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Old 23 April 2015, 15:03   #11
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They reckon 3 degree trim is the best, they being the guys on boatracingfacts. Ideally your trim would be when the centre line of the prop is parallel to the water surface, as this is when you're prop is delivering it's maximum forward thrust. Unless you have a highly optimized hull which developes exactly the right amount of lift at a certain speed you'll need to trim your engine to compensate for the lack of/too much hull lift.

With thundercats, I find that at speeds upto 50 knots its quite stable with about neutral trim. Anywhere over this and you start having to tuck the engine in as the amount of air flowing between the hulls generates too much lift. This is why the power required to achieve higher speeds is exponential, you're basically fighting the lift generated by the hull by using the engine trim to push the bow back down.

Without re-designing your hull to suit your setup specifically, the best you can do is trial and error . If you're chasing numbers props are the way to go, get yourself a tacho and a GPS
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Old 23 April 2015, 21:53   #12
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Interesting. My inclinometer is too sensitive for anything but a flat calm. Very difficult to get an accurate reading which I guess would only be an average over time since hulls moves around alot.

Some sort of dampening I guess or an artificial horizon from an aircraft.


Another problem is where on the hull to is the horizontal planing surface? (to zero the inclinometer)

I put my 24kg battery in the forward anchor locker and this helps alot. Might consider 2 fuel tanks say of 40-50 liters of fuel each. One as far forward as possible and one after. Then a transfer pump.

Cant understand why the prop would run at anything other than horizontal but how to measure this? short perhaps of using a artificial horizon (gyro)

My setup is a 2008 FT60 Yamaha on a 2008 Gemini 550 Waverunner. Best fuel economy appears to be at 3-4 knots, 800 rpm = 180ml/km. This is using a 25 inch pitch prop and 600 kg total displacement. I also use motor speed controller (Pulse width modulator) on the high pressure fuel pump and turn the pressure down to 18psi lower than this and she starts to misfire. I am looking for an air /fuel mix ratio meter to install into #26 plug on the exhaust cover. Not sure which is the best one.

Why is the water temp soooo cold at 67 C. ?? (Checked the sensor,OK in spec.) Checked the thermostat fully open at 70.


The best fuel economy planing at 18 knots loaded to 600kg and 2900 rpm is 218ml/km. Bottom clean and hullkoted (Mclube). I think this could be better. Would love to get my hands on a Smart 3 cylinder diesel 65hp engine! They weigh in at about 65kg not including heat exchanger or stern drive.
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Old 24 April 2015, 03:10   #13
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Hmm! I think you need to look at fitting a flux capacitor between plug 1 & 2. Adding a little jet1a to the fuel mix could help, about 2pico litres per litre of fuel should just about do it. It sounds as if your half housing bearing on the laffing shaft could do with balancing. As a last resort you could try enriching the Dilithium matrix in the GRP hull to reduce the friction co-efficient. Keep us updated how you get on, I'm off to the delta quadrant for a bowl of Gagh.


.....sh1t happens.......
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Old 24 April 2015, 03:53   #14
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Old 24 April 2015, 04:10   #15
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hahaa!

You should try moving as much weight towards the back of the boat as possible, that will help reduce the amount of hull in the water which will give better performance and economy. This is why trimming up often makes boats faster, although the prop is running at a less effective angle it's getting the hull out of the water. It's all a trade off, just go out and try various settings.
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Old 24 April 2015, 22:33   #16
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She runs faster with less load. Even one person less (40kg,wife) and will do 22 knots easily and this is with a 25 inch pitch prop and wot 2700 rpm. I try to reduce fuel pressure to compensate for the low vacuum (and richer fuel mix) in the intake manifold which is the only reason I think over propping gives you bad fuel economy.

Has anyone tried a small wing below the prop, something the size of the popular dolphin wing on the anti-cavitation plate?
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Old 25 April 2015, 04:45   #17
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On our deep V hulled Four Winns adding wings to the outdrive dropped planing speed from 24mph down to 16mph the difference was massive. Obviously some extra drag at higher speed. But the additional lift it provides at lower speeds made it hugely more practical, also means you can see where you are going lol

Next job I'm moving the 44gal fuel tank forward a foot or so, my bet is it will be better again.
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Old 25 April 2015, 05:17   #18
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Quote:
Originally Posted by Christiananthony View Post
with a 25 inch pitch prop and wot 2700 rpm.
What ever you do, do it quick, that engine hasn't got long
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Old 25 April 2015, 05:35   #19
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Oh! My 2nd boat happens to be a 268 vista. Has the Volvo duoprop. The Volvo petrol engine was damaged because the shipper didn't drain it properly. So bought the VW 2 liter CR TDI mapped to 180hp. Great solid boats. Will be trying a wing on one the stern drive but I think below the propeller since I suspect the cavitation plate mounting is too high
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Old 25 April 2015, 09:22   #20
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Have you actually looked at the Doelfins at WOT. You're trimmed out and there's no transom cavity refill that close to the boat at high speed (40+). They don't cause any drag because they're clear of the water altogether.
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