Leg Height Myths,-

[Locozodiac]

Having reached long time ago the sweet height spot and in constant pursuit of improving overall boat/engine performance tested same engine with standard factory delivered prop on light weight inflatables but sitting leg at 2 diametrally opposed transom heights.

Being reading about boaters advising to raise engine height so prop spins higher to water flow to achieve better top speeds. Not knowing exactly what to expect and counting with ideal close environments with flat, no wind water conditions went for the on-site water test.

Was pretty sure that running a prop with anticav plate riding close to water flow would increase slightly top speed on flat straight courses, but not necessarily better overall performance. A lightly loaded 420 rib, with just driver, a 18 HP 2 strokes engine and 2 gallons of fuel was used on both test trials.

Test 1: 1 mile run at wot, water flow passing slightly under small upper plate : 41.3 Km/h at 5,780 rpm

Test 2 : 1 mile run at wot, water flow passing slightly over anticav plate : 39.5 Km/h at 5580 rpm.

According to both test runs, the prop that sat at ideal transom height which is the sweet height spot for Tohatsu engines achieved best prop grip, top speed, + 200 rpm gain, faster hole shot and superb close turns than on same exact test 2.

The myth about achieving better engine performance with standard props and raised lower leg proved to be not true, at least with Tohatsu OB’s.

Happy Boating

[9D280]

Loco,

if I'm reading this right you have found that on a flat water straight run with the same prop the engine deeper in the water gives more speed & (logically) more engine RPM?

Can I just check that I am understanding this correctly - In your pic it's photographed as in test 1 - i.e the top "spray plate" at water level-ish and the "main" cav plate fully submerged. Test 2 you raised the engine so the "main" cav plate is now level with the dotted line you have called "test 1"?


If that is the case then I am gobsmacked, as I would have expected less drag with less of the engine being dragged through the water.



Do you have time and other props to see if that is repeatable both with other props and at different trim angles? (I assume you kept trim angle the same in this test)

[jyasaki]

Kind of goes against all the other tests that others have done. Not saying you're wrong, Loco, but I suspect that more testing may be of value here. Performance in other-than-straight-line runs would be of interest as well. Did you get ventilation running the motor higher? Blow outs in turns? More spray with the motor deeper (not that it will affect much)? Affect on fuel consumption (would probably need a much longer test period)?

Thanks for the report, though; gives everyone something to think about.

jky

[9D280]

I would agree that "something other than straight line" would be useful long term, but a straight line in flat water is a very repeatable test!

[simsy]

If I trim up my outboard my boat becomes dangerous to steer, (chim walk??) but the speed goes up 5 knots or so...... I do not trim all the way up for max speed unless I am on my own and in a mill pond...

Maybe when the motor is raised (not trimmed) and stays parallel with the water performance can be achieved and still be stable... triming up runs the motor at a step angle obviously.....

[Landlockedpirate]

Originally Posted by simsy

If I trim up my outboard my boat becomes dangerous to steer, (chim walk??) but the speed goes up 5 knots or so...... I do not trim all the way up for max speed unless I am on my own and in a mill pond...

.

I have been looking at a setup similar to yours, can you tell me a bit more ? Is it actually chine walking ?

[SimonStDavids]

Hi Loco, I have a different setup to you, a Mariner 40hp and SR4, and i have been moving my engine around to get the best out it but have had exactly the opposite result to you.

I started with anti ventilation plate about 2" below the hull, with the engine at that height, there was considerable spray from the leg and at WOT I could get about 28 knots. At WOT though in certain seas it could be a bit unstable and chine walk a bit!

Over the winter i tried the engine mounted a several heights and now the anti ventilation plate is level with the bottom of the hull. I now get 31/32knots and the boat is more stable at WOT

Maybe hull shape plays a role in optimum engine height?

[9D280]

Originally Posted by simsy

If I trim up my outboard my boat becomes dangerous to steer, (chim walk??) but the speed goes up 5 knots or so......

Originally Posted by Landlockedpirate

I have been looking at a setup similar to yours, can you tell me a bit more ? Is it actually chine walking ?

Doubt it. You need a "deep V" hull for that. Speed goes up as the engione is putting trh ethrust in a more efficient direction & has potentially less of it dragging in the water - a significant loss on a light boat.

Is it more like a "twitchy" unstable feeling on the tiller wehen you try to steer? I think I can get the same effect by swapping the std shaft on me dinghy to my longhshaft Aux...

Originally Posted by SimonStDavids

Hi Loco, I have a different setup to you, a Mariner 40hp and SR4,

<snip>

Maybe hull shape plays a role in optimum engine height?

It does, and as you have a deep V on the SR, yours will actually chine walk if the right conditions are present.

[simsy]

It looses steering and if you go over a wave it starts to oscillate left / right left/right left/right to the point that it feels as if it will throw you in the water.... It does none of this at all unless the motor is trimmed right up, say 15- 20 degrees and is at full power......

(it's helm steered)

My motor is raised above the transom to the point where the motor ventilation plate is around 25mm below the keel line. Not high by any standards.
(Raised as Tohatsu tend to be longer than most in leg).

10 dia x 12" pitch s/steel solas prop.....

[lightning]

I've got a little 9.8hp on my SIB and the only reason I raised the engine on the transom was to try and reduce splashback, and it worked.
The actual performance of the boat seemed the same.

[Locozodiac]

Some clarifications:

Test 1 is with small upper def plate as to water flow passes slight under which is the sweet height spot for Tohatsu, Merc, Mariner engines as many boaters having splash over transom issues have solved forever.

Test 2 is with raised engine as to water flow passes slightly over anticav plate.

Both water flows are being cut nicely through the edge angle. The speed difference is about 1Knot in straight flat water courses for both tests. Trin angle is same, being a mechanical trim engine.

The rib used for test is a shallow deep V light hull if would have used a 30 HP for same test would achieved more top knots, but with slight speed difference between both settings as both 18 & 30 have same lower leg shape.

On test 2 find that water flow doesn't pass even through plate as on test 1, on left side side has slight more water and bit of air than on right side, that's the spot where it's probably loosing grip and achieving less top speeds.

This test probably doesn't work on other brand engines, a matter to check under trial and error. Usually stay with factory delivered prop as find them delivering near it's max rpm at wot while solo speeding and rpm drop decrease accordingly while adding more mates or weight on rib.

Happy Boating