Page 1 of 13

Efficient cats

Posted: Mon Dec 29, 2014 6:05 am
by Saqa
Not sure if this is the right place for this. I have been looking at the Ecat and Cat 22 for a while now reading up all about cats on this forum. I came across a really interesting discussion on pondering a new model that Shine was pretty keen on as well, something about an efficient displacement cat that can handle a bit of offshore duty

The was some discussion on outboard leg length and drowning the motor. Got me pondering as well. Can a fixed rear facing jet outlet be fitted into the bottom of each hull a bit aft of the lowest hull point ( aft of the rocker ). I suppose the hull itself will act as fairing for the outlet. Steering in form of sailing cat type rudders. Maybe tee foils on the rudders to combat pitching a bit

Should remove the probs with leg lengths and other prop problems including leg and prop drag. A bridgedeck mounted engine could feed the jet outlets with either dual pumps or single pump with split output. I have no idea of the efficiency of running jet without the drag from legs and prop vs outboard efficiency vs jet efficiency

Re: Efficient cats

Posted: Mon Dec 29, 2014 9:42 am
by jacquesmm
You ask two different questions.
What size cat are you looking at and is it sail or power?

Power I suppose since you talk about the Ecat and Cat22.
The Ecat is limited to protected water, no offshore work.
The Cat 22 is close to the Australian outboard powered cats and will do well with twin outboards as designed. It is an offshore boat.
I don't know which post by Shine you refer too but it was certainly not applicable to the CT22.
For medium speed applications, look at the Skoota designs by Richard Woods.
They use a central outboard and are well fitted to medium speeds, let's say up to 15 knots.
Richard will elaborate on that.
I will post the study plans for the 20 and the 28 this week but there is also a 24 and a 32.

I would not bother with complicated jet pump systems: KISS (keep it simple) is a proven principle in boating.

Re: Efficient cats

Posted: Mon Dec 29, 2014 10:22 am
by Saqa
It was quite an involved discussion about cats like the supercats from africa, displacement types. The outboard leg length was one of the limitations being discussed. I have been very keen for a boat like this and was wondering if the idea faded away due to that limitation

I thot jets might solve that and mentioned it

I did previously contact Mr Woods regarding if it was possible to build the skoota as an open deck fishing boat instead of building all that cabin work but he declined to respond. I have been hoping you would have an efficient displacement cat suitable for offshore work in the pipeline as per the discussion I mentioned having read previously

Re: Efficient cats

Posted: Mon Dec 29, 2014 10:31 am
by jacquesmm
I see only 2 posts under your name.
1. Can you please link to that other thread? I am the designer and would have seen it if it was on this forum.
2. If not mentioned in the thread: what size boat, intended area of use, expected speed.
3. I know quite a lot about outboard legs like the Silette type but want to know the answer to the questions above first.
4. Richard Woods is present on this site. he is in another time zone but I am certain that he will reply later. We sell the Woods designs, supply kits, materials and give tech support but without a boat size and type, I can't answer.

Re: Efficient cats

Posted: Mon Dec 29, 2014 11:06 am
by jacquesmm
I am moving this to the category "Woods designs and other Multihullls".
We'll try to keep all the multihull discussions in one place.

Efficient cats

Posted: Mon Dec 29, 2014 11:10 am
by jacquesmm
Not that all others are inefficient but this topic is moved from another forum.

Re: Efficient cats

Posted: Mon Dec 29, 2014 11:36 am
by Saqa
This is the topic where I saw all the discussion regarding efficient displacement cat design and all the discussion on limitations on such designs due to outboard leg length, am surprised it faded away

I only joined this forum a few days ago. I had wanted to ask about the Cat 22 and if the hulls are efficient enough to run a pair of 40hp for a top speed of around 20knots and good cruise around 10-15knots with low fuel burn. Then I came across a topic where someone else had asked about running smaller engines to which the response was in the negative so found my answer there. Searching the forums, I also came across the discussion on the supercat type design in the link I posted above and the leg length issue had been in my mind so thot to pose the questions regarding jets

Re: Efficient cats

Posted: Mon Dec 29, 2014 12:17 pm
by jacquesmm
OK, I remember now. That was 5 years ago and we went all over the place in the discussion, from small planing sport fishing boats to large sailing cats.

The Cat22 needs a pair of 40 HP to go 20 knots in a formed sea. It will go 20 knots in flat water no wind with a pair of 25 but for offshore use, I recommend 40 or more.
Jet outboards are less efficient and you should increase those values by about 20%.

What is your ideal boat? Fishing with open deck, top speed 20 knots, offshore capable, around 22' long?
I need some figures as a starting point.
BTW, open deck is not my idea of seaworthiness and the best cross beam for a cat is a high one that goes all across as in a cabin frame.
Give me some figures first.
We exchanged a couple of ideas now but I still don;t know what kind of boat you want.
It is premature to discuss outboard shaft length if we don't know if this is for a 50' or a 20' boat etc.

Re: Efficient cats

Posted: Mon Dec 29, 2014 12:38 pm
by Saqa
I am looking for around 20-30' length, whatever gives best performance with a pair of 40s. Boat will be moored or beached depending on tides or times so no trailer limit. Running crossing a lagoon with persistent chop about 2' most days of the year with the prevailing 25 knot tradewinds, first stop 30mins after crossing the lagoon at current boat speed of 15knot to cast 200g poppers with 8' rods at the first reef with breakers. Then travelling the chain heading offshore fishing the next reef and so on. Coming back to the last reef at the top end of the lagoon and travelling back to base through the chop with wind on the back

Two fishing plus skipper plus decky

Looking to configure flat deck with all around rails and centre console driving position

2 stroke yamaha enduro 40hp as is the most common engine in the remote northwest and makes for less downtime compared to other engine options

Thanks for your time mate

Re: Efficient cats

Posted: Mon Dec 29, 2014 12:41 pm
by Saqa
btw, good to know about the Cat 22 with 40s. Do you have any milage numbers on that? Fuel efficiency will help for daily use