Cavitation at speed

[Dinga04]

Hey guys, after your opinions.

Just got my boat in the water after buying and restoring it.

16ft glass half cab with 1992 100hp merc.

Problem is it cavitates when I get to about 25knts (3/4 WOT). When trimmed fully down the hydrofoil is about 1 inch under the water. When I trim up the cavitation gets worse. I assume the engine is too low. Although it doesn't really seem it (see attached).

Pic1: Motor trimmed fully down
Pic2: Motor trimmed up slightly

My question is... is it that simple?

Thanks in advance

http://i257.photobucket.com/albums/hh231/hrc25/Boat/13122008018.jpg
http://i257.photobucket.com/albums/hh231/hrc25/Boat/13122008020.jpg

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[finga]

Gees that's a deep V on that hull.
What's the hull??
Could be a simple lift the motor (the cav. plate is supposed to just sit on top of the water) but a few other things can cause the problem.
Any repaired damage to the underside of the hull?

[FNQCairns]

Yeah massive V it has a couple of large planing strakes etc though.

I will go out on a limb and say you have too much pitch with not enough diameter - either/or or in combination, or even a reworked prop somehow and they stuffed it. That hull will need good diameter with the correct pitch high in the rpm range to get good trim/bow lift at speed or it will blow out -ventilate.

If you can throw up some ACCURATE figures like prop and diameter/prop numbers, GPS speed and RPM at those speeds and even wot, what ever you call that with your problem it might go some way to finding the cause.

cheers fnq

[Dinga04]

Thanks guys. Yep its a nice deep v on it. Goes through the water nice and gets up on the plane very easy. Its also very stable at rest. Its a 1976 Penguin Phantom hull that used to be made here in WA.

The prop has written on it "QUICKSILVER 48 16988 A5 18P". Can anyone decipher what this is? I assume 18" pitch?

I'm going out in the boat again tomorow, so will get GPS speed and rpm at cavitation. I don't think I can get WOT because it ventilates.

As far as unrepaired damage.. the bottom of the hull is the only part I havn't restored. It has a few bits of filler on the planing stakes and keel. Would this be enough to cause ventilation?

http://i257.photobucket.com/albums/hh231/hrc25/13122008015.jpg

[Blackened]

G'day

I'm not sure on the numbers but the guys here will look after you.

I can tell you quicksilver is the mercury brand, so maybe having a look online at their catalogs will give you an accurate diameter/model of prop.

Try it without the foil and see what happens

Dave

[Angla]

They are good pictures. You should get a good response from them here. My thoughts were that the motor may need to go up one hole but I see promise in FNQ's advice.

If it were too low then I think it would bog down and be hard to get on the plane relevant to how much too low. You say it gets on the plane easily so I am thinking the prop size and pitch is the answer in most part.

Maybe a 17 inch prop with more pitch and lift the motor one hole. Just my thoughts

Chris

[finga]

Ah, the illusive Phantom Penguin :D
Looks like a boat I'd like to have. :)

[Spaniard_King]

I would be checking to see that the prop bush hasnt begun to spin in the housing.

Have never herd of a prop cavitating because its too low in the water:P

The prop dosn't look worn and an 18 would seem ok for a 100hp on a 16ft boat

[Duckfish]

Hi Dinga 04,
Looks like the leg is right. The cav plate is in line with the bottom of the boat. Try taking off the foil then I would check and see if you could get full revs at wot. If not, a smaller pitch prop size could be the go. Give Steve at Solas Props a call.
Duckfish
P.S. The boat looks like an old Haines Hunter. Congrats on the restoration.

[FNQCairns]

yeah it's your diameter, that prop is only 13inch diameter, great for someone with a light boat who wants to go fast, you need more diameter without fail the engine is overpowering it on that hull and sucking air. are you running heavy or light? need to test heavy in the future.

If you do get some numbers from testing it might go some way toward working out just how much lower in P you need to go but truth be told a crystal ball will be needed at this point to get it right straight up.

Any 13.5 or 13.75 diameter prop will see you happy in comparison it's just assuming the pitch in concert so you can obtain full WOT prm that is the hard part right now, hopefully a 17 will do it for you, who knows even a larger diameter 18???? but you would probably need to be heavy right now.

Beg or borrow a couple props to test and record their test result accurately...then you may even be able to go up a hole in motor height, when you get closer to getting it sorted the final transformation will amaze you on that boat, it's a good one but the prop is all wrong IMO.

Wrong too in a rare way most are over diameter-ed, your setup is a standout just like the rest of the boat:).

cheers fnq

[TimiBoy]

I don't know a thing about the prop - but I wanted to congratulate you on the boat. She looks wonderful.

I had a trip on Friday on tropicrows' boat, another restored wonder. The work you guys do is amazing, hats off!

Cheers,

Tim

[Dinga04]

Thanks for all the tips and advise so far guys. Went out in in today. 4 POB + Dog (2 more POB than yesterday).

I can reach GPS 24knots fine. At 4500rpm I tryed to accelerate and it ventilated. I noticed it didn't ventilate as easily today with the extra 2+dog but still very frustrating to get anything over 26 knots.

Duckfish, thanks I will give steve at solas a call and see if I can do something.
FNQCairns that's good advise.... time to start experimenting :)

Unfortunatley for all you followers I'm off to NZ for 4 weeks, so will not have a chance to try props untill then. I'll keep you posted with the details for future reference