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Old 17 January 2015, 17:05   #11
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the transom should look like this
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Old 17 January 2015, 17:13   #12
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What's the big taboo with cutting down a transom to fit a short shaft? Is it something to do with the flooding hull?
Many boats this size were made short shaft & engines are often cheaper in short shaft
Quite often the centre of gravity is better with a short shaft
I've got a rib which was cloned from an sr 4 but doesn't have the flooding hull & it's absolutely fine with a 40 yam two stroke
Nothing wrong with short shaft boats & if you have a good short shaft engine there's no reason not to make the boat fit the engine
Unless I'm missing something peculiar to an sr4 of course?
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Old 17 January 2015, 17:25   #13
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The Avons are a bit older in the tooth when it comes to design.

The deck is below the waterline and when moored they can swamp fairly easily. If you cut 5 inches out of the transom this becomes more likely.

As the boat sinks lower into the water, the water gets closer to the bottom crank seal and exhaust of the engine.

If it does it in a harbour tied up, spare a thought of how it would be in a breakdown in borderline weather.

The engine and boat are also more likely to swamp in a following sea or under harsh deceleration.

There is a company in the Solent that burned through a few 250 Hp four stroke Yamaha's using longshaft rather than extra longshaft engines as water was entering the bottom cylinder and damaging the engines.
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Old 17 January 2015, 17:33   #14
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Quote:
Originally Posted by beamishken View Post
What's the big taboo with cutting down a transom to fit a short shaft? Is it something to do with the flooding hull?
Many boats this size were made short shaft & engines are often cheaper in short shaft
Quite often the centre of gravity is better with a short shaft
I've got a rib which was cloned from an sr 4 but doesn't have the flooding hull & it's absolutely fine with a 40 yam two stroke
Nothing wrong with short shaft boats & if you have a good short shaft engine there's no reason not to make the boat fit the engine
Unless I'm missing something peculiar to an sr4 of course?
Quote:
Originally Posted by RIB-Teccie View Post
The Avons are a bit older in the tooth when it comes to design.

The deck is below the waterline and when moored they can swamp fairly easily. If you cut 5 inches out of the transom this becomes more likely.

As the boat sinks lower into the water, the water gets closer to the bottom crank seal and exhaust of the engine.

If it does it in a harbour tied up, spare a thought of how it would be in a breakdown in borderline weather.

The engine and boat are also more likely to swamp in a following sea or under harsh deceleration.

There is a company in the Solent that burned through a few 250 Hp four stroke Yamaha's using longshaft rather than extra longshaft engines as water was entering the bottom cylinder and damaging the engines.
I've noticed on a few occasions just how close the SR4 is to swamping even with a full transom, I'd hate to think what might happen with one that had been cut to make a short shaft fit, the SR4 is a deep V hull to take a long shaft fir a reason so just fit one, there's not that much difference in price even on a new engine!
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Old 17 January 2015, 17:50   #15
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Ah I guess that's the penalty of the flooding hull then. your purely relying on the tubes for floatation
My boat actually self bails when it's empty at rest ie if you leave the boat afloat you can leave the trunk down & rainwater just runs out
Transom height is reduced only where the engine sits & I've never been worried about taking a wave over the back & the engine is well clear of the water.
I suppose if someone wanted to run a short shaft on an sr 4 then blocking the flooding hull would be the answer
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Old 17 January 2015, 17:50   #16
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The 5.4 is the same- I've been known to deliberately swamp mine by going astern and belt the throttle open with trunks down to wash the blood out after a messy string of mackerel. That's with the flooding hull blocked as well!
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Old 17 January 2015, 18:06   #17
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The 5.4 is the same- I've been known to deliberately swamp mine by going astern and belt the throttle open with trunks down to wash the blood out after a messy string of mackerel. That's with the flooding hull blocked as well!
But isn't that going to happen with any boat if you misuse/abuse don't watch what your doing?
We have a 38 ft cruiser & I'm pretty sure if I slammed the throttles shut at 30+kts I'd get wet feet at the helm
I've also been on the deck of a 100m+ DSV & had to dodge waves rolling over the deck
It doesn't mean that the freeboard is too low you just have to adapt to the capabilities of the boat
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Old 18 January 2015, 02:42   #18
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You don't have to misuse the boat. On a mooring some chop in an open harbour will slap off the tube and "jump" over the transom. As the craft sinks lower into the water the deck space will fill up. Fitting a bilge pump or blocking the hull will help. I liked the flooding hull when it came to stability. The thing is that short shaft engines are getting less readily available. If you want to swap or your old one dies then you have to raise the transom and its s ball ache. I've been through all of that with speedboat. I had a 40 yam SS and was planning on a 4.5m sib but a Valiant came along. After a bit of soul searching I sold the 40 and got a 55 at first and then the 50 I have now. I think cutting the transom would look a mess.Click image for larger version

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Old 18 January 2015, 03:02   #19
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As a further thought, manufacturers usually recommend 100mm clearance between the water and exhaust holes at rest. There is also the resale issue.
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Old 18 January 2015, 04:27   #20
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Quote:
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The 5.4 is the same- I've been known to deliberately swamp mine by going astern and belt the throttle open with trunks down to wash the blood out after a messy string of mackerel. That's with the flooding hull blocked as well!
look what happens when you catch too much Mackerel!
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