Cavitation Plate Height

[Seachange]

I have a DT115 on a Suzuki 23ft hull and have a problem with it running bow high and not getting onto the plane and not developing full rpm-- max's out at 3200
It has been suggested that I raise the motor so that the cav plate is above the bottom of the keel level,this is to give more forward thrust and not raise the bow (as much?) and the motor should gain more rpm by not being so deep under water and bogging down
Currently running a 13.5x15 prop
Any advice appreciated
The only other suggestion I received was that the engine spark is not advancing as the butterfly's open
cheers- Stephen

[boats.net]

You can try to raise engine as long it doesn't cavitate should be ok.

[seaox230c]

Hi, I hope you have more than 1 on the back of that 23footer..My 23 Sea Ox runs a DT225. The height of the cavitation plate depends upon how your boat rides in the water at full speed or on max plane. The prop should just clear the plate by less than an inch, and that is where the trim tab is so that plate should be at least an inch or two under the water level. Too little and the prop will spin out in turns or going over waves

[Seachange]

thank's guy's- sounds like the cav plate doesn't necessarily need to be in line with the bottom of the keel contrary to popular opinion.Which is what Solas here in Oz advised me,they recommended raising the motor also so that the prop is pushing in more of a straight line rather than lifting the bow
Still concerned that I am not geting onto the plane or reaching peak rev's.I am going to try running with the motor trimmed right out to near cavitating to see if my rpm increases
If that doesn't work I really think I have a spark problem

[Seachange]

Hi, I hope you have more than 1 on the back of that 23footer..My 23 Sea Ox runs a DT225. The height of the cavitation plate depends upon how your boat rides in the water at full speed or on max plane. The prop should just clear the plate by less than an inch, and that is where the trim tab is so that plate should be at least an inch or two under the water level. Too little and the prop will spin out in turns or going over waves

The boat is very light-around 1,000kgs.Just has a large centre console and no internal/external furniture
I wil look at the cav plate depth when the boat is at rest with full down trim and work from there
Hope the foil helps also as it is designed to make the cav plate longer rather than wider
Hard to decide what is wrong when 2 different mechanic's can't agree
Cheers- Stephen

[Swanie1975]

What pitch is your prop? Sorry just reread and saw you are running a 15p
There's got to be something wrong with the motor itself if that's all it revs to. Should be somewhere in the 5000 rpm plus range. Are you sure it's firing on all cylinders? Compression ok?

My old dt140 used to pull a 19p prop out to 5800 but when it dropped a piston it would only rev out to just over 4000rpm. Great motor with loads of balls.

Good luck with it in hope it's not as serious as my problem turned out to be

Cheers Ryan

[Moonlighter]

Read this- it is recognised as the authorative article on setting your engine to the correct height, and also how to test your prop.

You cannot be certain an engine is at the right height by looking at it in relation to the keel. You have to do the test as described in this article.

Propping correctly & how to test props.....

Frankly, though, if you are only truly getting 3200rpm with the throttle wide open, then you have bigger problems than engine height, i would suggest. You need to get your 115 revving to close to 5500, otherwise you will kill it in short time.

My DF115 4 stroke pulls 6200rpm with a light load, and 6000 solidly loaded, with a 20" Suzuki SS prop, and my Zuke mechanic here in Brisbane says thats exactly what he likes to see. The result is snappy acceleration and food fuel economy.

Mind you, mine is on a plate ally 18 footer, not a 23 footer like yours is. I am worried that you could be badly underpowered...... Plus, the old 2 stroke 115 might be getting on a bit.....

Id suggest you need to get a service done by a qualified Zuke mechanic to check it out properly and to clean all the fuel filters etc.

And to check if the tacho is working properly!!

Does it sound like its only getting 3200rpm? You would barely be able to plane most boats at that speed? It would be a total dog trying to get on the plane, too!

Wher in Oz are you Stephen? I can recommend a couple of great Zuke men in Brisbane are who could check it out for you.

[Seachange]

I will try and answer both the latest posts here-firstly I am in Sydney
I had 2 Suzuki mechanics have a look and got 2 different results
The compression was done by another marine guy at Xmas time and he is going to have a look next week to see what may be wrong,I told him to bring his timing light
The compression came in at 135psi on all cylinders and I was told that was o.k.?,I don't know what it should be
The motor is really labouring and won't get onto the plane.All the fuel lines and primer bulb have been replaced plus I changed the diaphram in the fuel pump
Once it gets over 2300rpm the stern drops and thats it-revs to 3200 at 2/3rd throttle and nothing changes even at wide open throttle,and the boat runs bow high

[Seachange]

Read this- it is recognised as the authorative article on setting your engine to the correct height, and also how to test your prop.

You cannot be certain an engine is at the right height by looking at it in relation to the keel. You have to do the test as described in this article.

Propping correctly & how to test props.....

Thank's for the link-it confirms what Solas Qld. told me
Won't hurt to go a few holes up,plus I will fit the Permatrim foil at the same time
cheers - Stephen

[Seachange]

update=== went for a fish today and tried running with the leg trimmed out as far as I could to minimise water pressure but without making a huge rooster tail
Didn't make any difference to the rpm,really starting to think the original diagnosis may be right in that the spark advance is not working - unless the motor has a jet problem-hard to believe 4 could be blocked

[catacom225]

Back your trailer into and see if it will rev over 3000 rpm in neutral . My old engine was once tuned under load strapped to the trailer

[Seachange]

The boat is on a mooring and yes it revs well over 3,000 rpm in neutral
I don't know what effect limp mode has in neutral if any
Going to see a mechanic tomorrow so it should be interesting
I am going to disconnect the throttle position sensor and reset the low oil alarm even though the alarm isn't going off just to see what happens

[Egorena]

Don't know where you are at with this, but here in South Texas, all we use is cavitation plates in shallow water. All cavitation plates should be above the keel. The further your prop is away from your transom, the higher your water flow will be when it hits the lower unit. I believe your cavitation plate is submerged and therefore keeping your stern down and your bow up. The 115 will push a 23 ft. boat, just not very fast, but you should be able to plane. I have a jack plate, so I start at 2 inches above tunnel (cavitation plate) and once running, I go as high as 4 inches. I have a water pressure guage to help me from cavitating. Good luck!

[Seachange]

had the mechanic look on Tuesday and he confirmed the spark is not advancing
I went and bought a workshop manual so he can check out the throttle position sensor and the gear counter coil next week to ensure they are as per factory specs
Hard to get anything done when I work through the week and the boat is not on a trailer,I have to wait till my Tuesday off then drive the boat down the bay to the mechanic
I am still going to raise the motor 1st chance I get even if only the last 2 holes on the swivel bracket
Add In== just found out that Solas in Australia import jacking plates from the USA (BOB'S).They don't look like they raise the motor much but will set it back 4"

[Seachange]

Don't know where you are at with this, but here in South Texas, all we use is cavitation plates in shallow water. All cavitation plates should be above the keel. I believe your cavitation plate is submerged and therefore keeping your stern down and your bow up. Good luck!

One of the water pickups for the pump is in the underside of the cav plate so I need to keep it partially in the water or I will possibly overheat
The inlets in the lower leg may not pick up enough water on their own
I do agree with you that the cav plate is dragging the stern down due to lack of rpm"s to push the boat up onto the plane
I will sort out the spark advance and progress from there
cheers == Seachange