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  #11  
Old 05-10-2011
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Thanks all for your feed back,
tried to find a waller forum but nothing although there are a few of them just try and find them.
havent seen a nissan down under
weight 1075kg displacement 770Kg dry.
Tohatsu ratio was 2.08:1.
Like the throttle theory.
why is an extra 15 kg over the old 6hp 2stroke too much? is it Drag?
Mr Waller said He had a 10hp in mind when he designed it?????But could not tie him down on specifics havent got a design hull speed either
Where is 4 hp going to leave me in a seaway?
Sorry if my relies are short but Ive had my detailed replies scrapped 3x
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  #12  
Old 05-10-2011
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Quote:
Originally Posted by Schlacky View Post
Thanks all for your feed back,
why is an extra 15 kg over the old 6hp 2stroke too much? is it Drag?
Mr Waller said He had a 10hp in mind when he designed it?????But could not tie him down on specifics havent got a design hull speed either
Where is 4 hp going to leave me in a seaway?
If you're saying that the boat had an old 6hp 2-stroke and the new 4-stroke is only 15kg heavier, you're right, it won't be a big deal. And it's good to hear Waller designed the mount to take up to 10hp.

I don't think hull speed will be different than the normal calculation (assuming a ~21ft LWL, looks like she's drawn so LWL is close to LOA), which gives you ~6.5kt.

4hp will probably get you to that just fine. Note that Waller indicate (Mike Waller Yacht Design - Waller TC 670 Trailer Catamaran - Plywood) this boat is not designed for rough stuff. That said, more power may get you more speed as you move out of displacement mode - this is where talking with current owners would help. But maybe not a fun way to travel in chop (memories of taking a Boston Whaler into the 6-8ft Atlantic chop off Bar Harbor, Maine ... fun but not for the faint-hearted)
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Old 05-10-2011
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Talking

To expand,
Firstly Ric thanks for your warm welcome and encouragement.
I am Planning to use the vessel around Hervey bay Moreton Bay etc. both funnel water = strong tidal flows.
found the 6hp yammy2stroke lacking when first time out (30 to 40 Km gusts and nice chop in local dam) I am not planning on racing so unless there is something safety wise that I dont know about I am not so worried about the extra 15 kg on the stern (please educate me)
so back to question of gear ratios and why does everyone think 9.9 is too big what is the rationale ? is 15 kg more- really that much more over the previous 27kg motor
In my state the vessel is rated to carry 7people for its length not that I would ever attempt it but even with 3 adults and 2 kids it is a nice stable platform.
I Need a motor strong enough to get me out of trouble and would rather run at low revs than try and punnish one at redline as I drift into the sea wall
I noticed no one has come back with any catamaran hull dynamics in consideration Like why I should not Ballast to the font with a heavy heavy9.9
at the back
so would the 2.08 ratio cavitate madly or would it just throttle the engine
would the 2.42 ratio rev the motors head off or just throw rooster tails while planing on the centre board?
Tug boat eh Toot Toot
Must sleep now
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  #14  
Old 05-10-2011
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Thanks zed (can I Call you that)
missed your reply while typing.
I realise it is built for inshore conditions and Im not intending on doing anything stupid But it is built really quite strong to cope with trailering
My uncertainties arise from sheer lack of experience and the fact that the purchase price of motors here are probably double to what I have seen advertised over the duckpond (online not reading fine print) so the motor is quite an investment so I dont want to get it wrong I am all theory and never mucked around with different props but mechanically I know there is always a trade off
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  #15  
Old 05-10-2011
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Ps. zed when I asked Mr W. about the hull speed He indicated that cats are quite different to monos and there are a number of (non specific) factors that influence the calc.
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Old 05-10-2011
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Quote:
Originally Posted by Schlacky View Post
Ps. zed when I asked Mr W. about the hull speed He indicated that cats are quite different to monos and there are a number of (non specific) factors that influence the calc.
Yeah I expect he would say so - because they're so much lighter, they can get out of displacement mode, so seeing "hull speed" as an upper end isn't relevant.

That said, 6hp correctly tuned and with the correct prop should have got you moving at a decent clip for a light boat. How fast are the currents you're dealing with? Over here, 3-4kts would be a strong current and your 6hp should get you moving enough faster than that to make decent headway.
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Old 05-10-2011
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Brother, I just went through this myself. If you can find a Nissan/Tohatsu/Merc 6HP with the sail package, you should be set. The shaft is 25 inches. They make a 20 inch too wich works for most just fine. Either one will be great. The real key is to make sure that you have the 8" diameter by 6" pitch prop. I tried to run the 6HP on my J27 with a 8" pitch prop made for planning boats and it was miserable. I get the J27 to 6.9 knots at full throttle and it sips gas. Your vessel will move out with that motor. It is not as nice as a 2 cylinder motor, but it is definitely better than a 2 Stroke motor in the noise/vibration area. It is also very easy to work on if you have any mechanical skills at all. There is nothing to it and you can reach all of nothing by opening the hood. The prop is the key. In order for the motor to achieve the proper RPM of around 5500, you can not have a prop that tries to "dig" too hard. It also only weighs 60 pounds. You can lift it off the transom easily while underway. Tie a line to it just in case!
Cheers PATSY
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  #18  
Old 05-10-2011
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Schlacky,
You have completely ignored the prop pitch???
I can not understand your thought process.
Do the math. Assume 50% slippage.
Now compare the prop pitch on your favored 9.9
vs a 6" pitch.
You will find that you will hit an rpm brick wall at hull speed+
with the former and the engine rpm will be way below max
speed and torque.
The assumption that you will get thrust proportional to rated hp
while ignoring ratio AND pitch is false.
But it's your $$.

Dick
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  #19  
Old 05-11-2011
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Dear Patsy and Dick thank you for your advice I will go back and re investigate the options don't kow if you got my reply this morning (I just replied on email notifications Im still figuring out how to drive this forum )
Dick I sugest you re read my initial request for help I don't see any assumption in it, nor am I guilty of ignorance. I only see a plea for experienced advice and and a brief explanation of the position I am at based on earlier poor advice and resources
In My defence I put it to you that I have signed up to this forum with the hope that at least one ofthe 100 something thousand members has faced my cunundrum before and might be willing to pass on advice. As it turns out I have already recieved more usefull advice in the last 2 pages than I have been able to gather locally from our so called experts over the last 5 weeks.
Yes my Hair is thinning.
Just for the record The motors I have been talking about will set me back between 4000 $Au and 5500$Au yea it hurts so there fore I am all ears. Not to mention some suppliers have told me to expect 8 week delivery, or... better be quick we have 1 in stock but a bloke was looking at it.
I get a little envious when I jump on line and see A, The variety of Models you have to choose from and B, the advertised prices that you have over the duck pond.
Neither am I a naval architect and the closest book to calculate the math on the issue our local store has is titled something along the lines of
"How to ride a Bull and not get Bucked"and "Better Yield from your Combine"
so please be nice.
Kind regards
Alex.
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  #20  
Old 05-11-2011
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Quote:
Originally Posted by Schlacky View Post
Just for the record The motors I have been talking about will set me back between 4000 $Au and 5500$Au yea it hurts so there fore I am all ears.
So given that, the thing to focus on is: how strong are the currents you're facing, and how fast can you get moving with your current setup?

What's the diameter and pitch on your current prop?

An old 2-stroke Johnson 7.5 gets my friend's Tanzer 26 moving nicely even through chop, and that's a boat displacing about 4X what you do.
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