Is my motor too high?

[Damned67]

Hi All,
When reading through a thread over in the saltwater section, about motor height, I came across this:
"Just make sure you try it in average conditions after lifting the motors.... You might find the Propellers will brake traction and the motor will rev up but you go no where to you back right off and go again... its a huge pain in the ass and could be dangerous if on a bar crossing when you lose forward momentum !!"

This is exactly what my boat does in very average conditions.
My rig is a 4.2m tinny with a 30hp Merc.
The engine was set up by the boat builder, and the builder placed a length of 25mm square bar between the top of the transom and the motor (lifting the motor 25mm off the transom).
I've attached two pics. One is a straight edge off the cavitation plate (?) demonstrating the difference in height between the cav plate and the hull.
In the second pic, I've used a 1500mm spirit level on the gunnel to set the hull so that it was horizontal/level. I've then placed a small spirit level against the leg and show that the leg is pretty damn close to vertical (a few mm gap between the top of the spirit level and the leg).
So, should I consider dropping the engine down 25mm and/or running the engine in a different tilt/trim position? Of the 6 different trim/tilt positions possible, it's in the 3rd lowest.

Comments? Opinions?

Cheers!7883878839

[gazza2006au]

looks fine to me your cavitation plate can be up to 50mm 2" below the bottom of the transom

[Chimo]

It looks a tad low to me, easy enough to lift it a couple of inches or so and try it.

C
C

PS Cleaned the specs hence the rewrite!

[Horse]

I would probably have it up a hole and run a small foil on it. I am yet to find a boat that needs the cavitation plate BELOW the level of the hull. I think your issue could be the need to trim it in one hole in rough conditions

[FishHunter]

I am somewhat confused by the above posts as the cavitation is clearly higher than keel line and looks too high to my inexperienced eyes. If the motor is breaking the surface of the water when its trimmed in then I would say its too high and needs to be dropped a bit.

Of course I may have the bull by the udders.

[Spaniard_King]

To answer the original question.... IMO... NO

Drop the pin down a hole and if needed re distribute the weight in the boat to compensate the ride

[bar raider]

I am somewhat confused by the above posts as the cavitation is clearly higher than keel line and looks too high to my inexperienced eyes.

i saw the line 3 inches above the ruler too, but i think the ruler is sitting on the cav plate

[gazza2006au]

i saw the line 3 inches above the ruler too, but i think the ruler is sitting on the cav plateyep the ruler is sitting on top of the cav plate

[netmaker]

i have a 30 merc on a 4.3 m tinny and played around with the mount height for a while before bolting on. had the same cavitating problems regardless of height although i only went as far as an inch higher and would expect any higher to only cavitate more. i wasn't going to cut into the transom to try lower either. my problem was solved with my se200 foil which i had on my old motor. dont go any faster but do plane at lower revs and have not cavitated since. was really impressed with the cornering difference it makes in the creek.
cheers

[Dicktracey]

Damned dont get to hung up on what works for one boat as it may not work for another !
I recently set up me new boat as out of the box the thing was a pig Cavitation shock the whole boat "140Suzuki 6m glass boat"
Long story short my leg was too deep much like your pics to be honest, causing too much drag and causing prop to slip like a mutha ;)
Lifted leg hole at a time videoing cav plate at different trim in flat sea's.
Found best setting was cav plate parallel at half trim setting on guage, dont get me wrong i enjoyed getting it sorted if I had my time over I would have booked it in with someone to sort it.
Maybe i expected too much from a 60K fit out.
Just to throw another curve ball at ya prop size & pitch has just an important input on cavitation as leg hieght.
cheers
Dick
PS;Fitting a foil is giving up;)

[stue2]

Doesnt look too high to me either. Infact it almost looks low.If you shift the pin it will be lower.
Is it a stainless prop? Im thinking a prop is more the trouble than height

[Damned67]

Stock prop. I have considered re-propping it in the past. I've got a few RPM up the top where it doesn't increase the speed of the boat, and I could use a bit more torque. Get's up on the plane quickly when I'm solo. By three POB, one needs to lean up on the bow to shift the weight forward to get it up on the plane. That said, I've never really screwed around with the tilt/trim positions either.

I always had the feeling that it was the prop breaking out of the water when crossing swell/wake, but it could be digging deeper coming down the wave? It always left me wondering why some boats can seem to get airborne, and not cavitate when they 'land'?
(That's not to imply that I ever get airborne!)

[Horse]

Tilt/trim is critical in how a vessel handles. Start playing around with that and you will have a totally different (and safer) boat. Basic rule for me is nose down into a chop and nose up in a following sea. What sort of sea was it playing up in. One person up will run a different pin hole compared to two or more. my boat has power trim but I am on it constantly to get the best ride and economy

[Fed]

They do mate it's just that it's so embarrassing sitting there at WOT doing 6000RPM with the boat blowing bubbles & standing still that we quickly learn to back off & hit it again.
Nothing better than the sound of twin Mercs getting airborne with the tacho on the stopper, I've often wondered what they used to rev to in free air, I bet it was way over 10,000RPM.

