Engine Height

[PROS]

Now that I am spending a bit more time to setup the boat for best performance, I have couple of questions about engine height.

Outboard is currently attached at top hole, can be lifted 3 more holes if needed.

Not sure if it is too low or right height.

Below pics showing the leg in water cruising at about 60km/h, normal trim not all the way down.

Is this looks too low as I cant see the flat plate skimming the surface?

What benefit do I get by lifting up the outboard?

image1.JPGimage3.JPG

Max

[Darren Mc]

Benifits can be more top end speed(less drag), better fuel economy and generally a lower bow attitude.

[kind_cir]

Down side can be cavitation of the prop especially in turns.
aim for the cav plate as near as level with the bottom for best results.

[Darren Mc]

That can happen, if so just go back down till it is managable.

[Chris Tucker]

How does the boat trim at speed and accelerating? A low outboard will lift the bow, a higher engine will lower it.

[Dogtoooth]

The water should just skim the cavitation plate, by the pic it looks like it could go up a hole. Check with a long spirit level on the bottom of your hull, take in account the deadrise of your transom first before you lift her, get a second apinion.

[raffles]

Hi PROS, like Dogtooth said water should just skim cav plate. I lifted my motor 2 holes and completely different boat, had no cavitation, fast hole shot and lighter steering. Both Mercury and Evinrude highly recommend doing this. Mercury has a good vid about it. Raffles.

[PROS]

I though boat trims fairly neutral.
There is enough adjustment on the button to lift to bow up and down far enough.

Cav plate is definitely lower than skimming the surface.
I never been able to check with level against the hull as I cant put the outboard all the way down, skeg hits the ground before.
Will try to find a drop off at some carpark to check this and take some photos.

I have a crane, easy to lift the outboard couple of holes and trial/error.
Will let you know of the outcome.

Raffles, besides the holeshot and lighter steering, any difference in ride quality?

Max

[Noelm]

The plate being level with the bottom of the boat is only a starting point, plenty of boats have the motor way above the bottom, all sorts of things come into play with engine height, trial and error is the best option for a one off home "tune" it does make a difference when it's right.

[GBC]

Having no plank you'll probably find the cav plate on the surface isn't going to work like other boats, hence why it is set a bit deeper. Looks like you have mechanical steering so you are going to feel real quick through the wheel what the motor is doing. Problem is, setting it up for least resistance in trim isn't always where you want the trim set in relation to the running angle of the hull so keep that in mind while playing. What h.p. is it?

[Chimo]

What Noelm said and if it were mine I would lift the motor, it looks way too low. As an indication of the subtlety of the adjustment I found that after I set up my last boat to its motor height sweet spot there was cavitation on tight turns in sea water and none at all when doing the same turns in fresh water. The difference was the bouyancy in sea water vs fresh.

Lift it as high as you can then try it. You can drop it anywhere you happen to do your trials just by sticking a block of timber under the skeg and with the trailer off the car and the wheels chocked and just adjust the motor / boat relationship by using your jockey wheel height on the trailer. Easy as and safe.

Much nicer boat to drive with the motor set up correctly and usually even the steering is lighter and easier. Boat dealers invariably have motors set low as its way easier and quicker and they avoid bitching customers complaining about a bit of cavitation on tight turns in the sea which for a fishing boat is no big deal IMHO.

Cheers
Chimo

PS Re Marchys comment, you can fit cupped lower water inlet screens same as are used for racing if you have a water pressure issue. I have done this although I had no lack of water pressure. Also reduced the pressure on thermostat springs by clamping two spring coils together to enhance second stage cooling.

[PROS]

I will lift up 2 holes this weekend and have a test run Monday hopefully.

Mechanical steering, 200hp

[Marchy001]

Just keep an eye on water pressure when you lift it. My last boat would loose water pressure if I trimmed out too far at it's happy engine height. I dropped it a touch just to ensure water pressure was solid.

[Moonlighter]

I agree with Noel and Chimo. The experts say that position of the anti-vent plate relative to the bottom of the hull when you look at it sitting on the trailer is no way to ascertain if the engine is at optimum height for any hull. At best, it is a starting position, from which you test and adjust.

The consensus of the experts is certainly that it can't be relied upon as an accurate method to set correct engine height, so ignore suggestions like that.

You photo, where the anti-vent plate seems to be well underwater, suggests the engine is too low. They can be hard to see, a trick is to put a re sticky dot on each side of the trailing edge of the plate to make it easier to see.

I would lift it and test again. If you end up with the plate just skimming or getting splashed that will be close to perfect. A bit of ventilation in turns can then be addressed with some extra cupping on the prop. But get the engine height right first, it will make very positive differences to how the boat handles overall and how well the engine performs too.

[gofishin]

... and generally a lower bow attitude.

... A low outboard will lift the bow, a higher engine will lower it.

Good to see this being stated (and correctly so) without you guys being shot down - like what was happening a few yrs ago on this topic, re what happens with the bow/boat trim due to a high or low motor.

... I never been able to check with level against the hull as I cant put the outboard all the way down, skeg hits the ground before...

What about if you drop the jockey wheel/hitch all the way down?

It's always a good idea, IMHO, to get 'a guide' of the start point when the A/V plate is parallel to the hull. Measure up/down and advise.

Yours is obviously a tin boat, with full transom (no pod). What sort of external keel does it have? I-beam, angle section (at 45/45), or half-pipe?

A single donk tin boat with keel will never be able to run as high a donk as a glass boat.

As a start guide:
Tin boat - A/V plate half way up the keel extrusion and go up from there, one hole at a time. My last tin boat (6m pressed 5mm bottom with external keel), I was able to go up to 25-30mm above the hull (can't remember exactly) but used a Permatrim (to counteract ventilation from keel), keel-cut, and a good SS prop.

Then it's down to water tests and visual inspection of where it runs in the water, as suggested by those above.

Even with a good alloy prop, you will not be able to run as high a donk as with a good SS prop. As Moonlighter has suggested, extra cupping can help - but prop needs to be a tad small in pitch to start with.

If your prop is bent/damaged/dinged etc, you will also suffer more ventilation (and even cavitation) than without prop damage, and a damaged prop will give problems with a higher donk where as with a low/buried donk it may not show up.

... besides the holeshot and lighter steering, any difference in ride quality?...

Mostly always better, except where bad design/build/damage/fit-out to hull causes a boat to tend to plough/bury the nose when running down hill. Then it is better to have the donk lower than where it should be - or better still, sell the boat .
Cheers
Brendon