Trying to design my own cat.

Richard
The boat has small sectional area if you compare it with something like a sailing boat with all its rigging. The Cd is not too bad either so windage is not that high. Windage is similar to a small motor car.

If I use a big prop with suitable gearing I can get a bollard pull of around 1kN (220lbf) from a 3.8kW motor. See the attached JavaProp screen image.

I expect you would find small sail drives would not be able to produce this sort of thrust. One of the gearboxes I am considering has a 1" shaft so it is solid if you compare with most outboards.

Many people do not understand that thrust and power are not the same thing. If you use a big prop you can get huge thrust for little power. I worked out that I could get a bollard pull of around 1000lbf using pedal power with a suitable prop.

So I expect that in heavy weather I will have the ability to make way into a strong head wind. I agree that a competent coastal cruiser needs that ability.

Rick W.

 

Before I jump to the wrong conclusion here, why do you ask and why are you wondering about it ?

 

Sounds good to me Rick, I think your project is quite exciting. Hurry up and finish all development so I can see it on you-tube! And then you could make it commercially available. Maybe I'll add it to my boat collection when I am filthy rich. Still like sails though.

 

I have decidedto show my wonderful catamaran in it's full glory as soon I have some images to send. The entire deck is an array of waterproof cushions which rest on an aluminium grill. (think of 4 double bed mattresses lying on one wall of a shark cage). This allows bad waves to pass through the cage and the shock from pounding will be eliminated by the mattresses.
There are sofa-like back supports which attach to the outer edges of the deck. The hulls have no solid cabin structure above the deck line (excuse my terminology). Instead I use a well sealed mini dome tent which folds over each hatch, providing ample headroom for temporary tasks like making a coffee or checking the laptop during a storm. These mini tents fold away to nothing when the hatch needs to be robustly sealed. (think of the folding hood of a baby's pram, or a convertible car). The hulls will be used primarily for storage. Larger low, flat tents are used for sleeping on deck. (no room to sit inside the sleeping tents...so not good for the clostrophobic) These tents are robust and attach to the back supports (mentioned earlier).
This design is very light and provides a huge amount of deck space for 8 lazy sunbathers. hmmm...that sounds a bit creepy

 

These are just pretty pictures (incomplete) to give some idea of my proposed deck layout. The tiller will go in between the 2 aft back supports. The helmsman will rest his albow on the point at which the tiller pivots.....so the whole tiller actually supports his arm. The spade rudders will turn effortlessly but will have very little feel. I can accept that trade off.
I apologise for the incomplete drawings at this stage. (these were done with house architecture software, as I am more of an architect than a boat designer.)
No numbers to give you yet. In the coming weeks I'll be getting familiar with Freeship.

 

The attachment below is my attempt at using Freeship. The fairing above the waterline is crap but below is not too bad (I think), and that's all I'm really interested in. If anyone out there has the skill to make these hulls faster (Rick???? or even you JCD baby designer), then feel free to play with it and send it back!! (am I dreaming)


Data that must not be changed:

boat length = 22.5 feet (does not include rudder)
boat weight + payload = 2240 lbs
boa = 12 feet
no dagger boards ( I'll be beaching it all the time)
must be canoe shape (for light conditions)


I'm not really sure if the design I have done is ideal.

Even just some plain old advice would be awesome.

Cheers

- Richard

 

Hello Richard,

If this is your first attempt...you have done a pretty good job of it brother. Very nice. Unless you're like me that asks for everyone's opinion but doesn't let anyone make up my mind for me, Rick's optimized hull below the waterline will be the way to go for a hull at maximum design paramater. With that said, here is my 2 cents.

Excellent work...stated with a bit of envy because my first attempt was a total failure and took lots of work. You're LWL/BM ratio is 11.2. Those hulls will be easily driven. Your LWL (22.35ft) to LOA (22.5ft) ratio is low, so reserve end bouyancy is also low. This is good because it reduces hobbyhorsing, but its bad because the ends will not recover quickly upon submersion. Obviously, carrying capacity is reduced on the ends so most weight should be centered amidships.

For me, the mini keels in the design will probably add more wetted area than may be worth for pointing ability. Perhaps a shallower but longer keel will provide the pointing ability that may be lacking from the absence of boards without the addittional wetted surface. The longer the keel, the wider the turning radius, so a compromise will be required.

You may also want to consider high aspect lee boards pivoting on the outboard sheer and eliminate all un-necessary underwater wetted surface while introducing just enough required surface by dropping the board. I would try to think of a way to place them on horizontal tracks to move them forward or aft to balance the helm for different sail areas leeds.

