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Transom Heights

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    Posted: 23 May 2020 at 9:58am
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Morning All,

We are in the process of having a boat built through Frewza in Invercargill. The boat comes with an engine pod for a 25" long shaft outboard.

Our current boat has a 20" on it. 

We are currently on the hunt for a reasonably priced 25" to mount on it once we have it but if we haven't got one by the time the boat is ready would it really be too much of a big deal if we chucked the 20" on for a little while?

I have done some googling which has said no worries but am after a couple more opinions/experiences.

Cheers.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 23 May 2020 at 11:00am
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Titanium
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you will very likely find the 20" will be still too high even right down...thats for ideal height.
 And may still not be very suitable.

I would suggest,  to measure the horizontal line off the keel of the new boat being built...
The vertical distance from that line to the mounting holes in the 'transom'
Measure distance from the top mounting holes on your current engine down to the top of the cavation plate.
 then add 1" to allow for the angle of the transom.

In practice ideally the cavitaion plate maybe slightly above the horizontal line off the keel line.

And /or speak with the builders see if they can or possible to add mounting holes for a 20"   may not be possible because the top edge height of the 'transom' mounting point.
This maybe able to be constructed a little lower.. by 2 to 3" and still have the required height above the mounting holes.


 
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Post Options Post Options   Likes (0) Likes(0)   Quote RC1 Quote  Post ReplyReply Direct Link To This Post Posted: 23 May 2020 at 11:40am
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No it will not work. Need a 25inch shaft engine
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 23 May 2020 at 5:08pm
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while boat under construction thought possible outside that totally agree unless mess with a engine jack.
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Post Options Post Options   Likes (0) Likes(0)   Quote sposman Quote  Post ReplyReply Direct Link To This Post Posted: 23 May 2020 at 5:52pm
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Brand new boat, get a new motor, I get Frewza to fit in Invercargill, Honda agent there great to deal with, you won't regret the money invested in the engine
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Post Options Post Options   Likes (0) Likes(0)   Quote Gappy Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2020 at 2:12pm
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Just done a frewza 18 cc fitout you will not get away with a 20inch it won't even be close to working

There is some serious deals on engines at the moment Suzuki are doing some really well priced engines in that mid range unsure what hp you are looking for.
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Post Options Post Options   Likes (0) Likes(0)   Quote CBF-Whk Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2020 at 3:26pm
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How about getting the pod done for 20 inch. Then when a new motor is purchased use a 25mm plate bolted to the pod to raise it to a 25 inch shaft.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2020 at 3:47pm
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Titanium
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Yep sort of what I was alluding to in posts above.
 Sort of pre set jack plate...
 how big is this boat? what size engine(s) current and new?
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Post Options Post Options   Likes (0) Likes(0)   Quote Gappy Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2020 at 5:19pm
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Certain boats don't suit shorter shafts as the engine ends up sitting to low in the water on these i wouldn't go to a 20inch shift you will end up swamping the engine.

20inch shafts are for flatter bottom boats and you just can't shove them on any boat
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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2020 at 8:54pm
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The boat I’m building is semi vee (8 degrees at the transom). I cut the transom at 20” as is standard. I wouldn’t want anything less, the bottom of the motorwell will be a lot closer to the water than I’d like. If I was doing it again I would have gone 25”.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 25 May 2020 at 8:13am
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Titanium
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Certain boats don't suit shorter shafts as the engine ends up sitting to low in the water on these i wouldn't go to a 20inch shift you will end up swamping the engine.

Had a good look at one on the weekend.. I have to go with the above comment.
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Post Options Post Options   Likes (0) Likes(0)   Quote MATTOO Quote  Post ReplyReply Direct Link To This Post Posted: 26 May 2020 at 4:33pm
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Long shafts I believe were developed to suit pods on yachts and powerboats to suit transom heights of the day.
But we're usually small cc capacity motors.

Early outboards for powerboats were short shafts.
There were inherent problems with this set up.

Because the motor had to be set so low to work for the hull shape they suffered numerous issues.

Flooding or dipping of the engine into the water through ballast movement, ie people and overloading.
But most common was the air intake issue with the high atomisations of water particles being sucked through the air intakes.
Obviously a fatal flaw.

So modern engines offer long legs and manufacturers of boats consider this with transom heights to keep the engine as high as possible out of the water.
And most modern engines now have sophisticated air intakes to reduce the draw of atomised sea water particles into the system.

So consider the principles above of issues and solutions for your needs.
Just cruising in my now sweetas pimped out Southern 755 HT0!
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Post Options Post Options   Likes (0) Likes(0)   Quote neil_cb125t Quote  Post ReplyReply Direct Link To This Post Posted: 26 May 2020 at 4:52pm
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Interestly when I purchased my new Suzuki they (suzuki NZ) mentioned that Short shafts are being banned in Aussie, due transom heights. I.e a short shaft 20" would be fitted to a lower transom and have a greater risk of swamping from a following wave. Therefore engine makers selling in aussie are no longer or soon to be no longer allowed to sell short shafts....
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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 26 May 2020 at 11:25pm
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20" isn't a short shaft. That would be 15". Only on dinghies really. 20" is the standard for most boats up to 5.5m. It's safe as long as there is a really good motorwell frame.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 2020 at 11:22am
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Im running a factory 2002 20" V6 Johnson 150 on the back of the commander
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