Building 23' catamaran. Build updates

[Chimo]

Steve
I didn't know that 9.8 hp Tohatsu outboards weighed 60KG! That seems a lot don't you think?
The two little pods on the tinny EACH support 100kgs with little depression of the rear of the tinny hull and even less depression if the weight is spread across the two.

Noel
That square jet of water is beside the clean water flow directly under the hulls that the motor (s) has to work in.
If the hull has more bouyancy at the stern end where the motors are operating would not the hull sit higher in the water at the stern?

[Apollo]

Bit out there, but would trim tabs work to keep her level once moving? Looking at the vid and the photo, you would think that it might, but could be a squeeze for space. Other option might be hydrofoils on the motors to give a bit of lift.

[Shark Poker]

60 lbs is more like it for a Tohatsu 9.8, (27kg) which makes them the lightest of their class by a mile.
(The Yammy and old Johnson Evinrude were about 37kg, early Honda four strokes weighed only around 54kg from memory.)

Beware of these styles of boat test, they often to great lengths to come up with exciting stats.
Did you notice that the fuel tanks are placed as far forward as the lines will allow? Another indicator of the above sentence.
A further thing, which many Shark Cat owners may not appreciate, is that composite constructions like Steve's have plenty of strength but greatly reduced weight. This makes them more fwd/aft weight sensitive and complicated.
I was looking at building a cat a few years back and the plan was to be able to move batteries, fuel tanks and water tanks around to fine tune this area. And a ute load of props to try out.
Pauly

[Noelm]

Lift when under way is not an issue (well as long as you have the HP) trim tabs would probably be of no use (well not much anyway) regardless of how fast you go, that square "jet" still exists, there was some research done on a sort of turned down "foil" in the tunnel, but it didn't work to any degree, so was never used in production boats.

[stevemid]

There is a whole lot of funny "stuff" happens to cats at planing speeds, most of the conventional theories go out the window! When a cat is up and running, when viewed from behind, it an be seen that it is not actually skimming across the surface, but in fact digging two "troughs" that the hulls are in, and the tunnel at the rear is full of water, kind of like a square jet of water "squirting"out the back.

Yep, I think that's what the shipwright at Brooklyn Marine was trying to say about catamaran hulls being (or acting like) displacement hulls not planing hulls. Clearly I saw this on Frank's boat when you powered up the BF40's they just dug in the back end and created that trough. My reaction was that more HP would help, but then I started to worry about the extra weight. Seeing these ultra light tohatsu 9.8's acting the same way on a 27' hull gave me a great deal of comfort, actually.

[stevemid]

I'd like to put some rails up around the boarding platform so that I could safely fish from back there. Also I'm thinking I'd like to incorporate a gate which could also be a ladder to get from the water up on to the boarding platform. Any ideas or especially pictures of solutions of this type would be greatly appreciated.

Here is one I've found. I have also included pictures of my boarding platform config. The walk-thru is 500mm wide and the boarding platform is 860 wide:
Safety Rails Boarding Platform.jpg
Boarding Platform Access.jpg
Boarding PlatformG.jpg

[Chimo]

[Chimo]

[gofishin]

...If the hull has more bouyancy at the stern end where the motors are operating would not the hull sit higher in the water at the stern?

Statically, it depends! If the additional buoyancy provided sufficiently offsets the mass of the motors (and their location further aft), yes. But remember, we are also playing with the longitudinal locations of the CoG (centre of gravity) and CoB (buoyancy). To answer this Q decisively we need to first know where these are, and then recalculate for the altered design.

Also remember that buoyancy and hydrodynamic lift (arising from a particular shaped/designed pod) are not the same.

As for the Q re whether cat hulls are disp. hulls, semi-disp. or planing, from what I recall of any technical papers/articles I have read on cat hull design, they are not normally classified as planing hulls. However, this was a long time ago, and opinions of Naval Architects may have changed since as designs have evolved.

I can say from experience that some cat designs do generate a lot more 'lift' than others - at speed I mean, including those with relatively low tunnels. Some designs also included tunnel kickers. However, these can come at a compromise/performance cost in certain situations.

What I can say for sure Steve, is that from the following of this thread there will be a lot of interest in how your cat performs when it finally hits the water
Cheers
Brendon

[Shark Poker]

Where "Naval Architects" have gained advancement in recent years is with computer software.
You can read about a lot of this with composite (fibreglass over foam and other materials) constructions of single and multihull Yachts and powered bridge-deck catamarans.
Being able to calculate the size and volume of a vessel - aka Displacement - is now possible and also allows buoyancy to be established automatically.
Further, these draftsmen can improve the design by making the cut-out list of the components more efficient.
Using same software to calculate balance, lift, amongst variable sea conditions and operator style, differing HP is NOT among their scope.
Plus there is a guaranteed variation exponent with items of fit out, things that the draftsmen may not be at all hip and groovy with the latest choices like many of the Ausfishers.
So a builder needs to keep something up his sleeve and be ready to fine tune.
That is what I think.

