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]

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]

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.com.au/gallery/main.php?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]

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.

[eastcape]

IMHO - try moving the weight and the driver back to just behind midship, allow the bow to rise up a bit, and this will give your two outside sponson a better chance of getting on a plane. I think your pushing to much water to get over the "hump" and onto a plane.

Hope you work it out, I give you full marks for trying something different!

All the best,

[lazeyjack]

nice posts rick as per, yes the man has a funnel hull, its a real shame feller, but hey one learns along the way Even if you make her go fast she is going to land very very heavily um,

[Guest625101138]

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.

I did some wave analysis on the hull. At around 6 knots with level trim and 8" draft you should find the boat is trying to carve a trench right at the outboard transom something like 460mm deep (17"). That is what you are up against. The actual power required to overcome this and get on the plane is only around 20kW. Problem is you will never deliver this with the prop ventilating.

The only way I can see to overcome this is to dramatically alter the trim to bow up so you get the concave section above water level. Once on the plane the hole moves behind the boat and you get a water hump at the motor. Hence you should be able to set the motor quite high to operate on the plane.

You could also try to set the motor tilt so the prop is further from the transom. This will also help lift the bow.

You have the power to push the boat with a highly exaggerated bow up attitude while you are getting on the plane. Once on the plane speed will pick up easily. However if the bow dips into the water you will slow down but you should still keep on the plane. If you lose the plane you will ventilate until you can force the stern down and bow back up.

As noted earlier this boat is LIKELY to behave in a nasty fashion. I have never seen such a small boat able to produce a wave that will be over 800mm (almost 3ft) from trough to crest. That is the difference between the trough and the crest trailing behind the boat. It exists throughout the speed range with level trim.

It would be useful if you could provide some photos of the wake when the ventilation occurs just to confirm my analysis. It might also be something you could boast about. I expect you will challenge the local tug fleet for size of wake.

Your proposed foil will be an airfoil in the normal trim and do nothing. If you do manage to get the stern down to get the foil and prop in water, the foil will tend to lift the stern and force the bow down so you get the opposite of what you need. And no setting it the other way adds downward force and you will get more drag. If you tried hard enough you could bury the stern.

So adjust the motor such that the cavitation plate is level with the hulls, tilt the motor to probably the last notch, shift as much weight aft as you can without swamping the stern of course, don a life jacket and push the throttle slowly forward. If you manage to get on the plane treat the controls gently.

This is only intended as a temporary solution (if it works) until you can correct the concavity and other defects in the hull shape.

Rick W.

[foxfish]

OK thanks, I might have to make a wedge to fit under the outboard bracket to get her going! I will try this on the weekend ---
PS, I did copy & roughly scale the design from a site found on the net but after 3 days of searching I cant find seem to find it anymore - wonder why!

[foxfish]

Well I think that I have to except that the bow shape has to be changed.
I think I might be able to take a mold off the boats inner hulls & then make a couple of foam filled - glassed over infill pieces.
I plan to dry the boat out on the beach & jack up the bow, wax up the area & take a glass fibre template. The I can work on the infill at home, once made up I can dry the boat out again & glue & glass them in place.
My question is - shall I make the bow symmetrical or is there a better shape for the infill pieces? Do I need to incorporate a spray rail?

[Guest625101138]

The main thing is to get rid of the concave portion. Symmetry is not crucial. You could get a better ride if you extend the bow of both lower hulls forward and upwards to get something more like the hulls linked in post #10 on this thread. They would then protrude forward of the existing hull. Maybe this is for the winter program.

Did you try to get it on the plane with weight aft - just curious for my own education.

Rick W.

[foxfish]

Ok Rick I have found that site http://www.johnsboatstuff.com/Boat%20Designs/TRIdent%2012/wp-10.htm
Yes I got her on the plane but the boat was very unstable & non responsive with erratic steering!
I agree about the winter work & in fact looking falward to her next set of improvements. I built a 20' cat a few years ago & have saved the bow section molds, I might be able to use these somehow?

