double universal joint to connect the drive shaft to the engine

[seadago]

Hi Northbound44, and everyone that offered input.

Many thanks for very pertinent and informative advice. Really much appreciated!

Just to add contextual info that may clarify the problem.

The boat is a Freedom 30 - the smallest of the breed. The US incarnation was fitted in most cases with a Yamaha 18HP indeed. The UK builds, for some incomprehensible reason, were fitted with smaller engines. When I bought the boat (built in the UK), it had a Volvo MD7A 12 HP. I believe this was the original engine, and was fitted with a two-blade prop.

As per literature, the boat has a hull displacement of just over 3 metric tonnes. Weight (mass) of the boat on the crane, is about 6 tonnes. I estimate actual displacement to be about 4.5 metric tonnes in cruising trim.
It is a straight long keel hull boat and, stable and sure-footed as she is, she is certainly not built for speed. Much as I love the boat, it's a chamber pot with masts, really.

So, following the 4 HP/tonne thumb rule, the Volvo was already undersized for the boat. The way I know the rule is: "4 HP (at the prop)/tonne + 10% overall". The “at the prop” and 10% elements are critical contingency factors here IMO.

Soon after I bought the boat, I replaced the Volvo 12 HP for a brand new Beta 16 HP, and fitted (as recommended by prop specialist!) a 2-blade 14 X 9 prop. Both choices turned out to be suboptimal. The engine capacity (16 HP) was at the upper limit according to the rule, considering all other factors mentioned above including hull shape. This was my choice. In my discharge, I must say that the 16 HP was the largest Beta engine I could fit in the bay without modifying the cradle as I described in the first post.

The engine itself is brilliant, and I am very happy with it. It runs on fumes, and it has performed flawlessly since day one. However, it is still clearly undersized for the boat. All other Freedom 30 owners I have (subsequently) corresponded with on the topic, have 20 HP or larger capacity installed.

The prop specs, despite being recommended by the prop manufacturer for that engine and gearbox combo, are also incorrect. The engine is supposed to peak at 3600 RPM. I can barely get it to 2500, and beyond 2300, the prop starts cavitating. At 2500 RPM I get 4.5 knots in dead clam, but can go down to 2.5 heading into a chop. Hull speed as per literature is 6 knots. BTW, in anticipation of other comments, my hull is clean as a whistle!

The prop is a known factor which I am soon to remedy. After consulting another expert (this time a supplier recommended by Beta UK themselves), I was assured that by fitting a 3-blade 12 X 8.5 prop, I should be able to achieve 5.3 knots at 3500 RPM. This would be an acceptable compromise. Still, I am hopeful but sceptical, given recent experience. Apart from that, a 3-blade prop will increase drag under sail by at least 0.5 knots. There are feathering props I can fit, but that’s a different debate.

That leaves for consideration the option of upsizing the engine, and whether it is worthwhile doing so cost-wise. As A64pilot suggests, I may be better off selling the boat and buying a bigger, faster one. I am considering it! but that would be again the topic for a different thread.

If I must modify the cradle as described above to do the upgrade, then it is a no-go for me, from all perspectives. Which brings me back to the original question of fitting a CV joint (thanks Northbound44) to avoid having to do structural modifications, and Billknny’s perceptive observation that engine installers are, by and large, not mechanical engineers. This is a precision engineering job; needs to be done right or it can turn into an unmitigated disaster!

Speed, in and of itself, is not the critical requirement here. Half a knot more or less will not save me nor kill me. But in this part of the world, where tidal currents regularly exceed 6 knots, and where chops of 2 and 3 mts with 4 sec periods are daily bread, having the power to plough through all that securely and with acceptable comfort, will add considerably to the enjoyment I get from sailing my beloved chamber pot with masts!

