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Old 24 March 2008, 11:00   #1
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Country: UK - Channel Islands
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Boat name: Martini II
Make: Arctic 28/FC470
Length: 8m +
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Transom height/shaft length on twins

Can anyone advise me on how to work out the required shaft length on a deep vee hull when using twin outboards?

On a single engine set up its easy, measure from top of transom to bottom of keel and that's your shaft length. But how is the vent plate supposed to sit in relation to the hull when the hull is at 21 degrees from horizontal?

Do you put the vent plate level with the bottom of the hull on the centreline of the outboard, ie. half the vent plate is hiding behind the transom and the other half is in the water flow?

Obviously every boat is different and minor tweaks need to be made to get it just right but where is the start point?

I bought a pair of 225hp Optimax's for my Arctic 28 before I had the boat delivered. I went for 25" (XL) thinking they were too long, the plan being to mount them on jacks to lift them the extra 5". The reasons behind this were:
a) keeps the engines from getting swamped so much
b) allows me to close in the transom cut out so I don't get water spilling over into the boat so much.
Trouble is it's just dawned on me that this won't work! With the motors set back on the jacks by 5.5" and the transom closed in, the hydraulic steering cylinder etc. will foul the transom when I try and tilt! At least I guess it will, I haven't got the motors yet.

So I'm now looking at fitting the motors straight onto the transom and I've been trying to work out how much I may need to raise the transom height. Now that I've taken a tape measure to the boat, I'm now wondering if I have actually bought the correct shaft length and can just bolt them straight on!

If this is the case then I won't be able to build up the transom height to keep the water out but looking at the tide mark on the back, it looks like I have around 8" above the waterline which isn't too bad I spose.

Is this post getting too long?? Hello.....? Anyone one there....? Zzzzzz...

Ps. scanned a sketch I made of the transom, not sure how it will look when it's resized. And no, it's not to scale!
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Old 24 March 2008, 11:17   #2
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Country: Canada
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I would fabricate an outboard extension out of aluminum. Not too many people seem to them in the UK.... They are used in Canada quite a bit. It would give you the chance to build up the transom height and it would give you about another 2' of length to the boat. It would enable you to tip the motors with no problems and get the perfect height. It also gives you more usable deck space. With a fiberglass boat it would need to bolted to the transom as well as using 5200 or sikaflex, I find 5200 much better. If you are interested I can take some pictures of both my ribs which are both done this way. One after market and one at the factory... Just a long thought.....
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Old 24 March 2008, 11:19   #3
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Why not give Barrus a ring later this week and ask what size engines they originally supplied to Halmatic for these boats. My money is on standard shaft engines though.

Pete
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Old 24 March 2008, 11:32   #4
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Country: UK - Channel Islands
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Quote:
Originally Posted by Pete7 View Post
Why not give Barrus a ring later this week and ask what size engines they originally supplied to Halmatic for these boats. My money is on standard shaft engines though.
That certainly is a good idea, I suppose they might be more forthcoming with info than halmatic themselves!

If by standard shaft you mean long shaft (20"), I'm not so sure. If you look at my drawing you'll see I have 26.5" on the centreline of the outboard. This would be just about right for an XL if my theory is correct.
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Old 24 March 2008, 11:52   #5
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You could be lucky, also worth asking them what the minimum distance apart you can rig the engines and what "toe in or out" they recommend because you don't want to mount them parallel.

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Old 24 March 2008, 12:03   #6
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Country: UK - Channel Islands
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Informative article herehttp://continuouswave.com regarding engine spacing and toe in/out for anyone interested.

Hopefully Turbodiesel will come along soon and enlighten me further
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Old 24 March 2008, 12:05   #7
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Quote:
Originally Posted by Pete7 View Post
Why not give Barrus a ring later this week and ask what size engines they originally supplied to Halmatic for these boats. My money is on standard shaft engines though.

Pete
And my money would be on you drawing a blank on that one...... you'd get more sense out of Manos.

if your sketch is correct, and you were mounting on those centres, then 25'' (xl) mids would be fine, giving you cav plates 1.5'' above the running surface. After all, it's not as if your putting together a performance boat!
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Old 24 March 2008, 12:06   #8
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Looking at the sketch, I agree with you, 25" XL outboards would be a good fit. I believe that the vent plate should be in clear water. You can always raise them an inch or ttwo later but cannot easily lower them. BTW if you do not have transom drains it would be good to position and put those in before fitting the engines. Your project seems huge compared with mine.
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Old 24 March 2008, 12:13   #9
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Thanks Limey, that's 2-1 to the 25" then! You say that the plate should be in clear water, but with 25", more than half of the plate will be out of the water when on the plane would it not?
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Old 24 March 2008, 12:30   #10
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Quote:
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Thanks Limey, that's 2-1 to the 25" then! You say that the plate should be in clear water, but with 25", more than half of the plate will be out of the water when on the plane would it not?
I believe that would be Ok. The alternatives are that the transom was made for XXL engines, but that is very unlikey, and 20 inch would be far too short.
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