Help?

Big swell- probably means some pitching, with the prop getting closer to the surface. Lower pressure near the surface, more chance of cavitation.

This might seem strange but to me if a simple outboard leg like a water snake was placed on either side of the widest part of the beam and the two control head were in the cockpit So they could be used like a joy stick and design the two legs so they can rotate 360 degrees.
This would be good for docking and also when motoring. And in the case of a catamaran Id fit at least four There fore no reverse is needed so a prop would only have forward thrust. When not in use rotate it above the water line. On a Cat Id fit them on the inside of each hull . Then add a required gen set and solar to run it the same gen set then could power your house system
No water cooling needed the diesel could be a air cooled system much like a Deutz tractor engine or a gen set with a radiator











So a boat could go side ways by rotating the outboard 90 degrees via a joystick a yacht could turn 360 degrees in its own length.
The motor could be place a a better position in the boat design for balance and human friendly

then add a Hamilton jet it would not need reverse just forward thrust rotate the jet 360 degrees for what ever direction you want one either side of the widest part of the beam


and still use your traditional rudder, the jets would be only for thrust for manoeuvring and rotated for reverse

to keep cost down just up size how many would be needed per length of hull or hulls
so any boat could go side ways or 360 degrees and still have great forward thrust
and swing them above the water line when not needed

Even coming into a bar you would have far better control of the hull it would work more like a earth moving tracked machine

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Agent nods said..
I had a similar situation...the bottom was fouled fairly badly - but location was not easy for haul out, on the berth everything fine.

But in the open water it cavitates....and if pushed just blew black smoke which it would not do at full throttle at the berth.

Not certain but when on the berth the prop just pushes water and can do that to without impedance. But on the water the prop can only push to hull speed...which is greatly reduced if it fouled. Therefore it now cavitates as it now can't get fresh undisturbed water.

yeah ,,, also the angle of attack of the prop blade changes with forward movement of the boat through the water

I'm sorry but why the thread drift why waste all that time posting theory's about bow thrusters and side thrusters? Perhaps read my first post on prop cavitation again ?

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southace said..
why is it when running the main engine in gear tied up in the marina I don't get cavitation yet when the hull is dirty , head winds , big swell I seem to get cavitation? I'm stumped. Bad design perhaps? I did notice over Christmas the encapsulated keel is around 1 meter wide perhaps that effects the free flow once I get to cruising speed?!

Are you sure your keel is 1 m wide?
That is very very wide. If it is 1 m wide it would definitely be effecting turbulence.
It is also amazing how a little growth can effect the performance of a prop.
On my previous boat it had a 2blade folding prop and the slightest growth on it and I could not get full throttle and would put out black unburned diesel.

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FreeRadical said..

southace said..
I'm sorry but why the thread drift why waste all that time posting theory's about bow thrusters and side thrusters? Perhaps read my first post on prop cavitation again ?

How about a funny cat video?

I do like cats for there ease of manovorbilty however they cost too much!
Skipped a sea trial on a Riv 48 for Cats on the weekend no manovorbilty!
Im stumped I suspect the low pitch 3 blade prop.

Back to the original question. What makes you think the prop is cavitating? Screaming high revs? Lots of bubbles in the wake? Vibration?
It is possible that a fat trailing edge from the keel will produce more disturbed flow in front of the prop, but that effect would be more noticeable on a calm day when your speed is higher.

Let's get back to the top of the page and the original question. Cavitation happens when the negative pressure on the prop surface so exceeds the ambient pressure that water flow cannot be maintained, so it is a factor of depth of immersion and the form of the prop and the HP applied.

Depth of immersion is obvious but does vary when under weigh due to wave action and stern squat.

Form of the prop takes into account stuff like curvature and blade area and fouling.

HP applied can be a funny one. Hydrofoils like pumps and props absorb less HP when the fluid is at rest - the angle of attack is all wrong. Get them going and more HP is needed to maintain a given RPM.

So when the boat is moving with head winds, swell, dirty hull that you were talking about, the prop can demand lots of HP at a relatively slow speed, and that translates to lots of negative pressure on the blade and very likely you will get flow separation and cavitation.

But I do love cat videos.

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southace said..
Ok so even with a clean hull if I increase rpm to quickly I hear cavitation (bubbles around prop) when I reduce rmp the bubbles stop. If I bring up rpm slowely no problem. If I hit headwind,turbulence,swell the bubbles start and loss of cruising speed as a result. My suspicion goes with Ciscos reply thinking my prop diameter is rather narrow bladed. Good for sailing not so good for motoring.

Exactly. All of those actions increase the load on the prop, and to fix that you need more blade area. Less pounds per square inch because more square inches. So fatter blades or longer blades or more blades. But you can't increase prop diameter unless you decrease pitch if you need to get the same revs from your motor.

So if you didn't like your the boat nest door anchoring to close or they used silicone on the boats you could clean them up a bit


you should get more than enough prop pitch

so as long as your up north you can beach shower

or image four pods on the inside of the hulls a 40 foot catamaran

Be good for budgie smuggler competitions and also wet T shirts

on an older yacht you could used beaching leg mounts as they do in UK and Europe







and store the light weight jets once your sailing and replace the old worn out diesel with a small diesel gen set
couple of Hendricks plug and power up
Or just bring out your Honda gen set on deck and one might also power your dinghy
Now I wish Id left my anchor well in place

The storage tank in this video should be a prawn farm @1.45 in so it pays for it self for Queensland prawn farms

be good for Bundy Cisco

I think Prawns hate living in the same bath water

My HT26 has a centre mounted Yanmar 3GM30 and twin hydraulic drives. Great manoeuvrability.

But thinking about it Id add a fuse to the main sheet and Genoa sheets if I was going around the world so at a set tension the sheet would break loose so it covered any high strength wind gusts and have a few spares on board
rather have a break away then do a 180 on a cat as they don't sail well upside down.
But I love them just the same that's the only thing Id add