Cavitation problem?

[wokii1]

I need some advice,

I have got my first boat, a 15ft cruise craft cobra 1500 run about with a 2000 mod merc 50hp with gauges. It a great boat but I have a problem.
IMG_0133.jpg

When i got it the prop was abit chewed, and it used to cavitate when trimmed up one bar, I thought it should be able to trim up abit further than that before it cavitated. ( is one bar normal?)


So I borrowed a new stainless prop off a friend to try to squeeze so more speed out of the 50 and maybe fix the cavitation.
I did gain more speed and it still cavitated at one bar ( I got 37mph via the gauge with 2 people on board). I then bought a se sport200 on sale and bolted that on to try to help with the cavitation.

But what happened next I don't understand, It now cavitates at half a bar in a straight line, But if I sit just on the limit before it cavitates in a straight line and take a corner at speed it does not cavitate like i thought it would.

I do know the prop selection does play a part as does the height on the motor.

All my examples are at wot about 5700rpm-5800rpm smooth water with 2 people.


I have attached some pictures to help show my setup and position.

Here is the munted orig propIMG_0325.jpgIMG_0324.jpg

Here is the stainless one I'm trying atm.IMG_0319.jpg

The cav plate is about 3cm above the bottom of the hull when trimmed down.
IMG_0320.jpg IMG_0312.jpg

This is when trimmed down all the way. about 11cm
IMG_0323.jpg

Trimmed half a barIMG_0322.jpg about 15cm
one full barIMG_0321.jpg about 21cm
IMG_0326.jpg This is how it is set up atm. The motor is mounted as low as it can.

Does anyone have any ideas?

I thought the se sport was meant to reduce cavitation but it seem to increased it. Could it be just a prop problem? would a different size/pitch make a difference? Any help appreciated.

[cormorant]

Hi

Sounds like airation not cavitation not that it matters.

Combination of prop, engine height, hull shape and trim can all cause what you are experiencing. Nicks or dings in the prop or edge of the skeg along with tranducers mounted too close or water pickups and anything else that can change / disturb the flow of water ahead of teh prop.

When it happens are you getting the prop to "blow out" ie loose traction and revs go to the limiter like it is in fresh air? Does the boat porpise up and down?

Which hole from teh top are the motor bolts in - take a photo?
What shaft length is the motor ? If you know .

Guages are rarely ( well the motor sender unit) calibrated on installation properly so it is just a indication.

Can you take a photo from the side of the motor / boat with the cav plate parallel to the bottom of the hull with the camera at the level of the bottom of the hull? The ruler or something longer to show the line along the lowest part of the hull? Trying to check if the engin eheigh is Ok. What is the approx distance from the bottom of the hull to the underside of cav plate?

The cav plate should be higher than the bottom of the hull.

The cure all cav plate addition may or may not help but best if you can get the boat set up properly without it and only put it back on if the boat has a bad habit you need fixed or if it makes a huge difference to your low speed planing / weight carrying.

[MyWay]

check out if you got any gel coat chip under your boat near transom

[robothefisho]

Flip those bolts on the foil the other way to, so the nut is on top and the smoother head is under the cav plate. Motor height looks pretty good really, A bit above the bottom of the hull, water will rise around an inch for every foot back from the transom. As was stated earlier trim guages can be no where near right. Does it indicate fully down when motor is and fully up when at the top of the trim range before lift. Unless the cav plate is out of the water when your planing i'd say whatever ventilation it has would be pretty normal. Nothing looks that far out from the pics. Is that foil clear of the water when planing or still in it? It does look like a pretty small diameter prop though, so that could be the issue. Or it could just be that hull, I'd ignore the trim guage and trim it off listening to it and where the boat is happiest.

[wokii1]

Thanks everyone for your thoughts,

@ Cormorant, There are no nicks or dings in the prop, it is brand new, Nor are there any water pickups or transducers close to the motor.

"When it happens are you getting the prop to "blow out" ie loose traction and revs go to the limiter like it is in fresh air? Does the boat porpise up and down? " It doesnt really sound like its "Blow out" it dosn't free rev, and I don't really know what porpise means.

It dosn't ride right at the moment, it is riding nose down to far, the water splash is right up near the front where I sit.
The motor is bolted via the top hole as shown in last pic. I do not know what length the motor is, how do you measure this? I have tried to show the position of the cav plate to the hull but will take some better picks tonight. I do relise the trim gauge is just a guage, that is why I have taken photos of how far it trim when 1/2 and 1 bar on gauge to show the different positions compaired to the transome.
The cav plate is about 3 cm above the bottom of the hull atm.

@ My Way There were some small chips that I just filled with flow coat and have left abit proud and still need to sand and buff. But they were heaps worse before I fixed them and I could trim up higher.

@ robothefisho , I will flip the bolts, As before I do relise the gauge is just a gauge but it does display correctly when full up n down and about haly way. I do not know if the plate is out of the water as I'm driving but I will try to check next run. When you say the ventilation would be normal, I dont think it is as i could trim up further without the se sport and plane nicly and with the sport on it, it rides like crap, the front is down like its planning wrong. There has been mention of trying to spin a larger prop and maybe a differnt pitch but I dont really know what to try.

I Will try to get better pics tonight, Agian thanks all for your ideas.

[Gon Fishun]

Is the diameter of the prop correct? Looks small in the pic.

[wokii1]

I have added some more pics, Hope this shows it better.

@ Myway, How much difference can a chip make? While placing the level so I could show the angle of the hull and cav plate I found this,

IMG_0327.jpg I never noticed it b4 as Ive never climbed under the back and it is right under the bung in the center. The other chips it did have were more on the back cnr of the outer chines and I have touched them up with some flowcoat. This lump of bog has been there from b4 I got the boat so would this cause my ventilation problem to get worse because I added a hydrofoil? After this pic I filed this down, It still need some more work, but not tonight.


