o/b postioning on transom

J

johnstop

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my o/b is postioned with the cavation plate 2" below the hull.I am told it should be level with the hull.
I think this pushes the stern down more than it should do.
Any thoughts.

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It depends on speed. If slow boat, doesnt matter. If a fast boat, say 25kts+, the cav plate should be level or about 1 inch higher of the hull surface. The consequences of being too low will be to push the stern up or down depending on where the power trim is set, but the more significant consequence is extra drag. I mean, a lot of drag, like in the order of 1-2kts worth.

It should ideal be set about 1.25inch to starboard of the centre line, if R/H prop

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If only it was that simple. There are NO hard and fast rules for O/B installation.

In theory, if the motor was installed by the boat manufacturer, then 9 times out of 10 it will be in the correct position, this would have been established by testing.

If the boat and motor is old tech, then you can normally increase the height by a few inches due to the advances in prop technology. The main thing to watch for is loss of water pressure.

As for offset, Some do & some don't. 1.25" may be correct, but nowadays a lot of boats that I work on have the motor in the centre. It used to be popular in high perf applications to offset an inch or so to counteract torque, doesn't seem so popular now.

Out of interest, I've just replaced the steering on a sportsboat that has the cav plate 6" above the keel line, and it's often run higher than that!!!

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low speed it makes no odds, but as Questor demonstrated in the recent Alum Bay World Tender Racing Championship, even on 9 foot tenders it matters. It would seem that cav plate level with, or at least within an inch either way of the bottom of the transom is where to start... Adjust the running angle with the trim, or consider Doelfin hydrofoils, if the stern really sits low.

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Wasnt that Jasper Carrot character called Wiggo - you know the one with the tank top.

<hr width=100% size=1>Adrian
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This is what the manufacturers recommend, I 'll think you'll find.

However, on race day, we reckoned that getting the cav plate as close to (or even out of) the water surface as poss was best, as less drag.

This provided better straight line speed, but hairier cornering, which wasn't an issue at that time. It was suggested that the cav plate is only there for enhanced cornering capabilities.

Now, who can I take on next?

R

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Agree, no fixed formula and all rigs differ. Twas a reply appropriate to the question - little info given on the rig so middle of road, non bespoke, answer supplied!

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