double universal joint to connect the drive shaft to the engine

[kristjan]

The big Irwin charter boats had two universal joints in the shaft. I just put grease in the nipple once a week. These boats put more miles than most on them, and at least the one I skippered didn’t have problems. The engine was a six cylinder Perkins, I don’t really remember more details. Irwin 65/68.

[seadago]

Originally Posted by 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

Hi Bill. Thanks. Clever gadget!! Will definitely get in touch with Thomson and see what they say for this application.
Rafa

[seadago]

Originally Posted by Pizzazz

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

Hi
Thanks for the offer! specs as follows:
Engine output (measured at the gearbox flange), 16HP (12 KW) @ 3600 RPM - max torque 37.8 Nm @ 2600 RPM
Gearbox TMC, 2.00:1 reduction ahead, 2.13:1 astern
Current prop: 2 blade 14 X 9
Suggested prop: 3 blade 12 X 8.5
Beta prop guideline for engine/gearbox combo: 3 blade 13 X 8
Boat: LOA 28'4", LWL 25'9", beam 9'4", draft 5'4" (keel down), hull displacement 3.1 tonnes - estimated 4.0-4.5 tonnes in cruising trim
I don't know what other info is necessary...
Rafa

[seadago]

Originally Posted by wingssail

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.

yeah.... that's why I bought it!

[seadago]

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?

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.

The one on the left; a ketch. Still a great downwind sailer. And yes, all those guys dealt with tides and shallows and chops, and wrecked and sunk and died in droves. Rhyme not intended.

[Uncle G]

seadago, you said: "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."

Whatever it weighs in the crane will be what it displaces when it is afloat. If it does weigh 6 tonnes and it is designed to be 3 tonnes, then it is seriously overloaded and that may be the problem. Or maybe there's a typo in your statement.

If you can only get 2500 rpm out of it, then as others have said, you probably have the wrong sized prop. Also, you aren't using the full hp of the engine.

It's better for a diesel engine to be driven hard than have an oversized engine that never gets properly warmed up because it's too lightly loaded or run at very low revs.

The 16hp should be adequate, if you aim to cruise at 3000 rpm and have a decent sized reduction on the gearbox.

We used to have a 10 ton 45 foot yacht that had a 28hp engine. We ran it at 3000 rpm and could do 8 knots.

My suggestion would be to get the correct sized three bladed prop, try it out - and if it gives you the speed you want then to get a feathering prop the same size and keep the fixed one on board as a spare. That will be far cheaper than changing engine or boat.

[Hesti]

A lot said already about prop dia and pitch. Another factor is blade area. Our current prop is also cavitating far before max engine power and the reason is simply not enough blade area. Before you shell out for a new engine, get a prop that fits and try it. And if the boat is 6 tons instead of 3, get the prop calculated for 6 tons - if you can not reduce weight. To have it calculated for more than listed weight makes sense anyway because hardly any boat sails at the listed weight. With 16 hp, there should still be huge room for improvement from the prop alone, even if 16 hp may be a bit on the small side, but with a cavitating prop, you are not putting 16 hp into forward drive. A bigger engine will not get you any more performance from a prop with insufficient blade area, instead, it will need a prop with even more blade area than you would need for your 16 hp. Which would increase drag under sail further than a good fitting prop for your current engine.


Prop choices on sailboats: The racer will go for a prop with insufficient blade area to keep drag down. The motorsailer / cruiser may well opt for a prop with a little safety margin on blade area to have the desired push without cavitating in bad weather. The third way is to spend for a folding / feathering prop with enough blade area.

[seadago]

Originally Posted by Uncle G

seadago, you said: "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."

Whatever it weighs in the crane will be what it displaces when it is afloat. If it does weigh 6 tonnes and it is designed to be 3 tonnes, then it is seriously overloaded and that may be the problem. Or maybe there's a typo in your statement.

If you can only get 2500 rpm out of it, then as others have said, you probably have the wrong sized prop. Also, you aren't using the full hp of the engine.

It's better for a diesel engine to be driven hard than have an oversized engine that never gets properly warmed up because it's too lightly loaded or run at very low revs.

The 16hp should be adequate, if you aim to cruise at 3000 rpm and have a decent sized reduction on the gearbox.

We used to have a 10 ton 45 foot yacht that had a 28hp engine. We ran it at 3000 rpm and could do 8 knots.

My suggestion would be to get the correct sized three bladed prop, try it out - and if it gives you the speed you want then to get a feathering prop the same size and keep the fixed one on board as a spare. That will be far cheaper than changing engine or boat.

Hi UncleG
No typo. That's what the literature (user manual, spec sheet, etc) clearly states: 7000 lb displacement (3.17 tonnes), of which 3600 are in the lift keel. My boat has weighted at "about 6 tonnes" in the crane. So, one of those figures is wrong, or... the 7000 lb refers to the displacement of the hull alone, not the boat. Go figure.
Honestly, I can't imagine how I have added 3 tonnes worth of fuel and provisions on a 30' boat. I can say that fully loaded for cruising, I sit on the water at the waterline as originally painted.
Rafa

[Uncle G]

- are you sure that the waterline as originally painted is where the designer intended it to be? Maybe the previous owner raised it up? Maybe the crane was weighing spreaders and strops as well as your boat?



there's a prop on ebay at the moment that you could get and try out to give you a better idea of where the problem really is


https://www.ebay.co.uk/itm/Propeller...AAAOSw-qldvWDy


- only of use if you need a RH prop of course!

