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tjotter
19-01-2014, 11:37 AM
another Motor Height query ...

I have read Posts about outboard motor height &
had a look at ours & now I am wondering if it is a bit too low.

Cav Plate is 60mm+ below Hull when level/parallel to hull.

On the Plane the Cav Plate is in the turbulent water
& above the solid water.
No photo/video on the Plane & no chance to provide
before she goes to Mechanic [who would do the lift].

Boat is 4.5m Alloy Runabout,
WOT 2POB + gear is 5900rpm,
ie inside 5500-6000 expected.
Motor 2002 4s 50hp, hull rated to 60hp.
Prop Solas S/S,
no foils.

Under the Hull is a Keel, sort of a like an I beam.


Q) Is it reasonable to try lifting a couple of holes ?
[3 holes would make it level with hull]


Thanks.

997829978399784

gofishin
19-01-2014, 12:55 PM
The good thing is that you have a cut-back keel extrusion (the I beam as you call it), so your setup will be able to tolerate a higher motor/prop (than otherwise) as this allows the air-entrained/spoiled water stream to disperse/spread out under the hull a bit before it gets to the transom/prop.

In relation to when you are planing and the motor is in ‘neutral trim’ (ie.~parallel to water, easy steering both ways - depending on how the steering tab is adjusted), does your boat run bow up or down a little?

If you raise it you will lose some ability to trim the bow up when trimming the motor up/out, as the A/V plate will no longer be slicing/carving through the water, and hence you will/may be solely relying on prop thrust to gain ‘additional’ bow lift when/if you need it. But I would be definitely be raising it!

I daren’t say any more :-X :) but I guess this will be another test for ‘the theory’….
Cheers

gofishin
19-01-2014, 01:09 PM
PS. I ask this
... does your boat run bow up or down a little? ... because I also see you have wobble rollers and not skids.

These are the biggest cause of hogs/hooks in the bottom sheets of tinnies. Make sure they are not taking too much load, especially the ones near the transom.
Cheers

Chimo
19-01-2014, 03:08 PM
tjotter

I'm not going to get into a big discussion about all sorts of theories but if it were my boat I would do what I always do with my tinnys and that is to lift it until it cavitates on corners with the motor in the middle of the trim travel and then either trim the boat so there is more weight in the stern or if that is not feasible go down one hole.99789997889978799790 This 12 ft tender tiny is an extreme example. With a 2.3 HP it planes with in excess of 100kgs of weight and runs flat with the weight on the centre thwart and cavitates on corners and as you can see the motor has been lifted a little.

Cheers
Chimo

Horse
19-01-2014, 04:22 PM
I always lift until it ventilates in normal use en drop it one hole. I would go the 3 holes to start with. Raising and lowering a motor is literally a 15min job

If you raise it 3 holes I'm thinking you may need a new prop as well to get the best out of the rig