If thing like chips can make a difference I sprayed the front of the boat with some flow coat b4 last trip to repair some bits at the front. This is right down to the first roller on the trailer as the boats been bouncing on the front roller and damaged the gelcoat there. I started sanding it but have not got down to the first roller and still need to sand and buff this and It is quite orange peeled. Could this also be causing the issue?

Here's some better pics to show the cav plate angle and height to the hull.

IMG_0328.jpg This is all the way on/down (1)


IMG_0329.jpg This is half a bar, and the max I can trim to with the hydrofoil on without ventilation (2)

IMG_0330.jpg This is one bar, I was able to trim this high without ventilation before I added the hydrofoil. (3)

IMG_0331.jpg This is how the motor is attached on the transom.

[wokii1]

@ Gon Fishun, I have thought it was alittle small to, But I don't really know whats meant to be on there and I have never looked into prop sizes. I am unsure of the size of the prop, It is the same size and pitch as the one that came on the boat when I got it. I borrowed it from a friend to test the dif between my munted alloy one and the new merc stainless that just happened to be the same pitch and diameter.

[oldboot]

If you are having cavitation or airation problems I doubt that it will be any of the minor imperfections mentioned.

I've seen props far worse than the original and I have run a prop that is far worse than the one picture with no cavitation issues.

I think you have an engine height issue.......the cavitation plate should be well a truly in the water while on the plane...generaly they get set up level with the bottom of the lowest point
of hull.

If the foil is causing more cavitation, it occurs to me that may be because it is providing lift and pushing the stern further out of the water.

I'd drop the motor a hole or two and see how you go.

as for the attitude of the boat..well that remains to be seen....work on your weight distribution.

Have you tried to get the genuine paper manual specific to that motor (hit and miss)...if you can get one it will have prop tables and WOT figures.

cheers

[Gon Fishun]

Just looking at your pics, the motor is to high, (the cav plate should be level with the keel) and the motor cannot be lowered, indicating it might have the wrong size leg on it ?
Maybe a trip to the local Merc dealer and pick his brain.

[Stik-ugly]

I think that you may find that a 50hp motor should have a se sport 3000 not a 2000. The web site states a 3000 for motors 40 hp and up. I have a 40 hp four stroke Merc and it has a 3000 and performs well.

[centrefire]

Have a read of 'Savage digging it's nose in' of June this year. I've experienced all of the problems you're having. I discovered a 'hog' in the hull about 50 cm back from the transom. Knocked it all out, hammered the cross braces down on the hull and shimmed them all hard against the hull throughout with 3mm plastic and silicone. Firmed the boat up 200%. Runs like a cut snake now.
Small imperfections don't mean much. Check for a concave shape in your hull near the back. Anything concave or convex simulates trim tabs.
The only fly in the ointment is that you've a glass hull. Unless something drastic happened near the stern, you shouldn't have any irregularity at all...

[wokii1]

Thanks everyone for your replies, I am going to try to ring a dealer tomorrow and see what they say.

I think that you may find that a 50hp motor should have a se sport 3000 not a 2000. The web site states a 3000 for motors 40 hp and up. I have a 40 hp four stroke Merc and it has a 3000 and performs well.

That maybe correct, I never checked thier web site But it does say
SE SPORT 200

1. "What size motor or outdrive will it fit?"

Our smaller 200 model is recommended for outboards from 8-40 HP. It will fit all outboards and sterndrives larger than 40 HP except Mercury 40, 50, and 60 four strokes. There are some smaller outboards (down to 4 HP) that it will fit as well.


I think I based my purchase off the bias boating cat. The se 200 9.9-150hp and the se300 35-300hp, So I thought my 50 was closer to the middle of the range for the 200 than the 300 as bigger is not always better. But it looks like maybe I should have got the 300.

[wokii1]

Have a read of 'Savage digging it's nose in' of June this year. I've experienced all of the problems you're having. I discovered a 'hog' in the hull about 50 cm back from the transom. Knocked it all out, hammered the cross braces down on the hull and shimmed them all hard against the hull throughout with 3mm plastic and silicone. Firmed the boat up 200%. Runs like a cut snake now.
Small imperfections don't mean much. Check for a concave shape in your hull near the back. Anything concave or convex simulates trim tabs.
The only fly in the ointment is that you've a glass hull. Unless something drastic happened near the stern, you shouldn't have any irregularity at all...

The Hull is in pretty good condition for an 85 mod, I'm the second owner, its been kept under cover all its life, it had only done 113 hrs in the last 10 years. Its never had the transom or floor done, no soft spots and no rot and dosn't take on water, It has small issues mostly caused from launching and retrieve before the previous owner put self centering rollers on. It would run off and grab on the roller brackets. I will check over the hull tomorrow but I doubt I will find much, But you never know !

[cormorant]

Hi

Good photos -thanks - bugger nothing real obvious

The photo where the motor cav plate is parallel with the hull bottom is your neutral trim position ( ie not trimmed in , pushing bow down or trimmed out, lifting the bow). If the trim guage was calibrated that would be dead middle. Many boats struggle ( with heavy bows) to go past neutral trim as they just can't lift the bow so the brop blows out.

Smoothing within reason any nicks at teh trailing edge will stop some airation.

The height looks OK . Saying that all hulls are different. I am never impressed with seeing boards mounted under motors as they are often hiding things. It may have just been placed there to stop damage to the top alloy edge cover but most are there to cover stress crazing in gel coat and loads of bogged up old bolt holes or a soft transom. Not always .

Merc dealer or someone who has a similar motor could tell you if the prop diameter is large enough and tehe WOT revs suggested.