New boat problems

[foxfish]

Hi guys I am new to the forum & would like some help!
I have just finished building a little catamaran of about 4.7 mts long, the problem is the motors prop just wont grip & is ventilating - cavitation?
I have tried to drop the motor deeper but the anti cavitation plate is now 150mm further down than the hulls & still the motor losses grip at 3500rmp (max 6500)
Top speed is now 17mph but only in a slight chop, on flat water she only manages 11mph.
Any ideas?

[Richard Woods]

What size engine are you using?

Richard Woods of Woods Designs
www.sailingcatamarans.com

[foxfish]

50 hp - Do you think one of the commercial anti cavitation adapter plates would help?

[alan white]

Is the cat your own design or has the design been proven elsewhere?
Hulls like that do better with small motors that weigh less. Even 10 hp would be better, enough to possibly exceed the current speed. You need a wide, flat bottom to get up on a plane. My guess is you're dragging badly.

[Richard Woods]

You certainly have a problem, possibly more than just a wrong prop/ventilation/leg drag one. I just had an email from one of my builders, in Lebanon of all places, who has just launched an open deck motor version of my 32ft Eclipse. Despite having sailing, not powerboat hulls and weighing 3 tons it motors at 14 knots with 2 x 60hp. And that is before fitting the correct props.

so a 14ft boat that only does 9 knots in flat water with one 60hp has something seriously wrong.

I don't know what to suggest, but more photos of the boat might help, it looks a bit unusual from the one photo we have seen

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Guest625101138]

Originally Posted by foxfish

Hi guys I am new to the forum & would like some help!
I have just finished building a little catamaran of about 4.7 mts long, the problem is the motors prop just wont grip & is ventilating - cavitation?
I have tried to drop the motor deeper but the anti cavitation plate is now 150mm further down than the hulls & still the motor losses grip at 3500rmp (max 6500)
Top speed is now 17mph but only in a slight chop, on flat water she only manages 11mph.
Any ideas?

A single motor on a cat with a tunnel between the hulls is not a good set up.

The prop will be getting air from any number of sources. Once a prop ventilates the power drops dramatically so thrust is lost.

I expect you will need a large "cavitation" plate from the tunnel down to the small plate on the outboard.

The other option is to fit smaller motors behind each hull.

A photo of the hull from the stern quarter would help to analayse.

Also look over the stern when your are at the point of ventialating and look how the waves from the hull are interacting. The outboard may be sitting in a trough.

Rick W.

[foxfish]

Thanks for the replies, yes the design is unique & was just built on a basic idea revolving around a miniature wave piercer!
I had great fun building the boat witch in fact started life as a 12' aluminium dingy, I have since cut away the bow piercer so that it no longer drags in the water while moving. I now realise it was a very ambitious to expect a home made design to work but I am reluctant to dump the boat because of my folly in preference to adapting the design or configuration.
Perhaps I could mount the motor on one of the hulls?

[Guest625101138]

Originally Posted by foxfish

Thanks for the replies, yes the design is unique & was just built on a basic idea revolving around a miniature wave piercer!
I had great fun building the boat witch in fact started life as a 12' aluminium dingy, I have since cut away the bow piercer so that it no longer drags in the water while moving. I now realise it was a very ambitious to expect a home made design to work but I am reluctant to dump the boat because of my folly in preference to adapting the design or configuration.
Perhaps I could mount the motor on one of the hulls?

I would not recommend mounting the outboard off centre.

The concave section in the bow will act like a trench digger until it gets on the plane. Not sure what you were aiming to achieve with this bit but it will not reduce drag.

The work looks reasonable without having a close-up look but the shape of the bow on the inside of the hulls is awful. I imagine this will make a trench that gets back to the outboard at about 10mph.

Lowering the height of the outboard might allow the boat to go faster but once the trench moves behind the outboard the leg will have a lot more drag.

Firstly you should remove the concave section at the bow. See if this gets you going.

