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Old 09 August 2004, 06:23   #1
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Country: UK - England
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Engine size/position

Can anyone advise me of the transom space required to fit two yamaha 25hp 2 stroke outboards?

I am looking to fit two 25hp's instead of a 50hp main & 4hp spare for added reliability/safety on a Narwhal 450. Looking at manufacturers specifications the 2x25hp combination will be lighter but burn slightly more fuel but I can find no info on the spacing required, just the physical dimensions of the engine.

Are there any obvious reasons why I shouldnt fit 2x25hp?

Thanks in advance!
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Old 09 August 2004, 06:45   #2
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Originally Posted by Craig H
Are there any obvious reasons why I shouldnt fit 2x25hp?

Thanks in advance!
It will be slower because of the drag of 2 gearboxes - if you uprate the total horsepower by 25 - 33% you should have the same performance figures - which of course will mean higher fuel consumption etc.

And if that doesn't put you off there is the higher cost of a twin rig double the servicing bill - shall I stop now?
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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 09 August 2004, 08:46   #3
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There is a plus; 2 or more props give a good hold on the water so acceleration is good and so is handling in waves.

A long time ago I ran 2 x 30's and then 3 x 30 for a while. Top speed was limited but it took off at least equal to my 200hp. I'll try and find a pic later to give you an idea of spacing.
----------
There you go. It worked well and I liked it but the need for speed overcame me.
You can probably work out the spacing, the props are 11"dia.
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Old 09 August 2004, 13:10   #4
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Thanks cookie, that pic helps a lot!

But Jwalker where do you come with the 25-33% figures from, there is obviously some extra drag but the gearbox on a 25 is smaller than a 50, I'm suprised there would be that much difference.

The boats recomended power is 40hp (60hp max) so being up by 50 should still give reasonable performance. As for servicing costs, I intend to service them myself so they wont be a problem.
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Old 09 August 2004, 17:05   #5
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Craig, you've got cookee and me crossed over. However, I do agree with him.

If you're a DIYer, go for it. With the servicing cost removed, the equation is different.
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Old 10 August 2004, 03:12   #6
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Percentage losses are industry accepted as correct - no idea who did the sums but it works out!

With counter rotating gearboxes the handling of any boat is greatly improved and low speed manouvrability is fantastic as well so it isn't all bad!

Good luck with whatever you do!
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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 11 August 2004, 11:23   #7
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Thanks guys, sorry 4 mixing u up!

Am still not really sure what to do, will need to do some serious thinking!
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Old 11 August 2004, 11:31   #8
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Did you happen to work for crompton at one time jw?

Quote:
Originally Posted by jwalker
There is a plus; 2 or more props give a good hold on the water so acceleration is good and so is handling in waves.

A long time ago I ran 2 x 30's and then 3 x 30 for a while. Top speed was limited but it took off at least equal to my 200hp. I'll try and find a pic later to give you an idea of spacing.
----------
There you go. It worked well and I liked it but the need for speed overcame me.
You can probably work out the spacing, the props are 11"dia.
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Old 11 August 2004, 16:04   #9
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Originally Posted by MadMat
Did you happen to work for crompton at one time jw?
I might be richer if I had....or in the jail
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Old 11 August 2004, 16:13   #10
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Was there an article the other day saying he (Davison) might get away with it on a technicality?
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