Polypropylene honeycomb panels for hull construction?

[fallguy]

I guess Ad Hoc might be needed to clarify if beam is waterline beam or some other.

I’d have to drop the boat down to get the best numbers.

Waterline beam is about 15’7”, so 10% of that is 18.7” .. Redfin is way over that for’d, and infinity at the extensions. Midship clearanxe was 17” as designed and we are meeting that afaik.

So 17”/187 = 9% >6%

I would need to measure to tell you for sure, but she doesn’t slam at all. I am installing splashrails; the water really grabs these hulls in all less than zero sea states.

For continued discussion, start a new thread…we had one started awhile back I can move the replies to as well.

 

[Fellies]

Bdeck wave slap is simple to avoid.

Minimum clearance at 6% of beam midship and citing @Ad Hoc 10% at the bow.

There are some caveats.

If the “midship” clearance is further aft from the bow; this reduces it.

If you avoid massive sea states; there is less likelihood to encounter.

If you install a forward trampoline, a webbed one means the boat can tolerate a blast of water better.

On my boat, I also added two vee shapes. The bottom of the bdeck is a mild vee of about 4-5” per 4’3” each side. The front is also veed.

Prior to the bow extension design offered by @Ad Hoc, the boat was pitching and the bdeck slammed one time in a nasty heaving boat wake.

Since then, no sea state has resulted in slam/shudder. I call it slam/shudder because the entire boat is affected by it.

Now, to be clear, my boat was 8.5” negative trim prior to the extensions, so it was way down and hobby horsing a horror.

The bow extensions keep the front of the boat higher sooner, if you will. So, while the 5-6%/10% rule is good general guidance, a longer distance to midships also helps.

A naval architect like Ad Hoc may have this pinned down to a harder science using length to midship.

This picture is best at hand photo, can’t really see the vees well. She sits on a lift aft end in so I can get another pic if I take a boat out. Maybe another day another thread.View attachment 209872

But surely you still getting some slap?
How do u reduce that with nidacore

 

[fallguy]

But surely you still getting some slap?
How do u reduce that with nidacore

If there is any slap, I’m not noticing it.

I suppose it is technically possible, but I also avoid seas over 3’ here.

Nidacore, no matter.

 

[brian eiland]

For continued discussion, start a new thread…we had one started awhile back I can move the replies to as well.

I think this subject thread should address your thoughts on that other thread to discuss bridge deck slamming,...

X

Thread 'Center nacelles and wave pounding in cats.'

Sep 1, 2007

«The idea is that the force of the wave collisions will be dampened by the cushioning effect of the V section (of the nacelle)". ( Chris White, The Cruising Multihull)
On the other hand, the wave collisions are more frequent on a catamaran bridgedeck with a center nacelle, aren’t t they?

• xarax
• Replies: 79
• Forum: Multihulls

I'm reading back thru it now, and in just the first page I have found LOTS of interesting postings. For instance here is a gentleman with a number of experiences on larger cats,..

Post in thread 'Center nacelles and wave pounding in cats.'

Oct 15, 2007

Hi everyone,
I am new to this forum, but not new to catamarans having owned a few power (7.5m x twin 225HP, 9.5m x twin 275HP and 12m x 320HP jet) and a few sailing ones (Windrush, Nacra 16sq and 18sq, Hobie17 and Schionning 12.5m) .
I always tell people cats should be compulsory!

I had the 12m jet cat designed and built during the America's cup days in Fremantle, she had asymmetric hulls (top speed 30kn). There were 4 ton of alloy in the factory before fitting out. She did not perform well and needed a lot of power to get up and go. she also used to slam a lot into the "Fremantle...

• Spiv

• 

BTW, those bow extensions of AdHoc remind me of my search for proper 'bulbous bows' extensions to the bows of catamaran vessels....both ideas seeking to cut down the height/size of the bow waves created by the individual cat hulls. (how about adding that photo of yours to that other subject thread?)

 

[fallguy]

I’d rather not tempt the devil. My boat is a Woods’ design with a major correction and another on the way and he posted there long ago. If you want to start a new thread, I would be glad to post there.

 

[brian eiland]

Are we talking about these bow extensions?

 

[fallguy]

Are we talking about these bow extensions?

View attachment 209906

Yes, we have fully hijacked. The design is great. It basically saved the boat. Lotsa lotsa work over 3 years as Mn seasons are short. I did the tips to get positive rake as lotsa weeds were collecting on them with negative rake. They also offer a bit of fun aesthetically when the boat is running flat water there are some mini bows running ahead.

But they really take care of a trim problem. They stopped hobby horsing that was present in the basic design. The bows were not quite full enough, imo. But the extensions made a longer boat..and it runs better in the sea.

I did use pp core for the bdeck double bottom and the for’d basic angle.

 

[Ad Hoc]

....might be needed to clarify if beam is waterline beam or some other.....

Waterline length, not the beam.

 

[fallguy]

Waterline length, not the beam.

Oh, jeepers. Some math to redo for me. Darnit and disgusting I posted that. Cannot fix the errant post, but this weekend, I’ll see if I can get a measurement somehow. I plan to head offshore on nearly flat seas to fish.

Thanks for the refresher.

Things must be okay with my boat nonetheless because not getting waves to hit the bdeck unless I get some wild stuff at the inlet at Packery Channel, like 3’ plus shallow water rollers and outgoing tide.

If we use 38 feet which is bow extensions and engines, then 5% is 1.9 feet or almost 2 feet at midships. This I doubt I am making. But then 10% of 38 feet is 3.8 feet at the bow. Not making that either…

Let me get some better pictures and numbers and report back.

