Wave slam

[W3un]

I am in the midst of buying a 5-10 year old cat in the 35-42 foot range. I'm curious as to wave slam. I delivered a Prout Quasar 50 from the Med to Brazil and, although a very comfy boat....and very slow... the wave slam was terrible. I don't want that again! I've briefly looked on the Forum and haven't found much...any feedback is greatly appreciated! Happy New Year to all and many thanks.

Alan
Santa Barbara
W3UN

[boatman61]

There will always be some wave slap in seas.. if not under the bridgedeck then the lee hull in beamish seas.
Idealy you want around 2 - 3ft clearance for minimum effect.

[W3un]

Thanks.....Yes, I'm sure it can never be totally avoided in a mid-size cat. I'm biased to avoid it as much as possible by almost 4 weeks of major pounding on the Prout 50... a very strong boat but no fun to sail.

[billknny]

Originally Posted by W3un

I am in the midst of buying a 5-10 year old cat in the 35-42 foot range. I'm curious as to wave slam. I delivered a Prout Quasar 50 from the Med to Brazil and, although a very comfy boat....and very slow... the wave slam was terrible. I don't want that again! I've briefly looked on the Forum and haven't found much...any feedback is greatly appreciated! Happy New Year to all and many thanks.

Alan
Santa Barbara
W3UN

This is definitely a case of try before you buy. If you haven't chartered boats of this type, it might be a really good idea to do so before you put down your money on a major purchase. And charter in places where you will be sailing in the ocean, not backwater lagoons.

The problem is the issue is completely subjective. What drives one person nuts, another doesn't care about. So asking is "waveslam a problem" doesn't really get useful information to you.

It is human nature that people who have invested heavily in something will minimize the problems. I suspect if a Prout 50 was not tolerable for you, smaller cats will not likely be any better.

[Saleen411]

Originally Posted by W3un

I am in the midst of buying a 5-10 year old cat in the 35-42 foot range. I'm curious as to wave slam. I delivered a Prout Quasar 50 from the Med to Brazil and, although a very comfy boat....and very slow... the wave slam was terrible. I don't want that again! I've briefly looked on the Forum and haven't found much...any feedback is greatly appreciated! Happy New Year to all and many thanks.

Alan
Santa Barbara
W3UN

I've been told 5-7% of LOA is a good rule of thumb. Example....

LOA (in feet) x 0.7 = Minimum Bridgedeck Clearance (in inches)

For example, for a 45-foot cat: 45 x 0.7 = 31.5 inches.

Unfortunately, my measuring of BD clearances at boat shows isn't realistic for cruising. I've not been able to determine how far a boat settles after being loaded for cruising.

[ozsailer]

G'day Alan,
IMO, you should have no less than 900mm to 1000mm of clearance under your bridgedeck to reduce slam. Be aware ALL cats slam at some point but bridgedeck clearance significantly reduces this. Be aware there is a big difference between deck slap and deck slam. Some designs such as our old Lagoon 410 would slap due to the design of the trailing edge of the inner hulls which was quite boxy. It was quite tolerable and once you got used to the sound you rarely noticed it. We now have a Lagoon 440 with a totally different bridgedeck profile. We have noticed a considerable reduction in slam compared to the 410 and its flat bridgedeck. Hope this is of some assistance.


Greg H
Lagoon 440

[Chotu]

There are a lot of factors here. Much more than posters typically think about.

1) Your course relative to the conditions is what causes or stops slamming. Pick a good one.

2) reservation buoyancy vs. total weight of the cat. If you carved a catamaran out of an enormous styrofoam block and made a small bridedeck clearance, would it slam? No. It would pop right over the waves. So heavy condomarans are more prone to slamming than light, well performing boats

3) Bridgedeck clearance. How much you need depends entirely on #2 above. It's different for every boat and every loading condition. You'll need to get out and see how they perform in chop yourself.

However, higher clearance on a lighter weight catamaran means less slamming.

The Prout is a dog of a cat in these areas.

[danielamartindm]

Originally Posted by Chotu

There are a lot of factors here. Much more than posters typically think about.

