Cat with Outboards - Performance Under Power ?

[pir8ped]

Here's a solution that offers zero cavitation using an outboard to power a catamaran - though it has obvious drawbacks, some might find it worth considering.

Put the outboard on the tender, and use the tender to push the cat.

First, my reason for choosing this arrangement. I am preparing a cat ( a Woods Sagitta ) to use as a commercial fishing boat. Here in the UK, you don't need a commercial fishing licence if your boat is under 10m and it doesn't have an engine. After extensive talks with the Marine and Coastguard Agency, they have accepted that if I use a powered tender to push the cat, they will accept that the cat itself is sail-only (this saves me several thousand pounds and avoids many administrative hoops - I considered renaming the boat 'Loophole', but not for long!).

The boat came with 2 x 9.9 HT Yamahas. I have removed them, and I am rebuilding the cockpit to get rid of all the lockers and make room for insulated fish boxes. (I need to save all the weight I can to allow for carrying the weight of the catch.) I am building a fishing platform (this is for line fishing, at anchor) aft of the aft beam. Under sail, I plan to carry a small rib there, which will have a 10 hp engine attached.

I plan to use the engine only to get in and out of harbour. Using it will require pushing the rib off the platform and attaching it to the middle of the beam that supports the new platform and use the rib to push the cat. The rib will have a loop of rope going from one side of its stern through blocks on the stern of the cat and back to the other side of the stern of the rib. Adjusting this rope will cantilever the rib and help steer the cat at low speeds. Otherwise, it just holds the rib straight onto the cat. The throttle will be controlled by an extension from the outboard handle made of that dense blue water pipe, which can reach to the cockpit of the cat (it is light and a bit flexible).

Obviously, the engine is a bit exposed and the whole arrangement susceptible in large waves, but I hope that in such conditions I will also have wind, and power the cat under sail. There is also the drawback of having to get the rib into the water and start the engine which obviously takes a little longer than just pressing 'Start'. However, I have experimented a little before I had the boat lifted into the yard for the alterations with a borrowed tender, and the cat was very easy to control being pushed from the middle of the stern. The outboard was 30hp, way bigger than I plan to use, but we used very little throttle to push the boat at 5-6 knots.

If I can come up with quick and simple arrangements for deploying the rib and starting the engine, I think this arrangement will suit my situation. Anyway, as I read about cavitation problems with outboards and cats, I realised that having an articulated floating outboard bracket (the rib) sorts this problem out (obviously creating other problems!).

[bastonjock]

only one problem with fishing off the UK,theres no fish left

[pir8ped]

I'm investing in a top fish finder!

[bastonjock]

i used to work as a diver in the North sea,during the early summer months the rigs would become a haven for fish,thousands of big juicy cod,you could hand feed them for a laugh and sometimes they were so numerous you could not see the things that you were working on.

i saw a video that a friend took two years ago,same time of the year same rigs.............no fish.

another friend back in Glasgow,one day said "stuff it" sold the house and went off to be a Lobster fisherman,he has earned a good living from it for the last 15 years.

That kind of life appeals

[pir8ped]

There's no shortage of pollack and bass in the Channel, and in fact the cod are coming back too though quota allowances won't allow fishermen to go for them.

Actually the loophole I'm in means the MFA can't apply quotas to me. I could legally come in with a ton of cod every day, though the quota currently is 50 kg a month. They did warn me though that if I found a wreck full of cod, I should just move on and look for the pollack, or something unpleasant might happen to my boat. I imagined a smouldering ruin tied to the mooring....

[rallard]

Originally Posted by Cadence

Rallard,
Just out of curiousity, does that configuration give enough prop wash for the rudders to be effective without headway. Or are the engines steerable?

The engines are not steerable. However they are 12 feet apart and close enough to mid-ship so that they are quite effective in maneuvering the boat without headway, it turns on a dime. Then from the previous photos you can see that the rudders are of fair size and will steer the boat with very minimal speed. The 9.9 HT Yamahas have huge tunnel hub type props which allow high power in reverse and this makes a substantial difference when using one engine reverse and the other forward. I have also received some information saying that adding Power Thrusters around the props will in fact improve performance and allow better control of cavitation.

[rallard]

Originally Posted by Calculator

So, if your boat is slender and lightly loaded, added power should improve performance in either speed or fuel efficiency. If it is not, well....

