Catamaran Conversion

Sorry, but this is just a misnomer based upon poor understanding. I read so often this 1.34 in conjunction with “hull speed”..what on earth is that??..there is no such thing as “hull speed”.

We talk about Froude numbers…..V/Sq.root(g.L)

The shape of your hull, in the sense of the hull lines, fine or full etc, coupled with the weight, giving you the L/D ratio defines what your hull can or can’t do. Nothing to do with 1.34. This is just some simplified method proposed by many to make things “fit” but does not relate to every boat, and as such is commonly misused and misunderstood.. I have never ever used this 1.34 –thingy in over 25 years of design.

To understand what a hull will do, you firstly need to understand the L/D ratio of the hull you’re looking at. Real figures NOT sales brochure values. The figures you see in sales brochures are just that..sales, and do not reflect reality and as such must treat them with serious caution and NOT absolutes.

If the hull has a vertical transom this will tell you about the hull too..shape is only important in the sense of what the hull looks like, where the LCB is, and hence the CB too. Is the hull like a brick..or like a plank of wood on edge..or like a flat plate etc. Each will dictate how the hull behaves coupled with its respective L/D ratio.

For example, a plank of wood on edge has a CB of 1.0…but if the L/D ratio is say 10 (a long plank)..she’ll go very smoothly through the water and fast. But if the L/D ratio is say 1.0, ie literally like a brick…you could surf behind the waves it makes as it attempts to go fast but fails.

 

Deering.....

First listen to what AdHoc is saying, he probably has more cat experience than all the rest of us put together.......

Second you need a lines drawing of your hull (as built not wishful thinking) and you current floatation measurements to establish real operating displacement, not what someone said or the travellift guy mentioned 2 years ago. Once you have that you can talk to a NA about how to improve economy.

Third...length is the cheapest speed you can buy, I would advocate adding to the bow (so called "wave piercing") as well as the stern, at least back the length of the swim grid......

 

Again, putting aside the hull mods, where I was going is a conventional strut/ /shaft drive via a V Drive gearbox keeping your current engines.

What this allows you to do is:

Leave the motor where it is, just spin it 180
leave it level and get a down angle V drive
Hook it up to a conventional prop strut drive and maybe put a skeg in front of it for stump jumping.

I would think this may allow you to keep all your soft connections to the engines (wires /hoses) intact and only have to re route the exhausts.

Steve

 

Thanks Tad. Good to hear that Ad Hoc has a lot of cat design experience. Wasn't clear if his comments were applying mainly to monohull design.

Wish I had a lines drawing, but that will have to be developed from the existing hull since the builder is now defunct and the designer is unknown. As far as displacement, right now all I have are travel lift readings from two separate lifts on three occasions - they all match up fairly well. Any reason why I shouldn't use those numbers? If they're w/in 5-10% of actual, that seems like it would be adequate for my purposes - the displacement of the vessel varies by that much or more with fuel/water/payload anyway.

My initial thinking has been to add onto the stern, back to the aft edge of the swimdeck as you suggested. Form that extension into a canoe stern, then extend beyond that a few feet with the flat section above the prop area, which would also support the rudder. That should help improve the L/D issue that Ad Hoc is referring to, adding perhaps 8 feet to the hull length. And with smaller engines, even with the hull extensions, I would expect the displacement to decrease.

How far forward would you extend the bow sections? If I add stern floatation, I'll have longitudinal trim issues. The bow extensions could help address that.

Also, I have limited experience with rudders...size, shape, and placement relative to prop. Any suggestions? I'd prefer to have a rudder shoe beneath it to help protect from debris and grounding, so if a deep rudder can be avoided that's best. At low speeds/tight quarters I use the twin engines for maneuvering very effectively.

My hope is to identify a qualified designer/NA who can assist with this modification.

 

Steve,

Got it. Since I don't currently have a rudder system, I'd also have to add that, and I have no room aft of the engine to do so since the engine mates up to the transom.

Keeping the engine in place, adding a gear box, and going straight out the canoe stern might be as simple. Welding on a hull extension isn't rocket science. Then I'd get the hull length increase benefits that others have been advocating here. Going with a smaller engine would save a lot of weight, as well as engine compartment space. Thinking I could swap the D-6's (with one good outdrive) for the D-3's. The D-6's are in great shape with low hours.

 

Understood, was just thinking with my fingers.

Steve

 

Ad Hoc,

If I extend the stern by approximately 2.5m and reduce the displacement by .5 tonnes (smaller engines), then I'm looking at a L/D ratio more in the range of 5.8. What affect do you anticipate that would have on my cruising speed? Thanks.

 

Deering

As Tad rightly points out you need to increase the length. This has the positive effect of reducing your L/D ratio and of course increasing the waterline length too. Improving your L/D ratio reduces your prismatic hump, so becomes more efficient, ie less power required to get over the hump...as well as reducing your residuary resistance too.

So, best way, if you say 2.5m, fine that’s your choice, you can decide how much you want/need to add. Just have it like and extension, as if you are extruding the hull, so keep it simple. Only thing I would do, is ad a bit of rise to the keel line, say 5 degrees. Also doesn't "drag" the transom as your Fns are too low for this type of transom....esp. for effecient fuel economy. This helps to get a lower angle for the prop to eng. Make sure you have around 17-20% tip clearance from prop to hull.



Adding up fwd, may be tricky, but could be simplified. If you take a vertical line somewhere above the dwl drop from the existing stem line, then as shown in XS, just make a simple bulb shape, if you can by bending 2 plates to form a diamond type shape. You then fair this back to the existing hull with no sharp changes.