[Damned67]

What sort of sea was it playing up in.
Got me thinking now.... You'd think I'd pay attention to that, wouldn't you? I guess, because the boat was set up by the builder, I assumed it was somewhat normal. If I had to guess, I'd say its more common in a following sea.

With a heavier load, I assume I trim it up to get the bow up?
(I've been wrong before, just ask my ex-wife!)

[gazza2006au]

Tilt/trim is critical in how a vessel handles. Start playing around with that and you will have a totally different (and safer) boat. Basic rule for me is nose down into a chop and nose up in a following sea. What sort of sea was it playing up in. One person up will run a different pin hole compared to two or more. my boat has power trim but I am on it constantly to get the best ride and economy
Horse i always thought it was raise the nose in chop to reduce the pounding by putting the nose down your just mushing your way thru the chop?

[Goochi]

I agree with the rest, more likley too low than too high. And as Dicktracey said, your prop can have a significant impact on cav issues.

[rowanda]

i just went through the same issues. Put a 130hp yamaha on my quinny....heaps of grunt but cavitated like no boat I've even had (i've had over a dozen) even when almost flat water couldn't trim it to fix it. spoke to my mechanic who came for a run with me as I was convinced it was too high and wanted to drop it down a notch. He convinced me it was probably a prop issue and almost needed to go up another notch. Looked at a foil but I wanted MORE bow lift not less which a foil can tend to do. Spoke to Shane at Solas and gee he knows his stuff. Motor had a stock 17" prop on it, which he said the yammy props can cavitate due to their design, after we spoke for a while, we worked out a 4 blade 15" would be best. I would've gone alloy, except the motor is counter rotating and they only do stainless. For the $600 outlay it's made the biggest difference to any boat i've even had. Made a good boat an awesome boat, heaps of grunt, takes off like a bullet, only lost a couple of kn/h which it had plenty of and NEVER cavitates. Used it in the Port Stephens trailerboat comp last weekend for 3 days off the coast and didn't look like cavitating once, over swells and everything....could be your answer and hey a phone call to solas might help??

[Horse]

Horse i always thought it was raise the nose in chop to reduce the pounding by putting the nose down your just mushing your way thru the chop?
In most boats the nose is the pointy bit that cuts through the wave. Trimming out exposes a broader flatter section to the waves so gives greater impact when you hit the steep front of a small wave or chop. Each vessel handles differently and especially in nswells and bar crossing safety overides comfort. Following sea sees a nose up position to reduce broaching and burying into the back of the wave in front of you

[fat-buoy]

Yep I agree the motor is too low if anything... I have never had a tinnie with too much issue with cavitation but I would also say lift the motor one notch at a time... when you are around an inch higher than the flat keel line (not the ribbing) that would be about as high as I would say to take it.. if no result then put a foil on the motor.. they really do make a big difference for the outlay..

[Damned67]

OK, I'll give I'll try raising it up.
Next question:
The motor has been bolted on. Raising it up will leave me with two holes in the transom. What's going to be the best way to seal the holes?

[centrefire]

Had a 4.3 with a Yamaha 30. Motor was to low by 2 inches so it ran nose down. It appears that you're not aligning the cav plate to the extreme bottom of the hull. It needs to be parallel with the tip of the centre chine on the hull. It appears like you may be and inch to high, the way you're measuring it. Give it a run after lowering the motor and start with the pin in the centre and go from there.

[Fed]

I wouldn't raise it.

Really strange why there's so many different opinions on this?

[fat-buoy]

OK, I'll give I'll try raising it up.
Next question:
The motor has been bolted on. Raising it up will leave me with two holes in the transom. What's going to be the best way to seal the holes?
Shouldn't have to drill new holes.. the motor normally has a range of holes to choose from with the existing holes so that you can make adjustments without drilling new holes?

If new holes are required maybe try the foil first.. some people argue that if the foil is in the water (as your will be at that motor height) that they can sometimes bite and cause the boat to behave irratically at times.. I have never experienced it myself and have had 3 boats with foils on them.. if you are thinking of putting a foil on anyway maybe put it on first as you may get away with it and not have to put any more holes in your boat than needed.

If you do then sikaflex will do the job of sealing..

[fat-buoy]

I was just thinking on the motor height maybe post a pic of a straight edge extending from the keel line of the boat to the motor... the boat line is what should dictate the height of the motor.. I have heard that for every foot back from the transom the cav plate should be up another inch in height.. I think the result will be very similar saying that it looks like it could go up another inch or two but it would be interesting to see the pic all the same.

[Damned67]

So, I did the unforgivable. I opened the owners manual.
See the attached pic regarding installation instructions: States cav plate should be 30-50mm BELOW bottom of boat.
Also attached a better pic, in the light, of the straight edge on top of the cav plate. Can see that the top of the cav plate is only a few mm below the hull line, and about 10mm from the bottom of the keel.
According to the manual (that was supplier with the motor), I should actually drop it 25mm?7886978870

[Fed]

Lowering it will stop the ventilation but I'd first try rearranging the deck chairs on the Titanic or make the fat people sit at the back.
Get a spare battery & a live bait tank in the back, fixed.