[stevemid]

Some very good comments here and I wanted to take a bit of time to digest before responding.

I think gofishin hits the nail on the head when he says, and this certainly applies to me, "there will be a lot of interest in how your cat performs when it finally hits the water" and I could just wait till I splash my boat to see how it performs. However, I'd like to presolve at least the motor height problem (if there is one) before I get to the ramp.

The experience of the two previous Spirited 230 composite catamarans, judging by the pictures in one case and my direct experience in the other showed what looks to me to be an excess of turbulence at the stern. The one I did sea-trial with BF40's on it, rode low at the stern and the cavitation plates were buried. Earlier Chimo pointed me to a thread where Chris20984 was getting the motor height set correctly on a Sharkcat with 60HP Suzuki's on it - a VERY similar situation to mine as the hulls on a sharkcat and the motor config are very similar to mine. The big revelation in this was the advice that with a sharkcat, the starting point for getting the motors at the right height is 70mm ABOVE the bottom of the hulls. My designer took the commonly accepted wisdom and drew the motor well plans for the Spirited 230 starting at the BOTTOM of the hulls. From earlier discussions I had decided to build up my motor wells so I could install 25" leg motors. BUT I still started my measurements from the bottom of the hulls. So to get ahead of the game, I am having a set of jack plates made which will raise up my motor placement starting point 70mm higher.

The other decision I've made is to go with 70HP motors. However, as FatBuoy says these composite boats are strong and light but one of the consequences of being light is that weight placement is even more important requiring the "builder" to keep some cards up his sleeve. Indeed. Putting on the jack plates will put the motors even further back from the centre of bouyancy and the extra 20KG will have even more effect on weiaht distribution. Hopefully, this will be offset by the greater HP, BUT as Noelm says, "no mater how fast you go you still got that square jet of water exiting the back." This was proven by the 27' ultra light cat with 27Kg motors (thanks FatBuoy) still having a square jet of water exiting the tunnel. I have requested from the designer the longitudinal centre of gravity and bouyancy which will give me more data to worry about. Grrrrr……

So, in spite of wanting to know how the cat performs when it finally hits the water, the fact is, as Michael Clark says, "no game of cricket has ever been played on paper." But that won't stop us talking about it for ever!

Steve

[Noelm]

There is a few things that come into play here as far as motor heights go, IF, the motor was bolted directly to the back of the boat, so there was minimal distance to the cav plate from the hull, then the cav plate needs to be level with the bottom of the boat, BUT, as you move the motor back via pods that are not extensions of the bottom of the boat, or some sort of jack plates, the further you go back, the higher your motors can be run, there is some sort of starting measurements that I think are something like, for every foot back, the motors come up two inches. My old cat had very long pods, that did not go all the way to the bottom of the boat, and my motor cav plates were way above the bottom of the hull, and I mean a long way up.

[Noelm]

I found an old picture of the pods side on, if you can imagine a line from the bottom of the boat, to the cav plate, you will see it is about 6-8" higher, your is a whole new ball game, so you will need a "best guess" and go from there.

[stevemid]

Hi Noelm,
Yep, exactly. My pods are back 300mm from the transom and are not extensions of the hulls. On Franks boat, and earlier pod design had the motors back about 200mm. My jack plates will put the motors back another 70mm. I'm going to drill the holes in the jack plates to give me lots of adjustment but I'm thinking to start 70 mm higher than plan. As FatBuoy said, because of the lightness, fore and aft weight distribution will have a larger impact.

FatBuoy:
On this design the LCB, is exactly 3.4m from cutwater/front of DWL @ bow. The LCG is positioned as close to this point as practical and in the calculations for the standard design with BF40's the LCG is located 150mm behind the LCB. Pushing my heavier motors further back will have an impact. I am wondering what the impact will be of the increased HP (+30HP).
Steve

[Shark Poker]

Hi Steve,
3.4m & 3.55m? Those specs are quite cheerful and conservative for a 7 metre hull.
But as per my comment, lift and tunnel action are unable to be commented upon from our point of view.

The composition and/or weight of your craft with almost central LCB/LCG specs means you are highly unlikely to have any problems where the vessel is too heavy in one area.
In any case you could actually add weight to a corresponding part of the hull, without disadvantage, really.
Yes, really!
Because you have doubled the HP with only 30kg approx. increase in weight per side! So power is on your side.
It is almost the perfect situation.
Sure you will have the same engine height/props specs to sort out but that is the same as for any new boat.
But just look at the pics with those beautiful chines!
So stop worrying about such things at this stage, there are 20 pages here of proof that you have been careful and conservative with every detail.
You are on the downhill run, and this is where worrying can slow things down and make things miserable.
And things like rear rails can be added later.
There are many cold, lonely hours in your type of project with difficult decisions and work. You know it.
Please keep going, keep achieving, keep posting.