Yamaha & transom placement/height

[Bassifier]

Calling all Yami / Quinie dealers and owners!!!
Can anyone help?

Yamaha have 4 holes on each side of the motor determining where the cav plate should sit for best overall performance correct?.

What I want to ensure is that when my motor gets fitted, that it gets fitted in the best possible position.
For instance, a friend has a 445 hornet fitted with a 60 yam 4 stroke and the engine is bolted in the second hole down position, problem is he is not getting the kind of revs he should be out of this motor so maybe the engine needs to be dropped a hole.
Of course it would have been better for the fitter to have ensured it was correctly located in the firts instance, but maybe they didnt.....

I am purchasing a 475 Hornet Trophy rigged with a 60 High Thrust Yamaha 4 stroke.
Surely there are owners out there that can give us some info on what hole level the bolts are in and what the rev range at wide open throttle should be, especially for this setup.
Please, pretty please?

Richard.

[Fishaholic]

The yammy 60 4-str is a real sweet motor, you'll love it, very frugal on petrol too.

I don't know about the hole but at WOT you want to be douing 5800-6000 RPM.

Cheers,
Gabriel.

[dantheman]

i have a 4 stroke yammie on a 475 quinnie, i dont get what you mean by which bolt.. mine has 4, if you could give me some more info i will just jump outside and check,,
cheers dan

he's not getting the full rpm he should be, that would tend to indicate one of either two things.

1. His OB is mounted too low and burying the leg too deep, which would increase drag and slow down the WOT rpms, indicating he needed to raise the outboard one hole rather than lower it as you suggested.

or

2. He is spinning a prop with too much pitch.

If he has the wrong pitch prop, a generalisation is that for every inch of pitch he decreases he will gain 150 ~250 rpm in WOT revs.

His OB Should be run in the upper 50% of the manufacturers secified WOT rev range, for "best performance / longevity".

As for where to mount yours...

Theres a general "rule of thumb" that says the ventillation plate should be either level with the straight line extended out from the bottom of the hull, Or anywhere up to 1 inch above that height - but seldom ever below hull height for best performance.

That can vary tho - with how far "back" from the transom your Ob is mounted. You can for example add transom spacers to give your outboard some setback from the hull. This allows the OB to be raised even more than 1 inch as the water leaving the underside of the hull raises up back to sea surface height..as the hull passes.

You can likewise add transom wedges that will give you a little setback but increase your trim angle for less porpoising..

You can add a manual screw thread adjusted transom jackplate - allowing you to "wind the OB up or down at will" with a socket and handle, until you find the ideal height, regardles of where the OB is mounted on the holes.... These also set your OB back from the transom a few inches. Often these sell new on places like Ebay for as little as US$150 (plus freight).

Then of course you can get the electric or hydraulic power lift jackplates from CMC or Bobs Machine shop, or the Panther plate thr SOLAS Props...etc etc which will raise and lower that 60 horse baby at the touch of a button..

The higher you raise it - the more RPM's you should get - BUT - you risk both "cavitating" (which is really ventilating) in turns, and rough conditions AND if you raise the OB too high, you risk getting the water pickup above waterline and overheating/cooking your new OB engine.

Any OB on a jackplate really needs a water pressure gauge, a water temp gauge and a modified lower unit , to take the water pickup down to the bottom of the skeg rather than in the middle of the leg above the centreline of the prop. These mods are available thru Bobs machine Shop..with the low water pickup nosecone adaptor kit.

Boats that need to run fast over flats are the ones that benefit most from this shallow draft jackplate / low waterpickup technology. Of course a large Hornet fits that category!

Power lift Jackplates cost anywhere from $2K - 4K Australian depending on what brand and where how you buy it.

I see you've gone with the high thrust model 60 Yam?

This is very much like Mercs bigfoot option in that you have a different (lower) reduction ratio gear in your lower unit...which in effect mean your top end WOT speed might be reduced somewhat, unless you increase the pitch on the prop to counter for it.

The high thrust & bigfoot options were for lil OB's driving barges, houseboats and the like at displacement speeds for long duration - where a higher gear ratio and a lot of prop slip would eventuate with a standard OB.

The gear reduction of the "high thrust" (read reduced top end) means that larger dia porps can be swung, for less prop slip and therefore less strain on the gears..in the lower unit.