Had a mate with a quintrex 4.9 that would do it sometimes on another boat's wake but 99% of the time it was fine.

Edit: What's the steering like, does it pull one way or the other?

[Damned67]

Fat person already sits at the back, along with the battery and 25L fuel tank.

Steering does pull to the right (tiller handle swings to the left if I let go of it)

[Fed]

If you can't live with the venting lower the motor one hole & trim it out one hole and if it's not already there move the trailing edge of your trim tab to the Starboard side.

[gazza2006au]

hes engine might only have the two mounting holes like the smaller engines have

[Damned67]

hes engine might only have the two mounting holes like the smaller engines have
Bingo!

I can drop it 25mm (which will then fall into factory specs). Although, I may wait until the funding situation allows for a new prop and give that a shot.

[nigelr]

I'd definitely try dropping it a notch for starters, my current boat cav'd like a sow until I replaced the motor and set it in a lower position. I don't like cavitation when bar crossing, not nice.

[Andy56]

Drop the motor OR go for a 4 blade prop. I had a quintrex 470 with a 60 4st yamaha on the back. Stock prop was 12". I dropped the motor due to cavitation and wolla, no more cavitation. Tinnies are definitlyl DIFFERENT to glass boats for motor setups. Level with bottom of keel is just the starting point. 4 blade props have more bite so less prone to cavitation. I cant see how raising the motor will stop the cavitation, makes no physical sense. Its cavitating because the prop IS OUT OF THE WATER. What does raising it do? doh. Drop it or use a jackplate

[centrefire]

Drop the motor by removing the 25mm spacer and I'm almost certain that the cavitation problem will be history. A 30hp on a 4 metre shouldn't need any prop greater than around 10 x 12'" to 13" pitch three blader. A four blade or a foil will only assist in masking a solvable problem.
Motor pull port or starboard can be nuetralised largely by adjusting the small skeg/ anode thing you see above and in front of the prop. Check the manual for direction.

[ifishcq1]

Look at Spaniard_King 's post
he is a professional motor man
setting motors up is his business where he makes a living at least try his educated suggestion before all the guesses
cheers

[Fed]

Well I wasn't guessing.:P

[ifishcq1]

Okay Fed not everyone was guessing
Garry "Spaniard King" knows his stuff is the point I was making
one post said about dropping the motor a couple of inches below the keel and that is just not right
cheers

[Fed]

Just having some fun ifish it must have been all those other bastards doing the guessing.;)

[stevemid]

Here is a reference to where your cav plate should be vis the bottom of the hull. http://www.onlineoutboards.com/Help.html?Question=Shaft-Length

BTW yours looks right per the above.

[Apollo]

Confused yet Jason?

Just for interest. Is the prop damaged in anyway?

[Andy56]

I certainly wasn't guessing or coming with no experience on this topic. I have now setup two boats, one a tinnie and one a high performance glass boat, at least some experience, lol
Raising or lowering the motor is easy to do at home and costs nothing. Before spending any money, get the height as close to right as you can, then maybe look at props, then you look at height again ha ha

[centrefire]

I'm convinced that I never guessed either. Damned67 is probably buzzing around as happy as Larry with his 1" lowered outboard by now. Here we sit still pondering the height of an outboard... gotta love it all.

[Damned67]

I'm convinced that I never guessed either. Damned67 is probably buzzing around as happy as Larry with his 1" lowered outboard by now. Here we sit still pondering the height of an outboard... gotta love it all.

Actually haven't touched it yet. I do think that the first thing I'll do is to try it in a different trim position, and go from there.

Steve, the prop is well polished from from sand bar action, and a couple of minor nicks, but it's had this cavitation problem as long as I can remember.

Cheers!

[bustaonenut]

It's your propeller Damed67, i have the same boat as yours but with a 40hp merc. I had the same problem getting ventilation/cavatation in choppy conditions to the point youd have to back right off the throttle so it grips again.

Changed the alluminium prop to a stainless one ( Apollo Michigan Wheel ) wich was about $270 from the states and have not had a problem since.

I even lifted the motor up another hole just to see how it goes and still dont get any ventilation, and thats the second last hole out of four holes

[WalrusLike]

Forgive my ignorance please... I know it must work since so many have proved it, but why does a Stainless prop work better than the same shape standard one?

[bustaonenut]

The stainless ones are usually a differant shape to the alloy ones, in saying that i should have mentioned in my earlier post that before i bought the apollo prop i tried a Michigan Wheel Ballistic prop wich was stainless as well.

It was no good on my boat as it would ventilate when i tried to trim up, i could get more trim with the alloy one.

I sold that one and got the apollo and never had a problem since.

[centrefire]

Stainless props inherently have a small amount of extra pitch over alloy size for size. They also resist deformation through water substantially better than an alloy equivalent. Less slip hence less cavitation and better forward propulsion.