Cal

Thanks for this info, it certainly makes sense. Jolie Julie's hulls are indeed slender as appears from the photos from construction days preparing for vaccuum bagging a second shoe-hull. The boat basically produces no wave. When loaded it does not produce dramatic changes as well as it simply sits higher on the flair part of the hull to better handle the extra weight.
By the way, I sent the female mold for the hulls to another builder in 2004 and I am told that it is still in fair condition and available if someone is interested. Without a mold, such flaired hulls with highly compounded curvatures are very difficult to make.

[Talbot]

My original outboard was a 30hp long shaft with a hi thrust prop. Bad weather meant that the cavitation problems = no thrust.

When I changed to a diesel outboard. I got the ultra long shaft, and I also added wings to the cavitation plate (normally fitted to speed boats to help them lift onto the plane faster). This combination worked really well, and I never had any cavitation problems after that.

[Cadence]

rallard.

My stupidity for even asking, about steerage, since you have twins.

She probably handles like a tractor tug, under power with the midships arrangement.

[rallard]

SMJ, thank you for letting me know that the Power Thruster tunnel devices around the props are called Kort Nozzles, I was completely ignorant about this subject. On the net I found plenty of info showing that such devices will in fact substantially improve performance and consumption. The Kort design is said to offer such benefits up to a speed of 10 knots and a newer improved design called the Rice Speed Nozzle is said to greatly reduce drag over the Kort and offer better performance up to speeds of 15 knots which sounds perfect for twin outboard cats . It is also obvious that it should as well greatly reduce cavitation.
Beeing a fanatic, indomitable old backyard builder (I'm just finishing a new cf/corecell wing mast), I will build a pair of "Allard"thruster nozzles cf/kevlar/e-glass fibers&foam core for my twin Yams 9.9 with a cross-section view looking as shown on the attachment. If this thing does not work I will laugh, but if the mast falls down, I may well commit suicide!

[Pat Ross]

Originally Posted by rallard

SMJ, thank you for letting me know that the Power Thruster tunnel devices around the props are called Kort Nozzles, I was completely ignorant about this subject. On the net I found plenty of info showing that such devices will in fact substantially improve performance and consumption.

I'd like to know more about the nozzels and thrusters please post a few links you personally liked best about these products.

Thanks,

Pat

[Tropic Cat]

Originally Posted by Pat Ross

I'd like to know more about the nozzels and thrusters please post a few links you personally liked best about these products.

Thanks,

Pat

ssullivan bought a Catalac 10M catamaran this past spring which had something very similar to these installed on the boat's Yanmar sail drives. The boat performed great under engine power but performance under sail was another subject entirely.

Ultimately, he decided that the additional drag they induced under sail canceled their benefit. He removed them immediately and was glad he did.

[Pat Ross]

I must have gotten lost in the discussion. I thought the Yamaha's he mentioned were outboards using these devices. I gather from your comments these devices cannot be raised out of the water when raising the outboard, is this correct? Where did I miss the boat, pun intended.

Pat

[sandy daugherty]

Kort nozzles were manufactured for Yamaha HT 9.9s and Honda 9.9s several years ago. The claims for enhanced thrust in forward gear were unfounded. There were reports that reverse thrust was improved to some degree. The company stopped production, and the remaining products (at $500-700 apiece) trickled out of the supply chain in the late 90's.

Discussions resolved that the problem was due to the use of stock propellers and overlarge gaps between the blade tips and the shroud. Effective Kort nozzles have squarish blades that keep the gap closer than 1/4", but have a lot of surface erosion on both components at that interface, and are susceptible to foreign object (flotsam, jetsum, fishing line and nets) damage.

Drag under sail was not an issue on retractable outboard engines, but was excessive on fixed saildrives. (That's big DOH, good buddy!)

And so,
They tried to make some, they didn't work as intended, they cost too much, and as a result, the unadorned Yamaha 9.9 High thrust is still the [numerical] outboard engine of choice for cats under 40 feet.

I think the reason the PDQ (and to a slightly lesser degree, the Seawind 1000) is better able to keep the bows into the wind is the fact that the engines are so far forward that they have a much better moment arm about the vertical axis. I have had to use reverse on one side to keep the bow into the wind in extreme gusts, but it was do-able. My engines cavitate only in the most confused seas, and rarely then.

Santa skipped my christmas wish again this year; all I asked for was a 150# 20hp 25" shaft diesel outboard with remote controls and power lift. Sheesh.

[amytom]

While we're talking about the 9.9HP. Let me show my ignorance about the prop ring. I don't have the plastic fairings installed with my props. I was given one in a kit of spare parts but haven't tried it yet. What benefit should I see with it? Will this reduce cavitation?.