In a light weight boat like a Hornet - you can probably drop prop diameter and increase pitch a fair bit to compensate for the gear reduction in the (high thrust) lower unit.

You'd probably swing a 14 or 16 inch pitch no problem

There are some neat prop slip - and WOT speed calc programs available online to work out the most appropriate pitch for your boat...might have a link here somewhere will have to look round for it tho.

Why do we buy "high thrust and bigfoot Outboards for light alloy bream boats? Why do dealers like to sell them to us when they are designed for barges and houseboats?.

Because they are an extra option that "costs more" and they SOUND like they will make your boat go faster...but they don't they actually make it go slower. The higher the $ value of the boat sale the more profit for the dealer. Less chance of anything in the gearbox blowing up because it's built tougher and into the bargain less chance of the new boater killing himself or any other water user if he's speed restricted a little bit!

All good reasons for selling high thrust and bigfoot options to new boat buyers...

All you need to know - is that your prop options in as far as a taller pitch are widely increased..and it weouldnt hurt to experiement with the numbers and see what possible for WOT top speed with the right combinations.

Normally - your manual tells you what height to rig the OB intitially.

I think you'll find some dealers bury the OB on lower unti below transom height on Hornets for a reason. That reason is the famous tunnel hull "air cushioned ride".

That compressed cusion of air gets passed out the back of the boat either side of the ONB where the tunnels finnish at the transom..

The water there is highly aerated...

Props don't like to grip in aerated water, it breaks their grip on the water and they can cavitate (ventilate) badly "in tight turns" as the aerated water passes on an acute angle through the water in the midle of the boat where the props situated.

So dealers like to bury the OB's an inch or so lower than transom to keep the prop in clean (non aerated) water so they don't break free and over rev, and lose traction in things like a following sea.

It does bog them down a bit having the gearcase buried so deep - but thats one of the tradeoffs for the air cushioned ride of the hornet.

The "ultimate" would be to have OB Height finger tip controlled from the helm with a power lift jackplate....but how much are you prepared to spend on the jackplate, height indicator gauge, water pressure gauge, water temp gauge, low water (skeg) pickup conversion etc?

Other than that it's a case of trial and error. Start where the dealer rigs it and raise or lower it a hole at a time until you get what you believe is the happy medium.

Theres no magic one hole suits all solution some folks ditribute the weight in their boat differently to others. Different strokes (OB mounting holes) for different folks.

Cheers!

[Bassifier]

Cheers for that trouty, very informative.

The main reason i went with the High thrust is that my experience with hornets in the past has been that they take a bit to get out of the hole.
Now the only option 4 stroke wise with a Hornet 475 is 60hp.
The transom weights do not allow for ant more HP unfortunatly

I usually carry a lot of weight in a boat and since this rig has twin big live wells etc, i went with the High thrust.
Now If I have an issue with the top end speed at some point I intend to do exactly what you have suggested and change the prop.
I previously did this on a 50hp high thrust to good effect when the flooring in the old horny got reduced.

Once again thanks for the info.
Richard.

the boat manufacturers put Max HP ratings on their hulls - and it's not ALL public liability issues in case of a prang or flipping etc.

Theres even an 'Australian Standard' which they have to abide bye.

I think it's Australian Standard 1799.5- 1990 Small pleasure Boats code. Image link below.



You'll notice that the Max power is the same as for my 16 ft (4.85M) punt at 60 Hp.

Makes you wonder how that 16 ft barra punt in the Nthn Territory featured in one of the Club Marine magazines a while back managed to get away with 200 HP doesn't it?

I wonder if Club marine would use that Australian Standard to avoid a payout on that vessel in the case of an accident or injuries claim?

I heard a rumour it flipped doing 70 mph around a bend in the east alligator river, but the passengers hit the water so fast they skipped across the surface and well up the bank into the pandannus palms before the crocks could eat em...

I was told the hulls still wrapped around a riverside tree trunk 15 ft above the waterline!

I have a photo kicking around here somewhere of the 200 HP equipped 16 ft "sum punt" and she's some rocketsled, built bye a feller in humptydoo from memory!

Might see if i can find it - would make any breamers boating blood run hot!

Here you go a couple of picks of the 200 Hp Sumpunt..



and on the plane!



Cheers!