The wetted area appears to be high and the draft is deep but, with that said, windage produces more drag than wetted area foot for foot and you have very little windage without a house on the hulls and getting the windward hull unstuck will require greater effort. Six humans on the tramps will however produce windage as will the tents, so staying low should be considered.

The LOA/BM ratio is a result of the width of the hulls and althought it is used often by many, I give it little thought unless the purpose is for trailering or slipping restrictions. The ratio I consider important is at the waterline. The ratio for your hull separation is 40% (9.1ft/22.35ft) which represents good maneuverability, but you will have to pay careful attention to the CE when designing the rig and I would make significant effort at the threat of lashing crew with a wet noodle, to keep crew or passengers on the windward side at all times. While anchored or not underway, make sure everyone doesn't decide to sit off on the same side or a titanic list is a possibility. This is ofcourse, my own paranoia and inate instinct to keep the boat right side up.

Take everything with a grain of salt since my intention is to provoke thinking. Well, except for the part that you did a good job. Keep it going, but be forewarned that this is addictive.

J

 

Great stuff J, thanks very much! You've definitely got me thinking. I never felt comfortable with those mini keels. Wetted surface is my main enemy. As you say, crew weight distribution will be important on my boat. I don't mind compromising a little on overall stability, but unnecessary wetted surface really bothers me.

quote "You may also want to consider high aspect lee boards pivoting on the outboard sheer and eliminate all un-necessary underwater wetted surface while introducing just enough required surface by dropping the board. I would try to think of a way to place them on horizontal tracks to move them forward or aft to balance the helm for different sail areas leeds."

I really like this idea. The boards would be very easy to access and could just kick up like the rudders? Is there any major performance compromise from using this type of system? I don't care less about visual appearance...I'm sure I could make them look tidy enough.

I'm going to take a look at your fbm files later tonight just for fun, now that I can view them.

- Richard

 

On my previous boat I had a matress inside the cabin. It had a cushion you could remove to get to a hatch where stuff was put in for storage. It was a hassle to move things around to get in there. The more loose things the more irritating it gets to move a lot of things to get to another.

I like your tent idea. A stowable tent however has to have a support structure, and losing one piece... You could also think in the direction of an infatable tent. The advantages of an inflatable tent is you can blow it up and reverse the action to deflate it as well. Just an idea... deflated it may be served to sit or walk on ?

If you plan on having a boom and not sail at night you may use the boom for the tent support as well - just an idea.

 

Leeboards

Why on the outboard side? If you put them on the inboard side you won't have to worry about bashing them while docking, and you won't have to look at them.
Multihulls are insensitive to lead, so just place them so that their center is under the CE of the rig. By the way, have you read any yacht design textbooks? Skene's Elements of Yacht Design, at least? If not, you should do so before continuing to try to design a boat.

 

Richard
Have you any thought on the construction material for the hulls?

Also a 6% section is a poor choice for a rudder. I have attached the polar plot from JavaFoil for the rudder shown. It could be worse than this because it is surface piecing so will also ventilate. You have a lot of drag for not much steering force.

You could improve pitch damping with fuller sections above the waterline both bow and stern.

Rick W.

 

Agreed...

I think 15% would be best. That is the section I intend to use on mine. The boards at 12%.

J

 

Hello Yachtie...

I suggested the outboard side because they're in clear water and not in the sideslip stream, and because if he needed to do a quick repair he wouldn't have to do it from under the deck. Bashing them is a matter of skipper error and easily avoidable with some caution.

Can you provide an authoritative reference that explains why multihulls are insensitive to leed? I would like to get my eyes into it since I have never found anything like that. The best range that I found for a multi is 12% to 15% of the LWL.

If the question about Skene was for me...yes. I agree 100% that it is at the very leats the book to read. But tell me this...how much help was it for multihulls instead of just yacht basic generalities? I tried to make connections with the material but fell short with my limited experience. I felt Principles of Yacht Design was better.

J

 

Hello Richard,

I think that you should reconsider compromising on stability even if just a little. Don't! Only million dollars thoroughbreds take the stability compromise to the edge to gain that extra 1/1000 of a knot. Wetted area is everyone's enemy but I will take that over instability anytime. Take your time and you will increase stability and reduce wetted area as the design evolves.

Tidy for the lees is nice. You may want to fair the trunk in. I'm not really sure of the performance reduction that may be encountered from having them above the waterline, but I would think that breaking the waterline plane could cause some cavitation or disruption of flow. I don't know how significant it can be. It may not be that serious for the design...unless it is a million dollar thoroughbred. Making them kick up is a must and really easy to set up.

Glad to hear the brain matter is firing up.

J

 

Thanks folks, I am thoroughly enjoying this thread and learning lots. Please consider yourselves all allocated brownie points.

Seasons salutations, safe sailing and fair winds