[seadago]

Originally Posted by Uncle G

- are you sure that the waterline as originally painted is where the designer intended it to be? Maybe the previous owner raised it up? Maybe the crane was weighing spreaders and strops as well as your boat?



there's a prop on ebay at the moment that you could get and try out to give you a better idea of where the problem really is


https://www.ebay.co.uk/itm/Propeller...AAAOSw-qldvWDy


- only of use if you need a RH prop of course!

Water line was painted right over the gell coat. I discovered it only when I sandblasted the hull down to gell coat to copper coat. Assume original. Makes sense that the crane weight includes the weight of the harness. Didn't think of that.
Irrespective, if displacement is considerably different from what specs say, then I have to size the new prop accordingly.
Thanks again

[Bill O]

Originally Posted by seadago

Hi UncleG
No typo. That's what the literature (user manual, spec sheet, etc) clearly states: 7000 lb displacement (3.17 tonnes), of which 3600 are in the lift keel. My boat has weighted at "about 6 tonnes" in the crane. So, one of those figures is wrong, or... the 7000 lb refers to the displacement of the hull alone, not the boat. Go figure.
Honestly, I can't imagine how I have added 3 tonnes worth of fuel and provisions on a 30' boat. I can say that fully loaded for cruising, I sit on the water at the waterline as originally painted.
Rafa

Personally would not trust the weight that the crane weighed you. They are not calibrated and highly inaccurate. If the lifts are anything like here, most of them do not have a functional ability to weigh anything (broken). But if you were able to find working ones, you will most likely find a large difference from reality. Would assume the additional 3 tonnes was way off.

Even w/full fuel and water (listed as std; 20 & 50 gal. respectively) that's ~ 560lb then add some weight for other gear, would really doubt an extra 3 tonnes in your boat.
Also you say the boat is sitting on its original waterline (was it ever repainted?) From online data, for every 953.7lbs your boat would immerse 1" in the water. Think you might notice the boat sitting >6" lower in the water if this were the case.

Based on all this would discount the exaggerated weight from the last haul out.
Sorry I'm late to the party on your discussion, but IMO doubt you need to change to a much bigger motor to get hull speed. Not really certain why you or others need to charge around doing hull speed under motor, but like most sailboats they can sail faster than they can motor. If the whole point is to be able to motor up into a chop/current, then switch to a 3 blade prop. A properly sized fixed 3 blade would be a cheap way to get more torque than w/the 2 blade, but has the draw back of extra drag under sail and may walk more in reverse. If you are going to keep the boat, possibly invest in a feathering or folding 3 blade prop.


Bill O.

[Leaff]

Originally Posted by Nicholson58

A cardan joint is just another name for U-joint. Read my post above. If you do this, only a CV joint can do this. It is really important that the prop thrust is carried by a thrust bearing in combination with the CV shaft. Thrust in a double U joint is a mechanical train wreck. Please go to the Aqua drive site and learn about these devices. They are made to separate and contain the thrust at the prop shaft’s shaft log and transmit pure torque so your crank bearings are not ruined. Without a thrust bearing, the Z offset shaft is like pushing on an S shaped noodle.

BTW, I am the chief engineer, PE, retired from a custom machine builder with over 40 years in the business. I have designed and built these things up to 5000 HP with 30 inch shafts.

It’s so nice to see a post by someone who knows what they are talking about rather than the general “I don’t know anymore than you do but I’ll have an opinion anyway” post.

Thanks for sharing Nicholson58.

[Cheechako]

U joints can be fine but you need enough length to get the angles right. IIRC driveshaft makers want at least 3 degree angle, they don';t like them straight!

[Mark Johnson]

Originally Posted by seadago

I'm thinking of up-sizing the engine on my sailboat. Problem is, installing the larger engine will require chopping the top off the current GRP mounting cradle, and probably eliminating the oil well underneath it; this in order to get the larger engine in line with the prop shaft, and within the inclination angle tolerances specified by the manufacturer.

I'm reluctant to do any of those things for a number of reasons.

Has anyone used a double universal joint to resolve this conundrum? Not an engineer, but intuitively, that's what universal joints were invented for; i.e. transmit power at different angles, and it works in a zillion different scenarios and applications.

This said, I never seen this solution on a small sailing boat (which does not, of course, means it's not used), and wonder why. Any insight much appreciated!

Cheers
Rafa

I built my own boat 24 years ago, using a CVA joint to connect the engine to the shaft.It allows BIG misalignment, but within a few degrees is best. It allows the engine to shake, without shaking the shaft, so the PSS shaft seal works better.
The thrust in both directions goes on the CVA thrust bearings, not the engine’s bearings or rubber feet.


No issues!
Win all around. I recommend them completely, if you know how to do it right.

[skenn_ie]

Not making hull speed ? Put your numbers into propking.xls before you change anything. Are you getting to max revs ? If so, a coarser prop might be all that is required. If not, a finer prop might do it, as you may not be getting to peak power either. Don't expect to get TO hull speed. It is a bit like the speed of light, as you approach it, each extra increment takes considerably more power. At hull speed, you are continuously climbing the steepest bit of your own bow wave.