Bridgedeck clearance in manufacturers specs

[Still Hopefull]

I see so often in discussions the issue of BD clearance on multihulls. From what I can gather it is obviously a factor of some importance when deciding which make / model one might want to buy. I think I have only seen the BD clearance mentioned on a manufacturer's website on one occasion.

When listing a boat's specs the manufacturers usually have LOA, LWL, draft, beam, mast height etc but not BD clearance. If this is so important, why is it that manufacturers don't list it as a standard spec on their websites?:h)

[Ozmultis]

Rough rule of thumb, if the manufacturer doesn't mention clearance - then its usually a safe bet it doesn't have enough. Seawind and lightwave in Australia both include it in their specs.

[Talbot]

It would also be useful if the spec's included how much additional boot topping is needed per additional 1000lb stores.

[MooseMoney]

As the bridgeck clearance increase, your windage increases. So it is always a compromise between two opposing forces. Regardless of the bridgedeck clearance, as the water builds in height, at some point or another, the bridgedeck will slam. The only way to do away with slamming is to go monohull. The question is how much slamming are you willing to tolerate. Most modern designs today generally cater for a bridge deck clearance of between 700 - 1200 mm. Having said this, I will never buy a catamaran with less than about 600 mm clearance. Some older designs are substantially lower.

[victor]

BDC or lack there of is one of the catamaran manufactures dirty little screw ups. Not necessarily the designers but sometimes the builders. This is due to the practice of 1. overloading 2. installing features and accommodations of lets say a 50 foot cat in a 40 footer, when this is the case the cat becomes a dog due to the lowering of the BD and raising the free board and top decks. The less sophisticated the sailor (buyer) the more this practice becomes prevalent just look at whats on the market. My feeling is that all these roomarands are wounderfuly comfortable at the dock or mooring. What happens when we go to sea in your home away from home and the bridge deck clearance is less what it should be, you develope stress on the structure and the crew. Again my personal opinion is that in the enthusiasum to sell more vessels the industry in many cases is offering up a vessel that appeals to thoughs of you that are looking for accommodations and comfort before sailing abaility and saftey.
I firmly beleive that a well designed cats proportions should be longer in the hulls and less in the cabin structure for example; 40 foot hulls with accommodations of a 30 footer. The problem of course few buyers want to pay for 40 footer with less that 40 foot room.
As more roomarans are offered for sale with massive in suite heads, etc.
What happens in the market as fewer and fewer proper sailing cats are made available there is less good examples of fast, safe and comfortable (at sea) vessels to choose from, with fewer and fewer sailors driving them.
Please lets not loose the plot of what multihull saing is about.

[downunder]

a good designer or manufacturer will be able to give you the lbs/Kg per inch/cm immersion figure for their vessel.

if they cannot ????????????????

[2hulls]

For the record, all cats slam when conditions get bad enough. Greater bridgedeck clearance can minimize the frequency and degree. While monos, by definition, cannot experience "bridgedeck" slamming, they are most certainly susceptible to wave slapping on the hulls, as are all multihulls. On a multi, the difference between waves slapping on the hulls and bridgedeck slamming is difficult to distinguish and is basically moot.

Rough rule of thumb, if the manufacturer doesn't mention clearance - then its usually a safe bet it doesn't have enough.....

I think Andrew is correct on this point, and I'll go a step further: Bridgedeck clearance, like low weight, even when published, is likely exaggerated. On a couple of occasions I have seen cats in cruising mode whose builders boasted of high bridge deck clearance on forums and in publications. I was astonished of how low they were in actuality.

The best determinant of bridge deck clearance is actual sighting, either at in-water boat shows or during chance observations on the water. As a surrogate to actual sighting, simply looking at magazine ads can be revealing. Pay particular attention to how low the sterns of bridgedecks can be as revealed in ad pictures - a pose frequently shown. If you can't imagine getting under the boat in a dinghy, it's probably too low.

2 Hulls Dave

[dmmbruce]

. If you can't imagine getting under the boat in a dinghy, it's probably too low.

