A plastic boat.

With a similar boat.. although I think we have established slightly skinner... my Mitchell has recorded 17.7knots WOT, but that is immediately after launch from fresh anti foul, polished prop and one up, empty water tank & no cruising gear.. (tender/outboards/general shite)

Engine for others info a Perkins sabre MT225... producing maybe 200-210hp.

With a seasons weed now on Tulana my trip Friday over to Yarmouth at 2050rpm produced 10knots... if I had opened the throttle I reckon I would have got 13.5knots and got to at 2400rpm..

So all I would say is how the heck 120hp would ever get the dawg to cruise at 13knots is a mystery to me..


And I’m a little more attached to the idea of the dawg with kitchen rudder than without.. I still think it’s a cool thing.
 
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Scarron said:
View attachment 80798
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ok, admit I'm confused!

fig 2 and fig4 show thing open (guess minimal drag) and boat going ahead, right?
fig 5 implies that there's no astern in prop rotation but you block and return wash back??? similar to my jet tender... must be a dog in reverse!
fig 6 is pointless (to me!) what's it trying to do? reduce thrust for some odd reason which should be achieved by reducing rpm on the engine? or is it troll mode or something?

BB, you're absolutely sure that when moving ahead contraption is fully open as fig4 and not slightly closed as in fig6?

cheers

V.
 
The same thought as vas's last line had occurred to me. would then be like dragging a parachute behind. Is there any way you can unbolt the appendages (temporarily) whilst in the water to try the theory. Scrubbing grid or scuba?
PS, Still hot & humid out here in TW
 
TheOrs said:
One thing that would be relatively easy to rule out and quite important to your calculations is the RPM figure. Can you check it somehow?

Then maybe do something temporarily with the rudder?
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Yes, a cheapo techo would be easy. That's one easy job to check. By ear, I would say its right. I can also use it to double check that the reduction box is 2:1 which I am fairly certain of.

Firefly625 said:
With a similar boat.. although I think we have established slightly skinner... my Mitchell has recorded 17.7knots WOT, but that is immediately after launch from fresh anti foul, polished prop and one up, empty water tank & no cruising gear.. (tender/outboards/general shite)

Engine for others info a Perkins sabre MT225... producing maybe 200-210hp.

With a seasons weed now on Tulana my trip Friday over to Yarmouth at 2050rpm produced 10knots... if I had opened the throttle I reckon I would have got 13.5knots and got to at 2400rpm..

So all I would say is how the heck 120hp would ever get the dawg to cruise at 13knots is a mystery to me..


And I’m a little more attached to the idea of the dawg with kitchen rudder than without.. I still think it’s a cool thing.
Click to expand...

I would be happy with that set of numbers at end of season.

What would your fuel burn be at 14 knots? If it was 6 gallons/hr then you pulling 120hp?

I agree its a cool thing, but if its costing me as many knots as I think it might, its heading to a skip.

Scarron said:
Well, having had the pleasure of being aboard 'The Dawg' last weekend at the Yarmouth speed trials [we weren't entered, we were the finish timing mark! but it took an age to get back from the far end of the course against the spring flood] I can confirm that she is dog slow.

In my own admittedly biased opinion a 4 cylinder diesel is the work of the devil so a 6BT it has to be, with tons of engine box sound proofing and a big bore well silenced exhaust.

I agree that it must be several tons of drag from the enormous kitchen rudder, have you a picture of the actual appendage you can post? for those not familiar with the principle [as I wasn't] this is the thing and it's like towing a blinking parachute.

View attachment 80798

Best thing for that thing is to take a gas axe to it and drop in a nice new blade job with a pair of thrusters.

Then your 220 hp Cummins should get you into the mid teens and the ability to punch the tide through Hurst narrows without a care in the world.
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Yes, I agree with that. Except, I think the higher revs of the 4BT mean I can keep the current gearbox. Shame the box with that 4BT is not the right ratio, although I'm not sure if its cost effectively changeable. That needs a bit of thought.

Firefly625 said:
https://youtu.be/L-tpFo9C4zE
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Thank you. Yes, mine exactually the same.

vas said:
BB, you're absolutely sure that when moving ahead contraption is fully open as fig4 and not slightly closed as in fig6?
Click to expand...


50/50!!
 
Not wishing to butt in on the thread, i simply find it interesting, it may be worth having a google at the Lochin 33 speed an keels on the web, similar type threads to this, goes into detail about props, engines and keels etc, the cutaway keel on the Lochin, as opposed to full keel band (for taking the ground) gives a difference of 2-5 knots using the same engine, The Kitchener rudder must slow the boat, i would have thought, if you are bored it may be worth having a google, it may give you some ideas, i am well aware the OP knows exactly what he is doing, i just thought i would mention it, getting my coat now:)
 
Davy_S said:
Not wishing to butt in on the thread, i simply find it interesting, it may be worth having a google at the Lochin 33 speed an keels on the web, similar type threads to this, goes into detail about props, engines and keels etc, the cutaway keel on the Lochin, as opposed to full keel band (for taking the ground) gives a difference of 2-5 knots using the same engine, The Kitchener rudder must slow the boat, i would have thought, if you are bored it may be worth having a google, it may give you some ideas, i am well aware the OP knows exactly what he is doing, i just thought i would mention it, getting my coat now:)
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Welcome input thank you, I will waste time at work googling Lochin keels.

I see you are in limbo, in which case I can help you out with a beautiful Huntsman 31! I broke the first rule of Fairey boats and bought my next boat before selling the previous one. Erk!
 
Don’t know what my fuel burn would be at 14knots.. I’ve never cruised at that.. at a guess 4.5 gph

I think if you junked your kitchen rudder it may just feel like you’ve let the handbrake off... but would be sad to see it go!
 
As was said, simple to calibrate the tacho against a cheap non contact one. My Levi Corsair has two na Ford's on shafts, at 1200rpm does about 6.5-7kn at roughly 6l/h. Flat out won't quite make 17kn. 2300rpm gives 15-16kn and roughly 40l/h. One engine wasn't right when I got her, overhauling the fuel pump made a big difference. I have messed about with the stops to make sure both will reach max revs in neutral and the fuelling to get max revs loaded without smoking.
 
An ignition timing strobe (that of course would be totally redundant on a diesel) should be able to confirm the max rpm at WOT. I don't know if there is an equivilent when it comes to marine diesels, but with avgas burning aircraft engines, the guide of 5usg per hour per 100hp is a pretty reliable indication of output? My guess is that under way, your rudder arrangement is essentially acting as a large sea anchor. I reckon glassing in a bow thruster and going back to a rudder blade will be your way forward... Looks like fun though.
 
ebay is full of Digital Photo Laser Non-Contact Tachometer RPM Motor, just bought one to confirm the accuracy of the VDOs on my engines.
around 15quid and a month to arrive from China :D

BB, the stock turning these bits of metal, aren't they visible inside the lazarette/engine bay? Can't you jam them open on some open water and do a test run?

V.
 
I found a pic I forgot about, it shows the rudder.

mid-599616-2.jpg
 
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