[Guest625101138]

I looked at the waves with the concave section removed. There is still a point between 6 to 7kts where there is a hole where the prop is. It is not as deep but it is still there. It is behind the motor at 10kts.

The hull floats are quite short relative to where the driver sits - does it float bow down? If it is doing this then it will tend to produce a larger hole at the outboard.

The advantage of extending the hulls forward is that you also get more buoyancy forward and this helps get the bow up.

Also by having a long tapering section at the bow you get lift up front. Your bows are more like a displacement hull than a planing hull. You want a shape more like a ski at the front than a wedge.

Rick W.

[foxfish]

Learning all the time! so I might be waisting my time making infills & might as well get her back home for more serious work?
Here is my first cat bow shape, like I say I have the mould.

[Guest625101138]

Learning all the time! so I might be waisting my time making infills & might as well get her back home for more serious work?
Here is my first cat bow shape, like I say I have the mould.

Have you tried that with an outboard? The shape looks superior to what you have with the modified aluminium boat.

The trident hull is somewhat different to what you have built. I still wonder how it performs.

Rick W.

[foxfish]

Well the little boat is coming out of the water next Tuesday, I have arranged for her to be taken to a local storage yard where I can carry out the work. I think I will go the whole hog & extend the hulls, I will post some pics when she is on dry land.
I found this pic, the prop looks very shallow?
http://www.breezecraft.co.nz/profile.html

[BWD]

Here's what you might do.
Mount a very narrow nacelle between the hulls, with a blunt transom end a couple feet forward of the OB lower unit. The nacelle will act as a mini hull and make a stern wave that will shoot solid green water at the OB prop instead of aerated water. I have seen some commercially made single outboard powercats in the US with this design, seems to work. The concept has some commonality to a tunnel on a flats boat, so likewise the exact dimensions of the nacelle are probably fairly critical.

[foxfish]

More delays as the hiab (mobile Crane) cant transport my boat until friday :rolleyes:
Bwd, yes, I would of thought that might work too?

[foxfish]

Here is a mail i have received from Breezcraft -
Hi Martyn,

Sorry for the delay in replying, I have not been at my desk much as I am sick.

Speed with a single 50hp outboard is 25 - 30 knots. We fit the single outboards to one side as there is less drag, and we have found that 40hp is max size and usually a lightweight model. We get better performance from a 40hp one one side than a 50hp in the middle as there is less drag in the tunnel.

Speed with twin 25hp outboards is 30 knots plus, these are the better option.

We can supply the boat in open form, I have attached a pic of a model we made like this, you do not have to have the longitudinal seat. I have also attached another pic of a hull with a cut down cuddy.

We would supply a twin engine bracket which could take a single to one side or twins.

There is not much else I can tell you as most of the info is on the website.

This boat is a great little runabout, very stable and plenty of room for its size, it has been a popular model.

Regards
Sharon

[Guest625101138]

The underwater shape of these boats look distinctly different to the ones in the referenced site showing the so-called wave piecing bow. These look almost conventional bow for a planing cat.

Also note the dead rise on the hulls rather than being flat.

Clearly they have a similar problem as you encountered because they are using the offset outboard. Would mean constant helm to hold a straight course but not much loss in performance.

I think you would get best results from a bow like the Thundercats and the stern with some deadrise like that in the Breezecraft.

You did not give a lot of information on the testing you did with yours at speed. I would be interested in more detail on what happened.

Rick W.

[Guest625101138]

I am confused between the Breezecraft and the Johnsboat.

The Breezacraft look nice. I have no doubt about the claims. Still interesting that they find they go better with a single offset drive. Supports what I found with the wave pattern from the symmetrical hull. Still a trough at the outboard at the 6 knot mark.

NZD6000 for the sportcat is a good price. It would be nice riding in chop.

I guess too far from you though.

Rick W.

