cavatation

[matt nz]

merc 140 blue band, mounted to hartly "gop" 18 ft medium "v" hull, goes great untill towing, cavation occurs when turning hard, left or right. locall dealer said to make up wedges to tilt motor under more, but that only made water spray up the back of the transom & lost a little speed. moved motor back to original position & added "turbo-hydrofoil" works great but sprays the person being towed. fully tucked in the motor is in line with the bottom of the hull, & is 1" behind the bottom of the transom. do i need to move the motor away from the boat, or your ideas please.

 

[steelespike]

Re: cavatation

If the Anti cav(ventilation) plate just above the prop is even with the bottom of the boat you could try lowering it just a fraction.But I think a prop change may be a better idea.If your prop is dinged up it may cause trouble too.Dont change props without knowing what RPM you are at now.Dont exceed the rpm range reccomended for your motor.Neither too high nor too low.A 4 blade is said to grip better and give good hole shot.Also a cupped prop bites better but will reduce your max rpm a little.You may need to go down a size in pitch with a cupped prop.If you need a tach "Tiny Tach" makes them for outboards and are priced reasonably

 

[Jdeagro]

Re: cavatation

Spike has given you some good pointers. Do not change the prop until you know what your wide open throttle(WOT) rpms are. Transom wedges and prochanges and hydrofoils have been the bandaids for small boat performance. The fact is that the hull dictates the performance of the boat, and this include design and balance.<br /><br />I have a few questions before we give you more things to do or check.<br /><br />1) when You put the boat into a turn, how much does it lean?<br />2) In the turn, does the boat slide or turn?<br />3) what is the configuration of the boat - ie: where is the seating , gas tanks, etc?<br />4) what is the top speed of the boat now and what was it before the hydrofoil?<br /><br />As a matter of information, the transom wedge is designed to allow the motor to be tucked under the boat more, which increases the stern lift and pushes the bow down.<br /><br />The hydrofoil is designed to increase the planing surface of the hull, which helps the boat plain easier, but it also increases the lift at crusing speeds which also drives the bow down.<br /><br />These not only cost you speed, they cost you money since the prop is not running perpendicular to the water surface (it is at an angle)and therefor not efficient. The added lift from the hydrofoil at crusing speeds makes the boat run bow down, adding more resistance to the hull because of attitude. Stop playing with bandaids that seem to help in one area and cause problems in others.<br /><br />Give me some additional information and i will try to help. <br /><br />Yes we make trim tabs and they may be the correct solution but we do not want you to use them as a bandaid either.

 

[BillP]

Re: cavatation

Those symptoms say the motor is too high. Its a typical setup for better speed but not so good for turning and towing. An overpitched prop aggravates the problem. Lowering the motor one hole on the transom usually takes care of the problem. <br /><br />Your foil is deflecting water down around the prop and letting the motor ride higher without cavitation...without loss of speed when mounted high on motor installations like yours.

 

[Jdeagro]

Re: cavatation

BillP is most likely correct, if the motor is too high you will loose bit in the corners. If you lower the motor the try to make sure that the hydrofoil does not run below the water or at water level when on plane. If it does you will likely end up with other handling issue, especially in corners (ie: bow steering). I think Billp would agree that you should get the boat set up as good as possible without additional devices, then work with add on if possible or needed. Lower the motor, remove the hydrofoil, and test. Then add the hydrofoil ( I can't believe I am saying this) and test.

 

[BillP]

Re: cavatation

nautiJohn,<br />Are you going into the foil business too? <br /><br />

 

[walleyehed]

Re: cavatation

If he did, they would be called "This is Nautifoil".......just funnin' with ya John! <br />You know I love my tabs.......