2 Hulls Dave

A lot of good points there Dave. Your last sentence is very thought provoking. I'm not sure I have ever seen that.

Is it not worth making the point that bridgedeck clearance is not just about 'slamming' on waves. I thought that was rare these days, you have to go back years to the likes of my Heavenly Twin for that.

The main point I have seen being made by some designers is that the clearance is needed for the height of the combined bow-waves, where they meet in the middle of the boat. The faster the cat, the bigger the problem. Long, thin, weak, wave-piercing hulls will help this but bring lots of other problems.

I don't know where that takes the discussion. But it certainly confuses the issues.

Mike

[Dave]

If you can't imagine getting under the boat in a dinghy, it's probably too low.

I put my boat on a sand bar in Belize. It was handy to be able to dingy under the anchor, drop the whole shooting macth in to the dink, motor underneath and out the back to drop the anchor. Used rolling hitches on the chain to the winches and got us off (with a bit of wiggling).

Dave L38 #38

[James Val Jackson]

I put my boat on a sand bar in Belize. It was handy to be able to dingy under the anchor, drop the whole shooting macth in to the dink, motor underneath and out the back to drop the anchor. Used rolling hitches on the chain to the winches and got us off (with a bit of wiggling).

Dave L38 #38

G'day Canuck. Good points you mention in your post. Sure hope 'StillHopeful' learns something from all this, however as said - all these opinions may well leave one a tad confused.

Now to all of you in this subject - please let me state some facts.

2 cats designed by David Barker (& yes he is a close friend of mine) DO NOT slam, smack or have ever made solid contact with the ocean. I could only guess at the ocean sailing miles these 2 cats have done however 'Sundancer' (40' cat) was built in 1967 & was sailing up until 10 yrs ago. 'Sundreamer (57' cat) was built in 1982. Each cat has sailed 10's of 1000's of miles in all conditions. There are many factors in the design & building which result in the bridge decks not being pounded. As already stated - rightly so - anything under 600mm is not very wise. Both these cats have 700mm or more - free board from - load waterline to bridge deck clearance. Both cats are light but strong & have most of their weight centeralized plus not being overrloaded & not over cabined - if their is such a word? My point here is - it can & has been done. Multihulls can be easily overloaded by the designers expectations, the builders lack of care & the owners taking to much on board. However it does not need to be so. Multihulls are great stable, get there quicker yachts & more comfortable than some monohulls BUT must not be overloaded & need to be allowed to sail well. IMHO There is some info on 'Sundreamer' go into 'google' by typing in - Sundreamer in New Zealand - can't seem to find my simple link to 'Sundancer' though. Hay guys - it's worth a look IMHO

Everyone keep smiling & keep your multihulls on the light side & not to overloaded, enjoy health, sailing, happiness & fair winds to all. Ciao, james

[multihullsailor6]

Hey James,

Now how could you have mislaid this link:

www.sundreamer.co.za

[James Val Jackson]

Hey James,

Now how could you have mislaid this link:

www.sundreamer.co.za

G'day Rog. Well might you ask. : Of all people to lose the 'Sundreamer' site. Phill & David would both disown me, for sure. I know where 'Sundreamer' site is but without getting everyone who might be interested - very lost & maybe me too - I can't find a simple link to 'Sundancer'. It's buried in a long book & a tad difficult to find. I still have the official working drawings of 'Sundancer' from way back in '66 when David, Michael & I built it in North Sydney, NSW in OZ. I also have a new set of drawings for a slightly modified version that David sent me earlier this year. I missed out buying 'dancer' some 20 yrs back - cause I didn't know it was for sale & David forgot to let me know. Unfortunately it's now stuffed & beyond recovery at an acceptable value/cost figure, being it's now 44 yrs old. It was way ahead of it's time back then & still a great design but the laminate is now old & a bit past it. I've talked to David about another (like a up-dated 'dancer' - 49' X 23') - still with canted hulls but built out of carbon/kevlar/foam/epoxy with pre stressed hulls & decks @ aprox 3600 kgs sailing weight. Not all that difficult for me as I can still remember how to do it after 45 yrs of practice in my yacht building/repairing business. However I don't think I can dig up all that amount of money these days. We are going to loose $350K when we sell the property cause of the world financial problems. I'll probably have to settle for a lesser multi but my heart still is in that direction or a newer similar one.