[foxfish]

Hi Rick thanks again for your interest, as soon as my boat is out of the water I will post some detailed shots.
I intend to completely remove the wave piecing protrusion & extend the hull falward so they at least reach the front of the boat.
I hope you can help me with the new shape but I will also fit a new transom so the motor can be slid along just in case we cant get the boat to work with it in the centre.
All help greatly appreciated - thanks Mart.
PS Yes a good price for the sportcat but it looks like a single skin, not like most poly boats? Still it would be a great base to work from & fit out as you please.

[Guest625101138]

Hi Rick thanks again for your interest, as soon as my boat is out of the water I will post some detailed shots.
I intend to completely remove the wave piecing protrusion & extend the hull falward so they at least reach the front of the boat.
I hope you can help me with the new shape but I will also fit a new transom so the motor can be slid along just in case we cant get the boat to work with it in the centre.
All help greatly appreciated - thanks Mart.
PS Yes a good price for the sportcat but it looks like a single skin, not like most poly boats? Still it would be a great base to work from & fit out as you please.

If you give me a better idea of what you want to do with the boat then we can work through some ideas. Would also be useful to know what construction techniques you use/prefer.

Information like the total weight, the intended speed, the preferred outboard, the typical operating conditions, etc

I have not evaluated the program I use for the wave analysis on hulls similar to yours hence my interest in your feedback on what you actually observed.

The Breezcraft hulls are certainly a good starting point. They might have tendency to yaw in a trough but should be a nice hull for 20 to 30 knots in chop. Also lots of buoyancy in the ends so stable platform.

Rick W.

Rick W.

[foxfish]

Ok well the boat was built with one sole purpose - spearfishing.
We are free divers (no tanks) so I had this idea to fit the floats under my boat for several reasons -
1 - to make the boat more stable when we get in & out over the side to dive.
2 - so the floats made a step along the length of the boat so we have easy entry to the boat from the water.
Both these aspects work very well, the main dive spot is a small group of islets about 3 miles off shore, there is very shallow water through some of the passages & we also dry out the boat for shore diveing.
A cat is a perfect tool for this type of use, the motor is new this year & I would loose out huge if I tried to sell it or part exchange it for twins motors.
On that basis I would like to keep my motor at least for this season.
Here are some pics of the build, I will be using the same techniques to altar the bow ie glass over wood or foam & sand down, top coat with gel.

[foxfish]

So this is what my hull look like & two possible designs for the mods?

[foxfish]

Sorry Rick forgot to say - we did seem to get the boat on the plane, my mate sat in the back to get the nose up & we ran against the tide in a small chop. The boats speed went up to about 22mph but there was so much spray comming out the back it looked like a jet boat, the water was being separated by the leg & jetting either side. however the motor at that time was sunk very low , cav plate about 4 " below the hull. The steering was very tight, i mean difficult to turn the wheel, as we turned around to run with the tide we could not get her to plane & only managed 10mph!
I took a few more pics but the boat will be dry on Friday when I can get better shots.

[Guest625101138]

So this is what my hull look like & two possible designs for the mods?

I would opt for the symmetrical bow shown in option 2. I would go as far forward with the hulls as practical to fit in with the line of the original hull. Then make the entries sharper than you have shown. Also take the entry of the bow of each hull a bit higher.

I would like to see some deadrise in the stern of the hulls but appreciate this would require more work. Without some lift in a turn there is a risk it will trip up and flip in a nasty fashion.

I prefer the outboard on the centreline but can see merit in the offset placement to get nice flow to the outboard.

What is the maximum length you feel would be sensible for the hulls? If you give me a number I will make a drawing of the hulls. Also what is the overall beam of the boat and spacing between the hulls?

Rick W.

[foxfish]

Thanks Rick, I will take the shots & post the measurements on the weekend, cant wait to get going.