Thanks again to all
Rafa

[Bill whitmore]

The solution to your problem is Thompson Couplings made in Australia. Study their website. I have used their coupling For many years with great success. Excellent design

[zstine]

Maybe I'm missing something here, but isn't the engine/transmission a load bearing member on a marine install in this application?? In other words, the shaft alignment and propeller thrust loads are transferred to the hull via the shaft-trans-engine... right? If so, you CANNOT use a CV (CV cannot take axial load, in cars anyway) or double U-joint (axial load in an angled shaft creates a side load and you would loose alignment) without adding a bearing into the system as well, in order to maintain shaft alignment and take the thrust. This would be a bit complicated and probably tough/impossible to fit in the space available.

[Michael Pope]

Years ago I raced Zamazan a Bruce Farr design 55ft with the engine (a large 6 cylinder smelly thing) mounted ahead of the single Mast and the drive took a turn port at a small angle then a turn to starboard . The drive line ( exactly the same as what you see hanging out under any commercial Truck) went about six feet aft past the Mast step. Then it did the reverse manoeuvre back to the center line and attached to a thrust bearing. on the end of the Propshaft in a nice strong GRP Mounting... Not Very challenging. The amusing part was that it was made with typical NZ ingenuity with standard truck parts all cleaned up and painted. It only cost a few hundred dollars and worked well. It was not silent and was almost vibration proof but it was almost free and gave a large advantage in the IOR rating . The RULE gave you a credit if the engine was a long distance from the vessels centre of gravity. As that would usually slow the boat when experiencing waves. I suspect in this case it was sort of like Hanging Ten on a long board ( getting ten toes over the front of the long heavy traditional Surf board) Knowing
the Kiwis as well as I do they probably made it with carefully checked parts from a Truck Junk yard. How sweet it is when you can do that to a rich Yank . Trouble was the owner was a fine gentleman Architect NZ Born who lived in San Fran. I had been press-ganged into Ram Roding the boat for its first Trans Pack when all the "Race Crew and Race
Sails" were stuck in NZ by the grounding of the DC10 ( I was born on a Friday the 13th)
The High Pressure died after three days and light air prevailed and I went the wrong way
initially so we had to Zag . Got a curt note from Farr three weeks after the finish saying he had studied our track and I had sailed 400 miles too Farr . No thank you for all my work and disappointing my friend who I had planned on racing with . My response referred to parts of his anatomy ! MP

[Pizzazz]

I think @a64pilot said it very well. You need CV joints and a thrust bearing. Installing and aligning the thrust bearing will be substantial work. After that you can use CV joints or even hydraulic power transfer. I believe though that you need to work on your prop/gearbox calculations.

Something does not compute. My boat weighs 9,700 lb, may be 11,000 now, so it is similar. Hull speed is 6.9 knots. It came to me with a 15 hp engine (2GM) and a 14x12 two bladed prop. At 3,600 rpm and 2.62 gear ratio it made 6.2-6.3 knots. I changed the engine to the 18 hp version, speed went up to 6.4 knots. Changed the propeller to folding 15x12 prop, speed went up to 6.5-6.6 knots.

You can get to 85-90% of the hull speed with little effort, then it becomes harder, especially above 95%. You should be able to get more than 90% of hull speed with your current setup. Check the gearbox ratio again, let us calculate the prop size again for you, then get a few props and start experimenting. You need to be able to achieve engine max rpm with a matched propeller.

SV Pizzazz

[roland stockham]

Depending on how you use the boat the other option may be to look at an electric drive or diesel/electric this could give hull speed on a combination of diesel and battery with the same size engine or a smaller one. Normally this is expensive but if you are looking at exotic drive lines so are they!
The other possibility is hydraulic drive which again gets away from all the alignment problems and although less efficient the shaft may not be much different once you factor in offset drives.
Before any of this I would get a good prop engineer to do an estimate of what you need. Frequently boats are using large engines to compensate for highly inefficient transmission. A large slow turning prop will transmit power more efficiently than a small fast one. There is also a lot of difference in engine size and shape for the same output. There may be a combination out there that would get you the performance without structural changes.


As a rough guide 2hp/ton should get hull speed on flat water, ie in harbour or on a river. 3hp/ton should get hull speed in coastal waters and mod sea and weather conditions. Say motoring into 12kn and a steady swell.
4hp/ton was always considered a 50/50 motor sailer and should give the same performance under motor as it would under sail.