What is so odd is I always felt like I was under 5%, so when I did the math, I wondered about it. When I did the original calcs, I would have used 32 feet before the extensions and discointing the engines.

Anyway, embarassing. But I have been off a bit lately and seeing a doctor.

 

[Ad Hoc]

...Things must be okay with my boat nonetheless because not getting waves to hit the bdeck unless I get some wild stuff at the inlet at Packery Channel, like 3’ plus shallow water rollers and outgoing tide.....

It was a combo of several issues really.
You needed volume to correct the trim, static trim at rest. And that would concomitantly also help the other seakeeping issue.

So your hull, is

9.37m on the dwl.....and I added a BB of 0.88m which is 9%-ish of Lwl, at the dwl...abotu above 10% if you take in the whole underwater length.

But it was about the reserve buoyancy that you needed up front, shown shaded:

So killing 2 birds with one stone.
Thus, because the bow is no longer is easily pushed down into troughs of waves and you gain benefit from a longer LWL for seakeeping and powering.

 

[fallguy]

It was a combo of several issues really.
You needed volume to correct the trim, static trim at rest. And that would concomitantly also help the other seakeeping issue.

So your hull, is
View attachment 210005

9.37m on the dwl.....and I added a BB of 0.88m which is 9%-ish of Lwl, at the dwl...abotu above 10% if you take in the whole underwater length.

But it was about the reserve buoyancy that you needed up front, shown shaded:

View attachment 210006


So killing 2 birds with one stone.
Thus, because the bow is no longer is easily pushed down into troughs of waves and you gain benefit from a longer LWL for seakeeping and powering.

It is working well, but for the splash rail matter. Those are ready for paints and the terrifying bonding challenge. Once completed, the boat will fair better in head seas where I ought to be able to run above displacement speeds a bit and in following seas; pretty close to wide open or 23.4 kts on small ones.

The operational challenge here is the fishing grounds for pelagics is a good 30 miles out, plus a harbor run of about 15 minutes, so 10 mph takes me 3.25 hours to get to fish, and at 15mph, 2.25 hours, so squeezing a bit more out of the boat is big savings in time. For first light fishing at 6:30am, I have to leave at 3am and run in the black of night. I don’t have radar, yet, so I usually leave about 5am and hope to get put there by 8 or so splitting the difference. This is why go fast boats are more popular here. But my boat feels real safe abeam in sick big rollers and is fun to fish on the big deck and I am getting 2+mpg, so I can run 120 miles and it costs me like $250. The go fast guys here spend $500+ to do same. The fishing grounds really have noone fishing for miles; too far to go! There are a LOT of fish here.

Many thanks, as always.

I will try to add some numbers today.

 

[brian eiland]

I think this subject thread should address your thoughts on that other thread to discuss bridge deck slamming,...

X

Thread 'Center nacelles and wave pounding in cats.'

Sep 1, 2007

«The idea is that the force of the wave collisions will be dampened by the cushioning effect of the V section (of the nacelle)". ( Chris White, The Cruising Multihull)
On the other hand, the wave collisions are more frequent on a catamaran bridgedeck with a center nacelle, aren’t t they?

• xarax
• Replies: 79
• Forum: Multihulls

I'm reading back thru it now, and in just the first page I have found LOTS of interesting postings. For instance here is a gentleman with a number of experiences on larger cats,..

Post in thread 'Center nacelles and wave pounding in cats.'

Oct 15, 2007

Hi everyone,
I am new to this forum, but not new to catamarans having owned a few power (7.5m x twin 225HP, 9.5m x twin 275HP and 12m x 320HP jet) and a few sailing ones (Windrush, Nacra 16sq and 18sq, Hobie17 and Schionning 12.5m) .
I always tell people cats should be compulsory!

I had the 12m jet cat designed and built during the America's cup days in Fremantle, she had asymmetric hulls (top speed 30kn). There were 4 ton of alloy in the factory before fitting out. She did not perform well and needed a lot of power to get up and go. she also used to slam a lot into the "Fremantle...

• Spiv

• 

BTW, those bow extensions of AdHoc remind me of my search for proper 'bulbous bows' extensions to the bows of catamaran vessels....both ideas seeking to cut down the height/size of the bow waves created by the individual cat hulls. (how about adding that photo of yours to that other subject thread?)

Rather wished you had taken these bow extensions discussions back over to this other subject thread,...as you had hinted at:

Post in thread 'Center nacelles and wave pounding in cats.'

Aug 23, 2026

There was a period of time when I thought the idea of adding 'bulbous bow' shapes to the front of a catamaran's hulls was a solution to keeping the height/size of the waves created by the bows down,...thus less contribution to bridgedeck slamming.

I'm now not so convinced,...but you might find a number of discussions/postings related to these ideas,..by using the search function with 'bulbous bows' inserted.

Here is another attempt to keep the size of those bow waves down.

I certainly what anything that size on my bows.

• brian eiland

• 

Post in thread 'Catamaran Bridgedeck Clearance'

Jan 19, 2026

On many ships the bulbous bow helps to keep the bow wave from being so large, and helps keep it from climbing up as high on the bow.

I would be looking for them to help keep the wave peaks the bows are creating,....down in size. I've seen a number of cases where the two peaks from each side join together in the center of the tunnel to contribute to slapping the bridgedeck.

• brian eiland

 

[Skip Johnson]

I've been there and seen the boat it is an impressive build.