Quote:
Originally Posted by 6789boat
On the website, it shows graphs, that says it gives it more power, and keeps top speed at a lower RPM.
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OK, think about this for a moment:
(1) If it actually made all boats better all of the time then the engine manufacturers would just build it in to the engine - the cost would be marginal as an OEM designed part. Indeed some suppliers offer a jet leg for this market - but with poorer performance / fuel consumption.
(2) More power out of the same fuel / engine combo is not possible by putting a bottomless bucket around the prop! I've seen it suggested that "ducting" the thrust from the prop could improve performance - but there is very little sideways 'thrust' from an outboard prop at speed.
(3) The only way you could get the same top speed with lower rpm (using the same prop) is if the prop was slipping too much to start with - i.e. using the wrong prop in the first place.
(4) Drag HAS to increase with the added cross sectional area. IIRC drag squares with speed - so the effect of the bucket in the water is 100x as bad at 20 knots as pottering round at 2 knots - which might be a sensible speed (or even too fast) when you have swimmers close by. BUT if you need to get to a casualty quickly, or get them ashore quickly you are going to notice the effect of the guard.
Its been discussed here before but guards make people complacent - training may be more appropriate as a risk control measure.
Now lets look at their test data and see if we can explain why they might "appear" to have got positive results:
(1) A static bolard / pole pull test
This doesn't really represent what most people do with ribs. With a normal prop not moving through the water but spinning at high revs you churn up a lot of air and get 100% slip on the prop. In reality a rib in normal use will get <20% slip, and is moving through constantly 'clean' water. The resulting drag is ignored in their test. If you are using your boat as a very low speed tug to push or pull very large objects this test might be relevant but if you are doing anything where your boat moves it will not be!
(2) Speed test
There is no X-axis on the graph so its meaningless.
The second set of tests - I doubt any of those speed differences are statistically meaningful (i.e. repeat them again and you'd get another set of slightly different results). None show appreciably higher top speed for lower revs as you suggested. It is not clear how the prop was selected, e.g. was it optimal prop for the guard but not ideal for use in the open?
(3) Generally
Noticeable that fuel consumption was not reported.
Noticeable that only sterndrives were tested with their biggest size (16"), and using 230 hp diesel engines... ...not quite the same as a SIB with a 9" version and 20HP petrol outboard...
There is much mention of improved trim control (which I could see is possible) but again that will be of little relevance on a small manually trimmed outboard. These are the type of engines people most often consider fitting prop guards to on here.
Yankee 154