If your boat seems well off these figures it may be that engine wear or prop setup is limiting performance not engine size

[Rapanui]

A double Hooks joint (universal joint) Cardan shaft is used in many military and commercial vessels up to 1000s of kW without problems. Provided you install the joints at the recommended angle (not in line) it is a reliable method. Because this intermediate shaft is only secured by the joints, unlike a conventional propeller shaft, you have to consider whether to design in some method of restraining a flailing shaft should a joint/shaft break, as the consequence could be major destruction or a big hole in the boat, don't ask how I know this. Unless you are talking large angular changes in direction, then Constant Velocity joints will be unnecessary.

[Madehn]

It can be done with universal joints but they will need to be reasonably aligned-- The Irwin 50 foot plus boats had this setup from the factory-- big thrust bearing that transferred force to the hull then two universal joints in a beefed up box setup-- the one i was on was very smooth and as i recall the parts used were replaceable with truck part as well.

[bcboomer]

Take a look at the SigmaDrive from Bruntons.
Not that expensive and without the stress of designing it yourself.

[wingssail]

Originally Posted by seadago

Hi Lepke
Thanks. That's exactly the point. I'm not making hull speed now!

That Freedom 30 is a great boat. It is a sailing auxiliary, not a power boat. It WILL MAKE HULL SPEED, under sail. It will go upwind!

Try cleaning the bottom, getting a new mainsail if needed, and sail more.

Reconcile yourself to motoring at a slower speed.

[wingssail]

[QUOTE=seadago;3009713 Much as I love the boat, it's a chamber pot with masts, really.
[/QUOTE]

Did someone hold a gun to your head when you bought it?

Wait a minute, did you say masts?

Is it the one on the right or the one on the left?

If it's the one on the left, well OK, do what you need but you won't improve or hurt the value of the boat in any case. If its the one on the right. Then its a great Gary Mull design. In either case, its a proper sailboat, you can sail it.

You know, When Captain Cook, Sir Francis Drake, and all the other great British mariners sailed from England they also had to deal with tide, chop, headlands...you name it. They did not have motors at all.

[Mike Banks]

There is only one thing that is a disadvantage to using two universals (make sure the yokes line up or the shaft will oscillate) and that is the amount of space they take up.

CV joints do similar things and take up less space. Even less space is taken up by a direct mounting. Engine mounts need to be adjusted properly and a rubber-bushed connector fitted properly. That takes only an inch or two.

[Brewgyver]

Originally Posted by zstine

Maybe I'm missing something here, but isn't the engine/transmission a load bearing member on a marine install in this application?? In other words, the shaft alignment and propeller thrust loads are transferred to the hull via the shaft-trans-engine... right? (snip)

Yes, apparently you missed the first several posts that brought that up...

[Brewgyver]

Originally Posted by wingssail

Did someone hold a gun to your head when you bought it?

Wait a minute, did you say masts?

Is it the one on the right or the one on the left?

It's the Cat Ketch, made a good 10 years before the fractional sloop. Freestanding masts with wishbone booms. Very different boat.

[kapnkarl]

As a few folks mentioned in this thread, since you're talking about significant alterations to the engine mounting bay, you might want to take a look at Aquadrive. There are several sizes to match engine horsepower ratings. I have one in my trawler running a Lehman Ford SP135 (135 hp). Unlike a typical shaft coupling installation that only allows being out about .003 inches, the Aquadrive CV setup allows up to .5 inches... yes, that's 1/2 inch! All of the thrust is born by a reinforced plate mounted between the stringers beyond the rear of the transmission. The engine is mounted on soft, flexible mounts since they don't need to take the thrust of the prop shaft to drive the boat forward (or aft) in the water. This considerably reduces vibration and stress on the engine & transmission. The result is a quieter, longer lasting power-plant. It's not a small undertaking, but if you're gutting the engine bay anyway, you might as well look at this option. Just mho.