Thanks for that info - maybe others will have a look at it. 'dreamer' is sure a great looking multi & is easily sailed & a good sea boat. Ciao for now. James Are you sailing at the moment

[dmmbruce]

Sundreamer link not working for me.

Mike

[multihullsailor6]

Sundreamer link not working for me.

Mike

Hi Mike,

It is not working even for me dumbass, so here is the correct one:

www.sundreamer.co.nz

Funny how two little letters at the end make all the difference .....:

[multihullsailor6]

Sometimes I think designers and boat yards of todays modern cats have gone back the learning curve when it comes to bridgedeck clearance.
This is what I call a good bridgedeck clearance, found on an already 25 year old 46' FP Casamance:

[dmmbruce]

Hi Mike,

It is not working even for me dumbass, so here is the correct one:

www.sundreamer.co.nz

Funny how two little letters at the end make all the difference .....:

Thanks! What a boat.

Now that does look a speed merchant. A real pratamaran at first sight.

However if JamesVJ says it has done thousands of miles it must be better than that!

James if this is your ideal cruising cat then we are on different wavelengths and I now understand your interest in speed, lack of large saloon, and as light as possible. She is listed as 4.8ton for a 57ft boat! She must have been fascinating to build, and then to sail.

Cheers.

Mike

[Tabs]

It occurs to me that we could qute easily build up a database of bridgedeck clearance for various cruisinng cats. If we all take a quick measurement whenever the opportunity arises, we'll soon have a good idea of the TRUE clearance for most models.

To get things going, I've just measured the cats nearest me in the marina, they're mostly older British cruising cats and all have surprising little clearance:

Heavenly Twins 26' -10.75" / 275mm Aft (Not counting the nacelle, which has zero clearance)
Patterson Twins 34' -15.75" / 400mm Fwd.
Woods Banshee 35' -16.5" / 420mm Aft (But seems pretty level)
Woods Flicka 37' -11.75" / 300mm Fwd
Dazcat 9.2M -19.5" / 500mm Fwd.

There are plenty of other cats around here, including some french boats, so when I get the opportunity, I'll measure a few more.
If the boats near you are the same model that someone else has already measured, it doesn't matter, it's still worth taking measurements, we'll get a range of results.

[multihullsailor6]

Tabs,

Where on the boats were these measurements taken, forward or aft end of the bridgedeck?

[Tabs]

Good point Roger, I've added a note to each one.

[georgetheleo]

It occurs to me that we could qute easily build up a database of bridgedeck clearance for various cruisinng cats. If we all take a quick measurement whenever the opportunity arises, we'll soon have a good idea of the TRUE clearance for most models.

To get things going, I've just measured the cats nearest me in the marina, they're mostly older British cruising cats and all have surprising little clearance:

Heavenly Twins 26' -10.75" / 275mm Aft (Not counting the nacelle, which has zero clearance)
Patterson Twins 34' -15.75" / 400mm Fwd.
Woods Banshee 35' -16.5" / 420mm Aft (But seems pretty level)
Woods Flicka 37' -11.75" / 300mm Fwd
Dazcat 9.2M -19.5" / 500mm Fwd.

There are plenty of other cats around here, including some french boats, so when I get the opportunity, I'll measure a few more.
If the boats near you are the same model that someone else has already measured, it doesn't matter, it's still worth taking measurements, we'll get a range of results.

My Woods 36 "Scylla" measures 28.5" aft 29.75 fwd" slammed once in 500 mile sea trip. Outboatd powered near fully retractable.However in a chop of more than 3ft I cant really use OB power due to cavitation. Everythings a compromise .