails, games, sail boards, clothing, spare food etc. Again the reduced weight will help. But ultimately a Gemini was designed and built with a low wingdeck and they pound as do Catalacs, Heavenly Twins etc. There are no simple answers. Hope you find some success.

oldmulti said:

Pete222. Yes, you can put on ribs, it may help. Other cats (eg older Prout designs) put a central underwing rib of larger dimensions to "split" a wave to reduce pounding a bit. There is also another way which may help. Reduce the weight of the cat. Get rid of anything you don't need. I have seen some cat of this size lose 500 lbs of extra rope, anchor chain, old barely used sails, games, sail boards, clothing, spare food etc. Again the reduced weight will help. But ultimately a Gemini was designed and built with a low wingdeck and they pound as do Catalacs, Heavenly Twins etc. There are no simple answers. Hope you find some success.

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Already junked the junk. Lightening her up further as we speak. Sillette drive was taken off today by the owner of the boatyard that cares for her over the winter and tomorrow he's lifting out the Westerbeke. Major weight savings going with OBs. I'm also selling the Highfield RIB that came with the boat and replacing it with 60 lb Takacat (or, more likely, a less expensive no-name version) and a lighter OB. Those two changes alone will shave off 440 lbs and probably give me another inch of bridgedeck clearance, not to mention allow the boat sail much better. But my objective in doing all the weight reduction is to improve sailing performance, not reduce slamming. Though you make a good point--more bridgedeck clearance will undoubtedly lessen the slamming. Thanks for your input. Much appreciated.

I don't profess expertise.

The Skoota was designed with a slight vee. I also had to come up with a way to finish the vee and not leave it blunt.

The bottom of the bdeck is veed about 4" and this shows how I finished it back to flush by adding another vee. I expect my boat will have some issues in big seas as I have a 2" gap between the bdeck and the hulls that will waterspout.. time will tell.

That 4" vee was expensive in terms of weight as I added basically a second sole to do it...and the glass had to be strong to deal with big waves...

You also ought to see if the bow is down vs the stern because bow down an inch means bdeck is down almost an inch...

If I were you; maybe try just making a vee to part waves instead of landing atop them.

Here is another view. The gap is closed by an aluminum panel not painted, yet.

I would think the length to beam ratio with the beam being quite narrow might also contribute to the issue you describe.

I’ve been on narrow planing power Cats that get a similar slap/pounding at slower speeds when not on plane with the water riding up the hulls slapping the underside of the deck.

You might try mounting a GoPro or something similar underneath to document exactly what is happening.

If it's hydraulic lock it will usually "sneeze", you get spray shooting out the front. If it's pounding it sounds like that.

Changing your underwing might help as mentioned above, but I'll second weight. 1 outboard weighs less that 2. Yep not as maneuverable, but lighter. Be ruthless. Allow a day or even 2. Take everything out of the boat that isn't nailed down. Put it back 1 item at a time and weigh every one. Dig around under floors, every void. Look for any little thing that is lurking in the dark. Are the keels hollow ? Great place for water tanks.

Remember think of it as an aircraft. Would you load that thing on a plane ? If not consider why you need it on the boat.

Best thing about this is it costs nothing/very little.

Pete222 said:

Already junked the junk. Lightening her up further as we speak. Sillette drive was taken off today by the owner of the boatyard that cares for her over the winter and tomorrow he's lifting out the Westerbeke. Major weight savings going with OBs. I'm also selling the Highfield RIB that came with the boat and replacing it with 60 lb Takacat (or, more likely, a less expensive no-name version) and a lighter OB. Those two changes alone will shave off 440 lbs and probably give me another inch of bridgedeck clearance, not to mention allow the boat sail much better. But my objective in doing all the weight reduction is to improve sailing performance, not reduce slamming. Though you make a good point--more bridgedeck clearance will undoubtedly lessen the slamming. Thanks for your input. Much appreciated.

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Hi Pete222! Can you tell me what yard removed your westerbeke and sillette? I have been wanting to do this very thing. I want something new on my gemini given the sillette is super difficult to service now as it's no longer made. Really appreciate your help here!!!

Hi Pete222! Can you tell me what yard removed your westerbeke and sillette and replaced with an OB? I have been wanting to do this very thing. I want something new on my gemini given the sillette is super difficult to service now as it's no longer made. Really appreciate your help here!!!