In both cases, you need to make sure you have adequate stiffing, especially longitudinally, other wise they fall off under a heavy load. So tie them back onto the main hull.

Without any real data of your hull, and depending upon your resources; once you do this, add your engine and drop box, if better angle to prop, and then launch her see how she sits. You may, or may not, find she may be bow down, owing to increase in buoy aft. Don’t worry about that. You can move fixed items already on board. Coupled with the weight of the stern gear (props, eng. g/box, shaft, P-Bracket bearings etc) may help, how much..depends upon what you spec. But you can add hand rails ontop of the ‘duck tail’….or worse case, even a bit of solid ballast.

Once you have done all that, try and establish the actual weight of the lightship, properly. That’ll give you some decent guidance on what engine size to spec for whatever SOR you want/need.

And take off that delta, just adds drag. The hull shape is awful too, not ideally suited for your needs, just easy to build, that’s all.. Why have a chine so low down and what looks like prismatic sections aft too? This isn’t a racer!!

 

In the cold light of day one should really look at any modification in terms of cost/benifit.......

How many hours will the boat get used per year, and how many years do you intend to keep her.....

Divide total modification cost by hours anticipated use and decide if it's more or less than the anticipated fuel savings per hour......does this make sense?

Of course the added reliability and comfort (longer hull is more comfortable at sea) are hard to quantify in dollars and cents.

I wonder if there's a way to make an almost double-ended extension aft, then notch it for the prop and rudder, cutting volume and creating a very minimal transom. Any aft extension will trim her down by the bow, to avoid having to add ballast you need to extend the bow.

 

Tad, you are 100% in line with my thinking about cost/benefit. And I have to factor in the cost of a new outdrive, and the likely cost of more outdrives in the future, along with the maintenance/reliability burden they bring.

But another 'cost' factor is the extended range a more efficient hull would give me. Here in SE Alaska fuel docks aren't around every corner. My trip last week totaled over 350 miles and I passed just one fuel dock in a small village where the juice was over $5 a gallon. It's not just dollars, but also flexibility. No fun cruising around with one eye anxiously watching the fuel gauge all the time.

Comfort isn't really an issue as the current hull is very comfortable in most of the seas I'm in, but more comfort is never a bad thing.

This is our retirement boat which we hope to operate for many years. In the future we may be running this boat all summer long so efficiency benefits will add up. And I consider this a hedge against future fuel price increases. Would be a shame to get to our golden years with our dream boat and find we don't have enough gold to run it.

Regarding your thought on the aft extension, I think you've just described the Tennant canoe stern design that I'm aiming for. Attached is a rough sketch I made of what I have in mind. I've also attached a pic and drawing from the PDQ powercats which use this hullform and have well documented performance numbers supporting its efficiency.

On the trim issue, I have considerable latitude to reposition weight such as batteries, fuel and water tanks, tender, etc. I already do this to some extent based on my payload.

Thanks for the input.

 

Deering

If your boat was a pure displacement hull, that would be fine. But the speeds you are looking at gives a Froude number roughly around thus:

8 knots - 0.39
10knots - 0.49
12knots – 0.59
15knots – 0.73


if you added 2.5m aft and say 1m fwd, the Fns would correspond to: 0.34, 0.43, 0.51, 0.64.

However you look at it, unless you wish to go around at less than or equal to 8knots almost all the time, you are either approaching, at or past the prismatic hump. What this means is that the wave system generated by the boat will create forces that will lower the stern and the transom will become ventilated and/or very turbulent. You cannot avoid this, hence the shape hulls end up being for that Fn to go with the flow, and not fight it.

But with the shape you have shown, unless you’re doing less than 8 knots, the transom will ventilate, ie be free of water, it will be dry when running, or try to be, so air will be sucked down directly and will probably cause ventilation of the prop leading to cavitation and a resulting drop in thrust and speed. The L/D ratio of your hull does not lend itself to this type of stern. If your L/D ratio was much higher, you could get away with it. That is how displacement hull type of vessels can go fast, with a high L/D ratio. That’s why I suggest that you just simply extend (extrude) the shape, but with a slight rise. You can have ‘tunnels for the prop’s if the draft is too much.

 

Ad Hoc,

With all due respect for your vastly superior knowledge on hull design, how do you explain the well documented performance of Tennant-designed canoe stern boats like the Maine Cat 47 (displacement 29,900 lbs) or the PDQ 41 (disp 26,900 lbs) which are achieving displacement speeds well in excess of 20 kts with reasonable efficiency? They are following the configuration I've sketched out, not what you are proposing, which appears to be more suited for a sailing cat or a displacement monohull.

I must be missing something obvious. Please enlighten me. Thank you.

 

You need to obtain independent actual facts and figures and test data to make such a statement. Otherwise, all you're doing is cut and pasting the sales bluurrb and buying into their sales pitch.

For starters, define reasonable efficiency...

Words are easy for any salesman. Facts are much harder to provide.

 

Um...I've talked to actual owners of the PDQs and other Tennant-designed cats who have verified similar numbers. Haven't had a chance to talk to any MaineCat owners yet as that hull is pretty new. I'll make sure to demand a notarized statement next time. These boats have been around for awhile so we're past the unsubstantiated marketing claims.

Reasonable efficiency in my book, for a boat of that size and fit-out is better than 2 nmpg at 12 kts. Your reasonableness may vary.

Now that we've settled that...how do you explain the performance? What am I missing?

 

Ad Hoc,

Looks like this designer http://www.noahthompsondesign.com/Production_boats.asp who happens to be a member of this board uses this same hull form and he's claiming similar performance numbers for his designs.

Perhaps you should ask him to provide the independent testing that you seek.