[foxfish]

The boat is now in the yard, I have sort of decided to move the motor to the port hull. I know this is not ideal but feel we will have more chance of success & I am willing to compromise the looks & handling in exchange for performance. On that basis I have already cut away the transom & will be building a full length new one to facilitate the engines new posision.
I am all up for modding the bows but adding dead rise would be a lot more difficult, do you think simply adding a keel rail might help, ie - I could glass a piece of timber down the length & in the middle of the hulls?
Anyway the hulls are at present 3.7 long & I can add a further 750 to the length befor the reach the bow.

[Guest625101138]

Don't add any keel.

The extra 750mm will be good.

What are the dimensions of the flat on each hull and the space between the flats?

Rick W

[foxfish]

Rick, click on the last pic I have drawn the dimensions on the boat.

[Guest625101138]

I attempted to draw what I think the hulls will look like - see attached. For simplicity the transom of the hulls is symmetrical.

Getting the bows to meet the aluminium hull might be interesting. I have guesstimated the shape.

These hulls reduce the depth of the trough by about 4". Likely to still be an issue for central mounted outboard. The ability to offset the outboard is certainly worth the effort.

Rick W.

[foxfish]

OK thanks Rick I am on the case!
I will post a progress report soon.

[kengrome]

I had this idea to fit the floats under my boat for several reasons -
1 - to make the boat more stable when we get in & out over the side to dive.
2 - so the floats made a step along the length of the boat so we have easy entry to the boat from the water.I guess I might have just left the original hull alone and added akas and amas at gunwale level, then tied the top of a boarding ladder to one of the akas. Lines tied to the ladder's bottom would limit its submerged angle and lift it out of the water while underway. Philippine dive boats use this setup, it's simple and cheap and works great. They already have the akas and amas of course ... :)

Interesting mods you're making.

[foxfish]

Here is my latest update - I have moved the motor to the starboard hull, I managed to use the existing steering & controls. I have not changed the hull shape.
Due to the circumstances, mid season & the cost of storing the boat in the yard I just wanted to get going, so the hull mods have been put on hold.
The result of moving the motor was quite dramatic! As the motor is new I have not revved it out but with 750 revs left to go she hits 34knt!
The point is she now works & in fact, works very well so far, very comfortable & stable traveling at 20kn.
Maybe over the winter I will rebuild the hull but to be honest I think a bigger cat is on the drawing board.
Thanks for all you help. Mart.

[kengrome]

It's a good thing you moved the engine ... :)

[Guest625101138]

Here is my latest update - I have moved the motor to the starboard hull, I managed to use the existing steering & controls. I have not changed the hull shape.
Due to the circumstances, mid season & the cost of storing the boat in the yard I just wanted to get going, so the hull mods have been put on hold.
The result of moving the motor was quite dramatic! As the motor is new I have not revved it out but with 750 revs left to go she hits 34knt!
The point is she now works & in fact, works very well so far, very comfortable & stable traveling at 20kn.
Maybe over the winter I will rebuild the hull but to be honest I think a bigger cat is on the drawing board.
Thanks for all you help. Mart.

That is good news. Just a matter of keeping air away from the prop.

I do think you would be better off putting effort into a nicely designed cat. Current one is at least useable now. I would be wary of turning hard at speed though.

Rick W

[foxfish]

Quite right Rick I do need a new design & I hope you will be there to guide me http://www.boatdesign.net/forums/showthread.php?t=23634

[rwatson]

Have you tried different props?

You can get adjustable props that allow you to experiment with the pitch. It sounds like the current prop has too little 'bite', and just thrashes at higher speeds.

In a conventional hull this would be the first thing you would look at.

[brian eiland]

Quite right Rick I do need a new design
I've not had time to read your subject threads yet, but I do have an idea to pass before you...its a RIB with a unique hull...but you might not have to build it as a RIB initially.

Please send me a private email as I don't wish to post it on the forum at this time.

brian.eiland@gmail.com

[BHOFM]

It looks to me like the engine is too far back, the sponsons
need to be extended.

???????????????

But what do I know??????

:P :P :P