The next thing would be to fill the gap between the hulls at the stern. It would need to be aligned with the bottom of the hulls for say 4" and then gently curve upwards. It should meet the tunnel top at about 30 degrees; certainly no more than 40 degrees.

Now you have a lot of power on this boat and I think it could be quite dangerous. I would not like to turn it very fast. The hulls have no lift in a turn so they will dig in and trip up causing a sudden capsize.

You have made radical changes to the hull apparently without too much study of the likely outcome. Hence you need to be very wary of the limits of the design.

If you want to keep at it then it would pay to list your objectives and see what can be evolved here.

Rick W

[foxfish]

Thanks for your interest Rick, are you suggesting a type of inverted "V" hull?
I am looking for a quick fix at least for this season - ie now till september. I could fit some sort of "something" between the hulls but changing the concave bows would involve serious work out of the water, the boat is on a mooring but I can beach her to take moulds or fit new attachments however it is not so easy to get her back home until September.
Is it possible to give some type of diagram to show what you mean please?

[Guest625101138]

I looked around to see if I could find a planing cat with a centre pod but did not find any.

I found this:
http://www.trickpowerboats.com/video
Notice how high the cavitation plate is mounted. There is a tiny pod in the tunnel. My guess is that the wake from the hulls fills the tunnel so the prop has nice clean flow to it.

I did think of the thundercats:
http://www.brisbanecoastthundercats....?g2_itemId=526

Both these type of boats show the sort of hull lines you should aim for. If you get the flow right then the single centre mounted outboard will do the job.

You may have taken the outboard too deep. Once you immerse the fat bit it will be its own plough and feed air to the prop.

Rick W.

[longliner45]

not to show my ignorance ,,,,,but is this more a tunnle hull than cat?,,,and maybe the solution lies therin

[Richard Atkin]

I once saw a monohull described as a 'tear drop'. It had a slightly concave bow and then progressed aft to a normal convex shape. It looked weird, but the builders claimed it was very efficient. Is there ever a time when a concave bow provides an advantage?

[longliner45]

once again ,,excuse ny ignorance ,,but the concaved part of the hull,,forces water to a piont,,,must give some lift ,,cant tell about the pontoons,,,are they lifting?at the same time water flows through them,,and must be some substantial forces at work there,longliner

[Guest625101138]

Originally Posted by Richard Atkin

I once saw a monohull described as a 'tear drop'. It had a slightly concave bow and then progressed aft to a normal convex shape. It looked weird, but the builders claimed it was very efficient. Is there ever a time when a concave bow provides an advantage?

A displacement hull that operates somewhat below hull speed is one example where concave waterlines provide an advantage. But it has a long and gentle transition I believe reduces the energy expended in the diagonal waves off the bow.

Water impinging on a surface that is angled more than say 40 degrees to the flow with an abrupt transition will have a negative pressure that fills with air. The attached shows pressure coefficients to give an idea of what I mean although this is only in one medium. When you have an air/water interface the low pressure area will be filled with air. So air is being drawn down behind the abrupt change while water is deflected off the surface. You can see the high pressure zone on the concave section that will cause a lot of drag. Compare the pressure differences to the convex side.

Also note that the water will be deflected downward in the same way so air will be getting under the hull and reducing lift at the bow.

The reason I know this is that I built a small planning boat that had planing surfaces meeting a 45 degree bow. If the bow ever hit a wave it would cause the nose to bury and boat would slow dramatically. Boat was fast in smooth water but as soon as the waves were large enough for the abrupt transition to be submerged it was not safe to plane. I ended up modifying it with a 30 degree transition and this avoided the large negative pressure.

Rick W.

[foxfish]

Well I have now tried various engine heights & although I found the limitations nothing really changed. On this basis I concluded the prop ventilating is caused but the present hull (bow) design.
Could I fit a naca type foil directly in front of the engine anti cav plate? I could hinge the foil so it could move up & down?
I figure the place to fit the foil would be flush with the bottom of the hulls, the boat draws 7'' of water at rest.
Any advice or comments please.