1) Your course relative to the conditions is what causes or stops slamming. Pick a good one.

2) reservation buoyancy vs. total weight of the cat. If you carved a catamaran out of an enormous styrofoam block and made a small bridedeck clearance, would it slam? No. It would pop right over the waves. So heavy condomarans are more prone to slamming than light, well performing boats

3) Bridgedeck clearance. How much you need depends entirely on #2 above. It's different for every boat and every loading condition. You'll need to get out and see how they perform in chop yourself.

However, higher clearance on a lighter weight catamaran means less slamming.

The Prout is a dog of a cat in these areas.

A very insightful take

[sailhand]

There is also a calculation for suitable bridgedeck clearance on multihulls which i think is possibly a better indicator of adequate clearance. Waves typically have a peak and a trough. If you imagine each hull in trough with the peak of the wave under the bridgedeck i think you have a better idea as to what is happening. The wider the bridgedeck the more height that is necessary as obviously clearance will be a function of both parameters. A good rule of thumb is a minimum of 20% of the gap between the two hulls and 25% should be comfortable. Not many cats meet these dimensions unfortunately.

[Dave_S]

Originally Posted by Chotu

There are a lot of factors here. Much more than posters typically think about.

1) Your course relative to the conditions is what causes or stops slamming. Pick a good one.

2) reservation buoyancy vs. total weight of the cat. If you carved a catamaran out of an enormous styrofoam block and made a small bridedeck clearance, would it slam? No. It would pop right over the waves. So heavy condomarans are more prone to slamming than light, well performing boats

3) Bridgedeck clearance. How much you need depends entirely on #2 above. It's different for every boat and every loading condition. You'll need to get out and see how they perform in chop yourself.

However, higher clearance on a lighter weight catamaran means less slamming.

^^^ Agreed

[contrail]

With all due respect, a lot of slamming is due to operator error. If you are motoring upwind, and you decide to go straight into the wind and waves, you will most likely slam. Bear away. The boat is designed to sail at sailing angles, and there is way less slamming there!

[boatman61]

Originally Posted by sailhand

There is also a calculation for suitable bridgedeck clearance on multihulls which i think is possibly a better indicator of adequate clearance. Waves typically have a peak and a trough. If you imagine each hull in trough with the peak of the wave under the bridgedeck i think you have a better idea as to what is happening. The wider the bridgedeck the more height that is necessary as obviously clearance will be a function of both parameters. A good rule of thumb is a minimum of 20% of the gap between the two hulls and 25% should be comfortable. Not many cats meet these dimensions unfortunately.

I think the conclusions of James Wharram in the 60's and his seaworthy designs set the benchmark for modern cats.. mind it did take till the early to mid 80's before designers and manufacturers finally admitted he was right..

[tomfl]

Originally Posted by boatman61

I think the conclusions of James Wharram in the 60's and his seaworthy designs set the benchmark for modern cats.. mind it did take till the early to mid 80's before designers and manufacturers finally admitted he was right..

Not sure what to make of this. Wharram designed cats had some real downsides. They were one of the worst designs for going to windward. Of course the closer you can sail to the wind the worse the bridge deck will be as a rule of thumb; as IT guys would say is this a bug or a feature.


Wharram cats also were narrower than modern designs. The upside of a wide cat is stability and the ability to carry more canvas resulting in more speed. Again the IT guys would say is this a bug or feature for a Wharram.


Due to the design of the stern/aft section of Wharram cats it was not all that easy to power them and as a result they were sailed more than some of the more modern designs; or stayed at anchor if there was no wind. Not sure if this is a bug or a feature.


Not trying to dis Wharram cats which have a lot of upside. But several modern designs offer real advantages over Wharram cats in terms of speed, space, comfort, and to some looks. Downside is the modern cats are a lot more expensive than most Wharrams.


As for slamming as has been previously posted while all cats/boats will slam the sea state is the biggest factor and often simply altering course 10-15 degrees will solve the problem.

[boatman61]

If memory serves it was Wharram who began the 50% beam to length ratio and also the bridgedeck clearance factor to minimise slamming.