 

[Jdeagro]

Re: cavatation

I think it is clear what I think about the hydrofoils, and I would never sell them, but let me explain in case someone does not know.<br /><br />Hydrofoil technology is not new and is valid, however they need to be incorporated into the overall design of the hull from the beginning. Simply adding one off the self is a crap shoot at best. Adding a fixed planing surface to a boat changes not only the low speed characteristics and handling but the higher speeds as well.<br /><br />The motor manufacturers understand this and that is why they do not incorporate larger cavitation plates to the motors. They sell because they are cheep and people put up with poor handling as if it were part of the boating experience and acceptable. With or without a hydrofoil!<br /><br />Poor performance is poor performance and should not be tolerated. The smaller boat manufacturers have been telling the same story for so long that they actually believe it. It is unfortunate because the consumer suffers. <br /><br />The sale of hydrofoils is in the 150,000 to 200,000 units per year range, add this to the trim tabs 75,000 sets per year, and prop sales (god knows how many) you begin to see that these wonderful boats may not be performing as wonderfully as they say. Since I have never seen an airplane without trim tabs and flaps, regardless of size or speed capability, it seems that trim tabs on a boat of any size would be a benefit. So far this assumption seem to be correct.<br /><br />I don't blame a consumer for buying a hydrofoil, his boat is not performing as it should, however the addition of this fixed planing device is a crap shoot. They are cheep to start with but the cost often increases every time the boat is used. most people report that the boat planes easier but runs slower at the top end. This means that the hull has lost efficiency at the top end, and that results in increased fuel consumption. Usually about 5%.

 

[Jdeagro]

Re: cavatation

See, You all got me started. I just can't help it!

 

[matt nz]

Re: cavatation

believe it or not, the fuel economy has improved, the top speed hasn't changed(40 mph), the boat does roll-over on turning. The cavitation plate on the motor (just above the prop) is dead in line with the bottom line of the hull. When tucked in further, the front of the cavatation plate is above the the line of the hull, but the rear of the cavatation plate is below the line of the hull( this would indicate that the motor needs to be dropped down the transom, in order to correct the c.plate angle to the hull.As per prop, the motor runs w.o.t. at 40mph @5800-6000rpm & has no problem getting out off the water, with or without towing(pulled 3 biscuits around lake on sunday, no problem in getting power down in straight line & easy cornering). as per wieght distribution, cabin boat 18 ft g.o.p. driver , co-pilots, fuel tanks, & gear behind cabin, would deffinetly be wieght biased to rear of boat. thanks for your replies mattnz

 

[BillP]

Re: cavatation

matt nz,<br />You have the perfect setup for foil operation. It's in the water for low speed take off and out for speed. If you lower the motor the foil may stay in the water at high speeds and scrub speed. It's a trade off. You could always go with a hydraulic jackplate for the best of both worlds...but it'll cost ya big $$$.

 

[rodbolt]

Re: cavatation

hello<br /> first off we gotta straighten out your terminology. what you have occurring is ventilation not cavitation. the foil, as long as it is running submerged, will help with the problem due to the expanded area of the motors anti-ventilation plate. I dont really like them but on certaian hull/motor combos its been the only solution. if you have power trim and its trimmed up much at all in a hard corner you will have ventilation. not much you can do to stop it. the hull is forcing the water away from the prop. try trimming down as you corner.and while the foil will assist with stern sqat at take off if its much below the surface at planing speeds it will drag. the ideal spot is at the waters surface when on a plane. the foil has 2 functions. one to help lift the stern at take off and 2 to give a wider area of water above the prop. the prop will try to suck air from the surface and is amazingly good at it under certain contions. when the prop sucks air we call it ventilating and the hull slows and the RPM's skyrocket. with stern drives its normally not a problem. what size and brand of prop are you running? is it stainless or aluminium?<br />and is the transom a v or flat ? I am not familiar with the hull you named.

 

[Jdeagro]

Re: cavatation

Matt; <br />As BillP said do not lower the motor. As rodbolt said the flange is an anti-ventilation not cavitation. It is to insure that the water presure above the prop is the same as below. You are bow steering the boat because of the excess lift on the stern. <br /><br />Your are leaning hard in the turns because the hydrofoil is acting like a rutter.<br /><br />I do not want to get into a debate on performance but the boat would go 2 to 4 MPH faster with Smart Tabs, reduce the time to plane in half, improve the handling and ride, and gain in fuel economy. If you change the prop to a higher pitch, the hole shot will continue to be good, and the top speed will increase even more, as will the fuel economy.

 

[BillP]

Re: cavatation

I'm always puzzled why motor mfgs call it a cavitation plate and others a ventillation plate. In my mind it really doesn't matter and I use cavitation as the general term. Either way, the prop is churning air and the fix is usually prop or elevation change.

 

[matt nz]

Re: cavatation

problem understood and now under control.Thanks for all help, Matt